FIXED AND BULK EMULSION WATER 47 interlamellarly fixed water molecules possess other properties than those of the bulk water phase. The surplus of cetostearyl alcohol, which is not part of the hydrophilic gel phase, builds up a matrix with lipophilic properties (Figure 1, region c) called the lipophilic gel phase. The inner aspect of the dispersed phase (Figure 1, region e) is mainly immobilized mechanically by this lipophilic gel phase. The lipophilic gel phase, consisting of pure cetost•aryl alcohol, is only able to form a semihydrate with water. It became evident that this four-phase model (hydrophilic gel phase, lipophilic gel phase, bulk water phase, and dispersed inner phase) is also valid for other O/W creams investigated as described for partly neutralized stearate creams (3,4) and O/W creams with non-ionic surfactants (5-7). Freeze fracture electron microscopy has added a new dimension to the studies of O/W cream organization. This technique allows the visualization of the above-mentioned structural elements (8-10). The interpretation of molecular structures visualized by this technique, however, has to be done very carefully. In Figures 2 and 3 the distances found for the interlamellarly fixed water layers and the mixed crystal bilayers (together forming the hydrophilic gel phase) are in good accordance with the interplanar spacings found by Small Angle X-ray Diffraction (SAXD) (2). Figure 2 shows the replica of a fracture face of emulsifying wax DAB 8 with a water content of 70 percent at a 30,000-fold magnification. The fracture plane preferentially reveals the hydro- phobic interior of the surfactant's bilayers. The lipophilic surface areas are to be seen as lure ß . •i•e 2. Eoeeeze fracture micro•raph of emu|sifyin• wax DAB 8 co•tai•i• 70% (w/w) water. a: lipophilic fracture face of the lipophilic moieties of the •ixed crysta| bilayer b• fracture edge of the hyd•ophilic phase c: fracture face of the interlamellarly fixed water layer of the hydrophilic gel phase.

48 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS weakly structured regions (a). Areas with irregular and strongly undulated structures (c) as well as with pronged lines (b), generally parallel, are also to be seen. These lines arranged in layers (b) show broken edges of the lameliar hydrophilic gel phase exposed by a favorable run of the fracture. The irregular shape of the lines (c) is due to the sublimation behavior of the frozen water, chilled in a vitrified state at --100øC (11). From Figure 3 the hydrophilic gel phase can be recognized very clearly. In this micrograph the alternating layers of the hydrophilic gel phase are nearly at right angles '* .\" - ' . .. . . • . .. •.:• • •:. • , • .... .* . • ß • . .... •. • .• • •.. • ... :.' :-. .... .... .•.• • .. • ,•- •..'.•"2 : •i ........ -• x,, .:.:. • • • '•" ::•' • •n • • •" '5-' '•- , '* ' , *. c'- ' : * ' *.•' -, -s: *'- " :• :•.,. -:'" :'* 4-*'-* •:'• ....... ,"_ ' .• ..:. '• .... :: 3 '•* '..,--.. •:. :* •...: - •'..•,{- - .?.:• ,, '-..?,. -w ", Figure 3. Freeze fracture micrograph of emulsifying wax DAB g wkh 70% water (w/w). a: bulk water phase b: fra•ure face of the lipophilic moieties of the mNed crystal b•ayer c: interlamellarly fixed water d: mixed crystal bilayer c+ d: together form the hydrophilic gel pha•. to the fracture face. Together with areas of bulk water (a) entrapped in the hydrophilic gel phase, the interlamellarly fixed water layers (c) and the bilayers of the surfactant molecules (d) are to be seen. Together (c) and (d) form the hydrophilic gel phase (b) is the lipophilic fracture face. It may be assumed that the ratio of interlamellarly fixed water and of the bulk water can be used as an impartial quality criterion for these O/W creams during production and storage. Most probably the swelling capacities of the hydrophilic gel phases of different O/W creams influence not only the water release to the skin but also the diffusion rate of drugs through the vehicle, as well as the penetration ability of drugs through the hydrated horny layer of the skin. A differentiation between interlamellarly fixed water and bulk water should be possible by means of thermogravimetry. Through the use of this technique the development of creams with controlled water release should be practicable.