Conformational Entropy Effects on the Desorption Kinetics of Polyethers from Graphite

• Published in: Langmuir 2002-12, Vol.18 (25), p.9799-9809
• Main Authors: Paserba, Kris R, Vaidyanathan, Nithya, Gellman, Andrew J
• Format: Article
• Language: English
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la026049v

The desorption kinetics of a series of poly(ethylene glycol)-dimethyl ethers (PEG-DMEs, CH3(OCH2CH2) n OCH3, n = 1−22) adsorbed on graphite have been measured to study the detachment of flexible linear heteropolymers from solid surfaces. Desorption rates were measured using temperature programmed desorption (TPD) and indicate that the desorption of the PEG-DMEs from graphite can be described using a first-order rate constant and a desorption barrier, , that is independent of adsorbate coverage. The average pre-exponent of the first-order desorption rate constant was measured to be v av = 1018.8±0.7 s-1 and is roughly independent of the PEG-DME oligomer chain length. In addition the scales nonlinearly with the oligomer chain length and can be represented by the expression = a + bN γ (where N = 3n + 3), with the exponent assuming a value of γ = 0.46 ± 0.01. A simple theory generates an analytical expression for the measured values of that accurately reproduces the measured nonlinearity of (N). The measured value of is determined by the difference in the average energy of the adsorbed state, 〈E〉, and the average energy of the transition state to desorption, 〈E ‡〉. The nonlinear dependence of the on chain length can be ascribed to conformational entropy in the adsorbed state. For long oligomers entropy favors conformations of the molecule that are partially detached from the surface at elevated temperatures and thus the average energy of the adsorbed state is quite different from that of the minimum energy configuration.