Effects of ionic strength and pH on the diffusion coefficients and conformation of chitosans molecule in solution

Abstract

The effects of ionic strength and pH on the diffusion coefficients and gross conformation of chitosan molecules in solution were studied. Chitosan with 83% degree of deacetylation (DD) was prepared from red shrimp (Solemocera prominenitis) processing waste. Ten different molecular weight chitosans were prepared by ultrasonic degradation, and their molecular weights were determined by static light scattering. The weight‐average molecular weight (Mw) were between 78 to 914 kilo dalton (KDa). Solution of different ionic strengths (I = 0.01, 0.10, and 0.20) but the same pH (2.18) and different pHs (2.37, 3.10, and 4.14) but the same ionic strength (I = 0.05) were prepared to measure their mutual diffusion coefficient (Dm). The diffusion coefficients for standard condition (D20,w) were derived from Dm. Intrinsic viscosities ([η]) were determined by a capillary viscometer in different pH solutions. The Mark–Houwink exponents a and ε were obtained from plots of Log [η] and Log D20,w versus Log Mw, respectively. The results show that diffusion coefficients increased with increasing ionic strength or with increasing pH or with decreasing Mw. Value of ε and a were between 0.503 to 0.571 and ranged from 0.543 to 0.632, respectively. The results indicates that chitosans conformation were in random coil in solutions in the ranges of ionic strength and pH studied. The values of a*, ε* and a**, ε**, Mark–Houwink exponents of smaller and higher than 223 KDa chitosans, respectively, were between 0.752 to 0.988 and 0.585 to 0.777 for smaller Mw chitosans and 0.406 to 0.428 and 0.430 to 0.518 for larger Mw chitosans, respectively. Molecular‐weight‐induced conformational transition occurred because smaller Mw chitosans was more extended than higher Mw chitosans.