Amphiphilic and amphoteric aqueous soy protein colloids and their cohesion and adhesion to cellulose

Abstract

Highly stable aq. soy protein (SP) colloids have been facilely prepared by high-speed blending (30k rpm, 15 min) to homogeneously disperse up to 97.7 % of soy protein isolate at up to 9 %. Aq. SP colloids were amphoteric with a 4.5 isoelectric point and ζ-potentials ranging from +32.4 mV at pH 2.2 to -39.1 mV at pH 7.1 with respective average hydrodynamic diameters of 157 and 233 nm. Diluted aq. SP colloids (0.0001 %) dried on hydrophilic mica appeared as 6.0 nm thick and 16.4 nm wide disk-like nanoparticulates. Aq. SP colloids were also amphiphilic, reducing water surface tension to 41.2 mN/m at > 0.98 % and could be solvent-exchanged to dimethyl formaldehyde or 4:1 v/v methanol/water. Colloidal SPs exhibited preferential affinity and adhesion to cellulose filter paper to significantly improve the respective tensile modulus and strength by 70 and 42 % at merely 2.8 % dried add-on. The improved cohesion of 6.2 % SPs was evident by drying at 110 °C to further increase the respective tensile modulus and strength of cellulose by another 78 and 41 %.