A Study by Contact Angle of the Acid-Base Behavior of Monolayers Containing <.r-Mercaptocarboxylic Acids Adsorbed on Gold: An Example of Reactive Spreadingr

Mixtures of HS(CH2)16CO2H and HS(CH2)1oCH3 adsorb onto gold from solution in ethanol and form densely packed, oriented monolayers. These monolayers expose a combination of polar carboxylic acid groups and nonpolar methyl groups at the surface. Varying the concentrations of the two thiols in solution allows the density of carboxylic acids at the surface and the polarity of the monolayer-liquid interface to be controlled. The technique of contact angle titration was used to study the acid-base properties of these mixed monolayers as a function of the composition of the monolayer. The contact angles of water were constant at low pH (<6) and decreased at higher values of pH. Similar behavior has been observed for polyethylene carboxyl ic acid (PE-CO2H). Unlike PE-CO2H. the contact angles on the monolayers on gold did not level out at high pH: f l c<,nt inued to decrease. -Carboxll ic acids af the monolayer-water interface were less acidic than carboxl ' l ic acids in solut ion. The pH at which carboxyl ic ac ids in the monolayer were f i rs t ion ized increased is the concent ra t ion o f carbrx l ' l i c ac ids in the monolayer decreased. The extent to which the f ree energ) 'o f ion izat ion o f ' the carbox l ' l i c ac ids is re f lec ted in the advanc ing contact ang le appears to be d i f ferent for PE-CO.,H and for monolavers o f ac idterminated thiols on gold. The general problem of reactive spreading is discussed.