The aggregation, rheological and structural properties of casein-dextran colloids induced by critical-zone-intensity ultrasound

Casein-dextran colloids, with various particle and filtration properties to simulate the main pollutant (protein-polysaccharide colloids) in the membrane separation process, were selected as the research object in this study. Effects of critical-zone-intensity ultrasound (CZ-US) on the aggregation, rheological and structural properties of casein-dextran colloids were investigated by ultrasonic immersion. The results suggested that CZ-US treatments unfolded the protein tertiary/quaternary structure to expose hydrophobic/hydrophilic amino acid groups, and partially affected the aggregation and flow behaviors, without altering the secondary structures of proteins and functional groups for casein-dextran colloids. CZ-US treatments enabled particle size distribution of casein-dextran colloids transform from multimodal to unimodal. CZ-US treatments' processes were strongly affected by pH of the colloidal mediums. When pH was at the casein isoelectric point, more hydrophobic amino acid groups of casein were exposed, the binding patches on the casein particle interface were reduced for the dextran. When pH was at other mediums rather than the casein isoelectric point, more hydrophilic amino acid groups were exposed under CZ-US treatments, more dextran adhered into the casein interface due to the presence of more available binding sites. Indeed, this work provides important theoretical guidance for the development of CZ-US and membrane separation coupling technology.