Study on the heterogeneity in mixture carrageenan gels viewed by long time particle tracking

Abstract

The network structure of mixed κ- (KC) and ι- (IC) carrageenan gels were investigated using particle tracking in a long time observation. The Brownian motion of 100 nm particles tracked for ~1400 s was utilized to probe the microenvironment in the mixed gels. The individual mean square displacement (msd) of particles in different mixing ratios of KC/IC showed a broad distribution, indicating spatial heterogeneity in the particle diffusion, suggesting the phase-separated network structure with KC-rich and IC-rich domains. The broad distribution of msd further suggests fast-moving particles in IC-rich and slow-moving particles in KC-rich domains. The distribution of displacement for each of the fast-moving particles showed a non-Gaussian distribution indicating the heterogeneous diffusion behavior. The time-course of the radius of gyration in a sliding time window with a duration time for the mobile particles revealed multiple confinement events. These were attributed to the sterical hindrance or the interaction with the surface of the KC-rich domain, which tolerates the breakaway from the confinement by the Brownian motion. Simulation of particle diffusion was performed to understand the interaction that causes the confinement period. Results revealed that the confinement was caused by the interaction between the particle and the KC-rich domain covered with KC chains. The KC chains have the negatively charged sulfonic group, but salt ions of KCl should satisfyingly shield the repulsive interaction by the sulfonic group to have cohesive interaction with the particle.