Comprehensive Study of the Chelation and Coacervation of Alkaline Earth Metals in the Presence of Sodium Polyphosphate Solution

Abstract

The effect of chelation of three alkaline earth metals (Ca, Sr, and Ba) by polyphosphates on the pH and viscosity of the solution is examined and correlated to the phosphate glass properties. Also, the impact of the polyphosphate average degree of polymerization (D(p)) as well as the type and amount of chelated divalent cation on the degradation rate of the chains is studied. Subsequently, the number of divalent cations required for polyphosphate chain agglomeration to form a coacervate, and the resulting composition of these coacervates, was investigated. A decrease in polyphosphate solution pH during chelation was routinely obtained, with a sudden shift in the rate of pH drop occurring around a divalent cation/phosphorus molar ratio of 0.18. Longer chains or cations with a smaller ionic radius accelerated the rate of D(p) reduction. The number of divalent cations required for coacervation depends on different variables such as the polyphosphate concentration, the D(p), and the type of divalent cation. The formed coacervate retains the D(p) of polyphosphate originally used for coacervation, and the resulting Ca/P molar ratio depends largely on the amount of calcium being used during coacervation. Overall, this article helps one to understand the coacervation of polyphosphates in order to exploit their potential as a biomaterial.

Degradation and hemostatic properties of polyphosphate coacervates

“…Another mechanism of degradation is intramolecular back-biting resulting in phosphate rings. Trimetaphosphate has been previously reported as one of the major byproducts of polyphosphate degradation in solution [4]. For our study, phosphate rings were not detected in significant amounts inside the degraded coacervates.…”

“…M II catalyze this reaction by neutralizing the charge of the chain, making phosphorus more susceptible to nucleophilic attack by H 2 O or OH À . In the absence of M II , the ability of water to attack the reactive center is very limited and as a result NaPP chains are highly stable in aqueous solutions [4]. Another mechanism of degradation is intramolecular back-biting resulting in phosphate rings.…”

“…The phase separation of polyelectrolytes in the presence of M II is governed principally by hydrophobic and electrostatic forces [3]. In the case of NaPP, the phase separation is induced by the screening of the negative charges on polyphosphate chains by condensed M II and also formation of hydrophobic M II -phosphate complexes [4]. Experimentally we have observed that polyphosphates coacervates rather than flocculates can still be obtained if some of the Ca 2+ is substituted with Sr 2+ or Ba 2+ , but their physical properties, including rheology, are profoundly different from their Ca 2+ -only counterparts [5].…”

Impact of trivalent ions on the stability and cohesion of calcium polyphosphate coacervates for embolization applications

“…Polyphosphate (PP) is a linear inorganic polymer that easily chelates metal ions and has been synthesized with a wide range of polymer length or degree of polymerization ( D p ) . Highly soluble sodium PP (NaPP) will readily undergo ion exchange with divalent and trivalent ions, separating out of solution to create a PP rich phase called a coacervate . This gel forming ability has recently been of interest for drug delivery and as an in situ setting liquid embolic agent whereby an embolic mass forms when a preloaded PP solution comes in contact with a calcium chloride solution .…”

“…Previous work in our group sought to optimize PP for use as a liquid radiopaque embolic agent by tailoring the alkali earth metals used and the degree of polymerization . Barium and strontium substitution for calcium up to 15 mol % was shown to increase the viscosity and improve the cytocompatability of the resulting coacervates . The change in properties with larger and heavier alkali earth elements may have been due to crosslinks formed by the metal ions, whereas calcium simply charge balanced the phosphate ions .…”