METAL-PHOSPHONATE CHEMISTRY: SYNTHESIS, CRYSTAL STRUCTURE OF CALCIUM-AMINOTRIS -(METHYLENE PHOSPHONATE) AND INHIBITION OF CACO₃ CRYSTAL GROWTH

Abstract

Organic (poly)phosphonates are additives in water treatment that find broad use as mineral scale and corrosion inhibitors for a plethora of industrial applications. The phenomenon of precipitation of scale inhibitors has been studied intensively. This study reports the synthesis, characterization, and crystal structure of a Ca salt of AMP (AMP = Amino-tris-Methylene Phosphonate). Ca-AMP crystals are monoclinic, space group P 2 ₁ n with unit cell dimensions, a = 11.3382(5) Å, b = 8.4555(4) Å, c = 15.5254(7) Å, β = 90.6551 ^o , V = 1488.33(12) Å ³ and Z = 4. The structure is polymeric due to chelation of multiple Ca ions by AMP. The Ca ^{2 +} center is slightly distorted octahedral and is surrounded by five phosphonate oxygens and a water molecule. CaO(P) bond lengths range from 2.2924(14) to 2.3356(14) Å. The CaO(H ₂ O) bond distance is 2.3693(17). Each phosphonate group is monodeprotonated, and the nitrogen atom is protonated. CaCO ₃ inhibition and crystal modification by AMP are also reported, together with synergistic effects with polyacrylate-based terpolymers.

Keywords:

• AMP
• calcium carbonate
• dispersant polymers
• phosphonate
• scale formation
• scale inhibition

The authors wish to acknowledge Dr. Peter White for collecting the X-ray data and Nalco Chemical Company for supplying samples of polymers A and B.

Notes

^a R = Σ F_o-F_cΣ F_o.

^b R_w = [Σ (w F_o-F_c)²/Σw(F_o)²]^1/2.

^c GoF = [Σw(F_o-F_c)²/(no. of reflections—no. of parameters)].^1/2

^a s, strong; m, medium; w, weak; sh, shoulder.