Polyelectrolyte–Dye Interactions: Comparison
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Please note this is a comparison between Version 2 by Dean Liu and Version 1 by Ranjit De.

Polyelectrolytes are polymers with repeating units of ionizable groups coupled with counterions. Recently, polyelectrolytes have drawn significant attention as highly promising macromolecular materials with potential for applications in almost every sector of our daily lives. Dyes are another class of chemical compounds that can interact with substrates and subsequently impart color through the selective absorption of electromagnetic radiation in the visible range.

  • polyelectrolytes
  • dye
  • interaction parameters
  • polymer

1. Polyelectrolytes

Polyelectrolytes (PEs) form an interesting class of macromolecules that dissociate in polar solvents to produce a large number of charged groups and their corresponding counterions [10][1]. The smaller counter ions neutralize the repeating charged groups and preserve the electro-neutrality. In an uncharged state, the behavior of PEs resembles that of normal macromolecules; however, the dissociation of the ionic groups, even to a small extent, may lead to dramatic changes in their physico-chemical properties [11][2]. Thus, polyelectrolytes can exhibit both the properties of polymers and electrolytes, which is advantageous towards their interactions with various types of dye molecules. Such polymer behavior can be modulated by the partial or complete dissociation of the ionic groups, which subsequently alters the electrostatic interactions leading to deviations in their polymeric properties [12][3]. The physical properties of PEs, such as viscosity, diffusion coefficient, solubility, pH, ionization constant, and ionic strength, can be modified by the introduction of ionic moieties into the polyelectrolyte environment [13][4].
Due to their excellent water stability and ability to interact with oppositely charged macromolecules and surfaces, polyelectrolytes have been extensively used in various fields, from materials science and colloids to biophysics. Their predominant applications include usage in optoelectronic devices [14][5], solar cells [15][6], rheology modifiers [16[7][8],17], adsorbents [18][9], coatings [19[10][11],20], biomedical implants [21][12], colloidal stabilizers [22][13], suspending agents [23][14], and for drug delivery and pharmaceutical uses.
Polyelectrolytes can be classified into different categories depending upon their origin, charge, pH dependence, morphology, position of ionizable sites, and composition. Some natural polyelectrolytes include carbohydrates, alginates, chitosan, carrageenan, pectin, and nucleic acids, while synthetic polyelectrolytes, such as poly(acrylic acid), poly(vinyl amine), poly(vinylsulfonic acid), and polyvinylpyridine, are also common.
Among the anionic polyelectrolytes, carboxylate –COO−, phosphonate (−PO3H−,−PO2−3), and sulfonate (−SO−3) are the most common functional groups, whereas cationic polyelectrolytes are mostly comprised of the primary, secondary, and quaternary amino (-NH2, =NH, and =N+=) groups. The types of ionic groups, their counter ions, and the structures of the repeating units determine the properties of polyelectrolytes, such as their solubility in water and other polar and hydrogen-bonding liquids (alcohols, etc.), electrical conductivity, and rheology. Unlike nonionic polymers, these properties strongly depend on the pH, solvent permittivity, and ion content [24][15].
The electrostatic interactions (attraction/repulsion) between charges present on the monomeric units of polyelectrolytes lead these macromolecules to be rich in a variety of physicochemical properties. For instance, in the absence of added salts (ions), the electrostatic repulsion between the same charges of monomer units of a macromolecule can result in significant chain elongation, which can vary almost linearly with the degree of polymerization [25][16]. Due to the strong influence of the degree of polymerization on chain morphology transition (coiling to elongation), which results in an increase in chain size, the crossover to the semi-dilute polyelectrolyte solution regime can be achieved at much low polymer concentrations than in the case of nonionizable polymers [26][17].

2. Polyelectrolyte–Dye Interactions

Polyelectrolytes and dyes comprise two of the most important classes of chemical compounds with the most versatile application in industrial chemistry. The interactions between polyelectrolytes and dye lead to formation of polyelectrolyte–dye complexes with modified physical and chemical properties. In the following sections, the applications of some materials prepared by the interactions between polyelectrolytes and cationic dyes, as well as polyelectrolytes and anionic dyes, are presented.

2.1. Polyelectrolyte–Dye Interactions and Their Applications

2.1.1. Polyelectrolytes and Anionic Dyes

Dye removal remains one of the most challenging aspects of industrial waste management. The strong interactions between polyelectrolytes and dyes were pushed further by Cai et al. to develop a chitosan-based cationic polyelectrolyte microsphere (CCQM) for the ultra-efficient removal of Congo red (1500 mg g−1) and methyl orange (MO, 179.4 mg g−1) [62][18]. Based on the strong polyelectrolyte–dye interactions, hydrogels fabricated using poly([2-(acryloyloxy)ethyl] trimethylammonium chloride) and poly(ClAETA) with cellulose nanofibrillation (CNF) had an efficiency of 96% in the removal of methyl orange dye, which remains a major industrial contaminant [63][19]. Schwarze et al. developed polyelectrolytic emulsions based on quaternary ammonium surfactants and demonstrated a dye removal efficiency of 90% for methyl orange [64][20]. The dye–polyelectrolytic complex aggregates have an important role in determining the spectral behavior of the dye. It was observed that methyl orange demonstrated an absorption maximum at 368 nm in poly(l-ornithine) (PLO), compared to 462 nm in poly(vinyl benzyl triethylammonium chloride) (PVBTEA). This observation was attributed to the formation of larger aggregates in PLO compared to PVBTEA, which promoted electrolytic dye stacking via ion-pair formation [65][21].
Microgels are three-dimensional cross-linked structures of polymer colloidal particles with an adjustable size and strong response to environmental stimuli, such as pH, ionic strength, temperature, light, and ultrasound [66][22]. Self-assembled microgels comprised of poly(N-isopropylacrylamide-co-2-(dimethylamino) ethyl methacrylate) and sodium alginate (SA) have demonstrated a highly pH-sensitive response [66][22]. Methyl blue is an anionic hydrophilic dye molecule which has been reported to be adsorbed onto microgel core star ionic covalent organic polymers, polymers, fibrous materials, and cross-linked polymer particles with cavity and ammonium functionalization [62,67,68,69][18][23][24][25]. A quartz crystal microbalance (QCM) investigation of the interaction between anionic dyes and the SA/microgel multilayers in the aqueous phase revealed an enhanced electrostatic attraction between the dyes and the microgels deposited on the QCM sensor surface compared to that with SA in the multilayers, which caused the release of microgels from the self-assembled structure and a mass loss ratio of 27.6% [66][22]. This study showed a promising application of the QCM-based sensors in the detection of dye contaminants in wastewater. In another report, a linear polysaccharide chitosan (CTS) composed of β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit), was chemically modified to form a cationic polyelectrolyte, viz., N-[(2-hydroxy-3-trimethylammonium)propyl]chitosan chloride (HTCC) [70][26]. A comparative investigation of the interaction of three anionic dyes, viz., Reactive Black 5, Reactive Blue 19, and Reactive Red 195, with HTCC and organoclay-modified montmorillonite (OMMT) demonstrated a high efficiency of dye exclusion (>91%) compared to sole polyelectrolyte and organoclay adsorbents. The study further showed that structurally distinct anionic dyes localized at separate sites within the hybrid organoclay adsorbents, enabling the simultaneous adsorption of different dyes with improved efficiency [70][26]. Thus, the materials designed via the interactions between polyelectrolytes and cationic dyes have exhibited success in various fields.

2.1.2. Polyelectrolytes and Cationic Dyes

Cationic dyes dissociate into positively charged ions and negative counterions in aqueous solutions and have been extensively explored to study their interaction with anionic polyelectrolytes. The strength of this interaction can be measured by the magnitude of metachromasy induced in its spectroscopic profile. Metachromasy, or the blue shift in the absorption spectrum, is one of the most common methods for spectroscopic detection of polyelectrolyte–dye interactions. Higher metachromic effects imply a stronger degree of interaction. Toluidine blue (7-amino-8-methylphenothiazin-3-ylidene)-dimethylammonium chloride) and methylene blue (3,7-bis(dimethylamino)-phenothiazin-5-ium chloride) both form a strong 2:1 dye–polyelectrolyte complex with polyacrylic acid polymer (PAA), exhibiting large hypsochromic shifts of 57 nm and 67 nm, respectively, in their UV-vis profiles. Consequently, due to the more hydrophobic nature of methylene blue, it formed a more stable complex with PAA compared to toluidine blue: the stability constants were 5332 dm−3/mol and 4358 dm−3/mol for methylene blue and toluidine blue complexes, respectively, at 298 K [71][27]. The interaction of toluidine blue with poly(potassium vinyl sulphate) (PPVS) resulted in observed metachromacy at 105 nm with a distinct color change of the blue uncomplexed form with a maximum absorption at 635 nm to a red-violet toluidine blue–PPVS complex with maximum absorption at 530 nm [72][28]. The metachromatic action of cationic dyes is particularly helpful in determining the charges of biopolymers and proteins [73][29]. The specific interaction of the polymerized cationic dye azure A and the biological polyelectrolyte DNA was utilized to detect and discriminate DNA damage [74][30]. The electrostatic forces and the difference in the negative charges on the repetitive polysaccharide units of sodium heparin and sodium alginate led to different degrees of metachromasy induced in the cationic dyes azure B and toluidine blue [75,76][31][32]. While azure B bonded more strongly with sodium alginate, a more favorable interaction was observed between toluidine blue and sodium heparin. The interactions between poly(2-acrylamide-2-methyl-1-propanesulfonic acid) (PAMPS) and poly(diallyl dimethyl ammonium) chloride (PDDA) and the highly versatile cationic dyes methylene blue (MB) and methyl orange (MO) have been employed for the purification of colored wastewater by the polymer-enhanced ultrafiltration (PEUF) technique [6][33]. The maximum removal efficiency under optimal conditions (pH 6.0, initial MB and MO concentrations of 3.5 mg L−1 and 80 mg L−1, respectively) was reported to be 98% and 90% for MB and MO, respectively, together with an ultrafiltration membrane (molecular weight cut off value: 10 kDa). Polystyrene sulfonate (PSS) adsorbed on laterite soil (polymer modified laterite, PML) showed efficiency in the removal of methyl blue (83%) and crystal violet (92%) [77][34].
Dye removal remains one of the most challenging aspects of industrial waste management. The strong interactions between polyelectrolytes and dyes were pushed further by Cai et al. to develop a chitosan-based cationic polyelectrolyte microscope (CCQM) for the ultra-efficient removal of Congo red (1500 mg g−1) and methyl orange (MO, 179.4 mg g−1) [78][35].

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