Self -assembly of amphiphilic molecules: Micelles and monolayers
Abstract
The self-assembly of short nonionic amphiphiles has been investigated. We study the thermodynamic behavior of micelles, the solubilization of small molecules into micelles, and the kinetics of monolayer formation. The Single-Chain Mean-Field (SCMF) theory was used to predict the cmc, and the size distribution of unloaded micelles, and solubilization amounts and sites. For unloaded micelles, cmc and size distribution quantitative agree with molecular dynamic results. The approach is quantitatively predicts the two different free energy scales responsible for micellization and for size distributions in model systems. We have systematically studied the dependency of properties on head and tail lengths, architecture of the molecule and temperature. The cmc is found to be slightly dependent on the head molecular architecture. However, the size distribution is found to be quite different for linear and branched heads. The micelle has a compact, hydrophobic core and a wide interface region. The core is found to be more compact and larger for longer tails, and more compact for lower temperatures. The molecular organization is not very sensitive to changes in the head architecture. For loaded micelles, our calculations predict a little influence in the cmc and size distribution by the presence of the solutes, as it is experimentally observed. A significant enhancing of the solubilization is predict for monomeric solutes. Contrarily, surfactant dimeric solutes are poorly solubilized. Solutes at low bulk concentrations are located mainly in the core-corona region. For some conditions, the formation of microdroplets is predicted. The kinetics of monolayer formation on the interface of two immiscible fluids has been investigated, combining the SCMF theory with a diffusion-like equation. We have found two adsorption regimes: A diffusion controlled regime, that corresponds to a dilute monolayer, and a kinetic controlled regime, much slower than the first, since the closely packed molecules at the interface impose a kinetic barrier to the upcoming amphiphiles.
Degree
Ph.D.
Advisors
Szleifer, Purdue University.
Subject Area
Chemistry|Chemical engineering|Polymers
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