AFM study of calcite surface exposed to stearic and heptanoic acids

Atomic force microscopy (AFM) was used to investigate the physical properties of calcite surfaces after exposure to short chain alkanes and organic acids and to compare micrometer scale behaviour with macroscopic surface wetting previously observed using contact angle measurement. AFM imaging shows changes in morphology and behaviour when organic compounds are present on freshly cleaved surfaces. Polished calcite is clearly rough at micrometer scale resolution compared with cleaved surfaces and this made it impossible to see differences in physical features when the organic compounds were added. On freshly cleaved samples exposed only to n-decane or n-heptane, spontaneous recrystallisation, a process observed on fresh calcite exposed to humidity in air, is inhibited. This change in behaviour is consistent with a slight increase in contact angle observed on similarly treated polished samples. AFM force measurements, however, were able to demonstrate that both freshly cleaved and polished calcite surfaces treated with heptanoic acid (HPA) and stearic acid (SA) were “stickier” than the heptane- and decane-treated surfaces. On samples equilibrated in deionised water where pH was adjusted to 5 and 10 before exposure to HPA and SA, no correlation between adhesion force and pH could be made for HPA-treated samples. However, for the SA-treated samples, adhesion was higher for samples equilibrated at pH 5 prior to SA exposure compared to those at pH 10. The results from AFM imaging and adhesion force measurement are consistent with results from macroscopic contact angle measurements and wettability studies reported earlier.