Surface behavior of hydrophilic silica nanoparticle-SDS surfactant solutions: I. Effect of nanoparticle concentration on foamability and foam stability

Hydrophilic silica nanoparticles have no tendency to adsorb at the water/air interface and they do not affect the surface behavior of pure water. In this paper, we show that this tendency remains unaltered by addition of anionic sodium dodecyl sulfate (SDS) surfactant, although the surface behavior of SDS solutions considerably changes with the addition of silica particles. Employing dynamic tensiometry, surface rheology, foamability, and foam stability experiments in presence of variable amounts of silica particles, we found that the surface activity of SDS molecules is significantly increased in the presence of nanoparticles and equivalently behaved as if higher concentrations of SDS are used. The so-called equivalent concentration values (EC) were separately measured using the tensiometry and foamability. The consistency of measured EC trends is discussed, and a new predictive tool for the foaming behavior of surfactant solutions is proposed based on the calculated EC trends from tensiometry and surface dilational rheology. The adsorption process and surface structuring of nanoparticles at the water/air interface were investigated by analyzing the surface pressure values as a function of drop surface area. We measured the standard deviation, STD, of drop shapes from their Laplacian profile during the drop compression process. It was found that STD-values abruptly change when particles form closely-packed surface structures. The STD trend is explained in detail and is shown it can be a very powerful tool for analyzing the surface adsorption and surface structuring of nanoparticles. (C) 2016 Elsevier B.V. All rights reserved.