Zwitterionic (ZI) polymers are a
promising class of fouling-resistant
and antiscaling coating materials. However, their mineral scaling
prevention mechanism is still not well understood, which hampers the
applications of ZI polymers. To fill this knowledge gap, we investigated
the scaling mechanisms of different types of ZI amphiphilic copolymers
(ZACs) synthesized from diverse combinations of zwitterion monomers
and hydrophobic monomers using free-radical polymerization (FRP).
The selected ZACs included poly(trifluoroethyl methacrylate-random-sulfobetaine methacrylate) (PTFEMA-r-SBMA, shortened as PT:SBMA), poly(trifluoroethyl methacrylate-random-2-methacryloyloxyethyl phosphorylcholine) (PTFEMA-r-MPC, shortened as PT:MPC), poly(methyl methacrylate-random-sulfobetaine methacrylate) (PMMA-r-SBMA, shortened as PM:SBMA), and poly(methyl methacrylate-random-2-methacryloyloxyethyl phosphorylcholine) (PMMA-r-MPC, shortened as PM:MPC). Coatings of these polymers
were tested for calcium sulfate (CaSO4) scaling in simulated
brackish water solutions with different saturation indices, defined
as the logarithm of the actual dissolved ion product (Q, i.e., calcium and sulfate ions) normalized by the solubility constant
of gypsum (K
sp). These experimental results
indicated that at high saturation indices (SI), the scaling deposition
on ZAC coatings mainly depended on the surface energy. Therefore,
PT:SBMA coatings, which had the lowest surface energy, exhibited better-scaling
resistance. At lower SI, the hydrophilicity of the ZAC coatings was
the dominant factor. Since ZAC coatings containing MPC have higher
surface hydrophilicity, they might have a relatively slower scale-forming
ion adsorption rate compared to SBMA containing ZACs, thus not markedly
enriching the scale-forming ion near MPC coating surfaces and hence
inhibiting heterogeneous CaSO4 precipitates. The results
demonstrated that the selection of ZAC coatings should consider the
feed water chemistry and focus on the optimal prevention of scaling
in addition to organic fouling.