Poynoret al.Reply:

Abstract

Poynor et al. Reply: Ocko, Dhinojwala, and Daillant agree with us about the role of methyl groups. Our unambiguous statement that [1] ''. . .the depletion thickness reported in Fig. 2(b) must be regarded as an upper bound, although the existence of the depletion layer is unequivocal. This is because protons on the methyl-terminated hydrophobic monolayer and on water adjoining the monolayer are virtually invisible. . .'' has been misquoted by Ocko, Dhinojwala, and Daillant, who assert [2] ''(their) entire electron density depletion is attributed to a depletion of water.'' Our Table I corrects additional factual mistakes in Table I of Ocko, Dhinojwala, and Daillant regarding data in the literature [1,3]. The product of the gap thickness and the electron density deficit is more consistent between the limits of the fitting range than Ocko, Dhinojwala, and Daillant have claimed. Table I also includes new experiments in which we tested experimentally our claim of a significant depletion by considering a counterexample, the interface of ethanol with octadecyltriethoxysiloxane (OTE) methyl-terminated monolayers. Measuring the reflectivity of this wetted structure under the same conditions as in our original work, we obtain the finding that methyl groups of the OTE monolayer explain all of the electron density gap for the wetting case but only half of that observed in our original [1] data.Ocko, Dhinojwala, and Daillant suggest a second contribution to the observed electron density depletion --surface-induced orientation of water [2]. We note that this phenomenon, if substantiated, would be an intrinsic aspect of hydrophobicity. The large literature on this question suggests a certain statistical bias for one of the two hydrogen bonds of the average water molecule to point towards a hydrophobic surface, giving a broad orientational distribution [4,5]-not the distinct smecticlike layering proposed by Ocko, Dhinojwala, and Daillant. These simulations therefore do not support the contention that orientational ordering of water is a dominant source of electron density depletion when water meets a hydrophobic surface. Ocko, Dhinojwala, and Daillant also assert that low-density regions at the calcite-water interface observed by x-ray experiments are associated with water orientation [6]. The low-density layer observed in that system is not, however, from number density depletion. It is, instead, due to the intrinsic structure of the interfacial hydration layer coupled with high (1 Å ) resolution in those measurements.We are gratified that the frame of discourse has shifted from controversies of whether a zone of depleted electron density exists, which was the main point of our Letter, and on which we and Ocko, Dhinojwala, and Daillant agree. It is generally agreed that water is attracted at best only weakly to a hydrophobic surface. The matter of soft modes and ensuing fluctuations of interfacial density, which might have very long wavelength, has been emphasized on the theoretical side [7], inferred from measurements [8], and...

How Water Meets a Very Hydrophobic Surface

“…However, Ocko et al 18 have pointed out that on a dry SAM, the terminal hydrogen atoms (which have few electrons) appear to X-rays as part of the ambient air or vacuum, but on a wet SAM the hydrogen layer appears as an intervening electron-depleted region with D eq ≈1.0Å. Indeed Poynor et al 19 find that the ethanol-OTE interface also shows a depletion (using their numbers, we find D eq ≈1.0Å), even though the contact angle of ethanol on OTE is small. Once the spurious "hydrogen gap" is subtracted, the data in ref.…”

The Dewetting Transition and The Hydrophobic Effect

“…Since this pioneering investigation, numerous studies have been reported with both finite solute and infinite plate models in water and other solvents. 4,5,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Of particular importance for the current study is the quantification of length scale dependent hydrophobic effects with respect to varying the interactions of water with the solute. Widely different views have emerged from numerous previous studies and there is no consensus 13 as yet.…”

“…One depicts that a large hydrophobic solute surface produces a thin vapor layer around it because of disruption of the local hydrogen-bond (H-bond) network of liquid water. 5,11,15,16 When two such solutes come close enough to each other, fluctuations in the individual solute-vapor interfaces expel remaining water molecules from the intersolute region leading to a dewetting induced collapse of the solutes.…”

The Hydrophobic Gap at High Hydrostatic Pressures

“…When water meets extended hydrophobic surfaces,t he electron density at the interface decreases below the level of bulk water. [8][9][10] Commonly,h ydrophobic substrates consisting of self-assembled monolayers (SAMs) with ad ense layer of saturated hydrocarbon chains at the surface have been studied. [5][6][7] Nowadays,t he existence of the hydrophobic gap,w ith as ize on the order of up to approximately 2 ngstrçms,i sg enerally accepted.…”

“…[11] However,ithas been shown that at least the first monolayer of the surface water is disturbed by the presence of ah ydrophobic surface. [13,17] Density oscillations have not been observed in experiments.W hile it was shown that gases dissolved in the liquid phase do not affect the hydrophobic gap, [8,9] an increase in temperature resulted in an increase in its size. [8] Simulations predicted an ordered and layered water structure at the interface,resulting in an oscillating electron density curve.…”