Polyelectrolyte complex characterization with isothermal titration calorimetry and colloid titration

Abstract

Isothermal titration calorimetry (ITC) and colloid titration (CT) were used to characterize aqueous polyelectrolyte complex formation between strong polyelectrolytes, poly(diallyldimethylammonium chloride) and potassium poly(vinyl sulfate) and between weak polyelectrolytes, polyvinylamine (PVAm) and carboxymethyl cellulose (CMC). ITC gives information about the evolution of polyelectrolyte complex properties but does not give good measure of the charge balance composition. By contrast, CT determines the charge balance endpoint but reveals nothing about the path to the endpoint. For strong polyelectrolyte complexes based on fixed charge polymers, the CT curves are very clean giving a sharp endpoint. The corresponding ITC curves are relatively simple although the endpoints are not as distinct as CT. ITC does, however, indicate the onset of phase separation. For weak polyelectrolyte complexes (PVAm/CMC) based on polymers with labile charges, ITC gives complicated curves which are sensitive to pH, ionic strength and the order of addition. The most dramatic effects occur at pH 5–5.9 where we propose that complex formation induces the CMC to ionize further during the titration.

Introduction

Mixing oppositely charged aqueous polyelectrolytes invariably leads to the formation of polyelectrolyte complexes. The resulting complexes can be water soluble; however, they are usually present as dispersed colloids or macroscopic precipitates. Since the pioneering work of Fuoss and Sadek [1], there have been many studies on polyelectrolyte complex formation [2], [3], [4], [5]. Recently, interest in polyelectrolyte complex formation has been rejuvenated by the development of layer-by-layer assembly of polyelectrolyte complexes [6]. For high molecular weight, oppositely charged polymers, the cooperative nature of the electrostatic interactions gives high affinity binding in which high fractions of the potential ion pairs are formed.

Many industrial processes involve polyelectrolyte complex formation. For example, in water treatment and in papermaking, cationic polyelectrolytes are routinely added to form complexes with anionic polymers indigenous to process streams. The resulting complexes can be isolated by sedimentation, filtration, or adsorption onto macroscopic surfaces. In these applications there is a continuing need for techniques to evaluate the polyelectrolyte complex formation process in order to optimize cationic polyelectrolyte addition.

From a more fundamental perspective, there is interest in understanding the detailed structures of polyelectrolyte complexes. Composition, degrees of swelling, and the concentration of ionic crosslinks are examples of properties which can be difficult to measure. Perhaps the most successful tool for following polyelectrolyte complex formation is the colloid titration (CT) in which an unknown polyelectrolyte is titrated with a known oppositely charged polyelectrolyte to give an endpoint corresponding to charge balance in the resulting polyelectrolyte complexes. In Terayama's [7] original paper describing the CT, endpoints were detected with a dye indicator. Although automated detection of dye indicators has been described [8], most practitioners of polyelectrolyte titrations employ a streaming current detector to determine the charge equivalence endpoint. First described in 1966 [9], streaming current detectors are commercially available from a number of suppliers both as laboratory instruments and online sensors [9], [10]. The heart of the streaming current detector is a reciprocating fluorocarbon piston in a loosely fitting fluorocarbon cylinder. When the piston moves in the cylinder, the solution is forced to move in the annulus between the piston and the cylinder wall. Electrodes embedded in the cylinder wall measure the induced streaming current.

Polyelectrolyte titration with streaming current detection relies on two primary assumptions:

• The reaction between the polycations and polyanions is complete. In other words, uncomplexed cationic and anionic groups do not coexist. The cooperative nature of polyelectrolyte complex formation gives very high effective binding constants, making this a good assumption.

• The species of interest in solution are also adsorbed on the fluorocarbon surfaces of the SCD and the titrant volume corresponding to the isoelectric point of the adsorbed layer, measured by streaming current, corresponds to the charge balance composition of the polyelectrolyte complex species in solution.

CT is widely used because it works well. Unfortunately, CT gives only the charge balance composition. CT gives no insight into changes which occur in a suspension during the complex formation process. Our interest in finding additional tools arose out of our study of polyelectrolyte complexes formed by mixing polyvinylamine (PVAm) with carboxymethyl cellulose (CMC). This is a fascinating example of a polyelectrolyte complex which can be prepared either as colloidally dispersed complexes [11] or as macroscopic homogeneous, transparent films [12]. In this paper, we describe our efforts to employ isothermal titration calorimetry (ITC) [13], [14] to increase our understanding of the PVAm:CMC polyelectrolyte complex formation process.

In an isothermal titration calorimetric experiment, a titrant is added to a solution and the induced heat flow required to keep a constant temperature is monitored. ITC is widely used to characterize biological interactions [15], [16]. The physical chemistry literature also reports many ITC investigations including ligand–macromolecule binding [17], [18], [19], polymer–solid interactions [20], surfactant–polymer association [21], [22], surfactant micellization [23] and amphiphilic polymer association [24]. For simple interacting systems involving low molecular weight ligands, ITC titrations can be used to calculate binding constants, binding ratio and thermodynamic parameters. Herein we consider the applicability of ITC for characterizing complex formation by high molecular weight, oppositely charged polyelectrolytes.

There are very few reports on the calorimetric characterization of polyelectrolyte complex formation. The low utilization of ITC perhaps reflects that polyelectrolyte complex formation is entropically driven by the release of counterions [3], [5]. In other words, the heat effects associated with polyelectrolyte complex formation are low.

In 1990, Oppermann and Shulz [25] used microcalorimetry to measure the enthalpy of complex formation between poly(trimethylammonium-2-ethylmethacrylate chloride), a quaternary ammonium polycation, and poly(styrene sulfonate), a strong polyacid. They observed a negative enthalpy (exothermic) with a peak value of about—2 kJ/mol of poly(styrene sulfonate) occurring at 1:1 stoichiometry. Furthermore, they investigated influence of cationic counterions on the complex formation. It was found that complex formation in the presence of divalent ions generates 30% more heat than that with univalent ions at stoichiometry.

Recently, Nystrom et al. [26] used ITC to measure the heat effect of the interaction between cationic starch and sodium polyacrylate in 0.01 mol/L sodium chloride and reported a small positive (endothermic) heat flow.

In this work, we report ITC results for two types of polyelectrolytes complexes formed from oppositely charged polymers. The first type was prepared from mixtures of poly(diallyldimethylammonium chloride), PDADMAC, and potassium poly(vinyl sulfate), PVSK. Both PVSK and PDADMAC are strong polyelectrolytes with constant charge densities over most of the pH range in water. The second polyelectrolyte complex was based on mixtures of polyelectrolytes, polyvinylamine and carboxymethyl cellulose. Both CMC and PVAm are weak polyelectrolytes whose charge densities are strong functions of pH and ionic strength [11]. The following sections will show that ITC gives new information about polyelectrolyte complex formation. The weak polyelectrolyte pair (PVAm/CMC) displayed particularly intricate behaviour which was sensitive to pH and ionic strength. Finally we demonstrated that colloid titration and ITC give complimentary information about weak polyelectrolyte complex formation