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Chain-Length and Solvent Dependent Morphological Changes in Sodium Soap Fibers Marc C. A. Stuart,*,†,‡ Jan van Esch,‡ John C. van de Pas,† and Jan B. F. N. Engberts‡ Formulation Unit, UnileVer Research and DeVelopment, OliVier Van Noordlaan120, 3133 AT Vlaardingen, and Stratingh Institute, UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ReceiVed February 16, 2007. In Final Form: April 16, 2007 Sodium soap fibers with varying alkyl chain lengths were studied by cryotransmission electron microscopy and differential scanning calorimetery in water and water-propylene glycol mixtures. The morphology of the lamellar fibers was found to be dependent on the chain length of the alkyl chain and the solvent polarity. Cryoelectron microscopy revealed that short-chain (C10-C14) sodium soaps have the bilayer plane perpendicular to the fiber width, which enables one to see the bilayer striations on the fibers, whereas long-chain (C16-C20) sodium soaps have bilayer planes parallel to the fiber width, and the bilayer striations are not visible. This change in morphology is accompanied by a change in dissolution enthalpy.
Introduction Soap was the first surfactant used for “hygiene” purposes, and it still is very important for the well-being of mankind. Apart from being a surfactant, soap is also used as a structuring agent, giving structure to detergent products ranging from (structured) liquids, relatively soft soap gels, to hard soap bars. In these products, the microstructure may range from concentrated micellar solutions via liquid-crystalline phases to crystalline soap networks. In this study, we focus on soap gel structures that are formed at relatively low concentrations of soap. Soaps are the salts, usually sodium or potassium, of (longchain) carboxylic acids. Below the Krafft boundary, the soap molecules are practically insoluble and appear in crystalline fibers of almost infinite length. Above the Krafft temperature, the fibers dissolve to monomers which can aggregate into micelles in water. At almost all concentrations, long-chain sodium soaps form in water so-called curd fibers.1 These fibers have intrigued researchers for many decades. One of the first who described these fibers, either by visual observation or by light microscopy, was Bachmann.2 The fibers were described as “being many centimeters long, but they are barely of microscopic diameter”. In the previous century, phase diagrams of soaps were produced, including areas containing curd fibers.3 These curd fibers were one of the first objects studied by electron microscopy.4 There has been much speculation about the structure of the soap fibers. Bundles of micellar fibers were suggested for sodium myristate,5 but also lamellar rolls with a “jellyroll-like” concentric cylindrical structure were reported.6 Overall, there is now sufficient evidence * Corresponding author. Dr. M. C. A. Stuart, Physical Organic Chemistry, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands,
[email protected], Fax +31 50 363 4296. Present address: Biophysical Chemistry, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands. † Unilever Research and Development. ‡ University of Groningen. (1) Madelmont, C.; Perron, R. Colloid Polym. Sci. 1976, 254, 581-595. (2) Bachmann, W. Kolloid Z. 1912, 11, 145-157. (3) McBain, J. W.; Elford, W. J. J. Chem. Soc. 1926, 421-438. (4) Marton, L.; McBain, J. W.; Vold, R. D. J. Am. Chem. Soc. 1941, 63, 1990-1993. (5) Trager, O.; Sowade, S.; Bottcher, C.; Fuhrhop, J. H. J. Am. Chem. Soc. 1997, 119, 9120-9124. (6) Liang, J. M.; Ma, Y.; Zheng, Y.; Davis, H. T.; Chang, H. T.; Binder, D.; Abbas, S.; Hsu, F. L. Langmuir 2001, 17, 6447-6454.
that the curd fibers formed from sodium laurate to stearate are lamellar in structure.1,7-9 At low concentrations, soap curd fibers can be used as low molecular weight organo-gelators, and are able to form thermally reversible gels in water and polar organic solvents.10 In polar solvents, the monomers might aggregate depending on the amount of water and on the apolar chain length.11 In the present study, the morphology of the fibers has been investigated, in particular, the orientation of the bilayers with respect to the fiber dimensions. Furthermore, the fiber morphology is linked to the dissolution enthalpy of the fibers in water and water-propylene glycol mixtures. Experimental Section Materials. Sodium decanoate, sodium dodecanoate, sodium myristate, sodium palmitate, sodium stearate, and arachidic acid were all from Sigma (purity >99%) and were used without further purification. All other chemicals were of the highest purity. Cryoelectron Microscopy. The desired amount of sodium soap was weighed and dissolved in 100 mM NaOH solutions in water or water-propylene glycol at elevated temperatures (above the Krafft temperature). The NaOH was used to ensure that all the soap is in the anionic form, especially because the pKa’s of long-chain soaps are rather high.12 After cooling to room temperature, the solution was kept for at least 24 h to gel. The soap concentration was chosen in such a way that a soft gel was produced (5% C10, 2.5% C12, 0.25% C14, 0.2% C16, and 0.1% C18 w/w). A few microliters of gel were placed on a bare 700 mesh hexagonal grid or on a Quantifiol 3.5/1 holey carbon-coated grid (Quantifiol GmbH, Jena, Germany) and blotted with filter paper. The grids were vitrified by plunging into liquid ethane and transferred to a Gatan model 626 cryostage. The grids were examined in a CM10 or CM120 (Philips, Eindhoven, The Netherlands) cryoelectron microscope under low-dose conditions. Cryoelectron diffraction was carried out at a camera length of 770 mm and exposure times between 5 and 10 s. Differential Scanning Calorimetry. For differential scanning calorimetry (DSC) measurements, a weighed amount of solid sodium (7) Luzzati, V.; Mustacchi, H.; Skoulios, A. Discuss. Faraday Soc. 1958, 43-50. (8) Luzzati, V.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13, 660-667. (9) Luzzati, V.; Husson, F. J. Cell Biol. 1962, 12, 207-219. (10) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3159. (11) Evans, D. F.; Miller, D. D. In Organized Solutions. Surfactants in Science and Technology. Surfactant Science Series; Friberg, S. E., Lindman, B., Eds.; Dekker: New York, 1992; Vol. 44, pp 33. (12) Kanicky, J. R.; Shah, D. O. Langmuir 2003, 19, 2034-2038.
10.1021/la063633l CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007
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Figure 1. Cryoelectron micrographs and cryoelectron diffraction patterns of curd fibers from sodium myristate (0.25% w/w) (a,c,e) and sodium stearate (0.1% w/w) (b,d,f) in water. Only in the sodium myristate (c) are lamellar striations seen. Bar: 100 nm (a-d), 5 nm-1 (e and f). soap was placed together with the solvent (100 mM NaOH in water or water-propylene glycol) in a stainless steel DSC pan. The samples were heated from 10 to 95 °C at 5 °C/minute. Heating and cooling scans were recorded on a Perkin-Elmer DSC 7 instrument. For each sample, several heating and cooling scans were recorded with an interval of 1 h.
Results In this study, the structure and dissolution enthalpy of soap gels with various alkyl tail lengths (C10-C20) were investigated. The gels were prepared in water and water-propylene glycol mixtures. All soaps used in this study form below their Krafft temperature thin, ribbonlike fibers with a lamellar organization (Figure 1a,b). Two morphologically different types of soap fibers can be distinguished using cryoelectron microscopy. Short-chain soaps (C10-C14) form lamellar fibers with the normal to the plain of the bilayer parallel to the fiber width. This can be clearly seen
by the lamellar striations on the fibers (Figure 1c). The longchain soaps do not display the lamellar striations on the surface of the fibers (Figure 1d). In these fibers, the lamellar organization is about 90° rotated with respect to the fibers from short-chain soaps. The normal to the plain of the bilayer is perpendicular to the width of the fibers. With cryoelectron diffraction, the orientation of the soap molecules in the fibers becomes even more clear. In the fibers from the short-chain soaps, the molecules are diffracted perpendicular to the hydrophobic tails, and therefore two spots are clearly present, reflecting the parallel packing of the molecules in the bilayer (Figure 1e). In the cryoelectron diffraction pattern of the long-chain soaps, the soap molecules are viewed head-on (perpendicular to the bilayer plane). From the diffraction pattern, it is clear that the molecules are packed orthorhombically in the plane of the bilayer (Figure 1f).13,14 (13) Pilgram, G. S. K.; Pelt, A. M. E. V.; Oostergetel, G. T.; Koerten, H. K.; Bouwstra, J. A. J. Lipid Res. 1998, 39, 1669-1676.
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Figure 2. DSC traces of C12-C20 sodium soaps (a). Calculated dissolution enthalpy for C10-C20 sodium soaps in water (b).
Besides the distinct morphological differences between longand short-chain soap fibers, a difference in dissolution enthalpy is observed. The dissolution temperature of the fibers increases with increasing chain length (Figure 2a). From the area under the dissolution peak, the dissolution enthalpy, ∆H, was calculated. The dissolution enthalpy was found to increase linearly with the chain length for both the short- and long-chain soaps, but with a discontinuity between C14 and C16 (Figure 2b). This discontinuity coincides with the morphological change found between the short-chain (C10-C14) and long-chain (C16-C20) fibers (Figure 1). When water was replaced by a mixture of water and propylene glycol (H2O/PG, 1/1, w/w) the dissolution enthalpies were almost similar to those found in water. However, sodium myristate has now a dissolution enthalpy on the linear extrapolation plot of the long-chain soaps (Figure 3a). The striations that were formerly seen on the myristate fibers have now disappeared (not shown). The morphology of the sodium dodecanoate fibers in the same solvent has not changed and striations can still be seen on the fibers (Figure 3b). Again, the discontinuity in the dissolution enthalpy coincides with a change in fiber morphology. Upon a further increase of the propylene glycol concentration, the dissolution enthalpies changed significantly. The long-chain soaps now have dissolution enthalpies that coincide with the extension of the linear plot of the short-chain soaps in water, whereas the short-chain soaps now have dissolution enthalpies consistent with a value obtained by extension of the linear behavior
of the long-chain soaps in water (Figure 3a). The long-chain soaps do show in water-propylene glycol (1/3, w/w) lamellar striation on the fibers (Figure 3c) indicative of a change of the fiber morphology. By contrast, the striations on the fibers of the short-chain soaps have disappeared. Although the overall contrast in the cryoelectron microscopic pictures of the fibers diminishes in water-propylene glycol (1/3, w/w), bilayer striations on the fibers are still observed. In water, all soaps form micelles above their Krafft temperatures, as demonstrated by their Nile Red excitation-dependent fluorescence15 (Supporting Information). However, in waterpropylene glycol (1/3, w/w) only the long-chain soaps (C16 and C18) form micelles, as indicated by the excitation-dependent Nile Red fluorescence. The short-chain soaps form random solutions in water-propylene glycol (1/3, w/w) above their Krafft temperature.
(14) Pilgram, G. S. K.; Van Pelt, A. M.; Spies, F.; Bouwstra, J. A.; Koerten, H. K. J. Microsc. (Oxford, U.K.) 1998, 189, 71-78.
(15) Stuart, M. C. A.; van de Pas, J. C.; Engberts, J. B. F. N. J. Phys. Org. Chem. 2005, 18, 929-934.
Figure 3. (a) Dissolution enthalpy of C10-20 sodium soaps in water and water-propylene glycol. In water-propylene glycol (1/3, w/w), C10 does not show a dissolution enthalpy here because it is completely soluble. (b,c) Cryoelectron micrographs of 2.5% (w/w) sodium dodecanoate in water-propylene glycol (1/1, w/w) (b) and 0.25% (w/w) sodium stearate in water-propylene glycol (1/3, w/w) (c). Bar 100 nm.
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Figure 4. Proposed model for the morphology of sodium soap curd fibers. All four types have a lamellar organization. In water (top), the short-chain (C10-14) soaps have lamellae with the normal to the plain of the bilayer parallel to the fiber width, whereas the longchain (C16-20) soaps have the normal to the plain of the bilayer perpendicular to the fiber width. In water-propylene glycol (1/3 w/w) (bottom), the morphology is exactly opposite. The short-chain soaps, lower left, are suspected to have an aditional monolayer of soap molecules with their tails directed into the solvent.
Discussion Long-chain soaps form below their Krafft temperature long “curd” fibers. These fibers all have a lamellar organization. The fiber morphology originates from a lamellar crystal growth in three dimensions. Since all fibers have an infinite length, we can consider the growth in this direction as fast compared to the growth in fiber width and fiber thickness. Due to a certain tilt of the soap molecules with respect to the fiber length,10 the growth in this direction is fast compared to the growth in the other direction in the bilayer plain and in stacking of bilayers. Surprisingly, their morphology changes with increasing chain length. As known from previous studies, the Krafft temperature increases with increasing tail length16 and the transition becomes more cooperative, as can be seen by a decreasing dissolution peak width with an increase of the soap chain length. In water, two distinct fiber morphologies were found. Shortchain soaps form fibers with the normal to the plain of the bilayer parallel with the fiber width, whereas long-chain soaps form fibers with the normal to the plain of the bilayer perpendicular to the fiber width. In Figure 4, top part, an artist impression of how the fibers are build up from bilayers is given. Short-chain soaps form in water lamellar fibers, and the largest surface of these fibers is composed of the ionic head groups and relatively short apolar alkyl tails. If the tail length increases, the largest surface, with unchanged morphology, becomes more apolar. Consequently, the fiber morphology changes in such a way that (16) Laughlin, R. G. In The Aqueous Phase BehaVior of Surfactants; Laughlin, R. G., Ed.; Academic Press: London, 1994; pp 328.
the largest surface is now made up of only ionic head groups, leaving most of the hydrophobic tails shielded from water. For both short-and long-tail soaps, hydrophobic interactions are the main driving force for the formation of curd fibers in water. A linear increase in dissolution enthalpy with increased apolar tail length is expected due to increased hydrophobic interaction. A similar effect was shown for the micellization enthalpy as a function of the hydrophobic tail length of sodium alkylbenzenesulfonates.17 We find a break in the dissolution enthalpy which coincides with the morphology transition of the fibers. It is obvious that the enthalpy change is due to the way the soap molecules are packed in the crystal. Buerger et al.18 designated seven different soap crystal phases on the basis of X-ray powder patterns, which is an extension of the well-known Furguson classification.19 A similar change in crystal types between short- and long-chain soaps was found. In water-propylene glycol (1/3, w/w) mixtures, the situation is completely different. Here, the solubility of the ionic head groups is limited (the logarithm of the solubility of a salt is inversely proportional to the reciprocal of the dielectric constant of a solution20), and therefore large surfaces of ionic head groups are unfavorable in mixtures with a decreased dielectric constant. Furthermore, hydrophobic interactions rapidly decrease with decreasing water concentration in mixed aqueous solutions. For this reason, the morphology of the long-chain soaps changes back to fibers with the largest surface of apolar alkyl chains. For the short-chain soaps, we propose (Figure 4) fibers with an additional monolayer of soap molecules with their apolar alkyl chains located in the solvent. This is likely, since the short-chain soaps above their Krafft temperature do not form micelles but a random solution, as was demonstrated by Nile Red fluorescence measurements (Supporting Information). Depending on the dielectric constant of polar solvents and the nature of the surfactant (i.e., alkyl tail length), surfactants can aggregate into micelles.11,21 Liang et al.22 also showed a change in crystal type for sodium stearate upon increased propylene glycol concentrations. Furthermore, an increased rigidity and an increased amount of trans conformation of the alkyl chains were found in the crystal type found at high propylene glycol concentrations, which can explain our observation of an increase in dissolution enthalpy for longchain soaps upon changing from water to water-propylene glycol (1/3, w/w). In conclusion, a chain-length and solvent dependent morphology of soap curd fibers was found. The change in morphology finds its basis in a different crystal packing, which is accompanied by a change in dissolution enthalpy. Supporting Information Available: Additional experimental data as discussed in the text. This information is available free of charge via the Internet at http://pubs.acs.org. LA063633L (17) Van Os, N. M.; Daane, G. J.; Bolsman, T. A. B. M. J. Colloid Interface Sci. 1988, 123, 267-274. (18) Buerger, M. J.; Smith, L. B.; Ryer, F. V.; Spike, J. E. Proc. Natl. Acad. Sci. U.S.A. 1945, 31, 226-&. (19) Ferguson, R. H. Oil Soap (Chicago) 1944, 21, 6-9. (20) Israelachvili, J. N. Intermolecular and surface forces, 2nd ed.; Academic Press: London, 1992. (21) Stuart, M. C. A.; van de Pas, J. C.; Engberts, J. B. F. N. J. Surfactants Deterg. 2006, 9, 153-160. (22) Liang, J. M.; Ma, Y.; Chen, B.; Munson, E. J.; Davis, H. T.; Binder, D.; Chang, H. T.; Abbas, S.; Hsu, F. L. J. Phys. Chem. B 2001, 105, 9653-9662.
