Rod Formation of Ionic Surfactants:  Electrostatic and Conformational Energies

Abstract

We present a thermodynamic model that describes the formation of micelles from ionic surfactants in aqueous solution at varying counterion concentrations. The micellar aggregates may be spheres, dumbbells, and rods. A former theory [Heindl, A.; Kohler, H.-H. Langmuir 1996, 12, 2464] is refined by the introduction of detailed models for the conformational energy of the surfactant chains and the electrostatic interaction of the ionic headgroups. The standard Gibbs energy of a surfactant ion is minimized under constraints imposed by the micelle shape. The conformational energy is calculated from an appropriately modified single-chain mean-field model proposed in another work [Ben-Shaul, A.; Gelbart, W. M. In Membranes, Microemulsions and Monolayers; 1994]. For the electrostatic interactions, we use a previously developed local balance model for a charged interface [Woelki, S.; Kohler, H.-H. Chem. Phys. 2000, 261, 411-419; 421−438]. This leads to a marked counterion specificity of the standard Gibbs energies of the micelles. Interfacial tension, steric headgroup repulsion, and direct counterion adsorption are taken into consideration. From the standard Gibbs energies, the size distribution of the micelles can be obtained by application of the law of mass action. This distribution is used to calculate the viscosity of the micellar solution at a given concentration of surfactant and salt. A single fitting parameterthe counterion dissociation constantis used to fit the model to experimental viscosity data for cetylpyridinium chloride, bromide, iodide, and nitrate. It is shown that two alternative models for the shape of the rods can be used to explain the observed counterion specificity.

Effects of Monovalent Anions of the Hofmeister Series on DPPC Lipid Bilayers Part II: Modeling the Perpendicular and Lateral Equation-of-State

“…2. We assume that the reference state for the calculation of free energy differences is a hypothetical lipid ''crystal'' containing n L molecules at the temperature of the experiment (26,39,40) and an infinite electrolyte solution with a monovalent salt concentration equal to C N . We write the total free energy difference per mol of lipid as follows:…”

“…The electrostatic free energy expression depends on the model assumed for the ionic adsorption at the lipid-water interface and will be discussed below. For the first three contributions we use models that were effective in the prediction of the area per surfactant molecule in lipid aggregates such as monolayers, bilayers, and micelles in the absence of electrolytes (24)(25)(26)(27)33,(38)(39)(40)(41). We thus set:…”

“…For DPPC we will set a 0 equal to 42 Å 2 and g equal to 50 mN m À1 , values generally acceptable for lipid-water interfaces (53,54,(70)(71)(72). The nonelectrostatic (e.g., steric) repulsions between headgroups have been modeled with various empirical equations in the past (25,26,33,(38)(39)(40)(41)54,73), but there is no rigorous theory for this term. Stigter and Dill (74-76) provided a complex formalism for the evaluation of the headgroup repulsion term, but it is not clear how to generalize this treatment in the presence of salts.…”

γ-Fe2O3 oriented growth by surfactant molecules in microemulsion

“…At low concentrations, the aggregates are generally globular micelles but these micelles can grow by increasing the surfactant concentration and/or upon injection of salt, alcohols, etc [8]. Then micelles grow to less flexible rod-like micelles as the theoretical prediction for micellization [9]. The amphiphilic molecules spontaneously assemble to form highly flexible locally cylindrical aggregates with the average size reaching several micrometers.…”

Analytical multi-scale method for multi-phase complex systems in process engineering—Bridging reductionism and holism

“…Simple as the model is, the amphiphilic property dominating surfactant self-assembly is accounted, and the dynamic process is realized in our simulations. Ionic surfactants are subject to future work since long-range interaction between the hydrophilic groups would affect both the system structure and the aggregation dynamics (Hsu et al, 2003;Bauer et al, 2004). Table 4 summarizes the exploration of the above six systems in this section and the gas-solid system discussed in Section 2, with the main results and characteristic diagrams.…”