6767
J. Phys. Chem. 1993,97, 6767-6769
An Upper Critical Point in a Lamellar Liquid Crystalline Phase J. Ockelford and B. A. Timimi Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington Wirral M 3 3JW, U.K,
K. S. Narayan Hindustan Lever Research Centre, Chakala, Andheri (East), Bombay 400 099, India
G. J. T. Tiddy' Department of Chemistry and Applied Chemistry, University of Salford, Salford M.5 4WT, U.K. Received: March 12, I993
An equilibrium between two lamellar phases is observed to occur in the binary system sodium dodecyl-5-pbenzenesulfonate/water. The coexistence region, extending from -25 to 6876, is characterized by an upper critical (or consolute) temperature of -39 OC. Deuterium N M R of DzO is a particularly suitable technique for revealing the existence of two lamellar phases in such systems. Samples in the above composition range have also a "white, milky" appearance typical of all emulsions. We attribute the coexistence of two lamellar phases to the presence of an interbilayer attractive component caused by a specific N a ion-head group interaction.
The observation of partial miscibility in binary surfactants/ water micellar solutions is comm~nplace.l-~There are frequent reports of both lower and upper consolutecurvesfor nonionic and zwitterionic surfactants.lJ There even exists a single report of a lower consolutecurve for an ionic surfactant in water.3 However, when the surfactant forms liquid crystal phases, such phenomena have been rarely observed. A few report^^.^ have been made of two lamellar phase coexistence in single and mixed surfactant systems, but no indicationof whether an upper or lower consolute curve exists is given. Recently, Uang et al. have reported6 two L, phase coexistence at room temperature and the absence of these at higher temperatures, implying the existence of an upper consolute loop, although this was not discussed. A second recent paper records that two coexisting lamellar phases are replaced by a micellar solution at higher temperature^.^ In this Letter we report the occurrence of an upper consolute loop within the lamellar phase region of a single ionic surfactant (sodium dodecyl5-p-benzenesulfonate) in water. The two separate phases are easily detected using ZH nuclear magnetic resonance (NMR) spectra of samples containing *HzO.
Materials and Methods Sodium dodecyl-5-p-benzenesulfonate(Na5-C12BS) was synthesized following the procedure of Gray et a1.8 by reacting n-heptyl phenyl ketone with butylmagnesium bromide followed by dehydration and hydrogenation of the product to obtain 5-phenyldodecane. This was then sulfonated and the product neutralized to give the crude Na5-CI2SB,which was recrystallized from acetone.8 The material was further purified by dissolution in dry 2-propanol filtering, and recovery, until no more inorganic electrolyte could be removed. Samples of NaS-ClzSB in 2 H ~(BDH 0 Spectrosol 99.75% isotopically pure) were prepared by weight in 5-mm NMR tubes and flame sealed off. Because of the hygroscopic nature of Na5CIzSB,the appropriate amounts of the material were introduced in the NMR tubes in a drybox under nitrogen gas. The samples were fully hydrated and homogenized by repeated up and down centrifuging and incubation at 100 "C for several weeks prior to measurements. NMR measurements were carried out on a Brucker WH360 spectrometer fitted with a variabletemperature unit and operating
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0022-365419312097-6767$04.00/0
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F i p e 1. Partial phasediagram of sodiumdodecyl-5-p-benzenesulfonate (NaS-ClzBS)in 2Hz0. The region L ' . + L"a is the area of two lamellar
phase coexistence.
at 55.28 MHz. The overall temperature accuracy is within f0.5 "C for temperatures up to 60 "C. Polarized optical microscopy was carried out on a Nikon (Japan) microscope fitted with a Linkam hot stage controllable to *O.l "C. Small-angleX-ray diffraction measurements were carried out using a Kratky camera fitted with a linear detector. Sample temperatures could be controlled to *l OC. ReSults At concentrations above ca. 30 wt 9% NaS-ClzBS forms a lamellar phase, which was identified by its oily streak or mosaic texture under the polarizing microscope: its low viscosity, and low-anglex-ray diffractionlines in theratio 1:1/2:1/3.10 A partial phase diagram for the system is shown in Figure 1. Most notable is the coexistence of two lamellar phases below ca. 40 OC and over a concentration range of ca. 3045%. Bulk samples within the two-phase region have a very white turbid (milky) appearance. On the optical microscope two distinctly different lamellar regions are observed, having very different viscosities. The definitive identification of the two-phase region relies on NMR data as discussed below. 0 1993 American Chemical Society
Letters
6768 The Journal of Physical Chemistry, Vol. 97, No. 26, 1993
C
Figure 2. 2H NMR spectra of 5-C12BS/2H20samples: (a) 50% Na5-C12BS, powder pattern, 90 "C; (b)-(e) 50.6% NaS-ClzBS, aligned sample. Note Occurrence of two doublets at and below 308 K.
NMR Measurement of z H P The NMR spectra from water in anisotropic media have been extensively discussed elsewhere,I1so only a brief outline is given here. Nuclei with a spin I> l / 2 have an electric quadrupole moment which causes the spectrum from equivalent nuclei to split into 21 lines in anisotropic media. For 2H, I = 1; hence, for 2H20the isotropic single line spectrum splits into two lines. The magnitude of the splitting (A) depends on the angle between the mesophase director and the magnetic field direction (OLD)
A = 3/4xS(3COS' OLD - 1) where x is the quadrupole coupling constant and S is an order parameter. The A value of a sample aligned with the director along the magnetic field (All) is twice as large as the value for a sample with director at right angles to the field ( A l ) . Typically, there is a random orientation of the director; hence, a powder pattern is observed (Figure 2a), from which both All and AI can be obtained. For the dependence of A on composition it is common to use the "bound/free" model of Lindman.11-13 Only the fraction of water molecules bound to surfactant head groups (Pb) have finite A values (Ab); hence A is given by A
Pbh, = n(cs/Cw)Ab
where n is the number of water molecules bound per surfactant,
while C, and C.,, are the mole fractions of surfactant and water. At very high concentration, where insufficient water is present to hydrate all the head groups, a maximum can occur in the dependence of A on C,/Cwat the value12 CJC, = l/(n - 1)
NMR is particularly sensitive to the presence of multiple ordered phases. Where two or more phases are present, a separate spectrum from each phase is observed with the spectrum intensity being proportional to the fraction of water in each state. Figure 2 illustrates NMR spectra observed with a sample containing 50.6% NaS-Cl2BS. At high temperatures (-90 "C) the powder sample (Figure 2a) becomes oriented with thedirector along the magnetic field (Figure 2b). On reducing the temperature, the spectrum remains unaltered (except for a very small decrease in A) until 308 K when two doublets are observed. On raising the temperature to 309 K, a single doublet is again observed. These observations are completely reproducible, with the double doublets always appearing below a particular temperature. Figure 3 illustrates the variation of A with temperature. Different compositions show the appearance of two doublets at different temperatures. The relative sizes of the two doublets vary according to composition, with the intensity of the larger splitting being reduced at low surfactant concentration. When the two doublets are present, the magnitudes of the two splittings are constant, at a given temperature. From the dependence of
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The Journal of Physical Chemistry, Vol. 97, No. 26, 1993 6769
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Figure 3. Water (*H20) quadrupole splittings for 54.6% NaS-&BS sample. Splitling/kHz
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our case the lamellar phase transforms to a micellar solution below ca. 25%). It is obvious that all we require to account for the two La phase coexistence is an interbilayer potential of the form used by Wennerstrom.1618 For the long-range repulsion the electrostatic forces are an obvious choice, supplemented by 'hydration" repulsion when the fraction of free water is small.12 The short-range attraction is less easy to find. Standard van der Waals attraction between bilayers is just not large enough; otherwise, multiple lamellar phase coexistence would be commonplace in nonionic surfactants (where large electrostatic repulsions are absent). We suggest that the attractive force may arise from a specific counterion/head group association. This will increase the concentration of counterions at the head group surface as surfactant concentration is increased. Wennerstrom and colleagues have demonstrated that correlations between ion positions on opposing layers give rise to attractive forces both for zwitterionicsurfactants17and for ionic surfactants with divalent counterions.18 It is possible that the additionalcounterion binding could give rise to a similar effect here. This would certainly give an upper critical loop, since an increase in temperature would increase the concentration of free counterions. Currently, we are investigatinghow the lamellar phase critical behavior varies with surfactant alkyl chain structure (smalleffect) and added electrolyte (large effect). We have observed similar behavior in other types of ionic surfactant systems. Although this is the first report of an upper consolute loop within a liquid crystalline region, it is likely that such phenomena can occur in any mesophase type, given a suitable surfactant. These systems provide a set of novel phase types for theoretical work, particularly on the nature of critical exponents.
c1
_. 3
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Figure 4. Water (*HzO)quadrupole splitting as a function of surfactant/ water molar ratio (1 at 298 K, 2 at 313 K, and 3 at 323 K).
A on composition (Figure 4) we note that the spectrum with the smallest A values corresponds to the low-concentration surfactant phase. Hence, there is no evidence of any marked structural difference between the two lamellar phases. However, the fact that A becomes small at low temperature and high concentration does indicate some unusual behavior. This points to a change in head group orientations, perhaps linked to counterion location.
Discussion The background to the coexistence of two lamellar phases has been discussed recently with reference to an "unbinding" transition of lipid system.1&l6 Mutz and Helfrich15 report that above a certain temperature lipid bilayers prepared from digalactosyldiacylglycerol swell continuously in 0.1 M NaCl solution while below this temperature they adhere to each other. Wennerstrom has given a theoretical treatment of this based on a mean field potential using a long-range repulsion and a short-range attraction.1618 His phase diagram16 (Figure 5 in ref 16) shows a good qualitative agreement with our Figure 1 (except that in
References and Notes (1) Degiorgio, V. In Physics of Amphiphiles, Micelles, Vesicles and Microemulsionr;Degiorgio, V., Corti, M., Eds.; North-Holland Amsterdam, 1985; p 303. (2) Laughlin, R.G. In Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1978; Vol. 3, Chapter 2, p 42 and Chapter 3, p 99. (3) Warr, G. W.; Zemb, T. N.; Drifford, M.J . Phys. Chem. 1990, 94,
3086. (4) Khan, A,; Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1985,89, 5180.
( 5 ) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, B. Acta Chem. Scand. 1986, A40, 246. (6) Uang, Yuh-J.; Blum, F. D.; Friberg, S. E.; Wang, J.-F. bngmuir 1992, 8, 1487. (7) Fuller, S.; Hopwood, J.; Rahman, A.; Shinde, N.; Tiddy, G. J. T.; Attard, G. S.; Howell, 0.;Sproston, S . Liq. Cryst. 1992, 12, 521. (8) Gray, F. W.;Gerecht, J. F.; Krems, I. J. J. Org. Chem. 1955,20,511. (9) Rosevear,F. B. J . Am. Oil Chem. Soc. 1954, 31, 628. (10) Luzzati, V.In Biological Membranes; Chapman, D., Ed.; Academic Press: London, 1961; Vol. 1, Chapter 3, p 71. (1 1) Halle, B.; Wennerstrom, H. J . Chem. Phys. 1981, 75, 1928. (12) Carvell, M.; Hall, D.G.; Lyle, I. G.; Tiddy, G. J. T. Faraday Discuss. Chem. Soc. 1986,81,223. (13) Lindblom, G.; Lindman, B.; Tiddy, G. J. T. J. Am. Chem. Soc. 1978, 100,2299. (14) Lipowsky, R.;Leibler, S . Phys. Reu. Lett. 1986, 56, 2541. (15) Mutz, M.; Helfrich, W. Phys. Reu. Lett. 1989, 62, 2881. (16) Wennerstrom, H. Langmuir 1990,6, 834. (17) Nilsson, U.; Jonsson, B.; Wennerstrom, H. Faraday Discuss. Chem. Soc. 1990,90, 107. (18) Guldbrand, L.; Jonsson, B.; Wennerstrom, H.: Linse. P. J . Chem. Phys. 1984,80, 2221.
