Structure of lyotropic phases formed by sodium dodecyl sulfate in

Karen J. Edler , Matthew J. Wasbrough , James A. Holdaway and Benjamin M. D. O'Driscoll. Langmuir 2009 25 (7), 4047-4055. Abstract | Full Text HTML | ...
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2385

Langmuir 1991, 7, 2385-2393

Structure of Lyotropic Phases Formed by Sodium Dodecyl Sulfate in Polar Solvents X. Auvray,. T. Perche, R. Anthore, and C. Petipas Facultk des Sciences et des Techniques URA 808 (CNRS), B P 118, 76134 Mont Saint Aignan C&dex,France

I. Rico and A. Lattes Uniuersitk Paul Sabatier, Laboratoire des IMRCP URA 470 (CNRS), 118 route de Narbonne, 31062 Toulouse C&dex,France Received January 31,1991.I n Final Form: April 3, 1991 The lyotropic liquid crystals formed by sodium dodecyl sulfate in various polar solvents (water, formamide, glycerol, ethylene glycol, N-methylformamide) were investigated by using low-angle X-ray diffractionand optical microscopy. The method for rapid and continuous observationsby X-ray diffraction of the surfactant-solvent system throughout their transitions from the micellar to the lamellar phase is described: the micellar and lyotropicphases are observed along the line of equilibrium with the crystalline phase. Complex phases were observed for the sodium dodecyl sulfate (SDS)/water system which displays an original sequence of two-, three-, and one-dimensional ordered phases: hexagonal (p6m) centered rectangular (cmm) rhomboedral centered tetragonal lamellar. The rhomboedral structure turns into a cubic phase Im3m. In the other polar solvents the phase sequence is very different. The SDS/ formamidesystem exhibited the following sequence: hexagonal cubic (Ia3d) lamellar. Only lamellar structures were observed in the other less structured solvents. This work shows the importanceof geometric constraints and head polar-solvent interactions.

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Introduction

The hexagonal phase is separated from the lamellar phase either by a reentrant micellar phase in watefl or by Cationic surfactants have been shown to form lyotropic a 3D phase of space group Za3d, in nonaqueous solvents The structures and often in water as well. This cubic phase is bicontinphases in nonaqueous polar uous, comprising two labyrinths whose separation surfaces of phases formed by cetyltrimethylammonium bromide are thought to be either minimal periodic surfaces (IPMS) (CTAB) and cetylpyridinium bromide (CPBr) in formor a Schwartz G surface.gJ0 amide and solvents with weaker hydrogen bonds than forThe micellization of sodium dodecyl sulfate (SDS) in mamide or even the aprotic solvent N-methylsydnone have water has been extensively studied a t low concentrabeen determined by optical microscopy,14 NMR spection,11J2 as well as in hydrazine13 or hydrazine/water .~ troscopy, and X-ray scattering or diffraction ~ t u d i e sAt mixtures.14 It has been shown that there is the formation increasing surfactant concentration, the most frequently of the Ha phase, a deformed hexagonal phase, and observed sequence of phases is as follows: micellar phase “intermediate” phases of unidentified structure prior to 2D Ha hexagonal phase (p6m) Qacubic phase of formation of the La phase.*&” Recently, KekichefPgm space group Ia3d Lalamellar phase. The CTAB/forhas reported the appearance of a 2D M a monoclinic or mamide or glycerol and CPBr/formamide4 or N-methdeformed hexagonal phase and a 2D 1D transition via ylsydnone systems5 all display this succession of phases. three 3D ordered phases: R, rhombohedral Qa cubic In other systems, such as CTABIN-methyl~ydnone,~ space group Im3m T, centered tetragonal (Figure 1). lecithin/alkanediols and ethylene glyco1,Sand dioleoylphosThis cubic phase is a bicontinuous phase of symmetry similar to surface P.9 phatidylcholine (DOPC)/formamide and N-methylformamide,7only a lamellar phase is observed after the isotropic (8) Ekwall, P. Advances in Liouid Crystals; Academic Press: New phase. A similar sequence of lyotropic phases is observed York, 1975. with these surfactants in water (e.g. CPBr, lecithin, or (9) Charvolin, J.; Sadoc, J. F. J. Phys. (Paris) 1987,48,1559. (10) Mariani, P.; Luzzati, V.; Delacroix, H. J. Mol. Biol. 1988, 204, DOPC), although the CTAB/water system forms a 2D 165-189. .. monoclinicphase between Haand Q,, which is not observed (11) Cabane, B. J. Phys. (Paris) 1981,42, 847. in formamide or glycer01.~ (12) Cabane, B.; Duplessix, R.; Zemb, T. J. Phys. (Paris) 1988, 46,

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(1) Rico, I.; Lattes,A.;Belmajdoub,A.; Marchal,J. P.; Canet, D. Nouv. J. Chim. 1987, 11,415. (2) Warnheim, T.;Jonsson, A. J. Colloid Interface Sci. 1988,125,627. (3) Auvray, X.; Anthore,R.;Petipas, C.; Rico, I.; Lattes, A. C. R. Acad. Sci. 1988,306,695; J. Phys. Chem. 1989,93,7458. (4) Auvray, X.; Perche, T.; Anthore, R.; Petipas, C.; Rico, I.; Lattes, A. 8th International Symposium on surfactante in Solution, Gainesville, FL. 1990. -($-Auvray, X.; Perche, T.;Anthore, R.;Petipas, C.; Marti, M. J.; Rico, I.; Lattes, A. J. Phys. Chem. 1990, 94, 8604. (6) El Nokaly, M.. A.; Ford, L. D.; Friberg, S. E.; h e n , D. W. J . Colloid Interface SCL1981,84, 228. (7) Bergenstahl, B. A.; Stanius,P. J . Phys. Chem. 1987,91,5944-5948.

0743-7463/91/2407-2385$02.50/0

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2161. (13) Ramadan, M. Sh.; Evans, D. F.; Lumry, R. J . Phys. Chem. 1983, 87, 4536. (14) Ramadan, M. Sh.; Evans, D. F.; Lumry, R.; Philson, S. J. Phys. Chem. 1985,89, 3505. (15) Husson,F.; Mustacchi, H.; Luzzati, V. Acta Crystallogr. 1960,13, 668. (16) Leigh, I. D.;MacDonald,M. P.; Wood,R. M. J. Chem. Soc.,Faraday Trans. 1 1981, 77, 2867-2876. (17) Wood, R. M.; McDonald, M. P. J . Chem. SOC.,Faraday Tram. 1 1985,81, 273. (18) Kekicheff, P.; Cabane, B. J . Phys. (Paris) 1987, 48,1571. (19) Kekicheff, P.; Gabrielle-Madelmont, C.; Ollevon, M. J. Colloid Interface Sci. 1989, 131, 112. (20) Kekicheff, P. J. Colloid Interface Sci. 1989, 131, 133.

0 1991 American Chemical Society

Auvray et al.

2386 Langmuir, Vol. 7, No. 10, 1991

2

50

60

0

70

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1 %SDS

Figure 1. Phase diagram of the SDS/water system according to k e k i ~ h e f f . ~ 9Hatched ~ ~ + ~ lines indicate the biphasic domain.

Are similar complex phases observed in other polar solvents? The present study was therefore undertaken to determine the lyotropic phases formed by SDS in four nonaqueous polar solvents with a progressive drop in structure: formamide, glycerol, ethylene glycol,and N-methylformamide. The SDSIN-methylsydnonesystemcould not be studied since SDS is insoluble in this solvent. Micellization of SDS in formamide has been evidenced by measurement of surface tension21 above 55 "C or by Raman spectroscopy.22 In a previous we demonstrated the formation of lyotropic phases of SDS in formamide. In the present study, we used the X-ray diffraction technique successfully employed for rapid detection of the lyotropic phases formed by cationic surfactants.3-6We have applied our method to the study of the behavior of a complex binary system such as the SDS/water system. Then we also compared the SDS/water system with SDS/ nonaqueous polar solvent systems.

Materials and Methods Materials. Sodium dodecyl sulfate (SDS)(Merck, 99% minimum purity) was used as supplied. Formamide (Aldrich, 995% spectrophotometric grade), glycerol (Aldrich, 99.574 spectrophotometric grade), ethylene glycol (Aldrich, 99% spectrophotometric grade),and N-methylformamide (Aldrich,99%) were kept over molecular sieves (0.3nm). Water was twice distilled. The X-ray diffraction study was carried out by use of a small angle diffraction setup with a linear localization detector with a spatial resolution of 136 pm. The radiation Cu K a (0.154nm) monochromatized by a quartz bent crystal is focused at the detector with a mid-height beam width of 80 pm. The detector was placed 26.5 cm away from the sample. The distance between cell and detector was maintained under vacuum to prevent scattering from air. A series of slita between the monochromator and the sample reduced parasitic scattering. With this geometry the range of scattering vectors s was 4 X lo-, nm-' 5 181 5 1.2nm-l with 181 = 20/X,20 being the angle between incident and scattered beams and X the wavelength. This setup was used to observe the diffraction spectra of the ordered liquid-crystal phases with lattice parameters ranging from 10 to 1.5 nm. Since the intensities of the strongest and weakest Bragg reflections were generally in a ratio of 100:1,the strongest reflections could be observed within a few minutes, although it took several hours to accumulate the whole spectrum. Focusing the beam on the detector produced narrow peaks and enabled observation of weak peaks above the background noise. A system of moving horizontal slits in front of the detector reduced effects of beam height and improved the signal to noise ratio. (21) Rico, I.; Lattee, A. J. Phys. Chem. 1986,90, 5870. (22)Amorim da Costa, A. M. J. Mol. Struct. 1988, 194, 195. (23) Auvray, X.; Danoix,F.; Perche, T.; Petipas, C.; Duval,P.; Rico, I.; Lattee, A. C. R. Acad. S i ,Ser. 2 1990,310, 471-476.

1 0

c2

%

*

Figure 2. Diagram of the transformations of the sample with change in temperature. The dotted parts represent limits of the domains of stability of a+ phases. They were adjusted to form a horizontal entrance slit of around 3 mm in front of the detector. The sample cell was made of brass with flat Mylar windows. The sample thickness was set at 1 mm with a Teflon spacer. The sample was heated from 13 to 98 OC by water circulating round the cell. The temperature of the water bath was regulated to *O.l "C by a microprocessor,and the temperature in the cell was measured before experiment with a thermocouple placed in the sample at the X-ray entry point. To check homogeneity and to observe effects of texture and the possible formation of large liquid crystals, the sample could be moved in the horizontal and vertical axes over an area of 25 mmp. A rotating movement of the cell about ita vertical axis allows an optical observation of the specimen to check either the transparency of lyotrope crystal phase or the presence of an excess of surfactant when ita Bragg peaks are weak or not observed because of texture effect. To enable observation of phases in a number of surfactant/ solvent systems, we employed a rapid method in which the solution was prepared directly in the sample cell by placing a drop of solvent on a layer of surfactant. If the slope of the equilibrium line between lyotropic phases and solid crystal on the temperature/concentration diagram is positive, all successive phases in equilibrium with excess surfactant can be observed by increasing the temperature (tracing 1 in Figure 2). This line was followed until the solid was completely dissolved. Above this point, concentration remains constant throughout the domain of existence of the last observed phase (tracing 2 in Figure 2). This method cannot be used to determine the temperature/ concentration diagram since at any given temperature the concentration is unknown and varies as the amount of excess surfactant falls. However, this method does enable observation of successive intermediate phases in a single sample even if they only exist over a very limited range of concentration. When the surfactant is totally dissolved, further surfactant can be added to enable observation at higher concentrations. Thus the entire diagram from the micellar up to the lamellarphase can be obtained with just two or three samples. With temperature changes of around 0.1 OC, the appearance and transformations of the phases can be followed with some precision, and the changes in structure can be observed all along the equilibrium line. It is also possible to observe the continuity of reticular distances of two successive phases. The rates of rise and fall in temperature and the length of time of an experiment at a constant temperature were controlled by the program. A number of spectra were recorded a t the same temperature to ensure equilibrium conditions and system stability. The highly complex SDS/water system could be readily observed by using this method, which could thus be extended to other systems. I. SDS/Water System. In water, two chemical problems are present together: (i) the acid-base equilibrium between the base ClzHsOS03- and the conjugated acid ClpHsOSOaH C,,H,,OSO~,Na+

+ H,O

s C,,H,,OSO3H

+ NaOH

(I)

(ii) the hydrolysis of SDS salt leading to dodecanol and sodium

Lyotropic Phases Formed by

SDS in Polar Solvents

hydrogen sulfate

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Ci2H,0SO~,Nat

+ H,O

Langmuir, Vol. 7,No. 10,1991 2387 L 'UI

C,,H,OH

+ NaHSO,

(11)

The equilibrium (I)is shifted to the right in acidicmedium, shifted to the left in basic medium. The reaction (11) is easier in an acidic medium and at high temperature, over 90 OC.U To limitate effect of temperature, we worked at temperatures below 75 O C . To examine the effect of pH on the temperature of appearance of ordered phases along the equilibrium line, samples were prepared with double distilled water or heavy water (pH = 7) or water containing sulfuric acid (2 5 pH 5 5) or sodium hydroxide (8 5 pH 5 10). The distance d, the opposite of the scattering vector scorresponding to the maximum of the scattered intensity, and the reticular distances dM1of the characteristic Bragg reflections of the ordered phases are marked along tracing 1 as a function of temperature. On the diffraction diagram, pure surfactant is characterized by four rings corresponding to the four orders of the reticular distance (dl = 3.85 nm). Immediately after addition of R drop of water at 13 OC, the diffraction diagram is that of pure surfactant. Scattering can be observed at this temperature as the solubility of SDS is around 2 % . As the sample is heated, a new system of peaks appears, corresponding to the three orders of the reticular distances (d2 = 3.26 nm) and the transformation of the starting salt into a hydrate. The two salts coexist over a range of a few degrees until complete disappearance of the initial salt. The temperature at which the second salt appeared was found to depend considerably on the initial conditions (temperature, proportions of surfactant and solvent, e).: 1. Micellar Domain. An isotropic phase in equilibrium with surfactant excess is observed along tracing 1 for SDS in H20 or D20 from 13 to 24 OC. At 13 OC the solubility of SDS is 2% and the isotropic phase contains spherical micelles. A t higher temperature and concentration, the micelles are elongated with a size distribution that is highly sensitive to surfactant concentration and temperature.% At low concentration (2%), the scattering intensity exhibits oscillations due to the form factor of spherical micelles in a medium in which the electron density of the solvent lies between the electron density of the outer head group layer and the aliphatic core.% The first minimum in form factor is close to the maximum of the structure factor which accounts for the intensity pattern observed. On further rise in temperature, the shifts in these two extremes alter the intensity until the appearance of a maximum characteristic of the structure factor observable at 22.5 OC for a concentration above 30%.The Bragg reflection 10 of the hexagonal phase emerges from this maximum at 24 OC (Figure 3). No discontinuity was observed between d, the mean distance between micelles, and the reticular spacing dlo of the Ha phase (Figure 4). 2. 2D Phase Domain. The hexagonal phase between 24 O C and 36.5 OC in H20 or DzO was characterized by Bragg reflections with reticular spacings in a d3:d4:.\/7 ratio. The lattice parameter ah fell rapidly from 5.46 nm at 24 OC to 4.62 nm at 27.5 OC to reach 4.39 nm between 32 and 36.5 OC. The changes in dlo crystal as a function of temalong the equilibrium line H, perature are shown in Figure 4. A transformation of the H, phase occurred at 36.5 "C with the appearance of two strong peaks on either side of peak 10 of H. (Figure 4), along with a number of much weaker peaks. This phase corresponds to the Maphase of the diagram shown in Figure 1, with a 2 OC shift, identified by Kekicheff as a monoclinic phase1*and by Luzzati as a centered rectangular phase.16 The centered rectangular structure can best be determined at the onset of formation from the maximum number of observable reflections. The values of reticular spacings dM at 37 O C are listed in Table I. Following the equilibrium line, the change in lattice parameters leads to a shift in positions of the peaks. Some peaks tend to mask others, which can lead to ambiguities in structural identification. Thus for a = b d 7 , d31= dm and d22 dal around 46 OC (Table 11). Continuous monitoring of the shift in reflections

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(24) Kekicheff, P. These d'sEtat, Orsay, France, 1988. (25) Mazer, N. A.;Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976, BO. 1075-1084. (26) Zemb, T.; Charpin, P. J. Phys. (Paris) 1985,46,249-256.

0 66 112 Figure 3. Scattering curves of a micellar solution of SDS in water in the presence of salt at 21 "C (dotted) and at 23 O C (solid line). The diffraction lines 10 and 11 of the Ha phase at 24.4 OC are represented by the dashed-dotted line. The diffractions peaks from the salt are not shown.

I I \ dnm

/

5.04

Figure 4. Lattice distances of strongest diffraction lines of the H,, M,, and R. phases along tracing 1 with aqueous (solid line) or acidified (dotted line) solutions of SDS. Some interplanar distances of R. are in a ratio: dlioldt11 = d 3 ; dlio/drlo = v'+ diioldszi = d 7 . Table I. Centered Rectangular Phase M. along the Equilibrium Line at 37 h k dmeudlnm d d d , nm I 2 1 3 4 0 2 5 4 6 1 3 7 6

0 1 1 0 2 2 1 2 0 3 3 1 2

4.46 3.65 2.38 2.23 1.98 1.83 1.62 1.47 1.32 1.205

4.46 3.65 2.39 2.23 2.00 1.82 1.63 1.49 1.49 1.32 1.22 1.215 1.19

+++ ++++ 0 ++++---

-----

a a = 8.92 nm and b = 4.00 nm. I = intensity; +, strong; -, weak; 0, moderate.

is the only way of observing the centered rectangular structure up to 47.2 "C, i.e. throughout the domain of existence of the 2D phase. From 36.5 to 47.2 "C, there was a change in lattice parameters (8.92 and 4.0 nm to 10.26 and 3.68 nm for a and 6, respectively). The Ha M, transition was similar to that observed with CTAB under the same condition^.^ Along the equilibrium line, the phase transition was characterized by a breaking up of diffraction line 10 of H, (Wood observed a continuous transition from Ha to Mal7).

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Auvray et al.

2380 Langmuir, Vol. 7, No. 10,1991 Table 11. Centered Rectangular Phase M, along the Equilibrium Line at 46.5 O C * h k dmeudt nm nm Z 4.98 4.98 +++ 2 0 1 1 3.45 3.45 ++++ 2.49 2.49 + 4 0 3 1 2.49 2.465 + ___ 1.89 1.84 0 2 1.73 1.752 + 5 1 2 2 1.73 1.725 + a = 9.96 nm and b = 3.68 nm. I = intensity; +, strong; -, weak. 'U,

220

11

decreased. The angle a,which is initially above 109.47O on the equilibrium line, fell progressively as the temperature rose, reaching 109.47O and continuing to decrease (Figure 7). The temperature at which a reached 109.47' depended on the concentration, since the value of a increases with increasing concentration along the equilibrium line. These results show that the cubic phase Im3m is obtained by continuous deformation of R,: at fixed concentration c, &I exist at only one temperature; its domain of existence is a straight line through the domain of existence of a R, phase in the phase diagram. The domains of existence of the various phases and their corresponding parameters can be modified without altering their structure by changing the ionic activity of the solution. The extent of the modification depends on the ionic strength. We report here the results with samples containing sodium hydroxide or sulfuric acid (8 5 pH 5 10; 2 I pH I5, respectively). In basic medium (pH of solvant 9), we observed the M a R, transition at 44.6 O C . At this temperature, the M, lattice parameters (a = 9.4 nm and b = 3.6 m) were below those observed in distilled water for this transition, and there was a greater discontinuity between reticular distance 11of M aand 211 of R, (0.1 nm versus 0.04 nm). We observed that the angle a at the appearance of R, was 109.42', close to that corresponding to a &I cubic structureof space group Im3m. This phase thus appears along the equilibrium line during the onset of formation of R,. In acid medium (pH of solvant 3),we found that the R, domain was shifted toward higher temperatures with a M a R, transition at 52.4 O C (Figure 4). The lattice parameters of M a(a = 8.75 nm and b = 3.8 nm) gave riscto a perfect continuity between reticular spacing 20 of M aand 211 of R,, and-a weak discontinuity between reticular spacing 11of M, and 211 of R.. At the onset of R,, a was 107.52O and rose to 1 0 9 . 8 5 O at 57 OC. In this case, the cubic phase occurs toward the end of the domain of existence of R,. The line of existence of the &I phase meets the crystal/Ra phase equilibrium line with samples prepared in basic or acid media. In acid media, the two meet at the start of the R, phase, whereas in basic medium they meet toward the end of this phase. These results indicate the influence of chemical conditions on both the domains of existence of the phases and their parameters.% This may explain the results obtained with different samples (shift in temperature of appearance of the various phases for example), as well as those where surfactant has been partially hydrolyzed after thermal cycling. The flat windows of the sample cell may orient the cylinders of the hexagonal phase and the M aphases. Leigh has reported that the glass sides have differing influences on the rods of the Ha phase oriented parallel to the walls and those of the M, phase perpendicular to the glass slides.l8 This alignment suggest that rod-rod interactions in M aare lower free energy than rod-surface interactions. Leigh invokes repulsive forces of hydration and hydrogen bonding. The textures observed from the favored orientations of large crystals are reflected by the ratios of intensity of lines 20 and 11, which are altered by a factor of 10. Although it is not possible to deduce the orientation of the axes of the rods from these measurements, they do show that the textures of M, induce those of R. giving rise to diagrams as different as those represented in Figure 6. The _extrastrong intensity of peak 11 of M, entrains that of peak 211 of R, (Figure 6, curve c). The orientation of R, gives rise to diagrams like that shown in Figure 6d, which can be interpreted in terms of a 2D complex hexagonal phase.16 This will be discussed below. In water around 49.5 "C,we observed the progressive disappearance of the diffraction lines of R, and the formation of a new spectrum characterized by three strong Bragg reflections, over a temperature interval of around 1"C. The overall diagram can be interpreted in terms of a T, centered tetragonal phase.%Along the equilibrium line, ita domain of existenceonlylasted for several tenths of a degree,although it extended over 1O C in basic medium. 4. Unidimensional Phase. The lamellar phase appeared at 51.8 O C , and the peak d l was ~ quite continuous with peak 002 ~ 3.20 nm) as reported by Kekicheff.ls The peak of of T. ( d l = the lamellar phase was accompanied by a diffuse band of strong intensity. On cooling,diffraction lines 101and 200 of T,emerged

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L L

47.6% 47.4% 47.2% 47.0"~

46.8% -11

4 -46.6%

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Figure 5. Diffraction spectrum showing the M a R, transition for a SDS/DaO sample at 7176 SDS. The line 20 of M adisappears progressively while line 11is enhanced by a particular orientation almost coinciding with line 211 of R,, which is also enhanced. 3. 3D Phase Domain. The method used enables observation of the transition of the M ato a 3D phase over a 0.3 O C interval between46.8and 47.3 O C (Figure 5). This plgse-wasccharactyized by four well-resolved strong reflections (110,211,220,211) and a complex band of lines more or less separated whose features depended on temperature and ionic activity of the solution (Figure 6a). The overall spectrum can be regarded as a R, rhombohedral phase.% The changes in reticular spacings in the domain of existence of this phase from 46.8 to 49.2 O C along the equilibrium line are reprssented in Figure 4. With pure water as solvent, reflection 211 of R. almost merged with reflection 11of M awith a jump of around 0.05 nm. The complex band contained reflections 311, 111, and 120 which were frequently not well resolved. The positions of these diffraction lines may be reversed along the equilibrium line. From 46.8 to 49.2 O C the lattice parameters a, and the angle a of the R, phases changed from 9.87 to 10.34 nm and from l l l . O o to 111.9O, respectively. It is thus impossible to obtain a rhombohedral lattice with an a angle of 109.47', corresponding to the primitive lattice of a Q, centered cubic phase (Im3m), on the equilibrium line even at a point as shown on Kekicheffs diagram. The possible presence of a cubic phase of space group Im3m in the SDS/water system is of interest as it has not been observed with other surfactantsin either water or other polar solvents. We therefore carried out a study at fixed concentration over a restricted part of the diagram (58-70%). In other experiments, we altered the activity of the solution by addition of sulfuric acid or sodium hydroxide to the water. In the domain of existence of the R, phase, the samples at fixed concentration are heated. The parameters a, and a

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Lyotropic Phases Formed by SDS in Polar Solvents

Langmuir, Vol. 7, No. 10,1991 2389

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Figure 6. Diffraction spectra for various orientation textures of the R. phase and with the cubic phase of space group Zm3m. (a) Diffraction spectrum of the R, phase (a, = 9.85 nm, a = 108O) at 53.3 O C with a sample prepared in acid medium containing 64% SDS. Lines 111,311,and 120 are unresolved and correspond to the wide diffuse bands observed by Kekicheff. (b) Diffraction diagram of the Q. phase of space group Zm3m. The interplanar spacings are in the ratios 1, d 3 , 4 4 , d 5 , d 6 , 4 7 , 4 8 , 4 9 , 410, 411, d13, the peaks corresponding to d 2 and d l 2 were below the detection threshold. The lattice parameterjs 11.75 nm. (c) Diffraction diagram of the R. phase at 47.4 O C for a sample of 71% SDS (a, = 10.14 nm and a = 111.55'). Line 211 is very strong (dzli = 3.49 nm). This diagram should not be confused with that obtained from a mixture of a 2D hexagonal phase and a cubic phase of space group Za3d formed by SDS in formamide (Figure lo), exhibiting a succession of narrow lines. (d) Line 211 and the diffused band 111-311-120 showed by an arrow are very weak. When the R. phase is in this orientation (a, = 10.1 nm, a = 111.5O), the diffraction diagram becomes that of a phase we refer to as 2D complex hexagonal (see Discussion). from this band, and all the previously observed phases appeared in reverse order with a temperature shift due to surfusion effects. 5. Analysis of Results. We confirmed the results of Kekich. phase whose domain of eff, except for the existence of the Q existence was reduced to a line crossing the R. domain: Ha M a R. Tu La. The Maphase was interpreted as a centered rectangular phase over its entire domain of existence along the equilibrium line. In water, the R. phase was observed continuously along the equilibrium line and in the domains marked R. and Q. on Figure 1. The parameters of the R. phase depended on those of the M a phase and varied with temperature and concentration over the whole domain of existence of this phase. The cubic phase represented a stage in the course of this deformation. The deformation of the R, phase can be interpreted in terms of a triple hexagonal cell with the following parameters: c = a,d3(1 + cos a)l/* and ah = a,(2 - cos During the deformation, parameter c = 3dlll only changed slightly, whereas ah was altered. The rhombohedral cell closes up as the temperature increases at fixed concentration and opens out as concentration increases along the equilibrium line (Figure 7). The type of diagram represented on Figure 6d can be viewed as a particular orientation of the rhombohedral lattice. If the 11111axis of the rhombohedral cell confluent with the lOOll axis of the triple hexagonal cell is perpendicular to the windows, only the hkO points of the reciprocal hexagonal lattice diffract the X-ray beam (Figure 8). In the (001)* reciprocal plane, these points form a planar hexagonal lattice of which the translations defining theiwo-dimensional unit cell are not points 300 and 030 but points 210 and 110, corresponding to a 2D hexagonal phase with parameter a' = (ahd3)/3. The diffraction diagram rep-

---

'r nm

10.0

,\I

II

\ \

\ \ \

\\

50

55

\

60

BEL T'C

Figure 7. Plots of a and a, as a function of temperature along the equilibrium line (solidlines) and at a concentration of around 68% SDS in water (dotted lines).

Auvray et al.

2390 Langmuir, Vol. 7,No.10,1991 I u.8.

030

211

4

x

32 s nm-1

2io

I

(iio)

0.6

Figure 8. Positions of points (hkO)in the reciprocal plane (001)* of the triple hexagonal cell. The Miller indices of the same reticular planes indexed in the rhombohedral lattice are shown in brackets. If HKL are the Miller indices in R,, hkZ in the triple hexagonal cell: H = (2H + k + Z)/3, K = (-h + k + Z)/3, L = (-h - 2k + Z)/3. The lattice shown in broken lines represents the reciprocal lattice of the 2D complex hexagonal lattice.

4.0

*

1.2

Figure 10. Diffraction spectrum of the Qaphase (Ia3d) a t 85 "C following tracing 2 (Figure 2). The observed lines are 211, 220,321,400,420,332,510, and 431. The intensities of the two curves are in a ratio of 1:32.

micelles

--_

45

55

65

75

85

T"C

Figure 9. Plot of d,

and lattice distance of ordered phases of SDS/formamide (dotted) and SDS/ethylene glycol (solid line) with temperature. resented on Figure 6d can be interpreted as being due to a 2D hexagonal phase with parameter a' = 8.97 nm, which can account for the confusion between a R, phase whose 1111axis is parallel to the incident beam and a 2D hexagonal phase. a' is comparable to the complex hexagonal phase parameter, which is a most improbable structure.lb17 In conclusion,the temperatures a t which the different phases appear and the extent of their domains of existence depend on the nature of the sample.24 Under certain conditions (after thermal cycling, pH), the line of existence of Q, can cross the crystal/R, phase equilibrium line. 11. SDS/NonaqueousPolar Solvent Systems. Depending on the nature of the solvent, quite different behavior is observed by either light microscopy or X-ray diffraction and scattering. The SDS/formamide system stands out from the other systems studied (SDS/glycerol, ethylene glycol or N-methylformamide). Immediately after addition of formamide, a new diffraction diagram was observed consisting of two Bragg reflections at 2.07 and 1.035 nm. The three diffraction lines of pure SDS were no longer observed. In solutions containing crystals of SDS and formamide, the scattering curves exhibited diffuse bands at temperatures above 38.5 "C. The intensity and position of the band depended on temperature (Figure 9). A t 57 "C and for a monomer concentration around 43%, diffraction peaks characteristic of a 2D Haphase were observed. In contrast to the continuity observed between d, and dlo of Hain water, a discontinuity of 0.16 nm was observed in formamide. The parameter of the Ha phase altered little ( a h = 3.89 nm) over its whole domain of existence up to 79.8 "C in spite of a marked change in concentration. No breaking up of the

Figure 11. Micrographs showing the Hahexagonal plane a t 68 "C for 62% SDS in formamide (photo a) and the lamellar phase in the presence of.an isotropic phase at 110 "C formed by SDS in glycerol. diffraction line giving rise to the Ma phase in water was observed. A phase transition occurred at 79.8 "C. The set of lines (Figure 10) can be interpreted in terms of a 3D phase of space group Ia3d. The spectrum,which was quitedifferent from that observed in water (Figure 6), resembled that observed with CTAB and CPBr.3-5 The discontinuity between dlo of Haand d211of Q, was around 0.1 nm. The lattice parameter of the cubic phase was 7.98 nm. The Q, L, transition took place at 84 "C, evidenced by the appearance of the dlm line between lines 211 and 220, corresponding to an interlamellar distance of 3.06 nm. Optical microscopy confirmed these results (Figure lla). Examination of identical samples that had been quenched and fractured showedthe existence of the Ha phase at a concentration of 62 % .5 In the three other solvent systems containing crystals of SDS, the scattering curves displayed a maximum (Figure 12) above

-

Lyotropic Phases Formed by SDS in Polar Solvents

Figure 12. Scatteringcurves produced by an isotropicsolution of SDS in ethylene glycol at 65.6 "C (solid line) and in N-methylformamide at 94 "C (dotted line). The diffraction spectrum of the Laphase waa observed in the SDS/ethylene glycol solution at 94 "C.

38.4 "C for ethylene glycol and above 47.4 "C for glycerol and N-methylformamide. The intensity of the scattering increased with temperature, although the positions of the maximum only changed slightly (Figure 12). No diagrams characteristic of the Ha or Qaphases were observed. In the SDS/ethylene glycol system, a lamellar phase was formed at 90 "C with a 2.68-nm interlamellar distance. The lamellar phase observed by optical microscopy formed around 104 "C in glycerol (Figure llb)and 136 O C in N-methylformamide (Table 111).

Discussion Depending on the nature of the solvent, association of molecules of SDS gives rise to different sequences of ordered phases with increasing SDS concentration. One-, two-, and three-dimensional ordered phases were observed in water and formamide, but only unidimensional phases were detected in glycerol, ethylene glycol, and N-methylformamide. In formamide, there was a clear-cut formation of rodlike micelles. At 42% SDS they turned into a hexagonal phase with a lattice parameter that was consistently less than that observed in water. If one assumes that the Ha phase is formed from rods of circular bases and infinite length, the radius of the rods and the surface area CT per polar head at the paraffin interface are related to the concentration c of surfactant by relationship2' r = (31/2c/2a)'/2ah u = 2v/r d v o / d v where v is the volume of a chain and uo is the volume of a molecule of surfactant.28 The values of the parameters are listed in Table IV. These results show that at low surfactant concentration there does not appear to be an ordering of infinite cylindrical micelles in formamide, although this does occur in water. In formamide as in water, the cylindrical micelles are infinite just before the Ha- Q, transition. Their radii and polar head surface areas are almost identical in these two solvents and are close to the theoretical value 2~111,(1, = maximum length of the chain = 1.67 nm and 2~11,= 0.42 nm2). The minimal distance, e, between charged surfaces is less in formamide (e = ah - 2r = 0.35 nm) than in water (0.91 nm). These results are comparable to those observed in the CTAB/water, formamide, and glycerol systems (e = 0.9 (27) Gulik, A.; Luzzati, V.; de R w . M.: Gambacorta. A. J. Mol. Biol. 1985, 282,131-149. (28) Cabane, B.; Duplessix, R.; Lmb, T. J. Phys. (Paris) 1985, 46, 2161-2178.

Langmuir, Vol. 7, No. 10,1991 2391

nm in water and 0.4 nm in formamide3). In formamide, the micelles formed by SDS like those of CTAB do not deform in a monoclinic structure as observed in water. The behavior of SDS in water is unusual since the Ma phase turns into a rhombohedral phase which gives by deformation a centered cubic phase of space group Im3m, followed by a tetragonal phase before giving rise to the lamellar phase. The bicontinuous Im3m phase withIPMS has only rarely been observed. It consists of two 3D labyrinths of intertwined but unconnected rods of aliphatic chains. One labyrinth connects the points with semiwhole coordinates, and rod axes parallel to the edges of the unit cube.9J0pB By extrapolation, it may be assumed that the rhombohedral phase is also formed from two 3D labyrinths whose junctions are the R, lattice points and the axes of rods parallel to direction of 11011 type which are orthogonal when the angle of R, is 109.47O. The distance between two neighboring junctions of the same labyrinth is 4 2 4 1 + cos and so a, in this particular case: cos a = -113. At 54 "C for a concentration of 68%, this distance is thus 11.26 nm. The distance between neighboring junctions not belonging to the same labyrinth is a, or (a,d3)/2 = 9.75 nm. MarianilO reported that the radius r of rods of hydrocarbon medium was 2.37 nm, which is considerably greater than the maximum length of the chain (1.67 nm), and thus not compatible with circular section rods. The orientation of the R, crystals depends on that of the M, structure. When the (11) planes of M, are perpendicular to the scattering vector giving rise to an enhanced diffraction line 11, the line 21T of R, is also enhanced. The two lines are of equal intensity, and the distances dll and dzll are similar in water and identical in acid medium (Figures 4 and 5). There is a similar configuration around these planes, and the (11) planes contain the axes of rodlike micelles with elliytical sections or in the form of ribbons. In R,, the (211) planes contain all the axes of the two labyrinths, parallel to the directions of type 10111. The intensities and positions of peaks 11 and 211, and the fact that the value of r is more than the maximum length of the chain lend support to the view of KekichefP4that in the Im3m phase the labyrinths consist of cylinders either of elliptical rather than circular section or in the form of interconnected ribbons. A T, phase as described by Luzzati30 and Kekicheff18 should consist of two planar lattices translated by l / 2 l / 2 '12 from unconnected rods with axes parallel to directions lOOl and lOlOl. The smallest distance between two unconnected rod axes is 4.17 nm with at = 7.6 nm and ct = 6.4 nm, giving a minimal thickness of water around 1.6 nm, which is quite large with respect to the 0.9nm thickness of the Haphase. The two models suggested for R, and T, thus do not appear to be representative. The R, phase may be related to a IPMS with rhombohedral symmetry, the H phase of Schwartz or D, phase produced by deformation of IPMS. Dr31generated by a rhombohedral distortion of cubic surface D related to space group Pm3n is a possible candidate. But the transition between the rhombohedral phase R, and tetragonal phase is not a Bonnet transformation since it is accompanied by a high enthalpy of t r a n s i t i ~ n . ' ~In* addition, ~~ these descriptions are essentially static, and some examples of static and/or dynamic disorders have been reported.'O Experimentally (29) Fontell, K. Mol. Cryst. Liq. Cryst. 1981, 63, 59. (30) Luzzati, V.; Tardieu, A.; Gulek-Krrywicki,T. Nature 1968,227, 1028-1030. (31) Lidin, S. J. Phys. (Paris) 1988,49,421-427. (32) Hyde, S. T.; Andersson, S.;Ericsson, B.; Larsson, K. Z. Kristall o p . 1984,168, 213-219.

2392 Langmuir, Vol. 7, No.10, 1991

Auvray et al.

Table 111. P h o ~ Observed e~ with SDS in the Four Nonasueous Solvents* formamide t, O

d,,

C

nm

Hat , O C Oh, nm Qa t , O

C

38.5 3.88 57.0 3.98 79.8

a,, nm La t , "C

alycerol

57 3.58 79.8 3.89 84

56 3.95

d , nm

>99 3.33

-

7.96 84 3.06

N-methylformamide

ethylene glycol 38.4 3.34'

104 b

90 2.91

56

90

d

2.68

-

90 2.68

136 b

*

a Domains of existence with temperaturealong the equilibriumline and the lattice parameters are indicated. The temperature was measured during optical microscopic observation. The position of the maximum was not determined at 38.4 O C , the value is that at 47 "C. In view of the spread of the curve, the position of the maximum was not determined.

Table IV. Parameters of Ordered phase^ in the SDS/ Water and SDWFormamideSystems concentration SDSJwater SDSJformamide

0.39 0.57 0.42 0.75

ah, nm

r, nm

u, nma

U O , nm2 ~

5.46 4.39 3.98 3.89

1.79 1.74 1.35 1.77

0.42

0.46 0.47 0.61 0.46 (5)

0.44 0.56 0.43

a uo is the surface area of each polar head at the interface with the solvent: uo = o d v o / d v = 2voJr.

one observes a narrowing of the diffraction lines of R, when this corresponds to a cubic phase, and hence a state of lower disorder impose by the symmetry (Figure 6a,b). In formamide, the situation is simpler. A cubic phase of space group Ia3d is formed at a concentration c of SDS of around 75 % with ac = 7.96 nm. This configuration of two unconnected labyrinths of rods has been described e1sewhere.s: Each junction involvesthree rods a t an angle of 120°, parallel to the direction of 11101type. The radius r of the rods can be calculated from the relations hi^^^ ca:/24 = *r21(1- 0.491r/l) 24 is the number of portions of rods in the unit cell of volume ac3,and 0.491 is a coefficient that depends on the bevel-shaped iunctions of the rods, 1 is the length of the rod, I = a c / d 8 = 2.82 nm, i.e. the distance between two neighboring junctions of the rods in the same labyrinth, giving r = 1.57 nm. The surface area S of the each rod allowing for intersection is given by the relationship27

5' = 2 4 1 - 0.735r/l) The surface area uooccupied by a molecule at the solvent interface is given by relationship

-

-

= S24u0/ca,3 giving uo = 0.42 ( 5 ) nm2 and u 0.39 nm2. The Ha Qa transition leads to a decrease in diameter 2r of the rods, and a fall in surface area per polar head at the solventpolar head interface (UO = 0.47 nm2 in Ha)(Table IV). The distances, I of the point in formamide furthest from the surface of the rod is 1,-= Dmax-r Dm, being the distance between the points of coordinates ( l / 2 '/2 l / 2 ) and (3/e l / 2 l / 4 ) giving 1, = 0.6 nm. The minimal thickness e of the layer of formamide is thus d 3 a c / 4 , the distance between two junctions of two labyrinths (3.45 nm), reduction for the diameters of the rods: e = 0.31 nm, a much lower value than that found in the Haphase. It should however be noted that the rod model is not good since the ratio r / l is not far from unity (0.56),and sothe rectilinear part of the cylinder is relatively bo

short. These results give an indication of the compact nature of the Q, phase. It should also be noted that the planes containing the axes of the labyrinths are of the family of reticular planes (2111,and the distance between parallel planes containing the junctions is a multiple of d211/4. The interplanar spacings dol of Ha and dzll of Qa (Figure 9) and the intensities of the diffraction lines are comparable, but the planes (211) contain all the axes of the rods. During the Ha Q, transition in formamide, the decoration and the distances of these planes are conserved, as is observed during the M a R, transition in water. The Q a L a transition for a surfactant concentration around 80% is still accompanied by a fall in surface area UO: u uo = 0.39 nm2,the values of u are the same in the Q, phase as in the Laphase. Assuming formamide does not penetrate into the lipid layers, the solvent layer is thus two molcules thick (0.6 nm). The complexity of the SDS/water system compared to the simpler SDS/formamide system is possibly due to the fact that SDS is stable in formamide but not in in which the reactions (I and 11)are always existing. In the three other solvents, glycerol, ethylene glycol, and N-methylformamide, no ordered 2D or 3D structureswere detected along the equilibrium line in the presence of a concentration gradient either by X-ray scattering or optical microscopic examination. The presence of micelles prior to formation of the lamellar phase was not demonstrated. The maximum in the scattering curve in the isotropic phase preceding the lamellar phase (Figure 12) could have been due either to micelles in the form of disks which order like those in the dodecylammonium bromide and chloride/ water systems33or to more or less structured aggregates, or even vesicles as in the lecithin/water system.34 In glycerol and ethylene glycol, SDS behaved quite differently to the cationic s ~ r f a c t a n t s . The ~ ~ ~sequence *~~~~ of ordered phases formed by CTAB in these solvents is the same as that observed in f ~ r m a m i d e The . ~ ~ differences ~ between SDS and CTAB can accounted for by the difference in chain length and the more solvophobic behavior in f0rmation.3~~38 Ethylene glycol has frequently been used with surfactants derived from lecithina37-39 giving rise to an isotropic phase of undetermined structure and a lamellar phase. The interlamellar distances for a given concentration are

-

-

-

-

(33) Holmes, M. C.; Charvolin, J. J. Phys. Chem. 1984,88,810. (34) Bangham, A. D. J. Mol. Bio. 1964,8, 660. (35) Ionescu, L. G.; Funy, D. S. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 2907. (36) Ray, A. J. Am. Chem. Soc. 1969, 91, 6511. (37) Lareen, D. W.: Fribera, - S. E.; Christereon,H.J. Am. Chem. SOC. 1980,102,6565. (38) Moucharafieh, N.; Friberg, S. E.Mol. Cryst. Liq. Cryst. 1979,49, 231-238. (39)Collins. J. M.: Tamura-Lis.. W.:. Lis.. J.:. Quinn. _ . P. J. J. Colloid Interface Sci. '1990, 134, 357.

Langmuir, Vol. 7, No. 10,1991 2393

Lyotropic Phases Formed by SDS in Polar Solvents lower in ethylene glycol than in water (at zero solvent concentration the difference is 0.9 nm37*38).Friberg attributes this difference to either the orientation of the chains or the state of disorder of the chains rather than to penetration of molecules of ethylene glycol into the liquid layer. SDS thus resembles phospholipids in this solvent. In general, the temperature of onset of formation of the lamellar phase is increased and the lamellar separation decreased when water is replaced by other solvents (Table 111). The lower the structure of the solvent the smaller the lamellar separation. The thickness of the solvent layer is determined by the range of the repulsive forces of solvation perpendicular to the planes of the lamellae.9 These fall with decreasing structure of the solvent. The SDS/formamide and N-methylformamidesystems are quite different even though solvation is essentially the same since the dipole moments of these two solvents are nearly equal ((12.5 and 12.8) X cm, respectively). Two possibilities can be advanced to account for this difference: (i) N-Methylformamide has only one hydrogen bond and is relatively unstructured. (ii) The molecule of N-methylformamide is more hindered by the presence of the methyl group. Eveh if solvation of the isolated polar head OSOS-is the same in the two solvents, the area a0 occupied by the polar head solvated by molecules of N-methylformamide at the polar head solvent interface may be too great compared to u, the area occupied by a chain in the curved interfaces of the Qa and Ha phases. u is solely determined by the curvature and hence by the geometric constraints in the Qaand Haphases; but in the La phase it may also depend on the nature of the solvent.39 The latter possibility could account for the absence of Qaand Haphases in various surfactant/nonaqueous polar solvent system^.^^^ The rigidity and Gaussian curvature constants, and particularly the spontaneous curvature derived from elastic energy depend on the solvent for a given polar head. In this case, the elastic energy of curvature would no longer compensate for the change in energy resulting from the difference in packing of chains in the lamellar phase and the rods of the Q.

phase or of the Haphase. The curvature of the interface is likely to be determined by both repulsive tangential forces between polar heads9and steric factors. However, the presence of a methyl group in the solvent molecule and the absence of hydrogen bonds do not necessarily prevent formation of ordered phases41 since they are observed in the CPBrlN-methylsydnone systeme6

(40) Heifrich,W .Z . Naturforsch., C Eiochem.,Biophys., Eiol., Virol. 1973,28, 693.

(41) Ray, A. Naturuissenschaften 1971,231,313. (42) Laughlin, R. G.; Mungon, R. L. J. Phys. Chem. 1987,91,3299.

Conclusion This method of observation of the diffraction diagrams of phases in equilibrium with a crystalline state enable accurate determination to be made of the appearance and disappearance of the various phases and the transitions between them in a complex binary system such as the SDS/water system. It can be applied in the same domain as method called “diffuse interfacial transport” or DIT method described in ref 42. Our results demonstrate the complexity of the SDS/ water system compared to the SDS/formamide system, which displays the conventional sequence of two-, three-, and one-dimensional ordered phases. For the same surfactant, two types of 3D bicontinuous phases were observed. One was thought to be a Schwartz F surface of space group Ia3d and the other a rhombohedral structure which turns into a cubic phase Im3m with a configuration of labyrinths that was not attributable to a Schwartz G surface. Lamellar phases were formed in all the polar solvents in which SDS was soluble, following either a 3D ordered phase or an isotropic phase containing structured clusters of surfactant molecules. The formatiOn of periodic curved interfaces characteristics of 3D lyotropic phases is complex. Our results show that the physical constants of solvents cannot readily be used to predict formation of the different phases. Geometric constraints are important, but polar head-solvent interactions also play a role. The solvent/surfactant ensemble should therefore be evaluated rather than the characteristics of the solvent alone.

Acknowledgment. The authors thank Mr.Bostel for his help with construction of the thermostated cell. Registry No. SDS,151-21-3;formamide, 75-12-7; glycerol, 56-81-5;ethylene glycol, 107-21-1;N-methylformamide,123-397.