Molecular Aggregation Equilibria. Comparison of Finite Lattice and Weighted Random Mixing Predictions

Molecular aggregation equilibria are described using finite lattice and mean field theoretical modeling strategies, both built upon a random mixture reference system. The resulting predictions are compared with each other for systems in which each aggregate consists of a central solute molecule whose first coordination shell can accommodate multiple bound ligands. Solute–ligand (direct) and ligand–ligand (cooperative) interactions are found to influence aggregate size distributions in qualitatively different ways, as direct interactions produce a shape-invariant transformation of the aggregate size distribution, whereas cooperative interactions can lead to a vapor–liquid-like transformation. When half the ligand binding sites are filled, the corresponding aggregate size distributions are invariably unimodal in the absence of cooperative interactions, but when the latter interactions are attractive, the distributions are predicted to be bimodal below and unimodal above a critical temperature. Mean field and finite lattice predictions are found to be in globally good agreement with each other, except under near-critical conditions, and even there, the predicted average aggregate sizes and equilibrium constants are remarkably similar. Potential applications of these theoretical predictions to the analysis of experimental and molecular dynamics aggregation results are discussed

Micelle Structure and Hydrophobic Hydration

Despite the ubiquity and utility of micelles self-assembled from aqueous surfactants, longstanding questions remain regarding their surface structure and interior hydration. Here we combine Raman spectroscopy with multivariate curve resolution (Raman-MCR) to probe the hydrophobic hydration of surfactants with various aliphatic chain lengths, and either anionic (carboxylate) or cationic (trimethylammonium) head groups, both below and above the critical micelle concentration. Our results reveal significant penetration of water into micelle interiors, well beyond the first few carbons adjacent to the headgroup. Moreover, the vibrational C-D frequency shifts of solubilized deuterated <i>n</i>-hexane confirm that it resides in a dry, oil-like environment (while the localization of solubilized benzene is sensitive to headgroup charge). Our findings imply that the hydrophobic core of a micelle is surrounded by a highly corrugated surface containing hydrated non-polar cavities whose depth increases with increasing surfactant chain length, thus bearing a greater resemblance to soluble proteins than previously recognized

Specific Ion Effects in Amphiphile Hydration and Interface Stabilization

Specific ion effects can influence many processes in aqueous solutions: protein folding, enzyme activity, self-assembly, and interface stabilization. Ionic amphiphiles are known to stabilize the oil/water interface, presumably by dipping their hydrophobic tails into the oil phase while sticking their hydrophilic head groups in water. However, we find that anionic and cationic amphiphiles adopt strikingly different structures at liquid hydrophobic/water interfaces, linked to the different specific interactions between water and the amphiphile head groups, both at the interface and in the bulk. Vibrational sum frequency scattering measurements show that dodecylsulfate (DS^–) ions do not detectably perturb the oil phase while dodecyltrimethylammonium (DTA⁺) ions do. Raman solvation shell spectroscopy and second harmonic scattering (SHS) show that the respective hydration-shells and the interfacial water structure are also very different. Our work suggests that specific interactions with water play a key role in driving the anionic head group toward the water phase and the cationic head group toward the oil phase, thus also implying a quite different surface stabilization mechanism