Self-assembly of β-lactoglobulin and acacia gum in aqueous solvent:: Structure and phase-ordering kinetics

Complex coacervation in beta-lactoglobulin (BLG)/acacia gum (AG) dispersions has been studied at pH 4.2 using time-resolved confocal scanning laser microscopy (CSLM) and small angle static light scattering (SALS). Two BLG/AG dispersions differing in the extent of charge neutralization were used in these experiments-BLG(1)/AG(1) and BLG(2)/AG(1) with a BLG/AG weight ratio of 1 and 2, respectively. A few seconds after mixing BLG and AG, both dispersions showed the presence of vesicular and unstable multivesicular coacervates (apparent diameters d(a) = 1-15 mum). In BLG(1)/AG(1) dispersions, the unneutralized anionic polysaccharide was located at the surface of vesicles, providing steric and electrostatic stabilization of particles. Sedimentation of vesicles occurred gradually and led to a great number of small stable coacervates (d(a) < 5 mum) in the focal plane after similar to30 min. In BLG(2)/AG(1) dispersions, coalescence of coacervates and rapid sedimentation of insoluble particles onto the observation slide were observed. After similar to15 min, an almost continuous layer of adsorbed coacervates was observed. Interactions between coacervates and precipitation led to the formation of rough surfaces. SALS experiments showed that the turbidity of BLG(1)/AG(1) dispersions was low and did not evolve markedly as a function of time. The scattered light intensity functions I(q) versus q displayed initially a correlation peak located at a length scale R (2pi/q(max)) of about 10 mum. Neither the maximum intensity I-max nor R changed significantly as a function of time. A transient increase of I. was however observed, suggesting a delayed emergence of coacervates. BLG(2)/AG(l) dispersions were initially very turbid and unstable. The scattered light intensity functions displayed initially a correlation peak that moved to smaller wave vectors. The length scale R increased from 14 to 50 mum within 3000 s and followed the power law R similar to t(alpha) with an a exponent value of similar to0.5, slightly larger than the 0.2-0.3 value characteristic of a purely diffusion-controlled growth of particles. During the same time interval, interfaces sharpened but remained fractal. At longer coarsening times, a new correlation peak appeared that slowly moved toward smaller q (alpha similar to 0.2). Interfaces became rough or fractal. The evolution of scattering patterns during the two growth processes was compatible both with late stage spinodal decomposition and nucleation and growth. Polydispersity of biopolymers, different rates of coarsening, and existence of an interfacial length scale did not allow dynamic scaling of data.