Reduced water density at hydrophobic surfaces: Effect of dissolved gases

Abstract

Here, direct noninvasive neutron reflectivity measurements reveal the presence of a reduced (deuterated) water density region, with a sigmoidal density profile at the hydrophobic silane-water interface that depends on the type and concentration of dissolved gases in the water. Removal of dissolved gases decreases the width of the reduced water density region, and their reintroduction leads to its increase. When compared with recent computer simulations, a locally fluctuating density profile is proposed, whereas preexisting nanobubbles are excluded. The presence of a fluctuating reduced water density region between two hydrophobic surfaces and the attractive ''depletion force'' to which it leads may help explain the hydrophobic force and its reported diminution in deaerated water. Our results are also quantitatively consistent with recent dynamic surface force apparatus results that drastically revise previous estimates of the slip length of water flowing past hydrophobic surfaces from microns to Ϸ20 nm. Our observations, therefore, go a long way toward reconciling three quite different types of experiments and phenomena: water depletion at hydrophobic surfaces, water slip at hydrophobic surfaces, and the hydrophobic interaction.interfacial water ͉ neutron reflectivity ͉ slip conditions

Quantitative determination of ion distributions in bacterial lipopolysaccharide membranes by grazing-incidence X-ray fluorescence

“…I illu q z ðzÞ is the vertical intensity profile of the incident illumination, which depends on the incident angle α i and on the electronic structure of the monolayer. For a monolayer at the air/water interface, this electronic structure can be obtained by GIXOS and parameterized using slab models § , where each slab represents a layer of constant electron density within the monolayer (28)(29)(30). Accordingly, once the illumination profiles I illu q z ðzÞ are known, the ion concentration profiles c j ðzÞ can be reconstructed from the measured fluorescence intensities I fluo j ðq z Þ using Eq.…”

What x rays can tell us about the interfacial profile of water near hydrophobic surfaces

“…Early neutron reflectivity studies [31][32][33] of hydrocarbon-water interfaces reported gap widths larger than 10Å, while later x-ray reflectivity measurements on similar systems reported gap widths of less than 5Å. 35, 36 Yet other X-ray 30 and neutron 34 studies reported that there is no gap.…”

“…In reality, although the water-hydrophobic interface has been studied by a number of groups, [30][31][32][33][34][35][36][37] the results are inconsistent and have not led to a consensus. Early neutron reflectivity studies [31][32][33] of hydrocarbon-water interfaces reported gap widths larger than 10Å, while later x-ray reflectivity measurements on similar systems reported gap widths of less than 5Å.…”

“…Thus silicon-supported SAMs provide excellent, well-defined hydrophobic surfaces that have been used in many experimental studies. [31][32][33][35][36][37] However, there is a high price: the system now contains at least three interfaces, namely silicon-oxide, oxide-SAM, and SAM-water. Only the last of these is of interest here, but all interfaces reflect X-rays or neutrons and must be accounted for during data analysis.…”

Nanobubbles are not a Superficial Matter

“…[3,4] On the other hand, incompatibility between a liquid and an adjacent solid-water against a hydrophobic surface, say-can lead to a depletion in the density of the fluid phase at the interface. [5][6][7][8][9][10][11][12][13] These deviations from bulk properties can exacerbate the chemical inhomogeneities of interfaces, altering for example the dielectric or solvation characteristics there. [14] Furthermore, ions may be depleted or enriched at the solidwater or air-water interfaces relative to the bulk, [15][16][17][18][19][20][21] an effect that involves subtle balances of enthalpic and entropic factors.…”