Effects of Alkyl Chain Length on the Adsorption ofn-Alkyl Isocyanides (RN⋮C) on Gold Powder

The chemisorption of the alkyl isocyanides n- butyl (C 4 H 9 NC), n- hexyl (C 6 H 13 NC), n- octyl (C 8 H 17 NC), n- dodecyl (C 12 H 25 NC), and n- octadecyl isocyanide (C 18 H 37 NC) on Au powder from 1,2-dichloroethane (DCE) was examined using both solution (FTIR) and surface (diffuse reflectance infrared fourier transform or DRIFT) spectroscopic techniques. For the isocyanides with fewer than 12 carbon units in their alkyl tails, equilibrium binding constants ( K 1 ), surface coverages ( n 1s and n a mol/g Au ), and qualitative rates of solution−surface equilibria in DCE solvent are the same. Competition studies between 13 C-labeled C 18 H 37 NC and nonlabeled C 4 H 9 NC show that the ratio of adsorbed isocyanides at equilibrium, C 18 H 37 N 13 C/C 4 H 9 NC, is 4.1. Adsorption studies show that isocyanides are bound strongly to the surface at low coverages (less than 70% of saturation coverage), but binding affinity is significantly lower at higher coverages. Studies of C 18 H 37 NC and C 4 H 9 NC in n- decane, DCE, and nitromethane (MeNO 2 ) solvents show that solvent polarity affects the rate of monolayer formation, surface coverage, and equilibrium binding constants.