Enhanced ordering of water at hydrophobic surfaces

Abstract

We study the properties of water molecules adjacent to a hydrophobic molecular layer with vibrational sum-frequency generation spectroscopy. We find that the water molecules at D2O/hexane, D2O/heptane, and D2O/polydimethylsiloxane interfaces show an enhanced ordering and stronger hydrogen-bond interactions than the water molecules at a D2O/air interface. With increasing temperature (up to 80 °C) the water structure becomes significantly less ordered and the hydrogen bonds become weaker.

Sum frequency spectroscopy of the hydrophobic nanodroplet/water interface: Absence of hydroxyl ion and dangling OH bond signatures

“…Above 2670 cm −1 one expects to find water molecules with a single or no H-bond donors [27]. As an example, the dangling OD bond vibrates at 2745 cm −1 [11]. Figure 2 shows SFS spectra in PPP (A) and SSP (B) polarization combination for different pD values.…”

“…The Richmond lab [8][9][10] has investigated planar water/CCl 4 and water/alkane systems and concluded that the hydrogen bonds between water molecules at these interfaces are significantly weaker than at the air/water interface. Bakker and coworkers however, found from SFG studies on the same systems that the ordering of hydrogen bonded water molecules is rather enhanced [11]. Thus, few experimental studies have been performed to elucidate the water structure at the hydrophobic/liquid water interface, and from these it appears that the surface structure of water is not yet very well defined.…”

“…The signal does not contain any ∼20 cm −1 broad features at the location of the free OD or OD − frequencies. The sharply increasing intensity edge originates from [25] and free OD modes [11] are indicated in the graph. The dashed line in panel B represents the envelope of the IR field.…”

Entropy Contribution to the Line Tension: Insights from Polymer Physics, Water String Theory and the Three-Phase Tension

“…X-ray absorption spectroscopy and xray Raman scattering demonstrated that water, consists of structures with two strong H-bonds, one donating and one accepting, thus promoting formation of chain-like structures [22][23][24]. There is much theoretical and experimental evidence that water molecules are strongly oriented in the vicinity of hydrophobic moieties [25][26][27], and this is the case in our treatment (recall, that we restricted our consideration by sessile …”

“…For the estimation of the entropic input into the line tension the threephase line is approximated by a polymer chain. This approximation is approximated by novel experimental data indicating strong orientation effects for liquid molecules located in the vicinity of hydrophobic moieties [22][23][24][25][26][27]. Thus, the polymer-chain model may be successful for sessile droplets placed on hydrophobic (say polymer) solid substrates.…”