Concentration dependence of the sol-gel phase behavior of agarose-water system observed by the optical bubble pressure tensiometry

We have studied an expansion behavior of pressurized bubbles at the orifice of a capillary inserted in gelator-solvent (agarose-water) mixtures as a function of the gelator concentration in which the phase transition points are included. The pressure (P) -dependence of the radius of the curvature (R) of the bubbles monitored by laser beam has shown a discontinuous decrease in the exponent (m) of the experimental power law R = KΔP(−m) (K: constant) from 1 to 1/2 and a discontinuous increase in the average surface tension (γ(ave)) obtained from the work-area plots of the mixtures exceeding that of pure water (72.6 mN/m) at 0.02 < [agarose] < 0.03 wt%, which is attributed to the disappearance of the fluidity. The apparent surface tension (γ(app) = ΔP/2 R) of the system in the concentration range of 0.03–0.20 wt% has been analyzed by a modified Shuttleworth equation γ(app) = σ(0) + τln(A/A(0)), where σ(0) is an isotropic constant component and the second term is a surface area (A) -dependent elastic component, in which τ is the coefficient and A(0) is the area of the orifice. The analysis has indicated that σ(0) coincides with the γ(app) value of the mixture of 0.02 wt% and the second term at >0.02 wt% is the dominant component. From the appearance of the elastic component and concentration dependence of τ, the plateau of τ for the agarose-water mixtures at 0.03–0.10 wt% (Region II) has been explained to the phase separation giving two-phase mixtures of 0.02 wt% sol and 0.10 wt% gel and the upward inflection of τ at 0.10 wt% has been assigned to an increase in the elasticity of the gel with the increase of the agarose concentration in the range of >0.10 wt% (Region III). On considering the concentration dependence of the surface tension of agarose-water mixtures, the discontinuous and inflection points were assigned to the 1st- and 2nd-order phase transition concentrations of the agarose gel, respectively. Given the results with our tensiometry based on the optical bubble pressure method, distinct gelation points for other systems could be determined both mechanically and thermodynamically which will provide a diagnostic criterion of sol-gel transitions.