Chemical heterogeneity in phosphosilicate gels by NMR magnetisation exchange

Nanometre scale chemical heterogeneity has been revealed in a phosphosilicate gel by P-31 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 P-31-P-31 dipolar coupling was rather ineffective, even with recoupling times of up to 30 ms, demonstrating the weak P-31-P-31 dipolar coupling and the effect of residual H-1-P-31 dipolar coupling. Effective P-31 spin exchange was possible in the laboratory frame through the H-1 spin interactions, when mixing times from 100 ms to 2 s were used.