NMR relaxation evidence for solute-induced nanosized superstructures in ultramolecular aqueous dilutions of silica–lactose

Abstract

We investigated 20 MHz water proton NMR longitudinal (T1) and transverse (T2) relaxation in ultrahigh dilutions (range 10 − 7 M–10 − 47 M) of a mixture of silica–lactose (Sil/Lac) in various media (water, 0.15 M NaCl, 0.15 M LiCl) and in various containers (glass, polyethylene). The samples were prepared by iterative centesimal dilutions under vigorous agitation and rigorously controlled laboratory conditions. Water and salt media were similarly and simultaneously treated, as controls. No significant effect on relaxation times was induced by the iterative dilution/agitation process in pure water and salt controls. By contrast, a slight increase in T1 and decrease in T2 was observed with increasing dilution in silica–lactose solutions, resulting in a marked progressive increase in T1/T2, especially in LiCl medium, distinguishable up to the ultrahigh dilution level. Cross-correlation analyses between T1, T2 and T1/T2 managed to demonstrate opposite behaviours of controls and Sil/Lac dilutions, even in the ultramolecular range of dilution. The effect seemed dependent on the medium (LiCl > NaCl > Water), and was observed in the glassware and polyethylene series as well. After a heating/cooling cycle directly in the sealed NMR tubes, the relaxation variations observed as a function of dilution totally vanished, and the T1/T2 ratio dropped, indicating a less ordered structure. These findings were interpreted in terms of nanosized superstructures with motional correlation time greater than 5.10 − 9 s, nucleated around the solute, and composed of water, ions and nanobubbles generated during the vigorous mechanical process. Incidentally, a striking catalytic enhancement of silica leaching was observed during the preparation of silica–lactose dilutions in glassware; but this did not influence the NMR relaxation results.