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A B C D E F

t(°C )

S (A2) <100* * 20 .1-20.3100-110 26 .3-29.4110-127 31.4-33.5133-165 34.7166-180 2 4 0 *43.8

W here A and B represent modifications of the crystalline lam ellar phase t = temperature stabilty range of the phase

* = the temperature for which the dimensions of the phase are given

* * = no values given, but is expected to be just less than the value of the phase B , which entails slightly less compact packing of the head groups

Table 6.8 The variation of area per polar group of calcium myristate (C14) with temperature and phase structure (values from ref. 166).

The surface area per polar group for the homologous series of straight chain calcium soaps (C12-C18) is relatively independent of chain length166. Hence, the values for calcium myristate (C14Ca) given in table 6.8, are representative of this series of soaps. For CaD4 at 25°C, the surface area per head group that would result in a lamellar structure was calculated to be 28.4A2 (see chapter 5). This is significantly larger than the area per head group of calcium myristate at the equivalent temperature, which would indicate that either:

- a less dense packing of the head groups in exists CaD4 than is present in the monomeric straight chain calcium soaps of this structure, or - there is some interdigitation of the soap molecules w ithin the bi layers.

A comparison of the bi layer thickness resulting from the proposed lamellar structure at 25°C (I.e. d0 = 30.8A), w ith the molecular dimensions of CaD4 (i.e two 'all trans' amphiphile repeat units, each of 20.25A in length; see section 6.2.1.2) would indicate an inclination of non-interdigitated hydrophobic chains to the basal planes of the lamellae, of 49.5°. This degree

of tiltin g is considerably less than the 73° found in calcium myristate (this value was calculated from data presented by Spegt and Skoulios166).

The structural parameters derived from the proposed lamellar structure for CaD4 at 25°C cannot be used to differentiate between the non-interdigitated or interdigitated bilayer structures and, do not preclude the possibility of either of these lamellar modifications. However, these parameters do indicate a packing of the molecules that is significantly different to that found in the crystalline lamellar structure of monomeric straight chain calcium soaps. As this tentative proposal of the lamellar phase is based on a diffraction pattern containing only two diffraction signals, further work is obviously required to verify this proposition and to fully characterise the crystalline structure of CaD4.

The proposal that, the optical texture of the solid film was not indicative of the equilibrium structure of CaD4 at 25°C, due to the previous quenching of the sample, cannot explain the different structures indicated by the optical and X-ray studies of the birefringent fluid phase of CaD4 (i.e. the samples were not cooled prior to carrying out the experimental observations at 200°C). Whilst a convincing explanation for these discrepancies eludes us, and it is not possible to propose a structure for the birefringent fluid phase of CaD4, the following points should be noted. Firstly, aggregates of rod micelles have been proposed as the structures of the intermediate phases formed by monomeric straight chain calcium soaps170. Hence, although the structures proposed for the monomeric calcium soaps are not regarded as definite, the possible occurrence of a hexagonal phase for CaD4 (as is indicated by microscopy) would be in keeping with the generally accepted

behaviour of these soaps (i.e. based on rod micelles). Secondly, the proposal of a lamellar structure for CaD4 at 200°C, would give rise to an area per polar group of 37.2A2, which is very sim ilar to that found in monomeric straight chain soaps at equivalent temperatures (see table 6.8). Although the phase structures involved may not be identical, this indicates sim ilar packing densities in these related amphiphiles at sim ilar temperatures and may therefore support the proposal of the lamellar phase.

In addition, the positions of the principle and second order reflection of the X-ray patterns of CaD4 and NaD4 in the range 25 to 200°C, are almost identical (see tables 6.4 and 6.7). Considering the obvious molecular sim ilarities between these sodium and calcium salts, this would seem to be strong evidence of a sim ilarity of phase structure. Unfortunately, this can be interpreted in one of two ways, firs tly , as supporting the proposition that the lamellar phase as the structure present in both classes of oligomeric amphiphilic siloxanes at temperatures at least up to 200°C, or, secondly, putting in doubt the previous proposal of the lamellar structure for the semi-crystalline phase of NaD4.

Although additional work is obviously required to characterise the structures of the individual phase regions of CaD4, some comparisons between the phase behaviour of CaD4 and the monomeric straight chain calcium soaps, may be made. Whilst the behaviour of the monomeric calcium soaps varies with the length of the hydrocarbon chain, the overall behaviour of this homologous series of amphiphiles is very sim ilar24'153’166*170*304. With increasing temperature, the structure of these anhydrous soaps changes from the lamellar crystalline state at 120°C,