Adsorption mechanism of aliphatic amino acids on kaolinite surfaces

The interaction of amino acids with clay minerals plays an essential role in many natural processes. Understanding the mechanisms of their adsorption on natural clays opens the way to a wide range of nanobiotechnological applications and helps to clarify the origin of life on Earth. In this work, the adsorption mechanisms and behavior of aliphatic amino acids (glycine, alanine, valine, leucine, and isoleucine) on kaolinite surfaces have been studied by the Density Functional Theory (DFT) method. The role of functional groups of aliphatic amino acids (AA) and their orientational behavior during the formation of hydrogen bonds with siloxane and hydroxyl surfaces of kaolinite have been systematically scrutinized. It has been found that the carboxyl group plays a crucial role in the mechanism of interaction between AA and kaolinite surfaces. The strongest hydrogen bonds are formed between the H-atom of the carboxyl group of AA and the O-atom of the hydroxyl surface of kaolinite. An additional hydrogen bond can be formed between the N-atom of the amino group and the surface –OH groups of kaolinite. The adsorption energy of AA on a hydroxyl surface is ~3 times higher than that on the siloxane surface. The obtained theoretical results comply with and help to explain the experimental data available in the scientific literature.