Bilayers in amphiphilic mixtures connected by threadlike micelles: a self-consistent field theory study

TL;DR

This study uses self-consistent field theory to analyze how amphiphile mixtures form connected bilayer structures with threadlike micelles, revealing how composition influences stability and segregation within these assemblies.

Contribution

The paper provides a detailed SCFT analysis of amphiphilic mixtures forming connected bilayer-micelle structures, highlighting the effects of composition and amphiphile architecture on stability and segregation.

Findings

Adding sphere-former lowers free energy of micelles

Increased sphere-former concentration causes amphiphile transfer to the thread

Higher sphere-former levels destabilize junctions between bilayers and micelles

Abstract

Binary mixtures of amphiphiles in solution can self-assemble into a wide range of structures when the two species individually form aggregates of different curvatures. A specific example of this is seen in solutions of lipid mixtures where the two species form lamellar structures and spherical micelles respectively. Here, vesicles connected by thread-like micelles can form in a narrow concentration range of the sphere-forming lipid. We present a self-consistent field theory (SCFT) study of these structures. Firstly, we show that the addition of sphere-forming lipid to a solution of lamella-former can lower the free energy of cylindrical, thread-like micelles and hence encourage their formation. Next, we demonstrate the coupling between composition and curvature; specifically, that increasing the concentration of sphere-former in a system of two bilayers connected by a thread leads to a transfer of amphiphile to the thread. We further show that the two species are segregated within the structure, with the concentration of sphere-former being significantly higher in the thread. Finally, the addition of larger amounts of sphere-former is found to destabilize the junctions linking the bilayers to the cylindrical micelle, leading to a breakdown of the connected structures. The degree of segregation of the amphiphiles and the amount of sphere-former required to destabilize the junctions is shown to be sensitive to the length of the hydrophilic block of the sphere-forming amphiphiles.