Abstract
The acid-base equilibrium of an optical probe, 5-dimethylamino-l-naphthalenesulfonyl group, conjugated to silk fiber was investigated by fluorescence measurements in water and in 0.1 M NaCl solution. The apparent pK values of the conjugated probe were smaller than that of the free probe in the aqueous solution and decreased with decreasing pH. The results were interpreted by considering the chemical potential of hydrogen ion and the electrical potential of the fiber phase in Gilbert-Rideal and Peters-Speakman theories. On the other hand, the affinity of hydrogen ion for silk fiber was determined from the adsorption experiments. It was suggested from the comparison of the affinity with the pK value of the conjugated probe that the activity coefficient of hydrogen ion in the fiber phase was reduced to ca. 80% of the coefficient in the aqueous solution.
