J. Phys. Chem. B 2008, 112, 14877–14887
14877
Structure/Property Relationships for the Thermotropic Behavior of Lysine-Based Amphiphiles: from Hexagonal to Smectic Phases Rodrigo O. Brito,† Eduardo F. Marques,*,† Paula Gomes,† Maria Joa˜o Arau´jo,† and Ramon Pons‡ Centro de InVestigac¸a˜o em Quı´mica, Department of Chemistry, Faculty of Sciences, UniVersity of Porto, Rua do Campo Alegre, no. 687, 4169-007 Porto, Portugal, and Departament de Tecnologia Quı´mica i de Tensioactius, IQAC-CSIC, Jordi Girona, 18-26, 08034 Barcelona, Spain ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: July 28, 2008
Amino acid-derived gemini surfactants arise as a potentially good alternative to the more conventional lipid and synthetic catanionic systems in view of their enhanced interfacial properties, increased chemical stability, and low toxicity. The presence of an amino acid as the polar headgroup allows toxicity reduction, with the simultaneous increase of biodegradability. For these compounds, the establishment of structure/function relationships from the assessment of their basic aggregation properties is therefore of the utmost interest, e.g., in the design of operative self-assembled systems (e.g., liposomes, nanotubes, etc). In this context, the study of the thermal phase behavior of the dry surfactants is a natural, straightforward first step, the more so as thermotropic liquid crystals are also relevant for practical applications. In this work, several lysine-based amphiphiles with a gemini-like configuration have been synthesized, with the amino acid side chain as the spacer group. The molecules are either esters (neutral, with C6-C12 even chains) or sodium carboxylates (anionic, with C6-C12 even chains). Upon increasing the temperature, different crystalline (cr) and liquidcrystalline (lc) phases have been detected and the corresponding thermodynamic and structural parameters determined by a combination of differential scanning calorimetry, polarizing light microscopy and smallangle X-ray scattering. The phase behavior of the amphiphiles is highly dependent on both the chain length and the presence of charge on the headgroup, with significant differences occurring within and between each group of molecules. The C6 and C8 esters form reverse hexagonal cr and lc phases, while C10 and C12 self-assemble into smectic cr and lc structures, with C10 showing also a reverse hexagonal lc phase prior to isotropization. All the carboxylate derivatives form smectic lc phases at high enough temperature prior to isotropization. The rationalization of the phase behavior and phase transition energetics of the compounds has been put forth on the basis of the intermolecular interactions at stake (van der Waals, H-bonding, electrostatic, and packing) and the molecular shape of the amphiphile. 1. Introduction High-end technological applications, increased efficiency, and environmental concerns motivate the search for new surfactants. Gemini amino acid-derived amphiphiles arise as an answer to this demand, mainly because their gemini configuration results in lower cmc values and enhanced surface activity,1,2 whereas the presence of an amino acid as the polar headgroup diminishes the biological and environmental hazard potential.3-6 Besides the different lyotropic phases formed by amphiphiles in polar solvents (typically water),7-11 mainly dictated by the surfactant packing parameter,12 these molecules can also form mesophases, some of liquid-crystalline nature, when heated in the absence of solvent13-17 from solid to isotropic liquid phases. Liquid crystals possess an intermediate degree of order between the crystalline solid and the isotropic liquid phase.18,19 Their assembly is driven by the differential character of molecules, namely shape, polarity, charge, and molecular geometry, which segregate incompatible portions of molecules19-21 while aligning others. Consequently, both long-range orientation and positional order is attained in these intermediate phases. * Corresponding author. E-mail:
[email protected]. † University of Porto. ‡ IQAC-CSIC.
Thermotropic studies of amphiphiles are relevant from a fundamental point of view, since they allow the establishment of structure/behavior/performance relationships. In particular, the relations between molecular geometry/chemistry and the number, nature and sequence of liquid-crystalline phases formed upon heating are of great interest. Furthermore, liquid crystals find a wide variety of applications in electro-optical devices, liquid crystal displays, and medical applications.22 Several classes of low molecular weight amphiphiles possess complex thermal behavior, namely, carbohydrate-derived and cholesterol-derived surfactants and metallic soaps. The stepped melting and the formation of mesophases are both dependent on the melting point of the amphiphile and the number of chains.23-25 The existence of different degrees of chain conformation (from which the formation of distinct mesophases depends) is directly connected to the possibility of the increase in thermal agitation of the molecules with increasing temperature. Carbohydrate-based surfactants are particularly interesting, as their structure strongly sways the structure of formed mesophases, not only by means of the chain number and chain length but also due to the possibility of forming long-range H-bond interactions. Single-chained amphiphiles tend to form smectic mesophases, whereas the more hydrophobic doublechained ones form columnar hexagonal mesophases.23 Cubic
10.1021/jp8042494 CCC: $40.75 2008 American Chemical Society Published on Web 11/01/2008
14878 J. Phys. Chem. B, Vol. 112, No. 47, 2008
Figure 1. Structure of the lysine-derived surfactants: (a) nonionic methyl ester derivatives and (b) anionic sodium carboxylate derivatives.
phases can also occur as intermediate structures between the lamellar and columnar organization. Columnar phases of varying nature, namely hexagonal and square ones, have also been observed for modified carbohydrate surfactants, where the volume ratio of the hydrophilic and hydrophobic moieties was varied.26 These particular mesophases are formed upon insertion of rigid domains in the molecule. Monovalent and higher valency metal soaps are also capable of forming liquidcrystalline phases, with very broad polymorphism,13,27,28 with the most commonly occurring phase being of smectic nature. This is due to the fact that lamellar arrangements are favored by the strong interactions between the ionic groups of the amphiphiles. Increasing chain length and decrease of counterion size also favors formation of smectic phases.28 Amino acid-derived surfactants are another class of small amphiphiles whose thermotropic behavior has been addressed. Previous studies include, for instance, alanine, arginine, aspartic and glutamic acid, asparagine, glutamine, and tyrosine-derived surfactants.14,29-31 It has been observed that these compounds form most commonly smectic phases for a chain length of C12, with interdigitation of the chains occurring extensively in the liquid-crystalline phases. In this work, we present the systematic study of two series of lysine-derived surfactants previously synthesized32 with varying chain length (C6 to C12) and headgroup charge (neutral or anionic), with the main goal of establishing a relationship between molecular structure and the thermotropic behavior. As will be seen, some significant differences occur between the two series, and even as the chain length increases within a series. 2. Experimental Section 2.1. Materials. The structure of the studied amino acidderived gemini surfactants is shown in Figure 1. Two series of compounds were studied: the nonionic ester derivatives (Figure 1a) and the negatively charged sodium carboxylates (Figure 1b), with chain lengths ranging from 6 to 12 carbon atoms. The surfactants were synthesized according to the procedure published in previous works,32,33 where chemical purity was checked by elemental analysis, spectroscopy (NMR and FTIR), and mass spectroscopy. The anhydrous form of the compounds was verified by the Karl Fischer method, which yielded trace amounts of water (less than 0.03 wt % for all the compounds). The compounds will be referred to by the abbreviation for lysine (Lys), preceded by s for sodium carboxylate or m for methyl ester and the corresponding number of carbon atoms on the hydrocarbon chains (as counted from the hydrocarbon chain of the fatty acid used in the synthesis process). 2.2. Differential Scanning Calorimetry (DSC). DSC scans were performed using a Setaram DSC141 calorimeter. About 3 mg of the solid was weighed to aluminum crucibles, and an empty crucible was used as a reference. Heating-coolingreheating cycles were performed at a scanning rate of 3 K/min
Brito et al. using air (p ) 0.5 bar) cooled with liquid nitrogen as the cooling fluid, allowing the scanning of the temperature range of -20 to 250 °C. The thermodynamic data represented in Figure 5 are the average for at least three independent measurements, with the experimental error always within (5.5%. 2.3. Polarized Light Microscopy (PLM). An Olympus BX51 polarized light microscope equipped with a Linkam TMHS 600 hot stage, controlled by a TP94 unit, was used. Images were acquired with an Olympus C-5060 Wide Zoom digital camera. Anisotropic liquid-crystalline phases give rise to typical birefringent textures under polarized light.18,22,34 2.4. Small-Angle X-ray Scattering (SAXS). SAXS measurements were done using a Siemens KF760 3 kW generator and a Kratky camera of small angle (M-Braun). Nickel-filtered radiation was used with wavelength of 1.542 Å (Cu KR line). The linear detector used was a PSD-OED 50 M-Braun and the temperature control was done via a Peltier KPR AP PAAR (precision better than (0.5 °C). Samples were placed between two Mylar sheets on the sample holder for analysis and allowed to equilibrate for 15 min at a given temperature, before each scan. The scattering curves are shown as they are obtained, that is, with the experimental beam line collimation. This produces small (81.2 25.0 68.8
0.845 0.621 0.842 0.619 0.845 0.621
12-mLys-12
75.6 80.0 25.0 47.2 71.1 >82.5
3.36 1.61 1.61 2.77 2.63 2.64 1.41 1.42 1.39 2.49
3.90 3.90 2.62 2.76 2.75 4.46 4.42 4.52
0.398 0.398 0.620 0.588 0.591 0.397 0.400 0.391
1.36 1.36 0.55 0.58 0.58 1.64 1.63 1.66
0.59 0.59 0.76 0.80 0.79 0.59 0.59 0.60
Cr [HI (1,3,4)]a Cr [HII (1,3,4)]a HI (1,3,4) HII (1,3,4) HI (1,3) HII (1,3) iso Cr [HI (1,3,4)]a Cr [HII (1,3,4)]a HI (1,3,4) HII (1,3,4) HI (1,3,4) HII (1,3,4) iso Cr [Sm (1,4,9)]a Sm (1,4,9) + HII (1,3) HII (1,3) + iso HII (1,3,4) + iso Cr [Sm (1,4)]b Sm (1,4) Sm (1,4) iso
compound 6-mLys-6
8-mLys-8
T (°C)
a Legend: Amol, area per molecule; lhc, hydrocarbon chain length; lhg, headgroup length; HI, hexagonal; HII, reverse hexagonal; Sm, smectic; iso, isotropic liquid. b Phase assignment in brackets refers to the packing observed for the crystalline solid phase of the amphiphile. Headgroup volume of 0.234 nm3, calculated on the basis of partial volumes addition; hydrocarbon chain volumes calculated as the sum of Tanford volumes.
region of the adjacent molecules upon the total isotropization of the surfactant. The nonlinear dependence of the total enthalpy and entropy of melting with the chain length is not fully
understood, but may be related to a nonlinear dependence of electrostatic contributions to phase stability as the chain length increases.
14884 J. Phys. Chem. B, Vol. 112, No. 47, 2008
Brito et al.
TABLE 2: SAXS Data and Structural Parameters for the mLys Series Surfactantsa q1 (nm-1)
d or a (nm)
Amol (nm2)
lhc (nm)
lhg (nm)
phase
25.0
2.49
96.0
2.33
2.52 2.91 2.70 3.11
0.444 0.428 0.415 0.443
0.73 0.51 0.78 0.50
0.53 0.95 0.56 1.06
8-sLys-8
118.0 >139.0 25.0
2.37 3.03 1.84
3.94
1.82 1.95 2.11
1.71 1.17 1.11 1.12
0.26 0.80 0.60 0.60
10-sLys-10
104.1 >108.9 25.0
0.508 0.347 0.391 0.387
128.5 137.2 >146.1 25.0 93.2 135.3 163.0 195 >215
2.23 2.24 2.69 1.48 1.53 1.75 2.33 2.36 2.25
2.98 2.82 2.80
0.704 0.431 0.521 0.551 0.553
1.54 0.78 1.04 0.98 0.98
0.18 0.94 0.45 0.42 0.42
4.25 4.11 3.59
0.416 0.431 0.492
1.56 1.51 1.32
0.56 0.54 0.48
Cr [Sm (1,4)]ba Cr [HII (1,3,4)]b Sm (1,4) HII (1,3,4) lc iso Cr [HI (1,3,4,7)]b,c Cr [HII (1,3,4,7)]b,c Cr [Sm (1,4)]c Sm (1,4,9) iso Cr [HI (1,3,4,7)]b,c Cr [HII (1,3,4,7)]b,c Cr [Sm (1,4)]c Sm (1,4) Sm (1,4) iso Cr [Sm (1,4,4)]b Sm (1,4,9) Sm (1,4,9) lc lc iso
compound 6-sLys-6
12-sLys-12
T (°C)
3.41 3.45 3.44
a Legend: Amol, area per molecule; lhc, hydrocarbon chain length; lhg, headgroup length; HI, direct hexagonal; HII, reverse hexagonal; Sm, smectic; lc, liquid crystalline; iso, isotropic liquid. b Phase assignment in brackets refers to the packing observed for the crystalline solid phase of the amphiphile. c Structural parameters calculated considering both an hexagonal and a smectic structure; details are given in text. Headgroup volume of 0.203 nm3, calculated on the basis of partial volumes addition. Hydrocarbon chain volumes calculated as the sum of Tanford volumes.
In the case of the s-series, the onset of long-range electrostatic interactions between the headgroup domains of the molecules, as well as (to a less extent) van der Waals interactions between the alkyl chains, may explain the upholding of some long-range order in an almost molten phase, in particular for C12. As described before, recrystallization of the isotropic liquid phase does not occur for the C6 and C8 carboxylates, an effect that could in principle be explained by packing constraints. Presumably, these chains do not immediately pack upon cooling to form an ordered lattice (as evidenced from SAXS data), partially due to the possibility of H-bond dimmer formation in the liquid phase. Summing up, one can state that in the case of the esters, the melting process and lattice packing is essentially dictated by van der Waals interactions, whereas in the case of the carboxylates, the melting process is dominated by their ability to establish long-range electrostatic interactions. 4.2. Molecular Shape and Sequence of Liquid-Crystalline Phases. Hexagonal phases in which the surfactant molecules are expected to aggregate in long-length cylinders, with macromolecular arrangement in a hexagonal or square lattice,22,26,45,46 are likely to be formed by surfactants with molecular geometries resembling a conical shape. This shape is typical of surfactants with either a large headgroup and short hydrophobic chains (direct hexagonal phases), or small headgroup and large/ branched hydrophobic chains (reverse hexagonal phases). For both C6 and C8 amphiphiles here investigated, the relatively large headgroup region and short alkyl chains could suggest a priori the formation of direct hexagonal phases. Yet, the hydrophobic spacer group and the possible protrusion of the headgroup on one side of the molecule allows more freedom for shape distortions. If so, and due to weaker van der Waals interactions (short chain lengths), the hydrophobic chains could adopt a more “loose” conformation, favoring instead reverse hexagonal phases. Regarding the smectic phases, these are mainly observed for the C10 and C12 surfactants, implying a
higher stiffness of the molecules both in the crystalline and liquid-crystalline phases. Thus, the C8 and C10 chain lengths can be regarded as the critical transition lengths between rods and lamellae for the crystalline structures. (a) Ester Series. For both 6-mLys-6 and 8-mLys-8 compounds, molecular parameter values from the SAXS data are shown for both direct and reverse hexagonal phases, for clarity of the discussion (Table 1). A critical assessment of these values leads to the conclusion that it is more reasonable to consider the occurrence of inverse hexagonal phase. We illustrate this point with C6. The molecular area is smaller (for all temperatures) for the HII phase than for the HI phase, being closer to twice the area of an aliphatic chain (ca. 0.2 nm2). For the crystalline phase, the molecular area of the HI phase, 0.575 nm2, would almost allow the packing of three hydrocarbon chains, whereas for the HII phase, the molecular area would just exceed the estimated value in 0.075 nm2. Furthermore, an excess in the molecular area would cope more easily with a reverse phase than with the direct one, in view of chain packing effects. Additionally, the hydrocarbon chain length in HI is excessive (1.13 nm) for a C6 molecule in direct aggregates, and an unrealistic molecular distortion would have to occur to accommodate for such an extension of the chain length. The calculated value of 0.47 nm to the HII phase is instead smaller than the C6 expected length, indicating that interdigitation of the chains of adjacent rods could take place. Irrespective of their nature, the hexagonal phases formed by the C6 and C8 do not change significantly their structural parameters with increasing temperature. The lower molecular areas for C6 imply a more efficient hexagonal packing than for the longer chained C8. It also is worthwhile noticing that the calculated chain lengths do not increase significantly for the C8, consistent with the presumed interdigitation between chains of different aggregates. Another peculiar aspect for the 6-mLys-6 surfactant is that poorly defined and broad Bragg reflections (Figure 7a) are seen for the crystalline solid, where molecules
Thermotropic Behavior of Lysine-Based Amphiphiles should be highly organized in rods, with fully crystallized hydrocarbon chains.28,39 There is also an isotropic band at 76.7 °C centered in a high q-value (4.40 nm-1). These broad peaks could be due to the formation of relatively short rods (or large globular aggregates) in a hexagonal array where the ratio between the major and minor axis is close to 1.28,47,48 Regarding 10-mLys-10, the preferred crystalline structure is a layered smectic one. Upon heating, a smectic phase is formed (Figure 6C), coexisting nevertheless with traces of a reverse hexagonal phase (Figure 7b and Table 1). For the smectic phase at 68.8 °C, the SAXS-calculated chain length is 1.38 nm, compared with 1.16 nm42 for fully stretched C10 chain. This means that the hydrophobic portion of the molecule must be elongated in some sense, likely via distortion of the spacer region. Such distortion together with the increased chain dynamics upon heating seems to kinetically allow also the formation of a HII phase, where the measured chain lengths are close to fully stretched chain values. This hexagonal structure present in traces amounts coexists both with the smectic phase prior to the melting, and with the isotropic phase after melting (i.e., above 75.6 °C). Since for a one-component system at fixed pressure two phases can only coexist at a fixed temperature (phase transition temperature), the most likely explanation for the presence of the hexagonal phase is that it is a metastable phase with very slowly kinetics of conversion to the equilibrium one. Similar slow kinetics of phase conversion has been seen for lyotropic liquid-crystalline phases formed by lipids.49 Alternative explanations lying on the basis of an amphiphilic impurity or water being present are ruled out in view of the purity checks described above. This metastable hexagonal phase is expected to be very fluid due to the high area per molecule displayed (ca. 0.620 nm2) and to the small intensity of the Bragg reflections. The 12-mLys-12 surfactant forms smectic phases with almost constant lattice parameters along the entire melting process (table 1). The measured chain length exceeds just slightly the theoretical length of a fully elongated C12 chain. When compared to similar surfactants,17,35,48 it can be expected that a slight deformation at the spacer region occurs. The spacer could be slightly bent, with one hydrocarbon chain being more deeply inserted in the hydrophobic region than the other, therefore increasing slightly the bilayer thickness. This deformation would also allow the molecules to pack more efficiently in the smectic phases (smaller molecular area values) than in the hexagonal phases. (b) sLys Series. The liquid-crystalline phases formed upon the melting of the crystalline solid are all of the layered smectic type for this series. In the crystalline solid state, however, diverse crystallization polymorphs are seen, namely for the C6. For the C8 and C10, hexagonal and smectic arrangements occur for the solid crystal, whereas for the C12 only a layered packing is observed. Table 2 displays the molecular parameters at 25 °C calculated considering both a hexagonal and a lamellar packing of the molecules in the solid crystalline phase. For the 6-sLys-6 amphiphile the SAXS curves for 25.0 and 96.0 °C display multiple peaks, where the fitting of a single specific phase is not feasible. The assignment of a smectic phase is based on the consistence of the calculated molecular parameters (molecular area and chain length) with the reasonably expected ones, on geometric grounds. The presence of a reverse hexagonal phase can also be argued, since the calculated molecular parameters are close to those of a smectic phase, namely the area per molecule, and the chain length is even closer to that of an extended C6 chain.
J. Phys. Chem. B, Vol. 112, No. 47, 2008 14885 Additionally, the high number of reflections could be explained assuming that the hexagonal phase would have an elliptical rather than a cylindrical cross section. Regarding the smectic packing, the reflections corresponding to the ratio 1:4 are fairly intense and the measured chain length (0.75 nm, T ) 25 °C) exceeds in just ca. 0.1 nm the value for a fully extended C6 chain.42 The proximity in the molecular parameters, the short chain length of the compound and the low molecular dynamics for the systems at lower temperatures might facilitate the formation of a kinetically trapped structure (hexagonal phase). The full conversion of this metastable phase into the stable smectic one could then be a very slow process at room temperature. At 96.0 °C the reflections corresponding to the smectic phase do not change significantly, whereas the nonsmectic pattern becomes visibly weaker, suggesting a faster conversion at higher temperature. The assignment of phases for the C8 and C10 soaps is done by combining PLM images (Figure 6 F and G) with the SAXS curves from Figure 7c,d. The SAXS data display feeble traces of a hexagonal packing and relatively intense Bragg reflections corresponding to a lamellar packing, whereas the optical textures at low temperatures point to hexagonal ones (paramorphic mosaic textures, Figure 6, F1 and G1). Overall, the calculated molecular parameters from SAXS render data that are essentially coherent with a smectic phase, namely chain length and molecular area values (Table 2). When the hydrophobic length of 8-sLys-8 is calculated, the value obtained assuming a direct hexagonal packing is 1.71 nm, largely exceeding that of a fully stretched C8 chain (1.03 nm); such phase is thus excluded. For a reverse hexagonal packing, the chain length (1.17 nm) is quite close to both the smectic arrangement and the theoretical value, implying a much smaller molecular distortion. The same analysis is valid for the 10-sLys-10 surfactant. The molecular areas, much smaller for the smectic and reverse hexagonal phases, appear to favor the packing of the amphiphiles under these geometries. Another interesting feature is that both for the C8 and the C10, for higher temperatures (104.1 and 128.5 °C, for the C8 and C10, respectively) the observed reflections correspond just to smectic phases. This means that for both surfactants the lamellar structure is easily accessible, but still the crystallization is not efficient and packing polymorphs may occur. The melting of the chains with increasing temperature allows the full conversion of the metastable packing to the stable smectic phase. We note here that the formation of HII phases by C6, C8, and C10, while not easily anticipated, is not unprecedented either. It has been reported before for other short-chain amphiphiles,39,50 where the inability of bringing the hydrocarbon chains together, owing to the stiffness of the molecules, also leads to reverse structures. Concerning the 8-sLys-8 amphiphile, we note that a difference in behavior was seen by Pinazo et al.50 for nominally the same amphiphile (at 25 °C, mixed cubic Pn3m and Ia3d structures were found). The surfactants were synthesized by different methods, and in the present case, the conditions used in the last synthetic step32,33 may allow a partial epimerization of the CR of the L-amino acid. The presence of D-enantiomer (as an optical impurity) and or different hydration states of the compound (here they are anhydrous) could justify this discrepancy, since a priori both samples are shown to be chemically pure. For all soaps, the clearing to the isotropic liquid occurs through smectic phases. The C12 surfactant shows a paramorphic fan-shaped texture (likely a smectic E phase) and
14886 J. Phys. Chem. B, Vol. 112, No. 47, 2008 the SAXS curves show that this defined smectic order is only observed until 135.2 °C. For higher temperature, the lc phase is very disordered, but isotropization does not occur until 202 °C. For temperatures above 135.2 °C the SAXS curves show one sharp peak centered at 2.34 nm-1. This observation is consistent with PLM, where a texture derived from a paramorphic fan-shaped texture with myelinic domains is seen from 165 °C up to 202 °C. It is expected that little order remains for this smectic phase, in view of the small transition enthalpy (Figure 3c). Two immediate differences regarding the smectic phases formed by the mLys series are the area per molecule and the hydrocarbon chain length values, which are closer to those of the corresponding fully elongated paraffin chains.42 The larger values of area per molecule are mainly the sum of two effects: (i) the presence of charge in the headgroups (electrostatic repulsions between negatively charged headgroups) and (ii) a less efficient packing of the molecules on the smectic phases. The larger proximity of the measured hydrocarbon chain lengths to the theoretical values implies that the molecules likely exhibit a small degree of distortion on the spacer region, as expected also for the mLys series. With the exception of the HII phases, for all the studied liquidcrystalline phases, neither chain interdigitation nor formation of kinks is expected to occur, as the chain length is always larger than the predicted value. Nevertheless, the small chain lengths in the reverse hexagonal phase suggest chain interdigitation between adjacent cylinders. 5. Concluding Remarks Synthetic lysine-derived ester and soap amphiphiles show a thermotropic phase behavior, which is both dependent on chain length and type of headgroup. While van der Waals and headgroup packing interactions dictate the self-organization of the esters, long-range electrostatic interactions in the lattice mainly influence the behavior of the soaps. The latter show a more gradual melting than esters, as well as much smaller overall melting enthalpy and entropy values. The short-chain C6 and C8 esters crystallize with the molecules forming hexagonally arranged cylinders, while the longer C10 and C12 compounds crystallize with a lamellar packing, melting to reverse hexagonal phases at high temperatures. By replacing the methyl group by a carboxylate one in the soap derivatives, significant changes occur. The short-chain C6 crystallizes under what can be seen as a poorly defined mixture of packing polymorphs, composed of a smectic phase with traces of a metastable hexagonal structure. The C8 and C10 carboxylates have a clear smectic arrangement, but traces of the metastable hexagonal structure can also be seen, albeit in weaker degree. The longer chain C12 crystallizes in a lamellar structure. The progressive dominance of the smectic phase has been interpreted as a result of an increase of the packing parameter of the surfactant, as the chain length increases. At higher temperatures, all the sodium carboxylates form smectic phases and no reverse hexagonal phase is observed. Acknowledgment. E.F.M. kindly acknowledges the Portuguese Science Foundation (FCT) and FEDER for financial support through research project POCTI/QUI/44296/2002. We are also grateful to CIQ(UP) L1 and L5 for financial support and to FCT for R.O.B.’s Ph.D. grant (SFRH/BD/16380/2004). We are thankful to Jaume Caelles for the SAXS measurements. References and Notes (1) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (2) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205.
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