Towards the determination of surface energy at the nanoscale

Surface energy, the work required to generate the surface of a solid or liquid, is an important property influencing a wide range of interfacial processes. This property determines physical interactions between phases, e.g., the degree of hydrophilicity of a solid surface and the surface adhesion of biological entities including bacteria and cells. The determination of surface energy by means of liquid/solid contact angle goniometry is well established, although only averaged values on the macro scale can be obtained and there may be practical limitations of the method, e.g., materials where liquid penetration occurs. The main aim of the research work described in this thesis has been to develop a technique to determine surface energy using atomic force microscopy, where the measurement may be obtained at a resolution of the order of the probing tip radius (50- 90 nm). To produce suitable substrate materials of known surface chemistry, self-assembled alkanethiol structures on gold have been prepared. Contact angle goniometry, atomic force microscopy, and infrared spectroscopy have been used to investigate the surface-preparation and solution-deposition conditions for the reproducible formation of the self-assembled molecular structures on gold-coated tips and substrates for atomic force microscopy. Procedures have been established for removing adsorbed material, including self-assembled monolayers, and for producing clean gold substrates. Preliminary data show that surface-saturated self-assembled monolayers form reproducibly on prolonged (> 16 h) exposure of gold-coated glass substrates to ethanolic solutions of lD-functionalised alkanethiols in the concentration range 80 - 160 mmol dm^-3. The data also show that exposure for 16 h to alkanethiol concentrations in the range 160-240 mmol dm^-3 promote bilayer formation whereas concentrations of 240-320 mmol dm^-3 result in the deposition of multilayers, the average orientation of which is parallel to that of the first molecular layer; the use of parent I-undecanethiol solutions at concentrations of 1-80 mmol dm^-3 results in incomplete monolayer coverage. The self-assembled alkanethiol structures deposited on gold from ethanolic solutions have been found to be susceptible to both chemical and structural changes: ethanol provides a medium for the formation of S-alkyl hydrogen thiocarbonates and related compounds via reaction with dissolved atmospheric CO₂. Deposition from ethanolic solutions results in multilayered structures, incorporating these thiocarbonates and thiocarbonate-derived compounds, which at room temperature are susceptible to time-dependent structural rearrangement and molecular migration. To assess the applicability of the atomic force microscopy method for determining surface energy, forces of adhesion have been measured for interactions involving self-assembled molecular structures or polymer-film structures that had each been deposited onto gold-coated glass substrates and onto the probing, gold-coated cantilever. The data have been fitted into mathematical models that allow the calculation of surface energy by considering the work done for the separation of the identically coated contacting surfaces. The obtained surface energy values are in close agreement with those from corresponding contact-angle determinations, highlighting the potential usefulness of the technique for the study of surfaces at a resolution level approaching 1000 atoms. Comparative studies have shown that the employment of the atomic force microscopy technique may be preferable for the study of materials that are susceptible to penetration by liquids or for the investigation of small samples « contact angle drop diameter).