Salt Addition Effects on Mesophase Structure and Optical Properties of Aqueous Hydroxypropyl Cellulose Solutions

Abstract

The effects of addition of neutral salts on the cholesteric mesophase structure and optical properties of aqueous solutions of hydroxypropyl cellulose (HPC) were investigated by spectrophotometry and an NMR technique with D2O as solvent. In the measurements of selective light-reflection spectra, the wavelength of maximal reflectance and therefore the cholesteric helical pitch (P), too, varied systematically with a change in strength of a so-called “chaotropic” effect of the additive salts; i.e., their effectiveness in increasing P was pronounced in the order of Cl-<Br -<NO3-<I-<SCN- and Cs+<K+<Na+ <Ca2+<Li+≤Mg2+<Al3+, when compared in a series of salts with a common sort of cation or anion. An LCST-type of phase separation behavior of the HPC/water system was also affected by the coexistence of salt ions; the cloud point shifted either down or up in almost the same manner as that found in the salt-sort dependence of the cholesteric periodicity. In the deuteron NMR study, a frequency width of the quadrupolar splitting, detected as a measure of the molecular orientation development within the mesophase, was found to be less affected by the salt addition. It is suggested that the hydrophobic bond-disrupting or promoting activity of the salts added could regulate subtly a conformational state of the cellulosic polymer, to induce a marked change in cholesteric twisting power without disturbing the orientational ordering in each of the pseudonematic layers.