Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

No abstract

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.