An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall.

  • Groth, P. (Paul), 1843-1927.
Date:
1906
    curves II and I in turn, first in the region of the meta¬ stable condition and then in that of the labile one. On reaching some point v in this region, a solid form must appear spontaneously; this, according to the above rule, will not be the form posses¬ sing the curve /, although it is the most stable at this tem¬ perature, but the form //will result, because it is the most adjacent. If II is then in the metastable region as regards /, there the matter will rest, and no further transformation will take place unless the product comes in contact with some of form I. At the same time there is the possibility that II may be in the labile region as regards 1; in that case a further spontaneous transformation will set in, and finally the most stable form / will be reached.” Fig. i represents the case as it exists with sulphur and many other dimorphous substances. Below the trans¬ formation temperature the metastable modification II possesses the greater solubility, and likewise, should it be volatile, the higher vapour pressure, p ; at the point the value of p is the same for both modifications I and //, so that the two are here in equilibrium ; beyond u the modi¬ fication /, stable at lower temperatures, becomes the metastable form with higher vapour pressure, while the previously metastable form II is now the stable one with the lower value for /, consequently the melting point i.e. the temperature of equilibrium between modification I and the fused substance, is lower than ;>?2, the melting point of the second modification ; thus, for sulphur, sl is 113-5°, and is 119-5° (see Page J8)- The case represented in Fig. 2 is also conceivable, how¬ ever, in which the fusion curve, Y, for the substance
    these circumstances the stable modification / will melt at sv without transformation being possible ; the metastable form //, on the other hand, will melt at a lower temperature s2, assuming that transforma¬ tion has not previously been induced by contact with /. Substances of this kind will therefore possess two crys¬ tallised modifications, one of which is stable at all tem¬ peratures below its melting point and never exhibits transformation into the other, whilst the second modification has a lower melting point and is metastable at all temperatures below that. Benzo- phenone, mentioned on page 23, is such a substance ; for it, consequently, transformation is not a reversible process, as it is for sulphur, etc., but can take place only in one sense, and at any temperature. Lehmann, who, by his researches with the crystallisation microscope (page 4), proved the existence of a considerable number of substances behaving in this way, called those of the first kind enantiotropic and those of the second kind monotropic ; but sub¬ sequently he advanced the conjecture1 that the two are not essentially different, since the transition points and melting points are dependent on the pressure, and there¬ fore it would probably be possible, by applying sufficient pressure to a monotropic substance, to bring the transition point below the melting point, whereby the substance would change into an enantiotropic one. According to the preceding considerations, which were propounded simultaneously by Ostwald2 and by Schaum,3 the difference between the two kinds of poly¬ morphous substances does consist merely in the posi- J Molekularphysik, I, 194. 2 Zeits.f phys. Chem. 1897, 22, 312. ;i A rten der Isomerie, page 24.
    Lion of the transition point relatively to the fusion curve ; and, since increase of pressure shifts the vapour- pressure curves away from the T-axis, it depends on the difference in the displacements of the various curves whether the transition-point // and the vapour-pressure curve S of the fused substance approach one another, or recede, with increasing pressure. There is therefore the possibility of one and the same substance appearing to be monotropic or enantiotropic, according to the pressure. The displacement of the transition point and the melting point by increasing pressure has been accurately investigated by Tammann1; he found that the melting point of monoclinic sulphur and the transition point between it and the rhombic form become steadily higher with increasing pressure, but that the second rises the more rapidly. In consequence of this, the curves representing the dependence of the two temperatures on the pressure meet at a pressure of 1400 kg. and a temperature of 1520. At this junction melting point and transition point are identical, and beyond it the transition point would lie above the melting point. Since the presence of a foreign admixture lowers the melting point of a substance, if the former is soluble in the fused mass, it follows that a displacement of the melting point below the transition point, and consequently the conversion of an enantio- tropically dimorphous substance into a monotropic one, can also be effected by means of such an addition. A conversion of this kind was first obtained by Schenck and Schneider2 in the case of p-azoxyanisole, which at 116-8 undergoes transformation into a second and “liquid crystalline” modification, which in its turn melts at 134°, i.e., it forms then a truly isotropic liquid. By addition of benzophenone this melting point can be lowered as far as 108-4°, that is to say 8-4° below the transition temperature, so that the substance then behaves as a monotropically dimorphous one. Carbon tetrabromide, CBr4, crystallises after fusion (m.p. 92-5°) in cubic crystals, which at 46-9° become transformed into the monoclinic modification which is stable at ordinary tem¬ peratures ; according to the investigations of Rothmund 3 this transi - 1 Krystallisieren un d Schmelzen, 269. 2 Zeits.f. phys. Chem. 1899, 29, 546 ; Journ. C. S. 76, ii. 637. 3 Zeits. f. phys. Chem, 1897, 24, 712 ; Journ. C. S. 74, ii. 158,
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    into the monoclinic takes place only with dilution of the nitric acid. In the dry state both forms are very stable, and no transforma¬ tion of the one into the other takes place.1 Ammonium fluosilicate, (NH4)2SiF,;, crystallises from aqueous solution at temperatures above 130, cubic, and below 6°, hexagonal ; between these two temperatures both modifications are formed side by side, and undergo no alteration at ordinary temperature ; the hexagonal form becomes transformed into the cubic only when it is heated along with some of the solution on the water bath. In the dry state both modifications can exist side by side for an indefinite period, provided the temperature is not high ; only towards 100" do the hexagonal crystals fall to a powder, which probably consists of the cubic form.2 Di -A77-nitro-s-diphenylcarbamide, CO(NH.CBH4NOo)0, exists in three modifications : a, yellow prismatic crystals ; j3, white needles ; 7, yellow tablets. When a solution of one of these forms, or of a mixture of them, is prepared in boiling alcohol of 95 per cent., and, after having been filtered while warm into a flask kept at constant temperature by immersion in an oil bath, is caused slowly to evaporate by leading through it a current of dry air, then, between 750 and 30°, crystals of the a and /3 modifications are always obtained side by side, even when the solution, saturated at the given temperature, has been inoculated with crystals of one kind. At the higher temperatures more crystals of the a modifica¬ tion are obtained ; at the lower ones, more of /3. Specially good crystals are obtained at 6o°. The inoculation with crystals of one kind merely results in increasing and accelerating the separation of that particular form, with retardation of the forma¬ tion of the other one ; in this way, however, the development of specially good crystals of the second form is favoured. If the filtered mother-liquor is allowed to evaporate by exposure to air at the ordinary temperature (130), then the third modification, 7, alone crystallises out. If the solution is warmed a few degrees, however, then a few crystals of /3 also appear ; at 40°, 7 entirely disappears, and a appears in small quantities, increasing with rising tempera¬ ture. Accordingly, so far as separation from alcoholic solution is concerned, the 7 modification is the most stabld at ordinary temperatures, the form /3 at 50-60°, and a at higher temperatures. This behaviour may be modified by the solvent, however ; for, from 1 Gossner, Zeits.f Kryst. 1903, 38, 501. * Gossner, loc. cit. 147*