Surface Patch Binding Induced Intermolecular Complexation and Phase Separation in Aqueous Solutions of Similarly Charged Gelatin−Chitosan Molecules

Abstract

The formation of selective surface patch binding induced complex coacervates between polyions, chitosan (cationic polyelectrolyte), and alkali-processed gelatin (polyampholyte), both carrying similar net charge, was investigated for two volumetric mixing ratios: r = [chitosan]/[gelatin] = 1:5 and 1:10. Formation of soluble intermolecular complexes between gelatin and chitosan molecules was observed in a narrow range of pH, though these biopolymers had the same kind of net charge, which was evidenced from electrophoretic measurement. This clearly established the role played by selective surface patch binding driven interactions. The temperature sweep measurements conducted on these coacervate samples through rheology and differential scanning calorimetry (DSC) studies yielded two characteristic melting temperatures located at approximately 68 +/- 3 degrees C and 82 +/- 3 degrees C. In the flow mode, the shear viscosity (eta) of the coacervate samples was found to scale with (power-law model) applied shear rate (gamma*) as eta(gamma*) approximately (gamma*)(-k); this yielded k = 0.76 +/- 0.2 (1 s(-1) < gamma* < 100 s(-1)), indicating non-Newtonian behavior. The static structure factor (I(q)) deduced from small angle neutron scattering (SANS) data in the low q (q is the scattering wavevector) (0.018 A(-1) < q < 0.072 A(-1)) region was fitted to the Debye-Bueche regime, I(q) approximately 1/(1 + zeta(2)q(2))2 that yielded a size of zeta approximately 215 +/- 20 A (for r = 1:10) and zeta approximately 260 +/- 20 A (for r = 1:5) samples, implying change in the size of inhomogeneities present with mixing ratio. In the intermediate q region, called the Ornstein-Zernike regime, I(q) approximately 1/(1 + xi(2)q(2)) gave a correlation length of xi approximately 10.0 +/- 2.0 A independent of the mixing ratio. The results taken together imply the existence of a weakly interconnected and heterogeneous network structure inside the coacervate phase separated by domains of polymer-poor regions.

Coacervation in Biopolymers

“…Figure 3 implies that as coacervation point is reached, the zeta potential of the aggregates that are formed due to associative interactions tends to a very low value indicating effective charge neutralization achieved due to strong electrostatic binding between oppositely charged segments of the polymer. In fact, occurrence of turbidity maxima coincides with minimum zeta potential which is in complete agreement with the requirement dictated by models of phase transition [11]. The experimental data indicate the interplay of at least two different types of interactions that precede coacervation:…”

“…In a small class of systems liquid-liquid phase separation gets initiated by surface selective patch binding [11,18,19] even though both the polyions carry similar net charge. This is often referred to as binding on the wrong side of pH.…”

“…In particular, protein-polysaccharide and protein-protein coacervates have attracted much attention because of their inherent potential in generating new biomaterials. In addition, such studies provide basic understanding of specific and non-specific interactions operating between complementary polyelectrolytes [1-6] or polyelectrolyte-polyampholyte [7][8][9][10][11][12][13][14][15] pairs. Normally, polysaccharides are strong polyelectrolytes whereas proteins, in addition, can be polyampholytes.…”

“…It has been shown that a polyelectrolyte, DNA and a polyampholyte, gelatin can undergo associative interaction and form complex coacervates with interesting thermal properties [15]. Further, it has been realized that in a class of systems coacervation transition is governed by surface selective patch binding even though both the polyions carry similar net charge [11,18,19]. In particular, in SPB interactions complementary polyions (normally a PA-PE pair) seek oppositely charged patches to bind overcoming the repulsion occurring between similarly charged surface patches.…”

Linear viscoelasticity of complex coacervates

“…This type of mechanical characterization can give insight into the molecular-level interactions present in the material, and is critical to enable effective processing of these materials. Generally, coacervates have been reported to have a Newtonian, or a shear-rate independent viscosity [71,111,171,172], as well as a shear thinning response [2,87,111,124,[171][172][173][174][175], depending on the shear rate. The occurrence of shear thinning is suggestive of a structural change in the material, and research has suggested that the tendency for shear thinning in coacervate-based materials may correlate with the strength of the underlying electrostatic interactions [2,174].…”