Structural Influences on the Fast Dynamics of Alkylsiloxane Monolayers on SiO₂ Surfaces Measured with 2D IR Spectroscopy

Abstract

There is widespread interest in alkyl chain surface monolayers and their applications. In many applications, alkyl monolayers are functionalized with active headgroups. Here we report the impact of major structural variations on the fast dynamics of alkylsiloxane monolayers. The monolayers were deposited with controlled structures on flat amorphous silica surfaces, and the terminal sites were functionalized with a metal carbonyl headgroup. The CO symmetric stretching mode of the headgroup served as a vibrational probe for detecting the fast structural dynamics of the monolayers using two-dimensional infrared vibrational echo spectroscopy (2D IR) to measure spectral diffusion, which is made quantitative by determining the frequency–frequency correlation function (FFCF) from the time-dependent data. Two methods of functionalizing the surface, independent attachment via a single Si–O bond formed with alkylmonochlorosilane precursors and network attachment via siloxane networks (−Si–O–Si–O−) formed with alkyltrichlorosilane precursors, were compared for several chain lengths. The two types of monolayers produced chain dynamics and structures that were independent of the manner of attachment. For densely packed monolayers, the FFCF decayed mildly slower when the alkyl chain length was decreased from C11 (chain with 11 methylenes) to C4. However, when the chain length was further reduced by one more methylene to C3, substantially slower dynamics were observed. When the chain density was reduced below 50% of fully packed monolayers, the single-component nature of the dynamics changed to a fast component plus an extremely slow component, possibly because of the collapse and entanglement of loosely packed alkyl chains