Optical characterization of structure and dynamics from single droplets to emulsions

Abstract

Aim of the present research is to investigate the intimate link between the properties of the single droplet interface and the mechanisms involved in emulsion formation and stability. Emulsion stability is a subject of enormous technological relevance, with potential impact over different types of industries (pharmaceutical, foods, petrochemical, cosmetics, and more). In particular, the target is to optimize the amount of additives used in order to reduce the potential negative impact and production costs, and to replace ionic with non-ionic surfactants for benefits to the environment and human health. Emulsions are dispersions of immiscible liquids in which one is dispersed in the other in form of droplets. Surfactants are molecules that, when adsorbed at the liquid-liquid interface, stimulate emulsion production and promote its stability. Stability is related to dynamic properties of the adsorbed layer. Under specific conditions, the stability of this layer contrasts droplets aggregation and coalescence that could cause droplet to growth and phase separation. Interfacial dilational viscoelasticity, namely interfacial tension response to droplets area variation, is of considerable importance against coalescence. Destabilization processes and emulsions evolution have been studied at different scales, from microscopic to macroscopic, by a combination of different optical techniques: microscopy and macro imaging, digital images elaboration, interfacial tensiometry and Diffusing Wave Spectroscopy (DWS). DWS is a non invasive optical technique based on the temporal analysis of coherent light scattered inside a turbid system, such as an emulsion. The technique detects the internal dynamic of the emulsion and allows to obtain an indirect measurement of structural (droplet mean size) and rheological (mechanical modulus) features. In this work, the long-term stability of a paradigmatic alkane in water emulsion stabilized by sodium dodecyl sulfate (SDS) at concentration well below the Critical Micellar Concentration (CMC) has been characterized. The most important result concerns the internal dynamic of emulsion in the long-time regime, explained also in terms of elementary process at the droplets interface. The aging of stable emulsions shows a slowing down of the dynamics, presumably related to progressive drainage of water and to the slowly increase of drop packing, as the value of average drop radius estimated remains constant. At constant aging, dynamics shows a peak of the relaxation time as a function of surfactant concentration. This implies that the bulk shear mechanical modulus shows a maximum; this maximum corresponds to an analogous maximum found in the dilational modulus of the single interface by drop tensiometry.