Habit Modifiers

Once the ratio of sodium ultraphosphate to calcium polyphosphate had been more or less standardized, the game of adding crystal habit modifiers was undertaken in an attempt to determine the influence of modifiers. No reliable generalizations could ever be stated as to how foreign ions influenced these systems, but 

If the objective is to prepare the safest mineral fiber that it is theoretically possible to manufacture, no trace of a toxic substance can be tolerated There was never any doubt in the author s mind that only food grade raw material could be used in this process, although the question of using purified wet phosphoric acid, derived from fertilizers, was raised from time to time. In my judgement, the best commercially available purified wet phosphoric acid is inferior to furnace acid and, although it probably could be used, it would be done with some loss of a safety margin that has been paramount throughout the history of this project. Safety is the hallmark of phosphate fibers. When safety is not an issue, some other fibers have superior properties. 

In the author s attempts to find substances that would modify properties of calcium polyphosphate fibers, additions of magnesium oxide or magnesium phosphates seemed to have the most favorable influence, but results were erratic. It seemed that sometimes there were vast improvements and, at other times, no influence could be seen. Answers to this behavior may have been similar to Mr. Ngo s discovery that two crystallizations were required to prepared outstanding fibers of [NaMg(P03)3]rt. These melts were first crystallized, remelted, and then crystallized again to yield some of the more beautiful fibers made in this work. Unfortunately, the doping of calcium phosphate remains a mystery. This project was terminated before it was completely understood. 

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TABLE 18-4 Some Impurities Known to Be Habit Modifiers... 

The feed preparation stage e.g. by mineral extraetion and evaporation, ehem-ieal reaetion ete. ean give rise to both dissolved and suspended solid impurities, either of whieh may affeet the erystallization step. Removal of suspended solids, e.g. by filtration, is usually the easier proeess. Dissolved impurities ean have by far the more pronouneed effeet, however, and may have to be removed e.g. by ehemieal means or by adsorption. Sueh impurities may, of eourse, aetually be benefieial to the proeess by indueing nueleation, habit modifie-ation ete. 

Exploiting ATRP as an enabling technology, we have recently synthesised a wide range of new, controlled-structure copolymers. These include (1) branched analogues of Pluronic non-ionic surfactants (2) schizophrenic polymeric surfactants which can form two types of micelles in aqueous solution (3) novel sulfate-based copolymers for use as crystal habit modifiers (4) zwitterionic diblock copolymers, which may prove to be interesting pigment dispersants. Each of these systems is discussed in turn below. 

Synthesis of Sulfate-based Block Copolymers as Crystal Habit Modifiers for BaS04... 

E.I. duPont de Nemours and Company Thickened Aqueous Inorganic Oxidizer Salt Blasting Compositions Containing Gas Bubbles and a Crystal Habit Modifier and Method of Preparation... 

Hindle, S.A. 1996. A Molecular Modelling Approach to the Prediction of the Effects of Habit Modifiers in Crystallisation in Foods. PhD Thesis, University of Leeds. 

Whetstone, J. Crystal habit modification of inorganic salts with dyes. II. Relationship between the structure of crystals and habit-modifying dyes. Trans. Faraday Soc. 1955, 51, 1142-1153. 

Highly charged and/or complex ions appear to have the greatest effect upon crystal habit. Thus, Al, Cr, Fe +, and Fe(CN)6" are commonly used in a variety of applications to modify habit. A number of excellent monographs have been published. This reference also includes a number of sources that provide further reviews that discuss the impact of trace concentrations upon crystallization. In Table 2, the range of initial morphologies, habit modifiers, and the crystal forms that can result from trace concentrations of several different trace modifiers are shown. 

Commodity Normal habit Habit modifier New habit... 

Buckley 1951), especially for inorganic systems. Known habit modifiers for inorganic materials are listed in Table 2.5. In recent years great strides have been made in developing a quantitative understanding of habit modification. More information relating to habit modification can be found in Chapter 3 of this volume, which deals with impurity crystal interactions. 

Substance Normal Habit Habit Modifier Changed Habit... 

Another example of control of crystal habit and size of cefinatilen hydrochloride hydrate (3, Scheme 24.2) with a habit modifier, an impurity, has been reported recently. It has been demonstrated that concentration of a related substance, a diphenyl methyl substitution at the N3 position in the triazole (4) (formed during deprotection of trityl and diphenyl methyl group (5) with AICI3 and anisole in methylene chloride at elevated temperature) is critical for formation of the particular crystal habit. 

The existing method for preparing crystalline zirconium phosphate is firstly to precipitate the amorphous form from a mixture of phosphonic acid and zirconyl chloride and secondly to reflux the amorphous form in 12M phosphoric acid for several days after washing the amorphous form free of chloride. This gives the a-form. Not only is this a time consuming process, but the amorphous form is difficult to filter. We aimed to develop a one step process to the crystalline material by using crystal habit modifiers. 

TTie above observations obey the Gay-Lussac Law which states that a compound that crystallises from a highly super-saturated solution first is the most soluble (from a saturated solution first it is the least soluble). In the above case, amorphous (2) is more soluble than (1). By choice of crystal habit modifier, it is possible to preferentially precipitate one form of the phosphate. 

The addition of crystal habit modifiers to the original phosphoric acid/zirconyl chloride solution gave catalysts which were mainly crystallinein less than one hour but still had some amorphous character (Figure 8). 

Table 5 - Phenol Hydroxylation Using Zirconium Phosphates Made with Crystal Habit Modifiers...

Crystal Habit Modifier % Phenol Conversion Selectivity Dihydroxy Ratio CAT.HO... 

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