Solvent Crystallisation Studies

Solvent crystallisation studies were conducted to identify the morphologies and the T^ values of the stable polymorphic forms. Theoretically, stable polymorphs crystallise from solvent (see Chapter 1). Plate 1-5, 6-11, 12-15, 16-17 are photomicrographs of C43/01, C54/02, C50/02 and C55/18, respectively. The values of T^ of gelucire samples crystallised from chloroform can be found in Table 2.2. The coefficients of variation (CV) of the values were less than 2% in all cases.

In the following discussion, crystals which have the same shape and T^ values are described as the same "Form" which was numbered I, II, III, etc. in order of increasing T^ values. It should be emphasised that these "Forms" of crystals were not of any particular pure components. They were likely to be complex mixture of components with similar properties and thus crystallised together (see later and also in Chapter 3).

Chapter 2 ... Morphological Studies/ 71

Table 2.2 values of gelucire samples at various periods of time/

Sample

CO'

0 days 14 days 30 days 60 days 90 days

A43/01 50.4 53.0 53.0 52.2 53.0 B43/01 47.7 47.6 50.1 49.2 49.6 C43/01 <--- 57. C ...> A50/02 49.8 51.5 54.3 - 54.4 B50/02 47.6 48.9 49.0 53.7 54.4 C50/02 <--- _{60.4 ---}---> A54/02 59.3 60.5 60.3 59.5 60.4 B54/02 55.9 56.3 56.7 57.0 59.2 C54/02 <--- _{66.4 ---...>} A55/18 51.7 50.7 50.9 54.5 53.7 B55/18 53.0 52.3 52.9 53.7 53.2 C55/18 <--- _{55.3 ---}---> A50/13 61.2 - - - - B50/13 59.1 - - - - C50/13 <--- _{-— 66.5 ---}--->

1) Samples were stored in the desiccator over silica gel at room temperature (18°C to 24°C). 2) A, B and C represent samples prepared by method A, B and C., respectively.

3) an average of three determinations. Coefficient of variation < 2% in all cases. 4) an average of ten determinations at various periods up to 60 days.

3.1.1 Gelucire 43/01

A typical appearance of gelucire 43/01 crystallised from chloroform is shown in Plate 1. Three distinct morphologies could be identified and described as Form I, II and III in order of increasing melting ranges, which can be found in Table 2.3. These melting ranges were determined from the melting of crystal aggregates (or spherulites) of the corresponding crystal forms. The lower temperature given in the melting range of each crystal form was only an approximation since this could not be observed with accuracy. It was given here merely to give an idea how these aggregates melted in relation to one another.

Form III (Plate 2, 3) was a platy-shaped crystal and usually formed large aggregates whose Tn, value was <57°C. Form II ( Plate 4) was a needle-shaped crystal and showed spherulitic pattern whose T^ value was <50°C. The morphology of Form I could not be described accurately because of their small size. However, other appearances than those of Form III and II (in Plate 1) which melted below 40°C could be described as Form I.

Certain parts of Form I and II were metastable in comparison with Form III, which

was stable. Form I and II partially transformed to Form III during storage, seen

qualitatively by both the increasing numbers of platy-shaped crystals on the microscope slide and the smaller proportion of mass that melted below 40°C as time proceeded. It was thought that the black areas in Plate 1 was undergoing polymorphic transformation. These areas can be seen more clearly in Plate 5 where many long needle-shaped crystals (on the right side of the plate) were transforming into platy-shaped aggregates. It should be emphasized that the transformation to stable forms was not complete even at 60 days after preparation, ie. Form I, II and III were found to exist altogether at all times during the studies.

Upon heating on a hot stage at a very low rate (0.2°C/min.) to 38-40°C when most of Form I had melted, transition of Form I to Form II and III could be observed. This was seen in the microscope as the occurrence of needle- and small platy-shaped crystals

Chapter 2... Morphological Studies / 73

Table 2.3 Melting ranges of various crystal forms found in gelucire samples

crystallised from chloroform.

Sample / Form Melting range (°C) Plate number

C43/01 I <35 - 40 1 II 42 - 50 4 ,5 III 47 - 57 2 ,3 C54/02 - < 52 6,7 I 45 - 57 7 II 52 - 64 8 III 54 - 67 9 IV 56 - 67 10, 11 C55/18 I 53 - 55.5 16 n > 72 17

from the melt. The transition was not extensive. The faster heating rate only resulted in melting of crystals at their melting ranges. The transition of Form II to Form III upon heating had never been observed.

It should be re-emphasised here that the classification of crystals into Form I, II and III was based on their shapes and T^ values only and for convenience in discussion. From Table 2.1, 43/01 consists of 6 fatty acids and can possibly contain as many as 180 triglycerides (ignoring stereoisomers and solid solution forming). If each triglyceride has 3 polymorphs, the number of species possibly found in 43/01 can reach 540. It is obvious, therefore, that the crystals classified as the same Form were not necessary the same components and they apparently have different polymorphic behaviour as seen above that not all Form I and II were metastable or transformed into another form. Form I, II and III should not be regarded as polymorph a , B' and 6 since this is clearly incorrect.

3.1.2 Gelucire 54/02

A number of crystal morphologies were identified for gelucire 54/02 crystallised from chloroform, the melting ranges of which are given in Table 2.3. There are 4 crystal morphologies that could be described accurately and whose melting ranges determined; Form I, II, III, and IV. Form I was seen as spherulites in Plate 6 and 7 and Plate 8, 9 and 10 represented Form II, III and IV, respectively. Considerable amounts of C54/02 melted below 52°C. However, their morphologies could not be described because of the small sizes and thus were not named (Plate 6 and 7).

Form II, III and IV were stable forms. During storage, the polymorphic

transformation of 54/02 was not as extensive as that of 43/01. Form IV (Plate 11) was thought to transformed from other crystal forms since its proportion increased as time proceeded. Upon heating on a hot stage, no transformation was ever observed.

Chapter 2 ...Morphological Studies / 75

3.1.3 Gelucire 50/02

The microscopic appearance of 50/02 was very much like that of 43/01 and 54/02 in combination (Plate 12 to 15). By comparison Plate 13 showed two types of crystals found in 50/02 which resemble Form II and III of 43/01 (Plate 1, 3 and 4). Not only there was a similarity between their morphologies, their polymorphic behaviours were comparable. The platy-shaped crystals in Plate 13 and 14, like form III, were stable polymorphs which increased their proportions with time. These were probably the triglyceride parts in 50/02.

Because of a number of crystal morphologies involved, no attempt was made to assign the melting ranges to those crystals found. Although this could be done, it would probably be very inaccurate. Upon heating on the hot stage, no transformation could be observed.

3.1.4 Gelucire 55/18

55/18 was crystallised from chloroform as a large continuous mass, the morphology

of which was not well defined (Plate 16). Small numbers of spherulites were found

(Plate 17) and named Form II in Table 2.3. They probably were free fatty acids.

Therefore, the value of T^ in Table 2.2 was taken as the temperature at which the main feature (Form I) completely melted. The melting range of Form I of 55/18 was relatively sharp, ie. melting occurred within 3°C. This is expected since 55/18 comprises only PEG diesters of palmitic and stearic acids. No polymorphic transformation was observed in C55/18 both on aging and heating.