External Reflection Fourier Transform Infrared Spectroscopy of Surfactants at the Air—Water Interface: Separation of Bulk and Adsorbed Surfactant Signals

Abstract

External reflection Fourier transform infrared spectroscopy (ERFTIRS) has been used to obtain spectra of monolayers of the hydrocarbon surfactant octaethylene glycol monodecyl ether (C(10)E(8)) and the fluorocarbon surfactant ammonium perfluorononanoate (APFN) at the expanding liquid surface of an overflowing cylinder. The use of target factor analysis (TFA) to separate out the contributions of water, adsorbed surfactant, and dissolved surfactant is demonstrated. For both surfactants, there is a linear relationship between the component weight of the adsorbed surfactant, obtained by TFA, and the surface excess determined independently by ellipsometry or neutron reflection. This linear relationship suggests that the monolayers behave like isotropic films with a constant density. A sensitivity of less than 10% of a monolayer is demonstrated. The benefits of using a multivariate curve fitting procedure to analyze sets of ER-FTIR spectra are discussed and some potential pitfalls are identified. This technique is also applicable to static interfaces.

Dynamics of Adsorption of an Oppositely Charged Polymer−Surfactant Mixture at the Air−Water Interface: Poly(dimethyldiallylammonium chloride) and Sodium Dodecyl Sulfate

“…Given the inherently weak signals from the adsorbed molecules, only the strongest part of the spectrumthe C−H stretching regionwas analyzed. Details of the application of TFA to monolayer reflection spectra may be found in refs and . We note two particular points here.…”

“…In recent years, we have extensively employed a device known as an overflowing cylinder (OFC) to study the dynamic interfacial properties of both pure and mixed surfactant systems. − In this study, we use the OFC to explore the dynamic interfacial composition of a strongly interacting polymer−surfactant system for the first time. The poly(dmdaac)−SDS system is interesting to study, both because polycation−SDS mixtures are used in conditioning shampoos, where dynamic properties are important, and because there is already an extensive body of work on the equilibrium surface properties from the work of Staples et al. − This system has recently been modeled theoretically by Bell et al…”

“…ER-FTIRS was used to quantify the amount of adsorbed SDS, as the spectra are insensitive to the weak polymer peaks (vide infra). ER-FTIRS yields precision similar to that of neutron reflection and can be carried out in our own laboratory without consumption of precious beam time at national facilities. − It has a secondary advantage that it does not require deuterated surfactants. Ellipsometry is sensitive to the total adsorbed amount of surfactant and polymer.…”

“…38 In recent years, we have extensively employed a device known as an overflowing cylinder (OFC) to study the dynamic interfacial properties of both pure and mixed surfactant systems. [39][40][41][42][43][44][45][46][47][48] In 25 The dotted and dashed lines mark the values of c SDS at which there is a sharp rise in γ e for the 0.02 and 0.08 g dm -3 poly(dmdaac) solutions, respectively. The arrow marks the cmc for pure SDS solutions in 0.1 M NaCl.…”

Total internal reflection Raman spectroscopy

“…In the case of SFG, the surface selectivity arises from the preferential orientation of molecules at the liquid-air interface while in reflection IR spectroscopy, the line-shapes of molecules at the surface and in the bulk are different and can be separated by chemometric methods (the former depends on the imaginary part of the refractive index and the latter on the real part). 75 Takenaka and coworkers used TIR Raman to look at monolayers on water of deuterated stearic acid and deuterated cadmium stearate under various amounts of compression. 76 Acquisition times were between 1 and 2 h and it was necessary to use deuterated layers to avoid spectral overlap between the O-H and C-H stretching bands.…”