A Theoretical Study on a Series of Alkylamines on Silica

Abstract

Silica surface covered with silanols round PZC detaches protons from the surface in alkaline ranges to increase surface negative charges of oxygen. The charge distributions of nitrogen of aminium ions are neutral, while those of amine molecules are negative. The positive charges of hydrogen connected to nitrogen of the aminium ions become greater than those of the amine molecules. The interaction between amines and silica is mainly coulombic. The bonds between amines and silica are almost the same as, or less in strength, than the hydrogen bond of a water dimer. The aminium ions are easy to adsorb on silica in alkaline ranges rather than round PZC. Both the aminium ion and the amine molecule prefer to adsorb on silica energetically by exchanging water molecules and sodium ions existing in the diffuse layer, or on the double layer. A series of aminium ions is easier to adsorb on silica as the chain length of the alkyl group becomes longer. The stabilization energy of the adsorption of a series of aminium ions is theoretically evaluated 0.25 kT per CH₂ group.