Probing the Molecular Orientation of Methyl Isobutyl Carbinol at the Air–Water Interface

Flotation surface chemistry of water-soluble salt minerals: from experimental results to new perspectives

The flotation separation of water-soluble salt minerals has to be conducted under the condition of saturation in brines which represents a challenging but exciting topic of colloid and surface chemistry. Despite several proposals on explaining the success of this industrial application for many decades, our understanding of the flotation separation is still far from complete yet, owing to the complexity of the highly selective collection of salt crystals by air bubbles in brines. Here, we thoroughly review the experimental results for halogen, oxyanion, and double salts and match them with the proposed theories on the flotation of soluble salts to identify the agreed and disagreed cases. The experimental results show that the flotation of these salts varies from collectors (surfactants applied to control the crystal hydrophobicity) to collectors and is strongly affected by the brine ion composition and pH conditions. We find some exceptional flotation results that cannot be simply explained by the crystal surface charge and wettability. Furthermore, we outline several disputes and discrepancies between the experiments and the theories when different collectors are applied. Apart from the extensive consideration of surface hydration, the presence of external ion species exhibits ubiquitous effects on the surface properties of salt crystals and the colloidal properties of collectors. We conclude that the interactions between salt ions, water molecules, collectors, and salt crystals must be considered more thoroughly, and the activity of collectors at the air-liquid interface should also be the focus. Advanced techniques such as molecular dynamics simulation, atomic force microscopy, X-ray photoelectron spectroscopy, and sum-frequency generation spectroscopy are expected to be promising research tools for future studies.

From Surface Tension to Molecular Distribution: Modeling Surfactant Adsorption at the Air-Water Interface

Surfactants are centrally important in many scientific and engineering fields and are used for many purposes such as foaming agents and detergents. However, many challenges remain in providing a comprehensive understanding of their behavior. Here, we provide a brief historical overview of the study of surfactant adsorption at the air-water interface, followed by a discussion of some recent advances in this area from our group. The main focus is on incorporating an accurate description of the adsorption layer thickness of surfactant at the air-water interface. Surfactants have a wide distribution at the air-water interface, which can have a significant effect on important properties such as the surface excess, surface tension, and surface potential. We have developed a modified Poisson-Boltzmann (MPB) model to describe this effect, which we outline here. We also address the remaining challenges and future research directions in this area. We believe that experimental techniques, modeling, and simulation should be combined to form a holistic picture of surfactant adsorption at the air-water interface.

A critical review of the model fitting quality and parameter stability of equilibrium adsorption models

We reviewed eight commonly used equilibrium adsorption models and examined their underlying assumptions, fitting qualities, and parameter stabilities. We compared several objective functions that have been applied to curve fitting analysis and a few statistics tests that have been performed to evaluate regression quality. The iteratively reweighted least squares algorithm was selected as the most suitable regression method for adsorption models in the presence of heteroscedasticity. The fraction of unexplained variance was selected to indicate the model fitting quality. Two sources of parameter instability were identified: residue instability and function instability. While the definition of the instability caused by residue is well established, we are the first to consider the instability caused by an adsorption model. The models discussed in this article can be applied to many surfactants, such as normal alcohols, polyglycol ethers, and sodium dodecyl sulfate at different salt concentrations. Our results show that both the model fitting quality and parameter instability increase with the number of parameters subject to curve fitting. For the Frumkin-type of reorientation model, the parameter instability can be as high as 25%. The high degree of instability in some complicated adsorption models may invalidate the estimated parameters. Therefore, additional measurements or simulations are required for complicated models to extract reliable model parameters.

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.