Precise Construction of Polyaspartic Acids with Α- or Β-Configuration: Study into Configuration Effect on Calcium Scale Inhibition

Abstract

Polyaspartic acid (PASP) has received considerable attention in the industrial water treatment field as an environmentally friendly and promising calcium scale inhibitor. Although previous studies have shown that the calcium scale inhibition efficiency of PASP can be significantly influenced by the introduction of side chains with different functional groups or varying molecular weights, the lack of an efficient method for synthesizing PASP with controlled α or β configuration hinders the investigation of the configuration's effect on calcium scale inhibition. In this work, a series of PASPs with controllable α or β configuration and degrees of polymerization (DP) have been obtained through ring-opening polymerization of α- or β-N-carboxyanhydrides (NCA), followed by selective deprotection with hydrobromic acid, enabling to evalaute the configuration effect on the inhibition of CaCO3 and CaSO4 scale for the first time. Static scale inhibition experiments showed that both α-PASPs and β-PASPs exhibit optimal CaCO3 scale inhibition performance at a DP of around 30, with α-PASPs being more effective than β-PASPs and normal PASP (α: β = 3:7) with the same DP. As a comparision, while both α-PASPs and β-PASPs exhibited optimal CaSO4 scale inhibition performance at a DP of around 20, a nearly 100% inhibition efficiency was observed for β-PASPs at a lower concentration (3 mg/L) than that of α-PASPs and normal PASP (α: β = 3:7) with the same DP again. Detailed mechanistic studies revealed that PASPs with specific configuration exhibited a distinct lattice distortion effect in CaCO3 scale inhibition, with the inhibition ability following the order α-PASPs > β-PASPs and normal PASP (α: β = 3:7) with the same DP. In CaSO4 scale inhibition, the threshold effect was dominant as no lattice distortion was observed, with the inhibition ability following the order α-PASPs and normal PASP (α: β = 3:7) < β-PASPs with the same DP of 20. The significant configuration effect of PASP on calcium scale inhibition could be beneficial for developing other more efficient polymeric antiscaling inhibitors.

Keywords: polyaspartic acid, Precise construction, α- or β-Configuration, Configuration effect, Antiscaling mechanism