Association of hydrophobic polymers in water and water-d2: fluorescence studies with (hydroxypropyl) cellulose

Abstract

The effects of substituting D{sub 2}O for H{sub 2}O on the fluorescence properties of the pyrene probe in aqueous solutions of (hydroxypropyl)cellulose (HPC) and of pyrene-labeled (hydroxypropyl)cellulose (HPC/Py) have been investigated as a function of temperature and of added surfactants. The polarity-dependence of the vibrational structure of the pyrene emission indicated (1) small differences in polarity between solutions of HPC in D{sub 2}O and H{sub 2}O below and above the lower critical solution temperature and (2) the formation of micelle like surfactant aggregates with sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (HTAC) at concentrations significantly below the critical micelle concentrations of the surfactants. The extent of pyrene excimer emission from solutions of HPC/Py in D{sub 2}O and H{sub 2}O was found to depend on temperature and on the amount of added surfactants. The excimer emission increased with increasing solution temperature from 4 to 35{degree}C and then dropped sharply before the cloud point of the polymer. Addition of SDS or HTAC to solutions of HPC/Py resulted in a decrease in excimer intensity, with a corresponding increase in monomer intensity. Significant solvent isotope effects were observed. The extent of excimer emission of HPC/Py solutions was larger in D{sub 2}O than in H{submore » 2}O for solution temperatures between 25 and 40{degree}C. The critical aggregation concentration of HTAC/HPC was lower in D{sub 2}O (4.9 {times} 10{sup {minus}4} M L{sup {minus}1}) than in H{sub 2}O (6.8 {times} 10{sup {minus}4} M L{sup {minus}1}). The results were interpreted in terms of increased hydrophobic interactions between the pyrene labels of the polymer and between HTAC and the polymer in D{sub 2}O compared to H{sub 2}O.« less