Adsorption Of Aminomethylphosphonic Acids On The Calcium Sulfate Dihydrate Crystal Surface
Authors: Liu Sung-Tsuen , Griffiths David W. 
Published: December 2007

Abstract
The adsorption of four phosphonic acids an the calcium sulfate dehydrate crystal surface has been studied as a function of pH. Tendencies toward adsorption increase as the pH of the solution is raised from 4 to 7, then level off or fall off as the pH is further increased from 7 to 9. Among the phosphonic acids studied, surface affinity is well correlated to the performance of these same materials as calcium sulfate dehydrate crystal growth inhibitors. Yet at surface saturation the amount of inhibitor adsorbed is sufficient to cover only one percent or less of the total available surface area. These findings are in accord with the idea that adsorption of inhibitor at active growth sites is responsible for inhibition.

Introduction
It has been known for many years that additives or impurities alter both the rate of crystallization and the morphology of crystals formed in solution. The now widely accepted use of a trace amount of an additive such as a polymeric carboxylic acid, a polyphosphoric acid, an ester of orthophosphoric acid, polyphosphoric acid, an ester of orthophosphoric acid, or any one of several phosphonic acids to control an industrial scale problem is an example of the intentional use of an additive to inhibit crystal growth. Although the exact mechanism of inhibitor action is not yet understood, adsorption of the inhibitor on newly formed nuclei, on existing surfaces, or in general on any active crystal growth site is often proposed as a primary step in the overall inhibition proposed as a primary step in the overall inhibition process. Supporting this proposal are a number of process. Supporting this proposal are a number of studies showing, for example, that polyphosphoric acids that are active as inhibitors are adsorbed on a variety of crystalline materials including calcium carbonate, strontium sulfate, calcium sulfate dehydrate, and calcium oxalate. Additionally, Crawford and Smith found that polyacrylic acid is incorporated into calcium sulfate dehydrate crystals grown in the presence of this additive, and Nestler, in a deliberate study of the adsorption of polyacrylic acid on the calcium sulfate dehydrate crystal surface, showed that tendencies toward adsorption are strong, particularly with increasing degrees of polyacrylic acid deprotonation.
As a class, the phosphonic acids have received relatively little attention, at least from the point of view of their adsorption tendencies. Exceptions are several studies concerned with the application of these materials by a squeeze treatment technique showing that adsorption occurs readily on the surface of silica sand. Another exception, and one that is more closely related to the process of scale inhibition, is a study by Weintritt and Cowan showing that nitrilotri (methylenephosphcnic acid) is rapidly adsorbed on the barium sulfate crystal surface. In the same system, Leung and Nancollas discovered that at an inhibitor concentration sufficient to prevent crystal growth, only a small fraction of the total available surface area is occupied by the inhibitor. The latter finding was interpreted to indicate that adsorption occurs selectively on active growth sites.
The present study is concerned with the adsorption of phosphonic acids, particularly -aminomethylphosphonic acids, on the calcium sulfate dehydrate crystal surface. It is specifically concerned with the influence of pH on adsorption and the correlation between surface affinity and inhibition activity. Four phosphonic acids of contrasting activity as calcium sulfate dehydrate crystal growth inhibitors have been studied. They are, in decreasing order of effectiveness: hexamethylenediamine- N,N,N',N'-tetra(methylenephosphonic acid); ethylenediamine- N,N,N'N'-tetra(methylenephosphonic acid); ethylenediamine- N,N di(methylenephosphonic acid); and ethylenediamine- N,N'-dimethyl-N,N'-di(methylenephosphonic acid). Following the abbreviated scheme of nomenclature used in the preceding article, these materials from this point on will be designated as HMDP4, EDP4, EDP2, and point on will be designated as HMDP4, EDP4, EDP2, and EDMe2P2, respectively.