Chemical heterogeneity in phosphosilicate gels by NMR magnetisation exchange

Abstract

Nanometre scale chemical heterogeneity has been revealed in a phosphosilicate gel by ³¹P spin exchange. Monomeric phosphorus was found near to phosphorus singly cross-linked with phosphorus. Phosphorus doubly cross-linked with phosphorus was found to be remote from both other types of phosphorus species. Radiofrequency driven magnetisation exchange through ³¹P–³¹P dipolar coupling was rather ineffective, even with recoupling times of up to 30 ms, demonstrating the weak ³¹P–³¹P dipolar coupling and the effect of residual ¹H–³¹P dipolar coupling. Effective ³¹P spin exchange was possible in the laboratory frame through the ¹H spin interactions, when mixing times from 100 ms to 2 s were used.