Theoretical study of mixed amphiphilic bilayers
Abstract
In this thesis, we use a molecular approach to calculate the elastic bending constants of mixed (two-component) amphiphilic bilayers. This method allows us to account for the molecular detail of the bilayer's constituents, while incorporating the intermolecular interactions inside the bilayer within a mean field approach. The aggregate's free energy is calculated and then expanded up to quadratic order in curvatures and compositions, choosing a flat symmetrical bilayer as the reference state. The bending constants are obtained from the derivatives of the free energy evaluated at this reference state. As a novelty, the local compositions are allowed to fully relax upon bending, so as to ensure chemical equilibrium between the two monolayers at every curvature. The compositional degree of freedom is found to affect the bending constant k, but not the saddle-splay constant k¯. The influence on the bilayer's elastic properties of various chain structural features, such as length, volume, and stiffness, is investigated. This may prove useful to model bilayers composed of various types of molecules, such as hydrocarbon/hydrocarbon and hydrocarbon/fluorocarbon mixtures. Additionally, we present a theoretical study of the electrostatic interactions in a system comprising two semi-infinite ionic solutions, separated by a dielectric medium of finite thickness (slab). Such system could serve as a model of an amphiphilic lipid bilayer immersed in an ionic biological solution. The space outside the slab may also include, besides the ions and the solvent (water), larger charged spherical particles intended to model certain types of proteins. A modified Poisson-Boltzmann approach is used, which allows us to treat the electrostatic interactions in an exact manner, while incorporating the finite size of the constituent particles within a mean-field approach. In particular, we focus on the study of the conditions under which the two ionic solutions are correlated across the dielectric slab.
Degree
Ph.D.
Advisors
Szleifer, Purdue University.
Subject Area
Chemistry
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