On the cooperative formation of non-hydrogen-bonded water at molecular hydrophobic interfaces

Abstract

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.

Micelle Structure and Hydrophobic Hydration

“…Figures 2C and 3C indicate that the average number of surfactant dangling OH groups (before micelle formation) has a nonlinear chain length dependence that is roughly consistent with that previously reported for n-alcohols (and ascribed to the cooperativity of the dangling OH formation process). 19 Comparisons of the open blue and solid red points in both Figures 2C and 3C imply that the excess number of dangling OH groups in the hydrophobic hydration shell of each surfactant in a micelle is comparable to that around a fully hydrated surfactant (below CMC). Most importantly, the number of such hydrophobic water molecules (per surfactant) clearly increases with chain length.…”

“…19 The ⟨k⟩ values were obtained from the area under the dangling OH peak in the SC spectrum, as previously described. 19 The solid points in Figures 2C and 3C represent the ⟨k⟩ values obtained after removing the contribution from free monomers that are in equilibrium with the micelles, assuming that the free monomer concentration is equal to the CMC (as further described in the Supporting Information). Figures 2C and 3C indicate that the average number of surfactant dangling OH groups (before micelle formation) has a nonlinear chain length dependence that is roughly consistent with that previously reported for n-alcohols (and ascribed to the cooperativity of the dangling OH formation process).…”

Water interaction with hydrogenated and oxidized detonation nanodiamonds — Microscopic and spectroscopic analyses

“…The stretching peak at 3690 cm −1 thus corresponds to the "free" OH bond of water molecules that straddle the interface between the water and hydrophobic surface of H-DNDs. The wavenumber of the "free" OH bond depends on specific condition at the interface; for instance, 3660 cm −1 for water/propan-1-ol interface [30] and 3669 cm −1 for water/tetrachloromethane interface [26]. Relatively higher wavenumber found on H-DNDs (3690 cm −1 ) may be due to rigidity of solid H-DND surface where the "free" OH bond stretching is less damped in contrast to the liquid-liquid interfaces.…”

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

“…[20][21][22] These simulations illustrate that at layer thicknesses below 1 nm, water molecules interacting with the outside surface (known as the exohedral surface) of the CNT have preferred orientations, and assume a spatially varying density profile (see Figure 1b) that is independent of CNT outer diameter. [21,22] On the other hand, these simulations show that water molecules interacting with the inside surface (known as the endohedral surface) are strongly influenced by confinement effects, [27][28][29] and assume density profiles that are a strong function of the CNT inner diameter. [20,21,30] While the scaling behavior of the water layer density observed in these simulations should be similar for CNTs with native wall defects (such as those ones used here), differences in interaction potentials mean that the results of these previous studies may not be representative of the current system of aligned CNTs.…”