Phase behaviors of agarose gel

We present evidence for the existence of phase separation in the gel state of agarose having the mixture of water and methanol as the gelsolvent. Firstly, the sol-geltransition line and the cloud point line are determined independently as a function of the concentration of agarose as well as the concentration of methanol in the mixed solvent by the quasi-equilibrium cooling of the solutions. Then the spinodal line is determined by quenching the solutions below the sol-geltransition line. We find that the spinodal line appears below the cloud point line and both lines are entirely buried below the sol-geltransition line in the aqueous agarose system. The concentration fluctuations are, therefore, frozen into the polymer network of agarose gel that promotes the opacity of the resultant gel. The structure of agarose gel is observed by the confocal laser scanning microscope (CLSM)imaging technique that reveals that the density fluctuations are grown up to micrometer scale in space. The phase separationboundary is found to shift to the higher temperature region than the sol-geltransition line when the concentration of methanol in the mixed solvent is increased. The results indicate that the position of the phase separation boundary in relative to the sol-geltransition line varies with the quality of solvent.These results are in agreement with the theory of the sol-geltransition in which both the divergence of the connectivity and the thermodynamic instability are taken into account.