Phase Segregation at the Liquid–Air Interface Prior to Liquid–Liquid Equilibrium

Abstract

Binary systems with partial miscibility segregate into two liquid phases when their overall composition lies within the interval defined by the saturation points; out of this interval, there is one single phase, either solvent-rich or solute-rich. In most systems, in the one-phase regions, surface tension decreases with increasing solute concentration due to solute adsorption at the liquid-air interface. Therefore, the solute concentration at the surface is higher than in the bulk, leading to the hypothesis that phase segregation starts at the liquid-air interface with the formation of two surface phases, before the liquid-liquid equilibrium. This phenomenon is called surface segregation and is a step toward understanding liquid segregation at a molecular level and detailing the constitution of fluid interfaces. Surface segregation of aqueous binary systems of alkyl acetates with partial miscibility was theoretically demonstrated by means of a thermodynamic stability test based on energy minimization. Experimentally, the coexistence of two surface regions was verified through Brewster's angle microscopy. The observations were further interpreted with the aid of molecular dynamics simulations, which show the diffusion of the acetates from the bulk toward the liquid-air interface, where acetates aggregate into acetate-rich domains.

An atomistic explanation of the ethanol–water azeotrope

“…We note that the behaviour for these two regions is what can be expected based on previous studies using SFG (sum frequency generation spectroscopy), XPS and MD, which all qualitatively agree upon ethanol content being surface enriched. [20][21][22][26][27][28]30 The exact slope of the linear bulk signal which increases in Fig. 2 is unknown due to uncertainties in determining the photoelectron escape depth in a cylindrical micro-jet.…”

“…We further support this hypothesis using molecular dynamics (MD) calculations and show that the surface composition of ethanol/water mixtures very closely mimics the vapour composition. The surface composition of ethanol-water mixtures has been studied with MD, 19 thermodynamics models, [20][21][22] surface tension, [23][24][25] sum-frequency generation 26,27 and X-ray photoelectron spectroscopy (XPS). 28 These studies agree that ethanol is surface enriched at low concentration and forms a monolayer.…”

General Adsorption Model to Describe Sigmoidal Surface Tension Isotherms of Binary Liquid Mixtures

“…An interfacial layer (or surface layer) exists between the two bulk phases, and its properties are different from those of the two bulk ones. Essentially, the surface layer is heterogeneous because its properties such as composition and density gradually change in the direction normal to the surface (from those of one bulk phase to another) although they do not in the direction parallel to the surface. , However, the surface layer is generally regarded as a homogeneous surface phase with a definite thickness so as to simplify the model processing. ,,, The simplifying assumption was adopted in this work.…”

“…It is worth noting that the S-type isotherms are often observed for the adsorption of surfactants at liquid–solid interfaces, which can be described by the surface micellization model. , In fact, the surface aggregation may also occur at the surface layer of liquid mixtures, ,,− probably resulting in the S-type isotherms, which is worth exploring.…”

Partially Miscible Water–Triethylamine Solutions and Their Temperature Dependence

“…Less familiar are partially miscible systems, in which liquids mix at some, but not all, proportions and temperatures. Nevertheless, these partially miscible systems are of considerable interest in academia and industry. − …”