Poly(allylamine)/tripolyphosphate coacervates for encapsulation and long-term release of cetylpyridinium chloride

Abstract

Complex coacervates formed through the association of poly(allylamine hydrochloride) (PAH) with pentavalent tripolyphosphate ions (TPP) have low solute permeabilities and enable multi-month release of small, water-soluble molecules. To this end, we have recently shown PAH/TPP coacervates to be highly effective in encapsulating and slowly releasing bioactive anionic surfactants. Here, we extend this work to the encapsulation and release of cationic surfactants and exploring the effects of the PAH/TPP/surfactant mixing order. Using cetylpyridinium chloride (CPC) as a model cationic amphiphile, we show that (like their anionic counterparts) cationic surfactants can be encapsulated in PAH/TPP coacervates in high concentrations (exceeding 20% of the coacervate weight). The high loadings evidently reflect the CPC/TPP association (which concentrates the CPC within the forming coacervates) and are maximized at high CPC concentrations, low PAH/TPP concentrations, and when the CPC is initially mixed with the TPP. This CPC encapsulation significantly affects the PAH/TPP coacervate properties (likely due to the considerable CPC/TPP association strength) and makes the coacervates opaquer and higher swelling, but less adhesive and less susceptible to spreading from their application sites. Besides increasing with their CPC content, the coacervate swelling varies with the PAH/TPP/CPC mixing order and is greatest when the CPC and TPP are mixed first. Once loaded, the CPC can be released over multiple months, with some control over the release rates (which, initially, are on the order of 10 µg/d, but tend to gradually fall below ~1 µg/d within the first 1 – 4 weeks). These findings suggest that PAH/TPP coacervates may be suitable for the low-dosage sustained release of cationic surfactants in applications where the release media has a low volume and is not frequently exchanged. More broadly, this work also indicates that: (1) effective encapsulation of active small molecules within polyelectrolyte/small multivalent counterion coacervates does not (when the payload associates with the small multivalent ions) require payload/polymer affinity; and (2) nonequilibrium effects during the formation of these payload-bearing coacervates can have long-term consequences on their properties and performance.