Study of sol-state properties of aqueous gelatin solutions

Abstract

Aqueous gelatin solutions have been studied in a systematic and exhaustive manner in a neutral buffer medium, in the temperature range of 30–60 °C, by static (SLS) and dynamic laser light scattering. It has been clearly established that upon cooling the sol from 60 °C, the gelatin chains form thermoreversible aggregates until the gelation is encountered below 30 °C. The radius of gyration of the chains (Rg) at any temperature was found to scale with corresponding molecular weight as Rg∼M0.57±0.03w. The ratio of Rg to hydrodynamic radius RH has been found to be Rg/RH=1.82±0.03 as expected for random coils. The translational diffusion coefficient of the chains Dz(c) exhibited linear concentration dependence; Dz(c)=D0(1+KDc) and D0∼M−0.57±0.03. The second virial coefficient (A2) of osmotic pressure obtained through SLS could be excellently related to KD through KD≂2A2Mw for all temperatures. The Flory–Huggins interaction parameter χ was determined to be χ≂0.48 and it showed negligible temperature dependence. The temperature dependence of sol viscosity η(T) could be fitted to Doolittle equation ln η(T)=A+B/T. At any given temperature, the concentration dependence of specific viscosity ηsp(c) followed ηsp(c)/c=1/([η]+k√c). The intrinsic viscosity [η] was found to scale with Mw as [η]∼M0.69±0.08w also yielding a typical overlap concentration of c*≂2.5% (w/w). The Flory–Fox constant Φ was found out to vary between 2.66×1021 at 35 °C and 1.27×1021 at 60 °C, against the standard value of 2.21×1021. The density values of the sol have been measured by a simple but novel method. Both the sol density and refractive index show monotonous increase as the gel point is approached from the hot sol state of gelatin.