The impact of P/Ca molar ratio on physicochemical and release properties of calcium polyphosphate coacervates
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This paper reports on the synthesis and characterization of gel-like calcium polyphosphate coacervates (Ca–PPCs) with three different [P]:[Ca] ratios as new matrixes for drug release. Caffeine and chlorhexidine were used as model drugs, showing encapsulation efficiency of up to 95.4% and 70.4%, respectively. Based on the correlation coefficient (r2) values, different in vitro release behaviors were determined. While the caffeine release followed the Weibull model, the chlorhexidine release was best described by the Korsmeyer-Peppas model. The caffeine-loaded coacervate sample with the highest Ca2+ concentration (CAFPCa0.5) showed the highest incorporation and a total drug release within 720 min. The Ca-PPC-drug interactions were evaluated by infrared and Raman spectroscopies. The Ca2+ concentration, drug molecule size and intermolecular drug/matrix interactions determined the incorporation and release of drugs in the polyphosphate coacervates. High calcium concentration led to a Ca-PPC with small and tight “cage-like” structure that better incorporated caffeine molecules because of their 4.3 times smaller size compared to chlorhexidine. In addition, caffeine interacted through van der Waals' forces with polyphosphate and its release from the coacervates was most likely to occur in relation to chlorhexidine, which interacted with the coacervates through hydrogen bonding. Thus, the calcium polyphosphate coacervates loaded with the proposed drugs can be competitive and promising as drug delivery systems.