Theoretical investigation of the phase behaviour of mixtures of a novel family of perfluoroalkyl–polyoxyethylene ether diblock surfactants in aqueous solutions of carbon dioxide

Abstract

The accurate knowledge of the thermodynamic properties, with special emphasis on phase equilibria, of aqueous solutions of carbon dioxide is essential from practical and theoretical points of view. These aqueous mixtures are of great interest in different industrial processes, including its use as supercritical solvents, CO 2 sequestration technologies and oil enhanced recovery, among many different applications. From a theoretical point of view, this mixture exhibits a variety of interactions, including dispersive forces, hydrogen bonding between water molecules, and also dipolar and quadrupolar electrical interactions, which determine its phase behaviour. In particular, the mixture exhibits type III phase behaviour, which leads to large regions of liquid–liquid immiscibility. In a number of practical situations, it is important to make miscible this mixture. This can effectively be done by adding some amount of a selected surfactant that stabilizes the mixture. An ideal candidate would be a surfactant formed by hydrophilic and CO 2-philic parts that interact favourably with water and carbon dioxide molecules, respectively. In this work we study a novel family of diblock amphiphiles, the perfluoroalkyl–polyoxyethylene ether non-ionic diblock surfactants, which have the general formula F(CF 2) i (OCH 2CH 2) j OH or simply F i E j . We employ the sophisticated and versatile Statistical Associating Fluid Theory for potentials of variable range, the so-called SAFT-VR approach, to study the phase behaviour of aqueous solutions of carbon dioxide and the novel surfactants. All molecules are modeled under the united-atom approach, in which different chemical groups are treated as hard-sphere attractive segments. The F i E j surfactant is modelled as a diblock compound, which consists of a perfluorinated alkane chain and a polyoxyethylene ether chain. The optimized molecular parameters for H 2O, CO 2, and the perfluoroalkyl, polyoxyethylene ether and the rest of chemical groups forming the surfactant are taken from previous works from the literature. Unlike interactions between different chemical groups are carefully chosen to reproduce the phase behaviour of similar associating surfactants previously studied in the literature. In this work we study the effect of temperature, pressure and composition of a selection of mixtures to understand the microscopic conditions that determine the stabilization of aqueous solutions of carbon dioxide when this kind of surfactants are added. To our knowledge, this is the first time the phase behaviour of mixtures containing this kind of surfactants is studied using a microscopic modelling approach.