Interfacial reactions in confinement:: Kinetics and temperature dependence of reactions in self-assembled monolayers compared to ultrathin polymer films

We report on a comparative study of the temperature dependence of the alkaline hydrolysis of N-hydroxy-succinimide (NHS) ester groups confined in self-assembled monolayers (SAMs) of 11,11'-dithiobis(N-hydroxysuccinimidylundecanoate) (NHS-C10) on gold and ultrathin films of poly(N-hydroxysuccinimidylmethacrylate) (PNHSMA) on oxidized silicon. The surface coverage of unreacted NHS ester following partial hydrolysis was determined quantitatively using ex situ FTIR spectroscopy, as well as contact angle measurements in conjunction with the application of the Cassie equation. The hydrolysis can be described as a pseudo-first-order reaction in all cases. The rate constant for the SAMs is 5 times smaller than that for the surface of the polymer films but differs by 3 orders of magnitude compared to those for similar reactions in solution. These results indicate that the reaction is affected by confinement effects both in the SAMs and in the ultrathin polymer films. The activation energies determined according to the Arrhenius equation (E-a(NHS-C10) = 30 kJ/mol and E-a(PNHSXU) = 61 kJ/mol) and in particular the activation entropies calculated according to transition state theory (AS'(NHS-C10) = -176 J(mol K) and AS (PNHSMA) = -59 J/(mol K)) reveal that steric crowding in the near-surface region and tightness of the transition state are more pronounced in the SAMs compared to the polymer films.