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Fig. 8. Complex coacervation model of foam cement formation. (A) Macroanion (red)
and macrocation (blue), pH 5. Mg2+ and Ca2+ are pumped
in to neutralize excess polyanion charges. (B) Neutralization leads to phase
separation of the coacervate (CO) from the equilibrium solution (E). CO is an
enriched blend of anion, cation and Mg2+. Both phases are fluid
although CO is viscous. (C) As volume of CO phase increases and CO desolvation
proceeds, more vacuoles of E form within CO instead of diffusing to E phase.
(D) Upon secretion into seawater at pH 8, electrostatic interaction of
Mg2+ and Ca2+ with phosphate groups becomes ionic due to
low solubility, increasing the viscosity of CO. (E) Oxidation of Dopa to
quinones leads to cement solidification by cross-linking.
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