Water adsorption and desorption on chemically inert, biorepulsive surfaces
Final Report Abstract
The main objective of the projects was a systematic study of water adsorption and desorption on/from the surface of oligo(ethylene glycol) (OEG) substituted alkanethiolate (AT) self-assembled monolayers (SAMs). This objective was formulated in context of well-defined biorepulsive properties of these systems, which, similar to analogous poly(ethylene glycol) surfaces, are generally ascribed to the hydration effect. As suitable test samples we selected a series of OEG-AT SAMs with a variable length of the OEG segment and variable termination (-OH or -OMe) on Au(111). These SAMs exhibit variable protein-repelling properties (depending on the length of the OEG segment) but persistent wetting properties (depending on the identity of the terminal tail group) which can be compared with those of the reference OH-substituted and non-substituted (CH3- termination) SAMs studied by us as well. As major experimental techniques we used synchrotron-based X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and temperature-programmed desorption (TPD). In the XPS and NEXAFS experiments, characteristic spectroscopic features of D2O were monitored upon adsorption of water (D2O) at 105-125 K and subsequent heating up to desorption temperature. In the TPD experiments, kinetics of water (D2O) desorption after its adsorption at low temperature was studied and some thermodynamical parameters were determined. All experiments were performed under UHV conditions. Upon adsorption at 105-125 K a predominant spectroscopic signature of amorphous ice was observed for all SAMs studied. The adsorption occurred almost exclusively onto the SAM surface, with a strong distortion of the spectral pattern for the first layer for OH-terminated surfaces and without any indication for extensive penetration of D2O molecules into the OEG part of the SAMs. The latter observation was in striking contrast with initial expectations that the water adsorption occurs at first into the hydrogel-like OEG part (hydration phase) and only then onto the SAM surface (wetting phase). The thickness of the ice films increased linearly with the D2O dose with the same rate for all OH-terminated SAMs and a somewhat lower rate for the CH3-terminated films, which was explained by the difference in the sticking coefficients and morphology of the ice films. The desorption of D2O in the given quasi-static temperature increase regime occurred at ~150-155 K for all SAMs studied. Specific features of the NEXAFS spectra could be interpreted as a signature of a hydration phase, formed by temperature-driven diffusion of the adsorbed D2O molecules into the hydrogel-like OEG part of the SAMs upon their heating. This tentative conclusion could be substantiated by the TPD experiments, which confirmed strongly dominant adsorption of water as an ice overlayer (wetting phase) at low temperature but exhibited a weak signature of the hydration phase, corresponding to the adsorption of D2O into the OEG matrix as well. This phase is characterized by a higher desorption energy (13.0 kcal/mol vs. 10.5 kcal/mol for the wetting phase) and a weight correlating with the length of the OEG strand and, consequently, with biorepulsivity. The adsorption into the hydration phase was found to occur over an activation barrier, being strongly suppressed for direct adsorption from the gas phase and occurred predominantly by diffusion from the wetting phase, with the latter process being promoted noticeably by a pre-desorption annealing. Not only qualitative but also numerical results, such as characteristic desorption energies could be obtained for all systems studied. The obtained results are both of significant scientific value (in our opinion) and of potential relevance for applications, in context of interfacial engineering, fabrication of biosensors, etc.
Publications
- (2018) Spectroscopic Study of Water Adsorption and Desorption on/from Oligo(ethylene glycol)-Substituted Alkanethiolate Self-Assembled Monolayers. J. Phys. Chem. C (The Journal of Physical Chemistry C) 122 (20) 10918–10928
Sayin, Mustafa; Nefedov, Alexei; Zharnikov, Michael
(See online at https://doi.org/10.1021/acs.jpcc.8b02514) - (2020) Interaction of water with oligo(ethylene glycol) terminated monolayers: wetting versus hydration. Physical chemistry chemical physics : PCCP 22 (15) 8088–8095
Sayin, Mustafa; Nefedov, Alexei; Zharnikov, Michael
(See online at https://doi.org/10.1039/D0CP00906G)
