CONTACT-ANGLE GONIOMETRY, ELLIPSOMETRY, XPS, AND TOF-SIMS ANALYSIS OFGOLD-SUPPORTED, MIXED SELF-ASSEMBLED MONOLAYERS FORMED FROM MIXED DIALKYL DISULFIDES

The self-assembly of mixed monolayers (mixed SAMs) on gold substrates using mixed dialkyl disulfide adsorbates is reported. This work was un dertaken to probe the factors that control monolayer formation in thes e systems and to provide a basis for comparison with mixed SAMs formed from mixtures of alkyl mercaptan adsorbates. Gold-supported thin film s were formed from three symmetrical and two mixed disulfides: di-n-oc tadecyl disulfide (4), di-n-butyl disulfide (5), di-tert-butyl disulfi de (6), n-butyl n-octadecyl disulfide (7), and tert-butyl n-octadecyl disulfide (8). As benchmarks with which to compare thin films formed f rom mixed dialkyl disulfides, two series of mixed SAMs formed from com binations of n-octadecyl mercaptan (1) and either n-butyl mercaptan (2 ) or tert-butyl mercaptan (3) were prepared. Surfaces were characteriz ed by contact angle goniometry with water and hexadecane probe liquids , optical ellipsometry, X-ray photoelectron spectroscopy (XPS), and ne gative ion time-of-flight secondary ion mass spectrometry (TOF-SIMS). It was found that the adsorption efficiencies of 1-3 varied in the ord er 1 > 2 >> 3. A 3-4-fold excess of 2 to 1, and a 40-100-fold excess o f 3 to 1 were required to produce mixed SAMs with equal concentrations of butyl- and n-octadecyl adsorbates, indicating the preferential ads orption of longer-chain species in these systems. In contrast, no pref erence for adsorption of longer-chain species was observed with the mi xed disulfide adsorbates 7 and 8, which produced mixed SAMs with appar ently equal surface concentrations of n- or tert-butyl- and n-octadecy l groups. Evidence from TOF-SIMS suggests that both butyl- and octadec yl mercaptide groups are present in thin films formed from 7 and 8, an d at concentrations similar to those present in 1:1 mixed SAMs formed from mixtures of alkyl mercaptans 1 + 2 or 1 + 3. SAMs formed from dia lkyl disulfides appear to be formed under kinetic control and not to u ndergo appreciable exchange with dialkyl disulfides in the deposition solution.