The surface saturation of solutions of three low-molecular-weight surface-active alcohols were identified independently using sum frequency generation spectroscopy. Tensiometry showed that solution surface tension kept decreasing uniformly and considerably even after full saturation of the adsorption monolayer. We employed Guggenheim's model of extended interface to attribute this observation to the ongoing adsorption of alcohol molecules beneath the saturated topmost adsorption layer. Our investigation of the dynamic behaviour of thin liquid films of alcohol solutions revealed the tremendous effect of sub-monolayer adsorption on the rheological characteristics of surface. Sub-monolayer region was found to function as a supplementary source of alcohol molecules. The fast diffusion of surfactants from sub-monolayer to the topmost adsorption layer imposed a buffering effect on the dynamic response of the surface through diminishing the surface tension gradient created by surface expansion. This resulted in sudden drop of the Gibbs elasticity and consequently faster decaying foam.
The foams and foam films stabilized by hydrophobically-modified inulin polymeric surfactant (INUTEC SP1) were investigated as a function of electrolyte concentrations. The Foam Pressure Drop Technique was applied to obtain the foam lifetime at constant capillary pressure. The Film Pressure Balance Technique was simultaneously applied to obtain the disjoining pressure as a function on film thickness. The results showed that at INUTEC SP1 concentration of 2 × 10 −5 mol.dm −3 and constant capillary pressure of 5 kPa the foam lifetime τ p was independent of electrolyte concentration in the range from 10 −4 to 10 −1 mol.dm −3 NaCl. Above 10 −1 mol.dm −3 NaCl, the foam lifetime increased rapidly with the increase in NaCl concentration and the most stable foams were produced from solutions containing 1 and 2 mol.dm −3 NaCl. The critical pressure P cr, foam above which foam collapse (avalanche-like) occurred increased with the increasing of electrolyte concentration and this explained the higher stability of the foam at high NaCl concentrations. The disjoining pressure (Π)-thickness (h) isotherms showed a different behaviour at low and high electrolyte concentrations. At 10 −3 mol.dm −3 NaCl, common thin films were obtained and the thickness decreased with increase in the pressure and rupture occurred at about 3 kPa. At 1 mol.dm −3 NaCl, black films were obtained with much smaller thickness that decreased slowly with the increase in Π and rupture occurred at about 2.7 kPa. These results indicated the transition from electrostatic to steric interaction as the electrolyte concentration was increased.
Biocompatible and biodegradable ingredients of natural origin are widely used in the design of foam and emulsion systems with various technological applications in the food, cosmetics and pharmaceutical industries. The determination of the precise composition of aqueous solution formulations is a key issue for the achievement of environmentally-friendly disperse systems with controllable properties and reasonable stability. The present work is focused on the investigation of synergistic interactions in aqueous systems containing Quillaja saponins and Apple pectins. Profile analysis tensiometer (PAT-1) is applied to study the surface tension and surface dilational rheology of the adsorption layers at the air/solution interface. The properties and the foam films (drainage kinetics, film thickness, disjoining pressure isotherm, critical pressure of rupture) are investigated using the thin-liquid-film (TLF) microinterferometric method of Scheludko–Exerowa and the TLF-pressure-balance technique (TLF-PBT). The results demonstrate that the structure and stability performance of the complex aqueous solutions can be finely tuned by changing the ratio of the bioactive ingredients. The attained experimental data evidence that the most pronounced synergy effect is registered at a specific saponin:pectin ratio. The obtained information is essential for the further development of aqueous solution formulations intended to achieve stable foams based on mixtures of Quillaja saponins and Apple pectins in view of future industrial, pharmaceutical and biomedical applications.
We studied silica suspensions with chitosan and biodegradable synthetic surfactant lauroyl ethyl arginate (LAE). Hydrophilic and negatively charged silica nanoparticles were neutralised due to the coating with chitosan. That presence of LAE led to the partial hydrophobisation of their surface, which favoured their attachment to the surface of a thin foam film. It was found that the presence of small and medium-sized (6–9 nm) hydrophobic particles in the interfacial layer of lamella foam film inhibited the coalescence and coarsening processes, which prolonged the life of the foam. Furthermore, hydrophobising of 30 nm particles allowed the formation of large aggregates precipitating from the mixture under steady-state conditions. These aggregates, however, under the conditions of the dynamic froth flotation process in the foam column, were floated into the foam layer. As a result, they were trapped in the foam film and Plateau borders, effectively preventing liquid leakage out of the foam. These results demonstrate the efficiency of using chitosan-LAE mixtures to remove silica nanoparticles from aqueous phase by foaming and flotation.
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