The morphology of aerosol particles consisting of hydrophobic and hydrophilic phases: hydrocarbons, alcohols and fatty acids as the hydrophobic component

Abstract

The morphology of bi-phase aerosol particles containing phase separated hydrophobic and hydrophilic components is considered, comparing simulations based on surface and interfacial tensions with measurements made by aerosol optical tweezers. The competition between the liquid phases adopting core-shell and partially engulfed configurations is considered for a range of organic compounds including saturated and unsaturated hydrocarbons, aromatics, alcohols, ketones, carboxylic acids, esters and amines. When the solubility of the organic component and the salting-out of the organic component to the surface by the presence of concentrated inorganic solutes in the aqueous phase are considered, it is concluded that the adoption of a partially engulfed structure predominates, with the organic component forming a surface lens. The aqueous surface can be assumed to be stabilised by a surface enriched in the organic component. The existence of acid-base equilibria can lead to the dissociation of organic surfactants and to significant lowering of the surface tension of the aqueous phase, further supporting the predominance of partially engulfed structures. Trends in morphology from experimental measurements and simulations are compared for mixed phased droplets in which the organic component is decane, 1-octanol or oleic acid with varying relative humidity. The consequences of partially engulfed structures for aerosol properties are considered.