13364
J. Phys. Chem. B 2007, 111, 13364-13370
Phase Behavior of Cetyltrimethylammonium Surfactants with Oligo Carboxylate Counterions Mixed with Water and Decanol: Attraction between Charged Planes or Spheres with Oligomeric Counterions Jens Norrman and Lennart Piculell* DiVision of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund UniVersity, P.O. Box 124, SE-221 00 Lund, Sweden ReceiVed: May 28, 2007; In Final Form: August 22, 2007
Cetyltrimethylammonium surfactants with a range of oligo carboxylate anions bearing 2, 3, or 4 negative charges have been synthesized, and their respective behaviors in binary mixtures with water and in ternary mixtures with added decanol have been investigated. In binary mixtures with water, all surfactants formed nearly spherical micelles at high water contents; however, the interactions between micelles varied strongly with the number of charges in the counterion. Micelles with divalent counterions were generally miscible with water, whereas micelles with tri- or tetravalent counterions demixed in one concentrated and one dilute phase. Addition of decanol resulted in all cases in the appearance of a lamellar phase, and all investigated oligo carboxylate anions (di-, tri-, and tetravalent) gave rise to a strong attraction between the lamellar planes, resulting in a limited swelling (up to 35-40 wt % water) of the lamellar phase in contact with excess water. These experiments confirm the theoretically predicted influence of aggregate geometry (spheres or planes) on the attraction between colloidal aggregates neutralized by multivalent counterions. Further addition of decanol resulted in the appearance of a second birefringent phase in equilibrium with the lamellar phase. SWAXS showed this phase to be lamellar and to display short-range order that disappeared upon heating. This phase is identified as a lamellar gel phase (Lβ-phase).
Introduction The behavior of polyelectrolytes mixed with oppositely charged surfactants in aqueous solution is a subject of intense investigation.1-3 Such mixtures of polyelectrolyte (polyion + counterion) and simple surfactant (surfactant ion + counterion) often phase-separate into a dilute and a concentrated phase, each of which can be described as a four-component system of water and three different salts.4 To simplify the mixtures, we have in our laboratory adopted a strategy wherein we use the pure complex salt (polyion + surfactant ion) as a starting point.5-9 This has the advantage of reducing the complexity of the investigated systems by using a minimum number of components. For example, the interactions between the polyions and the surfactant aggregates in water can be investigated without the influence of excess salt. If one adds a third nonionic component or an ionic component that shares one ion with the complex salt, one obtains a truly ternary system. From a slightly different perspective, aqueous mixtures of complex salts, which contain surfactant aggregates with polymeric counterions, represent a simple, versatile, and chemically very pure “model colloid” system. In this model system, one can change the geometry, tune the charge density of the charged surfactant aggregate, or both by, for instance, adding a longchain alcohol.8 By making the appropriate choice of surfactant counterion, or mixture of counterions, one can study the effect on the aggregate-aggregate interaction of varying, for example, the detailed chemistry6 or the degree of polymerization7 of the counterion. The interaction between charged aggregates in the presence of multivalent or polymeric counterions is a central problem in * Corresponding author. E-mail:
[email protected].
colloid science in which significant theoretical progress has been made during the last couple of decades. Thus, Monte Carlo simulations performed in the early 1980s showed that the contribution from ionic correlations should give rise to a net attractive force between highly charged planes neutralized by divalent counterions in water.10 This attraction, which is absent in the classical DLVO theory, has explained many experimental observations of attractive forces in colloidal systems.11 Of particular relevance here is the limited swelling, in excess water, of lamellar ionic surfactant phases neutralized by divalent counterions.12-14 More recent Monte Carlo simulations have predicted that for typical spherical micelles of ionic surfactants, divalent counterions are not sufficient to induce a phase separation of dilute mixtures, whereas solutions of micelles with trivalent counterions should, indeed, separate into one dilute and one concentrated phase.15,16 We have found no experimental investigation of the phase behavior of a salt-free system of spherical ionic micelles with trivalent counterions, but it has been shown that aluminum salt added to a micellar solution of sodium dodecyl sulfate indeed induces a precipitation.17,18 Another early theoretical work involving both Monte Carlo simulations and mean-field theory studied charged planar surfaces with a charge density typical for liquid crystalline surfactant systems, which were neutralized by flexible polymeric counterions. One parameter that was varied in this study was the connectivity, or degree of polymerization (DP), of the polymeric counterion. It was found that the distribution of charged monomers outside the charged surface rapidly converged with an increasing DP already for oligomeric counterions of a DP in the range 2-10, depending on the choice of other
10.1021/jp074114+ CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007
Phase Behavior of Surfactants, Water, Decanol parameters.19 The distance-dependent force between the surfaces was compared for DP values of 2 and 10. In both cases, attractive forces between the surfaces were found, with minima occurring at nearly the same (short) surface separation. The depth of the attractive minimum was, however, much deeper for the DP10 polyions, an effect that was ascribed to bridging effects. Recently, in connection with an experimental study in our laboratory of the phase behavior of surfactants with mixed polymeric and monomeric counterions, further Monte Carlo simulations were performed.6 As the fraction of polymeric counterions was increased, the force between the surfaces was found to change from repulsive to attractive, in agreement with the experimental findings. Again, an analysis of the simulation results showed that the dominating attractive component in the interaction potential was the bridging contribution. Prior to the present study, the situation is, thus, that there exists a wealth of theoretical studies of the interaction between charged colloidal aggregates of spherical and planar geometries (with parameters often chosen to mimic surfactant aggregates), neutralized by multivalent and polymeric counterions.10,11,15,16,19 On the experimental side, detailed studies on simple model systems exist for spherical and planar surfactant aggregates comparing mono-, di-, and polyvalent counterions.5-8,11-14 To fill an obvious experimental gap, we have in the present study chosen to focus on surfactant aggregates with oligomeric counterions. To make contact with the previous studies from our laboratory, which involved monovalent acetate and polymeric polyacrylate counterions,5-8 we here use di-, tri-, and tetracarboxylate ions as counterions to a cationic cetyltrimethylammonium surfactant ion. One advantage with these systems is that these carboxylate ions are commercially available and monodisperse so that the complication of polydispersitys notorious for synthetic polyionssis avoided. Another advantage is that a range of dicarboxylates, varying in the separation between the charged carboxylate groups, are available. In this work, we use the methodology developed by Svensson5 in our laboratory to make cetyltrimethylammonium surfactants with different oligo carboxylate counterions, and we investigate the phase behavior of all binary surfactant/water mixtures. In a recent study, we showed that by adding small amounts of decanol, a known cosurfactant, to complex salts of cetyltrimethylammonium ions with polymeric counterions, lamellar structures were obtained.8 Since decanol has a very low solubility in water (0.003%),14 virtually all decanol added to a lamellar phase will be incorporated into the surfactant aggregates, effectively diluting the surface charge. In this way, addition of decanol can be used, in addition to causing the changeover to a lamellar structure, to investigate what happens when the surface charge density of the planes becomes lower. Materials and Methods Materials. Cetyltrimethylammonium bromide (C16TABr) was purchased from Merck and used without further purification. Decanol, 99% pure, was purchased from BDH Chemicals Ltd Poole and used as received. Oxalic, malonic, adipic, and citric acids (all 99%+ pure) and suberic acid (98% pure) were all purchased from Aldrich; succinic acid (99%+ pure) was purchased from Lancaster; and butyl-tetracarboxylic acid (98% pure) was from Alfa Aesar. All carboxylic acids (shown in Figure 1) were used as received. Millipore water with a resistivity of 18.2 MΩ/cm was used throughout the study. Synthesis of the Surfactants. Surfactants of C16TA+ with oligocarboxylate counterions were prepared by titrating the hydroxide form of the surfactant with the acid forms of the
J. Phys. Chem. B, Vol. 111, No. 47, 2007 13365
Figure 1. Molecular structures of the carboxylic acids used in this study: (a) oxalic acid, (b) malonic acid, (c) succinic acid, (d) adipic acid, (e) suberic acid, (f) citric acid, and (g) butyl-tetracarboxylic acid.
carboxylates according to the procedure used in our previous work to make complex salts.5-9 The surfactants will be named (C16TA)xY, where x is the number of surfactant ions in an electroneutral formula unit, and Y is the carboxylate counterion: oxalate (Ox), malonate (Mal), succinate (Suc), adipate (Ad), suberate (Sub), citrate (Cit), or butyl-tetracarboxylate (BTC). The uptake of water of the surfactants after freeze-drying was measured by weighing the surfactant directly after freeze-drying and then after prolonged storage. This experiment indicated that the water uptake by the surfactants was 10 wt % for a salt stored in the desiccator and 20 wt % after prolonged storage in air. For this reason, care was taken to minimize the exposure of the components to air during sample preparation. The surfactants were assumed to have a water content of 10 wt % when calculating the sample compositions. Sample Preparation. Desired amounts of surfactant, water, and decanol were added to test tubes, which were then flamesealed. After flame-sealing, the samples were mixed by centrifuging the tubes at 3000g for 15 min and then turning the test tubes upside down and centrifuging again. This was repeated for 6 h. The samples were then left for 1 week, after which the samples were again centrifuged for 6 h, turning the test tubes over every 15 min. After this, the samples were left at 25 °C (unless otherwise stated) for at least 1 week before the number of phases was determined and SAXS was performed to study the nature of the different phases. Structures of Equilibrium Phases. All samples were investigated by visual inspection in normal light and between crossed polarizers to detect the number of phases present and which of these phases were optically anisotropic (in the present case, the hexagonal and lamellar phases). SWAXS (small and wide-angle X-ray scattering) measurements were performed with a Kratky compact small-angle system with linear collimation. The X-rays were detected with two different position-sensitive detectors: one for small angles (0.05 < q < 0.6 Å-1) and one for wide angles (1 < q < 2 Å-1). The wavelength of the X-rays was 1.54 Å, and the sample-to-detector distance, 277 mm. The sample cell had mica windows and was maintained at 25 or 60 °C for the duration of the measurement, with adequate equilibration times between measurements on the same sample at different temperatures. The spacing of the lamellar phases found was determined from
D)
2π q1
(1)
13366 J. Phys. Chem. B, Vol. 111, No. 47, 2007
Figure 2. The three different dilution lines used in this study. The gray solid arrow is the binary water axis, the black solid arrow is the water dilution line, and the black dashed arrow is the decanol dilution line.
Figure 3. Binary water axis for the different surfactants at 25 °C.
Here D is the distance between the lamellar midplanes, and q1 is the position of the first peak in the lamellar SAXS spectrum. Results The behaviors of the different surfactants were investigated by exploring samples along three different dilution lines, shown in Figure 2: The binary water axis, the water dilution line, and the decanol dilution line. The binary water axis is the left axis of the three-component phase diagram, that is, mixtures of surfactant with water (no added decanol). The water dilution line contained samples with a constant surfactant/decanol molar ratio (1.38:1), and the decanol dilution line samples, with a constant surfactant/water mass ratio (35:65). A previous investigation of similar systems has shown that this approach with three dilution lines is very powerful for getting information about the different phases in the phase diagram from a relatively small number of samples.8 For the water dilution line, we chose a relatively low decanol content to bring the systems all the way, but not too far, into the lamellar phase. Binary Surfactant-Water Mixtures. The different surfactants shared many characteristics on addition of water (Figure 3). At low water contents, there was a hexagonal phase, followed by a cubic phase. The latter was identified by SAXS to be of the Pm3n cubic space group (Figure 4). The same cubic structure has been found at the same location in the phase diagrams for the closely related alkyltrimethylammonium surfactants with acetate and polyacrylate counterions and has been shown to be
Norrman and Piculell
Figure 4. SAXS spectra of the cubic Pm3n phase with 44 wt % (C16TA)2Ox and 56 wt % H2O at 25 °C. The relative positions of the peaks x4, x5, and x6 are indicated.
a micellar cubic phase of small nearly spherical micelles.5-7 The extensions of the liquid crystalline phases varied between the different counterions, but the sequence was always the same. A substantial variation was seen among the dicarboxylate ions containing 0-6 methylene groups between the carboxylates, but with no monotonic trend. At still higher water contents, the different surfactants exhibited different behaviors, depending mainly on the number of charges in the counterion (Figure 3). Most of the surfactants with dicarboxylate counterions dissolved at high water contents, showing no miscibility gap in the binary water axis. The exception was the most hydrophobic Sub counterion, where a two-phase area with a cubic phase in equilibrium with a dilute water phase was entered at high dilution. In contrast to most dicarboxylate surfactants, both the tri- and tetracarboxylate surfactants displayed a limited miscibility with water. For (C16TA)3Cit, there was a miscibility gap at high water contents, with two liquid phases of different surfactant concentration in equilibrium with each other. A wider miscibility gap was present for (C16TA)4BTC; in this case, the two phases in equilibrium were the cubic phase and a very dilute solution of the surfactant. Ternary Phase Diagrams. For the investigation of ternary mixtures, the tri- and tetravalent ions and two of the divalent ions were selected. We wished to avoid the most hydrophobic dicarboxylate ions, in which hydrophobic interactions can be expected to play a significant role for the observed phase behavior. We therefore chose the small oxalate ion and the succinate ion, since these two ions represent the extremes in the quantitative variation of the phase boundaries among the dicarboxylate ions in Figure 3. The ternary phase diagrams shown in Figures 5-8 were constructed from samples with the global compositions indicated by the solid dots in the diagrams. The tentative positions and extensions of the three- and twophase areas, indicated by dashed lines in Figures 5-8, were estimated on the basis of these samples and the Gibbs phase rule. All the different surfactant systems changed over into lamellar phases upon addition of decanol. At high water contents and low decanol contents (along the decanol dilution line), each lamellar phase was in equilibrium with a mostly quite dilute aqueous micellar phase; for simplicity, this phase will henceforth be referred to as the water phase. Upon addition of more decanol, a second birefringent phase appeared on top of the lamellar phase. As will be justified below, the additional birefringent phase, which was visually different from the dominating liquid crystalline lamellar phase, was identified as a lamellar “gel” phase. In accordance with the conventional
Phase Behavior of Surfactants, Water, Decanol
Figure 5. Phase diagram for (C16TA)2Ox/H2O/decanol at 25 °C.
J. Phys. Chem. B, Vol. 111, No. 47, 2007 13367
Figure 8. Phase diagram for (C16TA)4BTC/H2O/decanol at 25 °C.
TABLE 1: D-Spacing of the Lr Phase in the Two-Phase Area Close to the One-Phase Border
Figure 6. Phase diagram for (C16TA)2Suc/H2O/decanol at 25 °C.
Figure 7. Phase diagram for (C16TA)3Cit/H2O/decanol at 25 °C.
terminology for lipids, we will henceforth refer to the gel phase as the Lβ phase and the lamellar liquid crystalline phase as the LR phase. For (C16TA)2Ox, (C16TA)3Cit, and (C16TA)4BTC, the LR and Lβ phases were in equilibrium with a third, isotropic liquid top phase containing essentially decanol, henceforth called the decanol phase. Increasing the decanol content along the decanol dilution line caused the LR phase to disappear and left the Lβ phase in equilibrium with the decanol phase. Within the LR-isotropic liquid two-phase area along the decanol dilution line, the D-spacing of the LR phase increased slightly (5-10%) when increasing the amount of decanol (data not shown).
surfactant
D-spacing (Å)
(C16TA)2Ox (C16TA)2Suc (C16TA)3Cit (C16TA)4BTC
44.0 44.3 43.6 44.0
The behavior with (C16TA)2Suc differed slightly from the above. When the Lβ phase appeared, the sample did not contain a separate third isotropic phase, as for the other surfactants, but the sample was cloudy. This cloudiness was interpreted as being due to a third phase, most probably the water phase, dispersed in the other two phases. Further addition of decanol caused the system to enter a one-phase area, containing only the Lβ phase, and then a two-phase area with the Lβ phase in equilibrium with the decanol phase. Along the water dilution line, all surfactants showed a singlephase LR area at approximately the same composition range at low water contents. Table 1, showing the D-spacing of the LR phase from samples in the two-phase (LR + water) area, confirms that the maximum water swelling of the LR phase was very similar for the different surfactants. At high water contents, the two-phase area disappeared for (C16TA)2Ox and (C16TA)3Cit and was replaced with a single isotropic water phase. Identification of the Two Lamellar Phases. For (C16TA)2Ox and (C16TA)2Suc, the two birefringent phases found upon addition of decanol were visually different when viewed under normal light, with a distinct border between them. For (C16TA)3Cit and (C16TA)4BTC, this border was not always that apparent, probably because of the higher viscosity of these mixtures, which made the separation of the two different phases difficult. When viewed between crossed polarizers, both phases showed strong birefringence. The birefringent phase at high decanol contents showed a typical lamellar SAXS spectrum with peaks at the relative positions of 1:2 (Figure 9) but also had a distinctive peak in the WAXS spectrum at 4.18 ( 0.03 Å that disappeared upon heating, indicating a crystalline order with a small unit cell. This behavior, a peak in the WAXS part of the spectrum that disappears upon heating, is typical for lipid “gel” phases in which the alkyl tails have crystallized.20 Visual observations showed that, upon heating a sample, the two lamellar phases macroscopically merged (at approximately 40 °C) into a single birefringent lamellar phase in equilibrium with an isotropic liquid. As the temperature was further increased, the volume
13368 J. Phys. Chem. B, Vol. 111, No. 47, 2007
Norrman and Piculell
Figure 10. SAXS spectrum of 11 wt % (C16TA)3Cit, 19 wt % H2O and 70 wt % decanol at 25 °C, showing a superposition of the spectra of the LR phase and the Lβ phase.
Figure 9. SAXS (a) and WAXS (b) spectra of 25 wt % (C16TA)2Suc, 45 wt % H2O and 30 wt % decanol at 25 and 60 °C.
TABLE 2: D-Spacing of the Coexisting Lr and Lβ Phases Recorded in the Three-Phase Areas with Excess Decanol (Ox, Cit, BTC) or Water (Suc) surfactant
L R, Å
Lβ, Å
(C16TA)2Ox (C16TA)2Suc (C16TA)3Cit (C16TA)4BTC
45.0 49.8 43.8 43.6
45.8 51.4 45.6 45.5
of the birefringent phase decreased while the volume of the isotropic liquid increased. This behavior was general for all surfactant mixtures containing water and decanol. The Lβ phase had a density that was lower than the density of the La phase, that is, the Lβ phase was on top of the LR phase. Comparing the D-spacing of the two different phases (Table 2), it is apparent that the D-spacing of the Lβ phase was slightly larger than that of the coexisting LR phase. As mentioned above, the Lβ phase was not always easily separated from the LR phase. This can be seen in Figure 10, showing the LR and Lβ phases in the same spectrum for a sample of (C16TA)3Cit. Discussion The Binary Water Axis. For the binary mixtures of the dicarboxylate surfactants with water, an increase in the number of methylene groups separating the carboxylate ions gave rise to a nonmonotonic variation in the extension of the liquid crystalline phases. Both the hexagonal and the cubic phases were most displaced toward low water content for the succinate surfactant. One may speculate that at least two mechanisms, giving rise to opposite effects, are at play: One mechanism would be the increasing distance between the oxalate group (a
Figure 11. Phase behavior for a number of C16TA+ surfactants with mono-, oligo-, and polycarboxylate counterions at 25 °C.
“spacer” effect), which seems to push the phase boundaries toward a lower water content, and the other would be the increasing hydrophobicity of the ion, on increasing the number of methylene groups. (The dicarboxylates with the longest spacers are quite hydrophobic, as indicated by the fact that the corresponding acids, adipic acid and especially suberic acid, have a low solubility in water.) However, we do not have sufficient information to pursue this argument further and conclude that in this investigation, we have not been able to isolate the spacer effect, which could be assumed to be important for a bridging attraction between aggregates.6,19 Instead, we content ourselves with the observation that micelles with divalent counterions generally (in the absence of other interactions, as for the strongly hydrophobic suberate) are fully miscible with water. This agrees with the previous results of Fontell et al., who found no miscibility gap in binary mixtures of (C16TA)2SO4 with water.14 In the further discussion of trends for counterions with differing degrees of polymerization, we find it useful to widen our perspective. To that end, Figure 11 gives an overview of the phase behavior of binary mixtures with water for a number of cetyltrimethylammonium surfactants with different carboxylate counterions. Here, we have included previously obtained data for monovalent and polymeric counterions carrying carboxylate groups.6,7 Among the divalent ions, we have included only the oxalate and succinate ions for reasons given above. From the bottom to the top, the DP of the surfactant counterion
Phase Behavior of Surfactants, Water, Decanol TABLE 3: Micellar Aggregation Numbers (Calculated6,7 from SAXS Data) for Various Surfactants and Complex Salts in the Cubic Phase surfactant/complex salt
Nagg
C16TAAc (C16TA)2Ox (C16TA)2Suc (C16TA)3Cit (C16TA)4BTC C16TAPA30
114 (from ref 6) 107 113 116 101 100 (from ref 7)
increases from monomeric via oligomeric to polymeric counterions. The hexagonal phase is present in all systems when the water content is low. This is probably because of packing constraints. At higher water contents, the hexagonal phase shifts into a cubic phase, found to of be the Pm3n cubic space group in all cases. As in previous work,6,7 we have here calculated the aggregation number, Nagg, of the micelles in the cubic phase from the unit cell dimensions obtained by SAXS (Table 3). It is notable that there is hardly any variation in Nagg between the various systems, indicating that there is little effect on the surfactant aggregates when changing from monomeric, via oligomeric, to polymeric counterions. As noted above, the extensions of the cubic and hexagonal phases vary between the different oligocarboxylate counterions, showing no clear trend. We have no explanation for the particular variation observed, but we note that the various oligomers do not contain exactly the same repeating units. Thus, not only the number of carboxylate groups, but also the detailed chemistry of the oligomers, seems to be important for the exact locations of the phase boundaries. At higher water contents, there are larger differences. With a monomeric or dimeric counterion (C16TAAc, (C16TA)2Ox, and (C16TA)2Suc) there is no miscibility gap at high water contents, as already mentioned. Increasing the charge of the counterion to three ((C16TA)3Cit) causes a miscibility gap to appear at high water contents, with two liquid phases of different surfactant concentration in equilibrium. When further increasing the charge of the counterion to four ((C16TA)4BTC), the miscibility gap remains, but now with a cubic phase in equilibrium with pure water. It is notable that an additional increase in DP of the counterion to 30 (C16TAPA30), has no effect on the sequence of phases along the binary water axis; already with four charges, the attraction is strong enough to condense the micelles into a crystalline cubic phase. However, when increasing the counterion charge to a very high value (6000, C16TAPA6000), the phase found in equilibrium with pure water is the hexagonal phase, not the cubic phase. Previous work showed that the switch to a hexagonal phase in equilibrium with water takes place at a degree of polymerization of ∼100.7 Apparently, polyion end effects play a role in the aggregate shape of this long-chain surfactant ion. It is important to note, however, that the cubic structure is retained, even with the long polyion as a counterion, for the C12TAPA6000 surfactant,7 and it should be kept in mind that C16TA micelles are quite easily persuaded to grow into rodlike aggregates, even by such subtle changes as using bromide as the surfactant counterion.5,6 The Water Dilution Line. In all surfactants, a single LR phase was found at low water contents (Figure 5-8) along the water dilution line, followed by a miscibility gap at high water contents. As has been shown previously by other authors, already divalent counterions give rise to a sufficiently strong attraction to yield a limited swelling of the lamellar phase in the presence of excess water.12,13 Specifically, for C16TA surfactants with added decanol, Fontell et al. observed that the LR phase with C16TABr swelled upon water addition, but (C16-
J. Phys. Chem. B, Vol. 111, No. 47, 2007 13369 TA)2SO4 did not.14 The extension of the LR phase was very similar for the different surfactants investigated in the present work. Since the samples investigated for the swelling of the LR phase were situated close to the phase border and the surfactantto-decanol molar ratio was the same for all surfactants, the composition (and therefore the surface charge density) should also be similar for all these LR phases. The D-spacing (Table 1) of these phases was also quite similar, and they also fall into the range previously observed for the C16TAPA30/decanol/water system (42-48 Å, increasing with an increasing decanol content).8 This insensitivity to the DP of the counterion is remarkable, but it does not necessarily imply that the bridging contribution to the attraction is insignificant for polyions with a high DP. Monte Carlo simulations of the interaction between charged planes with oligomeric counterions have shown, as pointed out above, that although the strength of the interaction differed considerably for dimer and decamer polyions, the minima in the potential curves occurred at the same distance.19 The Lr and Lβ Phases. A novel finding of this investigation was the transformation of the liquid crystalline LR phase into a lamellar “gel” Lβ phase at high decanol contents. The existence of a gel phase with chrystalline order among the alkyl chains was unexpected in this system, since the alkyl chains in decanol are only moderately long (neat decanol melts at 7 °C) and, moreover, do not match the much longer cetyl chains of the surfactant. Nevertheless, the WAXS part of the spectra clearly showed that there was a crystalline order of the alkyl chains in the Lβ phase that disappeared upon heating. A freezing of the alkyl chains has, furthermore, been confirmed by preliminary solid-state NMR studies in our laboratory.21 The fact that, upon heating a sample containing both LR and Lβ phases, the different lamellar phases fused together to form a single lamellar phase and again separated into two lamellar phases when cooled down strongly indicates that the Lβ phase is, indeed, an equilibrium state. As can be seen in Table 2, the D-spacing for the LR phase was consistently smaller (that is, the lamellar mid-planes were closer) than the D-spacing for the coexisting Lβ phase. Since the distance between two surfaces is related to the force between them (and therefore to the charge density of the surfaces) and there is no extra salt to modulate the force between the surfaces, this result supports the notion that the decanol is situated in the surface of the surfactant aggregates and that there is more decanol in the Lβ phase than in the LR phase. This is supported by the observed density difference between the two phases, although the packing itself could also influence this density difference. Earlier investigations of complex salts with alkyltrimethylammonium and poly(acrylate) of different lengths have shown that the density of the complex is very close to the density of water (1 g/mL).5,7 The density of decanol, however, is much lower (0.83 g/mL), and the observation that the Lβ phase has a consistently lower density than the LR phase supports the hypothesis that the Lβ phase contains more decanol per volume than the coexisting LR phase. Other studies of closely related systems have reported on additional (in addition to the LR phase) phases with a lamellar symmetry. When adding water and decanol to C16TABr and (C16TA)2SO4, Fontell et al. found a birefringent phase at high decanol content that they called the K phase.14 This phase displayed a lamellar spectrum in the SAXS, but no WAXS data were reported. Fontell et al. concluded that this phase was different from the LR phase found at lower decanol contents, but they gave no conclusive explanation of how the surfactant aggregates were ordered. When investigating the lamellar phase
13370 J. Phys. Chem. B, Vol. 111, No. 47, 2007 found with complex salts, water, and decanol, Bernardes et al. saw two superposed lamellar spectra in the SAXS for some of the samples investigated, but no WAXS was performed on these samples.8 These two lamellar phases had a very similar D-spacing, and the phases reappeared after heating and cooling the sample. In some cases, these two lamellar phases were found in equilibrium with pure water, indicating a three-phase sample. Although the additional lamellar phases found in the two mentioned investigations were not located at precisely the same positions in the phase diagram as the Lβ phase of the present investigation, they all had a rather high decanol content. It is thus possible that the additional lamellar phases found previously were, in fact, gel phases similar to those found in the present investigation. Further investigations by WAXS, calorimetry, or NMR of the different systems could test this possibility. Concluding remarks Cationic surfactants with a range of different oligo carboxylate ions as counterions in aqueous solutions and in aqueous solutions with added decanol have here been investigated. To the best of our knowledge, this is the first experimental investigation of the phase behavior of a salt-free pure ionic surfactant with tri- or tetravalent counterions. When changing the degree of polymerization of the polyion and the geometry of the surfactant aggregate (by adding decanol), we have preserved, as far as possible, the basic chemistry of our colloidal system in that we consistently study the behavior of C16TA surfactant aggregates neutralized by carboxylate counterions of varying degree of polymerzation. We thus regard the present study as the cleanest and most extensive experimental study to date for testing theoretical predictions of the interactions between charged aggregates of spherical or planar geometry, neutralized by counterions of different degrees of polymerization in an otherwise salt-free environment. From a fundamental point of view, it would have been interesting to compare our results with those for monatomic counterions of a valency of three and above, since Monte Carlo simulations indicate that there could be differences in both the strength and the range of the attraction.11 However, monatomic anions with a valency above two do not exist in an aqueous environment. In conclusion, we find that the results of our studies agree with the predictions from computer simulations, as far as these are available, and may be summarized as follows: • Spherical, micellar aggregates with dimeric (divalent) counterions are generally fully miscible with water (with the possible exception of very hydrophobic counterions, which probaby enter the micelles). Trimeric (trivalent) counterions give rise to a liquid-liquid demixing of the micelles in one concentrated and one dilute liquid phase. With tetrameric (tetravalent) counterions, the concentrated phase is condensed into a crystalline cubic phase. Essentially, the latter behavior is preserved with longer polymeric counterions. • Planar aggregates demix from water (that is, they show a limited swelling in excess water) already with dimeric counterions. At least for the systems studied here, the separation of lamellar planes in the maximally swollen structures does not vary significantly with an increasing DP of the polyion up to at
Norrman and Piculell least DP30. From this, we cannot conclude that the strength of the attraction is necessarily the same in all cases, but the minimum in the potential-vs-distance curve seems to be located at the same (short) distance. A quite unexpected finding of our study was that, upon addition of larger amounts of decanol, a lamellar gel phase with crystalline order among the surfactant alkyl chains appeared. The gel phase could be melted into a liquid crystalline lamellar phase but reformed on cooling, which strongly indicates that it is an equilibrium structure. This finding raises several questions to be addressed in future studies: How can one understand the appearance of crystalline order at room temperature in lamellae containing mostly relative short decyl chains and a few much longer cetyl chains? What is the exact stoichiometry of the gel phase? Does it appear also in other similar systems, specifically in decanolic mixtures of the same surfactant bearing other counterions? Acknowledgment. The authors thank Martin Dahlquist for his keen eyes and steady hands, which detected the two different lamellar phases in a similar system prior to this investigation, and Håkan Wennerstro¨m, Bernard Cabane, and Bo Jo¨nsson for valuable discussions and suggestions. L.P. thanks the Swedish Science Council for financial support. References and Notes (1) Goddard, E. D. Colloids Surf. 1986, 19, 255. (2) Thalberg, K.; Lindman, B. Polymer-surfactant interactionssrecent developments. In Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL. 1993; p 203. (3) Kwak, J. C. T. Polymer-surfactant systems; Marcel Dekker, Inc.: New York 1998. (4) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 6004. (5) Svensson, A.; Piculell, L.; Cabane, B.; Ilekti, P. J. Phys. Chem. B 2002, 106, 1013. (6) Svensson, A.; Piculell, L.; Karlsson, L.; Cabane, B.; Jo¨nsson, B. J. Phys. Chem. B 2003, 107, 8119. (7) Svensson, A.; Norrman, J.; Piculell, L. J. Phys. Chem. B 2006, 110, 10332. (8) Bernardes, J. S.; Norrman, J.; Piculell, L.; Loh, W. J. Phys. Chem. B 2006, 110, 23433. (9) Norrman, J.; Lynch, I.; Piculell, L. J. Phys. Chem. B 2007, 111, 8402. (10) Guldbrand, L.; Jo¨nsson, B.; Wennerstro¨m, H.; Linse, P. J. Chem. Phys. 1984, 80, 2221. (11) Jo¨nsson, B.; Wennerstro¨m, H. When ion-ion correlations are important in charged colloidal systems. In Electrostatic effects in soft matter and biophysics; Holm, C., Ke´kicheff, P., Podgornik, R., Eds.; Kluwer Academic Publishers: London, 2001; p 171, and references therein. (12) Khan, A.; Fontell, K.; Lindblom, G.; Lindman, B. J. Phys. Chem. 1982, 86, 4266. (13) Khan, A.; Fontell, K.; Lindman, B. Colloids Surf. 1984, 11, 401. (14) Fontell, K.; Khan, A.; Lindstro¨m, B.; Maciejewska, D.; PuangNgern, S. Colloid Polym. Sci. 1991, 269, 727. (15) Linse, P.; Lobaskin, V. Phys. ReV. Lett. 1999, 83, 4208. (16) Linse, P.; Lobaskin, V. J. Chem. Phys. 2000, 112, 3917. (17) Talens, F. I.; Pato´n, P.; Gaya, S. Langmuir 1998, 14, 5046. (18) Angelescu, D.; Caldararu, H.; Khan, A. Colloids Surf., A 2004, 245, 49. (19) Åkesson, T.; Woodward, C.; Jo¨nsson, B. J. Chem. Phys. 1989, 91, 2461. (20) Larsson, K. LipidssMolecular organization, physical functions and technical applications, 1st ed.; Bell and Bain Ltd.: Glasgow 1994; Vol. 5. (21) Topgaard, D.; Martin, R. W.; Norrman, J.; Piculell, L. Unpublished results.
