Accessibility of the residual silanols can be estimated by the ligands' bonding density. This parameter can not be measured directly, it is calculated on the basis of carbon loading measurements.
Carbon loading is the weight percent of carbon atoms on the adsorbent sample, measured by elemental analysis. Usually data of the elemental analysis is very precise (<0.2%). But high or low amounts of carbon on the adsorbent do not tell anything about bonding density, because we have to account for an adsorbent surface area.
If, for example, we have 10% carbon on the adsorbent with a surface area of 200 m2/g and the same 10% of carbon bonded to the adsorbent with a surface area of 100 m2/g, we obviously can conclude that second adsorbent has bonding density two times higher than the first one.
Question about the accessibility of the whole adsorbent surface during modification
process is also important. If original silica has wide pore size distribution, and some
amount of mesopores are present, then the surface inside this small pores will not be
accessible for alkylchlorosilane molecules, and this surface will not be covered.
Calculations of the bonding density based on the carbon content and surface area of this
silica will have a large error.