Abstract :
The bonding of molecularly thin (10 A) dipropylamineterminated ZDPA 4000 and the hydroxyl-terminated Zdol 4000 lubricant films to amorphous hydrogenated carbon (CHx) are compared from 20 to 150 C. The kinetic results demonstrate that the rate at which both ZDPA 4000 and Zdol 4000 bond to CHx is nonclassical with a time-dependent rate coefficient that is best described by the general form: k(t)(alpha)kBt-h. For ZDPA, the bonding rate coefficient is observed to scale as t0.25 while for Zdol the bonding rate coefficient is observed to scale as t-1. The t-0.25 bonding rate is shown to be consistent with diffusion-limited kinetics that originate from a high density of ZDPA-CHx reactant pairs that leads to a segregation of reactant pairs, while the t-1 bonding rate results when bonding occurs from a more liquidlike (no segregation) Zdol film structure. Surface energy measurements are indicative of initial intermolecular interactions between the dipropylamine end groups and the CHx surface despite a low bonded fraction. These results indicate that the lubricant end group can profoundly dictate the physical state of the confined lubricant films on the CHx surface. The kinetic studies are accompanied by both experimental data and ab initio calculations on possible bonding interactions between the lubricant end groups and possible CHx surface functional moieties. Infrared sprectroscopy is used to show that the ZDPA end group, in the presence of carboxylic acid, is capable of forming the NH+--OOC salt as characterized by an intense absorption band at 1673 cm-1. H-bond energies for ZDPA-CHx(-OH) and Zdol -CHx(-OH) dimers are computed to be -5.3 and -4.4 kcal\mol, respectively. The energetics of the dimer interactions are discussed within the context of the observed lubricant bonding and debonding kinetics.
