An Infrared Spectroscopic and Dynamic Contact Angle Study of the Water-Ferrihydrite Interface

Abstract

An improved understanding of the influences upon the structure of water at interfaces is becoming increasingly important for further advances in materials chemistry and for surface engineering applications. As hydration forces influence the surface properties and reactivity of materials, a clearer understanding may enable greater control over chemical processes such as corrosion and catalysis and in the design of materials with specific wetting properties. Three influences on the orientation of interfacial water have been identified from sum frequency vibrational spectroscopy (SFVS) data; the tendency to maximise hydrogen bonding interactions in the bulk; the strength of hydrogen bonding interactions with the surface; and net surface charge, which forces an alignment of the water dipole. The orientation and structure of interfacial water are necessarily co-dependant which is reflected by the ice-like structure of water at hydrophobic surfaces where puckered hexagonal water layers combine with dangling OH bonds, directed towards the interface. Although SFVS data have indicated that net charge and variation in the interaction strength with surface species influence the orientation of interfacial water, the further influence of this change in orientation on interfacial water structure has not been determined.

In this thesis multivariate curve resolution (MCR) has been applied to attenuated total reflection infrared (ATR-IR) spectra to probe the above-mentioned influences on interfacial water structure. The variation with pH in the ATR-IR spectra of high surface area ferrihydrite particle films revealed that decreasing pH increased the proportion of Fe-OH2 +1/2 surface species relative to Fe-OH-1/2 and influenced the structure of water in the first hydration layer which became more like that of bulk water with decreasing pH. The low wavenumber of IR absorptions corresponding to interfacial water hydrogen bonded to Fe-OH-1/2 surface species indicates a strong hydrogen bonding interaction which reflects the distribution of charge at the interface. The influence of variation in relative humidity (RH) on the ATR-IR spectra and contact potential difference (CPD) between ferrihydrite particle films and a Kelvin probe further indicated that the distribution of charge at the interface was influenced by hydration and associated changes in water structure. A different variation with pH was observed in the ATR-IR spectra above and below the point of zero charge (PZC) indicting that the net surface charge influenced the orientation of interfacial water further from the surface and that the stability of an ice-like structure was reduced by the positively-charged surface. Variation with pH in dynamic water contact angles on thin ferrihydrite particle films also highlighted the influence of net surface charge on interfacial water structure with advancing contact angles effected by pH > PZC and receding contact angles effected by pH < PZC.