Isotherm analysis of the solution-phase uptake of chlorotrimethyl silane on a photosensitive glass

Abstract

We report on analysis of the uptake of chlorotrimethylsilane (CTMS) on a photosensitive glass substrate. Contact angles of distilled water (DI) are measured on substrates immersed over a range of concentrations of CTMS in acetonitrile (ACN) for different immersion times. Contact angles are also measured for coatings stored in anhydrous ACN and ambient air. Samples are characterized with x-ray photoelectron spectroscopy and atomic force microscopy. Characterization supports a patchy layer for the glass chemistry and coverage of CTMS. Contact angles are converted to coverage accordingly. Different isotherms are subsequently tested to fit the coverage data. A modified Freundlich isotherm fcp=1−exp(−(Kco+Kctt)C1/n) is found best, where fcp is fractional coverage, C is solution concentration, t is time, and Kco, Kct, and n are fitting constants. A weighted, chi-squared minimization method is used to fit coverage versus time and concentration comprehensively. The results support a two-step reaction of CTMS depositing in to a patchy monolayer, with best fit values as Kco=0.11832±0.0062 (mg/mL)1/2 and Kct=0.0279±0.0011 (mg/mL)1/2 min−1 for n=2. In summary, these studies provide an optimal protocol to silanate a photosensitive glass and demonstrate the stability of the CTMS coating in ACN, with a focus on applications to functionalize micro-fluidic channels.