Influence of Alkyl Chain Length on Phosphate Self-Assembled Monolayers

Abstract

A series of alkyl phosphates with alkyl chain lengths ranging from C₁₀ to C₁₈ have been synthesized. Self-assembled monolayers (SAMs) of these molecules were prepared on titanium oxide surfaces by immersion of the substrates in alkyl phosphate solutions of 0.5 mM concentration in n-heptane/isopropanol. The SAMs were characterized by means of dynamic water contact angle (dCA) measurements, variable-angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), and polarization-modulated infrared reflection−absorption spectroscopy (PM-IRRAS). A higher degree of order and packing density within the monolayers was found for alkyl phosphates with alkyl chain lengths exceeding 15 carbon atoms. This is reflected in a lower dCA hysteresis, as well as a film thickness measured by VASE and XPS close to the expected values for SAMs with an average alkyl chain tilt angle of 30° to the surface normal. Additionally a shift of the symmetric and antisymmetric C−H stretching modes in the PM-IRRAS spectra to lower wave numbers was observed. These findings imply a higher two-dimensional crystallinity of the films derived from alkyl phosphates with a longer alkyl chain length.