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(12) Contact angle titration experiments of carboxylic acid terminated alkanethiol surfacessuggest that the pAfa of the MHDA surface is pH 8-9.9 d This unusually high value has been attributed to hydrogen-bonding effects. It is likely, however, that the MHDA surface possesses a net negative charge at pH 7 due in part to dissociation of non-hydrogen-bonded carboxylic acid functionalities (M. D, Porter, personalcommunication).(13) MHDA was synthesized and purified according to published proce-dures. 90 A Cr layer of~ 50 Á was deposited on the silicon to improve adhesion of the Au layer.