Formation of Monolayers by the Coadsorption of Thiols on Gold: Variation in the Length of the Head Group, Tail Group, and Solvent

Abstract : Long-chain alkanethiols, HS(CH2) (n)X, adsorb from solution onto gold and form oriented, ordered monolayers. Although alkyl chains terminated by other functional groups (e.g., trialkylphosphines, dialkyl disulfides and dialkyl sulfides) also form monolayers on gold that are stable at room temperature, thiols are adsorbed preferentially from solutions containing mixtures of a thiol and one of these other adsorbates. Surfaces containing more than one functional group can be generated by coadsorption of two or more thiols from solution. In general, the ratio of the concentrations of the two components in a mixed monolayer is not the same as in solution but reflects the relative solubilities of the components in solution and interactions between the tail groups, X, in the monolayer. Multi-component monolayers do not phase-segregate into single-component domains large enough to influence the contact angle (a few tens of angstroms across), but also do not act as ideal two-dimensional solutions. From dilute solutions in alkanes, adsorption of HS(CH2)10CH2OH is strongly preferred over HS(CH2)10CH3, probably due to the stabilization afforded by intra-monolayer hydrogen bonds between the hydroxyl tail groups. The wettability of mixed monolayers is not linear in the composition of the surface. In a surface comprising a polar and a nonpolar component, the polar component is more hydrophilic when its concentration in the monolayer is low than when the monolayer is composed largely of the polar component.