Density of Bonded Ligands

Bonding density is a surface concentration of bonded ligands, and it is expressed in either number of moles per square meter (pmohm ) or in number of groups per square nanometer (groups/nm ). Unit conversion is shown in equation (3-7). 

The term bonding density is constantly used throughout this book. This is probably the most important characteristic of the bonded phase. The higher the bonding density, the more hydrophobic the adsorbent surface, the better the shielding of residual silanols, and the higher the hydrolytic stability. 

Above we briefly discussed the limitations on possible values of bonding density (Section 3.6.1). Molecular volume of bonded ligands is one of these limitations. If the most stretched conformation of a ligand has length / and its molar volume is v, then the minimum area, co, it occupies on the surface will be ft) = v/l and the maximum bonding density will be d ax = 1/ft). This maximum density is calculated for the flat surface, while on the concave internal surface of the pore the maximum density is lower and could be expressed as 

Bonding density cannot be measured directly the measured value is either (a) carbon content on modified material, (b) weight loss after ashing in the oven, or (c) thermogravimetric weight loss between 200°C and 700°C. Carbon 

TABLE 3-3. Comparison of the Carbon Load and Surface Density Values 

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The efficiency of the substrate shielding is dependent on the local density of bonded ligands and their overall distribution on the surface. Three different types of surface ligand distribution could be distinguished random, uniform, and island-like. These distributions are illustrated in Figure 3-14. 

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