The effect of neutral salts on gelatin-gum arabic complexes*1
Swollen gelatin-gum arabic (GGA) complexes behave like typical polyelectrolyte gels in that observed swelling values are a strong function of pH and neutral salt concentration. At pH 3.5 and 24°C., GGA complexes swell very slowly in many salt solutions. True equilibrium swelling values are never obtained in many cases. Increasing concentrations of NaCl or Na2SO4 cause GGA swelling values to pass through a maximum, thereby giving swelling curves similar in shape to the typical salting-in and salting-out curves frequently encountered with proteins and polypeptides. CaCl2 concentrations as low as 0.25 M grossly weaken and appear to partially melt GGA complexes at 24°C., whereas higher CaCl2 concentrations (>0.75 M) rapidly destroy GGA complexes. Irreversible swelling changes observed when GGA complexes are subjected to cyclical swelling treatments in buffer and buffered NaCl solutions, are due to displacement of gum arabic (GA) from the complex as Bungenberg de Jong and Landsmeer concluded previously. Only 83% of the gum arabic initially present in the complex can be displaced under a variety of conditions, indicating that a substantial amount of GA is difficult to remove from a GGA gel. This may be due to physical entrapment of GA in the gel or may be a characteristic feature of GGA gels. The initial rate of GA displacement is a strong function of neutral salt concentration even though marked shrinkage of the gel occurs at higher salt concentrations. Chloride analyses show that diffusion of salt ions into the swollen complex is rapid and does not control the rate of GA displacement. In contrast, the large size of GA and the multiplicity of charged groups on each GA molecule suggest that its diffusion from the gel controls the over-all rate of displacement. Limited experiments at 17°C, suggest that decreasing the temperature from 25° to 17°C. does not noticeably affect the rate of GA displacement.
Portions of this paper were presented at the Winter Meeting of the American
Chemical Society, Polymer Division,