Abstract
The equilibrium phase behaviour of a model binary fluid is investigated through Monte Carlo simulations and by developing a molecular thermodynamic model. Both fluid components interact through a hard core with short-range attractions (SA), but one of the components exhibits an additional long-range repulsion (SA+LR). We find that phase behaviour for this system is controlled by the cross-interaction between the two types of particles as well as their chemical potentials. For a weak cross-interaction, the system displays behaviour that is a composite of the behaviour of the individual components, i.e. the SA component can display bulk vapour/liquid phase separation, while the SALR component can display giant micelle-like clusters for a suitable combination of SA and LR interactions. For a strong cross-interaction, qualitatively different behaviour is observed, with the resulting clusters typically composed of a more equal mixture of SA and SALR particles. Moreover, these mixed clusters can exist even when the SA component by itself would be undersaturated or supercritical, and/or when the SALR component by itself would not form giant clusters. These insights should help to identify the mechanisms for clustering in experimental systems where giant equilibrium clusters are observed.
Highlights
This work investigates the equilibrium phase behaviour of binary fluids where one component exhibits shortrange attractive and long-range repulsive (SALR) interactions
For a weak cross-interaction, the system displays behaviour that is a composite of the behaviour of the individual components, i.e. the short-range attractions (SA) component can display bulk vapour/liquid phase separation, while the SALR component can display giant micelle-like clusters for a suitable combination of SA and LR interactions
We developed a thermodynamic model to study the equilibrium phase behaviour of a model SA+SALR binary fluid
Summary
This work investigates the equilibrium phase behaviour of binary fluids where one component exhibits shortrange attractive and long-range repulsive (SALR) interactions. Such fluids are suitable for modelling the formation of giant clusters in solutions of biomolecules [1,2], and other soft matter systems where solutes can become charged in solution (i.e. by proton exchange with the solvent). In these systems, attractive interactions between solutes can arise through a variety of mechanisms, such as hydrophobicity, hydrogen bonding (or specific binding interactions) or a depletion interaction (induced by solvated polymer, for example), whereas long-range repulsion is generally the result of screened coulombic charges. The SALR mechanism for cluster formation is relatively novel, and reports of giant clusters observed in small molecule solutions, such as glycine [4,5,6] and urea [7], suggest this SALR mechanism might be more universal.
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