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Talk from the 2011 American Oil Chemist's Society meeting (Surfactants and Detergents Division). Reviews the basics of FT-IR spectroscopy and how it can be used in a wide range of applications to surfactant science.
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Applications of Fourier Transform Infrared Spectroscopy to Studies of Surfactant Behavior
AOCS Meeting – May, 2011
D.R. Scheuing – Clorox
Outline
How Does a Fourier Transform IR work?
Working with Aqueous Systems – Really?
How Can I Use FT-IR to Probe -
Structure and Properties of Surfactant Aggregates
Interfacial Behavior of Surfactants = Performance!
Moving Mirror
Direction
Beamsplitter
IR source
To Detector
Fixed Mirror
Distance – Fixed to Beamsplitter
Distance – Fixed to Moving Mirror
Phases of Two Beams
Michelson Interferometer Recombines Split Beams with Different Pathlengths
Optical Retardation = Difference in Beam Paths -
Measured in cm ! =
Output
Beam Focus in Sample
Area
IR source
Laser source
Beamsplitter
Laser detector
IR detectors
EM drive for moving mirror (voice coil)
Moving Mirror
Modern FT-IR Benches Are Compact, Deliver High Energy Throughput and Versatility for Sampling Optics
Fourier Transform = from cm to cm-1
Ratio Single Beam Spectra
5 mAU ! An adsorbed
polymer layer
Record I’gram - FT to Single Beam - Ratio Sample/”Background” Single Beams
Background
Sample
Spectra of Surfactants in Water Are Possible with “Short” Pathlengths ! (Transmission or ATR)
CH stretch
S-O stretch
C-O-C stretch
CH deformation
C=O stretch
Sample as film between windows
Jacket for circulating fluid or
heaters.
Innovations with Surfactants Require Understanding (Controlling) - -
Aggregate Structures = “Bulk Properties”
Micelle Size, Shape – Packing Parameter
AND -
Interfacial Activity of Aggregates =
Detergency, Surface Modification, Solubilization
Monitoring Sphere – Rod Transitions with FT-IR
Graphic- O'Reilly R Phil. Trans. R. Soc. A 2007;365:2863-2878
Decreasing “tail disorder” = “Straighter” tails
Increasing “tail disorder” = more gauche conformers
Dluhy, R.A., Mendelsohn,R., Casal, H.L., Mantsch,H.H. Biochemistry, (1983), 22, 1170-1177
Snyder,R.G, Strauss, H.L., Elliger,C.A., J.Phys.Chem, (1982), 86,5145
Shape/Position of CH2 Bands Correlate to “Disorder” in Methylene Chains & Packing Parameter
Peak Locations Can Be Measured to +/- 0.05 cm-1
(Laser reference!)
“Center of gravity” of peak can be calculated
Cameron,D.G., Kauppinen,J.K., Moffatt,D., Mantsch, H.H, Applied Spec, (1983) 36,245
CH2 str. symm
CH2 str. Asymm.
CH3 str. Asymm.
Mixed SDS – DTAC (quat) Micelles (B = -25!)
“Tail Ordering” Increases with Aggregation Number
Increasing Mole Frac. SDS
Scheuing,D.R., Weers, J.G. Langmuir, (1990) 6,3, 665-671
Separated CH2 and CD2 Bands Show Same Trends
S
OO
Net Transition Moment Vector
Net Transition Moment Vector
Asymmetric S-O stretch, 1220 cm-1
Symmetric S-O stretch, 1061 cm-1
S
OO
+
+ +
+
+
++
+
++
+
+
+
+-
--
--
-
S-O Bands Sensitive to Counterion Type, Location – Vibrational Modes Have Direction
Mantsch,H.H. et.al., J.Phys.Chem. 1980,84,227 Scheuing,D.R., Weers,J.G., Langmuir,1990,6,665
-
-
-
-
O
O
-- -
- - -+
+ ++
++
+
Shifts in S-O Bands Confirm Lateral Crowding of Headgroups in SDS/DTAC rods
SDS-rich
DTAC -rich
S-O asymm. stretch
S-O symm. stretch
DTAC rich minus SDS
SDS-rich minus SDS
SDS rods – SDS spheres
S-O symm. stretch
Difference Spectra*
(Spectrum A) – X (Spectrum B)
X=Subtraction Scalar
Scheuing,D.R., Weers,J.G., Langmuir,1990,6,665
* Cameron, D.G., Casal, H.L., Mantsch, H.H., Biochemistry, (1980) 19,3665
Interfacial Activity of Aggregates
IRE (Ge)Air
Sampling depth, dp= 736 nm at 1650 cm-1
dp = /2 (sin2 n21 2 )1/2
Refractive index = n2 = 1.5
Refractive index = n1= 4.0
n21=n2/n1
Internal Reflection Optics Key To Analysis of Surfaces – Including the IRE Surface Itself !
50 mm
Trough on Horizon rig
Multiple Reflections Aid Sensitivity with Versatile Horizontal IRE
Add a layer of “model solid soil” for detergency studies
Add a layer of nanoparticles to study adsorption on them!
Solid Fats – Major Challenge in Detergency at Lower Temperatures!
Beta-tristearin differs from alpha form – even in thin layers on Ge IRE.
Extensive FT-IR literature available for interpretation of changes.
Before and after exposure to water for 15 minutes Beta crystal
structure – most stable,
high m.p.
Time-resolved Analysis of Soil-Solution Interface During Detergency Process
Tristearin Removal from Ge Surface By Alcohol Ethoxylate (C12E8) in DI Water - High and Low
Conc.
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20 25 30 35
Time, min.
No
rmal
ized
Ab
sorb
ance
, C=O
Est
er 0.3% 0.03%
Tristearin Removal from Ge Surface By Alcohol Ethoxylate (C12E8) in DI Water - High and Low
Conc.
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40
Time, min.
No
rmal
ized
Ab
sorb
ance
, C
H2
Def
. B
and
0.3% 0.03%
(TS on IRE under surfactant solution) – X (water only on IRE) = Spectrum of solid soil and adsorbed surfactant every 20 seconds!
Ironically – Strong Interaction of Alcohol Ethoxylate Induces Formation of Beta -TS – Slowing Detergency!
Sequential difference spectra = (Spectrum of Interface at longer times) – X (First spectrum of Interface under surfactant solution)
Scheuing, D.R., Langmuir (1990),6, 312-317
Using ATR – Structure of Mixed CTAB/d-SDS Hemi-micelles on Fumed TiO2 Surfaces Determined
Quantitative Adsorption on particles Determined from CH2 and CD2 Bands
Lateral Headgroup Interactions Derived from S-O stretching and CH3-N+ Deformation Bands by H.Li & C.P.Tripp
Li, H., Tripp, Carl P., J.Phys.Chem B. 2004, 108, 18318-18326
Surface Compositions Assessed With FT-IR
How Does Modification of Surfaces (within 5 minutes) Depend on Location in Phase Boundary Diagram ?
Cationic Polymer = pDADMAC
Thomas, R.R., et. al, Langmuir, 2002, 18, 5933-5938
“Formulation” = Surfonic L12-8 (alcohol ethoxylate) + Fluorinated oxetane + pDADMAC
Poly(DADMAC) Adsorbed on Ge – Adequate Detection Limit < 0.5 mg/m2
Freely Adsorbed from 3 mM Solution
DADMAC Detection Limit (CH3-N+) < 0.3 mAU
Intense “coupled” S-O and C-F bands used to detect fluorosurfactant
Maximum Adsorption Near Boundaries, But High [Salt], Net Anionic Complexes Inhibit Adsorption
0
0.002
0.004
0.006
0.008
0.01
Abs
orba
nce
DADMAC CH3-N C-F 1236 C-F 1136
R = anionic charges from fluorinated surfactant/cationic charges from DADMAC
AT 1002/Surfonic Interactions with 0.3 mM DADMAC
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6 7 8
Equivalents, Anionic/Cationic
Na
Cl,
M
clr 2, clr + coacervate 2, clr+ppt
Modern FT-IR Spectrometers -
“It’s All Done With Mirrors”.
Can Drive Fundamental Understanding of Aggregate Structures in Water
Probe the actions of surfactants and formulations at solid interfaces
Can Help Us Drive Innovations In –
Surfactant structure/performance
Formulation cost/performance
Will You Ask More of Your Spectroscopist and Spectrometer?
Thanks
AOCS – Analytical, S&D Divisions
Clorox
You –
The Audience & Consumer!