Hydrophobic hydration is considered to have a key role in biological processes ranging from membrane formation to protein folding and ligand binding. Historically, hydrophobic hydration shells were thought to resemble solid clathrate hydrates, with solutes surrounded by polyhedral cages composed of tetrahedrally hydrogen-bonded water molecules. But more recent experimental and theoretical studies have challenged this view and emphasized the importance of the length scales involved. Here we report combined polarized, isotopic and temperature-dependent Raman scattering measurements with multivariate curve resolution (Raman-MCR) that explore hydrophobic hydration by mapping the vibrational spectroscopic features arising from the hydrophobic hydration shells of linear alcohols ranging from methanol to heptanol. Our data, covering the entire 0-100 °C temperature range, show clear evidence that at low temperatures the hydration shells have a hydrophobically enhanced water structure with greater tetrahedral order and fewer weak hydrogen bonds than the surrounding bulk water. This structure disappears with increasing temperature and is then, for hydrophobic chains longer than ~1 nm, replaced by a more disordered structure with weaker hydrogen bonds than bulk water. These observations support our current understanding of hydrophobic hydration, including the thermally induced water structural transformation that is suggestive of the hydrophobic crossover predicted to occur at lengths of ~1 nm (refs 5, 9, 10, 14).

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.

Experimental MethodsRaman Spectral Measurements: Raman spectra were obtained using a home-built, micro-Raman system similar to that used in previous studies (1). The system used in the present studies includes an Ar-Ion laser source (514.5nm, ~ 50 mW power at the sample) and a thermoelectrically cooled CCD detector (Princeton Instruments Inc., Pixis 400, 1340x400 pixel) mounted to a 300 mm focal length imaging spectrograph (SpectraPro300i, Acton Research Inc.), with a 300 g/mm grating, such that the dispersion is approximately 5 cm -1 per CCD pixel. Liquid samples were analyzed in spectroscopic 1 cm glass cuvettes contained within a thermoelectric, temperature-controlled, translating cell holder (Quantum Northwest). The solution temperature was controlled to within ± 0.01 °C over a 0°C to 60°C temperature range. Such temperature control was required in order to avoid spurious spectral features in the solute-correlated (SC) spectra resulting from small differences in temperature between the solution and pure water samples. A helium lamp was placed behind the sample so that two He lines at 587.562 nm and 667.815 nm were visible in each Raman spectrum (on either side of the OH stretch band). The He lines were used to correct for small wavelength drifts (resulting from small changes in ambient temperature and pressure) which, if uncorrected, would also produce spurious features in the SC spectra within the OH stretch band. The frequency shifts were corrected by introducing a sub-pixel shift the wavelength axis of the solution spectra so as to precisely match the He peak positions in the solution and pure water spectra. The latter shifts were performed using IgorPro (Wavemetrics Inc.) which facilitates duplication of waves with sub-pixel shifts, using a command such as wave1=wave0(p+d), where p is the pixel number (variable) d is a real constant whose magnitude is typically less than 0.1, and wave1 and wave0 are the shifted and un-shifted solution spectra, respectively.Benzene: Spectrophotometry grade benzene (99.93 % assay, EMD Chemicals Inc., Germany) was used without further purification. Water was ultra-purified (Milli-Q UF plus, Millipore Inc.) to an electrical resistance of 18.2 MΩ •cm. Saturated benzene in water was prepared by thoroughly stirring a sample consisting of water with a small excess benzene for 5 min. The solution was then allowed to equilibrate at the desired temperature for at least one day (in equilibrium with the excess benzene phase). Raman spectra were collected from the aqueous phase of the resulting solution (with an excess benzene layer at the top) as well as from a pure water sample (at the same temperature). All samples were equilibrated for at least 5 min in the temperature controlled sample cell holder, and measurements were made in the same cell position. Total (un-polarized) Raman scattering spectra of each solution of benzene in water

Molecular dynamics and electric field strength simulations are performed in order to quantify the structural, dynamic, and vibrational properties of non-H-bonded (dangling) OH groups in the hydration shell of neopentane, as well as in bulk water. The results are found to be in good agreement with the experimentally observed high-frequency (∼3660 cm(-1)) OH band arising from the hydration shell of neopentanol dissolved in HOD/D(2)O, obtained by analyzing variable concentration Raman spectra using multivariate curve resolution (Raman-MCR). The simulation results further indicate that hydration shell dangling OH groups preferentially point toward the central carbon atom of neopentane to a degree that increases with the lifetime of the dangling OH.

Raman multivariate curve resolution (Raman-MCR), as well as quantum and classical calculations, are used to probe water structural changes in the hydration shells of carboxylic acids and tetraalkyl ammonium ions with various aliphatic chain lengths. The results reveal that water molecules in the hydration shell around the hydrophobic chains undergo a temperature and chain length dependent structural transformation resembling that previously observed in aqueous solutions of n-alcohols. Deprotonation of the carboxylic acid headgroup (at pH ∼ 7) is found to suppress the onset of the hydration-shell structural transformation around the nearest aliphatic methylene group. Tetraalkyl ammonium cations are found to more strongly suppress the water structural transformation, perhaps reflecting the greater intramolecular charge delocalization and suppression of dangling OH defects in water's tetrahedral H-bond network. The observed coupling between ionic and hydrophobic groups, as well as the associated charge asymmetry, may influence the hydrophobicity of proteins and other materials.