ABSTRACT

Contact angles (CAs), measured using buffered solution droplets on the same surfaces, also showed a sharp transition (an increase in wettability) as the droplet pH was varied. Both local force microscopy measurements using a modified probe tip and macroscopic wetting studies provided very similar values for the pK\/2 (i.e., the pH at which adhesion or C A changes by a factor of two) of the surface amine group for disordered amine-terminated siloxane SAMs, although these values were 6 pK units lower than the typical bulk solution values. Vezenov et al also used CFT to determine p ^ values for COOH-terminated surfaces and found that the pK\/2 of the surface-confined carboxylic acid was approx. 5.5, very similar to the pKa of the COOH functionality in aqueous solution. Similar results for these groups were obtained by van der Vegte and Hadziioannou [11] and by He et al [12].