Interpolyelectrolyte complex formation: From lyophilic to lyophobic colloids

Highlights

• IPEC particles were formed from chitosan (CS) and an anionic polyelectrolyte (PSSMA).

• Turbidity, conductivity, and viscosity indicated best PSSMA/CS ratio for PEC formation.

• At this very ratio zeta potential suggested IPEC system to be stoichiometric.

• ICF detected a second relaxation process and narrowing of relaxation rate distributions.

• SAXS suggested particles initially have a globular nature before phase segregation.

Abstract

Interpolyelectrolyte complexes (IPEC) have been used in different fields, ranging from biomedicine to oil exploration. Particle dimension is a key parameter for the use of an IPEC for a particular application. In this work, dynamic and static scattering (in the form of optical DLS and SAXS, respectively) were used to follow particle growth as a function of anionic/cationic polyelectrolyte mass ratio, $w_{AC}$, coupled to turbidimetry, conductometry, viscometry, and zeta potential measurements. Turbidimetry, conductometry, and viscometry showed that at $w_{AC}\cong 2$ the IPEC dispersions presented changes that were associated to a massive production of IPEC particles. At the same $w_{AC}$ value, zeta potential measurements indicated an apparent isoelectric point, showing that this $w_{AC}$ must be related to a stoichiometric point for IPEC formation. Double KWW equation was adjusted to data from DLS intensity correlation functions, and parameters related to the distribution of relaxation rates presented characteristic changes at the same value of $w_{AC}$. SAXS was used to follow particle growth until massive IPEC production. It showed that particles firstly were in the form of small, globular particles, tending to unfolded chain geometry, which can be related to the occurrence of flocculation, just at the point of massive IPEC production.

Introduction

Interpolyelectrolyte complexes (IPEC) nanoparticles have already been used in (or proposed for) applications related to drug release [1], [2], [3], [4], [5], [6], oil recovery [7], membranes [8] and environment remediation [9], [10], among others [11], [12], [13]. In fact, dimensions of IPEC clusters may be so small that they may exist in the form of solubilized structures or solid IPEC particles dispersed in a continuous liquid phase [14], [15]. This flexibility of behavior makes them, apart from being used as solid particles, to be used in the form of solutions, in order to obtain membranes [16], [17].

IPEC’s may be formed by simply mixing solutions of macro-ions [18]. These mixtures may be directly carried out, or they can be prepared using a dispersed aqueous phase, such as in emulsion-based process for IPEC preparation [19], [20]. Examples of anionic polyions used in these systems are poly(acrylic acid) [21], sodium salt of poly(styrene sulfonic acid-co-maleic acid) [22], [23], alginate [24], [25], dextran sulfate [26], and carrageenan [27].

Chitosan has widely been reported to be used as a cationic macro-ion to obtain IPEC’s [28], [29], [30], [31]. This natural polymer has wide applicability [32], [33], [34] and has excellent physical and biological properties [35], [36]: in fact, it has been shown that specific interactions with drugs make it an excellent candidate for effluent treatment of drug loading for posterior drug release [37], [38], [39]. The cationic nature and high density charge in acid solution make chitosan extremely adequate to form polymer networks in IPEC’s with water-soluble polyanionic species [40].

Chitosan-based IPEC particles have initially been characterized as a result of electrostatic interactions between cationic groups from chitosan an anionic groups from the polyanion. A more representative, comprehensive mechanism of IPEC particle formation depicts it as being entropically driven by the delocalization of both polycation and polyanion counter-ions. In this manuscript, we intend to carry out a study on the formation of IPEC particles, following the approach already developed by Izmorudov et al. [41], involving the usual techniques of conductometry, zeta potential, viscometry, and static light scattering. In addition to this, we explore dynamic aspects of IPEC formation using DLS, correlating them with the deconvolution of pair-distance distributions from SAXS.