A. Nikolov

Calculation of the surface potential and surface charge density by measurement of the three-phase contact angle.

The silica/silicon wafer is widely used in the semiconductor industry in the manufacture of electronic devices, so it is essential to understand its physical chemistry and determine the surface potential at the silica wafer/water interface. However, it is difficult to measure the surface potential of a silica/silicon wafer directly due to its high electric resistance. In the present study, the three-phase contact angle (TPCA) on silica is measured as a function of the pH. The surface potential and surface charge density at the silica/water surface are calculated by a model based on the Young-Lippmann equation in conjunction with the Gouy-Chapman model for the electric double layer. In measurements of the TPCA on silica, two distinct regions were identified with a boundary at pH 9.5-showing a dominance of the surface ionization of silanol groups below pH 9.5 and a dominance of the dissolution of silica into the aqueous solution above pH 9.5. Since the surface chemistry changes above pH 9.5, the model is applied to solutions below pH 9.5 (ionization dominant) for the calculation of the surface potential and surface charge density at the silica/aqueous interface. In order to evaluate the model, a galvanic mica cell was made of a mica sheet and the surface potential was measured directly at the mica/water interface. The model results are also validated by experimental data from the literature, as well as the results obtained by the potentiometric titration method and the electro-kinetic measurements.

Effect of Depletion Force on the Stability of Food Emulsions

Abstract The stability of food emulsions containing milk proteins and sucrose ester was studied by measuring the creaming velocity of an oil‐in‐water emulsion. The inter‐droplet interaction was quantified by measuring the radial distribution function using an advanced digital imaging technique. The phenomenon of protein sub‐micelles layering inside the film formed between the adjacent droplets was observed by using the capillary force balance apparatus. The attractive interaction of the droplets was attributed to the depletion force induced by the protein sub‐micelles in the film. The experimental results on creaming velocity were rationalized in terms of a statistical thermodynamic model, which takes into account the attractive inter‐droplet interactions.

Superspreading: Role of the Substrate Surface Energy

Trisiloxane surfactants are a class of nonionic surfactants that have the unique ability to spread quickly on difficult-to-wet surfaces (i.e., hydrophobic surfaces). There are numerous studies on the spreading of trisiloxane surfactants, but the mechanism of spreading is not fully understood. The trisiloxane surfactant containing 7.5–8.0 mol of ethylene oxide groups (e.g., Silwet L-77) has been found to be one of the best spreading agents for substrates with an intermediate wettability, and is an excellent herbicide adjuvant, causing an aqueous solution to completely spread on leaf surfaces (e.g., “velvetleaf”). It was observed that, at any substrate wetting angle between 35∘ and 115∘ (referred to as the “water phase”), the spreading rate vs. concentration had a maximum. The value of the concentration corresponding to the maximum rate of spreading did not depend on substrate’s wetting angle. The spreading rate had an optimum at a substrate wetting angle between 60–65∘. The optimum in the spreading rate for L77 reported in the literature had a maximum at a surfactant concentration of 0.45 wt% and a substrate wettability of 60–65∘. Here, we present a simple model based on the Marangoni flow over a curved surface illustrating why the rate of spreading vs. substrate wettability has an optimum at a wetting angle of 60–65∘. The model predicts well the value of the spreading rate and the optimum value of the substrate wetting angle.

Macroions Under Confinement

The layering of like-charged particles or macroions confined by two plane-parallel and two inclined surfaces is studied using a canonical Monte Carlo method combined with a simulation cell that contains both the confined and bath regions. The macroion solution is modelled within a one-component fluid approach that in an effective way incorporates a conventional double layer repulsion due to the ions of suspending electrolyte as well as an extra contribution due to the discrete nature of an aqueous solvent. The plane parallel and wedge-shaped geometries mimic the confinements that naturally occur in large number of systems widely known as colloidal dispersions. The effects of macroion charge, macroion and electrolyte concentrations on the particle layering and in-layer structuring are analyzed. The relation of obtained results to experiments on confined ionic micelle solutions and suspensions of charged polysterene spheres is discussed.

Colloidal suspensions confined to a film: local structure and film stability

This paper summarizes recent experimental and theoretical research conducted in our laboratories on understanding the properties of colloidal suspensions confined to a film. The results of statistical mechanics modelling to explain some experiments on thinning liquid films formed from concentrated monoand bidisperse colloidal suspensions are reported. The effect of colloidal particle size bidispersity on the local density distribution and film stability is discussed in detail.