Helical aggregate formation of cholate salts in poly(N-vinyl-2-pyrrolidinone) gel and its effect on conductivity enhancement

The polymer gels (cross-linked poly(N-vinyl-2-pyrrolidinone) (c-PVP) containing sodium deoxycholate (NaDC) or sodium taurodeoxycholate (NaTDC), and N-methyl-2-pyrrolidinone (NMP) were prepared by radical polymerization using 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator. Conductivity of the gels were measured as a function of temperature and cholate salt concentration. NaDC and NaTDC molecules in the c-PVP films formed aggregates above a concentration of not, vert, similar 2 × 10−2 mol kg−1 (per kg of gel) and (8–9) × 10−2 mol kg−1, respectively. The conductivity of the NaDC/c-PVP gel increased from 7.73 × 10−8 to 2.88 × 10−6 S cm−1 with the formation of NaDC aggregates in the gel. The conductivity of the NaTDC/c-PVP gel also increased from 5.30 × 10−9 to 6.33 × 10−7 S cm−1. Furthermore, the induced circular dichroism measurements of the surfactant gels containing bilirubin as a probe dye suggested that structure of aggregates in the gel is a helical structure. The conductivity jump is induced by an increase in number of charge carriers produced by increased dissociation of cholate salt molecules and formation of helical aggregates in the gel.