Adsorbent surface area significantly changes after modification. Considering a cylindrical pore model we can theoretically calculate the new surface of the adsorbent after modification.
If we assume the highest possible bonding density for C18 chains, then these chains most probably will be straightened or slightly bended, and average thickness of bonded layer will be 21 - 22 Å.
Schematic of the cylindrical pore
If, for example, original surface area of silica adsorbent is 300 m2/g and mean pore diameter is 120 Å, then total length of all pores in one gram of adsorbent will be L=S/3.14D = 7,960,000 km. After modification effective pore diameter will be 78 Å and total pore length will remain the same. Thus, the new surface area will be 3.14DL = 195 m2/g.
We can see that after modification the adsorbent surface area decreases by 34%.
The term "endcapping" was introduced in the late 70's, when it became obvious that some silanols are still accessible after surface modification with long-chain ligands.
Trimethylchlorosilane is the most common reagent to cover accessible residual silanols. This molecule is relatively small and it can penetrate to the silica surface and cover "cap" some spotty areas of unreacted silanols. Generally it leads to the significant improvement of the reversed phase adsorbent properties (less tailing and more predictable retention of polar compounds).
We have to emphasize
here that even after endcapping, silica based adsorbents contain
almost 50% of its original silanols, but they are mainly not accessible for analytes.