Self-Assembly and Foaming Properties of Fatty Acid− Lysine Aqueous

pubs.acs.org/Langmuir © 2010 American Chemical Society

Self-Assembly and Foaming Properties of Fatty Acid-Lysine Aqueous Dispersions Bruno Novales,† Alain Riaublanc,† Laurence Navailles,‡ B
er
enice Houinsou Houssou,† C
edric Gaillard,† Fr
ed
eric Nallet,‡ and Jean-Paul Douliez*,† †

UR1268 Biopolym eres Interactions Assemblages, INRA, e
quipe Interfaces et Syst emes Dispers
es, rue de la G
eraudi ere, F-44316 Nantes, France, and ‡CNRS, Centre de recherche Paul-Pascal, 115 avenue Albert-Schweitzer, F-33600 Pessac, France Received September 14, 2009. Revised Manuscript Received March 17, 2010

We report on dispersions of fatty acid-lysine salts in aqueous solutions which are further used to produce foams. The alkyl chain length is varied from dodecyl to stearic. In aqueous solutions, the lysine salt of the dodecyl chain yields an isotropic solution, probably micelles, whereas for longer alkyl chains, vesicles formed but crystallized upon resting at room temperature or when kept at 4 °C. Solid-state NMR showed that in vesicles fatty acids are embedded in a lamellar arrangement passing from a gel to a fluid state upon heating; the transition temperature at which it occurs was determined by DSC. Those results are confirmed by small-angle neutron scattering which also give additional information on the bilayer structure. Incredibly stable foams are obtained using the palmitic acid/Lys salt whereas for other alkyl chain length, poor or no foam is formed. We conclude that the foamability is related to the phase behavior in aqueous solution.

Introduction Despite that fatty acids are widely used in the industry, for instance as soaps, their self-assembly in water has been poorly studied on a physicochemical point of view. Dispersing fatty acids in aqueous solutions is an interesting challenge in the search for environmentally safer surfactants from renewable source.1,2 However, long chain fatty acids form crystals in aqueous solution, and this feature often hampers their use for biochemical and physicochemical studies. Vesicles can be produced by using short chain fatty acids or those bearing a double bond.3-9 Membrane (bilayer) dispersions are also obtained in particular conditions requiring the formation at the water/membrane interface of a hydrogen-bond network.3,10-12 This feature has been emphasized in mixtures of fatty acids and cosurfactants bearing H-group *Corresponding author: Ph þ33 240 67 50 83, Fax þ33 240 67 50 84, e-mail [email protected]. (1) Johansson, I.; Svensson, M. Curr. Opin. Colloid Interface Sci. 2001, 6 (2), 178-188. (2) Infante, M.; Pinazo, A.; Seguer, J. Colloids Surf., A 1997, 123-124, 49-70. (3) Apel, C. L.; Deamer, D. W.; Mautner, M. N. Biochim. Biophys. Acta 2002, 1559 (1), 1-9. (4) Morigaki, K.; Walde, P. Langmuir 2002, 18, 10509-10511. (5) Morigaki, K.; Walde, P. Curr. Opin. Colloid Interface Sci. 2007, 12 (2), 75-80. (6) Namani, T.; Ishikawa, T.; Morigaki, K.; Walde, P. Colloids Surf., B 2007, 54 (1), 118-123. (7) Morigaki, K.; Walde, P.; Misran, M.; Robinson, B. H. Colloids Surf., A 2003, 213 (1), 37-44. (8) Gebicki, J. M.; Hicks, M. Nature 1973, 243 (5404), 232-234. (9) Hicks, M.; Gebicki, J. M. Chem. Phys. Lipids 1977, 20 (3), 243-252. (10) Hargreaves, W.; Deamer, D. W. Biochemistry 1978, 17 (18), 3759-3768. (11) Cistola, D. P.; Atkinson, D.; Hamilton, J. A.; Small, D. M. Biochemistry 1986, 25 (10), 2804-2812. (12) Cistola, D. P.; Hamilton, J. A.; Jackson, D.; Small, D. M. Biochemistry 1988, 27 (6), 1881-1888. (13) Fukuda, H.; Goto, A.; Yoshioka, H.; Goto, R.; Morigaki, K.; Walde, P. Langmuir 2001, 17, 4223-4231. (14) Ouimet, J.; Croft, S.; Pare, C.; Katsaras, J.; Lafleur, M. Langmuir 2003, 19 (4), 1089-1097. (15) Douliez, J.-P. Langmuir 2004, 20, 1543-1550. (16) Douliez, J.-P.; Barrault, J.; J
er^ome, F.; H
er
edia, A.; Navailles, L.; Nallet, F. Biomacromolecules 2005, 6, 30-34.

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donors.3,13-16 An alternative to this route is to produce salts of fatty acids. Sodium salts of fatty acids are poorly soluble in the case of long alkyl chains, crystallizing at a temperature lower than the Krafft temperature.17 However, it has been reported that tetrabutylammonium salts of long chain fatty acids can form both a gel phase and micelles.18,19 Moreover, we have recently shown that amine salts of fatty acids and their hydroxyl derivatives can self-assemble into various supramolecular structure among which are cones, tubes, vesicles, twisted ribbons, etc.20-26 Fatty acids have also been shown to self-assemble into vesicles, disks, and icosahedra in catanionic systems.27-29 Dispersing fatty acids in aqueous solutions is also a springboard for further applications like foam production. Aqueous foams are dispersions of gas bubbles in a liquid stabilized by surfactants.30 The surfactant goes to the air/water interface, allowing the formation of stable aqueous foams. The properties of the surfactant are often discussed in terms of the HLB balance, but their physical state is also very important in order to (17) de Mul, M. N. G.; Davis, H. T.; Evans, D. F.; Bhave, A. V.; Wagner, J. R. Langmuir 2000, 16 (22), 8276-8284. (18) Zana, R. Langmuir 2004, 20, 5666-5668. (19) Zana, R.; Schmidt, J.; Talmon, Y. Langmuir 2005, 21 (25), 11628-11636. (20) Douliez, J.-P. J. Am. Chem. Soc. 2005, 127, 15694-15695. (21) Douliez, J.-P.; Gaillard, C.; Navailles, L.; Nallet, F. Langmuir 2006, 22 (7), 2942-2945. (22) Douliez, J.-P.; Navailles, L.; Nallet, F. Langmuir 2006, 22 (2), 622-627. (23) Douliez, J.-P.; Pontoire, B.; Gaillard, C. ChemPhysChem 2006, 7 (10), 2071-2073. (24) Gaillard, C.; Novales, B.; J
er^ome, F.; Douliez, J.-P. Chem. Mater. 2008, 20 (4), 1206-1208. (25) Douliez, J.-P.; Navailles, L.; Nallet, F.; Gaillard, C. ChemPhysChem 2008, 9 (1), 74-77. (26) Novales, B.; Navailles, L.; Axelos, M.; Nallet, F.; Douliez, J.-P. Langmuir 2008, 24, (1), 62-68. (27) Dubois, M.; Deme, B.; Gulik-Krzywicki, T.; Dedieu, J.-C.; Vautrin, C.; Desert, S.; Perez, E.; Zemb, T. Nature 2001, 411 (6838), 672-675. (28) Zemb, T.; Dubois, M.; Dem
e, B.; Gulik-Krzywicki, T. Science 1999, 283, 816-819. (29) Rico-Lattes, I.; Blanzat, M.; Franceschi-Messant, S.; Perez, E.; Lattes, A. C. R. Chim. Org. Chem. 2005, 8 (5), 807-814. (30) Weaire, D.; Hutzler, S. Phys. Foams 1999.

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understand their behavior at interfaces. The physical state affects the solubility and mobility of the surfactants as well as their ability to spread at an interface. In particular, it has been shown that the adsorption behavior of surfactant from vesicle structures is strongly dependent on the physical state of the vesicle membrane.31 Rapid surfactant adsorption, which is a requisite for many biological or technological processes, only occurs if the lipid bilayers are in the liquidlike state at a given temperature and pressure. Surfactants embedded in a solidlike gel state in vesicle membranes exhibit an extremely slow adsorption at the interface, and consequently, the resulting foam is unstable. Once formed, foams can undergo multiple destabilization phenomena.32 Liquid drainage occurs because of gravity and leads to dried foams. As the liquid drains, foam films become thinner and bubbles approach toward each other and may collapse. Liquid crystals have been reported to strongly improve the foam stability of aqueous foams.33 Lamellar liquid crystals have high viscosity and could therefore stabilize foams by decreasing the liquid drainage. Small solid particles are also very effective for that and have been shown to be good foam stabilizers.34 Finally, interfacial crystallization of lipids could led to a better stabilization of foams.35 Herein, we report on dispersions of fatty acid-lysine salts which were further used to produce foams, the stability of which is described. These systems were investigated by using transmission electron microscopy, differential scanning calorimetry, and deuterium solid-state NMR to study the thermotropism, as well as the structure and dynamics of bilayers, and small-angle neutron scattering to get quantitative structural data on the bilayer aggregates. Foams were obtained from the fatty acid-lysine dispersions by bubbling nitrogen through a glass filter, and the results were analyzed in terms of foamability and stability.

Materials and Methods Sample Preparation. Protonated (Sigma-Aldrich, both 99% purity) or deuterated (Eurisotop) fatty acid (Sigma-Aldrich) was weighted exactly in a tube, and ultrapure water was added so that the concentration was 10 mg/mL (1%). Then, the desired volume of a 1 M stock solution of L-lysine (Sigma-Aldrich) prepared in ultrapure water was incorporated in order to reach the equivalence (molar ratio fatty acid/base = 1). Samples were heated and vortexed at 70 °C for at least 30 min and frozen at -20 °C. This procedure was repeated three times, and samples were stored at -20 °C. Prior to be used, each sample was heated at 60 °C for 10 min. Differential Scanning Calorimetry. We employed differential scanning calorimetry (DSC) to characterize the lipid-phase transition. Experiments were performed on a microcalorimeter from MicroCal Inc. Only heating cycles can be recorded with this apparatus. The amount of 1.5 mL of the lipid solution was injected in the sample cell with a syringe. The reference cell contained 1.5 mL of ultrapure water. The sample was scanned between 8 and 80 °C at 1 °C/min. The DSC is controlled by an intelligent user interface, and data analysis is performed with Origin. Solid-State NMR. Deuterium solid-state NMR experiments were performed at several temperatures from 20 to 70 °C on a 400 MHz Bruker spectrometer operating at 61 MHz for deuterium using a static double channel probe. The sample coil of the (31) Lee, S.; Kim, D. H.; Needham, D. Langmuir 2001, 17 (18), 5544-5550. (32) R. K. Prud’homme; Khan, S. A. Foams 1996. (33) Shrestha, L. K.; Acharya, D. P.; Sharma, S. C.; Aramaki, K.; Asaoka, H.; Ihara, K.; Tsunehiro, T.; Kunieda, H. J. Colloid Interface Sci. 2006, 301 (1), 274-281. (34) Horozov, T. S. Curr. Opin. Colloid Interface Sci. 2008, 13 (3), 134-140. (35) Rodrı´ guez Patino, J. M.; Rodrı´ guez Ni~no, M. R.; Carrera S
anchez, C.; Navarro Garcı´ a, J. M.; Rodrı´ guez Mateo, G. R.; Cejudo Fern
andez, M. Colloids Surf., B 2001, 21 (1-3), 87-99.

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Novales et al. probe was adapted to load a 7 mm rotor such as those used for magic angle spinning probes equipped with a stretched stator. Typically, lipid dispersions were previously heated to 70 °C, and a volume of ca. 500 μL transferred into the rotor which was sealed and then end-capped. A Hahn quadrupolar echo sequence19 was used with an inter pulse delay of 40 μs. 8K points in 1K accumulations (every 2 s) were done with a 90° pulse and spectral width of 8 μs and 250 kHz, respectively. Free induction decay signal were zero-filled to 16K points prior to Fourier transform after a broad-line exponential multiplication of 200 Hz. For deuterium spectroscopy, the general theory for lipid systems can be found in the literature.36,37 Briefly, the deuterium NMR signal is composed of doublets with a splitting, Δν, which depends on the orientation of the C-D bond with respect to the magnetic field. In an anisotropic but disoriented medium, all the orientations are allowed, and these doublets are superimposed to form a powder spectrum having two main peaks with an increased intensity corresponding to the 90° orientation, separated by Δν90. The edge of the spectrum corresponds to the 0° orientation, with a splitting Δν0 equal to twice Δν90. In the case of perdeuterated systems, the spectrum is composed by the superimposition of signals from each labeled position. Transmission Electron Microscopy. For TEM experiments, a drop of aqueous dispersion was first placed on a carbon-coated TEM copper grid (Quantifoil, Germany). The dispersion was previously heated at 70 °C to prevent the formation of crystals. The grid was then air-dried before introducing it in the electron microscope. Samples were stained with uranyl acetate and were viewed using a JEOL JEM-1230 TEM operating at 80 kV. Small-Angle Neutron Scattering (SANS). Small-angle neutron scattering (SANS) experiments were performed at Laboratoire L
eon-Brillouin (laboratoire mixte CEA/CNRS, Saclay, France) on spectrometer PAXY. The neutron beam was collimated by appropriately chosen neutron guides and circular apertures, with a beam diameter at the sample position of 7.6 mm. The neutron wavelength was set to 4 or 8 A˚ with a mechanical velocity selector (Δλ/λ ≈ 0.1), the 2D detector (128  128 pixels, pixel size 5  5 mm2) being positioned at 1.4 or 6.7 m, respectively. The scattering wave vector, Q, then ranges from typically 0.005 to 0.4 A˚-1, with a significant overlap between the two configurations. The samples, prepared with deuterated water, were held in flat quartz cells with a 2 mm optical path and temperature-controlled by a circulating fluid to within (0.2 °C. The azimuthally averaged spectra were corrected for solvent, cell, and incoherent scattering as well as for background noise. The general theory for fitting the SANS data can be found in the literature.38 Briefly, the scattered intensity can be described as the product of a structure factor S(Q), characteristic of the correlations between objects, and a form factor P(Q), describing the shape of the objects. In the case of a single bilayer of height δ in a vesicle, virtually infinite basis and no preferred orientation (i.e., a randomly oriented flat platelet, a model appropriate for describing independent bilayers), the form factor is Pplatelet ðQÞ µ sin2

Qδ=2 Q4

Form factors asymptotically merge to the Porod limit, that is to say a Q-4 decay, when the scattering wave vector Q is much larger than the reciprocal of the smallest structural size;the thickness δ;up to oscillations usually rapidly damped in scattering experiments by finite resolution, fluctuations in size, etc. Note that, in the Q f 0 limit, with a well-defined Guinier regime Pplatelet is diverging as Q-2. Such a singularity only signals that the approximation made in considering the platelet as virtually infinite no (36) Davis, J. Biochim. Biophys. Acta 1983, 737 (1), 117-171. (37) Seelig, J. Q. Rev. Biophys. 1977, 10 (3), 353-418. (38) Glatter, O.; Kratky, O. Small Angle X-ray Scattering; Academic Press: London, 1982; p 3.

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Figure 1. TEM (A) and cryo-TEM (B) micrographs of a C14-Lys dispersion. The solution was heated at 70 °C prior to the experiment being performed in order to prevent the formation of crystals. On the two cryo-TEM micrographs (B), the dark parts stand for the carbon grid and two vesicles are clearly observed. The sample for cryo-TEM was diluted 10 times compared to that for TEM, which was at 10 mg/mL. Bar represents 200 and 500 nm for TEM and cryo-TEM micrographs, respectively. longer holds true as Q approaches and becomes smaller than the reciprocal of the platelet radius of gyration. Since we shall not attempt to describe quantitatively the scattering data in this wave vector range, the mathematical divergence has no physical relevance. Foams. Foaming experiments were conducted on a “Foamscan” apparatus developed by IT Concept (Longessaigne, France). With this instrument, the foam formation, the stability, and the drainage of the liquid from the foam are determined by conductometric and optical measurements. After calibration of the conductometric electrodes, foams are produced from 15 mL of the solution at a fatty acid concentration of 1 g/L by injection of gaseous N2 through a porous glass filter. The flow rate is fixed at 35 mL/min. All foams are allowed to reach a final volume of 60 mL; after that, gas flow is stopped and the evolution of the foam is analyzed as a function of time. The foamability corresponds to the time needed to reach 45 mL of foam volume. After foam formation, in a timespan of 1200 s, several parameters are automatically recorded by the “Foamscan” analyzing software. The volume of the foam is determined with a CCD camera (Sony Exwave HAD). A pair of electrodes at the bottom of the column is used to measure the quantity of liquid that is not in the foam, while the volume of the liquid in the foam is measured by conductimetry using three pairs of electrodes located along the glass column. The liquid drainage of the foam is followed via conductivity measurements at different heights in the foam column. For foam preparation, samples were heated just before the experiment and cooled down at ambient temperature for 10 min to avoid possible solid crystallization.

Results and Discussion Phase Behavior. Dispersions of fatty acid-lysine salts from C12 to C18 were studied at a molar ratio of 1/1, that is, for an equimolar ratio (neutralization point). In the case of the dodecyl alkyl chain length, an isotropic liquid solution was observed whatever the temperature (from 5 to 70 °C), suggesting the formation of micelles in this system. For longer alkyl chain lengths, turbid birefringent solutions were obtained. They were formed of vesicles as suggested by phase contrast microscopy (not shown) and further demonstrated by transmission electron microscopy, TEM, and cryo-TEM (Figure 1). The formation of fatty acid vesicles is rare5 and is a strong challenge for various technological applications involving green and/or generic molecules and even for understanding the origin of life on the early earth.10,39,40 Indeed, several models postulate that fatty acid vesicles might have been present in the prebiotic soup serving as compartments and reacting medium for the early synthesis of (39) Segr
e, D.; Ben-Eli, D.; Deamer, D. W.; Lancet, D. Origins Life Evol. Biosphere 2001, 31, 119-145. (40) Monnard, P.-A.; Deamer, D. W. Preparation of Vesicles from Nonphospholipid Amphiphiles Methods in Enzymology; Academic Press: New York, 2003; Vol. 372, pp 133-151.

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Figure 2. Differential scanning calorimetry experiments for fatty acid-lysine salts from C14 to C18 (from top to bottom, respectively). As expected, the transition temperature increased with the alkyl chain length.

Figure 3. Solid-state deuterium NMR spectra obtained below (top) and above (bottom) the phase transition for the C16-lysine salt (see text for details). Similar spectra were obtained for the C14 and C18 chain length.

biological materials. This may be of particular interest since amino acids as lysine may have also been present in the prebiotic conditions. However, those fatty acid-lysine salts solutions crystallized after several hours when kept at room temperature or at 4 °C. This is a common feature of fatty acid salt solutions.17,22,24,26 Then in the following, except when mentioned, all the samples were heated at 70 °C and rehomogenized prior to an experiment being performed. This allowed to investigate the behavior of the fatty acid vesicles and not that of the crystals. The thermogram of the samples from C14 to C18 is shown Figure 2. The C12-Lys system was not submitted to DSC since micelles form and then no transitions are expected. The sharp peak indicates a phase transition of 34, 51, and 64 °C for respectively the C14, C16, and C18 chain length. This suggests a transition from a gel (Lβ) to a fluid (LR) bilayer phase upon increasing temperature as commonly observed for phospholipid vesicles or membranes and even fatty acid-containing systems.15,21,22,24,26,36,41 This was further confirmed by small-angle neutrons scattering and deuterium NMR. Interestingly, in the present case, the transition temperature Lβ f LR is only slightly lower than for the corresponding sodium salt of the fatty acids. For instance, (41) Koynova, R.; Caffrey, M. Chem. Phys. Lipids 2002, 115 (1-2), 107-219.

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Figure 4. SANS intensity profile for C16-Lys salt below (left) and above (right) the phase transition. Both the Porod region (top) and the scattering profile in a logarithmic scale are shown (bottom). The continuous line corresponds to the model discussed in the Materials and Methods section and in the text.

sodium myristate forms micelle above a temperature of about 40 °C and crystallizes below. Deuterium NMR. To get deeper information on the structure and dynamics of the birefringent phase, we employed deuterium solid-state NMR using deuterated fatty acids. This is known to bring valuable information on the lipid phase behavior36,42 and has been successfully employed in the case of other fatty acid systems.15,22,26,43 For the dodecyl chain length, the spectrum (not shown) returned an isotropic line consistent with the formation of micelles, since the tumbling and dynamics of small dimensional assemblies averages the solid NMR signal. For longer chain length, at low temperature below the phase transition, the systems exhibited a spectrum similar in shape to the one obtained for other systems containing fatty acids (Figure 3).15,22 It was basically composed by the superimposition of two main patterns with a splitting of 57 and 12 kHz, respectively. The larger value stands for the methylene groups of the fatty acid alkyl chain, while the lower corresponds to the terminal methyl group.15,22 Such a width of the NMR pattern shows that fatty acids are embedded in a gel (Lβ) bilayer phase below the phase transition with their alkyl chains in an all-trans conformation.36 However, note that several weaker doublets having a splitting of about 40 kHz can be assigned and account for methylene positions at the end of the alkyl chain. This shows that the end of the alkyl chain is slightly mobile as was already observed in the case of other fatty acid mixtures.15 After a period of rest of 1 day at room temperature, the shape of the spectra remained similar; i.e., it was still composed of the superimposition of two powder patterns (not shown); however, the quadrupolar splitting of both components increased (42) Davis, J. H. Biophys. J. 1979, 27 (3), 339-358. (43) Douliez, J.-P. J. Colloid Interface Sci. 2004, 271 (2), 507-510.

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to 125 and 37 kHz, respectively. Those values are typical for alkyl chains embedded in a crystal structure,36 confirming the observations made by visual inspection of the sample tubes. After heating the sample to 60 °C and cooling it back to room temperature, the spectrum was identical to that previously recorded prior the crystallization occurs. Above the phase transition, the spectra were composed by the superimposition of well-assigned doublets having quadrupolar splitting markedly lower than those acquired below the phase transition. This feature is typical of alkyl chains embedded in a fluid (LR) bilayer phase.15,22,36 Note that for both spectra the shoulders which generally occur for powder patterns are of weak intensity, suggesting a self-orientation of the bilayers in the magnetic field. Once again, this is a common feature in systemcontaining fatty acids.15,22 This suggests that vesicles adopt a prolate shape induced by the strong magnetic field. This may appear surprising since the bending modulus in a bilayer gel phase is greater than in the fluid phase.44 Then, the self-orientation occurs in both the gel and the fluid phase, in other words, whatever the rigidity of the bilayer. Small-Angle Neutron Scattering. To get another and more complete quantitative view of the structure of the phases observed in those systems, we performed SANS experiments in the case of the palmitate-lysine salt below and above the transition. At both temperatures, the spectra are qualitatively and even quantitatively very similar. They undergo a strong small-angle scattering signal with a slope at Q-2 for Q f 0, and no Bragg peaks as observed for other fatty acid systems22,23,26 (Figure 4). This suggests that the vesicles are not multilamellar but confirms the bilayer structure. (44) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. Proc. Natl. Acad. Sci. U.S.A. 2001, 98 (4), 1353-1357.

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Figure 6. Evolution of liquid volume in the foam for foams made of fatty acid-lysine salts at 1 g/L.

Figure 5. Evolution of foam volume for foams made of fatty acid-lysine salts at a concentration of (A) 1 g/L and (B) 0.25 g/L.

The continuous lines displayed in Figure 4 correspond to fits of the large-angle part of the experimental Q-range where the approximation S(Q) = 1 can be considered as valid to the relevant form factor, using the characteristic size δ (and an arbitrary intensity prefactor) as a fitting parameter. The good quality of the resulting fits, already apparent in the double-logarithmic representations of Figure 4, is perhaps better illustrated in the so-called Porod representation, where Q4 times the scattering intensity is displayed as a function of Q (see Figure 4). The fits yielded values of the thickness δ equal to 40 and 33 A˚ for the low- and hightemperature phases, respectively. A thickness of 40 A˚ is consistent with fatty acids embedded in a gel phase since it corresponds to twice the alkyl chain length of the palmitic acid in its all-trans conformation. At higher temperature, that value is markedly lower, showing that the alkyl chains undergo motions; i.e., they are in a fluid (LR) state. Altogether, this confirms the results obtained by deuterium NMR. Foaming Properties. Foams were prepared from the fatty acid dispersions and their stability was assessed. At a concentration of 1 g/L, for the lysine salt of stearic acid, almost no foam was produced and the maximum foam volume (60 mL) could not be reached. For the three other samples, the foam stability depended on the chain length. The time evolution of the foam volume for samples C12, C14, and C16 in the presence of lysine is shown Figure 5 and is commented on hereafter. For C12-Lys, the final foam volume was reached after about 120 s. However, the foam presented large bubbles and was not stable as the foam volume decreased very rapidly. A fast coalescence was also observed as the bubble size rapidly increased in the sample column. Finally, the foam completely collapsed after 800 s. For C14-Lys and C16-Lys, the fixed foam volume of 60 mL was reached after about 110 s, and the foams were very stable with small bubbles of low polydispersity. At the end of the experiments, the remaining foam volume did not decrease anymore and was still above Langmuir 2010, 26(8), 5329–5334

55 mL. These foams could be observed without any change of foam volume for several hours. However, the liquid volume incorporated in the foams differed for these two samples (Figure 6). For C14-Lys, more than 4 mL was incorporated in the foam which corresponded to a foam containing 6% of liquid. For C16-Lys, the liquid volume in the foam was much lower with only 2 mL of solution being incorporated during the foaming process. A slow drainage was observed leading after 1200 s to a very dried foam. The foam obtained from the C16-Lys contained less than 0.3 mL of liquid at the end of the experiment, which corresponded to a foam with a liquid content less than 0.5%. A visual observation of the foam showed that a slight coalescence had occurred in the upper part of the foam which exhibited a shiny aspect. In order to check the effect of fatty acid concentration, each sample was diluted by a factor of 4 and foamed in the same conditions as previously done. Figure 5 shows the remaining foam volume as a function of time. At that concentration (0.25 g/L), it was not possible to reach the fixed volume of 60 mL with the C12/ Lys mixture. With the C14/Lys mixture, the fixed volume was reached, but the resulting foam was not stable and rapidly collapsed (after 260 s). On the contrary, with the C16/Lys mixture, we still observed a stable foam formed by small bubbles. At the end of the experiment, the remaining foam volume was above 55 mL even if the foam was very dry (less than 0.5% of the fatty acid solution in the foam). These experiments show that the physical aspect of the surfactant is very important for foam formation and stability. The C12/ Lys mixture which formed micelles at any temperature ranging from 5 to 70 °C led to the formation of foams with large bubbles and a low stability. On the contrary, C14/Lys and C16/Lys, which formed vesicles at the temperature at which foams were produced, led to stable foams with small bubbles. Vesicle forming surfactants seem to provide foams more stable than micelle forming surfactants. This phenomenon has been already observed by Bezelgues et al.,45 who compared the foaming properties of vesicle and micelle forming surfactants. The differences observed between the three vesicles forming mixtures (C14, C16, and C18) could also be explained by the physical aspect of the surfactant solutions. At the temperature at which foams were produced (around 35 °C), C18/Lys exhibited large solid crystals visible by visual inspection of the sample. Under this form, fatty acids are not able to spread at the interface, resulting in poor or even no foam formation. At the same temperature, as viewed by visual (45) Bezelgues, J.-B.; Serieye, S.; Crosset-Perrotin, L.; Leser, M. E. Colloids Surf., A 2008, 331 (1-2), 56-62.

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inspection and phase contrast microscopy, C14/Lys was perfectly dispersed, whereas for C16/Lys, crystallization started to occur although vesicles were still present. In both cases, stable foams were produced from these solutions, but the stability was higher in the case of the palmitic chain length (Figure 5). However, if we cooled down the mixtures at 20 °C for more than 1 h, large crystals formed for both samples and no foam was formed. The coexistence of both crystals and vesicles in the solution then appears determinant for the formation of a very stable foam. The relative amount of both assemblies is difficult to determine in our present experiments. We could perform foaming experiments at different times in order to study the kinetics of crystallization and then the effect on the foam stability.46 However, the bubbling procedure during the foam formation may accelerate the crystallization of the fatty acid vesicles since the gas we use is at room temperature. Then, we plan in the future to perform such kinetics experiments by coupling the foam formation and a systematic SANS study of the foams. This should give us valuable information on the foam itself47 and the structure of the assemblies which are still present. In the case of the C16/Lys system, we expect that the fatty acid salts which were embedded in the vesicles are spread at the interface. Then, the crystals also present in the solution help to slow down the drainage within the foam. Indeed, solid particles have been shown to slow down the drainage rate due to an increase of the surface viscosity of the continuous phase.33 In the same way, Shrestha et al. have shown that the solid surfactants

were better foam stabilizers than LR liquid crystals.33 Kunieda et al.48 have also demonstrated that superstable nonaqueous foams were stabilized mainly by surfactant solid instead of lamellar crystalline phase. Interfacial crystallization can also increase the foam stability, mainly because of the rigidification of the foam film. It has been shown that the formation of surface active crystals at the interface results in a reduction of the interfacial tension.35 Krog et al.49 have shown that crystallization at the interface can be expected to be faster than nucleation and crystal growth in the bulk phase. We also expect that crystallization of fatty acids occurs at the interfaces with time as it occurs in solution, and we will further investigate this phenomenon by studying foams via SANS experiments. Summary. We have shown that lysine fatty acid salts selfassemble in water depending on the alkyl chain length into micelles and “unstable” vesicles which further crystallize. This finding is similar to previous results on fatty acid salt dispersions: although fatty acids can be dispersed into various supramolecular assemblies, they tend to crystallize when they are kept for a prolonged time in their gel (Lβ) phase. Of more practical considerations for industrial applications, those fatty acid-lysine salts exhibit strong foaming properties depending on the alkyl chain length. We expect this finding to be of interest in the field of detergency. It is noteworthy that both fatty acids and lysine are bio and green molecules which may increase this practical interest for this field.

(46) We thank one of the reviewers for the suggestion of such experiments. (47) Axelos, M. A. V.; Bou
e, F. Langmuir 2003, 19 (17), 6598-6604. (48) Kunieda, H.; Shrestha, L. K.; Acharya, D. P.; Kato, H.; Takase, Y.; Gutirrez, J. M. J. Dispersion Sci. Technol. 2007, 28 (1), 133-142. (49) Krog, N. Cryst. Process Fats Lipid Syst. 2001.

Acknowledgment. We thank Laboratoire L
eon-Brillouin for the allocation of neutron beam time on the spectrometer PAXY. We gratefully acknowledge the assistance of our local contact, Laurence Noirez, during the neutron scattering run.

5334 DOI: 10.1021/la9034613

Langmuir 2010, 26(8), 5329–5334