Investigation of the role of filtration in suppressing laser-induced nucleation of glycine

Abstract

Laser-induced non-photochemical nucleation is an intriguing phenomenon, and various mechanisms have been proposed to be responsible. It was previously reported that filtration using syringe nanofilters suppresses laser-induced nucleation of glycine in aqueous solutions, but it is not clear why. In this work we have investigated possible causes of this effect, namely the presence of mesoscale clusters, which may be relevant for an isotropic polarisability mechanism, and the effects of potential impurities which could facilitate a cavitation mechanism. Using syringe filtration with filters of various sizes, we found no direct relationship between the mesoscale cluster population or hydrodynamic radius and the likelihood of laser-induced nucleation of glycine. Furthermore, nanofiltration of deionised water prior to solution preparation was equally effective in suppressing laser-induced nucleation as equivalent filtration of the glycine solutions. This shows that the effect of filtration is not due to removal of impurities originating from glycine itself. Various sources and purification procedures for the water used to prepare glycine solutions were also investigated. Whilst using nanofiltered water or tap water decreased nucleation likelihood, the use of HPLC-grade, distilled, refluxed or degassed water increased nucleation likelihood compared to deionised water. Surprisingly, when either the distillate or the residue fraction from water distillation were used, each of them showed increased nucleation probability. These results taken together show that water treatment is an important determinant of the likelihood of laser-induced nucleation of glycine. We hypothesise that dissolved air is responsible for these observations, whereby syringe filtration leads to supersaturation by air under applied pressure while other treatments lead to degassing. The physical mechanism for this effect is not yet clear and while impurities originating in glycine may facilitate cavitation upon laser irradiation, we suggest that dissolved gas has a key role to play in laser-induced nucleation of glycine, perhaps through cavitation assisted formation of nanobubbles which assist with glycine nucleation.