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!