Structure of monolayers formed by coadsorption of two n-alkanethiols of different chain lengths on gold and its relation to wetting

We have characterized and correlated the structure and wetting properties of self-assembled monolayers (SAMs) on gold derived from two different mixtures of n-alkanethiols (C12SH and CI3SH; Cr2SH and C22SH); in each of these SAMs, one thiol had a perdeuterated alkyl chain and one a perprotonated chain to allow the two alkanethiols to be distinguished in the SAM by polarized infrared external reflectance spectroscopy (PIERS). The hydrocarbon parts of the SAMs consist of well-defined regions exhibiting structural characteristics reminiscent of those formed in crystalline and liquidlike states. In each SAM, both the shorter alkanethiolate and the section of the longer alkanethiolate closest to the gold contain very low densities of gauche conformations (and presumably are ordered) over all compositions of the SAM. The terminal portion of the longer alkanethiolate-that which extends beyond the terminus of the shorter alkanethiolate-is disordered. The density of gauche conformers present in this mmponent decreases as its concentration in the SAM is increased. The advancing and receding contact angles (0" and d,, respectively) of water and hexadecane (HD) respond differently to the disordered region: for hexadecane, 0.(HD) and d,(HD) reach minima when the interface is present in the disordered, liquidlike state; for water, d"(H2O) and 0,(H2O) appear insensitive to the presentr of this disorder. The IR spectra of these mixed SAMs are consistent with previous inferences from wetting that the components of the SAMs, as prepared here (adsorption of 1 mM thiol from ethanolic solutions over 24-48 h at -25 oC), do not phase segregate into macroscopic islands. The larger interpretation which follows from the data, however, is that this result reflects a structural architecture derived from a kinetically controlled growth process. Under suitable conditions, phase segregation is important as might be expected from consideration of elementary thermodynamics. The importance of kinetic and thermodynamic factors in the structural characteristics of multicomponent SAMs is discussed.