Thermo-reversibility, ergodicity and surface charge–temperature dependent phase diagram of anionic, cationic and neutral co-gels of gelatin–BSA complexes

Abstract

We have investigated the gelation behavior of polyampholyte gelatin B (GB) in the presence of colloidal plasma protein bovine serum albumin (BSA) as a function of mixing ratio (r = GB : BSA = 1.5, 2, 3, 4), pH (acidic, basic and neutral), and temperature (20–45 °C). Both the bio-molecules have identical isoelectric pH, and similar zeta potential profiles. Formation of a BSA–GB intermolecular complex arising from hydrogen bonding, and surface selective patch binding between the flexible gelatin chain and colloidal protein BSA, with BSA acting as junction zones was probed. We observed the cationic, neutral and anionic gelation occurring at pHs (3, 5 and 7) corresponding to surface charge of the intermolecular complexes (seen through their zeta potential, ζ). The kinetics and dynamics of gelation were observed from the intensity of scattered light, and time-correlation function which evolved with waiting time (t_w ≥ t_gel), and indicated rapid gelation for pH 5 (neutral gels) compared to other sols at pH 7 (anionic gel) while at pH 3 (cationic gel) very slow gelation was evident. For all gels with a waiting time of t_w → ∞ the gels moved from an ergodic to non-ergodic state. At t_w = 0 (nascent gel), the correlation function exhibited a single relaxation mode due to the system residing deeply inside the ergodic phase, and mirroring purely Brownian dynamics. After a characteristic waiting time (ergodicity breaking time, τ_EB), an additional relaxation (slow mode) appeared which was attributed to inter-chain interactions induced by reorganization of entanglements. The rigidity (low frequency storage modulus, G₀), and melting temperature T_m of neutral gels was higher than anionic, but was least for cationic gels. Finally, we capture a unique surface charge versus temperature (ζ vs. T) phase diagram at different pHs, and noticed thermally activated phase transitions which indicated multiple self-assembled states of this pair of bio-molecules leading to evolution of several new soft matter phases, all of which were not thermo-reversible.