Trends in Colloid and Interface Science V
-
Upload
others
-
View
7
-
Download
0
Embed Size (px)
Citation preview
PROGRESS IN COLLOID & POLYMER SCIENCE
Editors: H.-G. Kilian (Ulm) and G. Lagaly (Kiel)
Volume 84 (1991)
Trends in Colloid and Interface Science V Guest Editor: M. Corti
(Pavia) and F. Mallamace (Messina)
0 Steinkopff Verlag • Darmstadt Springer-Verlag • N e w York
4
ISBN 3-7985-0885-2 (FRG) ISBN 0-387-91399-8 (USA) ISSN
0340-255-X
This work is subject to copyright. All rights are reserved, whether
the whole or part of the material is concerned, specifically these
rights of translation, reprinting, reuse of illustrations,
recitation, broadcasting, reproduction on microfilms or in other
ways, and storage in data banks. Duplication of this publication or
parts thereof is only permitted under the provisions of the German
Copyright Law of September 9, 1965, in its version of June 24,
1985, and a copyright fee must always be paid. Violations fall
under the prosecution act of the German Copyright Law.
© 1991 by Dr. Dietrich Steinkopff Verlag GmbH & Co. KG,
Darmstadt. Chemistry editor: Dr. Maria Magdalene Nabbe; English
editor: James Willis; Production: Holger Frey.
Printed in Germany.
The use of registered names, trademarks, etc. in this publication
does not imply, even in the absence of specific state- ment, that
such names are exempt from the relevant protective laws and
regulations and therefore free for general use.
Type-Setting: Graphische Texterfassung, Hans Vilhard, D-6126
Brombachtal Printing: betz-druck gmbh, D-6100 Darmstadt 12
Preface
The first meeting of the European Colloid and In- terface Society
(ECIS) was held in Como, Italy, in 1987. Three years later,
following meetings in Ar- cachon and Basel, the ECIS Conference was
again held in Italy at Copanello di Catanzaro in September 1990.
This gathering was attended by participants from 21 countries,
including the USA and Russia. More than 150 papers were presented
either orally or as posters. This volume includes most of these
papers, which have been rather ar- bitrarily subdivided into six
sections: Micelles, Microemulsions, Application of Colloids,
Interac- tion and Ordering, Biological Macromolecules, and Layers
and Interfaces. The interdisciplinary nature of these fields
bordering between physics and chemistry is evident. Unfortunately,
it was, of course impossible to reproduce in this volume the
lively, friendly atmosphere of the meeting; discus- sions outside
the conference room were wide-rang- ing and fruitful.
On behalf of the ECIS, we thank: all the par- ticipants for their
contributions; the scientific c o r n -
mittee: M. Almgren, S. J. Candau, R. Klein, R. H. Ottewfll, R.
Strey and M. Zulauf; from the Italian Ministry of University and
Scientific Research, Prof. A. Ruberti, and the President of the
Regional Government of Calabria, Dr. R. Olivo, both for their
dedicated patronage; from the city of Catanzaro, Mayor Dr. M.
Furriolo and Cultural Attach6 G. Guerriero; Prof. G. Stagno
D~lcontres, Rector of Messina University; and the generous sponsors
who made the Copanello meeting possible: The Italian Consiglio
Nazionale delle Ricerche support- ing the publication of this
issue, the Departiment of Physics of Messina University, the
Assessorato Agricoltura della Regione Calabria, IBM-Italy, Spec-
tra Physics, dB Electronics, and Chemifarm. Finally, particular
thanks go to our hosts and the staff of Villaggio Guglielmo in
Copanello for their hospitality.
Mario Corti Franco Mallamace
Micelles
Safran SA, MacKintosh FC, Pincus PA, Andelman DA: Spontaneous
vesicle formation by mixed surfactants. 3 Thalberg K, Lindman B,
Karlstr6m G: Electrolyte dependent phase separation in aqueous
mixtures of a polyelec-
trolyte and an ionic surfactant . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 8 van Stare J, Almgren M,
Lindblad C: Sodium dodecylsulfate-poly(ethyleneoxide) interactions
studied by time-
resolved fluorescence quenching . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 13 Cantu L, Corti M, Musolino M,
Salina P: Spontaneous vesicle formation from a one-component
solution of a
biological surfactant . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 21 Hoffmannn H, Hofmann
S, Rauscher A, Kalus J: Shear-induced transitions in micellar
solutions . . . . . . . . . . . 24 Lin T-L, Liu C-C, Roberts ME
Chen S-H: Mixed short-chain lecithin/long-chain lecithin aggregates
studied by
small-angle neutron scattering . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 36 Appell J, Porte G:
Polymer-like giant micelles. An investigation by light scattering .
. . . . . . . . . . . . . . . . . . . . . . . 41 Glatter O:
Scattering studies on colloids of biological interest (Amphiphilic
systems) . . . . . . . . . . . . . . . . . . . . . . 46 Baglioni P,
Dei L, Ferroni E, Kevan L: Electron spin echo modulation and
electron spin resonance studies of
sodium dodecylsulfate and dodecyltrimethylammonium bromide micellar
solutons: Effect of urea addition 55 Hill A, Candau F, Selb J:
Aqueous solution properties of hydrophobically associating
copolymers . . . . . . . . . . 61 Despotovi4 R: On mixed surfactant
systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Miinch C, Hoffmann H, Ibel K, Kalus J, Neubauer G, Schmelzer U,
Selbach J: A shear-induced structure transi-
tion on a micellar solution measured by time-dependent small-angle
neutron scattering . . . . . . . . . . . . . . . . . 69 J6hnannsson
R, Almgren M: A fluorescence and phosphorescence study of
AOT/H20/aikane systems in the L 2
reversed micellar phase . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 72 L6froth J-E, Johansson
L, Norman A-C, Wettstr6m K: Interactions between surfactants and
polymers. I: HPMC 73 L6froth J-E, Johansson L, Norman A-C,
Wettstr6m K: Interactions between surfactants and polymers.
Ih
Polyelectrolytes . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 78 Malliaris A,
Binana-Limbele W: Solubilization of aprotic additives in aqueous
micelles . . . . . . . . . . . . . . . . . . . . 83 Tsiourvas D,
Paleos CM, Malliaris A: Aggregation of polyamphiphiles with the
polar head on the main chain 86 Onori G, Ronca M, Santucci A:
Properties of water solubilized in reversed AOT micelles from
near-infrared
spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Onori G,
Ronca M, Santucci A: Shape and solvation of water-containing
reversed AOT micelles from viscosity
measurements . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 92 Ravey JC, Gherbi
A, St6b6 MJ: Mixed systems of fluorinated and hydrogenated nonionic
surfactants: The
air/water adsorbed film and micelles . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 95 Rauscher A, Rehage H, Hoffmann H:
Stretched exponential relaxation processes in viscoelastic
surfactant
solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 99 Schubert
K-V, Strey R, Kahlweit M: 3PHEX: A new surfactant purification
technique . . . . . . . . . . . . . . . . . . . . . . 103 Tondre C,
Derouiche A: Solubilization of electrolyte solutions in AOT
reversed micelles. Conductivity percola-
tion and phase behavior . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 107 Treiner C, Bury R:
Peculiar micellar solubilization of benzyl alcohol in binary
benzyldimethyltetradecylam-
monium chloride and trimethyltetradecylammonium chloride solutions:
A calorimetric investigation . . . . . . 108 Korolenko EC,
Shokhirev NV: Spin-controlled reactions on the micellar surface . .
. . . . . . . . . . . . . . . . . . . . . . . . . 112
Microemulsions
Teixeira J, Alba-Simionesco C, Angell CA: Glass transition in
microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 117 Senatra D, Lendinara L, Giri MG: W/O microemulsions as
model systems for the study of water confined in
microenvironments: Low resolution 1H magnetic resonance relaxation
analysis . . . . . . . . . . . . . . . . . . . . . . . . . .
122
VIII Progress in Colloid & Polymer Science, Vol. 84
(1991)
Atkinson PJ, Clark DC, Howe AM, Heenan RK, Robinson BH:
Characterization of microemulsion-based organo- gels . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 129
Baglioni P, Gambi CMC, Goldfarb D: Pulse electron spin resonance
and quasi-elastic light scattering of Winsor microemulsions . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 133
Rouch J, Safouane L, Cametti C, Codastefano P, Tartaglia P, Chen
SH: A dynamic transition at the percolation threshold of a
three-component microemulsion . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 139
Paillette M: Phase electric birefringence measurements in
attractive-type W/O microemulsion systems . . . . . . . 144 Liano
P, Duportail G: Fractal models for luminescence probing of
organized assemblies. Studies with respect
to the nature of the assembly, the temperature, and the quencher
concentration . . . . . . . . . . . . . . . . . . . . . . . . 151
Mallamace F, Magazu S, Micali N, Salvetti P: Microemulsion as model
system for the study of the glass-like tran-
sition: Refractive index and calorimetric measurements . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 155 Mallamace F, Micali N, Vasi C, D?~rragio G,
Paparelli A: Hypersound velocity measurements in dense
microemulsions, evidence of a viscoelastic behavior connected with
the percolation process . . . . . . . . . . . . . . 159
Interfaces
Woermann D: Critical phenomena in associative binary liquid
mixtures with miscibility gap . . . . . . . . . . . . . . . 165
Kuzmin SV, Malomuzh NP: Surface-induced polarization properties of
highly viscous liquids . . . . . . . . . . . . . . 171 Dgkrrigo G,
MaUamace E Micali N, Paparelli A, Teixeira J, Vasi C: Aggregation
phenomena in water-alcohol solu-
tions. Thermodynamic and dynamic studies . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 177 Aveyard R, Binks BP, Fletcher PDI: Effects
of subphase pH on the successive deposition of monolayers of
docosanoic acid onto mica . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 184 Gambaro M, Gliozzi A,
Robello M: Effect of surface charges on the electroporation process
in lipid bilayers. 189 Meunier J, Henon S: Optical study of
monolayers at liquid interfaces: Direct observation of first order
phase tran-
sitions and measurement of the bending elastic constant . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 194 Nahrinbauer h The interaction between polymer and
surfactant as revealed by interfacial tension . . . . . . . . . .
200 R6hl W, von Rybinski W, Schwuger MJ: Adsorption of surfactants
on low-charged layer silicates. Part I: Adsorp-
tion of cationic surfactants . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 206 Bartolotta A, Di
Marco G, Carini G, Tripodo G: Study of local and cooperative
molecular movements in Po-
ly(ethylene oxide) -- Potassium thiocyanate complexes by mechanical
measurements . . . . . . . . . . . . . . . . . . . . 215 Caminati
G, Tomalia DA, Turro NJ: Photo-induced electron transfer at
polyelectrolyte-water interface . . . . . . . 219 da Gra~a M.
Miguel M, Burrows HD: Luminescence study of fluidity in the L a
mesophase and liquid phase of
lead(H) decanoate . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 223 Bartolotta A, Di
Marco G, Carini G, Tripodo G: Relaxation processes in polymeric
electrolytes: Effect of the ca-
tion size and of the thermal history . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 227 Gabrielli G, Puggelli M,
Prelazzi G: Mono- and multi-layers containing ion carriers . . . .
. . . . . . . . . . . . . . . . . . . 232 Gallegos C, Nieto M,
Nieto C, Mufioz J: Influence of surfactant concentration on the
time-dependent theological
behavior of the lamellar liquid crystal . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 236 G6bel S, Hiltrop K: Influence of
organic counterions on the structure of lyotropic mesophases . . .
. . . . . . . . . . 241 Miller CA, Gradzielski M, Hoffmann H,
Kr/imer U, Thunig C: L 3 phases: Their structure and dynamic
properties . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 243 Kaeder U,
Hiltrop K: Alignment of lyotropic nematics by surface action . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
Lachaise J, Sahnoun S, Dicharry C, Mendiboure B, Salager JL:
Improved determination of the initial structure
of liquid foams . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 253 Papirer E,
Perrin JM, Siffert B, Philipponneau G: Surface characteristics of
colloidal aluminas and barium
titanates determined by inverse gas chromatography . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 257 Sch6n G, Peschel G, Stobbe H:
Impedance-spectroscopic investigations of water structure near
silica surfaces 262 Porte G, Appell J, Bassereau P, Marignan M,
Skouri M, Billard I, Delsanti M, Candau SJ, Strey R, Jahn W,
Snabre
P: Scaling laws for some physical properties of the L 3 phase . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 264 Paluch M: Effect of halogeno substituted ethyl
alcohols on the surface potential and on the surface tension
at
the water/air interface . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 266 Burger A, Rehage H:
Two-dimensional model networks . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
Rolandi R, Dante S, Maga L, Robello M: Domains formation in
polymerized monolayers revealed by
fluorescence microscopy . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 273 Schroder A, Candau SJ:
Study of the swelling of latex particles by means of ultrasonic
techniques . . . . . . . . . 275 Vandevyver M, Roulliay M, Bourgoin
JP, Barraud A, Morand JP, Noel O: Structure-reactivity relationship
in
Langmuir-Blodgett films of bisethylenedithio-tetrathiafulvalene
(BEDT-TFF) derivatives . . . . . . . . . . . . . . . . . . . 279
Has M, Lfidemann H-D: p,T dependence of the hydrophobic interaction
in a model solution . . . . . . . . . . . . . . 283 Shokhirev NV,
Burshtein AI: The change in density and pressure tensor at the
liquid-vapor interface . . . . . . . 285
Contents IX
Vituhknovsky AG, Sluch MI: Optical properties of Langmuir-Blodgett
films: perylene excimer formation . . . . 288 Churaev N, Kotov A,
Solometsev Y, Starov V: The influence of charged gel layers on the
electrokinetic
phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 290 Kotov A,
Solomentsev Y, Starov V: Direct approach of two particles covered
with a porous layer . . . . . . . . . . . 293
Application of Colloids
Bongiovanni R, Ottewill RH, Rennie AR: Small-angle neutron
scattering from dispersions of organophilic clays 299 Carpineti M,
Giglio M, Paginini E, Perini U: Low-angle static light scattering
by fast aggregation of polystyrene
latex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
Siffert B, Badri F: Competition between micellization and
adsorption of alkyl-PEO diblock copolymers on
titanium dioxide particles . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 309 Fucile E, Denti P,
Saija R, Borghese F, Sindoni OI: Density dependence of the
extinction coefficient of a disper-
sion of spherical metal particles . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 318 Carri6n Fit6 FJ:
Electrokinetic behavior of polyester and solid impurity during
washing process in the presence
of cellulose ethers and NTA . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 319 Herzog B: Micelle shape and
capacity of solubilization . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
Callejas-Fern~indez J, de las Nieves FJ, Martinez-Garcfa R,
Hidalgo-Alvarez R: Electrokinetic characterization and
colloid stability of calcium oxalate monohydrate dispersions in the
presence of certain inhibitors . . . . . . . . . . 327 Jenta TR-J,
Robinson BH, Batts G, Thomson AR: Enzyme kinetic studies using
lipase immobilised in microemul-
sion-based organogels . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 334 Mendiboure B, Graciaa
A, Lachaise J, Marion G, Bourrel M, Salager JL: Influence of the
intensity of mixing on
the droplet size distribution of emulsions: Theory and experiment .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 338 Lisiecki I, Lixon P, Pileni MP: Synthesis in situ in
reverse micelle of copper metallic clusters . . . . . . . . . . . .
. . . 342 Tondre C, Claude-Montigny B, Ismael M, Scrimin P, Tecilla
P: Metal-ion complexation by micelle-solubilized
long-chain complexing agents . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 345 Te~ak D, Heimer S, Derek V,
Strajnar F: Precipitation of aluminium with surfactant in sea-water
. . . . . . . . . . . 348 Palberg T, Hartl W, Deggelmann M,
Simnacher E, Weber R: Comparison of charge numbers of interacting
latex
spheres from different experiments: Conductivity, electrophoresis,
torsional resonance detection, and static light scattering . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 352
Schulz SF, Maier EE, Hagenbfichle M, Graf Ch, Weber R: Structural
properties of dilute aqueous solutions of charged rods studied by
light-scattering techniques . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
356
Di Biasio A, Bolle G, Cametti C, Codastefano P, Tartaglia P: Light
scattering from aggregating colloids: Stretched exponential
behavior of the time correlation function . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 359
Weyerich B, DAguanno B, Canessa E, Klein R: On the structure of
suspensions of charged rodlike particles. 362
Interaction and Ordering
Candau SJ, Ilmain F, Moussa'id A, Schosseler F: Structure and
properties of partially neutralized poly(acrylic acid) gels . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 369
Ostrowsky N, Gamier N: Brownian dynamics close to a wall, measured
by quasi-elastic light scattering from an evanescent wave . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 371
Arauz JL, Ruiz-Estrada H, Medina-Noyola M, Nagele G, Klein R:
Tracer-diffusion in binary mixtures of charged spherical
macroparticles . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 377
D~guanno B, M6ndez-Alcaraz J, Klein R: Structure and thermodynamics
of mixtures of charged spherical col- loidal particles . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 381
Granfeldt MK, J6nsson B, Woodward CE: The interaction between
charged colloids with adsorbed polyelectrolytes . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 391
Palberg T, Simon R, Leiderer P: Forced Rayleigh scattering in
mixtures of colloidal particles . . . . . . . . . . . . . . . . 397
Mimouni Z, Mathis C, Bossis G: Analysis of alignments of colloidal
spheres by light scattering . . . . . . . . . . . . 402 Peschel G,
van Brevern O: The contribution of hydration forces to
particle-particle interaction in a silica hydrosol 405 Chang S-L,
Chen S-H, Rill RL, Lin JS: Measurement and interpretation of
counterion distribution around cyclin-
drical polyelectrolytes . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 409
Chabalgoity-Rodrfguez A, Marffn-Rodrfguez A, Galisteo-Gonz~lez F,
Hidalgo-Alvarez R: Electrophoretic mobili-
ty, primary electroviscous effect and colloid stability of highly
charged polystyrene latexes . . . . . . . . . . . . . . . 416
Lemaire E, Paparoditis C, Bossis G: Yield stress in magnetic
suspensions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 425 Mallamace F, Micali N, Vasi C: Role of the ionic
strength in the viscosity of charged colloids . . . . . . . . . . .
. . . . 428
X Con~n~
Bio log ica l Macromole s
Margheri E, Bonosi F, GabrieUi G, Martini G: Spectroscopic
investigation on the effect of the addition of ceramide into lipid
vesicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 435
Giordono R, Grasso A, Teixeira J, Wanderlingh E Wanderlingh U: SANS
in lysozyme solutions . . . . . . . . . . . . 439 Huruguen JP,
Pileni MP: Changes in the percolation threshold by cytochrome c
addition in AOT reverse micelles 442 Gallardo V, Bolivar M, Salcedo
J, Delgado AV: A study of the effect of different amino acids on
the electrical
properties of nitrofurantoin suspensions . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 447 De Cuyper M, Joniau M: Effect of
dimethylsulfoxide on the kinetics and thermodynamics of
asymmetric
phospholipid fluxes between magnetic and non-magnetic vesicles . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 456 Deriu A, Cavatorta E Cabrini D, Middendorf HD: Molecular
structure and dynamics of biopoylmer gels by
neutron scattering . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 461 Cametti C, De
Luca E D'nario A, Macri MA, Briganti G, Maraviglia B: The ripple
phase in model membrane
systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 465 Domazou
AS, Mantaka-Marketou AE: Fluidity variation of DODAB vesicular
membranes with estrogen hor-
mone using the lucigenin chemiluminescent reaction . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 470 Edwards K, Almgren M: Solubilization of lecithin
vesicles by C12E8 . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 472 Fisicaro E, Pelizzetti E, Lanfredi E,
Savarino P: Osmotic coefficients of N-nonyl- and
N-decyl-nicotinamide
chloride surfactant aqueous solutions . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 474 Aliotta E Fontanella ME, Magazu" S,
Wandeflingth F: Hypersonic properties in macromolecular
aqueous
solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 483 Giordano R,
Grasso A, Wanderlingh F, Wanderlingh U: Static and dynamic
properties in thixotropic structures 487 G~ilvez-Ruiz MJ,
Cabrerizo-Vflchez MA, Galisteo-Gonz~lez F, Hidalgo-Alvarez R: Study
of temperature and pH
effects on phase transition liquid expanded/liquid condensed of
cholesterol, lecithin and lithocholic acid mixed monolayers . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 494
Seras M, GrabieUe-Madelmont C, Paternostre M-T, Ollivon M,
Handjani-Vila R-M: Study of non-ionic monoalkyl
amphiphile-cholesterol vesicles solubilization by octylglucoside .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502
Staunton S, Quiquampoix H: The use of a trace amount of methylated
bovine serum albumin as a probe of the state of bovine serum
albumin adsorbed on montmorillonite . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 506
Xenakis A, Valis TP, Kolisis N" Microemulsions as a tool for
enzymatic studies: The case of lipase . . . . . . . . . . 508
Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 512
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 514
Progress in Colloid & Polymer Science Progr Colloid Polym Sci
84:3--7 (1991)
Spontaneous vesicle formation by mixed surfactants
S. A. Safranl'4), E C. MacKintosh1), P. A. Pincus2), and D. A.
Andelman 3)
1) Exxon Research and Engineering, Annandale, New Jersey, USA 2)
Materials Department, University of California, Santa Barbara,
California, USA 3) Raymond and Beverly Sackler Faculty of Exact
Sciences, School of Physics and Astronomy, Tel Aviv
University,
Ramat Aviv, Israel 4) Polymer Dept., Weizmann Institute, Rehovot,
Israel
Abstract: Although single surfactants rarely form vesicles
spontaneously, mixtures of two surfactants can lead to spontaneous
vesicle formation. By considering the curvature elasticity of the
surfactant bilayer, we show theoretically how the energetic
stabilization of mixed vesicles can occur. In- teractions between
the two species (of the proper sign and magnitude) are crucial to
stabilizing these vesicles. These interactions lead to composition
asymmetries and effective spontaneous curvatures of the inner and
outer layers that are of equal and opposite signs.
Key words: _Vesicle; surfactant; selfassociation
I. Introduction
Since vesicles rarely form as the equilibrium structure of simple
surfactant-water systems, non- equilibrium methods, such as
sonication of lameUar liquid crystalline phases, are usually
necessary to obtain a metastable phase of vesicles, which may
reequilibrate back into the multilamellar, liquid crystalline
structure. Recently, however, Kaler et al. [1] have reported a
general method for producing equilibrium phases of vesicles of a
controlled size. The vesicles form spontaneously upon mixing sim-
ple surfactants with oppositely charged head groups. Most previous
reports of spontaneous vesi- cle formation have also involved
surfactant mixtures [2--5]. Using the charge as a control parameter
has both chemical and physical advantages since a wide variety of
head group, counterion, and salt chemistries can be prepared and
studied.
In this paper, we use the concepts of curvature elastic theory [6]
to explain the stability of vesicles formed in mixed surfactant
systems. In systems composed of a single surfactant, the curvature
energy of a bilayer dictates that the energy of a phase of
spherical vesicles is never lower than that of a multilamellar,
liquid crystalline phase [7, 8]. This is because the bilayer is
composed of two am-
phiphilic monolayers which, in the single surfactant case, have the
same spontaneous curvature [6]. Since the two layers have curvature
of opposite sign (e.g., the inner one being concave with respect to
the water, and the outer one convex), the system is frustrated.
Small vesicles, where the vesicle radius is of the order of the
surfactant size, can be of lower energy than fiat bilayer, as
discussed in [9--12]. However, they may be of higher free energy
than small micelles. In this work, we consider the case of large
vesicles and discuss their stability with respect to lamellar
phases; this feature can be compared with the experimental phase
diagrams [13]. We find [7, 8] that the stabilization of the
vesicles by surfac- tant mixtures only occurs when interactions of
the surfactants is considered; ideal mixing of the two components
does not yield vesicles as the ground state. These results can be
used to see how the in- teractions can be exploited to control and
stabilize the vesicle phase.
II. Mixed vesicles
In contrast to the situation for single amphiphiles, where large
vesicles are usually not energetically stable in comparison with
fiat bilayers, vesicles
4 Progress in Colloid & Polymer Science, Vol. 84 (1991)
composed of two amphiphiles can have lower cur- vature energies
than fiat films. The curvature energy [6--8] per unit area of the
vesicle is given by
fc = 2K[(c + c0) 2 + (c - -c i ) 2], (1)
where K is the bending elastic modulus [7, 8], q and c o are the
spontaneous curvatures of the inner and outer monolayers, and c is
the actual curvature of the inner layer. For the case of single
surfactant systems, in the limit of small curvatures, c o = c~. In
this case, the minimum of fc with respect to c im- plies that c =
0; fiat bilayers are the lowest bending energy state. For mixed
surfactants, constitutive relations for effective spontaneous
curvatures of the inner and outer layers, c i and c o are
needed.
For simplicity, we consider a model where the spontaneous
curvatures of films composed of each, single surfactant are equal,
cl = c 2, and define ¢/as the volume fraction of surfactant type
"2" in the system. In addition, we define ¢/~ and ¢/0 as the volume
fraction of surfactant "2" in the inner and outer layers,
respectively. The composition differ- ence between these two layers
is rp = 1/2(¢/0 -- ~i), with the constraint of fixed ~, --- 1/2(¢/0
+ ~,~).
We now describe a simple statistical model for the surfactant
head-head interactions which allows for a unified treatment of the
free energy of the system including both the elastic, entropic, and
interaction contributions. Our basic assumption is that the in-
teraction between head groups alone determines the spacing between
surfactants at the interfaces, while the resulting compression of
the surfactant tails determines the spontaneous curvature of each
monolayer. (In [11], we shall relax this assumption.) In this case,
the spontaneous curvature depends directly on the mean spacing
between surfactant head groups as a function of composition,
¢/.
We first consider a monolayer with a repulsive in- teraction +J
between like head groups, and an at- tractive interaction, - J
between opposite head groups. This suggests an Ising model
description for the energy H of a two-component mixture:
H = ~ JS~Sj, ( 2 )
where the sum over (i]) includes only nearest neighbor pairs. The
constituents are labeled by i, and S i = +1 (--1) denotes the
presence of surfac- tant (2). Furthermore, the attractive or
repulsive
interactions result in a local deformation of the bond distances
compared to their values for the pure surfactants (which are
assumed to have the same bond lengths). We describe this by a
quantity Aij, which is the change in the bond length bet- ween
surfactants at nearest-neighbor sites i and j. Finally, there is an
elastic-restoring force, with spring constant k:
H = ~. S ~ S j - B(1 -- SiSj)A 0 + - ~ A . (3) (ij~
Here, B represents the strength of the coupling be- tween the
composition and elastic degrees of freedom. Equation (3) represents
the compressible Ising model.
The mean-field value of (A,) is found by minimizing Eq. (3) with
respect to (Ais):
(A~j) = B(1 -- (S~Sj))/k ; (4)
the resulting expression for the free energy per sur- factant h
is
B 2 h = l ( S , - (1 - ( s , sj ) 2 . (5)
In random mixing, the nearest-neighbor correlation function (SiSj)
can be found by weighting the two possible values by the
appropriate product of in- dependent probabilities for finding
surfactants 1 and 2 at each site:
( s i s j ) = (1 - + C - 2 ,(1 -
= ( 1 - - 2 ¢ / ) 2 . (6)
Simple models for the packing of surfactant molecules at a surface
yield a spontaneous cur- vature which depends linearly on the mean
spacing between polar head groups. Within the model of the previous
section, the change in the spon- taneous curvature depends on
(Aij), and hence on I s i s j l :
/ / ( 1 - - ( S i S j ) ) = - - (7) - - c ( 0 ) = 7
The parameter ]/is of order a -1, where a is a micro- scopic
length. The precise value of ] /can be obtain- ed, although it is
somewhat model specific [11].
Safran et al., Spontaneous vesicle formation by mixed surfactants
5
Considering now the propert ies of a bilayer, and using the
definitions of the composi t ion asym- metries discussed above, we
arrive at the following expressions for the effective spontaneous
cur- vatures:
(8)
a = (c 1 - c2) - 8 ( 1 - 2 ~ , ) , (10b)
These formulae are writ ten for the general case where the
individual spontaneous curvatures are unequal . For the case where
q = c 2, the effective spontaneous curvature of the interacting
system is reduced (for fl > 0) compared with q . This reduc-
tion is just what is necessary to stabilize the vesicle so that the
effective spontaneous curvatures of the inner and outer layers are
equal and opposite, thus relieving the frustration present in the
single surfac- tant case. For ideally mixed, or non-interacting
sur- factants (fl = 0), a vesicle composed of a single sur- factant
has an outer layer which satisfies the spontaneous curvature, but a
frustrated inner layer. Interactions be tween the two surfactants,
however, can result in a contribution to the spontaneous cur-
vature which is opposi te in sign to both q and c 2. If more of
these pairs are placed on the inner layer, one can stabilize the
vesicle so that w h e n c = ci --- --c o , the system is at its
lowest curvature energy state and the frustration is relieved. This
is seen quantitatively from Eqs. (8) and (9) where the choice
¢ = + (e/p) v2 (11)
results in c i = --c 0. Note that this stabilization is only
possible if the interaction terms are con- sidered.
With this model , the curvature free energy of Eq. (1) then
becomes
fc = 4K[(c - - a(o) 2 + (5(q]) -- fl~a2) 2] . (12)
Thus, the spontaneous curvature of the bilayer is c = a(a. This
describes a flat bilayer, unless (p #0. We must now determine the
value (o*, which minimizes
the free energy as a function of ¢. When c = a~o, the free energy
per surfactant F c is
F c = 2 K a ( c ( u / ) - - fl¢2)2
= 2 K a [ c ( ¢ ) 2 _ 2flc0g)¢2 + fl2¢4], (13)
where a is the area per polar head group. The con- tribution of the
interaction terms of Eq. (5) to the free energy per surfactant
is
F ~ = / ( 1 - - 2 V ) 2 - - 8B z
k • ~ ( 1 - ¢ ) :
~p2
k
Similarly, for small values of ¢, the contribution due to the
entropy of mixing is
U
F m = kT i ~ , l og ~/ + (1 - - ~) log (1 - - ~u) i_
1 ( 1 -)rp2 •
12 (1 - - ~,)8 •
The total free energy per surfactant can be wri t ten a s
F = F o --~,~p2 + Acp4, (16a)
where
8 c = 4 K a B c ( ¢ ) - - 4J + - - B2(1 - - 6~(1 - - ~))
k
6 Progress in Colloid & Polymer Science, Vol. 84 (1991)
and F 0 is independent of ~a. Equations (15) and (16) are valid in
the high "temperature" limit. This cor- responds to interaction
terms J and B/k, which are small compared with kT. In this limit, e
- ( T c - - T )
and B - T, where T c --- Kaf l c (g] ) . Then, a spon- taneous
vesicle phase characterized by ¢ ~= 0 will occur below a second
order phase transition at T = To. This suggests that it will be
fruitful to more ful- ly examine the case of low temperatures, or
the case of strong interactions between the constituents
[11].
III. D i scuss ion
For e < 0, the minimum free energy state is com- posed of flat
bilayers where the two monolyers have identical compositions (~a =
c = 0). When e > 0, the free energy is minimized by a non zero
value of ~a and hence a non-zero curvature. However, the vesi- cle
phase is limited to a finite region of the phase diagram as a
function of the relative composition ¢/, as well as the absolute
concentration of amphiphile ~s. This limitation arises from the
imposition of packing constraints on the vesicles. This enables an
estimate of the phase diagram at fixed values of temperature, ]/,
cl, and c 2 as a function of concen- tration. Neglecting
polydispersity, the volume frac- tion of the system occupied by
vesicles is
4~ ¢~ = - - n R 3 , (17)
3
where R = 1/c* is the vesicle radius and n is the number density of
vesicles. For large vesicles, the volume fraction of surfactant
is
Cs = 8 n n r ~ R 2 . (18)
Eliminating n, we find that 6r3/R = CJcp. The vesicles cannot be
overpacked (¢ must be less than one); we take the value of ¢ = 1 as
the bound of stability of the vesicles with respect to the lamellar
phase where steric constraints are much weaker. An approximation to
the phase boundary as a function of (a s (the total volume fraction
of surfactant) and g~ (the fraction of surfactant that is type "2")
is then given by the locus of points which satisfy
r G = 6~c*(g]) , (19)
where
c* = a(g~)~a*, (20)
where ¢*(¢/) is the value of (a that minimize Eq. (16). A more
detailed discussion of the phase diagram can be found in [8,
11].
In summary, we have shown how interactions between surfactants can
stabilize a phase of spherical vesicles with respect to a fiat
lamellar phase. These interactions require that the effective
spontaneous curvature of the film have a term quadratic in the
composition. The physical origin of this stabilization is the
tendency of "1--2" surfactant pairs to have a different bond
distance from the average of "1--1" and "2--2" pairs. It is then
possi- ble for the effective spontaneous curvature of a film
composed mostly of "1-2" pairs to be quite different (even in sign)
from the spontaneous curvature of the pure films. In the case where
the curvature energy dominates, the vesicle is then stable; the
outer layer, for example, may consist mostly of "1--1" pairs and
the inner layer of the vesicle may be mostly "1--2". The
concentration asymmetry of the two layers is such that the
effective spontaneous curvatures of the inner and outer layer are
equal and opposite; the frustration of one of the layers that
destabilizes vesicles composed of a single sur- factant is thus
prevented.
Even within the context of this model, several outstanding issues
remain. The first is to explore the interactions and mixing effects
more generally for both the strong and weak interaction case [11].
In addition, the case of mixed amphiphiles of long and short chains
should be studied. Finally, the microscopic interactions which
determine the dif- ferent head spacings in ionic systems should be
ex- plored so that the interaction parameters fl can be related to
charge and salinity.
Acknowledgements
The authors acknowledge useful discussions with J. Israelachvili,
E. Kaler, D. Lichtenberg, Y. Talmon, and J. Zasadzinski. The
support of the US-Israel Binational Science Foundation under grant
no. 87-00338 is acknowledged. D. Andelman is grateful for the
support of the Bat Sheva de Rothschild Foundation.
References
1. Kaler EW, Murthy AK, Rodriguez BE, Zasadzinski JAN (1989)
Science 245:1371
Safran et al., Spontaneous vesicle formation by mixed
surfactants
2. Carnie S, Israelachvili JN, Pailthorpe BA (1979) Biochirn et
Biophys Acta 554:340
3. Gabriel NE, Roberts MF (1984) Biochemistry 23:4011; Hargreaves
WR, Deamer DW (1978) Biochemistry 17:3759
4. Miller DD, Bellare JR, Kaneko T, Evans DF (1988) Langmuir 4:1363
and J Phys Chem, in press
5. Jain MK, de Haas GH (1981) Biochim et Biophys 642:203; Alrnog S,
Kushnir T, Nir S, Lichtenberg D (1986) Biochemistry 25:6597
6. Helfrich W (1973) Z Naturforsch 28a:693 and in J de Phys (Paris)
47:321 (1986)
7. Safran SA, Pincus P, Andelman D (1990) Science 248:354
8. Safran SA, Pincus P, Andelman D, MacKintosh FC (1990) Phys Rev A
43:107 (1991)
9. Israelachvili JN, Mitchell DJ, Ninham BW (1972) Trans Far Soc II
72:1525
10. Israelachvili J, Mitchell DJ, Ninham BW (1977) Biochim et
Biophys Acta 470:185
11. A unified theory which accounts for both the cur- vature energy
and the in-plane interactions is given in E C. MacKintosh, S. A.
Safran, P. Pincus, to be published
12. Wang ZG, to be published 13. Kaler EW, unpublished
Authors' address:
Dr. S. A. Safran Department of Polymer Research Weizmann Institute
Rehovot 76100, Israel
Progress in Colloid & Polymer Science Progr Colloid Polym Sci
84:8--12 (1991)
Electrolyte dependent phase separation in aqueous mixtures of a
polyelectrolyte and an ionic surfactant
K. Thalberg, B. Lindman*), and G. Karlstr6m 1)
Physical Chemistry 1 and 1) Theoretical Chemistry, Chemical Center,
Lund University, Lund, Sweden
Abstract: A mixture of a polyelectrolyte and an oppositely charged
ionic surfac- tant generally phase separates from an aqueous
solution due to the strong at- tractive interaction between the two
solutes. Under certain conditions the con- centrated phase is a
transparent gel. Redissolution can be achieved by electrolyte
addition or by a high surfactant concentration. Over a wide range
of electrolyte concentrations, there is no phase separation.
However, at high elec- trolyte concentrations, separation into two
isotropic phases occurs. While phase separation at low electrolyte
contents results in one dilute solution and one phase concentrated
in polymer and surfactant, phase separation at high elec- trolyte
concentrations is of a different nature and results in one solution
rich in sttrfactant and one rich in polymer. The phenomenon is
related to, but different from that displayed by two polymers in a
common solvent; called "polymer in- compatibility", and can be
referred to the elimination of electrostatic interactions. The
phase diagrams can be modelled in Flory-Huggins type calculations
with reasonable assumptions of the intermolecular
interactions.
Key words: Cationic surfactant; polyanion; polyelectrolyte; phase
separation; phase _behavior; coacervate
Polymer-surfactant interactions are important in biology and in
several applications, such as emul- sions, pharmaceuticals,
cosmetics, detergents, paints, thickeners and foods.
Polymer-surfactant systems have been extensively studied with
respect to the binding of surfactants to polymers for relatively
dilute systems [1]. A range of physico- chemical parameters has
been used to map binding in terms of concentration of onset of
binding (the ciritical aggregation concentration, CAC) and of
saturation of binding. For many systems, binding isotherms have
been obtained, notably by the use of surfactant-selective
electrodes [2]. On the other hand, studies of more concentrated
systems as well as of phase behavior are sparse, although it is
well recognized that separation into two or more phases may easily
occur and be of significant biological and technical interest. As
regards structure of the systems, it can be inferred indirectly
that surfactant molecules self-assemble to micelle-like clusters
along the polymer chain, but there is also direct evidence for this
type of structure, notably from
neutron scattering [3], but also from fluorescence quenching
studies [4].
In systems containing a charged polymer (i.e., a polyelectrolyte)
and an oppositely charged surfac- tant, the interactions are
considerably reinforced, as is primarily seen in the low value of
the CAC relative to the critical micelle concentration (CMC) of the
surfactant. Also, for solutions of an ionic polymer and an
oppositely charged ionic surfac- tant, phase separation is commonly
observed over a wide range of mixing ratios. A conspicuous, but
general feature is the redissolution and formation of a single
isotropic solution phase at higher surfac- tant concentrations.
Redissolution can also be ef- fected by electrolyte addition, which
can be ac- counted for by the screening of the attractive polyion
-- surfactant ion interaction.
We have studied the phase behavior for polyelec- trolyte -- ionic
surfactant systems over a wide range of electrolyte concentrations
and report a further type of phase separation, which is distinct
from the one which is found in the absence of electrolyte
Thalberg et al., Electrolyte dependence of polyelectrolyte - -
surfactant systems 9
or at low electrolyte concentrations. The pheno- menon is compared
with other types of phase separation in colloidal systems and
attempts have been made to model the behavior in theoretical
calculations of phase diagrams applying the Flory- Huggins theory
of polymer solutions for a three- component system of solvent,
polymer, and cosolute.
Phase diagrams are presented here with hyalu- ronan, which appears
to show a typical polyelec- trolyte behavior, and cationic
surfactants of the alkyltrimethylammonium type. Hyaluronan (or
hyaluronic acid, here abbreviated Hy) is a linear anionic
polysaccharide, built of alternating units of glucuronic acid and
N-acetylglucosamine [5]. It plays an important role for the
physico-chemical properties of the extracellular matrix [6], and is
found in all mammals.
Samples containing polymer, surfactant, and water were thoroughly
mixed and equilibrated. Under certain conditions, there is
separation into one low-viscous supernatant phase, which is dilute
in polymer and surfactant, and one concentrated, often gel-like
bottom phase. The two phases were analyzed with respect to all the
components and from the composition of the phases the phase diagram
was traced. Systems of polyelectrolyte, ionic surfactant, and
solvent must strictly, in ther- modynamic considerations, be
treated as four-com- ponent systems. However, an adequate
representa- tion for many purposes will, for an isothermal case, be
in terms of a two-dimensional representation in a triangular
diagram.
The phase diagram for the system hyaluronan- tetradecyltrimethyl
ammonium bromide (C14TAB) - water is shown in Fig. 1. The two-phase
region is located close to the water corner, and has a droplet-
like shape. The supernatants are located at the left boundary of
this region, and the concentrated phase to the right and upper
sides of the region. The size of the two-phase region decreases
when a surfactant analogue of shorter chain length is used, but its
shape and location are largely retained. This phase behavior
applies also to other systems of polyelectrolyte and oppositely
charged surfactant, and seems to be of a wide generality.
We have been able to model the observed phase behavior in
calculations based on the Flory-Hug- gins theory for polymer
solutions [7]. Calculations for systems of polymer, solvent and
cosolute show that it is impossible to obtain anything near the
observed behavior without assuming the cosolute
Surfactant
7i
% Polyel.
Fig. 1. Phase diagram for the system hyaluronate-
tetradecyltrimethylammonium bromide-water [8]. The compositions of
some samples are indicated. Open circles refer to initial sample
compositions, and filled circles connected by tie lines refer to
the composition of the two phases in equilibrium. The dashed part
of the phase boundary indicates larger uncertainty in this
region
Polymer B /
/ -/z S.d
Water ~ - - - P o l y m e r A 10 20 30 40 % Polymer A
Fig. 2. Theoretically calculated phase diagram for a system of two
polymers in a common solvent (water) [8]. Polymer A represents the
polyelectrolyte and polymer B represents the surfactant. Index I
refers to the solvent, in- dex 2 to polymer A and index 3 to
polymer B. The interac- tion parameters used are w12 = i200 J/mol,
w23 = --5200 J/mol and the polymerization numbers are 300 for
polymer A and 25 for polymer B
to have a high molecular weight, which is in agree- ment with our
general view of a cooperative bind- ing of surfactant to a polymer
and the formation
10 Progress in Colloid & Polymer Science, Vol. 84 (1991)
of micelle-like clusters. The system is, therefore, treated as a
system of solvent and two polymers, A and B, representing the
polyelectrolyte and the sur- factant respectively, and an effective
interaction parameter wij is introduced between each pair of
species in the system. (wij is related to the normal Flory
interaction parameter by wij = R TXij). In ad- dition to the
interaction parameters, two other parameters have a major influence
on the phase behavior, namely the polymerization numbers for the
two polymers. Phase diagram calculations are performed by
minimizing the total Helmholtz free energy of the system with
respect to the composi- tion in the different phases. (For a full
description of the model and its limitations, see [8].)
By a proper choice of the five parameters, a theoretically
calculated phase diagram is obtained (Fig. 2), which shows a
reasonable agreement with the experimental one concerning the shape
and the location of the two-phase region, as well as the slope of
the tie-lines. The model is also able to ac- count for the changes
in phase behavior observed when the surfactant chain length is
varied [9]. The driving force behind the phase separation is the
favorable interaction between the polyelectrolyte and the
surfactant molecules, relative to the interac- tions of these
species with the solvent (water). A phase concentrated in both
these components, which enables a higher degree of contact between
polyelectrolyte and surfactant micelles, is then favored, in spite
of the loss in entropy for redis- tributing the components of the
system.
The phase separation seen in systems of a poly- electrolyte and an
oppositely charged surfactant thus seems to be of the same origin
as phase separation between two oppositely charged polymers or
other colloidal species. Such systems were thoroughly in-
vestigated by Bungenberg de Jong in the 1930s, and he was able to
qualitatively explain the observed phase behavior [10]. The
resulting concentrated phase was called a "coacervate" and the
phase sepa- ration was referred to as "complex coacervation'.
In Fig. 3, the phase diagram at addition of 75 mM of NaBr to the
system is shown. The area of the two- phase region is reduced while
its location in the phase diagram and the slopes of the tie-lines
are rather unaffected. If we add more salt, the two- phase region
shrinks and will finally vanish com- pletely. Investigations show
that this has occurred at 250 mM of added NaBr in the present
system.
Very striking is that, at addition of a rather large salt
concentration (i>500 mM of NaBr for this
Surfactant /
Poly- electrolyte
Fig. 3. Phase diagram for the system presented in Fig. 1 in the
presence of 75 mM of NaBr. Symbols as in Fig. 1. The contour line
of the two-phase region in the absence of salt is indicated
Water
Surfactant /
% Polyel. ,.
Fig. 4. Experimental phase diagram for the system presented in Fig.
1, in the presence of 1.0 M of NaBr. Symbols as in Fig. 1
system), two-phase separation reappears, but now the phase behavior
is of an entirely different type. The two phases in equilibrium are
again clear and isotropic, but now the supernatant phase is enrich-
ed in surfactant while the bottom phase is enriched in
polyelectrolyte. A phase diagram is shown in Fig. 4, corresponding
to a salt concentration of 1.0 M NaBr. The two-phase region is
still located close to the water-surfactant axis of the phase
diagram, but is not "anchored" at the water corner. Furthermore,
the tie-lines now have a different
Thalberg et al., Electrolyte dependence of polyelectrolyte - -
surfactant systems 11
direction. At a high salt concentration, a separation into one
polyelectrolyte-rich and one surfactant-rich phase is the case,
while the driving force for the phase behavior in the absence of
salt (Fig. 1), is the attraction between these two species. It is,
there- fore, evident that we here deal with a totally dif- ferent
phase separation mechanism.
For systems of two nonionic polymers in a com- mon solvent, a
familiar phenomenon is the polymer incompatibility, leading to the
separation of the polymers into two different solution phases (Fig.
5) [11]. Apparently, a behavior related to this can be expected for
two oppositely charged colloids or polymers when the electrostatic
attraction is eliminated. Although the phenomenon reported here to
some extent can be considered as related to the common two-phase
separation of polymer solu- tions, it can also from the phase
diagram be seen to be distinct from that.
polymers A and B, representing the polyelectrolyte and the
surfactant, respectively, remain unchang- ed, and, therefore, also
the interaction parameter between these two species.
Addition of salt is known to facilitate the forma- tion and growth
of ionic micelles in surfactant- water systems [12]. In the model
this corresponds to an increase in the polymerization number of
polymer B. (Besides, the interaction parameter bet- ween water and
polymer B has been slightly disfavored). Furthermore, the
interaction between the polyelectrolyte and the salt-containing
water will be more favored when the salt concentration is
increased, due to the electrostatic interactions bet- ween the
polyion and the salt ions. (This translates into a more favorable
interaction between polymer A and the solvent in the model.) By
appropriate changes in the interaction parameters according to the
above reasoning, the phase behavior can be modelled (Fig. 6).
Water
Fig. 5. Phase diagram for the system PEG 6000 -- Dextran D17 --
water at 20°C. Experimental points (triangles) and tie lines (.. •
), and theoretical phase separation curve and tie line (full lines)
calculated with the Flory-Huggins theory. See reference [11] for
further information
Polymer B Poly (ethylene glycol) uu60 ~ /
j o o , o,o / \
/ / ~ ~ . ' . . " . ~ : ' - , "~.... \ Water t , , , ) - - - P o l
y m e r A 10 20 30 40
lo is 2o 25 Dextran % Polymer A .
% Dextran
Fig. 6. Theoretically calculated phase diagram for a system of two
polymers in a common solvent (water). In- dices as in Fig. 2. The
interaction parameters used are W12 = --7200 J/mol, W13 = 1200
J/mol and W23 = --5200 J/mol and the polymerization numbers used
are 300 for polymer A and 100 for polymer B
The influence of salt has also been modelled as described above. In
order to circumvent the pro- blem of the fourth component, we have
chosen in the calculations to incorporate the added salt into the
water component. In this way, the two
We can thus qualitatively rationalize the phase behavior of the
system also at high salt concentra- tions. The dominating factor is
that the polyelec- trolyte prefers the salt-containing water to the
sur- factant. This, in combination with the poor inter-
12 Progress in Colloid & Polymer Science, Vol. 84 (1991)
action between water and the surfactant and the in- crease in size
of the micellar aggregates, which allows the surfactant to separate
out without too much loss in entropy, is the physical explanation
to this new phase separation mechanism it is in- dicated in the
model calculations that this phase behavior is very delicately
balanced, with respect to both differences in the interactions
between the three components and to the aggregation number of the
micelles, as modelled by the polymerization number of polymer B.
Especially, we note that for a large range of parameter values
between those used in obtaining Figs. 2 and 6, phase diagrams
without a two-phase region are calculated.
These results illustrate a rich new area of phase behavior in
surfactant-polymer-solvent systems. Several features have been
documented for systems other than that of hyaluronan and
tetradecyltri- methylammonium bromide (mainly considered in this
report), suggesting a considerable generality of the results. The
fact that it can be successfully reproduced by a quite simple and
general model (which does not depend on assumptions about the
structure in the systems) supports this view, as well as provides a
picture of the molecular interactions underlying the phase
separation phenomena.
3. Cabane B, Duplessix R (1982) J Physique 43:1529; Colloids Surf
(1985) 13:19
4. Turro NJ, Baretz BH, Kuo P-L (1984) Macromolecules 17:1321;
Abuin EB, Scaiano JC (1984) J Am Chem Soc 106:6274; Chu D, Thomas
JK (1986) J Am Chem Soc 108:6270
5. Hyaluronan was provided by Pharmacia AB, Upp- sala, Sweden in
the form of sodium salt (i.e. sodium hyaluronate). It was of a
highly purified quality, con- taining no appreciable amounts of
protein or other impurities. The molecular weight of the Hy
prepara- tion used in this work is about 250000
6. Comper WD, Laurent TC (1978) Physiol Rev 58(1):255; Laurent TC
(1987) A_cta Oto-Laryngol, Suppl 442:7
Z Flory PJ (1953) Principles of Polymer Chemistry; Cor- nell
University Press: Ithaca, NY
8. Thalberg K, Lindman B, Karlstr6m G (1990) J Phys Chem
94:4289
9. Thalberg K, Lindman B, Karlstr6m G, J Phys Chem, in press
10. Bungenberg de Jong HG (1949) In: Colloid Science, vol II, Ed:
Kruyt HR, Elsevier, Amsterdam; Chapter 10, p 259
11. Guastafsson .h,, Wennerstrtim H, Tjerneld F (1986) Polymer
27:1768
12. Lindman B, Wennerstr6m H (1980) Top Curr Chern 87:1
References
1. Goddard ED (1986) Colloids Surf 19:255/301; Hayakawa K, Kwak JCT
(1991) In: Rubingh D, Holland PM (eds) Surf Sci Ser, Marcel Dekker,
New York, chap 5
2. Hayakawa K, Kwak JCT (1982) J Phys Chem 86:3866; Hayakawa K,
Santerre JP, Kwak JCT (1983) Macromolecules 16:1642, Malovikova A,
Hayakawa K, Kwak JCT (1984) J Phys Chem 88:1930
Authors' address:
Prof. Dr. B. Lindman Physical Chemistry 1 Chemical Center
University of Lund Box 124 S-22100 Lund, Sweden
Note added in proof:
We have recently noted an analytical error in the chemical analyses
of the separating phases at high salt concentrations. Therefore,
the two-phase region in Fig. 4
is underestimated and the phase behavior comes even closer to that
of two nonionic polymers in a common sol- vent. The theoretical
model accounts for the behavior with a lower w u than given in Fig.
6. For a full account of the phase behavior see ref. 9.
Progress in Colloid & Polymer Science Progr Colloid Polym Sci
84:13--20 (1991)
Sodium dodecylsulfate-poly(ethyleneoxide) Interactions studied by
time-resolved fluorescence quenching
J. van Stam, M. Almgren, and C. Lindblad
Department of Physical Chemistry, Uppsala University, Uppsala,
Sweden
Abstract: The interaction between sodium dodecylsulfate (SDS) and
po- ly(ethyleneoxide) (PEO) has been studied by time-resolved
fluorescence quenching at 20°C and 40°C in the dilute regime, i.e.,
0.2% w/v, and in the semi-dilute regime, i.e., 2% w/v, with respect
to PEO. Lifetime measurements show that PEO wraps around the
micelle-like cluster formed by SDS upon interaction with PEO -- the
polymer shields the probe, pyrene, from quenching by bulk water
solubilized oxygen. The aggregation number determined at the
SDS-concentration when interaction starts, CAC, is much lower than
predicted by current theory. As the surfactant concentration is in-
creased, the aggregation number is simultaneously increased in the
dilute regime, but remains constant at low additions of SDS in the
semi-dilute regime. This indicates a certain number of locations
for the clusters on the polymer chain. At CAC clusters are formed
at these locations. Added surfac- tant is consumed by forming new
clusters until all locations are filled in com- petition with the
growth of excisting clusters.
Key words: Fluorescence quenching; sodium dodecylsulfate;
poly(ethylene- oxide); "_interactions; aggregation numbers
Introduction
Polymer-surfactant systems are of great interest, both from a
fundamental point of view and for ap- plications in a variety of
industrial fields, e.g., enhanced oil recovery, paint, and
medicine. Surfac- tants and polymers may interact with each other
in a way where the surfactant forms micellar-like ag- gregates (in
the following referred to as clusters to distinguish them from
ordinary micelles) in contact with or in the vicinity of the
polymer. In the case of ionic surfactants and a polyelectrolyte of
opposite charge, electrostatic effects are of prime importance.
Interactions between an ionic surfactant and an un- charged
polymer, on the other hand, cannot be ex- plained in this way, and
theories have been sug- gested for this type of systems [1, 2]. For
many reasons the systems of the anionic surfactant sodium
dodecylsulfate (SDS) and the uncharged poly(ethyleneoxide) (PEO, in
the literature some- times referred to as poly(ethyleneglycol))
have been used as model systems in the study of these
interac-
tions. Many studies have appeared, both with classical methods [3,
4] and with new ones, such as small-angle neutron scattering, SANS,
[5] time- resolved fluorescence quenching [6] and fluorescence
quenching [7]. Some good reviews in this field have also been
published, e.g., [8]. The over-all picture for interaction between
an anionic surfactant and a neutral polymer is that the interac-
tion starts at a critical aggregation concentration (CAC) which is
lower than the ordinary critical micellization concentration (CMC)
for pure surfac- tant solution. Moreover, it is also found that the
ag- gregation numbers for the polymer-interacting dusters are lower
than for ordinary micelles.
Even if results and suggested theories point in the same general
direction, they do not agree in the detailed picture of the
systems. For this reason it is of interest to continue studies of
the SDS-PEO system, aiming at a detailed description and
understanding of the interaction.
In this study, we present results for the natural fluorescence
lifetime of an excited probe, pyrene,
14 Progress in Colloid & Polymer Science, Vol. 84 (1991)
the aggregation numbers for the clusters obtained from
time-resolved fluorescence quenching measurements with dimethyl
benzophenone as quencher, and the III/I vibronic peak ratio from
pyrene steady-state fluorescence spectra, giving in- formation of
the micropolarity around the pyrene molecule for solutions without
polymer, with 0.2% w/v PEO, i.e., a dilute solution, and 2% w/v,
i.e., a semi-dilute solution.
Materials and methods
Poly(ethyleneoxide) (PEO) was purchased from Fluka (molecular
weight 35000) and was used as supplied. Pyrene (Aldrich) and
dimethylben- zophenone (DMBP) (Aldrich 99%) was twice
recrystallized from ethanol. Sodium dodecylsulfate (SDS) was from
BDH, specially pure. As conduc- tivity measurements gave CMC in
accordance with literature values (= 8 mM), the surfactant was used
without further purification. All solutions were prepared with
distilled water. For the deoxygenized experiments pure nitrogen was
used to remove oxy- gen from the solutions prior to measurement.
Dilute PEO solutions were 0.2% w/v and semi- dilute solutions were
2% w/v.
The preparation of samples for fluorescence measurements was
described earlier [9]. To allow pyrene to dissolve completely in
the micellar phase the solutions was stirred for at least 12 h. The
pyrene concentration was kept low enough (< 10 -5 M, or less
than one pyrene molecule per 50 clusters or miceUes) to prevent
excimer formation. The DMBP concentrations were chosen to be less
than one DMBP molecule per cluster or micelle.
Static fluorescence measurements were carried out on a SPEX
Fluorolog 1680 combined with a SPEX Spectroscopy Laboratory
Coordinator DMIB.
Time-resolved fluorescence decay data were col- lected with the
single photon counting technique, as described earlier [10]. The
set-up uses a mode- locked Nd-YAG laser (Spectra Physics, Model
3800) to synchronously pump a cavity-dumped dye laser (Spectra
Physics Models 375, 344S) for the excita- tion, using DCM as dye,
and a KDP crystal for fre- quency doubling. The excitation
wavelength was 320 nm and the pyrene monomer emission was measured
at 395 nm. The pulse width was less than 1.5 ns, which can be
treated as a 0-pulse compared to the lifetime of pyrene in our
experiments, about 150 ns. The excitation rate was low enough to
pre- vent multi-photon excitation. The temperature was
held constant by thermostatting the cuvettes and the cuvette holder
by the same standard water- bath. The measurements were performed
at two temperatures, 20°C and 40°C.
All data were analyzed on a Digital Equipment VAXstation 2000 with
the same method as describ- ed earlier [11].
Conductivity measurements were used to deter- mine the CMC or CAC
of each sample differing in either polymer concentration or
temperature. For these measurements a standard platinum conduc-
tivity probe connected to a Philips PW 9505 con- ductivity meter
was used. All solutions were ther- mostatted in a water bath and
stirred to allow equilibrium conditions.
The method of time-resolved fluorescence quen- ching in
microheterogeneous solutions is well described in the literature
[12, 13]. Under the condi- tions that the excitation pulse is
narrow compared to the fluorescence lifetime and that both probe
and quencher molecules are stationary in their host micelles during
the time window measured, the in- terpretation with the well-known
Infelta model [14] is straight-forward.
In the Infelta equation,
F(t) = A l e x p [ - - A 2 t + A3(exp( - -A4t ) - - 1)] , (1)
and the parameters have the following meaning under the
circumstances stated above:
A 1 is the fluorescence intensity at time t = 0, i.e., F(0). This
has no physical meaning, but is only dependent on the time one
allows each measure- ment to take.
A 2 is the decay rate at long time, i.e., when the decay shows an
exponential tail.
A 3 is the average number of quencher molecules per micelle. If one
knows the amount of bound sur- factant molecules and the
distribution of quenchers between micelles and the bulk phase, the
aggrega- tion number, (a), can be calculated from
A 3 x [surfactant]bo~ d (a) = ; (2 )
[quencherlbound
A 4 is the first-order quenching rate constant L. If it is assumed
to be inversely proportional, roug~aly, to the hydrophobic volume
of the micelle, the values can be used to check the reliability of
the estimated aggregation numbers.
The natural lifetimes r 0 were determined in separate experiments
without quencher, and the
1 a difference between 1/A 2 and r 0 is a measure of the condition
that the probe and quencher are sta- tionary during the time window
of the quenching experiments. If this difference is close to zero,
the quencher does not migrate between micelles, or micelles and the
bulk phase. The probe, pyrene, is certainly stationary under our
conditions, but the quencher, dimethylbenzophenone, have some
solubility in the water bulk phase. It turned out, however, that
also the quencher was stationary under the conditions of the
investigated systems. The water-solubility of the quencher turned
out to be significant only in solutions with a surfactant
concentration close to CMC or its analogue in polymer-surfactant
solutions, CAC. In all solutions the quencher concentrations were
corrected for this solubility.
It should be noted here that the solutions in general were not
deoxygenated, which means that the fluorescence decay was quenched
by oxygen. However, this only affects the natural lifetime r 0 and
does not influence the use of the Infelta model.
Results and discussion
The effect of PEO on the pyrene fluorescence quenching in SDS
micelles is immediately seen in Fig. 1. Figure la shows the set of
fluorescence quen- ching curves when PEO is absent, and Fig. lb
when it is present. As all other parameters, i.e., SDS and DMBP
concentrations, were the same, the decreased quenching in Fig. lb
shows that the ag- gregation number is much lower when PEO is pre-
sent.
For the quantitative discussion, the results can be divided into
three parts: lifetime measurements, ag- gregation numbers, and
III/I vibronic peak ratio from steady-state pyrene fluorescence
spectra.
l b
4
2
0 1 O0 200 300 400 500 600 t ime / ns
4
0
I I I I I
0 100 200 300 400 500 600 t ime / ns
Fig. 1. Time-resolved fluorescence quenching measure- ments at
20°C. Both figures are 22.9 mM with respect to SDS and have the
same DMBP concentrations. The upper curve is 0% and the lower curve
2% with respect to PEO, respectively. The DMBP concentration in mM
is from above: 0, 0.18, 0.27, and 0.36
Lifetime measurements
In aerated solutions, the presence of PEO in- creases the lifetime
of cluster-solubilized pyrene compared to pyrene in ordinary
miceUes (Fig. 2 and Table 1). This increase is more enhanced in the
semi-dilute solutions. Comparing the results from two temperatures,
the same behavior is found, but is more pronounced at the lower
temperature. In deoxygenated samples no difference in lifetime bet-
ween the different systems is observed, as is also in
Fig. 2 and Table 1. Clearly, the polymer shields pyrene in the
micellar aggregates from quenching by oxygen. Normal micellar
solutions offer little protection in this respect. Assuming the
oxygen concentration in air-saturated water at 20 °C to be 5.7 • 10
~ M [15] the lifetimes in Table 1 for pyrene in SDS without PEO
give a second-order quenching constant of 4.7 • 109 M -1 s -1,
which is reduced to 2.9 • 109 M -1 s -1 in 2% PEO. The former value
is as expected for a diffusion-controlled process in a homogeneous
aqueous solution. The protective ac- tion can be understood if one
imagines that the
16 Progress in Colloid & Polymer Science, Vol. 84 (1991)
th r =
[ S D S ] / raM
Fig. 2. Lifetime of pyrene. The symbols denote: circles = 0% PEO,
squares = 0.2% PEO, triangles = 2% PEO, open symbols = 20°C, and
filled symbols -- 40°C. The two up- per curves refer to
deoxygenated samples, squares with diagonal line = 0.2% PEO, 20°C;
squares with cross mark = 0.2% PEO, 40°C. Inserted line shows
lifetime in deox- ygenated ordinary micelles
polymer wraps around the cluster in a rather com- pact layer, as
has been suggested by others [5]. In this way the polymer will
replace water molecules from the interface between the cluster and
the bulk, and decrease the possibility for oxygen to quench the
fluorescence.
It could be argued that oxygen contained in the clusters, and not
molecules approaching from the bulk solution, were responsible for
the quenching. There are two strong arguments against such a
mechanism: i) the fluorescence decay is single-ex- ponential, and
ii) even if the solubility of oxygen in the micelles was 10 times
that in water, very few of the aggregates would contain an oxygen
molecule.
The fluorescence lifetime decreases with increas- ing
surfactant/polymer concentration ratio. This is already seen at low
concentrations of SDS in the dilute regime, but also at the highest
concentrations of SDS in the semi-dilute regime. The protective ac-
tion of the polymer starts to decrease before it is saturated and
free micelles form.
Aggregation numbers
In the dilute polymer solutions, it is found that the aggregation
number at CAC is about 20 (see Fig. 3a and Table 2) at both
temperatures. The ag-
Table 1. Natural lifetimes, r 0, for the systems in- vestigated.
All lifetimes are given in ns-units
I. Air-saturated samples [SDS] 0% PEO 0.2% PEO 2% PEO mM
20°C 4 0 ° C 2 0 ° C 4 0 ° C 2 0 ° C 40°C
9.0 10.2 15.3 17.5 20.0 20.4 25.0 25.5 30.6 40.8 60.0
100.0 200.0 300.0 4O0.0
217.2 169.1 179.2 129.0 217.2 165.3 181.6 131.2 220.9 166.3
210.0 160.0 204.7 158.2
180.7 131.0 219.9 163.4 202.9 153.9
182.3 134.7 216.6 162.1 179.0 138.6 219.0 163.8 185.6 134.2 214.9
157.2
192.4 147.9 218.1 164.0 201.2 150.0 194.3 150.2 189.6 153.6
[SDS] mM
20°C 20°C 40°C
9.0 338.1 330.5 17.5 345.0 347.9 327.0 60.0 312.7 342.8
gregation numbers increase gradually upon in- creasing the SDS
concentration, up to a limiting value. This value, approximately
60, is equal to that found in ordinary SDS micelles at this
concentra- tion.
The same behavior is seen in the semi-dilute solu- tions (see Fig.
3b and Table 2), but shifted to much higher surfactant
concentrations; the value found in ordinary micelles at this
concentration, approx- imately 100 [16, 17], is not reached until
the SDS- concentration is as high as 300 raM. The only temperature
studied in the semi-dilute solutions was 20°C, but reference
measurements at 40°C show the same pattern.
A source of uncertainty in the interpretation in this case is the
relatively high CMC of SDS. It is known from NMR measurements [18]
that the amount of free surfactant decreases in aqueous solutions
when the surfactant concentration is in- creased -- thus an
uncertainty is introduced by set-
van Stare et al., Sodium dodecytsulfate-poly(ethyleneoxide)
interactions 17
10
0 6O [SDS] / mM
0 400
Fig. 3. a. Aggregation numbers vs SDS concentration in 0.2% PEO.
Open circles refer to 20°C and filled circles to 40°C,
respectively. Dashed lines show results from model simulations.
Inserted line shows aggregation number for a free micelle in this
concentration range, b. Aggregation numbers vs SDS concentration in
2% PEO and at 20°C. Inserted line shows aggregation number for a
free miceUe at the upper part of the concentration range
Table 2. Aggregation numbers, quenching rate constants and
polydispersity indexes from fluorescence quenching measurements.
For the dilute solutions, i.e., 0.2% PEO, the limiting duster
aggregation number is 56 and 42 at 20°C and 40°C, respectively,
from model simulations. Or- dinary SDS micelles have an aggregation
number of about 60 in the concentration range in the dilute
solution, and of about 100 in the upper part of the concentration
range in semi-dilute solution. Ordinary STS micelles have an
aggegation number of about 90 for the STS micelles in the
concentration range investigated
I. SDS and 0.2% PEO [SDS] (a)w kq " 10 -7 S -1 G/~a)w mM
20°C 40°C 20°C 40°C 20°C 40°C
9.0 26.4 20.3 4.5 10.0 0.76 0.96 12.0 30.1 22.9 4.3 9.0 0.35 0.19
17.5 41.2 39.2 3.7 8.1 0.37 0.37 20.0 49.2 41.4 3.6 7.4 0.37 0.22
25.0 56.1 49.2 3.3 7.2 0.44 0.49 60.0 62.1 54.7 3.4 7.3 0.40
0.39
II. SDS and 2% PEO [SDS] la~w kq • 10 -7 s -1 a/la)w m M
20°C 40°C 20°C 20°C
15.3 24.5 3.8 0.93 22.9 20.6 5.1 1.32 51.0 26.0 3.9 0.32
100.0 58.3 55.4 3.2 0.51 200.0 83.4 77.5 2.8 0.43 300.0 99.0 92.4
2.6 0.39 400.0 103.8 97.5 2.5 0.37
III. (a)w for STS at 40°C [STS]/mM 0.2% PEO 2.0% PEO
9.0 54.8 28.7 25.0 89.4 33.9
ting the concentrat ion of free monomers equal to the CMC. This mus
t play a great role w h e n the SDS concentrat ion is close to CMC,
and there is no data on ho w the amoun t of free surfactant behaves
w h e n PEO is present . To overcome this problem, some reference
measurements were per fo rmed with the more hydrophobic surfactant
sod ium tetradecyl- sulfate (STS), for which the two additional
methylene groups have reduced the CMC to about 2 raM, and the CAC
to about 1.5 mM at 40°C; at 20°C, STS is insoluble. The pat tern is
just the same
as in the case of SDS - - the aggregation n u mb er in- creases
already cont inuously at very low concentra- t ions in the dilute
solut ion and was constant in the semi-dilute (Table 2).
At CAC, where only clusters and no micelles are present , the
aggregation numbers were about 1/3 of those of the ordinary
miceUes. This is lower than predicted by the theory of Nagarajan
[1], whose model gives aggregation numbers in the range
18 Progress in Colloid & Polymer Science, Vol. 84 (1991)
45--55 for the SDS-PEO system, but in the same range as those found
by Zana et al. [6] in a time- resolved study of pyrene excimer
formation.
At higher concentration of surfactant, one must keep in mind that
the method measures an average over the whole system. This means
that if both small clusters interacting with the polymer and big-
ger ordinary micelles are present, one gets an average aggregation
number, approximately weighted by the hydrophobic volume of the two
states. In this case, when the aggregation number for the clusters
at CAC is about 20 and that for the ordinary micelles about 60,
equal weights for the two states is reached when the number ratio
bet- ween polymer-interacting clusters and ordinary bulk micelles
are 3:1. On the other hand, it is possi- ble to use this feature to
understand the aggregation behavior. In the dilute solutions, a
number of small
c 20.0 clusters are formed at CAC; this number is equal to .~ or
just below the maximum number of clusters in ,- o the system. The
main effect of increasing the surfac- ~ 15.0 tant concentration is
not to form more clusters, but E to increase their aggregation
number up to a limiting value, lower than the aggregation number o.
10.0 for ordinary micelles. When the clusters reach the limiting
size, ordinary micelles are formed. In the semi-dilute solutions,
also small clusters are formed ~ 5.0 at CAC, having the same size
as in the dilute solu-
O} tions. But, added surfactants are first consumed by < forming
more clusters; as the amount of polymer is 0.0 10 times higher, the
maximum number of clusters is also 10 times higher, and the growth
occurs much slower than in the dilute solution. This rough model is
tested for the systems investigated by = 8.0 simulation and gives
excellent concordance with "~
'- 7.0 the experimental points in the dilute solutions o (dashed
lines in Fig. 3a) at both temperatures. In ~ 6.0 the semi-dilute
solutions the model cannot E ->" 5.0 reproduce the experiments,
as the model tested o Q.
4.0 does not take into account the possibility of forming more
clusters, o. 3.0
There is also a temperature dependency in the ag- 2.0 gregation
numbers. In the dilute solutions the P
model gave the limiting sizes for the cluster ag- m 1.0 gregation
number to be 56 and 42 at 20 °C and 40 °C, < 0.0 respectively.
This explains why the average aggrega- tion number is always lower
at the higher temperature at corresponding surfactant concentra-
tions. The lower cluster size limit at higher temperature can be
explained by the increased hydrophobicity of PEO at elevated
temperatures. The polymer will then shrink and be less
flexible,
avoiding water contact. Thereby, the interaction bet- ween the
polymer and the ionic cluster is also restricted to lower cluster
aggregation numbers.
The number of aggregates per polymer chain is calculated from
A3-values in Eq. (1) and the quen- cher concentrations. According
to the model, this value should be constant up to the point where
they reach their maximum aggregation number. This is also indicated
in the dilute solution; in Fig. 4a, this is most pronounced at 40
°C, but not seen at all in the semi-dilute solution (Fig. 4b).
Evidently, growth of existing clusters and formation of new
clusters occur simultaneously. This differs from the picture
suggested by Cabane [5], as our interpretation does
0 60
4 a
[SDS] / mM
4 b
0 400 [SDS] / mM
Fig. 4. a. Number of aggregates per polymer chain at 0.2% PEO. Open
circles refer to 20°C and filled circles to 40°C, respectively, b.
Number of aggregates per polymer chain at 2% PEO and 20°C
van Stare et at., Sodium dodecylsutfate-poty(ethyleneoxide)
interactions 19
not predict a stoichiometric composition of the cluster. Instead,
our model is more like that sug- gested by Winnik and Winnik for
the systems SDS- hydroxypropyl cellulose [7].
In a system consisting of two sets of aggregates of different
sizes, the polydispersity would be of in- terest. From
time-resolved fluorescence quenching measurements it is possible to
calculate the poly- dispersity [13, 19] (Table 2). Due to the very
small difference between the aggregation number for the free
micelle and the limiting aggregation number for the cluster, no
clear indication of a broader size distribution is seen in that
concentration range. In- stead the data in Table 2 indicates a
higher poly- dispersity at very low surfactant concentrations in
the semi-dilute solution. The competition between cluster growth
and cluster formation causes a very broad polydispersity in this
region. When increas- ing the surfactant concentration, this
polydispersity is first decreased and then increased again, which
may be due to the fact that two different size sets are present in
the system.
The values of the quenching rate constants, also collected in Table
2, would be expected to decrease with the increasing micellar size.
This is also found, but not to the extent expected. Zana et al. [6]
similarly found the excimer formation rate constant k¢ to be
decreased much less than expected. Fur- thermore, in an
investigation of the system CnTAB- hyaluronan [9], with n = 10, 12,
a similar slow change of the quenching rate constant with the
duster aggregation number was found. The interac- tion with polymer
seems to slow down the quen- ching process in these systems.
Static fluorescence
The temperature also affects the III/I-values, the ratio between
the third and the first vibronic peak in the pyrene steady-state
emission spectrum. This value decreases when the surrounding
environ- ment experienced by the pyrene molecule becomes more
polar. Typical values in water are about 0.6, and in a SDS micelle,
about 0.9--1.0 [20].
Comparing the aqueous solution, the dilute solu- tion, and the
semi-dilute solution at the two temperatures (Fig. 5), it can be
concluded that the differences in III/I are greater at 20°C. For
the aqueous SDS solution, the values are only slightly higher at 40
°C, but the difference is increased in the polymer solutions. In
the dilute solution at high
1.0
0.9
z
0.8
0.7
40 [SDS] / mM
Fig. 5. III/I vibronic peak ratios from pyrene steady-state
fluorescence spectra. The symbols denote: squares = 0% PEO,
tirangles = 0.2% PEO, circles = 2% PEO, open sym- bols = 20°C, and
filled symbols = 40°C
SDS concentration the ratio comes very close to the ratio in the
aqueous solution at both temperatures, but in the semi-dilute
solution this is true only at the higher temperature. This supports
the idea of a limited number of interacting dusters on each polymer
chain, as the III/I-values in aqueous micellar solution are
approached in the dilute system at higher surfactant concentration,
but not in the semi-dilute system. At higher temperature the
approach to the values of the aqueous SDS solution occurs at lower
surfactant concentration. This can be explained by a higher
hydrophobicity of PEO at elevated temperatures, which would lead to
a stronger interaction with the hydrophobic parts of the cluster
interface, in accordance with the fin- dings from the lifetime
measurements mentioned above -- the stronger interaction with the
polymer leads to a less polar cluster with less difference bet-
ween the lifetimes at higher temperature. The polymer itself does
not interact with pyrene, as is seen from the III/I-values at zero
surfactant concen- tration.
In the small clusters, with a higher curvature than ordinary
micelles, pyrene would have been more exposed to water than in
micelles, were it not for the shielding effect of the polymer. The
increased shielding explains the small, but significant dif-
ferences in III/I-values between the different PEO- concentrations.
At higher temperature there will be a less polar pyrene
microenvironment due to stronger interaction between the polymer
and the
20 Progress in Colloid & Polymer Science, Vol. 84 (1991)
cluster, thus leading to a decreased difference in III/I-values at
different PEO concentrations.
Conclusions
Each polymer chain can host a certain number of clusters that
depends only on the chain length. At the critical aggregation
concentration (CAC), surfac- tant aggregates or dusters start to
form almost simultaneously at this number of locations.
At the CAC the aggregates, or clusters, have very low aggregation
numbers -- about one-third that of the ordinary micelles. The
polymer wraps around the cluster and replaces water at the
cluster/bulk in- terface. Added surfactant is consumed by the
growth of all clusters simultaneously, and not by the build-up of
more clusters; almost all duster locations on the polymer chain
become immediate- ly occupied in the dilute solutions. When the
clusters grow, the polymer will wrap around a decreasing part of
the duster interface area, thus leading to a decreased shielding
from oxygen quen- ching. In the semi-dilute solutions, however, at
CAC, not all locations are occupied because of the 10 times higher
polymer concentration. In this case, added surfactant will first
mainly be consumed by occupying the remaining locations before the
clusters start to grow, but a certain amount simultaneously takes
part in the cluster growth.
From comparing model simulations and the ex- perimentally found
aggregation numbers at the two temperatures, we conclude that the
limiting size of the clusters is smaller than for ordinary
micelles. This difference increases with increasing tempera-
ture.
Acknowledgement
This work was supported by the Swedish Natural Science Research
Council and the Swedish Natio