Binary solvent crystallization

Chapters 6 and 7 dealt with solid state reactions in which the product separates the reactants spatially. For binary (or quasi-binary) systems, reactive growth is the only mode possible for an isothermal heterogeneous solid state reaction if local equilibrium prevails and phase transitions are disregarded. In ternary (and higher) systems, another reactive growth mode can occur. This is the internal reaction mode. The reaction product does not form at the contacting surfaces of the two reactants as discussed in Chapters 6 and 7, but instead forms within the interior of one of the reactants or within a solvent crystal. 

Dimethylnaphthalene concentrate contains significant amounts of 2,6-dimethylnaphthalene bound in a binary eutectic with 2,7-dimethylnaphthalene. This eutectic cannot be broken by distillation or solvent crystallization. A practical method for separating this eutectic mixture of 2,7-dimethylnaphthalene and 2,6-dimethylnaphthalene has been achieved. Selective adsorption of 2,7-dimethylnaphthalene from a dimethylnaphthalene concentrate is obtained with sodium type Y molecular sieves. 2,6-Dimethylnaphthalene then can be crystallized from the unadsorbed raffinate fraction. Separation factors of 6 to 8 are obtained, indicating the high selectivity of these particular molecular sieves for this adsorption. Previous work in this area achieved a separation factor of 2.7. A continuous method has been developed for adsorption and desorption of 2,7-dimethylnaphthalene. Toluene has been selected as the optimum desorbent. This process makes 2,7-dimethylnaphthalene potentially available. 

Solvent selection for the micronization of drugs is crucial because the molecules may be multifunctional and polar with a tendency toward hydrogen bonding, resulting in specific solute-solvent interactions (61). However, it is generally believed that the solute-antisolvent interaction is negligible compared with the solvent-antisolvent interaction for the S-F-V equilibrium in antisolvent crystallization systems. Thus the solvent-C02 interaction is solely responsible for the reduction of solvent power of the mixed solvent. Accordingly, the PMVF of solvent in a binary (solvent-antisolvent) mixture depicts the solute mole fraction in the ternary (solute-solvent-antisolvent) liquid phase. 

In this type of batch crystallization, a solute is crystallized from a primary solvent by the addition of a second solvent (antisolvent) in which the solute is relatively insoluble. The antisolvent is miscible with the primary solvent and brings about a solubility decrease of the solute in the resulting binary solvent mixture. 

The major advantage of the anti-solvent crystallization is that the process can be carried out at the ambient temperature, which—aside from the convenience and economical aspects—is of a paramount importance for heat-sensitive substances. The disadvantage of this process is that the binary solvent mixture must be subsequently separated in order to recover and recycle one or both solvents. Frequently, however, the added cost of the separation operation is fully absorbed by the valuable and expensive products, such as pharmaceuticals. 

Different polymorphic forms of solid benoxaprofen, nabilione and pseudopolymoiphic (i.e. including solvent) crystal forms of cefazolin were analyzed in 1985 by Bym et al. [33]. Solid state C NMR spectra of ciystal forms I and II of benoxaprofen were recorded a spectrum of the pharmaceutical granulation exhibited the signals of excipients but was identical to that of form II. The spectra of nabilone I and II allowed for a determination of which polymorph is present in a binary mixture. Hydrated crystal forms of cefazolin, the sesquihydrate, pentahydrate and monohydrate, gave different signal positions the amorphous form can be distinguished from crystalline ones because it showed broadened resonances. 

A mixture of two or more solvents will occasionally be found to possess the best properties for a particular crystallization purpose. Common binary solvent mixtures that have proved useful include alcohols with water, ketones, ethers, chlorinated hydrocarbons or benzene homologues, etc. and normal alkanes with chlorinated hydrocarbons or aromatic hydrocarbons. 

Both the cooling rate of the process and the thermodynamic properties of the binary system solvent—polymer (eg, relative melting temperature, phase separation temperature) influence scaffold porosity and micro- and macroarchitecture. For example, binary systems where the solvent crystallization temperature is lower than the liquid—liquid phase separation temperature (called liquid—liquid separation) result in a scaffold with micro- and nanometric features. Composite scaffolds manufactured via phase separation have been investigated mainly for bone TE apphcations. ... 

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or crystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binary mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirectly cooled, and/or solvent is evaporated to effect crystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt crystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining section of the apparatus. 

Hydrides of variable composition are not only formed with pure metals as solvents. A large number of the binary metal hydrides are non-stoichiometric compounds. Non-stoichiometric compounds are in general common for d,f and some p block metals in combination with soft anions such as sulfur, selenium and hydrogen, and also for somewhat harder anions like oxygen. Hard anions such as the halides, sulfates and nitrides form few non-stoichiometric compounds. Two factors are important the crystal structures must allow changes in composition, and the transition metal must have accessible oxidation states. These factors are partly related. FeO,... 

Eutectic point (Tc) A single point on a temperature concentration phase (or state) diagram for a binary solution (e.g., water and sugars or salts) where the solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at Te results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred (based on Fennema, 1996). 

Transition Region Considerations. The conductance of a binary system can be approached from the values of conductivity of the pure electrolyte one follows the variation of conductance as one adds water or other second component to the pure electrolyte. The same approach is useful for other electrochemical properties as well the e.m. f. and the anodic behaviour of light, active metals, for instance. The structure of water in this "transition region" (TR), and therefore its reactions, can be expected to be quite different from its structure and reactions, in dilute aqueous solutions. (The same is true in relation to other non-conducting solvents.) The molecular structure of any liquid can be assumed to be close to that of the crystals from which it is derived. The narrower is the temperature gap between the liquid and the solidus curve, the closer are the structures of liquid and solid. In the composition regions between the pure water and a eutectic point the structure of the liquid is basically like that of water between eutectic and the pure salt or its hydrates the structure is basically that of these compounds. At the eutectic point, the conductance-isotherm runs through a maximum and the viscosity-isotherm breaks. Examples are shown in (125). 

The successful conversion of graphite to diamond involves crystallizing the diamond from a liquid melt. The solvent most often used is nickel metal, or alloys of nickel with other ferrous metals. The reason for this success can be seen by referring to Figure 15.7, the binary (solid + liquid) phase diagram for (nickel + carbon).u8 We note from the figure that (Ni + C) forms a simple... 

Solid solutions with complete solid solubility, i.e., solid solubility over the entire range of the composition, are possible to form, but always of the substitutional kind. For a metallic binary solution to exhibit a complete solid solubility, for instance, both metals must have the same type of crystal structure, because it must be possible to replace, progressively, all the atoms of the initial solvent with solute atoms without causing a change in crystal structure. 

In many cases, atom size, crystal structure or other factors restrict the ease with which solute atoms can be dissolved in the solvent in the solid state. Thus it is much more common to find that solids are partly soluble in one another rather than be either completely soluble or completely insoluble. The following is an example of a phase diagram for a binary system which shows partial solid solubility ... 

We have carried out "direct" crystallization of aluminum hydride in one stage using interaction of binary metal hydride with aluminum chloride in the medium of ether-toluene at 60-100°C and using solvent distillation. In the reaction of LiH with AICI3, we achieved output of pure crystal A1H3 of hexagonal modification, which was close to quantitative. 

In many cases, there is partial solid solubility between the pure components of a binary system, as in the Pb-Sn phase diagram of Figure 11.5, for example. The solubility limits of one component in the other are given by solvus lines. Note that the solid solubility limits are not reciprocal. Lead will dissolve up to 18.3 percent Sn, but Sn will dissolve only up to 2.2 percent Pb. In Figure 11.5, there are two two-phase fields. Each is bounded by a distinct solvus and liquidus line, and the common sofidus line. One two-phase field consists of a mixmre of eutectic crystals and crystals containing Sn solute dissolved in Pb solvent. The other two-phase field consists of a mixture of eutectic crystals and crystals containing Pb solute dissolved in Sn solvent. 

An attractive method to produce single crystals (in the dimension range from pm to several nun) is the high-temperature solution (flux) method, becanse of its simphcity and the low temperature required. The elements are dissolved in the solvent metal (often Al) and subseqnently the solntion is slowly cooled to room temperature. A prerequisite is, of course, that the solubility of the used solvent in the desired boride is insignificant. The solubility of Al in most boron-rich binary borides has been found to be extremely small. Crystals prepared in this manner are suitable for measurement of physical properties, for instance, microhardness, electrical resistivity, and so on. 

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