Water structural transformation at molecular hydrophobic interfaces

Spectroscopic measurements now reveal that at low temperatures, the water in hydrophobic hydration shells has greater tetrahedral order and fewer weak hydrogen bonds than the surrounding bulk water; this structure disappears at higher temperatures and around alcohol chains longer than 1 nanometre. Action at the oil–water interface The hydrophobic interactions that prevent oil and water from mixing also play a central part in biological processes ranging from cell-membrane formation to protein folding and drug binding. Yet little is known about how an oil molecule changes the structure of water, and thus facilitates such processes. Joel Davis et al . report spectroscopic measurements revealing that at low temperatures, the water in hydrophobic hydration shells has greater tetrahedral order and fewer weak hydrogen bonds than the surrounding bulk water. As the temperature increases, this structure disappears and another appears that is more disordered and has weaker hydrogen bonds than the bulk — but only around nonpolar chains longer than ∼1 nanometre. Hydrophobic hydration is considered to have a key role in biological processes ranging from membrane formation to protein folding and ligand binding 1 . Historically, hydrophobic hydration shells were thought to resemble solid clathrate hydrates 2 , 3 , 4 , with solutes surrounded by polyhedral cages composed of tetrahedrally hydrogen-bonded water molecules. But more recent experimental 5 , 6 , 7 , 8 and theoretical 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 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) 17 , 18 , 19 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