Effect of pH on the Complex Coacervation and on the Formation of Layers of Sodium Alginate and PDADMAC

Abstract

In this study, we investigated the thermodynamic features of a system based on oppositely charged polyelectrolytes, sodium alginate, and poly(diallyldimethylammonium chloride) (PDADMAC) at different pH values. Additionally, a comparison of the effects of the thermodynamic parameters on the growth of the layers based on the same polymers is presented. For this investigation, different techniques were combined to compare results from the association in solution and coassembled layers at the silicon surface. Dynamic light scattering (DLS) and isothermal titration calorimetry (ITC) were used for studies in solution, and the layer-by-layer technique was employed for the preparation of the polymer layers. Ellipsometry and atomic force microscopy (AFM) were used to characterize the layer thickness growth as a function of the solution pH, and interferometric confocal microscopy was employed to analyze the topography and roughness of the films. The titration of both polyelectrolytes in two different sequences of additions confirmed the mechanism; it involved a two-step process that was monitored by varying the enthalpy, as determined by ITC experiments, and the structural evolution of the aggregates into coacervates, according to DLS. The primary process is aggregation to form polyelectrolyte complexes having a smaller hydrodynamic diameter, which abruptly transit toward a secondary process because of the formation of coacervate particles that have a larger hydrodynamic diameter. Independent of pH and the sequence of addition, for the first process, both directions are entropically driven. However, the binding enthalpy (ΔH b ) decreased with a decrease in the pH of the solution. The layers grown for the PDADMAC/sodium alginate system demonstrated pH sensitivity with either linear or exponential behavior, depending on the pH values of the polyelectrolyte solutions. The more endothermic process at pH 10 afforded layers with a smaller thickness and with linear growth according to the increase in the number of layers from 5 to 20. Decreases in the pH of the solution resulted in the layers growing exponentially; additionally, a decrease in the ΔH b of the association in the solution was observed. The layer thicknesses measured using ellipsometry and AFM data were in good agreement. Additionally, the influence of pH on the roughness and topography of the films was observed. Films from basic dipping solutions resulted in surfaces that were more homogeneous with less roughness; in contrast, films with more layers and those formed in a low-pH dipping solution were rougher and less homogeneous.

Microencapsulation of <i>Ruellia tuberosa</i> L. Extracts Using Alginate: Preparation, Biological Activities, and Release

“…Thus, the increasing ionic bonds might strengthen the matrix, escalate the amount of encapsulated bioactive ingredients, and result in the greatest encapsulation efficiency [26][27]. The results agreed with previous research [28], which stated that a higher pH solution would produce less protonated forms. The protonation of the carboxylate anions can interrupt the coordination with calcium cations and reduce the existing sites for cross-linking.…”

Salt Resistance as a Measure of the Strength of Polyelectrolyte Complexation

“…The Δ H term reports the sum of all specific interactions: electrostatic, hydration, hydrophobic, hydrogen bonding, and dipolar. Calorimetry studies of polyelectrolyte complexation rarely reveal an athermal process, ,,,− although this condition is almost met by a system comprising poly­(diallyldimethylammonium) (PDADMA) and poly­(styrene sulfonate) (PSS) in KBr …”

Weak Polyelectrolytes as Nanoarchitectonic Design Tools for Functional Materials: A Review of Recent Achievements

“…In the case of LbL film buildups relying on electrostatically driven interactions, assembly conditions with smaller polyelectrolyte charge densities result in less crosslinks and more loops between the layers, leading to larger thickness increments at each adsorption step of weak polyelectrolytes compared to conditions where an extended conformation of the polyelectrolyte is favored (corresponding to higher charge density). Accordingly, the growth mechanism of a polydiallyldimethylammonium chloride (PDADMAC)/ALG multilayer changed from linear to exponential behavior when the assembly pH value was changed from pH 10 (where both polyelectrolytes are ionized) to pH 3 (where ALG is protonated) [ 97 ]. It follows that the multilayer thickness, chemical composition, mechanical properties, permeability, and adhesion are largely determined by pH and ionic parameters during the LbL deposition process.…”