Covalent Grafting to μ-Hydroxy-Capped Surfaces? A Kaolinite Case Study

Abstract

All μ-hydroxyl groups are frequently encountered capping groups found on the external surfaces of various minerals that are often used as fillers in composite materials. Covalent grafting to this functional group would therefore offer a versatile and attractive route to surface modification. The octahedral layer of kaolinite is composed of μ-bridged aluminol groups. In particular, intercalation compounds of kaolinite, where all basal planes are exposed and may be modified, are ideally suited to study the feasibility of such covalent graftings. The huge (internal) specific surface area greatly improves the sensitivity of the analytics and renders kaolinite an ideal model compound. Herein we analyze the mode of bonding of ethylene glycol (EG), intercalated into kaolinite (EG kaolinite), by solid-state NMR techniques. 27Al MQMAS allows for distinction between intercalated and grafted EG molecules because the chemical surroundings of octahedrally coordinated aluminum nuclei in the layer are significantly changed by the formation of a covalent bond. Moreover, the temperature-dependent dynamics of the EG molecules in the interlamellar space are examined by wide-line solid-state 1H NMR measurements. The EG molecules perform a circular motion around the covalently bonded hydroxyl group in the interlamellar space. Analysis of the 13C–27Al REAPDOR measurement in conjunction with the EG dynamics allows for determination of the 13C···27Al distance between octahedral aluminum and the bonded carbon atom of EG. This distance is 3.1 Å. A thorough description of the bonding mode of the EG molecules is provided and proves beyond any doubt the covalent grafting. This suggests that the reactivity of μ-hydroxyl groups, in general, is sufficient to realize a covalent surface modification of a wide range of minerals.