LIQUID CRYSTALS 589 extinction crosses illustrated in Fig. 8*. As the microscope stage is rotated, the broad-centcred crosses progress to the pinwheel-shaped crosses which are an identifying feature of the neat phase. Such a netx•'ork of crosses is usually much finer (Fig. 9) giving the characteristic and common appearance on which the term "mosaic" texture is based. Middle phase, not being smectic, can assume neither planar nor focal conic arrangements, t an important distinction from neat phase. Furthermore, middle can_ assume nongeometric textures, e.g., Fig. 10, whereas neat is never less geometric than a fine mosaic. The closely related angular (Fig. 11) and fanlike (Fig. 12) textures of middle are based on arrangements of the cylindrical molecular aggregates. Friedel's "nematic" structure, the "fiiissige Kristalle" of the early liquid-crystal literature, can be found with certain surfactants, e.g., on the higher-water and higher-electrolyte side of the middle phase. In keeping with its threadlike molecular structure--elongated molecules, all parallel but not arranged in lines, planes, or other groupings (1)-- nematic material is soft and often stringy in consistency. It is optically birefringent and exhibits characteristic microscopic textures, such as in Figs. 13 and 14, which distinguish it from neat, middle, and the iso- tropic phases. From evidence on certain surfactants the nematic compositions may consist of two phases in a submicroscopic dispersion. OCCURRENCE OF MESOMORPHIC PHASES IN SURFACTANTS Now that some of the characteristic mesophase structures have been examined, it is useful to consider how their existence is controlled by temperature and the proportion of surfactant, water and additional components such as electrolyte or fatty acid. Figure 16 shows the combined effect of temperature and composition on a simple system in the form of a phase diagram (10-13). It is seen that there are several mesomorphic phases which can be produced even in this simple system. In addition to the more usual neat and middle phases at higher water- contents, there are two neat-type (smectic) and three somewhat similar "waxy"-type phases along the anhydrous axis this behavior is rem- iniscent of the "thermotropic" mesomorphism encountered early in the * Figures 8-11 are from Reference 9. t In Reference 9, focal conic structures are attributed to middle phase. This is on the basis of the earlier literature, e.g., Friedel (1) in which certain middle-phase textures, super- ficially similar to certain neat-phase textures, were interpreted as focal conic, but with poorly developed focal-conic geometry. The structure of middle is now well established and seen to be inco•npatible with focal-conic geometry. This reinterpretation does not invalidate the microscopic scheine for distinguishing neat from middle.

590 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS : ' '-' ' '•.•.x ........ -• • • .. ..•,:•.:... . .. . : .... ., ..•:...--: • ..... ..de•.2•:•7•,: .:i .... •. ---: -:: -: :.• ....... : . :•:•- .... •'"2 .'""• ß • .'T•.:•:..:•.: ............. •.• ........ •: '•":: •":•5•::.:.:. :.5.'-::-- Figure 1•. Middle pha•. Fanlike tex- ture. Crossed polarizers, magnification ca. 150X Figure 13. Nematic structure (right) precipitating from isotropic solution. Stippled texture. Crossed polarizers, magnification ca. 80X •.• .... $• • • ...... :.. •1• • . •?• ß •. .-..:., • •'• .....:...: • :• • •'•.• • . • • ' . •:::.• ::.:•-:."'• .-.. * .• • • •' • .• :: :::%': :.:::...: .• ::.....--..:- , ::,:.:• .•. * •i:C: :-- .. : • .:- ,•.:-•. ...• ...... ... e'... • . ... .... ..... ,:. Figure 14. Nematic structure. Sinuous texture. Note distortion of extinction crosses in individual droplets. Cros•d polarizers, magnifi•tion ca. 80X ß ...,.,•.. :.:• .• ' • .: . .•.: . .:. .. ::•,::• . .- •,•..•',)• •,,•*' ..: .--=. :• . , •. •'•,J•..•' ..•. '. "v '•:• .'•:'. •.. ':•' :'•.:',-.--'. • •.•:.• •'•... • '•'• :. ' • '. "'5' •. ' • .'.• o. •. :• .• • • • '. ' ,. • •,• • , .• . ' •. ß . . ß .... • '• • .... •. ? ..}•:..• .., . . . :• •: .... . .• ....... ß .. .•? . •. . •.. ., ........ ß .•.. .,. • • ..... .•. . . .-•., ,,. • ß , :, .... :• ."•.'. ,• ..: ..•. .- • 1','..• •. ..• •.'..•. •.•. .•.•.. .e-,:•.,. •.• . • .... • .•.. •.•. ß ..(•.:•.. : • •. • ,. .• . •.•.. •.• .. .... ß : •. •,• . ... • •.. • . . •.. ... :• ,,• • :. : , . : ..- • . . • '• . .Fig•re 1•. Neat/lye emulsion. Crossed polarizers, magnification ca. lOOX