Abstract
The polyelectrolyte poly(sodium 4-styrenesulfonate) undergoes aromatic–aromatic interaction with the drug chlorpheniramine, which acts as an aromatic counterion. In this work, we show that an increase in the concentration in the dilute and semidilute regimes of a complex polyelectrolyte/drug 2:1 produces the increasing confinement of the drug in hydrophobic domains, with implications in single chain thermodynamic behavior. Diafiltration analysis at polymer concentrations between 0.5 and 2.5 mM show an increase in the fraction of the aromatic counterion irreversibly bound to the polyelectrolyte, as well as a decrease in the electrostatic reversible interaction forces with the remaining fraction of drug molecules as the total concentration of the system increases. Synchrotron-SAXS results performed in the semidilute regimes show a fractal chain conformation pattern with a fractal dimension of 1.7, similar to uncharged polymers. Interestingly, static and fractal correlation lengths increase with increasing complex concentration, due to the increase in the amount of the confined drug. Nanoprecipitates are found in the range of 30–40 mM, and macroprecipitates are found at a higher system concentration. A model of molecular complexation between the two species is proposed as the total concentration increases, which involves ion pair formation and aggregation, producing increasingly confined aromatic counterions in hydrophobic domains, as well as a decreasing number of charged polymer segments at the hydrophobic/hydrophilic interphase. All of these features are of pivotal importance to the general knowledge of polyelectrolytes, with implications both in fundamental knowledge and potential technological applications considering aromatic-aromatic binding between aromatic polyelectrolytes and aromatic counterions, such as in the production of pharmaceutical formulations.
Highlights
At polymer concentrations between 0.5 and 2.5 mM, the strength of the PSSn/CPMn/2 reversible interactions given by j and the irreversibly bound fraction of chlorpheniramine maleate (CPM) bound to poly(sodium 4-styrenesulfonate) (PSS) (u) are directly correlated, showing a linear tendency of positive slope, evidenced upon increasing the system total concentration
Lower affinity is found for CPM and the non-aromatic polyelectrolyte PAA, j and KdissCPM/PAA values showed high standard deviations, and correlations with u could not be found, highlighting the role of aromatic-aromatic interactions in the system behavior
Synchrotron-SAXS results display an outstanding increase in characteristic chain correlation lengths, static screening lengths ξ1 in the range 0.5–1.5 nm, and correlation lengths ξ2 in the range of 1–4 nm, following an aggregation pattern with a fractal dimension of 1.7
Summary
We have studied the interactions between aromatic polyelectrolytes, such as poly(sodium 4-styrenesulfonate) (PSS), and low molecular-weight aromatic species (LMWS) acting as counterions, among which we can find xanthene dyes [1,2,3,4,5,6,7], redox-active tetrazolium salts [8,9,10,11], and different drugs [12,13,14,15,16]. Despite the different systems containing polymers, whose conformation properties in solution have been studied, there is no report in the literature, to the best of our knowledge, concerning th3eofb1e8havior of aromatic polyelectrolyte chains subjected to aromatic-aromatic interactions with aromatic low molecular-weight counterions as a function of the concentration. The significance of the correlation of the independent variables u and j (and KdissCPM/WSP) was evaluated by the Pearson correlation coefficient method applied to the experimental data [69]
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