Interaction of Water with Self-Assembled Monolayers:  Neutron Reflectivity Measurements of the Water Density in the Interface Region

Abstract

We applied neutron reflectivity measurements to hydrophilic and hydrophobic self-assembled monolayers (SAMs) to probe water density at these interfaces. The measurements were motivated by a previous theoretical study which reported a reduced water density at a hydrophobic surface (Lum, K.; Chandler, D.; Weeks, J. D. J. Phy s. Chem. B 1999, 103, 4570) and by our own computer simulations on the hydration force and water density between two methoxy tri(ethylene glycol) terminated undecylthiolate SAMs adsorbed on gold substrates used to study protein adsorption. These simulations predicted that the surfaces are slightly hydrophobic and are characterized by a reduced water density at the interface (Pertsin, A. J.; Hayashi, T.; Grunze, M. J. Phys. Chem. B 2002, 106, 12274). In disagreement with the marginal reduction in water density derived in the simulations, the neutron reflectivity measurements reported here indicate an unexpectedly extended (∼4 nm) water layer at the SAM surface with a noticeably reduced density (85−90% of the density of bulk water). The reproducibility of the experimental results with the methoxy tri(ethylene glycol) terminated undecylthiolate SAMs was confirmed with four different samples and by one measurement using a contrast-matched D2O/H2O water mixture. We also used neutron reflectivity measurements to study the water density at the water/SAM interface of hydroxy hexa(ethylene glycol) and hydroxy tri(ethylene glycol) terminated undecylthiolate SAMs. Except for one of the hydroxy hexa(ethylene glycol) samples studied, the results are consistent with the presence of bulk water in direct contact with the interface. We also discuss possible artifacts and problems in the analysis. Our results on nonfunctionalized hydrophobic octadecanethiolate and hydrophilic hydroxy-terminated undecylthiolate SAMs give physically unreasonable and nonconclusive models for the water interface on these surfaces, respectively. The best fit of the data for the hydrophobic surface gives an unreasonably low water density, possibly due to the presence of air inclusions in the film and/or adsorbed air “nanobubbles”. The results obtained on the hydrophilic hydroxy-terminated surface can be fitted equally well with a model assuming an interface water density that is higher or lower than that of bulk water, demonstrating the ambiguities associated with describing the organic/liquid interface with a box model for the Q range accessible in our experiment.

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Neutron [547] and X-ray [548] reflectivity experiments confirmed the observed reduced density of water at the interface, while experiments with an ellipsometer excluded nanobubbles on silanated silica [549]. There is now strong evidence, that bridging bubbles contribute to the long-range hydrophobic attraction in many experiments [495] but the whole hypothesis and the effect of nanobubbles are still being debated [550].…”

Water? What's so special about it?

“…10%; the layer thickness is, however, less than 1.5 nm. Other neutron reflectivity work (Schwendel et al 2003) shows a depletion layer of ca. 2 nm, but one with such a density deficit as to be unphysical.…”

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.…”