Reconsideration on structural anisotropy of silica hydrogels prepared in magnetic field

TL;DR

This study reevaluates the anisotropic structure of silica hydrogels prepared in a magnetic field, suggesting that the gel network aligns along the magnetic field based on birefringence and crystallite orientation evidence.

Contribution

It provides new insights into the orientation of silica gel networks in magnetic fields, challenging previous assumptions about the direction of birefringence.

Findings

Silica gel networks align along the magnetic field.

Crystallite orientation supports network alignment along B.

Size of crystallites indicates pore dimensions affected by magnetic field.

Abstract

In a previous paper [A. Mori, T. Kaito, H. Furukawa, Mater. Lett. 62 (2008) 3459-3461], we carried out birefringence measurements of Pb(II)-doped silica hydrogels prepared in a magnetic field (B). For a 5T sample, we observed a negative birefringence with the optic axis along B. At that time, providing a positive intrinsic birefringence of silica, we speculated that in the birefringent gels the gel network extended perpendicular to B. The purpose of this paper is to reconsider this speculation on the basis of previous and recent results [T. Kaito, S.-i. Yanagiya, A. Mori, M. Kurumada, C. Kaito, T. Inoue, J. Cryst. Growth 289 (2006) 275-277; T. Kaito, A. Mori, C. Kaito, J. Chem. Chem. Eng., 9 (2015) 61-66]. In the former, the silica gels were used as a medium of a crystal growth of PbBr2 and aligned arrays of crystallites with long axis parallel to B were obtained. In the latter, Pb(II) nanocrystallites were formed in silica xerogels by electron irradiation. Both of the short axis of PbBr2 crystallites and the diameter of Pb(II) crystallites were a few tens of nanometers. This size must be a size of short axis of pores in the gel networks elongated affected by the magnetic field. Since the PbBr2 crystallites were elongated along the magnetic field, we conclude that the Pb(II)-doped silica gel networks aligned along the magnetic field.