Phase Behavior, Lyotropic Phases, and Flow Properties of Alkyl

Langmuir 1995,11, 4250-4255

4250

Phase Behavior, Lyotropic Phases, and Flow Properties of Alkyl Glycosides in Aqueous Solution Gerhard Platz," Jurgen Polike, and Christine Thunig Institut fur Physikalische Chemie, Universitat Bayreuth, Universitatsstrasse 30, 95440 Bayreuth, Germany

Rainer Hofmann, Dieter Nickel, and Wolfgang von Rybinskj Henkel KGaA, Henkelstrasse 67, 0-40191 Diisseldorf, Germany Received November 14, 1994. I n Final Form: June 8, 1995@ Micellar aggregation, rheological properties, and phase behavior of two commercially available alkyl polyglycosides (APG Trade name of Henkel KGaA, Dusseldorf) have been studied. Light scattering and viscosity measurements show that the short chain Cs/lo-APG (n = 8-10) forms up to high surfactant concentrations spherical micelles which are highly hydrated whereas the long chain C12/14-APG (n = 12-14) forms rodlike micelles above 0.04 wt % surfactant. The isotropic phase of Ca/lo-APGwhich extends t o 98 wt % above 20 "C is a Newtonian liquid. C12/14-APGsolutions are viscoelastic even at low concentrations. The liquid crystalline phases which appear at high surfactant concentrations (Ca/lo-APG75-85 wt % below 20 "C, C12/14-APG above 60 wt % at all temperatures) are characterized by their polarization microscopic textures. As well as in ternary surfactant-fatty alcohol-water systems we find three different lamellar regions with three characteristic textures. Only the Luh region is able t o generate the pseudoisotropic orientation between two glass surfaces which is characteristic for isotropic lamellar planes. Polarization microscopic pictures of the Ld-h region show a broken focal conic texture of thermotropic smectic-E phases. The Ld region has a biaxial woven or a schlieren texture, similar t o that of a nematic rod phase, which is characteristic for thermotropic smectic-C phases.

1. Introduction Alkyl polyglycosides (APG, trade name of Henkel Corporation, USA) are surfactants that are arousing increased interest for a number of applications, because in addition to their excellent ecological and toxicological properties1they exhibit interesting interfacial properties.* A comprehensive physicochemical characterization of surfactants must be based on a knowledge of not only their interfacial properties but also their phase behavior. Micellar aggregation, cloud-point, and phase formation have been ~ t u d i e d . ~I n- ~this paper the phase behavior oftwo alkyl glycosides inwater was studied in the complete concentration region. At temperatures above 20 "C, alkyl glycosides with a Cs/lochain are fully miscible with water. They form low-viscous solutions. Alkyl glycosides with long alkyl chains exhibit much more complex phase behavior with water; liquid-crystalline phases are detected and high-viscous solutions are formed even at low concentrations. The aim of this study was to characterize the phase behavior and the rheological properties of technical alkyl polyglycosides in aqueous solution.

2. Experimental Section Two industrial products with mixed alkyl chain lengths were studied: &/IO- and C12/14-APG. They were manufactured by *Tel. 0049 921 55 2768; FAX 0049 921 55 2780; Email, [email protected]. Abstract published in Advance ACS Abstracts, September 1,

Henkel KGaA by means of Fischer glyc~sidation.~ The samples were frozen for 2 days and then dried in vaccum for 3 days. The surfactants were dissolved under stirring and heating to 6 0 "C. The cmcvalues were obtained by surface tension measurements. The densities of the surfactant solutions were measured with the aid of a OCR-D Oscillating Capillary Rheo- and Densimeter (fivesignificant digits). The surfactant densities were calculated from density values in the concentration region 0- 10 wt % at 25 "C using the equation

The headgroup distributions of c8/10 (C12/14)-APGare 44 (63) wt % monoglycosides, 14 (16) wt % diglycosides, 9 (8) wt % triglycosides, 7 (3.5) wt % tetraglycosides, 3 (1.5) wt % pentaglycosides, 1.5 (1.5) wt % hexaglycosides, and 2 1 (6.5) wt % glycosides with higher molecular weight. The chain-lengthdistributions are 75 wt % C12, 25 wt R C14 (C12/14-f%PG)and 45 wt % c8, 55 wt %, Clo (cs/l~).a/P for the monoglycosides is 2.511 (2.211).

cmc values: C8/10-f%PG,0.05 wt %; C12/14-APG, 0.003 wt %. Densities: Csllo-APG, = 1.31 glmL; C12/14-i\PGI = 1.21 glmL. The turbidity behavior of the surfactants in water was measured in a thermostated glass vessel; the turbidity of the solution was followed spectroscopically. The phase behavior studies were carried out with a Zeiss standard polarizing microscope. An Ubbelohde capillary viscometer and a Bohlin and a Contraves rotational viscometerwere available for carrying out rheological measurements. A Brookhaven instrument was used for light scattering studies. Electrical double refraction measurements were carried out in an already described apparatus.8

@

1995. (1)Andree, H.; Middelhauve, B. Tenside, Surfactants, Deterg. 1991, 28, 413-418. (2) Nickel, D.; Nitsch, C.; Kurzendorfer, C. P.;von Rybinski, W. Prog. Colloid Polym. Sci. 1992,89,249. (3) Eastoe, J.; Rogueda, P. Langmuir 1994,10, 4429-4433. (4) Balzer, D. Langmuir 1993,9, 3375. (5) Fukuda, K.; Soderman, 0.;Lindman, B.; Shinoda, K. Langmuir 1993.9.2921-2925. (6j h i o r l e i W. G.; Tiddy, G. J. T. J. Chem. SOC.,Faraday Trans. 1993,89(15), 2823-2831.

3. Results The phase diagrams of the Cs/lo-APGand a C12/14-APG in bidistilled water are shown in Figure 1 and Figure 2. In the temperature range above 20 "C up to very high (7) Fischer, E. Chem. Ber. 1893,26,2400. ( 8 ) Schorr, W.; Hoffmann, H. Electric birefringence of micellar solutions. In Physics of Amphiphiles: Micelles Vesicles and Microemulsions; SOC.Italiana di Fisica: Bologna, Italy, 1985; pp 160-180.

0743-746319512411-4250$09.00/0 0 1995 American Chemical Society

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Alkyl Glycosides in Aqueous Solution

lP

Figure 1. Phase diagram of C~IIO-APG.

40

20]

__........

-

I

CAPG ,oo/wt%

0

I

I

1

1

I

I

1

I

I

I

20 LO 60 80 100 Figure 2. Phase diagram of C1y14-APG. The dotted and dashed lines depend on storage time and heating and cooling rates. (a) Crystalline precipitates which result a liquid-cristalline Ld texture after crumblingby shear between microscope slide and cover glass; dotted line: upper melting point of precipitate after cooling down to 5 "C for 12 h. (b) La1-h texture after cooling which changes permanently to L,1 after shear. (c) L d texture after cooling and La1-h texture after heating from Ld. The boundary moves to higher temperatures in very thin samples. The low-temperature behavior was determined under the polarization microscope with heating and cooling rates of 2 Wmin from 58 wt % to the highest concentration under investigation (95 wt % Cl2/14-APG). 0

concentrations, the Cs/lo-APG(Figure 1)has an isotropic phase whose viscosity increases steeply. At about 95 wt % APG a double refractive lyotropic phase with nematic texture occurs, which is joined at approximately 98% by a turbid two-phase region with solid CS/lo-APG. At lower temperatures a lamellar liquid-crystalline phase also appears between 75 and 85 wt %. The phase diagram of the system Cly14-APG/water (Figure 2) differs clearly from that of the short-chain APG. At low temperatures over a wide range of concentrations there is a region which looks like a solid-liquid region below the Krafft point. The system changes to an isotropic liquid phase as the temperature rises. Because crystallization is kinetically strongly inhibited, the position of the phase boundary changes with storage time. Above this two-phase region is an isotropic liquid phase which changes to a two-phase region with two liquid phases at low APG concentrations above 35 "C. At concentrations above 60 wt % APG a sequence of liquid-crystallinephases was passed through a t all temperatures. It is worth mentioning that, in the isotropicone-phaseregion at C12/14APG concentrations just below the lyotropic phases, marked streaming birefringence occurs. When the reagent glass was given a gentle lateral push, the solution becomes bright between crossed polarizers for some tenth

Figure 3. (top) Texture with pseudoisotropism (Lah). 75 wt % C12/14-APGobserved between microscope slide and cover glass at 25 "C after heating up to 70 "C. (middle)Texture of Ld 90 wt % C12/14-APG at 25 "C. (bottom)Texture of La1-h 68 wt % CIU~~-APG at 70 "C.

of seconds. This phenomenon is attributable to enormous streaming birefringence, which rapidly disappears again after the shearing. However, no multiphase regions were found that separate this region from the L1phase. In the diluted L1 phase there is also another region with noticeable streaming birefringence. This is in the vicinity of the minimum of the liquid-liquid miscibilitygap. Weak double refraction in slowly flowing solutions was observed here. Figure 3 contains polarization micrographs of typical textures of the detected lyotropic phases (magnification 100). Figure 4 shows the viscositiesof Cwlo-APG and C12/14APG in the dilute range, measured using an Ubbelohde viscometer. Oscillating measurements on the Bohlin instrument show that the solutions behave as Newtonian fluids at moderate shear rates up t o very high concentra-

Platz et al.

4252 Langmuir, Vol. 11, No. 11, 1995

G ' , G" bel 1Hz T = 25°C

T: 3OoC 100

6001wt%

'0

I

10

20

30

50

40

60

APG 600 I Wt.-%

1

q/mPas

Figure 7. G and G ' values at 25 "C. I o4

-.

I

C p c l wt % I

I

I

I

I

)

I

I

1

I"

I

3 L 5 Figure 4. Ubbelohde measurements ofviscosityof dilute C i m APG solutions at 30 "C and Cs/lo-AF'G solutions at 25 and 30 "C.

0

0

2

20 30 40 50 APG 600 Jwt.-%

10

60

Figure 8. G of C12/14-APGat 25 "C at point of intersection G' =G

,i.e. at ut = 1. 1 " " " " " ' .

IU

IO'

-

G'IPa G"IPa 'J Iq'llPas 0

x

0

IO0 -

w v v v v w v v a6 v v w v v

-

10-l-

0 0

xx

0

APG 600 I W t . - % Figure 9. C1Ul4-AF'G structure relaxation times at 25 "C.

0

G , G , and Figure 5. 10 wt % CIWM-APG. of frequency o at 25 "C.

v* as a function

20

40

60

80

APG 600 I wt.-%

Figure 6. Zero viscosities from oscillating measurements at 30 "C.

it was possible to determine not only the viscosity component G but also the shear elasticity G in the high viscosity range by oscillating measurements (Figure 7). The elasticity components G' increase with concentration but remain smaller than G" up to high frequencies. Increasing G values were also observed as the temperature increased. Only in the isotropic phase does the frequency behavior of the moduli G and G correspond to that of a Maxwell fluid. Figure 8 shows the points of intersection G = G resulting from the frequencydependent moduli. The structure relaxation times based on this are shown in Figure 9. These times are extremely short. In the case of CBllo-APG it was not possible to determine these values. For light scattering studies the refractive index increments dnldc, were determined in the concentration range up t o 50 mg/cm3:

C,/,,-APG: dnldc, = 0.1440 m u g C,,/,,-APG: dnldc, = 0.131 m u g

tions. Figure 5 shows the typical frequency behavior of the moduli G and G . Figure 6 shows the zero viscosities determined with the Bohlin rheometer. With C12114-APG

In the case of CBllo-APG the KclRo values obtained in the 0-5 wt % range showed only slight dependence on

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Alkyl Glycosides in Aqueous Solution

7 I-

10-6

7 0-7

0,OO 0,Ol

0,02

0,03 0,04 0,05 APG I g l m l Figure 10. KdRo values for C m - A P G (upper)and C12114-AF'G solutions at (lower curve) 25 "C.

-

concentration (Figure 10). For c cmc: Me, = 31 300 is obtained. From the density of CEllo-APG (e = 1.31 g/m& it follows that the spherical micelle radius is 23 A. Dynamic light scattering gave a constant mean diffusion coefficient of 5.9 x m2/s in the concentration ran e 2-5 wt % and hence a spherical micelle radius of 32.5 . The form of the correlation function indicates some polydispersity. Electrical double refraction gave no utilizable signals with C~/~O-APG. C12114-APG exhibited much more light scattering than Ce/lo-APG. The scattering decreased appreciably as the scattering angle increased. Rod lengths and diameters were determined from scatter intensities, radii of gyration, and hydrodynamic radii. The transient electrical double refraction An(t)gives relaxation curves which can be used to calculate rotation diffusion coefficients 0 with the help of the equation

x

l/t = 6 0

(1)

and to estimate rod lengths.8 For C12/14+iPG a rod diameter of 25 A was obtained. The rod lengths increase linearly with the C12114-APG $oncentration up to 0.15 wt % and remain constant (2000 A) above this overlapping concentration. The lengths obtained from birefringence measurements are in agreement with the values from scatter measurements up to 0.075yt %but somewhat smaller above this concentration (1000 A).

4. Discussion 4.1. Rheological Behavior. The flow behavior ofAPG solutions can be characterized by distinguishing between three viscosity regions. At low APG concentrations the viscosity increases linearly with concentration. The results shown in Figure 4 were obtained from measurements with the Ubbelohde capillary viscometer and demonstrate that short-chain Ce/lo-APGfollows this linear relationship to more than 10 wt %, whereas the longchain C12/14-APG follows it barely to 0.05 wt %. In the case of CEllo-APGthe slope of the line obtained by plotting the specific viscosity against the volume fraction of the added surfactant was 3.9. For nonhydrated spherical micelles a value of 2.5 would be expected according to the 2.54)). Above these Einstein equation ( q = qo(1 concentrations is a region in which the viscosity of both surfactants increases strongly with concentration. In the case of C12/14-APG this rise is limited to a narrow concentration range up to about 15wt %, while Cs/lo-AF'G exhibits such behavior up to the highest concentrations. The third viscosity region is only observed with c12114APG. In contrast to the second region, above about 15 wt % and almost up to the lyotropic phase, the viscosity increases only linearly with the concentration. At very

+

high concentrations, therefore, the viscosities of CE/lo-APG and C12/14-APG are almost identical on the logarithmic scale. The especially large increase in viscosity of C12/14APG is caused by steric hindrance to the shearing of rod micelles, which are formed even at very low concentrations and overlap. There are almost only spherical micelles in CE/lo-APG,and for this reason its viscosity remains low until the added concentration is high. As soon as the mutual steric hindrance of the spherical micelles takes effect, the viscosity increases enormously and even reaches that of C12114-APGsolutions with the same concentration. This behavior is typical for spherical micelle dispersions. Extensions of the Einstein equation have been developed for the highly concentrated region, which give very high viscosity value^.^ It is interesting that, with CE/lo-APG, the formation of a lamellar phase owing to steric interaction can only be forced a t low temperatures and in a very narrow concentration range. Because this phase only occurs in a narrow concentration range around 80 wt %, it can be assumed that a cubic arrangement of normal and inverse micelles is present at Cwlo-APGconcentrations below and above the lamellar phase. This assumption explains the very high viscosities in the high-concentration region. The structure relaxation time measured for C12/14APG as a function of the concentration has a maximum value. This lies a t an C12114-APG concentration that roughly corresponds to the minimum of the miscibility gap. The attractive interaction between the rods formed from the spherical micelles as the concentration increases becomes so strong that, when the above mentioned maximum is reached, the rods start to break down thus forming disklike aggregates like small vesicles as more surfactant is added. Only in the vicinity of the maximum of the structure relaxation time is a clear streaming birefringence also to be observed when the solutions are shaken between crossed polarizers. 4.2. Micelle Shape. With the help of classical light scattering it was possible to determine molecular weights for the CE/lo-APGsolutions and from these to determine micelle radii r. These radii are much smaller than the hydrodynamic radii rH obtained from the diffusion coefficients. The hydrodynamic volume of the micelles is larger than the surfactant volume by a factor ( r ~ h - 1This ~. factor is bigger than the value #/& = 1.6, which can be obtained from the viscosity measurements if spherical hydrated micelles are assumed. From the comparison with the viscosity data it can be concluded that spherical micelles with a 9 A thick hydration shell are present in Cs/lo-APG. The result is understandable if it is assumed that the hydrophilic glucose groups penetrate as far as possible into the water phase. In the case of CIZ/I~-APG the viscosity curve a t very low concentrations indicates that no spherical micelles are present even a t a concentration of 0.05 wt % C12/14-APG. The bend in the curve indicates the growth of rod micelles. Similar results can be obtained by measuring electrical double refraction. Readily evaluable measurement curves are obtained above 0.1wt % Ciy14-APG. The relaxation times can be explained if it i: assumed that rods with lengths of approximately 100 A are present. In the whole dilute concentration range, CEllo-APG gives no signal during electrical double refraction measurements that is appreciably higher than the Kerr effect of the solvent water. This observation supports the hypothesis that only spherical micelles are formed in CE/lo-APG. Deviations from Einstein behavior occur if the shape of the micelles varies only slightly from (9) Hiemenz, Paul C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, Basel, 1986; p 198.

Platz et al.

4254 Langmuir, Vol. 11, No. 11, 1995 the spherical. SimhalOhas calculated the viscosity values (7 - vo)lvo@ for ellipsoids. Tables of values are given in ref 11. For ellipsoids of rotation with semiaxes a , b, b the extensive algebraic expressions can be replaced by a function that yields very exact values for axis ratios p = alb between 0.1 and 10

r-70- 2.5

ro@

+ 0.75(111p)~+ 0.15(lnp)*+ 0.044(ln pI5 (2)

The viscosities of the Ca/lo-APG solutions could be explained without assuming hydration if elongated or compressed ellipsoids of rotation with, respectively, p = 3 or p = 0.25 were present. Translation diffusion coefficients change to a much lesser extent than specific viscosities if there is any deviation from the spherical shape, provided the particle volume remains constant. The Perrin equation12 for the coefficient of friction f of ellipsoids of rotation as a function ofp a t constant volume can be approximated in simple form for 0.1 < p < 10

flf, = 1 + 0.089(lnp)2

(3)

From this it follows that radius calculations from diffusion coefficients D = kBT1f for an axis ratiop = 3 give a deviation of only 10% from the radius of a sphere with the same volume. These considerations confirm the assumption of the strong hydration (0.6 g of waterlg of APG) of C8/lo-APG. Similar results have already been reported for octyl ma1t0side.l~ However, in that case hydration was 0.44 glg. Nonionic surfactants have been known to undergo hydration of up to 0.7 g1g.l4 4.3. Characterization of the Lyotropic Phases. Samples of the lyotropic phases formed a t concentrations above 60 wt 5% were prepared on microscope slide and cover glass. Under the polarizing microscope these samples exhibited textures that were strongly dependent on the concentration and temperature. The results of the studies indicate that three lamellar regions can be distinguished in binary concentrated C12/14-APGsolutions as well as in dilute systems containing fatty alcohol: Lal, Lu1-h and L,h.l5,l6 Therefore we are able to characterize all liquid crystalline regions of C8/10- and C12114-APG by these three different polarization microscopic textures. In dilute ternary systems, La],L+h, and Lah occur at the same surfactant concentration a t increasing fatty alcohol concentrations. A typical lamellar lyotropic phase develops dark pseudoisotropic regions after a long period of storage. These regions are sharply separated from strongly doublerefractive sites. The L b phase that occurs in the medium concentration range of the lyotropic region of C12114-APGwater mixtures, preferentially a t higher temperatures, exhibits such textures (Figure 3a). Schlieren textures are never observed, although strongly double-refractive oily streaks are usually present. Pseudoisotropism is characteristic of lamellae that are isotropic in their plane and orient themselves parallel to the walls of the microslide. Such lamellae contain component units that, on (10) Simha, R. J . Phys. Chem. 1944, 4 4 , 25. (11) Scheraga, H. A. J . Chem. Phys. 1955, 23 (81, 1526-1532. (12) Perrin, F. J . Phys. Rad. 1936, 7 , 1-11. (13) Kameyama, K.; Takagi, T. J . Colloid Interface Sci. 1990,137(1). (14) Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. Colloidal Surfactants; Academic Press: New York, London, 1963; Chapter 2. (15) Platz, G.; Thunig, C.; Polike, J.;Kirchhoff, W.; Nickel, D. Colloids Surf. A: Physicochem. Eng. Aspects 1994, 88, 113-122. (16) Platz, G.; Thunig, C.;Hoffmann, H. Ber. Bunsen-Ges.Phys. Chem. 1992, 96(5),667-677.

average, are oriented a t right angles to the lamellar plane. Fluid lamellar structures fulfill these conditions. If the stage of the slide is tilted, there is no longer any pseudoisotropicorientation in the direction of observation. Strong double refraction occurs. Pseudoisotropic orientation can be distinguished from isotropic phases in this way. If a sample with a phase is cooled in order to determine a Krafft point, the texture changes below a characteristic temperature. The pseudoisotropic sites disappear, as do the sharply separated, strongly doublerefractive oily streaks. However, no C1~14-11PGcrystallizes out but instead a new lyotropic phase (L,l, Figure 3b) is formed, which only exhibits slight double refraction. This phase expands a t higher C12114-AF'G concentrations up to high temperatures. The transition from the lyotropic L,1 to the solid C12/14-APGand to thermotropic phases (different smectic phases from 115-165 "C) was not investigated in this study. The highest concentration under investigation was 95 wt % C12114-APG. The L,1 texture is similar to that of a nematic rod phase. In contrast to a genuine rod phase, however, it was not possible to achieve any orientation with the help of strong magnetic fields. In thermotropic liquid crystals such a texture is typical of optically biaxial thermotropic smectic C-phases that contain regularly inclined component units in their lamellar structure, causing anisotropism to occur in the lamellar ~ l a n e . ' ~Indeed J~ the biaxiality of the La, texture can be clearly proved by connoscopic observation through the polarization microscope. When polarizer and microslide are parallel, a dark cross in an illumated field can be seen. The cross opens as soon as the microscope table with the microslide is rotated for some degrees. In the case of a Lah phase there is a dark cross which does not change on rotating the table. An ordered uniaxial phase like a hexagonal phase with its optical axis parallel to the microslide is completely dark at zero position and changes to bright on small rotations of the table. It must be noted that biaxial polarization microscopic textures are no proof to lamellar structures with biaxial orientations of the surfactant molecules. For example, flattened tubuli could be formed by shearing a two-phase system containing uniaxial lamellae. However the existence of biaxial lamellar structures is well-known. Deuterium-NMR measurements could be helpful to clear the situation.20 The presence of partially crystalline angled and therefore biaxial lamellae in lipid systems was detected with the help of electron micrographs.21The Ekwall B-phaselQ is a lyotropic lamellar phase whose texture is similar to that of a L,1 phase. This B-phase contains a dispersion of multilamellar droplets (vesicles).22We assume that it should be possible to force any vesicular dispersion with isotropic lamellae to pseudoisotropic orientation, for example, by shear. Therefore we have indications that L,1 may have a tilted partially crystalline lamellar structure. The lyotropic region which appears a t the lowest C12/14APG concentrations shows a texture with fanlike structures. As is the case with the L,1 phase, no pseudoiso(17) Demus, Dietrich; Richter, Lothar Textures of Liquid Crystals; Verlag Chemie: Weinheim, 1978. (18)de Gennes, P. G. The Physics of Liquid Crystals; Clarendon Press: Oxford, 1974; pp 276-277. (19) Ekwall, Per In Advances in Liquid Crystals; Brown, Glenn H., Ed.; Academic Press: New York, 1975; Vol. 1, pp 37-41. (20) Blackburn, J. C.; Kilpatrick, P. K. Langmuir 1992,s (61,167987. (21) Gebhardt, C.; Gruler, H.; Sackmann, E. 2. Naturforsch. 1977, 32c, 581-596. (22) Tiddy, G. J. T. J . Chem. Soc., Faraday Trans. 1 1972, 68, 369.

Alkyl Glycosides in Aqueous Solution

Langmuir, Vol. 11, No. 11, 1995 4255

tropism is exhibited in this lyotropic region. The fanlike temperature phases whose formations are strongly instructures observed under the polarizing microscope hibited. The given dotted line represents the upper (Figure 3c) give cause for suspecting that a hexagonal melting courve of the systems after cooling to 5 “C for 12 configuration is present. Such “broken focal conic texh. It is of interest that the precipitation becomes much tures” are however characteristic of biaxial, smectic slower in more dilute solutions. thermotropic phases. The fact that Ld.h is no hexagonal The phase observed a t low temperatures in Csllo-APG but a lamellar phase is supported by the formation of systems with approximately 80 wt % is a lamellar phase lamellar droplets in two-phase region L,l-h-isotropic of the type Ld-h. A L,1 phase is probably present in the solution. A hexagonal phase develops b a t h n e t s . The apparently nematic texture that is formed a t about 95 wt textures themselves show also significant differences in % Cglo-APG. their fine structures. When a hexagonal lyotropic phase Three different lamellar phases are also found in lipid is sheared, a complete orientation is obtained which systems: La, Lp, and L,.24 The La phase corresponds to remains permanently unchanged. A La1-h texture is the L d phase. According to our concept the La, phase transformed by shear to Ld which goes back to Ld-h several could have a similar structure to the Lp phase. The L, hours after shear. phase is thoughtto consist alternatively of lamellae of the If Ld and L b are present in ternary systems, Ld-h occurs L, and Lp phases. It should be mentioned that there are between these two lamellar phases. It is known that Lal-h a large number of configurations for biaxial lamellar can exhibit flow limits. This property probably points to textures.21 It must also be takeninto account that planar the presence of a spatially very extended angled lamellar structures with a lower degree of symmetry can be formed structure, in which the lamellae cause mutual shearing during the transition of rods to lamellae. Regularly hindrance. It is possible that the L,1 and Lah lamellae are arranged rods or stripes are then present in a lamellar joined to each other in the La1-h region. Such structures plane.25However, differentiation is not possible by means could stabilize vesicular dispersions which are formed of polarizing microscopic analyses. Lamellar phases with under shear. regular structural defects, which can occur as a result of When cooled under the Krafft temperature the Lal-h the transition from rods to plates, are described with phase, just as the L b phase, changes into a L,1 phase. In reference to the system: sodium lauryl sulfate, water, the vicinity of the phase boundary to L d , heating causes and decanol.26 X-ray and deuterium-NMR studies about a L h phase to be formed from a La1-h phase. On cooling, the structures of the L, phases will be part of a separate the texture becomes that of a Lu1-h phase again. The study. transition temperatures on heating are appreciably higher 5. Conclusions than those on cooling. This shows that strong kinetic inhibitions dominate, making it difficult to determine the Different APG/water systems have been studied in the phase boundaries more precisely. For instance, the region whole concentration region. It is shown that the phase of the L d phase in preparations with a very small layer behavior can be strongly influenced by variations of thickness is more extensive than is the case with thicker concentration and alkyl chain length. Micelle aggregation samples. The steric interaction with the planar glass wall dominates rheological behavior. Low-viscous solutions apparently facilitates the formation ofthe pseudoisotropic contain spherical micelles. Rod micelles produce highL b phase. This shows that strong kinetic inhibitions viscous systems. The micelle shape itself depends on the dominate, making it difficult to determine the phase combination effects of terminal-group area and surfactant boundaries more precisely. chain length and volume. Because of this reason shortThe formation of such a L,1 phase during the cooling of chain APGs form low-viscous solutions and long-chain a lamellar phase with fluid lamellae (Lab) below the Krafft AGPs form high-viscous solutions. Owing to strong steric temperature can be explained if it is assumed that, for interaction in systems with rod micelles, liquid-crystalline crystallization or partial crystallization to occur, on steric phases are formed in systems with high concentrations grounds the molecules have to be at an incline, leading of long-chain APGs. It was shown that lyotropic lamellar to a lamellar structure with less symmetry, Le., to a biaxial phases with completely different textures were formed. system. No boundaries with multiphase regions could be Only one of these phases possessed isotropic lamellae. found between the different lyotropic regions that were The two other phases exhibit a lower degree of symmetry observed in C12/14-APGsystems. This may be because the and show textures of thermotropic biaxial smectic phases. high viscosity of the system inhibits separation. However, Transitions from rods to lamellae and angled lamellae it is possible that the zones are only separated from each are plausible suggested structures. Similar behavior is other by higher order phase transitions. observed in the dilute concentration range when fatty It is ofinterest that the crystalline precipitations which alcohol is added.15 The knowledge of the volume phase are obtained when the dilute C12114-APGsolutions are behavior gives information on how APG can be used for cooled below the “Krafft-point”can be transformed to a technical applications. liquid-crystalline L,1 texture after crumbling by shear LA940902B between microscope slide and cover glass. A true solid crystalline phase cannot be changed by this procedure. Strongly hydrated C12/14-APGforms a “qua~i-crystalline”~~ (24) Luzatti, V. T.; Gulik-Krzywicki, T.; Tardieau, A. Nature (London) 1968.218.1031. state. We did not investigate in detail these low(25) Funari, S. S.; Holmes, M. C.; Tiddy, G. J. T. J. Phys. Chem. (23) Sadler, D. E.; Shannon, M. D.;Tollin, P.;Young, D.; Edmondson, M.; Rainsford, P. Liq. Cryst., 1986,1 (6), 509-520.

1992,96, 11029-11038. (26) Quist, P. 0.;Fontell, K.; Halle, B. Lzq. Cryst. 1994,16(21,235256.