A sum-frequency generation spectroscopic study of the Gibbs analysis paradox: monolayer or sub-monolayer adsorption?

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

A quantification of immersion of the adsorbed ionic surfactants at liquid|fluid interfaces

The electrical-double-layer (EDL)-based adsorption models for ionic surfactants propose all surfactant heads ideally aligned in the Stern layer, creating a single surface potential which controls the distribution of counterions within the diffuse layer. Despite their successful application to many surfactant systems, these models exhibit a noticeable shortcoming when it comes to explaining the larger surface excess of ionic surfactants at air|water interface than oil|water interface. Experiments and computation simulations have already shown that some surfactant head groups tend to immerse into the deeper layers of interfacial water at high surface coverage. Such an immersion alters the surface potential distribution and characteristics of EDL. However, a theoretical study of this phenomenon is not available yet. This paper presents a useful modeling approach to quantification of the surfactant immersion. We combined the ionic surfactant adsorption models with the theory of equilibrium adsorption constant. We modified several non-ionic surfactant adsorption models for describing the ionic surfactant adsorption by accounting for their immersion. The adsorption parameters were determined by fitting to experimental adsorption data and were then used along with the theory of equilibrium adsorption constants to successfully explain the reported difference in the surface excess of sodium dodecyl sulphate (SDS) at air|water and oil|water interfaces. The surfactant immersion directly affected the intermolecular interaction parameter which was closely related with the adsorption energy of surfactants at the interfaces. The collaborative effect of these two parameters was also found to be responsible for the immersion of surfactants and shaping their different concentration profiles across various liquid|fluid interfaces. The immersion of surfactants was found to enhance their surface adsorption effectively. Also, our model successfully elucidated why SDS adsorption at the oil|water interface unexpectedly decreases as the hydrophobicity of the oil phase increases

Modifying effect of imidazolium-based ionic liquids on surface activity and self-assembled nanostructures of sodium dodecyl sulfate

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

Combined sum frequency generation and thin liquid film study of the specific effect of monovalent cations on the interfacial water structure

Some salts have been recently shown to decrease the sum frequency generation (SFG) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here, we report a similar trend for the chloride salts of monovalent cations, that is, LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin-film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration dependency by considering the direct ion-water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. Although the former was dominant only at the low-concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broad-band intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye-Hückel lengths using the TFPB technique

Modifying Effect of Imidazolium-Based Ionic Liquids on Surface Activity and Self-Assembled Nanostructures of Sodium Dodecyl Sulfate

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

Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure

Some salts have been recently shown to decrease the sum frequency generation (SFG) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here, we report a similar trend for the chloride salts of monovalent cations, that is, LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin-film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration dependency by considering the direct ion–water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. Although the former was dominant only at the low-concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broad-band intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye–Hückel lengths using the TFPB technique