CONTACT-ANGLE TITRATIONS OF MIXED OMEGA-MERCAPTOALKANOIC ACID ALKANETHIOL MONOLAYERS ON GOLD - REACTIVE VS NONREACTIVE SPREADING, AND CHAIN-LENGTH EFFECTS ON SURFACE PK(A) VALUES

A nonreactive spreading protocol for measuring contact angles of buffe red water droplets on low-energy surfaces is described. The protocol c onsists of immersing the sample (prior to contact-angle measurement) i n a buffer solution of the same pH as that of the buffered water dropl et that will be used to measure the contact angle. Contact-angle titra tion data acquired using this protocol for acidic self-assembled monol ayers of alkanethiolates on gold exhibit a smooth transition in contac t angle between plateau regions at low and high pH. This is in contras t with data acquired for identical monolayers using the more common re active spreading protocol, for which a plateau region could not be obt ained at high pH. It is postulated that the buffer pretreatment leaves the surface id a partially deprotonated state even though the surface is not macroscopically wet; this postulate is supported by infrared r eflection-absorption spectroscopy data which show a conversion from pr otonated to deprotonated carboxylate in a dry acid-containing monolaye r after exposure to a basic buffer solution. The nonreactive spreading protocol has proved particularly useful for deriving equilibrium para meters, e.g. pK(A) values for surface acid groups, from contact-angle titration data. Mixed monolayers of 11-mercaptoundecanoic acid with th e alkanethiols nonanethiol, decanethiol, undecanethiol, and dodecaneth iol exhibited well-defined contact-angle titration curves from which p K(A) values for acid dissociation in the monolayers could be obtained. A strong dependence of pK(A) on alkanethiol chain length was observed , with pK(A) values ranging from approximately 6.5 for a mixed monolay er with nonanethiol to approximately 11.5 for a mixed monolayer with d odecanethiol. The latter value reflects a shift of 6.7 pK units relati ve to the pK(A) for butyric acid (a representative alkanoic acid) in b ulk, water. The pK(A) shifts are interpreted in terms of two physical processes, one involving solvation of carboxylate anions in the monola yer microenvironment and another involving interfacial potentials at t he monolayer-solution interface.