Thermotropic Phase Behavior of Cationic Gemini Surfactants and

J. Phys. Chem. B 2006, 110, 1151-1157

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Thermotropic Phase Behavior of Cationic Gemini Surfactants and Their Equicharge Mixtures with Sodium Dodecyl Sulfate Yujie Wang and Eduardo F. Marques* Centro de InVestigac¸ a˜ o em Quı´mica, Department of Chemistry, Faculty of Sciences, UniVersity of Porto, Rua do Campo Alegre, No. 687, P-4169-007 Porto, Portugal ReceiVed: September 16, 2005; In Final Form: NoVember 21, 2005

The lyotropic phase behavior for the neat cationic gemini surfactants alkanediyl-R,ω-bis(alkyldimethylammonium bromide), designated here as m-s-m, has been investigated previously in several works, but the thermotropic behavior has not been well characterized. Only for 15-s-15 and 14-s-12 have thermotropic liquid crystals (Lc) been reported. In this work, for the first time and in contrast to previous reports, we observe thermotropic Lc formation for m-2-m geminis with m ) 12, 14, 16, and 18, by means of polarizing microscopy and differential scanning calorimetry (DSC). Furthermore, we investigate mixtures of m-2-m and SDS, m-2-m Br2‚2SDS, which exhibit crystal-to-crystal phase transitions at lower temperature and, at high temperature, smectic Lc phases. The transition temperatures and enthalpies for Lc phases, obtained by DSC, present clear trends upon increase of the chain lengths. Combining Langmuir film experiments, possible lamellar arrangements for the different phases are tentatively discussed.

Introduction Many surfactants with long alkyl chains, in the absence of solvent, show a complex behavior when structural disorder is introduced by thermal treatment. Great interest has been focused on thermotropic mesophases, which are considered as an intermediate state between the crystalline solid and the isotropic liquid. In fact, structural order of headgroups and structural disorder of chains are combined in liquid-crystalline (Lc) phases. The type of structural disorder involved in Lc phases may be positional, orientational, or conformational,1,2 depending on the length of the alkyl chain, the type of headgroup, and the original lattice structure at low temperature. Compounds able to form thermotropic mesophases are known as mesogenic molecules. It has been known that certain ionic soaps exhibit thermotropic mesophases provided the “melting” point is sufficiently high to enable thermal motion of alkyl chains.3 Already in 1938, Knight and Shaw reported examples of liquid crystals of long-chain alkylpyridines.4 These cationic surfactants form thermotropic mesophases at relatively low temperatures (about 100 °C).5 Most carbohydrate derivatives with one or two alkyl chains of sufficient length (usually C6 or larger) form a mesophase upon heating.6-9 Phosphorus catanionic amphiphiles (catanionic phosphinate and phosphonate salts) form a liquid-crystalline thermotropic phase at high temperature (>180 °C).10 Sodium oleate11 and several metal alkanoates12 are also mesogenic molecules that have been investigated. In the Lc’s of ionic surfactants and their mixtures (including cationic and anionic surfactants), the stabilization of the mesophases is provided by the strong ionic interactions of the headgroups (including counterions), the van de Waals interactions involved in the hydrocarbon chain-chain packing that stabilize the lattice,11 and, in some cases, hydrogen bonding. The magnitude of the van der Waals interaction is much smaller * Corresponding author: e-mail, [email protected]; fax, +351 226082959.

than that of the ionic one, but its importance is clearly demonstrated by the fact that the clearing points rise rapidly with increasing alkyl chain length.7,8 For carbohydrate surfactants, interactions between the layers are also governed by hydrogen bonding.7 The effect of hydrogen bonding on mesophases has been, for instance, elucidated by IR absorption studies of guanidinium alkylbenzensulfonates.13 All the mesophases composed by surfactant molecules appear to involve some degree of disorder of the long alkyl chain. On the type of disorder, some models have been proposed based on experiments,7,8,13-19 as well as on Monte Carlo and molecular dynamics calculations and theoretical thermodynamic considerations.20-24 In a recent investigation, two types of disorder, comprising clusters of chains with a kink defect and conformational distortions, have been proposed.13,14 Moreover, the chain length of the surfactant is an important factor to the temperature range and energy of formation of the mesophase.13 A series of guanidinium alkylbenzensulfonates from ethyl to tetradecyl (GABA-n) appears to melt sharply, transforming either into an isotropic liquid for chain lengths n e 6 or into a strongly birefringent fluid stable up to 300 °C for chain lengths n g 8; also, the phase transition temperature decreases as n increases. However, the measured enthalpies were not analyzed in detail for crystal-to-crystal transition, particularly as a function of the chain length, because of a complex polymorphism in the solid state difficult to control, and no clearing temperature from smectic phase to isotropic liquid was detected. For long-chain alkyl glycosides, the clearing points rise rapidly with increasing alkyl chain length.7,8,19 The mesophase behavior of N-(n-alkyl)pyridinium hydrogen sulfates depends on the chain length n, increasing with n.5 The different results on chain length dependence are directly related to the mechanism of disorder. Dilatometric investigation of the smectic phases showed that the cross-section areas of the molecules actually increase a little with chain length.13 The bulkiness of the chains influences the headgroup area and does play an important role in the packing of the ionic moieties.

10.1021/jp0552729 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/24/2005

1152 J. Phys. Chem. B, Vol. 110, No. 3, 2006 For carbohydrate derivatives, the type of mesophase strongly depends on the number of alkyl chains and chain length attached to the carbohydrate moiety: smectic mesophases with a bilayer arrangement, for single chain and short double-chain moieties; columnar mesophases (HII), for long double-chain ones.9,25,26 The cubic thermotropic mesophases, which are optically isotropic, have been described and considered as an intermediate phase between smectic and columnar phases.27-29 At low temperature, a crystal-to-crystal transition appears often before crystal-mesophase transition, which signifies a rearrangement of the packing of alkyl chains.7 As for cationic gemini surfactant alkanediyl-R,ω-bis(alkyldimethylammonium bromide), designated here as m-s-m, the thermal phase behavior relative to aqueous systems has been investigated by differential scanning calorimetry.30-32 The lyotropic Lc phases depend strongly on the chain length and spacer. However, only a few works have been reported on the crystalline structure and thermal properties of the anhydrous surfactant.33-37 The melting temperature of m-s-m increases with the carbon number of the alkyl chain, m. The variation of melting temperature with s is remarkable with a maximum at s ) 5-6, for both the 12-s-12 and 16-s-16 series, and a minimum at s ) 10-12 for 12-s-12.33,35 It was shown that 12-s-12 surfactants manifest no thermotropism, and this behavior was interpreted by geometric constraints on the headgroup arrangement associated with the presence of the spacer.35,37 But for 15-s-15, thermotropism has been obtained.36 Another example showing thermotropic mesophase formation, strongly dependent on the spacer length, is the dissymmetric gemini surfactants 12s-14.37 However, upon comparison with other mesogenic molecules, it is reasonable to suppose occurrence of smectic phases for the cationic gemini surfactants, m-s-m. Also, the mixing of cationic gemini surfactants with anionic sodium dodecyl sulfate (SDS) will alter the interactions between headgroups and between alkyl chains (chain asymmetry), and accordingly the thermal phase behavior will be changed. This paper is a follow up of works on the phase behavior38 and thermodynamics of micellization39-41 of aqueous gemini surfactants and their mixtures with SDS and also of thermotropic phase behaviors of catanionic surfactants.42 It focuses on the mesophase formation and their relative stability with respect to chain length. Also, we discuss the respective transition enthalpy changes and present some interpretation on the phase behavior. Experimental Section Materials. Gemini surfactants were synthesized and purified according to the method of Menger.43 The following materials were used for the synthesis, without further purification: N,N,N′,N′-tetramethylethylenediamine (99%, Aldrich), 1-bromooctadecane (96%, Aldrich), 1-bromohexadecane (97%, Aldrich), bromotetradecane (97%, Aldrich), and 1-bromododecane (97%, Aldrich). The good purity of the geminis was checked and confirmed by NMR, elemental analysis, and surface tension, which revealed no minima in around the critical micelle concentration break. The anionic surfactants SDS (>99%) was purchased from Sigma and was used without further purification, yielding a critical micelle concentration of 8.0 mM. Preparation of m-s-mBr2‚2SDS. The mixtures of gemini surfactant and SDS were prepared by mixing a stock solution of gemini surfactant in chloroform with a stock solution of SDS in methanol, both with twice the final desired concentration. The molar ratio of gemini surfactant to SDS is a constant value of 1:2. The mixing of the solvents, chloroform and methanol,

Wang and Marques was adjusted until no precipitate was shown in the solution. Then, the solvent was slowly evaporated at room temperature, and the mixture kept at 50 °C for more than a week and was vacuumized for 24 h. A white crystalline compound was obtained. Considering the presence of Na+ and Br- on the surfactant mixture, we denote the formed compound as m-smBr2‚2SDS. Differential Scanning Calorimetry (DSC). The thermograms in the temperature range from 20 to 200 °C were recorded with a differential scanning calorimeter from Setaram, model DSC141, properly calibrated. The scanning rate was always 3 K/min. Every sample was scanned at least three times, and all the results of enthalpies shown in the text were mean values with an uncertainty of (5%, unless stated otherwise. The results are shown in heat flow vs temperature plots. Polarizing Light Microscopy (PLM). Observation of mesophases was performed with a Nikon polarizing microscope (Optiphot-Pol model), with a calibrated heating stage from Linkam (model TH600). The images were obtained with a Nikom Coolpixx 995. Measurement of π-A Isotherm. A Langmuir trough (KSV Instruments Ltd, Helsinki) with 364 mm length and 75 mm width was used. The trough was thermostated at 20.0 ( 0.1 °C by a Julabo F12-MV bath. A surfactant solution with chloroform as solvent was spread onto the subphase with a SGE gastight syringe. After 30 min for solvent evaporation, the isotherm was recorded with a constant compression rate of 5 mm min-1. Results and Discussion 1. Pure Gemini Surfactant, m-2-m. DSC Trace and Mesophases of m-2-m. Differential scanning calorimetry (DSC) and a polarizing microscope were used to check for the presence of mesophases for the m-2-m surfactants. Figure 1 shows the recorded thermograms and Figure 2 shows the observed optical textures. In Figure 2, only the images for 18-2-18 are shown, as representative examples, since similar textures were obtained for all the m-2-m surfactants. In Figure 1a, the heating trace of 18-2-18 shows two peaks, a sharp one at 108 °C and a broad one at 170 °C. The polarizing micrographs show that the first sharp peak belongs to a crystalto-crystal transition, and the solid does not show birefringence. At the temperature range of the second peak, pseudoisotropic (homeotropic) regions separated by oily streaks are exhibited (Figure 2a). Once the clearing temperature is reached, an isotropic liquid (I) is formed. When the sample is cooled, fanshape textures appear at the same temperature range and remain unchanged until room temperature is attained (Figure 2b). The observed textures are characteristic of a disordered smectic liquid crystal (Sm Lc) phase, either SmA or SmC. The heating traces of 16-2-16 and 14-2-14 have similar features to the former, showing a sharp peak assigned to a crystal-to-crystal transition, and a higher temperature peak corresponding to a liquid crystal (Lc) with an oily streak texture. The heating trace of 12-2-12 differs from the previous only in that two peaks assigned to crystal-to-crystal transitions, at 97 and 105 °C, respectively, are observed, whereas the peak top for the mesophase temperature occurs at 160.5 °C. For all four surfactants, the peaks corresponding to the Cr-Sm and Sm-I phase transitions cannot be resolved, even at the lowest heating rate used, 0.5 K/min. Transition Temperatures and Enthalpies of m-s-m. Figure 3 shows the phase transition temperatures and a view of the thermal phase behavior as a function of the chain length, m. Note that the lines are only guides for the eyes. At low temperature, for m ) 18, 16, and 14, the crystal-to-crystal

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Figure 1. The DSC heating traces of m-2-m: a, m ) 18; b, m ) 16; c, m ) 14; d, m ) 12.

Figure 2. PLM micrographs of 18-2-18: a, at 170 °C with oily streaks texture; b, at room temperature after cooling with fan-shapes texture. For the surfactants with m ) 16, 14, and 12, similar textures have been obtained. Figure 4. Transition enthalpies as a function of chain length for m-2m. The lines are only guides for the eyes.

Figure 3. The transition temperature as a function of the chain length for m-2-m. The note I means isotropic liquid phase.

transition, denoted here TCr1/Cr2, is shown, whereas for m ) 12, two crystal-to-crystal transitions are shown, TCr1/Cr2 and TCr2/Cr3. TCr1/Cr2 increases slightly with the chain length. It should be stressed that the crystalline structures of m-2-m have not been investigated in this work, so the same subscript only means the possible similarity of the structures. We also point out that polymorphic behavior is a well-know phenomenon for amphiphilic compounds, in particular with lipids, such as n-alcohols and fatty acids.44,45 It can be connected with alternative chain packing in the crystal layers or variation in the tilt angle of the molecules in the smectic layers. At high temperature, all the surfactants exhibit a region of occurrence of the Lc phase, as shown in the shadow area. Since the peaks are broad and cannot be well resolved, this range is defined by three values, the onset temperature of the broad peak,

the top temperature, Tt, and the terminal temperature, here denoted as clearing temperature, Tc. The uncertainties for TCr2/Lc are of the order of (1 °C, and for Tc somewhat higher, (2.5 °C. These temperatures are in good agreement with PLM observations. It should be noted that the “melting” temperatures of 16-2-16 (175 °C) and 12-2-12 (159 °C) that have been measured in previous work33 are in the Lc region, and the Lc phases have been altogether previously missed. The transition enthalpy as a function of chain length is shown in Figure 4. The total enthalpies of the crystal-to-crystal transitions increase linearly with the chain length with a slope of 6.8 kJ/mol of CH2. Note that 12-2-12 has two crystal-tocrystal transitions (Figure 1d). The enthalpies of Cr-Lc plus Lc-I transition increase linearly for m ) 12 to m ) 16, and then decrease a little for m ) 18. The overall enthalpy follows the latter trend. π-A Isotherm of m-2-m. The molecular arrangement in a thermotropic smectic Lc is correlated with the molecular geometry in a similar way to lyotropic lamellar liquid crystals.46-48 The mean area per molecule of a condensed monolayer phase can provide some information on molecular geometry, even though the hydrated headgroups in the monolayer must have slightly different areas from the anhydrous headgroups in the thermotropic phases. In Figure 5, the π-A isotherms of the m-2-m geminis are shown. For surfactant 12-2-12, with a Krafft temperature TKr ) 14.4 °C33,49 lower than the experimental temperature (20 °C), no surface pressure is detected, just as with 12-6-12,50 due to the

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Figure 5. The π-A isotherm of m-2-m at 20 °C. The curves from top to bottom correspond, respectively, to m-2-m surfactants with m ) 18, 16, 14, and 12.

dissolution of 12-2-12 into water. Surfactant 14-2-14 forms an unstable monolayer. 16-2-16 (TKr ∼ 45 °C34,49) exhibits similar phase behavior to 18-2-18 but higher dissolution at lower surface pressure range and a lower pressure at the “soluble point” (denoted here as the pressure for which the dissolution is clearly present). It is reasonable to consider the monolayer of 18-2-18 as a stable monolayer before the soluble point on the high pressure of about 40 mN/m. The extrapolated molecular area of 18-2-18 on the condensed phase is 0.67 nm2. If one considers possible dissolution before this defined soluble point, the real extrapolated area would be slightly higher. Since the limiting area per alkyl chain is, according to previous works,51-55 of about 0.18-0.20 nm2, a limiting doublechain area of about 0.36-0.40 nm2 indicates that for the m-2-m surfactants the hydrocarbon cross section area is much smaller than the headgroup area. From surface adsorption measurements for 12-3-12, an area per molecule of 1.05 nm2 has also been measured.34 Altogether, these data indicate a relatively high mismatch between the cross-section areas of polar and apolar moieties, a fact that will have consequences in molecular packing in the layers and thus thermal behavior. Also, from the lyotropic viewpoint, it is in agreement with the fact gemini surfactants with short spacers (s ) 2 or 3) always form micelles at low concentrations (entangled, threadlike, or wormlike micelles), i.e., aggregates of higher curvature, whereas bilayer aggregates only occur for s > 12.54-56

Wang and Marques 2. Equicharge Mixtures of m-2-m and SDS, m-2-mBr2‚ 2SDS. DSC Trace and Mesophases of m-2-mBr2‚2SDS. Figure 6 shows the DSC heating traces of m-2-mBr2‚2SDS. The first general observation is that the thermotropic behavior of the ionic pair is much more complex than that of the gemini surfactants similar to the mixtures of cationic 1,2-dimyristoyl 3-trimethylammonium propane (DMTAP) and zwitterionic dimyristoylphosphatidylcholine (DMPC),59 with five, nine, five, and seven endothermic peaks observed, respectively, for m ) 18, 16, 14, and 12. All the DSC traces of the mixtures show an endothermic peak at about 180 °C, sharp for m ) 18, with a left shoulder for m ) 16, broad and short for m ) 14, and broad and noisy for m ) 12. Morevover, the peak becomes broader as the chain length decreases. PLM shows that for the temperature range of this peak, two birefringent textures corresponding to a Lc phase, distinguished as Text1 and Text2, and an isotropic liquid have been observed. Text1 is obtained for all the mixtures below the temperature range where the endothermic peaks appear, whereas Text2 appears within the peak range. They will be further characterized below. The Cr-Lc transition of the mixture with m ) 18 shows an endothermic single peak at 120 °C, while for m ) 16 and 12 it occurs at 118 and 112 °C, respectively, followed by a small endothermic peak. It is observable under a microscope that the small particles of the crystalline solid soften already at the main Cr-Lc transition temperature, indicating that the second small peak may correspond to a minor rearragement in the mesophase. Therefore, it is reasonable to consider the small peaks as belonging to the Cr-Lc transition. The DSC trace of 14-214Br2‚2SDS differs from the previous ones with a TCr/Lc at 120 °C that is somewhat out of trend with the TCr/Lc change with chain length (Figure 9). As for the crystal-to-crystal transitions, the profiles are rather complex, and the structures cannot be resolved in this work. However, the transition temperatures and corresponding enthalpies are reported below. The microscopic textures from room temperature to clearing temperature are now discussed for all the mixtures. The crystalline solid phases of the m-2-mBr2‚2SDS compounds always shows anisotropic properties with birefringence, as

Figure 6. The DSC heating trace of m-2-mBr2‚2SDS: a, m ) 18; b, m ) 16; c, m ) 14; d, m ) 12.

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Figure 8. Transition temperature of m-2-mBr2‚2SDS as a function of chain length, m. The line and shadows are only guides for the eyes.

Figure 7. PLM observations for the m-2-mBr2‚2SDS: (a) the anisotropic crystal of 16-2-16Br2‚2SDS at room temperature; (b) the Lc phase of 12-2-12Br2‚2SDS at 165 °C; (c) cracking of the anisotropic crystal of 16-2-16Br2‚2SDS at 120 °C; (d) textures Text1 and Text2 of 18-218Br2‚2SDS at 180 °C; (e) the fan-shaped tetxure of 16-2-16Br2‚2SDS at 180 °C; (f) the fan-shape of 12-2-12Br2‚2SDS at room temperature after cooling from the liquid phase; (g) nonbirefringent NaBr salt in 18-2-18Br2‚2SDS under nonpolarized light at 178 °C.

shown in Figure 7a, as an illustrative example. These observations rule out the presence of residual solvent in the solid compounds. There are no differences between the crystalline textures observed under the microscope. At TCr/Lc, the crystal softens first and then exhibits Lc textures with gradual increase in birefrigence as temperature is raised, as shown in Figure 7b. After TCr/Lc some larger crystallites crack into pieces as Figure 7c and then become Lc in texture. Until the temperature range where a mixture of Text1 and Text2 appears, the Lc domains with Text1 gradually lose their birefringence, while inside the pseudoisotropic regions a fine fan-shaped Lc appears, occasionally with some Maltese crosses, at a narrow temperature range, as shown in Figure 7d. Figure 7e shows the fan-shaped texture of 16-2-16Br2‚2SDS. All the mixtures have a coarse mosaic texture with some fan domains on cooling after clearing temperatures and maintain this texture even at room temperature (Figure 7f). The observations suggest that Text1 and Text2 are polymorphic textures of the same Lc, which is of the smectic type.

A remaining question is the location of counterions Br- and Na+ located in the mixtures. The mixtures of m-2-mBr2‚2SDS are obtained by evaporation of mixed solvents, methanol and chloroform. It is possible that the counterions codeposit with the surfactants or deposit as a salt NaBr. At room temperature there is always some nonbirefrigent solid on the anisotropic crystal (Figure 7a). When the flowing isotropic liquid forms, some nonbireringent solid domans remain unchanged and are visible under normal light in the microscope (Figure 7g). We ascribe these solid domains as NaBr crystals separated from the Lc, even though some evidence from other techniques could be useful to confirm this. Transition Temperatures and Enthalpies of m-s-mBr2‚ 2SDS. Figure 8 shows the transition temperatures as a function of chain length, m, of m-s-mBr2‚2SDS. Similar to the pure m-2m, at high temperature, the terminal temperatures of the DSC peaks for m ) 14 and m ) 12 have an uncertainty of (2.5 °C. But the obvious fact is that the temperature range for the occurrence of the Lc narrows with chain length increase, due to the onset temperature increase and the clearing temperature (Tc) decrease. The Cr-Lc transition temperatures are practically constant. As for the crystal-to-crystal transitions, the corresponding temperatures are obtained for m ) 12, 16, and 18, but for m ) 14, the temperature is out of trend with the chain length change. The Lc-I peaks have long shoulders for m ) 14 and 12, so that the integral enthalpies have some nonnegligible uncertanties. However, the enthalpies for the crystalline solid and Lc transition as a function of the chain length have well-defined values, increasing with chain length. The overall transition enthalpy among solids is more difficult to analyze in detail, particularly as a function of the chain length, due to the observed complex polymorphism in the solid state. But the sum of the enthalpies of crystal-to-crystal transitions does increase with increasing chain length due to the van der Waals contribution between the chains. π-A Isotherm of m-2-mBr2‚2SDS. The solubility of m-2mBr2‚2SDS is very small (below 10-5 M), so that it forms a stable monolayer on the interface between air and water, just as other previously investigated mixtures of oppositely charged surfactants.51,60-63 A common behavior is observed for all m-2mBr2‚2SDS surfactants, so here we only present and discuss the π-A isotherm of 12-2-12Br2‚2SDS as an illustrative example. The π-A isotherm of 12-2-12Br2‚2SDS in Figure 10 exhibits two phases connected by a plateau. The initial areas for which the surface pressure increases, 1.02 nm2 at 14.0 °C, 1.08 nm2 at 20.3 °C, and 1.11 nm2 at 25.0 °C, increase with temperature. Also, the pressure of the corresponding phase, at constant area,

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Figure 11. Possible bilayer (smectic) arrangement for m-2-mBr2‚2SDS.

Figure 9. The transition enthalpies of m-2-mBr2‚2SDS between crystalline solid and Lc and between Lc and isotropic liquid, and the sum of transition enthalpy among crystalline solids.

Figure 10. π-A isotherm of 12-2-12Br2‚2SDS at different temperatures.

increases with increasing temperature, suggesting the phase is an expanded phase. On the plateau, the initial areas decrease, the surface pressures increase, and the area range becomes narrower, as the temperature increases, and all this is caused by the extended phase. But at the left end of the plateaus the corresponding areas are almost constant. This steeply rising part of the π-A isotherm shows two characteristics: a small compressibility with a mean area decrease about 0.05 nm2, and a small thermal dilatability according to the superposition of the steep parts at different temperatures. These characteristics are typical of a condensed phase, either solid or liquid. The extrapolated areas are 0.63 nm2 at 14.0 °C, 0.62 nm2 at 20.3 °C, and 0.62 nm2 at 25.0 °C, respectively, so that the mean area per chain is about 0.16 nm2, approximately equal to the limiting chain area of 0.18 nm2. It is well-known that the headgroup areas of m-2-m (discussed above) and SDS (which forms spheroidal micelles in solution) are larger than the respective chain areas. However, the headgroup area of the mixture is smaller than the sum of both respective headgroup areas, due to the strong electrostatic attractions, so that the molecular area is effectively determined by the chain area. Hence, from these data we infer that the polar-apolar mismatch in the catanionic complex is much less significant, compared with the neat geminis. From this fact, we speculate that a more compact ordering of the molecules in the layers is possible, also possibly justifying the wider Lc region observed. 3. Comparison of m-2-m and m-2-mBr2‚2SDS Systems. Some comparisons can be made between the gemini m-2-m surfactants and stoichiometric catanionic pairs with SDS (in the presence of salt). Comparing Figure 3 and Figure 8, one can observe a transition at about 100 °C that increases slightly with chain length, m, implying a rearrangement of the packing of the alkyl chains.7 This change should be relative to the conformational state of the chains but not of the long-range crystal structure. The Lc appears immediately for m-2-mBr2‚

2SDS after this temperature but not for m-2-m. The mixtures of m-2-mBr2‚2SDS show two birefringent textures of Lc on heating, but one texture on cooling, so it is reasonable to consider both the textures as polymorphic of the same phase. The neat gemini m-2-m has a broad peak comprising the Cr-Lc and Lc-I transitions. For both types of compounds the Lc phase is of the disordered smectic type, but they may have some structural differences. According to Langmuir experiments, the area of the hydrated headgroup of m-2-m is larger than the limiting area of the chains, whereas the area of the hydrated headgroup of m-2-mBr2‚2SDS is not. Assuming that the anhydrous headgroups have their areas in the same order as the hydrated headgroups of the condensed monolayer state, the data can be used qualitatively to discuss the thermotropic Lc phases. Therefore, we suggest that the molecules of m-2-m have a tilted arrangement in the lamellar (smectic) structure (larger headgroup areas), whereas the molecules of m-2-mBr2‚2SDS may have an arrangement perpendicular to the lamellar plane. Furthermore, we know the chain length of gemini moiety is longer than that of SDS moiety, except for the m ) 12 case. A plausible structural sketch of m-2-m‚2SDS is shown in Figure 11. The cationic and anionic headgroups are compactly alternatingly arranged due to strong electrostatic interactions, and the repeating distance would comprise the sum of the long and short chain length. The complementation of chain lengths renders the interface between the ends of every unilayer, alternatingly convex and concave, and obviously not planar. Accordingly, one finds here an explanation for the fact that the respective transition temperatures (except clearing temperature) decrease as the chain length m decreases (and hence the asymmetry between gemini chains and SDS chain decreases). Also, for the fact that the Cr-Lc and Lc-I temperatures for 12-2-12Br2‚2SDS are several degrees lower than the expected trend. The thermal motions of the intertwisted chains for m > 12 need more energy than in the case of the chains in a planar interface, for m ) 12. Conclusion We report here for the first time liquid crystalline formation for m-2-m compounds with m ) 12, 14, 16, and 18, by means of polarizing microscopy and DSC. Both the transition temperature and enthalpy for the Cr-Lc transition increase with the chain length. The mixtures of m-2-mBr2‚2SDS exhibit anisotropic crystalline solids at low temperature and a Lc phase at lower temperature and with a large thermal stability. The Lc phases of both m-2-m and m-2-mBr2‚2SDS have fan-shaped textures characteristic of disordered smectic phases. From Langmuir experiments, the areas per molecule of both m-2-m and m-2-mBr2‚2SDS in condensed state are obtained. By comparison of the areas with the limiting areas per chain, we present some structural inferences for the smectic layers of m-2mBr2‚2SDS. Acknowledgment. We are grateful for financial support from Fundac¸ a˜o para a Cieˆncia e Tecnologia (F.C.T.), Portugal, for a

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