Effect of selected monovalent salts on surfactant stabilized foams.

Surfactant-stabilized foams have been at the centre of scientific research for over a century due to their ubiquitous applications in different industries. Many of these applications involve inorganic salts either due to their natural presence (e.g. use of seawater in froth floatation) or their addition (e.g. in cosmetics) to manipulate foam characteristics for the best outcomes. This paper provides a clear understanding of the effect of salts on surfactant-stabilized foams through a critical literature survey of this topic. Available literature shows a double effect of salts (LiCl, NaCl and KCl) on foam characteristics in the presence of surfactants. To elucidate the underlying mechanisms of the stabilizing effect of salts on foams, the effect of salts on surfactant-free thin liquid films is first discussed, followed by a discussion on the effect of salts on surfactant-stabilized foams with the focus on anionic surfactants. We discuss two distinctive salt concentrations, salt transition concentration in surfactant-free solutions and salt critical concentration in surfactant-laden systems to explain their effects. Using the available data in literature supported by dedicated experiments, we demonstrate the destabilizing effect of salts on foams at and above their critical concentrations in the presence of anionic surfactants. This effect is attributed to retarding the adsorption of the surfactant molecules at the interface due to the formation of nano and micro-scale aggregates.

Monolayer formed by l-Asp-based gemini surfactants self-assembled in 1D nanostructures.

Herein, studies on the surface activities of newly synthesized l-Asp-based gemini surfactants, both nonionic and anionic, are presented. Conductometry, tensiometry, and the Langmuir-Blodgett (LB) film technique were applied for this purpose. π-A isotherms were obtained with a Langmuir trough and Wilhelmy balance. The structures of the monolayers assembled at the air/water interface and those deposited as LB films were studied via Brewster angle microscopy (BAM) and atomic force microscopy (AFM). The 2D films formed by the anion-active compounds show a well-known pattern of a monolayer film, whereas the nonionogenic amphiphiles have been found to be 1D structures with nano-widths and micro-lengths that align with each other during the process of compression; this is the first study where the organization of 1D fibrils in 2D films during compression is reported. The scanning electron microscopy (SEM) study reveals that 1D nanostructure formation is an intrinsic tendency of these molecules as not only nonionogenic surfactants, but also the anion active representatives have been constructed in the solid state by fibrillary structures.

Surface tension and surface Δχ-potential of concentrated Z+:Z- electrolyte solutions.

Schmutzer's model for the surface of aqueous electrolyte solutions is generalized to Z+:Z- salts. The thickness of the ion-free layer is calculated from the thickness of the "hydrophobic gap" at the water surface (1.38Å) and the radii of the ionic hydration shells. The overlap between the adsorption and the diffuse double layers is accounted for. The proposed model predicts the dependence of the surface tension σ and the surface Δχ-potential on the electrolyte concentration c(el) in agreement with the available data, without adjustable parameters. The Hofmeister effect on σ for salts of the same valence type is explained with their ion-specific activity coefficients. The negative value (toward air) of the Δχ-potential of most 1:1 electrolytes originates from the dipole moment of the water molecules at the surface. The negative χ-potential due to water dipoles is inversely proportional to the dielectric permittivity ε of the solution. Since ε diminishes as c(el) increases, most 1:1 electrolyte solutions exhibit a more negative χ-potential than pure water (Δχ<0). The Hofmeister series of Δχ of 1:1 salts (Δχ(LiCl) ≈ Δχ(NaCl)<Δχ(KCl)<Δχ(KF)) follows the corresponding series of ε (ε(LiCl) ≈ ε(NaCl)<ε(KCl)<ε(KF)). The theory allows the estimation of the surface potential χ0 of pure water from the experimental data for electrolyte solutions; the result, χ0 ≈ -100 mV, confirms the value currently accepted in the literature.

Delta-comb potential in modeling three-phase contact line (TPCL) on periodically patterned surfaces.

This work is a study of wetting of small water droplets on smooth glass surfaces with periodic patterns in the form of imprinted net with hydrophilic cells and hydrophobic bars. Microcover slides consisted of soda lime glass were used. The imprinted images of the net were with cell sizes in the range 40-200 µm, which corresponds to a quite narrow scope of hydrophilic surface fractions f(1)(30-36%) due to the relative increase in the size of the hydrophobic bars. The receding contact angles θ(R) of small water droplets, positioned on the patterned surfaces, were measured. The experiment showed significantly lower receding contact angles as compared to the theoretical expectations by the Cassie formula, which accounts for the contribution to the contact angle of the surface fraction of the imprinted hydrophobic/hydrophilic net. For this reason, we developed new theory accounting for the periodicity of the surface and the contribution of the three-phase contact line on the contact angle. This new theory considered delta-comb potential energy Δ(x,y) of the surface, effective line tension κ, and the lattice parameter a. The restriction of theory was discussed as well. It was pointed out that the theory is not valid for very small and very large lattice parameters.