1 INTRODUCTIONAmphiphilic molecules can self-assemble into long cylin-

drical micelles in aqueous solutions, commonly known aswormlike micelles, which eventually entangle to form vis-coelastic networks 1-3). The equilibrium and dynamics ofthese structures can be affected by changes in tempera-ture, dilution, variation of salt concentration, or the addi-tion of oils or polymers 4,5). In the case of charged micelles(ionic surfactant systems), micellar growth occurs as a

consequence of reduction of surfactant head group’s inter-action, which is in general induced by salt ions, stronglybinding counterions or sometimes by cosurfactants 1,2,6-11).The molecules packed at the hemispherical ends of longaggregates have an excess free energy (so-called “end-capenergy”) in comparison to the molecules at the cylindricalpart; therefore, the “end-cap energy” acts as a drivingforce for the linear growth of the cylindrical micelles. Thesystem can minimize the free energy by reducing the num-

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Journal of Oleo ScienceCopyright ©2009 by Japan Oil Chemists’ SocietyJ. Oleo Sci. 58, (5) 243-254 (2009)

Wormlike Micelles in Mixed Amino Acid Surfactant/Nonionic Surfactant Aqueous Systems and the Effect of Added ElectrolytesRekha Goswami Shrestha1, Carlos Rodriguez-Abreu2 and Kenji Aramaki1＊1 Graduate School of Environment and Information Sciences, Yokohama National University (79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPAN)

2 Institut de Química Avançat de Catalunya. Consejo Superior de Investigaciones Científicas (IQAC/CSIC) (Jordi Girona, 18-26, 08034Barcelona, SPAIN)

Abstract: The formation of viscoelastic wormlike micelles in mixed amino acid surfactant/nonionicsurfactant aqueous systems in the presence of different counterions and salts is reported, and the effects ofthe different electrolytes on the rheological behavior are discussed. N-dodecanoylglutamic acid (LAD) isneutralized with biologically relevant L-lysine and L-arginine to obtain anionic surfactants (LAD-Lys2,LAD-Arg2) which form aqueous micellar solutions at 25℃. Addition of a nonionic surfactant, tri-ethyleneglycol mono n-tetradecyl ether (C14EO3), to the aqueous solutions of both LAD-Lys2 and LAD-Arg2 causes the zero-shear viscosity (h0) to increase with C14EO3 concentration gradually at first, and thensharply, indicating one-dimensional growth of the aggregates and eventual formation of entangledwormlike micelles. Further addition of C14EO3 ultimately leads to phase separation of liquid crystals. Sucha phase separation, which limits the maximum attainable viscosity, takes place at lower C14EO3

concentrations for LAD-Lys2 compared to LAD-Arg2 systems.It was found that the rheological behavior of micellar solutions is significantly affected by the addition of

Na+X– salts (X=Cl–, Br–, I–, NO3–). The maximum viscosities obtained for the systems with added salt are all

higher than that of the salt-free system, and the onset of wormlike micelle formation shift towards lowernonionic surfactant concentrations upon addition of electrolyte. The maximum attainable thickening effectof anions increases in the order NO3

– >I–>Br–>Cl–.The effect of temperature was also investigated. Phase separation takes place at certain temperature,

which depends on the type of anion in the added salt, and decreases in the order I–>NO3– >Br–≈Cl–, in

agreement with Hofmeister’s series in terms of amphiphile solubility. The thermoresponsive rheologicalbehavior was also found to be highly dependent on the type of anion, and anomalous trends, i.e. viscosityincrease with temperature, were observed for all anions except Br–.

Key words: amino acid surfactant, phase behavior, wormlike micelles, rheology, viscoelastic solutions

＊ Correspondence to: Kenji Aramaki, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPANE-mail: aramakik@ynu.ac.jpAccepted January 15, 2009 (recieved for review December 11, 2008)

Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 onlinehttp://www.jstage.jst.go.jp/browse/jos/

R.G. Shrestha, C. Rodriguez-Abreu and K. Aramaki

ber of free end-caps. Hence, wormlike aggregates arefavored over short rod-like micelles. Above a certain con-centration, these wormlike micelles entangle with eachother and form a transient network responsible for its vis-coelastic properties.

Wormlike micelles cover a wide range of applications.They are mostly used in food, emulsions, cosmetics, paints,pharmaceuticals, adhesives, and many household products.Therefore, knowledge on the structure and dynamics ofwormlike micellar systems is important for the optimiza-tion of these applications12-16). Salt induced wormlikemicelles in ionic surfactants have been widely reported17-20).A few papers are available in the literature on the rheologi-cal behavior of wormlike micelles in mixed system ofionic/nonionic surfactant systems21-25). Cationic surfactantin the presence of strongly binding counterions or excesssalts, forms viscoelastic wormlike micelles 26,27). Similarly,the formation of viscoelastic wormlike micelles in mixedsystems of cationic/anionic surfactants is well studied10,28).However, anionic surfactants are used in a greater volumethan any other surfactants, because of their highly potentdetergency and low cost of manufacture. Amino acid basedanionic surfactants (both natural and synthetic types) areone of the environmentally friendly anionic surfactantshaving many applications in cosmetic and toiletry productsdue to their mildness to the skin. Recently, we have foundthat the N-dodecanoylglutamic acid, designated as LAD(an amino acid based anionic surfactant) upon neutraliza-tion with triethanolamine (TEA) or L-lysine forms vis-coelastic wormlike micelles in mixed systems ofLAD/cationic surfactant or LAD/nonionic surfactantwithout addition of any salts 29,30). However, there is noreport on this kind of surfactants concerning the effects ofsalts (often present in formulations) or the effect of chang-ing organic counterions, on the phase behavior and rheo-logical properties of amino acid surfactant aqueous sys-tems.

In this paper we report on the phase behavior and for-mation of wormlike micelles in mixed LAD-X/nonionic sur-factant systems where X are biologically relevant organiccounterions (i.e., L-lysine and L-arginine), and the effect ofdifferent salts on the rheological behavior.

2 EXPERIMENTAL2.1 Materials

N-dodecanoylglutamic acid (designated as LAD) and L-arginine were obtained from Ajinomoto Co., Inc. Japan.Homogeneous tri-ethyleneglycol mono n-tetradecyl ether(C14EO3) was purchased from Nikko Chemicals Co., Ltd.Japan. Sodium Chloride, Sodium Bromide, Sodium Iodideand Sodium Nitrate were purchased from Wako PureChemicals Industries, Ltd. Japan. The purity of all chemi-

cals was higher than 99%, so they were used as receivedwithout further purification. The molecular structure ofLAD is presented in Scheme 1.

2.2 Methods2.2.1 Phase behavior

First of all, aqueous binary phase behavior of neutralizedLAD was studied at 25℃. For this purpose, different sam-ples (~0.50 g each) of composition range 2-50 wt% wereprepared in clean and dry glass ampoules (size 12 mm ×100 mm) and immediately flame sealed. The chemicalswere properly weighed and the total uncertainty for thevalues of concentration of each component is ±0.01%. Thesamples were mixed with vortex mixer, and repeated cen-trifugation in order to get homogeneity. After mixing, thesamples were put into a water bath with temperature setto 25℃ with the accuracy of the thermometer ±0.5℃. Thesamples were left for 24 h before identifying the equilibri-um phases. Next, partial ternary phase diagrams were con-structed at 25℃. Samples with compositions ranging from5 to 40 wt% surfactant (1 g each) were prepared in cleanand dry glass ampoules with screw cap. Samples wereproperly mixed with vortex mixer, and repeated centrifu-gation. Finally, ternary phase diagrams were constructedby adding C14EO3 and following the method of titration. Inthe titration method, first ~ 0.2 wt% C14EO3 was addedinto the micellar solution of neutralized LAD, and thenmixed properly, and finally kept into a water bath at 25℃for an hour prior to identify the equilibrium phase. Theaddition of C14EO3, mixing, phase identification processeswere continued until phase separation to liquid crystaltakes place. In order to confirm the phase boundary, sealedampoules containing a required amount of reagents werehomogenized and kept in a water bath at 25℃ for 24 h forequilibration. Equilibrium phases were identified by visualobservations (through crossed polarizer). Millipore waterwas used in preparing all the solutions.2.2.2 Rheological measurements

All the samples (3 g each) for rheological measurementswere prepared taking required amount of chemicals inclean and dry glass bottles (20 mL) with screw cap. Thechemicals were properly weighed so that the total uncer-tainty for the values of concentration of each component is±0.01%. Samples were homogenized and kept in a ther-mostated bath at 25℃ with the accuracy of the thermome-

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Scheme 1 Molecular Structure of N-DodecanoylglutamicAcid (LAD).

Wormlike Micelles in Mixed Amino Acid Surfactant/Nonionic Surfactant Aqueous Systems

ter ±0.5℃ for at least 24 h before the measurements. Vis-cous samples were mixed using a magnetic stirrer. Therheological measurements were performed on an AR-G2stress-controlled rheometer (TA Instruments) equippedwith a Peltier-based temperature controller using cone-plate geometries (cone diameter: 40 mm; cone angle: 1°). Asample cover provided with the instrument was used tominimize the change in sample composition due to evapo-ration during the measurement. Frequency sweep mea-surements were performed in the linear viscoelasticregime of the samples, as determined previously by dynam-ic strain sweep measurements.

3 RESULTS AND DISCUSSION3.1 LAD/non-ionic surfactant/water systems3.1.1 Phase behavior

LAD possesses a relatively high Krafft point (58℃). How-ever, when both the carboxylic acid groups of the LAD areneutralized with TEA or lysine, the Krafft point is reducedbelow room temperature, and a micellar solution is formedat 25℃ 29,30). In the present study, we have used arginine asa neutralizing agent (counter ion) to compare its phase andrheological behavior with that of the LAD-lysine system.Figure 1 shows the effect of neutralization degree of

LAD on the Krafft temperature for L-arginine and L-lysine.The Krafft point of LAD-arginine systems keeps around50℃ up to a neutralization ratio of Arg/LAD＝ 1; abovethis value, the Krafft point decreases sharply and forArg/LAD＝ 2, isotropic micellar solution is found above0℃. On the other hand, when the counterion is L-lysine(Figure 1b), the Krafft point decreases almost monotonical-ly with the neutralization degree up to Lys/LAD＝ 1.5;

above this value, the Krafft points remain constant around0℃. Note that there are regions in which a solid phase andan isotropic solution coexist; such regions are narrower forLAD-lysine system in comparison to the LAD-arginine sys-tem. The neutralization ratios to obtain isotropic solutionsat room temperature are Arg/LAD＝ 2 for LAD-argininesystem and Lys/LAD＝ 0.8 for LAD-lysine system. In thefollowing experiments we have used LAD-arginine andLAD-lysine surfactants with a neutralization degree equalto 2, abbreviated as LAD-Arg2 and LAD-Lys2, respectively.

To explore the effects of mixing with nonionic surfac-tants, we constructed partial ternary phase diagrams ofLAD-Arg2/water/C14EO3 and LAD-Lys2/water/C14EO3

system in the dilute regions at 25℃ as shown in Fig. 2. Inthe LAD-Arg2/water binary system, isotropic solutions areformed over a wide range of surfactant concentrations.The viscosity of dilute aqueous solution of LAD-Arg2 iscomparable to pure water, which suggests that micellesare spherical above the critical micelle concentration(CMC).

For the binary LAD-Arg2 aqueous system (LAD-Arg2-water axis), the liquid crystal region just adjacent to the L1

region is a direct micellar cubic phase, as confirmed bySmall Angle X-ray (SAXS) measurements (see Fig. 2c).Such a cubic phase has been found in other binary aminoacid surfactant-water systems as well 31). On the otherhand, C14EO3 is insoluble in water and therefore two liquidphases coexist in the C14EO3－ water axis.

Upon successive addition of C14EO3 to dilute aqueoussolution of LAD-Arg2, the viscosity increases gradually atfirst, and then sharply. Further addition of C14EO3 resultsin phase separation of birefringent liquid crystals.

The binary system LAD-Lys2/water (LAD-Lys2-wateraxis) also forms an isotropic solution at low concentrations

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Fig. 1 Effect of Neutralization Degree of LAD on the Phase Behavior of(a) LAD-Arginine and (b) LAD-Lysine Aqueous Systems. S is a solid phase, and L1 is a micellar solution, respectively. TheLAD concentration is fixed at 10 wt%.

R.G. Shrestha, C. Rodriguez-Abreu and K. Aramaki

but transforms into a cubic phase at lower surfactant con-centrations as compared to LAD-Arg2. The other featuresof the LAD-Lys2/water/C14EO3 phase diagram are similarto that of LAD-Lys2/water/C14EO3 system, namely, highlyviscous solutions were also found in the vicinity of the LCregion.3.1.2 Rheological behavior

In Fig. 3, we present the steady shear-rate (･g ) － viscosi-ty (h) curves for 10 wt% LAD-Arg2 + C14EO3 system at dif-ferent mixing fraction of C14EO3, expressed in wt% ofC14EO3 in total system at 25℃. At lower C14EO3 concentra-tions, h is small, close to that of pure solvent (water) andindependent of ･g , i.e., Newtonian flow behavior is observed.However, as concentration increases, h increases andshear thinning is observed, which is a typical behavior ofwormlike micelles. As can be seen from Fig. 3(a), the flowbehavior is Newtonian over wide range of shear rate at14.5 wt% C14EO3, but shear thinning occurs at ･g ≥ 100 s-1.With further increasing concentration, the critical shear-rate ･g c (the shear-rate at which shear thinning occurs)shifts gradually to the lower values, and the viscosity inthe plateau region (low ･g region) increases. This phe-

nomenon highlights the fact that the system is gettingmore structured. The observed steady-shear rheologicalbehavior is a typical of network structure formed by worm-like micelles. The network structure is deformed by apply-ing a shear, and hence, shear thinning occurs due to align-ment of aggregates under flow if the deformation is fasterthan the time required for regaining equilibrium.

The effect of adding C14EO3 to LAD-Arg2/water andLAD-Lys2/water systems on zero-shear viscosity (h0) isalso shown in Fig. 3(b). An almost monotonic increase in h0

with C14EO3 concentration can be seen in both systems;further addition of C14EO3 leads to phase separation, whichoccurs at lower nonionic surfactant concentrations forLAD-Lys2 than for LAD-Arg2 system, as already indicatedby the phase diagrams in Fig. 2. Therefore, the maximumattainable viscosity is higher for LAD-Arg2 at the givencomposition.

3.2 LAD-Arg2/non-ionic surfactant/water systems withadded salt

3.2.1 Rheological behaviorDue to the usual presence of electrolytes in formulations,

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Fig. 2 Partial Ternary Phase Diagrams of (a) LAD-Arg2/water/C14EO3 and(b) LAD-Lys2/Water/C14EO3 Systems at 25℃ where L1 is MicellarSolution with High Fluidity (possibly spherical micelles). LC represents a liquid crystal region. I1 is discrete cubic phase(Figure b is adapted from ref. 30). (c) SAXS pattern of discrete cubicphase in LAD-Arg2/water binary system at 25℃.

Wormlike Micelles in Mixed Amino Acid Surfactant/Nonionic Surfactant Aqueous Systems

the effect of a series of salts with a common cation (Na) onthe rheological behavior of LAD-Arg2/C14EO3 aqueous sys-tems was studied. As can be seen in Fig. 4, salts modify therheological behavior significantly. Namely, h0 curvesbecome steeper and shift towards lower C14EO3 concentra-tion compared to the salt-free system, indicating that alower amount of C14EO3 is required for micellar growthwhen salt is added. In addition, the amount of C14EO3 need-ed for the onset of thickening for the different salts seemsto increase in the order I－ >NO3

－ >Br－ >Cl－, which is inagreement with the Hofmeister series in terms ofamphiphile solubility. Hence, the salting-out effect is play-ing a role in the onset of micellar growth. For NaCl andNaI slight viscosity maxima are observed, whereas forNaNO3 and NaBr, viscosity increases continuously untilphase separation takes place. The maximum thickeningeffect of anions, i.e. the maximum attainable viscosity,increases in the order NO3

－ > I－ > Br－ > Cl－.The effects of salt concentration on the steady-shear

rheology, and hence, on h0 for a particular salt (NaNO3) isshown in Fig. 5. For the sake of comparison, data of thesalt free system is also included in the figure. The h0

curves shift towards lower C14EO3 concentration withincreasing salt concentration indicating that the saltfavors micellar growth. In order to figure out the net saltconcentration effects on h0, we have plotted h0 as a func-tion of salt concentration at fixed concentrations of LAD-Arg2 and C14EO3 (see Fig. 5b). It can be seen that at fixedLAD-Arg2/C14EO3 mixing ratios, there is an inflexion pointat a certain NaNO3 concentration, which can be assignedto the onset of micellar growth. Curves shift to higher vis-cosities with increasing C14EO3 concentration. It is gener-

ally admitted that increasing the salt concentrationamounts to an increase in the curvature energy of surfac-tant molecules in the end-cap region relative to the one inthe cylindrical body of the micelle. This leads to anincrease in micellar length.

As can be seen in Fig. 5c, the maximum attainable zero-shear viscosity, h0max (indicated by small arrows in Fig. 5a)first increases with NaNO3 concentration up to 0.3 M, andthen decreases. It is possible that the salt first favorsmicellar growth, but after a certain concentration, micellarshortening or branching occurs and maximum zero-shear

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Fig. 3 (a) Viscosity vs. Shear Rate of the LAD-Arg2/Water/C14EO3 System at DifferentMixing Fractions of C14EO3 (expressed as wt% besides each set of data). (b) Zero-shear Viscosity vs. wt% of C14EO3 for LAD-Arg2/Water/C14EO3 and LAD-Lys2/Water/C14EO3 Systems at 25℃. The LAD-Arg2 and LAD-Lys2 concentrations in water are kept at 10 wt%.Arrows indicate the C14EO3 concentration at which phase separation takes place.

Fig. 4 Zero-shear Viscosity vs. C14EO3 Concentration forDifferent Salts in LAD-Arg2/C14EO3 AqueousSystems. The concentration of LAD-Arg2 is fixed at 10 wt%,and salt concentration is fixed at 1 M. All themeasurements were carried out at 25℃.

R.G. Shrestha, C. Rodriguez-Abreu and K. Aramaki

viscosity decreases. h0max is also related to the C14EO3 con-centration for phase separation, which decreases withNaNO3 concentration.

Viscoelastic properties of the wormlike micelles as afunction of nonionic surfactant concentrations were inves-tigated by oscillatory-shear (frequency sweep) measure-ments. Generally, dynamics of wormlike micelles underoscillatory-shear is described by considering two differentprocesses. When a small strain is applied, the stress relax-ation occurs by reptation, that is, a reptile-like motion ofthe micelle along its own contour, with an associated relax-ation time, trep～（L－）3, where L－ is the micellar contourlength 3). Beside reptation, micelles may also undergoreversible scission, with a characteristic time, tb～（L－）－1 32).When the time scale of the scission for an average micellarcontour length is too short in comparison to the time scaleof the reptation (trep>> tb), the viscoelastic micellar solutionsbehave as a Maxwell fluid 33,34) with a single relaxationtime 3), given by

(1)

The variation of the elastic or storage modulus G´ (w) andthe viscous or loss modulus G´́ (w) as a function of oscillato-ry-shear frequency, w, is described by the following rela-tions:

(2)

(3)

where G0 is the plateau modulus. At high frequencies, G´tends to attain a constant value equal to G0. The relaxationtime tR may be estimated from the G´-G´́ crossover fre-quency, that is tR = 1/w, when G´= G´́ .Figure 6a shows a plot of the elastic (G´) and loss (G´́ )

moduli as a function of oscillatory frequency (w) for differ-ent C14EO3 concentrations in the LAD-Arg2/C14EO3 /0.3 MNaNO3 system at 25℃. There is a clear viscoelastic behav-ior: at low frequencies, the viscous modulus G´́ is largerthan the elastic modulus (G´) and the system behaves like aliquid; at high frequencies, G´>G´́ and the system behaveslike a solid. The system follows the Maxwell model of relax-ation in the low frequency range, but departs from it athigh frequencies, which might be attributed to the exis-tence of multiple relaxation processes. This deviation isthought to have arisen from a transition of the relaxationmode from “slower” reptation to “faster” relaxation modes,such as the Rouse mode 11,35). Maxwellian-type oscillatoryrheological behavior of viscous micellar solutions, such asthat shown in Fig. 6, can be related to the transient net-work formed by the entanglement of wormlike micelles11).

As evident from the Maxwell equations, at low frequency

′′ =+

G GR

R

( )ω ωτω τ1 2 2 0

′ =+

G GR

R

( )ω ω τω τ

2 2

2 2 01

τ τ τR b rep L= ( )12 ~

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Fig. 5 (a) Zero-shear Viscosity vs. C14EO3 Concentration atDifferent Concentrations of NaNO3. The concentration of LAD-Arg2 is fixed at 10 wt%,(b) effects of salt concentration on h0 at fixed LAD-Arg2 concentration (10 wt%) for two differentC14EO3 concentrations, and (c) h0max (indicated bysmall arrows in panel (a)) as a function of NaNO3

concentration. All the measurements were carriedout at 25℃.

Wormlike Micelles in Mixed Amino Acid Surfactant/Nonionic Surfactant Aqueous Systems

region, w << wc, therefore G´ and G´́ scale with w accordingto G´~w2 and G´́ ~w. On the other hand, in the region of w>> wc, G´ attains a plateau value equal to G0, whereas G´́shows a monotonic decrease. The shear frequency corre-sponding to the G´ and G´́ crossover, wc, is equivalent tothe inverse of relaxation time, tR. Considering reptation ordiffusion of wormlike micelles along its own contour as themechanism of stress relaxation in the entangled network,the magnitude of tR is related to the average length of thewormlike micelles, whereas G0 is related to the numberdensity of entanglement in the transient network. Theparameters G0 and tR are related to h0 by following rela-tion:

h0 = G0tR (4)

As can be seen from Fig. 6a, with increasing C14EO3 con-centration, the frequency at which the G´ and G´́ meetshifts towards lower values and attains a minimum at thecomposition for the viscosity maximum (12.7% of C14EO3).With further increase in C14EO3 concentration, viscositydecreases and finally phase separation occurs. As can beseen from Fig. 6b, G0 and tR increase with C14EO3 concen-tration, show a maximum, and then decrease similarly tothe case of viscosity as a function of C14EO3 concentration.The initial increasing trend can be attributed to the one-dimensional micellar growth and increased density (i.e.,decreased mesh size) in the network structure of wormlikemicelles. On the other hand, the decrease could be attribut-ed to morphology changes in the entangled micelles,

although the reasons of this behavior are not clear yet; apossibility would be micellar breaking as suggested fromEq. 1.

The rheological parameters derived from the Maxwellmodel fitting to the experimental data for salt added sys-tems are presented in Fig. 7. The figure shows the varia-tion of G0 and tR as a function of C14EO3 concentration atdifferent salt concentrations for a given salt (NaNO3). Forthe sake of comparison G0 and tR of the salt free system arealso included in the figure.

Both G0 and tR increases with C14EO3 concentration forall concentrations of the salt. The increase of G0 withC14EO3 concentration indicates that the number of entan-glements in the system increases and a more “rigid” struc-ture is formed. Besides, the micellar length increases asanticipated from the increasing trend of tR. Thus, the vis-cosity increase with C14EO3 concentration in all the sys-tems can be attributed to both micellar growth andincreased network density of the micellar structure, i.e.,number of entanglements.

As mentioned before, the increase of salt concentrationin a particular salt system reduces the amount of nonionicsurfactant needed to induce the viscosity increase, i.e., vis-cosity curves shift towards lower C14EO3 concentrations.Irrespective of the salt concentrations, G0, and tR follow anincreasing trend with C14EO3 concentration and are higherthan the values obtained for salt free system. Thus, it canbe concluded that the higher viscosity observed with saltincorporated systems are due to higher entanglement den-sity of wormlike micelles and also due to longer axial

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Fig. 6 (a) Variation of Storage Modulus (G′), and Loss Modulus (G″) as a Function ofOscillatory Frequency (w) for Different C14EO3 Concentrations in the LAD-Arg2/C14EO3/0.3 M NaNO3 System at 25℃. The LAD-Arg2 concentration is kept at 10 wt%. The solid lines represent thebest fit to the Maxwell model. Arrows indicate the G′- G″crossover (b)Variation of plateau modulus, G0 (closed symbols) and relaxation time, tR (opensymbols) for the system mentioned in (a).

R.G. Shrestha, C. Rodriguez-Abreu and K. Aramaki

length of the wormlike micelles. Minute observation of Fig.7 reveals that although the values of G0 are comparable atall salt concentrations studied, tR differ, and it has maxi-mum at 0.3 M NaNO3. Note that this is the composition atwhich the maximum thickening effect (maximum h0max) wasobserved (see Fig. 5c). Thus, the increase of h0max with saltconcentration at the beginning can be attributed to themicellar growth and the entanglement between them. Onthe other hand, the decrease in the maximum zero-shearviscosity above a certain salt concentration is possibly dueto micelle shortening or branching. Addition of salts at theinitial stage screens surfactant head group interaction andfavors the micellar growth by reducing the interfacial cur-vature of the aggregate. However, after a certain concen-tration (say when a saturation of salt at the micellar inter-face is reached), the interaction among the salt ionsbecomes crucial, which in turn, could reduce the screeningtendency of the salt. Hence, the interfacial curvature tendsto increase, which results in shortening of rod-likemicelles. However, still there is a possibility of micellarbranching.3.2.2 Thermoresponsive behaviorFigure 8 displays the results on the effect of temperature

on rheological behavior of LAD-Arg2/C14EO3 aqueous sys-tems in the presence of salts. The experiment was con-ducted for all the samples up to the temperature at whichphase separation takes place. It was found that the phaseseparation temperature decreases in the order I－ > NO3

－

> Cl－ ≈ Br-. This is in agreement with Hofmeister’s series

in terms of amphiphile solubility; the same trend has beenobserved for aqueous solutions of thermoresponsive poly-mers 36,37). It is interesting to note that the temperaturedependence of viscosity shows different trend dependingupon the nature of anion species present in the systems.The viscosity of the system with NaI and NaNO3 firstincreases with temperature, reaching a maximum and thendecreases. For NaCl samples the behavior is qualitativelysimilar at low temperatures; however, no maximum isobserved below the phase separation temperature. On theother hand, for NaBr samples, the viscosity decreasesalmost continuously with temperature. The exact reasonbehind the phenomenon is not clear at present.

Oscillatory measurements as a function of temperaturefor NaBr and NaNO3 systems are shown in Fig. 9. The val-ues of G0 and tR calculated by fitting the experimental datato the Maxwell equations are plotted in Fig. 9(a) and (b). Inthe case of NaBr system, it is observed that the relaxationtime, tR, decreases with increasing temperature while theplateau modulus G0 remains almost constant. The micellarlength is proportional to the relaxation time, tR (see Eq. 1).Since tR decreases with temperature, it can be argued thatmicelles become shorter 1,32), and hence, the viscositydecreases.

On the other hand, in the case of NaNO3 system, therelaxation time increases on increasing temperature firstslightly and then abruptly at 30℃, which corresponds tothe temperature at which the highest viscosity for thesample is attained. Further increase in temperature causesa decrease in the relaxation time. The plateau modulus alsoshows the same trend with an increase in temperature.Thus, it is inferred that first the micelle grows in onedimension and get entangled, causing an increase in vis-

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Fig. 8 Variation of h0 as a Function of Temperature for 10wt% LAD-Arg2/water + 9.5 wt%C14EO3 at FixedSalt Concentration (1 M) for Different Salt Systems. The arrows in the figure indicate the phase separa-tion temperatures for the systems.

Fig. 7 Variation of Rheological Parameters G0 and tR asa Function of C14EO3 Concentration at DifferentNaNO3 Concentration. Closed and open symbols represent G0 and tR,respectively. Circle = 0.3 M, diamond = 0.5 M,triangle = 1 M, square = 1.5 M, and invertedtriangle = without salt. The concentration ofLAD-Arg2 is fixed to 10 wt%. All the measure-ments were carried out at 25℃.

Wormlike Micelles in Mixed Amino Acid Surfactant/Nonionic Surfactant Aqueous Systems

cosity, and then beyond 30℃, the micelles are disrupted.Therefore, the viscosity of the sample decreases.

The studied systems are multicomponent, and therefore,several complex mechanisms could affect the thermore-sponsive behavior. The temperature dependence of theMaxwellian relaxation time is usually described by anArrhenius-type relationship:

(5)

where A is a pre-exponential factor, R is the gas constantand Ea is the total activation energy. The temperaturedependence of zero-shear viscosity for a Maxwellian fluidcould then be estimated by Eq. 4, if G0 is known as a func-tion of temperature.

The decreasing part of the tR curve in Fig. 9a followsroughly Eq. 5, indicating a positive value of Ea associatedwith micellar disruption. As a matter of fact, we foundArrhenius behavior in our previous report on worm-likemicelles in salt-free LAD-Lys+C14EO3 aqueous systems 30).As the case of Fig. 9a, G0 has been found to be rather tem-perature insensitive for cationic wormlike micelles38,39). Onthe other hand, the maximum in the tR curve of Fig. 9bsuggests the presence of two opposed contributions to Ea,namely, the activation energy could be expressed as Ea=Ed－Eg, where Ed and Eg are activation energies for micel-lar disruption and growth, respectively. On this basis, fortemperatures below the maximum, Ed < Eg, whereas fortemperatures above the maximum, Ed > Eg. The differencein contributions of Ed and Eg depending on temperaturecould also serve as a qualitative explanation of the maxi-mum in h0 observed in Fig. 8.

The behavior of G0 with temperature in NaNO3 systemsalso departs from previous reports on wormlike micelles.For dilute polymer solutions in a good solvent, it has been

shown that40).

G0～ kBTx–3～ kBTf2.3 (6)

where kB is Boltzmann’s constant, x is the mesh size of thetemporary network formed by the entangled polymerchains and f is the polymer volume fraction. On the basisof Eq. 6, the trend for Fig. 9b could be only explained by anon-obvious dependence of x on temperature.

The presence of nonionic surfactant, C14EO3 in the sys-tem can play a role in the increase of viscosity with tem-perature 41). In the case of nonionic surfactant system, onincreasing the temperature, the dehydration of ethyleneoxide chains induces a decrease in the interfacial curva-ture of aggregates. This in turn would lead to a sphere- rodtransition in aggregate shape or promote one dimensionalgrowth if the rod-like aggregates are already formed. Theformation of end caps in the cylindrical aggregatesbecomes unfavorable with increasing temperature becauseof high free-energy cost of the formation of hemisphericalends, and consequently one-dimensional growth is favored.However, at high temperatures, due to a decrease in solu-bility, a migration of C14EO3 molecules towards the micellarcore cannot be ruled out, which would contribute to thedisruption of the long micelles42).

4 SUMMARYN-dodecanoylglutamic acid neutralized by arginine in 1:2

mole ratio (LAD-Arg2) forms an aqueous micellar solutionover a wide range of concentrations at 25℃. Upon additionof lipophilic nonionic surfactant (C14EO3) to the semi-dilutesolution of the LAD-Arg2, the interfacial curvaturedecreases and LAD-Arg2 micelles grow, forming ultimately

τ RaA E

RT=

exp

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Fig. 9 Variation of Plateau Modulus, G0 (circles), and Relaxation Time,tR (diamonds) as a Function of Temperature (T) for 10 wt% LAD-Arg2/Water + 9.5 wt%C14EO3 at Fixed Salt Concentration (1 M)for (a) NaBr and (b) NaNO3 Systems.

R.G. Shrestha, C. Rodriguez-Abreu and K. Aramaki

a transient network of viscoelastic wormlike micelles.Zero-shear viscosity (h0) increases with C14EO3 concentra-tion continuously till a liquid crystal phase separates out.Dynamic oscillatory-shear rheological measurements haveshown formation of wormlike micelles in the viscousregions, and the Maxwell model of single stress relaxationmode fits the data in the low frequency range.

Addition of salts enhances the micellar growth andshifts the h0 curves towards lower C14EO3 concentration,which indicates that lower amount of C14EO3 is requiredfor viscosity increase in the salt-added systems. In a par-ticular salt system (NaNO3), increasing salt concentrationincreases the maximum attainable zero-shear viscosity(h0max), but after a certain concentration, the h0max decreas-es. The increase in h0max can be attributed to the onedimensional micellar growth induced by screening effect ofthe salt. On the other hand, the decrease of h0max after themaximum could be attributed to micellar shortening orbreaking, as indicated by the decrease in relaxation time,tR. However, we cannot exclude the possibility of micellarbranching.

Phase separation takes place at a certain temperature insalt-added systems studied; the phase separation tempera-ture decreases in the order I－ > NO3

－ > Cl－ ≈ Br－. Thethermoresponsive rheological behavior was also found tobe highly dependent on the type of anion, and anomaloustrends, i.e. viscosity increase with temperature, wereobserved for all anions except Br－.

More detailed studies using other techniques (such asCryo-TEM) are needed to get more insight into the originsof the observed rheological behavior.

ACKNOWLEDGEMENTAuthors are thankful to the Ajinomoto Co. Ltd. Japan for

supplying the amino acid surfactant. Technical assistanceof Dr. Lok K. Shrestha (Yokohama National University) isacknowledged. KA is thankful to Ministry of Education,Culture, Sports, Science and Technology, Grant-in-Aid forYoung Scientists (B), No. 18780094 and partly supported byCore Research for Evolution Science and Technology(CREST) of JST Corporation.

APPENDIXThe effect of salt concentration on the steady-shear rhe-

ology, and hence, on h0 for other salts (NaCl, NaBr and NaI)is also observed and is shown in Fig. A1. For the sake ofcomparison, data of the salt free system is also included inthe figures.

The h0 curves shift towards lower C14EO3 concentrationwith increasing salt concentration indicating that the salt

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Fig. A1 Zero-shear Viscosity vs. C14EO3 Concentration atDifferent Concentrations of (a) NaCl, (b) NaBrand (c)NaI. The concentration of LAD-Arg2 is fixed at 10wt%. All the measurements were carried out at25℃.

Wormlike Micelles in Mixed Amino Acid Surfactant/Nonionic Surfactant Aqueous Systems

favors micellar growth. Curves shift to higher viscositieswith increasing C14EO3 concentration. It is generallyadmitted that increasing the salt concentration amounts toan increase in the curvature energy of surfactantmolecules in the end-cap region relative to the one in thecylindrical body of the micelle. This leads to an increase inmicellar length.

The maximum attainable zero-shear viscosity, h0max as afunction of salt concentration has been plotted for otherNa-salts too and presented in Fig. A2. As can be seen in thefigure, the h0max first increases with Na-salt concentra-tions, and then decreases. The viscosity increase can beattributed to one-dimensional micellar growth. On theother hand, the viscosity decrease is caused by micellarshortening or branching.

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