Water adsorption and desorption on chemically inert, biorepulsive surfaces

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The main objective of the proposed project is a better understanding of the mechanism behind the inertness of poly- and oligo(ethylene glycol) (PEG and OEG, respectively) based organic surfaces to protein adsorption and biofouling. Assuming, in accordance with the most accepted viewpoint, that the key factor behind these properties is hydration of the PEG/OEG moieties, we plan a systematic study of the kinetics and thermodynamics of water adsorption and desorption on a series of model OEG surfaces as well as investigation of the wetting and nucleation behavior of water during the adsorption. As suitable test systems we will take a series of self-assembled monolayers (SAMs) of OEG substituted alkanethiolates (OEG-ATs) on gold and silver substrates. The biorepulsive properties of these monolayers will be tuned in a well-controlled fashion by either varying the length of the OEG segments or by changing their packing density and diluting the OEG-AT moieties with other species. The above changes will be performed in such a way that the macroscopic wetting properties of the target surfaces will be kept constant or vary slightly only, which should make possible to untangle the hydration and subsequent interfacial wetting. Additionally, the terminal group of the OEG-AT SAMs will be varied to obtain a further tool to untangle hydration and interfacial wetting. A particular attention will be paid to the nucleation behavior of water and to the initial stages of adsorption, where the hydration should play the dominant role since the adsorption of water should first occur in the form of hydration of the OEG part of the OEG-ATs SAMs followed by the formation of the ice film at the SAM-ambience interface (interfacial phase). Varying the water coverage and systematically varying the capability of hydration by design of the OEG-ATs molecules and respective SAMs, we should be able to monitor the transfer from the hydration to wetting regime as well as to distinguish between the hydration and interfacial phases and to derive specifically their parameters. The observed behavior and derived kinetic and thermodynamic parameters as well as extent of hydration, etc., will be correlated with the proneness or inertness of the OEG-AT SAMs to protein adsorption and biofouling. Finally, using several complementary spectroscopic techniques, information about the binding configuration of the water molecules in the hydration phase and their structure and morphology in the interfacial phase will be derived. In addition, we plan to monitor the changes in the structure and conformation of the OEG segments at the initial stages of the water adsorption. The experimental results and derived parameters will be compared with predictions of available theoretical simulations, both as a test of correctness of the respective models and as a means to stimulate a further improvement and development of theory.