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.

Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) has been used to study the protein resistance of poly-and oligo(ethylene glycol) (PEG and OEG) terminated alkanethiolate selfassembled monolayers (SAMs) on Au and Ag in the temperature range from 0 to 85 °C. These experiments extend previous room-temperature studies by Harder et al. 1 who related the protein adsorption characteristics of OEG-SAMs to the lateral density and corresponding molecular conformation of the ethylene glycol (EG) moieties in the film. In addition to the short oligomer OEG-SAMs, we investigated PEG-derivatized alkanethiolate monolayers with an average chain length of 45 EG units and a mean molecular mass of 2000 g/mol (PEG2000). We observe that films, which are protein resistant at room temperature, maintain their protein repulsive characteristics up to 85 °C but may adsorb significant amounts of protein if the temperature is lowered.

We determined the density profile of a high-molecular-weight globular protein (bovine serum albumin, BSA) solution at the methoxy tri(ethylene glycol)-terminated undecanethiol SAM/protein solution interface by neutron reflectivity measurements. Information about the interactions between oligo(ethylene glycol) (OEG)-terminated self-assembled monolayers (SAMs) and proteins is derived from the analysis of the structure of the solid-liquid interface. The fitting results reveal oscillations of the protein density around the bulk value with decaying amplitude on a length scale of 4 to 5 nm. The amplitude, phase, period, and decay length are found to vary only slightly with temperature and the ionic strength of the protein solution. Adsorption is reversible within the limits of detection, which suggests that the hydrated ethylene glycol surface inhibits the protein from unfolding and irreversible bonding. The insensitivity of BSA adsorption toward the ionic strength of the solution contrasts with observations in surface force experiments with a fibrinogen-coated AFM tip, where electrostatic repulsion dominates theprotein/OEG SAM interaction. As reported previously, irreversible BSA adsorption takes place below 283 K, which we interpret as indicative of the presence of dynamic effects in the protein resistance of short-chain OEG-terminated surfaces.