Surface tension and neutron reflection measurements have been used to study the surface composition of aqueous solutions of mixtures of sodium dodecyl sulfate (SDS) and n-dodecyl--D-maltoside (C 12 maltoside) and C 12 maltoside and n-dodecyl-N,N′-dimethylamino betaine (C 12 betaine). From measurements of surface tension and mixed critical micelle concentrations (cmc) the pseudo-phase separation model has been used to calculate values of the interaction parameters in the micelle, M , and at the surface, σ . SDS/C 12 maltoside mixtures behave nonideally and both M are negative, indicating attractive interactions between the two surfactants, but the C 12 maltoside/C 12 betaine mixtures are closer to being ideal. Direct measurements of the surface excess using neutron reflection on isotopic mixtures of the surfactants are shown to be consistent with the surface tension measurements using the integrated form of the Gibbs equation. These direct values of the surface excess were found to agree with predictions from the partial phase separation model in the case of the nonideal pair, SDS/C 12 maltoside, but were different for the ideal pair, C 12 maltoside/C 12 betaine. It is suggested that this may result from the relatively large errors in the partial phase separation model when the mixture is very unsymmetrical (very different cmc's coupled with weak interaction). The C 12 maltoside/ C 12 betaine mixture was found to behave regularly in that σ showed negligible variation with either surface pressure or composition. On the other hand for the nonideal pair, SDS/C 12 maltoside, the magnitude of σ decreased with surface pressure, consistent with reduced interaction as the molecules are more widely spaced, and also decreased with higher SDS fraction in the layer, consistent with an increasing contribution from electrostatic repulsion.

The interactions between oppositely charged surfactant/polymer mixtures have been studied using neutron reflectometry with supplementary surface tension measurements. The cationic surfactant dodecyl trimethylammonium bromide (C12TAB)/anionic polyelectrolyte sodium poly(styrene sulfonate) (NaPSS) system is compared with a system containing anionic sodium dodecyl sulfate (SDS) and cationic poly(vinyl pyridinium chloride) (PVPmCl). The PVPmCl/SDS mixture has been studied both with and without added electrolyte. Neutron reflection shows that for both systems, the surface consists of a mixture of polyelectrolyte and surfactant over a range of surfactant concentrations from above the critical micelle concentration (CMC) to CMC/100 for polymer concentrations between 10 and 140 ppm. In the lower surfactant concentration range, the amount of surfactant adsorbed approximately corresponds to a surfactant monolayer (area per molecule ∼ 35−45 Å 2 for SDS in the presence of 0.1 M NaCl, 35−60 Å2 without NaCl, and 50−60 Å2 for C12TAB with 0.1 M NaBr). However, at higher concentrations and in the presence of electrolyte, this increases to an amount approximately corresponding to three adsorbed layers (area per molecule = 12 Å2 for SDS and 17−20 Å2 for C12TAB). This increase is not observed for PVPmCl/SDS in the absence of 0.1 M NaCl. The structure of the higher concentration layer is a sandwich structure with an outer surfactant layer and a submerged polymer/micellar (spheres or rods) or polymer/defective bilayer. The surface tension and neutron results can be interpreted qualitatively in terms of three species in the system, a surface active complex PSS, a bilayer complex , which can only adsorb on a preformed PSS layer, and a bulk solution complex PSM. PSS is adsorbed at very low concentrations of surfactant, possibly even before any PSM is formed in the bulk solution. At high concentrations, there are two effects. There may be adsorption of complexes to the layer of PSS already at the surface. However, the formation of is in competition with the formation of PSM. If the latter is dominant, there is no secondary adsorption of , as is the case for PVPmCl/SDS in the absence of electrolyte, and the surface tension may increase very sharply with surfactant concentation at the point where the formation of PSM in the bulk solution is complete. If there is secondary adsorption of PSM or PSS, as for NaPSS/C12TAB with or without electrolyte and PVPmCl/SDS with electrolyte, the surface tension should show a more modest increase at this concentration.

Aqueous solutions of the strongly interacting anionic/zwitterionic surfactant mixture of sodium dodecyl sulfate (SDS) and dodecyl dimethylamino acetate (C12betaine) have been studied by means of surface tension and neutron reflection. The mixed critical micelle concentrations (cmc) were used to derive the interaction parameter βM for micellization, which was found to be large and negative. It was also found to be composition dependent and therefore not to obey the first-order approximation for the activity coefficients in the pseudo-phase separation approximation. The temperature dependence of the mixed cmc gave the thermodynamic excess functions for micellization; S E was found to be positive over most of the composition range. Application of the pseudo-phase separation model to surface tension data was used to show that the interaction parameter βσ in the surface layer is similarly large and negative. However, direct measurement of the surface concentrations using neutron reflection disagree with the predictions of the pseudo-phase separation model and indicate that βσ, although negative, is much smaller in magnitude. The structure of the mixed layer was determined at three compositions and found to be significantly dehydrated in comparison with layers of the single surfactants, which may explain the positive excess entropies observed for both micellization and surface mixing. It is also suggested that changes of hydration on mixing invalidate the use of the pseudo-phase separation model and may be responsible for the deviations from the first-order model observed for both micellization and surface interaction.

A thin chromium oxide hydroxide colloid film has been used as a model of the passive stainless steel surface for studies of anion adsorption from aqueous solutions. The adsorption of perchlorate, sulfate, and thiosulfate ions has been investigated by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. Surface charge was monitored from the surface excess concentrations of tetramethylammonium ions and of perchlorate ions using the infrared spectroscopic STIRS technique. The colloid film showed a high positive charge at low pH and a low negative charge at high pH. The adsorption of sulfate was only observed for a positive surface charge. The infrared spectrum of adsorbed sulfate was significantly altered by the interfacial electric field, but there was no evidence of sulfate coordination to surface Cr(III) ions. Thiosulfate gave adsorption behavior analogous to sulfate. Adsorption isotherms for sulfate and for thiosulfate were determined from infrared spectral data, and Langmuir binding constants of (2.3 ± 0.4) × 105 and (1.4 ± 0.4) × 105 M-1 were obtained for the respective adsorbates. The lack of chemical binding of sulfate and of thiosulfate to the chromium oxide hydroxide surface may be part of the basis for the corrosion-promoting properties of these ions at stainless steel surfaces.