| Abstract
| - An aqueous solution of an amphiphilic polysaccharide derivative, hydrophobically (stearyl alkyl group) andhydrophilically (sulfonic-acid salt group) modified hydroxyethylcellulose (HHM-HEC), showed increased viscosity,elasticity, and thixotropic properties in response to the addition of monovalent and divalent salts. Furthermore, theHHM-HEC solution had a transparent appearance at a NaCl concentration of 7 wt %. Since it showed superior salttolerance to HEC, we focused attention on the two substituents of HHM-HEC and prepared HEC derivatives, namely,hydrophobically modified hydroxyethylcellulose (R-HEC), hydrophilically modified hydroxyethylcellulose (S-HEC),and nonmodified hydroxyethylcellulose (HEC). In addition, we used oscillatory, thixotropic, and fluorometric methodsto compare the rheological properties of HHM-HEC with those of other derivatives in the presence of NaCl and ZnCl2,and attempted to elucidate the respective roles of the two substituents of HHM-HEC solution in the salt-tolerancemechanism. As the NaCl concentration in the HHM-HEC solution increased, the values of the elastic modulus G‘and the viscous modulus G‘ ‘ increased, and, moreover, the relative intensities of the first (I1 = 372 nm) and the third(I3 = 383 nm) vibronic bands of the pyrene monomer emission spectrum (the I1/I3 ratio) decreased. These resultssuggested that the added salt strengthened the three-dimensional network structure of the HHM-HEC polymer by theformation of cross-linkages through the association of hydrophobic substituents. This hydrophobic substituent wastherefore essential in allowing HHM-HEC to exhibit a high viscosity in a salt solution. Although the R-HEC solutionshowed a higher viscosity than did the HHM-HEC solution in the absence of added salts, it became cloudy and lostits viscosity at high NaCl concentrations, apparently because of the shrinkage of its network structure. This signifiedthat the hydrophilic substituent was essential for the sufficient solubility of HHM-HEC to show its rheological propertiesin a salt-rich solution. We propose to explain how the viscosity of HHM-HEC increases in the presence of salts asfollows: Added salts weaken the electrostatic repulsion between the hydrophilic substituents, thereby enhancing theinteractions of hydrophobic substituents and consequently increasing the rigidity of the HHM-HEC solution.
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