The main purpose of surface modification is shielding of the active silanol groups and attachment to the accessible adsorbent surface organic ligands which are responsible for specific surface interactions.
Thus, one of the main questions is how dense is the bonded layer. Before we mentioned, that there are about five -OH groups per 100 Å2 (1 nm2) of the fully hydroxylated silica surface. Bulky alkylsilane ligands can not react with all available silanoles due to the steric hindrance.
Van-der-Vaals volume of dimethyloctadecylchlorosilane is about 925 Å3, and its length along the main axis is 22 Å. Thus, being attached to the surface this molecule will occupy ~42 Å2. Which means that we roughly can attach ~2.5 alkylsilane molecules per 100 Å2 of the silica surface. So, we will have also about 2.5 unreacted -OH group on the surface underneath the bonded layer.
Does these residual silanols accessible for the analyte molecules or not is one of the main questions ?
Picture of dodecilsilane model on the silica surface and three octadecylsilane