The Gibbs adsorption isotherm (GAI) has been considered as the foundation of surfactant adsorption studies for over a century; however, its application in determining the limiting surface excess has recently been intensively discussed, with contradictory experimental evidence either supporting or refuting the theory. The available arguments are based on monolayer adsorption models. In this paper, we experimentally and intellectually propose and validate the contribution of sub-monolayer adsorption to the GAI paradox. We utilize a powerful intrinsically surface-sensitive technique, vibrational sum-frequency generation spectroscopy (SFG), complementing with conventional tensiometric measurements to address these controversies both quantitatively and qualitatively. Our SFG results revealed that the precipitous decrease in surface tension directly corresponds to surface occupancy by adsorbates. In addition, the Gibbs analysis was successfully applied to the soluble monolayer of a surface-active alcohol to full saturation. However, the full saturation of the topmost monolayer does not necessarily mean that the surface adsorption was completed because the adsorption was observed to continuously occur in the sub-monolayer region soon after the topmost monolayer became saturated. Nonetheless, the Gibbs isotherm failed to account for the excess of alcohol adsorbed in this sub-monolayer region. This new concept of surface excess must therefore be treated thermodynamically.

The effect of four cationic ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride (BMImCl), 1-butyl-3-methylimidazolium bromide (BMImBr), 1-hexyl-3-methylimidazolium chloride (HMImCl), and 1-hexyl-3-methyl-imidazolium bromide (HMImBr) on surface activity and micellization of an anionic surfactant, sodium dodecyl sulfate (SDS), is studied. The thermodynamic data on micellization and surface adsorption are obtained from tensiometry and conductometry. The applicability of UV-visible spectroscopy to study of SDS/IL systems is also investigated using Crystal Violet as the probe. Cyclic voltammetry, dynamic light scattering, and TEM imaging are employed to investigate the size and morphology of aggregates. According to the findings, addition of butyl-chained ILs to aqueous SDS results in only a slight gradual increase in average aggregate size whereas the size of SDS assemblies are dramatically increased upon addition of hexyl-chained ILs. It is proposed that BMIm(+) cations of the IL undergo Coulombic attractive interactions with anionic headgroups adsorbed at the micellar surface in aqueous SDS whereas HMIm(+) interact through hydrophobic chain-chain attractions as well. Thus, mixed micellization results in formation of vesicles. A micellar phase change from vesicles to micelles takes place at higher [SDS]/[IL] ratios. All of these processes are successfully tracked by the employed techniques.

The aggregation behavior of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), in aqueous solution of various imidazolium-based ionic liquids (ILs) was investigated using tensiometry, conductometry, transmission electron microscopy (TEM), and cyclic voltammetry (CV) methods. The data obtained from tensiometry and conductometry measurments were used to specify the critical micelle concentrations (CMCs) and other physicochemical parameters. The CMC of the surfactant was found to significantly decrease in the presence of ILs. The surface activity parameters of CTAB were observed to be a function of structural properties of ILs. Furthermore, intermicellar interaction parameter (k d) was measured using cyclic voltametry. The intermicellar interaction parameter was also found to decrease slightly with addition of IL. However, in the presence of HMImBr or HMImCl, the intermicellar interaction parameter is not changed significantly, as a result of phase transition from spherical to cylindrical micelles, which was also confirmed by transmission electron microscopy (TEM) micrographs. The size of CTAB nanoparticles grew from 5 nm to 20–30 nm in the presence of ILs.