.I. •h'oc. Cosmetic Chemists, 19, 581-594 (Aug. 19, 1968)

Liquid Crystals: The Mesomorphic Phases of Surfactant Compositions

F. B. ROSEVEAR, Ph.D.*

Presented December G, 19G7, New York City

Synopsis-"Liquid crystals" are "mesomorphic" structure types between liquids, which are random, and crystals, which are periodic in all three dimensions. The mesomorphic state is surprisingly widespread in organic, and even certain inorganic, compositions. In the surfactant industry, mesomorphic material by the ton occurs as an intermediate in processing. The layered "smectic" structure of the industrially important "neat" phase will serve as an introduction to mesomorphic structures. In each layer, elongate•t molecules or other units are arranged parallel to each other and with their head ends at the interface between layers; the stacking of layers is periodic. Laterally, however, the molecules have a random, liquidlike arrangement. Thus a single structure has a hybrid liquid and crystalline nature. The layers, however, slide easily over each other and the structure flows under its own weight even while maintaining crystallike periodicity. The chemical and physical factors leading to this and other mesomorphic structures in surfactant compositions are reviewed.

INTRODUCTION

Even those with some scientific background are likely to think in terms of just three states of matter: gas, liquid, and solid. Even if "solid" really means "crystal," one is likely to overlook the important structural types between liquid and crystal. Yet glasses, and natural and synthetic polymers, are familiar materials whose structures are intermediate between the randomness of liquids and the periodicity of crystals.

"Liquid crystals" or, more descriptively, mesomorphic structures (1) (meso • between, morph • form) are among the less well-known

* The Procter & Gamble Co., Miami Valley Laboratories, Cincinnati, Ohio 45239.

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582 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

intermediate types. Yet these, too, occur in some relatively coralnon materials and in this paper the important role of these structure-types in soaps and other surfactants will be examined.

NEAT PHASE

Mesomorphic structures will seem less unreal and mysterious if we consider a relatively simple example: the "neat" phase of surfactant systems. This material has been known for many years as the soap- rich upper layer of the soap kettle. Still prepared by the ton., although in many cases the kettle has given way to continuous processes, neat is the starting phase for crutching, spray-drying, chilling, milling and other operations which lead to the many and varied detergent products. A notable feature of the neat phase is its relative fluidity in spite of high surfactant-content. When a tube of neat phase is inverted to a tilted position, it is seen to flow readily under its own weight. However, the flow and the surface are not smooth as with honey, but are more like those of a soft petroleum jelly. Yet the latter is a two-phase system of oil thickened with hydrocarbon crystals, whereas neat is a single phase whose flow properties arise from its structure. While neat has a yield value, it has ample fluidity to permit pumping in plant operations.

The mesomorphic structure responsible for the ready flow of the neat phase is the type known as "smectic" (1). On a molecular scale, the smectic structure consists of flexible layers of equal thickness. Within each layer the molecules are (a) essentially parallel to each other, hence perpendicular to the surface of the layer, but (b) not regularly spaced in the lateral directions; this arrangement has been likened by one writer to that of the people on a crowded dance floor. The lateral ar- rangement is thus liquid-like, but the stacking of the layers has a crystalline regularity manifest in sharp X-ray long-spacings. Even in this direction, however, the structure is not fully crystalline, as the in- dividual molecules in one layer do not have a definite relationship to those in the next. This allows the layers to slide readily over one an- other while maintaining their crystal-like periodic arrangement, and helps account for the relative fluidity of even concentrated surfactants in the neat phase. In a sense, the layer structure can be seen directly under the microscope in the interesting terraced droplets often exhibited by neat phase (Fig. 6). The step-heights are, of course, multiples of the smectic-layer-thickness, but they provide a striking contrast to the smooth, rounded surface of drops of water, oil and other conventional liquids.

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Figure 1. Structure of neat phase Figure 2. Structure of middle phase

When surfactants exist in a smectic structure, as in the neat phase, molecules in adjacent layers alternate in direction. If water is present, the polar heads of adjacent layers share the same aqueous interface (Fig. 1'); this means that the nonpolar tails of molecules in adjacent layers similarly share a common nonpolar environment (2-4). It will be seen that all of the mesomorphic surfactant structures show such polar and nonpolar zones.

MIDDLE PHASE

A second important mesomorphic phase in surfactant/water systems is the "middle" phase. In spite of a substantially higher water-content, middle phase is much stiffer than neat. Middle does not flow under the influence of gravity, though it does flow plastically if subjected to a sufficient external force. This stiffness renders pumping impossible and, coupled with an inherent translucency, gives rise to the term "gum soap." It is easy to picture the consternation caused by even partial conversion of a kettle-full of neat phase to middle because of inadvertent overhydration.

The stiffness of middle phase lies in the absence of a smectic layer- structure. Its structure is shown in Fig. 2. The basic unit of this structure can be thought of as neat-phase layers rolled up into cylinders. Normally, the hydrophili½ heads of the molecules are on the outsides of these cylinders and the tails comprise a hydrocarbon interior. These

* Figures 1-5, courtesy of K. D. Lawson, Miami Valley Laboratories.

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cylindrical aggregates, approximately two molecules in diameter and o[ indefinite length, are aligned parallel to each other in bundles having a hexagonally packed cross-section (2-4). The bundles are thus crystalline in two dimensions; however, since the component cylinders are not in register longitudinally and are possibly of assorted lengths, the bundles are liquid-like in their long direction. Thus, whereas neat phase is liquid in two dimensions and crystalline in one, middle phase is liquid in one dimension and crystalline in two.

The latter type of structure was first demonstrated (5) in a meso- morphic phase of tobacco mosaic virus whose molecules, like the mo- lecular aggregates of middle phase, are elongated cylinders. It should be noted that this structure was among a number of theoretically possible mesomorphic structures deduced by C. Hermann (6, 7) in 1•,t31 on the basis of the three types known at that time and named smectic, nematic and cholesteric by Friedel (1).

The neat and middle structures, each of which has one unique structural direction, are in the class of "anisotropic" structures whose properties vary with direction. Possibly the most strikin• evidence of this anisotropic character is the double refraction, or birefringence, by which the sample appears bright when viewed between two light- polarizers arranged in the so-called "crossed" position. Isotropic materials, such as ordinary solu•rions and those crystals which possess no unique direction, appear dark between crossed polarizers.

OTHER NON-CRYSTALLINE PHASES

If any of the mesomorphic phases are sufficiently diluted, they dis- solve to form an isotropic solution. Usually middle phase is the im- mediate precursor of this solution; in this case one may thin. k of the soap cylinders moving sufficiently far apart that the hexagonal bundles disintegrate, leaving the individual cylinders randomly distributed as "cylindrical micelies" (Fig. 3). On further dilution, the cylinders be- come shorter, until at some fairly high dilution they degenerate into spheres, a geometrical shape of minimum surface energy for a given volume. Below a certain concentration, the spherical micelles disappear entirely to leave only molecularly dispersed surfactants.

In addition to neat and middle, and usually at compositions between those phases, some surfactants exhibit a third mesomorphic phase which is actually isotropic. Its structure is interpreted as a face-centered cubic arrangement of spherical micelles with water filling in the voids (3, 4) (Fig. 4). The close packing of the micelles accounts for the

LIQUID CRYSTALS 585

Figure 3. Structure of rodlike and spherical micelies

Figure 4. Structure of viscous isotropic phase

marked flow-resistance of the phase, which is stiffer even than middle phase.

Another stiff, isotropic region sometimes adjoins the middle phase on the dilute side. Its highly elastic nature results in a characteristic feel and sound when struck. This is presumed to be a concentrated, random suspension of individual cylindrical micelies.

In hydrophobe-dominated compositions such as Na-soap greases, where a soap/hydrocarbon middle phase is known (8) it is easy to con- ceive an inverted mesomorphic structure in which the soap tails point outward toward a continuous hydrophobic medium.

It will Ee noted that all these miceliar and mesomorphic structures

586 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

Vis. Iso.

.... . . .. .. . . • . .. •.• ,• , .. .• • .. :•... . :-. . •

.• •-. •-. • . •:.y• . -,,• .... ..

-- • • • . '.•' .... '.•, ' .... " • "'•' ' ...... '• { ..... '"X" ' :-• .• : -• :• • • . , - . •, • • : ;.:.• ' '-•:. ':•...-•

• . .• .•..• .... .. • '•' .• ..;'• ß .... ::•? ,.-• . • •'• .•. •, • '• •.

Figure 3. Effects of hydration of dodecyldimethylamine oxide as obsen-ed by the polarizing microscope

exhibit the structural features of polar heads in a hydrophilic environ- ment and nonpolar tails in a hydrophobic environment.

RECOGNITION OF THE MESOMORPHIC STRUCTURES

Among the several physical techniques, e.g., X-ray diffraction, nuclear magnetic resonance, calorimetry, and vapor pressure measurements,

LIQUI1) CRVSTALS 587

.½.

Figure 6. Neat phase (left) showing te•aced edge at interface with isotropic solution (right). Some te•aces are above co•ect focus and some below. Ordinary

light, magnification ca. 150X

Figure 7. Neat phase. Bright bands (oily streaks) with complex focal conic detail. Black circles are air bubbles.

Irregular black areas are neat phase in planar orientation. Crossed polarizers,

magnification ca. 80X

used to identify or locate mesomorphic phases, the most widely used is the polarizing microscope. The structural differences between phases are evident in the microscope as patterns or "textures" which constitute a convenient and dependable method for identification (9).

Such differentiation of several mesomorphic phases is illustrated in Fig. 5, which was obtained by allowing a drop of water to diffuse into a group of surfactant crystals placed under a cover glass on the micro- scope slide. In succession can be seen crystals, neat phase, viscous isotropic phase, and middle phase.

Figures 7 through 14 are representative polarized-light textures of individual phases. Figure 7, although it is all neat, gives the impression of two phases. However, the irregular black areas arise from the fact that the smectic layers are parallel to the surface of the slide ("planar"

ß

arrangement) and the light is traveling along the "optic axis" of the structure, its nonbirefringent direction. The bright bands in the same figure are so-called "oily streaks" (1) in which the molecular layers are not planar but arranged in the complex "focal conic" (1) pattern, a characteristic equilibrium configuration of the smectic structure. Another characteristic focal conic appearance is the complex network of

588 JOURNAL OF THE SOCIETY OF COSMETIC CHEhlISTS

ß

Figure 8. Neat phase. Coarse mosaic texture showing "positive" extinction- crosses (narrow centers) and "negative" crosses (broad centers) with both straight and curve4 arms. Crossed polarizers,

magnification ca. 1.50X

.,'..-22. • •' ..ß. .: "22'..-2 'ß. "i ' - ' ? ' '

Figure O. Neat phase. Typical fine mosaic. Crossed polarizers, magnifica-

tion ca.

.. . .';,•:" •:' •* -'; '-•*. '?..-'.' .. l'" • .

•, .l•: I ,,l

F•gure 10. Middle phase. Nongeo- metric [ex[ure, s[ria[ed. Cros•d polar-

izers, magnifica[ion

-' i "-- •. - . ': .. •- •. •½ :-- •' ;; ;.•' :: ...... '

.:....;{ , '. -• ?< ..... . . -•..

Figure 11. •iddle phase. Angular tex- ture. Crossed polari•rs, magnification

ca. 50X

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 •. .-..:., • •'•

.....:...: • :• • •'•.• • . • •

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Figure 14. Nematic structure. Sinuous texture. Note distortion of extinction

crosses in individual droplets. Cros•d polarizers, magnifi•tion ca. 80X

ß ...,.,•.. :.:• .• ';• .: . .•.: . .:. .. ::•,::• . .- •,•..•',)• •,,•*' ..: .--=. :• . , •.;•'•,J•..•' ..•. '. "v '•:• .'•:'. •.. ':•' :'•.:',-.--'. •;•.•:.• •'•...

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. .-•., ,,. • ß , :, .... :• ."•.'. ,• ..: ..•. .- •

1','..• •. ..• •.'..•. •.•. .•.•.. .e-,:•.,. •.•

. • .... • .•.. •.•. ß ..(•.:•..

: • •. • ,. .• . •.•.. •.• .. .... ß : •. •,• . ... • •.. • . .

•.. ... :• ,,• • :. : , . : ..- • . . • '• .

.Fig•re 1•. Neat/lye emulsion. Crossed polarizers, magnification ca. lOOX

LIQUID CRYSTAI. S b91

THE SYSTEM:SODIUM PALMITATE-WATER TEMP.•C,

NEAT 11•1-•-- / "• N ]

SU•NEATr• ',• / •]• •,. A (ISOTROPIC) /•

SUPE RWA XY•-• • -'t NEAT F•uu • • +

• (NEAT I) • •- PHASE I' •u•w•xY-• • •'• I

+ -- • '• I00

80 60 40 •0 % SODIUM PALMITATE •

Figure 16. A composite phase diagram of sodiron palmitate-water compiled from

several sources (10-8)

80[ THE SYSTEM: TALLOW SOAP-NoCI-H20 , IO0*C

A = NEAT SOAP

60 B = NEAT-MIDDLE C = NEAT-MIDDLE-NIGRE

% D = MIDDLE SOAP SOAP E = MIDDLE-NIGRE

F- NIGRES OR SOAP SOLUTIONS

40 G = NEAT- NIGRE H = NEAT-NIGRE-LYE K = NEAT-LYE N = NIGRE- LYE

20

2 4 6 8 I0 12 % NoCI

Figure 17. •hree-component soap-water- salt diagram

history of "liquid crystals" with such compounds as p-azoxyanisole and ethyl p-azoxybenzoate.

Figure 17 shows the effects of adding NaC1 as a third component (14). On addition of NaC1 at relatively low surfaetant concentration (a few per cent) there is at first no change from homogeneous isotropic liquid. At several per cent N aC1, the solution disproportionates into two isotropic layers, one somewhat richer in soap. As NaC1 content increases, the two layers gradually separate farther in composition. Since the two layers are really different compositions in the same iso- tropic liquid phase, some means of differentiating them is needed. The phase-name for the less soapy layer is "lye" because in the early stages of soap-boiling, the electrolyte is actually a soda-lye. The phase-name for the soap-richer layer is "nigre," referring to the accumulation of the coloring matter and glycerine in this layer in soap-boiling. When sufficient salt is added to these layered compositions, a long, narrow, three-phase triangle (neat/nigre/'lye) is entered; in this triangle the composition of the three phases is fixed and, on settling, the topmost phase is neat.

On the high-NaC1 side of this triangle, neat and lye coexist until, when sufficient NaC1 is present, the lye reaches saturation and additional NaC1 remains crystalline in the neat/lye/solid-NaC1 triangle.

If NaC1 is now introduced at higher surfaetant levels, e.g., about

592 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

40% where middle phase is present in the absence of NaC1, middle phase dissolves progressively until with several per cent NaC1 only a con- centrated isotropic solution remains; middle phase has been "salted in." Additional NaC1 now "salts out" neat phase and the two-phase neat/nigre region is entered. Here the compositions of upper and lower layers vary with total composition but, for a given total composition, are fixed at the ends of the appropriate tie-line. With further NaC1 the long, narrow neat/nigre/lye triangle, encountered earlier, is entered but now the proportion of the neat layer is higher. Soap-boiling oper- ations are carried out close to this three-phase triangle but nearer the neat-phase end. Kettle-finishing is done in the triangle or on the neat/ nigre side and the settled neat phase is relatively free of entrained iso- tropic solution. On the other side of the triangle, in the neat/lye region, the consistency of the neat is grainy or curdly with much entrained lye- phase. This is desirable for replacing residual caustic with salt but requires hydrating back into the three-phase triangle or the neat/' nigre region for finishing as a smooth, homogeneous neat layer.

Free fatty acid and fatty alcohol have an effect somewhat similar to that of sodium chloride on the soap phases. For example, addition to a soap-water middle phase suppresses middle and in sufficient quantity leads to a layer of neat phase.

Formulators will be interested in some generalizations on the effect of composition variables on the occurrence of mesomorphic phases. All three types of surfactants---anionic, cationic, and nonionic--exhibit mesomorphic phases. Soaps, alkyl ammonium halides (15), alkylamine oxides (16), and polyoxyethylene esters and ethers are representative.

Among the soaps, the mesomorphic phases of the unsaturated soaps, such as the oleate, occur closer to room temperature than do their sat- urated counterparts; this is true also of the shorter-chain soaps, such as the laurate. In general, the sodium soaps require heating for attain- ment of mesomorphic phases. In the potassium, ammonium and tri- ethanolammonium soaps, however, the mesomorphic phases occur at or near room temperature.*

PRACTICAL APPLICATIONS OF MESOMORPHIC COMPOSITIONS

The practical applications of the neat phase have been in processing rather than in formulations for sale, although it has been marketed in a

* Because of the existence of mesomorphic phases of potassium soaps at ordinary tempera- tures, the making of the early soaps by the action of wooct-ash leachings, largely potassium carbonate, led to mesomorphic soft soap.

LIQUID CRYSTALS 593

tube as a shampoo. With such a product the hydration to gummy middle phase in use offers interesting possibilities.

Middle phase, on the other hand, has been marketed as an auto- mobile soap, as a concentrate for suppliers of liquid soap, and as a shampoo (17).

The stiff, elastic isotropic phase-type on the high-moisture side of the middle-phase region of potassium coconut soap has been marketed as a textile soap. The clear, elastic hair-dressings are presumably the same phase type, possibly inverted.

Lotions and creams are ordinarily two-phase compositions, and often involve fine crystals rather than mesomorphic phases. Neat/nigre or neat/lye emulsions do exist, however, as in some "liquid detergent" products. Figure 15 shows a representative neat/lye emulsion in the polarizing microscope.

ACKNOWLEDGMENT

In the preparation of this paper, the author appreciates the valuable help of Messrs. T. J. Flautt, K. D. Lawson, and A. P. Murphy, and of the art and photographic staff at Miami Valley Laboratories.

(Received December $, 1967)

REFERENCES

(1) Friedel, G., Les •tats m•somorphes de la matiare, Ann. Phys. (Paris), 18, 273-474 (1922).

(2) McBain, J. W., and Marsden, S.S., The structure types of aqueous systems of surface- active substances and their x-ray diffraction characteristics, Acta Cryst. 1,270-2 (1948).

(3) Luzzati, V., Mustacchi, H., and Skoulios, A., The structure of the liquid-crystal phases of some soap and water systems, Discussions Faraday Soc., 25, 43-50 (1958).

(4) Clunie, J. S., Corkill, J. M., and Goodman, J. F., The structure of lyotropic mesomor- phic phases, Proc. Roy. Soc. (London), Ser. A., 285,520-33 (1965).

(5) Bernal, J. D., and Fankuchen, I., X-ray and crystallographic studies of plant virus preparations, J. Gen. Physiol., 25,111-65 (1941).

(6) Hermann, C., Die Symmetriegruppen der amorphen and mesomorphen Phasen, Z. Krist., 79,186-221 (1931).

(7) Mabis, A. J., Structure of mesomorphic phases, Acta Cryst. 15, 1152-7 (1962). (8) Doseher, T. M., and Void, R. D., Phase relations in the system: sodium stearate-

cetane, J. Colloid Sci., 1,299-312 (1946). (9) Roseyear, F. B., The microscopy of the liquid crystalline neat and middle phases of

soaps and synthetic detergents, J. Am. Oil Chemists' Soc., 31,628-39 (1954). (10) McBain, J. W., Lazarus, L. H., and Pitter, A. V., Die Anwendung der Phasenregel auf

das Seifensieden, Z. Physik,. Chem., A147, 87-117 (1930). (11) McBain, J. W., and Lee, W. W., Vapor pressure data and phase diagrams for some

concentrated soap-water systems above room temperature, Oil &' Soap 20, 17-25 (1943). (12) Vold, R. D., and Vold, M. J., Successive phases in the transition of anhydrous sodium

palmitate from crystal to liquid, J. Am. Chem. Soc., 61,808-16 (1939).

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

(13) Roseyear, F. B., unpublished data. (14) Adapted from Ferguson, R. H., Phase phenomena in commercial soap systems, Oil

& Soap, 9, 5-8, 25 (1932). (15) Ralston, A. W., Hoffman, E. J., Hoerr, C. W., and Selby, W. M., Studies on high mo-

lecular weight aliphatic amines and their salts, J. Am. Chem. Soc., 621, 1598-1601 (1041). (16) Lutton, E. S., Phase behavior of the dimethyldodecylamine oxide-H20 system, J. Am.

Oil Chemists' Soc., 4il, 28-30 (1900). (17) Wood, R. C., to Procter & Gamble Co., ?lastic Detergents and Method of Making Same,

U.S. Patent 2,580,713 (January 1, 1952).