Thermoreversible and salt-sensitive turbidity of methylcellulose in aqueous solution

The turbidity of methylcellulose (MC) aqueous solutions without or with salt was investigated by optical transmittance. The optical transmittance was found to decrease with heating, while it recovered upon cooling. This phenomenon reflected the micro-phase transition from an unassociated state to a hydrophobically associated state in the solution when the system underwent the sol–gel transition. The derivative of absorbance (dA/dT) was used to determine the clouding point during gelation and the melting point during degelation. A salt-out salt (NaCl), a salt-in salt (NaI) and a salt mixture (NaCl+NaI) were, respectively, added into a MC solution to further investigate effects of salts on the gelation behavior. The critical transition temperatures obtained from rheological and micro thermal measurements were found to be consistent with the clouding points obtained by turbidity measurements. In the MC solutions with the salt mixtures, the critical temperatures followed a linear rule of mixing, indicating that the effects of salts on the sol–gel transition of MC are completely independent.