Pressure Effects on Self-Assembly in Mixtures Containing Zwitterionic Amphiphiles.

To understand the responses of self-assembly in mixtures containing zwitterionic amphiphilic chains to high pressure, we introduce a self-consistent field theory in combination with a molecular equation-of-state model for them in a primitive way. The free energy density for those in the bulk state is first formulated. Its locally equilibrated excess part is then incorporated into Edwards Hamiltonian along with the electrostatic energy contributions to elicit the saddle point approximation to the partition function with proper self-consistent field equations. It is shown that charge-charge correlations enhance self-assembling tendency for the amphiphiles with the opposite charges on one component side, as the medium dielectric constant ε decreases. Those with the opposite charges at the two chain ends respond in a more complicated way to ε. Densification by applied pressure strengthens the self-assembly for both at a moderate ε, similar to typical phospholipids, but pressure effects are strongly dependent on the position of charges along the chains at a lower ε. It is argued that the manipulation of the dielectric environment and disparity in component dispersion interactions can yield useful materials exhibiting various types of baroresponsivity or thermoresponsivity with re-entrant self-assembly.