Langmuir 1995,11, 3667-3675
3667
Sodium Sulfopropyl Octadecyl Maleate-A Long Chain Surfactant with Unusual Aggregation Behavior in Aqueous Solution Hans von Berlepsch" Max-Planck-Institut fur Kolloid und Grenzflachenforschung, Kantstrasse 55, 0-14513 Teltow, Germany Received February 13, 1995. I n Final Form: July 5, 1995@ The long chain surfactant sodium sulfopropyl octadecyl maleate (SSPOM)shows above the KrafR boundary
(TK% 37 "C)with respect to its phase behavior in aqueous solution many similaritiesto other ionic surfactants. An isotropic liquid micellar phase L1 followed by the normal hexagonal Ha-phaseare observed. Uncommon properties appear in the temperature range below TKwhere crystals and a gel-like state with lamellar structure exist. The gel state is metastablebut long-lived,so that its microstructure can be well-characterized by application of many different techniques. The Krafft boundary of mixtures of SSPOM and the C14 homologue shows the typical chain length dependence of the chain-melting phase transition of lipids. Wide-angleX-ray scattering data suggest the molecules within the bilayers of the gel state to be densely packed and interdigitated. The experimentally estimated bilayer thickness is in well agreement with atomic data. The swelling ofthe gel state in water is nonlinear and shows that the system has to be viewed as a dispersion of lamellar aggregates.
I. Introduction Surfactants are amphiphilic molecules which adsorb a t interfaces and lower the interfacial tension. This property is the basis of many technical processes and is extensively used in several applications. Polymerizable surfactants are a n interesting class of surfactants, allowing enhancement of the stability of formed organized structures by polymerization. A homologous series of sodium sulfopropyl alkyl maleates with n = 12,14,18was
with the aim of applying them as polymerizable emulsifier for latex stabilization preventing surfactant desorption during application of the p ~ l y m e r . ~During -~ investigations of the physicochemical properties of this new class of surfactants in aqueous solution,2,6-8the C18 derivative sodium sulfopropyl octadecyl maleate (SSPOM) attracted our special attention because of its unusual behavior. The critical micelle concentration (cmc) of SSPOM is very small, being in the range of 5 x m o m a t 43 "C.2 The Kraffl boundary at TK =Z 37 "C was estimated by conductivity measurements and differential scanning calorimetry.2 SSPOM forms above TKin aqueous solution a n isotropic, low-viscosity liquid phase L1 extending up to
* Present address: Institut Charles Sadron (CRM-EAHP), 6 rue Boussingault, F-67083Strasbourg Cedex, France. Abstract published in Advance A C S Abstracts, September 1, 1995. (1)Kanegafuchi, Kogaku, Kogyo, Kabushiki, Kaisha, G. B. Patent, 1427 789,1976;US.Patent, 3 980 622,1976. (2) Goebel, K.-H.; Stahler, K.; von Berlepsch, H. Colloids Surf. A: Physicochem. Eng. Aspects 1994,87,143. (3)Greene, B. W.; Sheetz, D. P.; Fisher, T. D. J . Colloid Interface Sci. 1970,32,90. (4)Tauer, K.; Goebel, K.-H.; Kosmella, S.; Stahler, K.; Neelsen, J. Makromol. Chem., Macromol. Symp. 1990,31, 107. (5) Urquiola, M. B.; Dimonie, V. L.; Sudol, E. D.; El-Asser, M. S. J . Polym. Sci., Part A: Polym. Chem. 1992,30, 2619,2631. (6)von Berlepsch, H.; Strey, R. Ber. Bunsenges. Phys. Chem. 1993, 97,1403. (7)von Berlepsch, H.; Dautzenberg, H.; Rother, G.; Jager, J . J . Phys. Chem., submitted. ( 8 )von Berlepsch, H.; Hofmann, D.; Ganster, J. Langmuir 1996,11, 3676 (followingpaper in this issue). @
20 wt % and at higher concentrations a normal hexagonal phase Ha. The micellar nature of the liquid phase L1 has been demonstrated r e ~ e n t l y . ~Using light-scattering techniques spheroidal micelles were detected, which upon addition of sodium chloride grow in size, turning into wormlike micelles after exceeding a certain threshold salt concentration. After passing the Krafft boundary by cooling from the L1- or Ha-phase, respectively, needle-like crystals precipitate which transform after further lowering of the temperature and without the application of mechanical shear into a gel-like state of lamellar structure.6 We have called this metastable and long-lived state the Gg-phase, in analogy to the usual lamellar phase Lg with frozen chains within the plane of the bilayers. A very similar gel-formation phenomenon is known also for the dioctadecyldimethylammonium chloride/water s y ~ t e m . ~ J ~ Laughlin et a1.l0have described the gel state as a colloidally structured dispersion of crystal hydrates in the dilute liquid phase. SSPOM solutions of around 1wt % display iridescent colors when illuminated with white light due to Bragg refraction of incident light from the periodic lamellar structures, showing the high state of order of the Gg-phase.'l The light-scattering investigations revealed the existence of very small crystallites at surfactant concentrations in the range of 0.01 wt % below the Krafft boundary. While the crystallites were anticipated, they are not visible to the naked eye a t these compositions, which made it difficult to show their existence. This is done here. Presumably we have to assume that a certain portion of crystals coexist with the bilayers in the gel state, although the crystallites cannot be seen by visual inspection. The gels formed a t higher compositions (above 5 wt %) are transparent, but show some turbidity which rises with increasing concentration. This finding suggests that the gel-formingreaction is incomplete. When the gel state is a colloidally structured dispersion of hydrated crystals or sheetlike aggregates in the dilute liquid phase but not a single phase, the incompleteness is not surprising. The inclination to form a gel state is obviously connected with the buckled conformation of the molecule forced by the ( 9 )Kawai, T.; Umemura, J.;Takenaka, T.; Kodama, M.; Ogawa, Y.; Seki, S. Langmuir 1986,2,739. (10)Laughlin, R.G.; Munyon, R. L.; Burns, J. L.; Coffindaffer,T. W.; Talmon, Y.J. Phys. Chem. 1992,96,374. (11)von Berlepsch, H.; Strey, R. Colloid Polym. Sci. 1994,272,577.
0743-7463/95/2411-3667$09.00/0 0 1995 American Chemical Society
von Berlepsch
3668 Langmuir, Vol. 11, No. 10, 1995 cis-double bond, which allows two molecules to interdigitate, but also with the long length of one of the two chains.8 The Krafft temperature of the derivative sodium sulfopropyl tetradecyl maleate (SSPTM) is lower than 0 "C, and a gel-like state could not be observed. The present paper is primarily concerned with the binary phase diagram of SSPOWwater. We summarize the essential results of recent investigations without considering details and extend the studies to include effects which are related with the process of structure formation below the Krafft boundary. We will show that the Krafft temperature is a nonlinear function of chain length agreeing with the chain-length dependence of the chainmelting transition temperature of lipids12but in contrast to many single-straight hydrocarbon chain ionic surfactants for which a linear relationship is often found to hold.13 Another unusual property ofthe gel state is its sensitivity to the conditions of sample preparation, which in certain cases markedly influences the repeat distances of bilayers. Such effect leads obviously to the recently observed and unexplained disagreement between the spacings determined by light and X-ray scatterings6 These and some further features point to the metastable nature of the gel state as the physical reason and will be discussed.
11. Materials and Methods The sodium sulfopropyl alkyl maleates were synthesized following a procedure described in ref 2. The surfactants have been purified by recrystallization at least three times from wateracetone mixtures. The purity was checked by elemental analysis and 'H NMR spectrometry, indicating at least 99% purity. In addition, t h e purity of SSPOM was checked by thin-layer chromatography, and no impurities were detected. Highly deionized water was used to prepare the surfactant solutions. While the Clz and c14 derivatives are fully soluble in water at room temperature, SSPOM required a more sophisticated technique for sample preparation due to its high Krafft temperature. In the latter case and for surfactant concentrations up to about 2 wt %, a mixture of water and detergent crystals is slightly warmed to about 30 "C and vigorously shaken until the crystals dissolved. After cooling to 17 "C and maintaining that temperature, the transparent and more or less viscous solution forms in less than 1 day. For higher concentrations the slurries of crystals and water had to be homogenized first by strong shaking or centrifuging back and forth through the sample tubes. For the complete dissolution of crystals, temperatures well above the Krafft temperature (37 "C) are necessary. Cooling to 17 "Cleads again to gel-like solutions. The reproducibility of the results obtained on samples prepared in the described way is sufficient. In particular, samples of low concentration which have been first heated to temperatures above TK are nearly identical to those which were only warmed up to 30 "C and held there for 0.5 h. This finding obviously reflects the fact that on cooling from above TKto 17 "C the thermodynamic state of the system a t 30 "C is passed and the time needed to find the dynamic equilibrium during the structure building process is well within the required time scale. Larger deviations between both types of samples may be expected and have indeed been obsemed when the required time to attain equilibrium at 30 "C was not sufficient. These effects point to the nature ofthe colloidal structures ofthe present system below TKas being so-called "irreversible colloidal structures". 14 The density of SSPOM in the Gg-phase at 20 "C was determined using a'Paar DMA 60 density meter (Paar, Graz, Austria). The cmc values of the sodium sulfopropyl alkyl maleates have been estimated from electrical conductivity and surface tension studies, as described in detail in ref 2. The Krafft boundary of samples with a surfactant concentration ( y ) ranging between 0.05 and 30 wt %was determined by conductivity measurements, differential (12) Cevc, G.; Marsh, D. Phospholipid Bilayers. Physical Principles and Models; Wiley: New York, 1987; p 231. (13) Gu, T.; Sjoblom, J. Colloids Surf. 1992,64, 39. (14) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994; p 13.
60
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001
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01
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10
1
100
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Figure 1. Partial phase diagram of the water/SSPOM system. L1, Ha, Gb, and X denote the isotropic liquid phase, normal hexagonal phase, gel state, and crystalline phase. scanning calorimetry,2 and visual inspection of samples in a thermostated water bath, and for the most diluted samples of y < 0.05 wt %, static light scattering was used.7 The Krafft temperatures for t h e mixtures of C14 and CISderivatives to be reported below were estimated from the change ofturbidity versus temperature by visible inspection. The other data points in the SSPOiWwater phase diagram were determined by visual inspection of samples between crossed polarizers, backscattering of light, and small-angle X-ray scattering (SAXS). The light backscattering experiment is illustrated in'detail in ref 11, and the SAXSinstrument used (Kratky low-angle camera, Paar, Graz, Austria) is described in ref 15. The samples for light backscattering were contained in quartz cuvettes (Hellma, FRG) of 10 mm path length (standard) and for SAXS in thin-walled glass tubes (diameter 1.0 mm, wall thickness 0.01 mm; Mark-tubes, Hilgenberg, FRG). The static and dynamic light scattering techniques used to characterize the micellar solutions ofSSPOM above TK, the freeze-fracture electron microscopy (FFEM), and the cryogenic transmission electron microscopy (cryo-TEM),both applied to visualize colloidal structures, are described in detail el~ewhere.~J6-~9 Wide-angle X-ray scattering (WAXS) and molecular dynamics methods were applied to study the packing ofsurfactant molecules in the Gb-phase. These results have been discussed in detail in ref 8.
111. Results and Discussion
A. Phase Diagram. In Figure 1 a partial phase diagram is shown which summarizes the various observations in the interval 0-70 "C and 0.004 wt % < y < 30 wt %. Above the crystal solubility boundary, the so-called Krafft boundary, SSPOM is highly soluble and forms the liquid phase L1. Above y 20 wt % the normal hexagonal phase Ha forms. The Krafft boundary terminates a t its upper temperature limit a t an eutectic discontinuity, which is called the Krafft discontinuity o r Krafft eutectic accordingto Laughlina20Below the discontinuity the liquid phase and a c r y s t a l phase coexist, while above a liquid crystal phase, in our case, the Ha-phase appears. The crystal solubility curve of surfactants shows in general a nearly vertical part followed by a more or less pronounced knee and a plateau region, whereas along the plateau the slope is small and the temperature coefficientof solubility (15)Wiirz, U. Prog. Colloid Polym. Sci. 1988,76,153. (16) Magid, L. In Dynamic Light Scattering. The Method And Some Applications; Brown, W., Ed.; Monographs on the Physics and Chemistry of Materials, Vol. 49; Clarendon Press: Oxford, 1993; p 555. (17) Jahn, W.; Strey, R. J.Phys. Chem. 1988,92, 2294. (18)Hoffmann, H.; Thunig, C.; Munkert, U.; Meyer, H. W.; Richter, W. Langmuir 1992,8,2629. (19) Bellare,J. R.; Davis, H. T.; Scriven,L. E.; Talmon,Y. J . Electron Microsc. Tech. 1988,10, 87. (20) Reference 14, p 102.
Sodium Sulfopropyl Octadecyl Maleate is very large. The relatively weak slope of the Krafft plateau is evident for the present system. The intersection of the cmc versus temperature curve and the Krafft boundary within the knee region has been termed the "Krafft point".21 The cmc of SSPOM is very low and therefore difficult to determine. The conductivity method gives no unique value and the surface tension method suffers from very long equilibration times on the one hand and elevated temperatures on the other hand. Interpreting the second break point in the conductivity versus concentration curves around 5 x m o m (43 "C) as the cmc, we obtain agreement between the usually observed chain length dependence for a homologous series of straight-chain alkyl surfactants (about half a decade per two methylene groups) and our data.2 That value is also in accordance with the light-scattering data,' and it seems that it is the most reliable one. There are similar difficulties in estimating the Krafft boundary in the very diluted region. We measured the scattered light intensity a t an angle of 0 = 30" and at a fxed heating rate as a function of temperature between 20 and 40 "C. When passing the Kram boundary from below, the intensity decreases strongly and approaches a constant value above T K , The decrease reflects the melting of crystallites. The more or less pronounced breakpoints in the heating curves indicate the position of the Krafft boundary. The lowest concentration for which we obtained a reliable data point for the Krafft boundary was 4 x wt %. In particular, the exact location of the Krafft point is unknown, leaving open the phase diagram in the extremely diluted region. For higher concentrations the Krafft boundary is simply obtained from a scan of the electrical conductivity versus temperature showing a steep increase at TK, DSC, or visual inspection.2 Below the Krai-3 boundary one typically finds the coexistence of a liquid phase and a crystal phase. Depending on the position of the Krafft point, the liquid phase should be either a molecular solution of surfactant molecules in water or a micellar solution. Due to the unknown Krafft point, we cannot exactly specifythe liquid phase in our diagram. The proof of the presence of surfactant crystals between 20 "C and the Krafft boundary for low concentrations has been given by the static light scattering measurements. Highly concentrated samples (3 wt % < y < 30 wt %) become opaque and a white precipitate is observed within a few hours if the sample is kept at rest just below the Krafft boundary. The precipitate has been identified under an optical microscope with phase contrast enhancement as needlelike crystals of micron size. Because the crystals have not been quantitatively characterized until now, we can only speculate about their structure. Surfactants invariably pack within crystals, the lipophilic groups of different molecules being associated with each other in lipophilic regions, and the hydrophilic groups are collected within polar regions. The most frequently found packing structure is the bilayer.22 But the bilayer crystal is not the only one that exists. Alternative possibilities include interdigitated structures and also monolayer struct u r e ~ . In ~ an ~ , interdigitated ~ ~ packing, the head of one molecule is adjacent to the tail end of the next one within the structural layer of the crystal. Due to the buckled conformation of our molecule, this type of structure is highly probable for the present surfactant. A further open (21) Shinoda, K. Solvent Properties ofsurfactant Solutions; Marcel Dekker: New York, 1967;Vol. 2, p 12. (22) Krog, N. J.Food Emulsions; 2nd ed.;Marcel Dekker: New York, 1990;p 127. (23)Jeffrey, G. A.;Maluszynka, H.Acta Crystallogr. 1989,B45,447. (24)Muller-Fahrnow,A,;Zabe1,V.;Steifa, M.; Hilgenfeld, R. J . Chem. SOC.Chem. Commun. 1986, 1573.
Langmuir, Vol. 11, No. 10, 1995 3669 Table 1. Krafft Temperatures for Different Effective Chain Lengths (ned
TKW) neff
4.5 14.45
13.0
14.9
24.5 15.8
30.5 16.6
34.0 17.3
37.3
18.0
question is, what amount of hydration water is necessary to stabilize the packing? In Figure 1 the Gp-phase extends from the vertical broken line up to 30 wt % and terminates on its high temperature side a t the broken steplike line. Between these boundaries the gel state is long-lived. Ifthe samples are stored a t 17 "C, they are stable over several months. We have prepared a 60 wt % sample which also exhibits the characteristic lamellar SAXS reflections of the Gbphase, a sign that the gel state obviously extends to much higher compositions. In the region on the left side of the vertical broken line the samples phase separate macroscopically during a few days. The upper phase is an isotropic liquid and the lower phase the gel. The lowest concentration for which the gel state has been resolved from backscattering of light is 0.1 wt %. If the steplike upper boundary is crossed from the low temperature side, the gel state collapses. Concomitantly, both light scattering and SAXS reflections sharpen, the peaks shift to larger scattering vectors, their positions become functions of temperature and time (with a characteristic time of 1 h), and lamellar peaks of higher order appear. After some hours needlelike crystals have been detected. The collapse of the gel state is not visible in the DSC spectrum.
B. Chain-Length Dependence of the Krafft Boundary. Because the Krafft boundary of the c14 homologue is located below the freezing point, a gelforming process is excluded. To understand the mechanism of gel formation it is interesting to study its chainlength dependence. An effective chain length between n = 14 and 18 can be adjusted by mixing the homologues SSPTM and SSPOM. Such an approach has been justified by experiments with nonionic surfactant^,^^ but it should be kept in mind that specific effects might be leveled out. In Table 1 the Krafft temperatures obtained for different effective chain lengths (n,ff)are given. neff is calculated from neg= nlX1+ na2, where n, and X,denote the chain lengths and mole fractions of the two components, respectively. The total surfactant concentration was YT = 1wt %. The values have been estimated from the change of turbidity versus temperature detected by visual inspection. The dependence is nonlinear. Both linear as well as nonlinear dependencies have been measured in several families of surf act ant^.^^^^^ The present data follow the empirical relation:
where c1, c2, and c3 are numerical constants. Analogous functional dependencies have been observed for the chainmelting transition temperature (TJoflipids.12 Moreover, a further similarity exists, namely that the enthalpy and entropy changes associated with the Krafft boundary of AH = 36 kJ/mol and AS = 117 J/mol K for SSPOM2 are within the same range of typical values of lipids.12 In a second series of experiments a gel state SSPOM solution has been mixed with increasing amounts of the (2x4 homologue and the shift of the first-order Bragg reflection as measured by backscattering of light was followed as a function of composition. For very low (
