Surfaces have different predilections for different compounds. In terms of surface characterisation, these differences are a function of the affinity a surface has for a particular compound and the accessibility to specific sites on the surface that both the solid structure and the fluid system can provide, co–operatively. The energy changes and matter transfers associated with 'satisfying' this affinity can reveal much about the physico–chemical structures of surfaces down to the Angstrom level and it is these changes that are measured by our flow sorption microcalorimeter, the capabilities of which are set out more fully in What is calorimetry? – following on from this introduction.
To simplify the experimental procedure, the compounds which are offered to a surface are frequently well known molecules (in liquid solution, gaseous form, or a mixture of vapour and gas) and we refer to them as probes. By measuring the energy of the interaction and the quantity of probe transferred between the flowing fluid stream and the stationary adsorbent, we are able to categorise a surface in terms of its affinity for the probe, whilst also obtaining an overall picture of the interaction kinetics. By adjusting the probe concentration, the level of surface coverage can be altered and this provides information on the distribution of active surface sites.
When a probe mixture contacts a surface competitive segregation occurs – the most strongly adsorbed compound depositing preferentially at the surface and excluding (partially or totally) other weaker competitors. As a result we can saturate a surface with one fluid (called the carrier) and use this to transport another material (the probe) to the surface. In this way we measure the preferential adsorption of the probe from its carrier. The thermal data which we obtain are the differential and integral heats of adsorption.
Thanks to the flow–through design of the Flow Microcalorimeter (FMC), an appropriate concentration monitor, when connected to the cell effluent, can measure the concentration changes in the fluid stream. These data can be used to calculate the quantity of matter transferred between the fluid carrier and the solid surface, which has caused the thermal effects just measured by the FMC.
Interestingly, thanks also to its 'open system' or 'flow–through' design, the FMC can also make the same measurements for desorption, so that the level of reversibility can be determined in terms of both heat and matter transfer, as well as the rapidity with which desorption occurs. These data also contain information on the mobility of the probe. The profiles of the traces which result show the kinetics of adsorption and of desorption. The integral values obtained are measures of the energy change and matter transfer caused by that interaction.