Eutectic Behaviour

During the early stages of process development it is very unlikely that relevant impurities will have been synthesized in sufficient quantities that a thorough analysis of their eutectic behaviour can be performed. Eutectics should be investigated when possible and are particularly relevant in the purification of stereo-isomers and in classical resolution using diastereomeric salts [5, 24, 25]. 

Some examples of mixtures with eutectic behaviours are silicon/ aluminium and silicon dioxide / aluminium oxide. 

A typical pharmaceutical formulation will serve as model, frozen aqueous solutions that are contained in loosely stoppered glass vials. They stand directly on the freeze-drier shelves. Such a model appears deceptively simple but is in fact subject to considerable complexities that bedevil the modelling of optimum processing conditions and the estimation of a realistic drying cycle time. As emphasised in earlier chapters, for the case where the product does not display eutectic behaviour, it is imperative to freeze the solution to below Tg, in order to ensure maximal freeze-concentration and minimise deleterious chemical reactions that have been shown to occur at high rates in part frozen, supersaturated mixtures.By the same token, the temperature during the removal of ice by sublimation must on no account be allowed to rise above T at... 

Other more complex systems may be encountered, however, including minimum melting solid solutions, eutectics with compound formation, etc., but in a comprehensive survey of binary organic mixtures, Matsuoka (1991) estimated that over 50% exhibited simple eutectic behaviour, about 25% formed inter-molecular compounds and about 10% formed solid solutions of one kind or another. Interestingly, fewer than 2% formed simple solid solutions Figure 8.15b). 

Reports of kinetic studies do not always include an explicit statement as to whether or not the reactant melted during reaction or, indeed, if this possibility was investigated or even considered (cf. p. 1). This aspect of behaviour is important in assessing the mechanistic implications of any data since reactions in a homogeneous melt, perhaps a eutectic, usually proceed more rapidly than in a crystalline solid. It is accepted that the detection of partial or localized melting can be experimentally difficult, but, in the absence of relevant information, it is frequently impossible to decide whether a reported reaction proceeds in the solid phase. 

The concept of a characteristic reaction temperature must, therefore, be accepted with considerable reservation and as being of doubtful value since the reactivity of a crystalline material cannot readily be related to other properties of the solid. Such behaviour may at best point towards the possible occurrence of common controlling factors in the reaction, perhaps related to the onset of mobility, e.g. melting of one component or eutectic formation, onset of surface migration or commencement of bulk migration in a barrier phase. These possibilities should be investigated in detail before a mechanism can be formulated for any particular chemical change. 

Claes P, Dewilde Y, Glibert J (1988) Chemical and electrochemical behaviour in molten alkali hydroxides Part II. Electrochemistry of chalcogenide ions in the molten NaOH + KOH (49 moI%) eutectic mixture. J Electroanal Chem 250 327-339... 

Weaver MJ, Inman D (1975) The sulphur-sulphide electrode in molten salts-1 Chronopotentiometric behaviour in lithium chloride-potassium chloride eutectic. Electrochim Acta 20 929-936... 

Positive deviations from ideal behaviour for the solid solution give rise to a miscibility gap in the solid state at low temperatures, as evident in Figures 4.10(a)-(c). Combined with an ideal liquid or negative deviation from ideal behaviour in the liquid state, simple eutectic systems result, as exemplified in Figures 4.10(a) and (b). Positive deviation from ideal behaviour in both solutions may result in a phase diagram like that shown in Figure 4.10(c). 

The Te-S system is peculiar it is a simple eutectic-type diagram and shows (like an island completely surrounded by the single-phase field of the liquid) a small oval insolubility region situated between —37 and 41.5 at.% S and between two critical temperatures (upper Tc = 740°C and lower Tc = 690°C). This behaviour (often observed for instance in organic systems) among the different pairs of elements has been described only for Te-S. 

Temperature, pressure, and concentration can affect phase equilibria in a two-component or binary system, although the effect of pressure is usually negligible and data can be shown on a two-dimensional temperature-concentration plot. Three basic types of binary system — eutectics, solid solutions, and systems with compound formation—are considered and, although the terminology used is specific to melt systems, the types of behaviour described may also be exhibited by aqueous solutions of salts, since, as Mullin 3-1 points out, there is no fundamental difference in behaviour between a melt and a solution. 

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). 

Figure5.4.1) Contact angle versus time for a eutectic (Ag-Cu) dropon polycrystalline W at 900°C in a high vacuum. Before the experiment, the W substrate was heat-treated in high vacuum at 1100°C for 2 h. Despite this treatment, the surface remained oxidised and a slow spreading, controlled by W deoxidation, was observed. 2) The same without prior heat treatment of W. In this case segregation of O at the W surface, by fast grain-boundary diffusion, prevents deoxidation of the substrate, resulting in non-wetting behaviour. From Lorrain (1996).

Such excellent or at least adequate capillary behaviour is also typical of the process variant known as eutectic bonding in which the transient creation of a liquid phase is caused by the interdiffusion of two chemically different metal alloy component materials. In the laboratory variant process known as partial transient liquid phase bonding, (Shalz et al. 1992), a coated interlayer is used for ceramic-ceramic or ceramic-metal joints. In this process the interlayer is a ductile metal or alloy whose surface is coated with a thin layer of a lower melting temperature metal or alloy, for example Ni-20Cr coated with 2 microns of Au. The bonding temperature is chosen so that only the coating melts and the ductility of the interlayer helps to accommodate mismatches in the coefficient of thermal expansion of the component materials. 

A third compilation intended to be devoted to polymorphic materials is that of Kuhnert-Brandstatter (1971). The body of this book is an identification table for hot stage studies of pharmaceutical materials (see Sections 4.2 and 7.2), in which materials are arranged by increasing melting point, with eutectic data for mixtures with azobenzene and benzil. There is considerable descriptive detail on the melting behaviour and identification and description of polymorphic forms, albeit only microscopic determinations, for approximately 1000 pharmaceutically important... 

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