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25°C 200°C

do 30.8 27.7

d1 15.4 13.8

Table 6.7 Spacings observed in the X-ray diffraction pattern for NaD4 at 25 and 200°C. 15 O 17 . 5 2 0 . 0 2 2 . 5 2 5 . 0 1 7 .0 C A M E Eft H T ( M M ) b) 11.5 15.0 17.5 20.0 22.5 25.0 27.0 CAME.?A H T I N M )

Figure 6.19 X-rav diffraction pattern obtained for CaD4 in the low angle region at a) 25°C and b) 200°C.

At 25 and 200°C the main peaks observed were very broad. In each case, the major peaks were in the ratio of 1:1/2, and there was some evidence of a very weak broad peak around d0//3 . No peaks of any real Intensity were observed in the wide-angle region at either temperature. An attempt to record an X-ray diffraction pattern at 300°C was abandoned, as considerable thermal degradation of the sample had obviously occurred.

6.2.2.2 Discusion

Optical microscopy of film s of CaD4 indicated that this material exists as an ordered solid at room temperature. The non-geometrlc texture of this birefringent solid was indicative of a hexagonal phase16. At about 154°C, a birefringent fluid phase formed. The non-geometric texture and the viscosity of this fluid phase were also indicative of a hexagonal phase. The birefringency of this phase gradually diminished over a broad temperature range from 275 to 295°C. The loss of birefringent was not accompanied by a significant change in viscosity. This optically Isotropic viscous fluid phase was stable up to the transition to the low viscosity Isotropic liquid at about 480°C. It was not possible to be precise about the temperature of this transition because of the severe thermal degradation of the sample.

The transitions observed during the DSC heating of previously unmelted samples of CaD4 were in general agreement with these observations. The temperature of the two main endothermic transitions occurring in the range -170 to 550°C (i.e. T1 and T2 at 159 and 480°C, respectively), corresponded to the observations under the microscope of the transition to the fluid birefringent phase and the formation of the low viscosity isotropic liquid, respectively. No transition was identified for the loss of birefringence of the viscous fluid phase.

There is some evidence of a small endothermic transition occurring over a broad temperature range at around 50°C. A transition occurring at a sim ilar temperature was observed in NaD4 and NaD5 and was believed to be an intercrystalline transition or the initiation of the step-wise melting of the non-polar chains of the crystalline phase (see section 6.2.1.2). However, the intensity of the DSC signal occurring in CaD4 is such that no firm conclusions may be drawn with respect to this 'transition1.

As the transition to the isotropic liquid occurred at a temperature at which considerable thermal degradation of the samples had also occurred, all subsequent DSC investigations of the effects of repeatedly heating and cooling CaD4 were carried out on samples that had not been heated to greater than 320°C. The thermograms obtained on the reheating of CaD4 from -50 to 320°C, gave no transitions.

During controlled cooling of samples from 200 to 20°C (20C.min_1), there was some evidence of two small, broad enthalpy transitions occurring at around 155 and 55°C. As the higher temperature transition corresponds to the T 1 transition occurring during initia l heating, it may be that the thermal events occurring around 155°C corresponded to this same transition. This being the case, no significant supercooling of the solid/fluid phase transition occurred. The weak signal occurring at around 55°C on cooling, may Indicate that a transition does indeed occur at a sim ilar temperature during the initia l heating of CaD4. If this were the case, the absence of any evidence of this transition during the reheating of a sample-remembering that these samples were in itia lly heated and then cooled rapidly-may be explained by a quenching of the sample during the rapid cooling that

followed the initia l heating analysis (see section 6.2.1.2). However, the low intensity of these signals do not allow for an unambiguous interpretation of these 'transitions'.

Having used thermal analysis and optical microscopy of CaD4 to establish the temperature 'boundaries' of individual phase regions, and to supply some initial indication of the structures present within these boundaries, X-ray diffraction data was used as corroborative evidence for these observations. At 50 and 200 °C, the low-angle patterns of CaD4 exhibited two strong broad peaks (see table 6.7 and figure 6.19). At both temperatures, the positions of these reflections were, within experimental error, in the ratio of 1:1/2. No peaks of any real intensity were observed in the wide-angle region at either temperature.

A number of phase structures have been proposed, whose principle and second order reflections in the low-angle region are in the ratio of 1:1/2. These include the crystalline and mesomorphic lamellar and the tetragonal phases (i.e. C and K). However, as discussed in section 6.2.1.2, only the generally accepted lamellar phase structure w ill be invoked here.

In attempting to rationalise the behaviour of CaD4, the optical studies-which indicated a hexagonal phase below the transition to the viscous isotropic phase at 275 to 295°Ocannot be reconciled with the X-ray diffraction patterns at 25 and 200°C, which did not exhibit the strong d0/7 3 reflection that is generally characteristic of the hexagonal phase24*149. In attempting to proposing a structure for the crystalline phase of CaD4 (i.e. at temperatures less than 155°C) on the basis of this

conflicting data, it is worth remembering the experience with the equivalent sodium salt (see 6.2.1.2). This work indicated that the optical study of solid film s may have been misleading because of the method employed in the preparation of the film s themselves (i.e. the rapid quenching of the sample from the higher temperature fluid phase, freezes the structure of this high temperature phase into the solid film ). A sim ilar quenching of molecular orientation during rapid cooling has been noted in a number of studies153’304*305. In fact, during an early X-ray study of calcium stearate153, in the absence of a cell for obtaining diffraction patterns at elevated temperatures, this effect was used to advantage, w ith samples which had been quenched from various higher temperatures being examined at room temperature, in the hope that the molecules would be frozen into the relative orientations characteristic of each of the higher temperature forms. This, and the fact that the crystalline lamellar is the phase structure encountered in monomeric straight chain soaps of the alkali and alkaline earth metals, may indicate that the optical texture of the solid film of CaD4 at 25°C was not characteristic of an equilibrium structure. Hence, the X-ray diffraction pattern obtained from a unmelted sample at 25°C, may be more representative of the equilibrium crystalline structure of CaD4 On this basis, the experimental evidence indicates that a lamellar phase sim ilar to that encountered in monomeric calcium soaps24*149*166, and NaD4 and NaD5 (see 6.2.1.2), may also be characteristic of the crystalline form of CaD4

Having proposed the lamellar structure at 25°C, the area per polar group that would result in this structure can be calculated149. The comparison of the area per polar group of monomeric calcium soaps of the lamellar structure (see table 6.8) w ith this derived value Is obviously of Interest.