Study by Contact Angle of the Acid-Base Behavior of Monolayers Containing W-Mercaptocarboxylic Acids Absorbed on Gold: An Example of Reactive Spreading.

摘要

Monolayers formed by the spontaneous assembly of organic thiols from solution onto gold provide a high degree of control over the chemistry and structure of an organic surface. In this paper we exploit the flexibility of these self-assembled monolayers to study the influence of the polarity of an interface on the acidity of carboxylic acids. Methyl-terminated thiols, such as HS(CH2)10CH3, adsorb from solution onto gold and form non-wettable, hydrophobic surfaces composed predominantly of nonpolar methyl groups. Thiols terminated by carboxylic acid groups form hydrophilic monolayers that are wetted by water. By coadsorbing mixtures of HS(CH2)10CH3 and HS(CH2)10CO2H in various mole fractions, we can synthesize surfaces in which the carboxylic acid groups are surrounded largely by nonpolar methyl groups, by other polar acid groups, or by mixtures of the two. This system allows us to examine the effect on the advancing contact angle of a chemical reaction occurring at the interface between a drop and a surface- in this case, neutralization of a carboxylic acid by aqueous base. Reactive spreading appears to be complex and may lead to different wetting behavior for the same chemical reaction on different substrates. A knowledge of the acid-base properties of carboxylic acids and other organic acids and bases at solid-liquid interfaces is important for understanding phenomena as diverse as the stability of colloids, the folding of proteins and the mechanisms of catalysis by enzymes. Keywords: Ethanols; Polarity; Polyethylene carboxylic acids; Contact angles; Reactive spreading; Titration. (aw)