In this study, the effect of phosphonate additives on the crystallization of calcium sulfate dihydrate (CaSO 4 3 2H 2 O, gypsum) has been investigated in aqueous solutions. Ethylenediamine-tetrakis(methylenephosphonic acid) (EDTMP), hexamethylenediamine-tetrakis(methylenephosphonic acid) (HDTMP), octamethylenediamine-tetrakis-(methylenephosphonic acid) (ODTMP), and dodecamethylenediamine-tetrakis(methylenephosphonic acid) (DDTMP) have been used as additives. It was found that they are very effective retardants for the crystallization of calcium sulfate dihydrate. The inhibition efficiency is directly proportional to the number of methylene groups in the organic chain that connects the aminobis(methylenephosphonate) moieties. The degree of inhibition of crystallization was measured as an increase in induction time and reduction in crystallization rate. Particle size and crystal morphology were determined with a particle-sizer and scanning electron microscopy. According to experimental results, phosphonate additives tested in this study are very effective retardants for the formation of calcium sulfate dihydrate scale. The crystal structure of [Ca(EDTMP)(H 2 O) 2 ] 3 H 2 O is also reported. This is a one-dimensional coordination polymer in which EDTMP acts as both a bidentate chelate and a bridge for Ca 2þ centers.

Barium sulfate is a common scale in oil production installations that is treated and controlled with phosphonate inhibitors. A fundamental understanding of how these inhibitors operate, however, is only slowly emerging. In this paper, we investigate the effect on barium sulfate crystallization of two very similar phosphonate molecules that only differ in their backbone spacing, ethylenediamine-N,N,N′,N′-tetra(methylenephosphonic acid) (EDTMP) and hexamethylenediamine-N,N,N′,N′-tetra(methylenephosphonic acid) (HDTMP). It was found that the inhibitory efficacy of the organic molecules depends on their structural differences but also on the presence of other cations such as Zn 2+ . It appears that both stereochemical considerations and complexation strength differences between the two phosphonate additives result in different inhibitory powers. In the presence of zinc cations and EDTMP, it is found that inhibition is related to the concentration of uncomplexed ("free") organic.

The structures of various layered calcium tetraphosphonates (CaH6DTMP; H8DTMP=hexamethylenediamine tetrakis(methylenephosphonic acid)), have been determined. Starting from CaH6DTMP.2H2O, thermal treatment and subsequent exposure to NH3 and/or H2O vapors led to four new compounds that showed high storage capacity of guest species between the layers (up to ten H2O/NH3 molecules) and a maximum volume increase of 55 %. The basic building block for these phosphonates consists of an eight-membered ring chelating Ca2+ through two phoshonate groups, and the organic ligand is located within the layers, which are held together by hydrogen bonds. The structural analysis revealed that the uptake/removal of guest species (H2O and NH3) induces significant changes in the framework not only by changing the interlayer distances but also through important conformational changes of the organic ligand. An anisotropic breathing motion could be quantified by the changes of the unit-cell dimensions and ligand arrangements in four crystalline derivatives. Complete characterization revealed the existence of interconversion reactions between the different phases upon gas uptake and release. The observed behavior represents, to the best of our knowledge, the first example of a breathing-like mechanism in metal phosphonates that possess a 2D topology.

Syntheses and structures of alkaline earth metal ions and EDTMP, ethylenediamine-tetrakis(methylenephosphonate), are reported. The isostructural Ca 2+ and Sr 2+ analogs have 1D topologies, with EDTMP acting as both chelating and bridging ligand. The M-EDTMP compounds act as Fe-oxide removers from corroded surfaces.