The unique structural, dynamical and chemical properties of air/water and oil/water interfaces are thought to play a key role in various biological, geological and environmental processes. For example, non-hydrogen-bonded ('dangling') OH groups--which create surface defects in water's hydrogen bonding network and are experimentally detected at both macroscopic (air/water or oil/water) and microscopic (dissolved hydrophobic molecule) interfaces--are thought to catalyse some chemical reactions. However, how the size, curvature or charge of the exposed hydrophobic surface influences water's propensity to form dangling OH defects has not yet been established quantitatively. Here we use Raman multivariate curve resolution to probe spectroscopically the hydrophobic hydration shell and, using a statistical multisite analysis, we show that such interfacial dangling OH structures are entropically stabilized and their formation is cooperative (the probability that a non-hydrogen-bonded OH group will form depends nonlinearly on the hydrophobic surface area). We thus expose an important difference between the chemical properties of molecular and macroscopic oil/water interfaces.

Given the importance of water-mediated hydrophobic interactions in a wide range of biological and synthetic self-assembly processes, it is remarkable that both the sign and the magnitude of the hydrophobic interactions between simple amphiphiles, such as alcohols, remain unresolved. To address this question, we have performed Raman hydration-shell vibrational spectroscopy and polarization-resolved femtosecond infrared experiments, as well as random mixing and molecular dynamics simulations. Our results indicate that there are no more hydrophobic contacts in aqueous solutions of alcohols ranging from methanol to tertiary butyl alcohol than in random mixtures of the same concentration. This implies that the interaction between small hydrophobic groups is weaker than thermal energy fluctuations. Thus, the corresponding water-mediated hydrophobic interaction must be repulsive, with a magnitude sufficient to negate the attractive direct van der Waals interaction between the hydrophobic groups.

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 n-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 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.

Guilty as charged: Water is often modeled as a dielectric continuum, but the molecular structure of water is asymmetric. Two ions that have a virtually identical size, shape, and structure, but an opposite charge sign have been investigated to see whether charge makes a fundamental difference to water structuring. The spectroscopic data for the hydration and interface structures are found to be remarkably different for opposite charges.