Effect of the Spacer Length on the Solid Phase Transitions of

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Langmuir 2003, 19, 10044-10053

Effect of the Spacer Length on the Solid Phase Transitions of Dissymmetric Gemini Surfactants Maja Sikiric´,† Ivan Sˇ mit,‡ Ljerka Tusˇek-Bozˇic´,† Vlasta Tomasˇic´,† Irina Pucic´,‡ Ines Primozˇicˇ,§ and Nada Filipovic´-Vincekovic´*,† Department of Physical Chemistry, Rud-er Bosˇ kovic´ Institute, Bijenicka c. 54, 10001 Zagreb, Croatia, Department of Materials Chemistry, Rud-er Bosˇ kovic´ Institute, Bijenicka c. 54, 10001 Zagreb, Croatia, and Department of Organic Chemistry, Faculty of Science, 10000 Zagreb, Croatia Received May 9, 2003. In Final Form: September 16, 2003 Three dissymmetric gemini surfactants (abbreviated as 12-s-14) in which n-dodecyldimethylammonium bromide and n-tetradecyldimethylammonium bromide are connected at the polar headgroups by a flexible -(CH2)s- spacer (s ) 2, 6, or 10) have been synthesized. The influence of the spacer length on the structural and thermal properties of 12-s-14 surfactants was investigated by means of IR and NMR spectral analysis, X-ray diffraction, thermogravimetry, differential scanning calorimetry, and polarizing optical microscopy. Geminis with s ) 2 or 10 form monolayers in which two alkyl chains are in the trans configuration, while the gemini with s ) 6 forms interdigitated bilayers with two alkyl chains in the cis configuration with respect to the spacer. All compounds exhibited a complex polymorphism and thermotropic mesomorphism from the stable crystalline form to the liquid crystalline phases of smectic type. The number of thermal phase transitions and the sequence of phases are markedly affected by the spacer length; that is, they depend on the configuration of the two alkyl chains with respect to the spacer.

Introduction Considerable attention has recently been paid to the solution behavior of a new class of surfactants, gemini surfactants of the bis(quaternaryammonium) type, abbreviated as m-s-m, where m denotes the number of C-atoms in the alkyl chains and s denotes the spacer.1-15 A number of papers have shown that the crystal structure,6 thermal properties,7,8 adsorption at interfaces,9 and selforganization in solutions of m-s-m surfactants connected by a flexible -(CH2)s- spacer strongly depend on the spacer length.10 Two characteristic s values have been recognized, s ) 5-6 and s ) 10-12. The first s value corresponds to a maximum of the melting temperature of the solid surfactant,11 a maximum of the critical micelle concentration (cmc),9 a minimum of some thermodynamic parameters of micellization,12 and a maximum of micelle * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Physical Chemistry, Ru]er Bos ˇ kovic´ Institute. ‡ Department of Materials Chemistry, Ru]er Bos ˇ kovic´ Institute. § Department of Organic Chemistry. (1) Menger, F. M.; Littau, C. A. J. Am. Chem Soc. 1991, 113, 1451. (2) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. (3) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (4) Zana, R. J. Colloid Interface Sci. 2002, 248, 203. (5) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 203. (6) Hattori, N.; Masuda, H.; Okabayashi, H.; O’Connor, C. J. J. Mol. Struct. 1998, 471, 13. (7) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940. (8) Fuller, S.; Shinde, N. N.; Tiddy, G. J. T.; Attard, G. S.; Howell, O. Langmuir 1996, 12, 1117. (9) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (10) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (11) Zana, R. J. Colloid Interface Sci. 2002, 252, 259. (12) Bai, G.; Wang, J.; Yan, H.; Li, Z.; Thomas, R. K. J. Phys. Chem. B 2001, 105, 3105. (13) Hattori, H.; Hirata, H.; Okabayashi, H.; Furusaka, M.; O’Connor, J. C.; Zana, R. Colloid Polym. Sci. 1999, 277, 95. (14) Sikiric´, M.; Primozˇicˇ, I.; Filipovic´-Vincekovic´, N. J. Colloid Interface Sci. 2002, 250, 221. (15) Sikiric´, M.; Primozˇicˇ, I.; Talmon, Y.; Filipovic´-Vincekovic´, N. J. Colloid Interface Sci., submitted.

micropolarity.4 The second s value corresponds to a maximum in the curve of the surface area occupied by one molecule of a dimeric surfactant at the air/solution interface,9 the lowest value of the packing parameter, the maximum stability of the micellar phase, the minimum tendency toward micelle growth and a complete disappearance of lyotropic mesophases,7 and so forth. The manner in which surfactant molecules are arranged in the crystal phase correlates with their solution behavior; that is, there is a strong relationship between the structure of a surfactant molecule, the crystal structure, and the type and structure of supramolecules self-organized in aqueous solutions. Although the effect of spacer length on the solution properties of gemini surfactants was the subject of the most recent investigations, only a few papers7,8 tackled the structural and thermal properties of the solid phase. It was shown that 12-s-12 surfactants manifested no thermotropism, and this behavior was attributed to geometric constraints on the headgroup arrangement associated with the presence of the spacer.7 Unlike 12-s-12 surfactants, 15-s-15 surfactants showed thermotropism.8 This paper is a follow-up of our work14,15 on the phase and structural behavior in aqueous solutions of dissymmetric gemini surfactants and its dependence on the length of the flexible hydrophobic spacer. It will be shown that the spacer length markedly affects structural and thermal properties of dissymmetric gemini surfactants (referred to as 12-s-14, with s ) 2, 6, and 10). Experimental Section Materials. Dissymmetric surfactants C12H25N+(CH3)2(CH2)sN+(CH3)2C14H29‚2Br- (12-s-14, s ) 2, 6, and 10) were prepared in two steps as described previously.14,15 Their purity was checked by elemental analysis expressed as mass fraction in percent (12-2-14: calcd C 58.62, H 10.82, N 4.56%; found C 59.8, H 10.8, N 4.5%. 12-6-14: calcd C 60.88, H 11.12, N 4.18%; found C 61.9, H 10.5, N 4.1%. 12-10-14: calcd C 62.79, H 11.37, N 3.85%; found C 63.8, H 10.4, N 3.6%) and mass spectra obtained by electrospray ionization (ESI) (Table 1). Results of elemental

10.1021/la034799e CCC: $25.00 © 2003 American Chemical Society Published on Web 10/31/2003

Solid Phase Transitions of Gemini Surfactants

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Table 1. Relative Molecular Mass (Mr) and Measured (expt) and Calculated (calcd) m/z of the 12-s-14 Surfactants: Mass Ions Obtained from Mass Spectra

Table 2. FTIR Spectral Analysis of the 12-s-14 Surfactants (Characteristic Absorption Bands are Assigned)a wavenumber/cm-1

MS m/z compound

Mr

expt

calcd

12-2-14

12-6-14

12-10-14

assignation

12-2-14 12-6-14 12-10-14

642.14 698.18 754.18

241.4 269.7 297.9

241.4 269.8 297.8

3005 w-m 2950 m-s 2921 vs 2852 s 1495 w-m 1471 m 1424 w 1387 vw 1374 vw

3021 w 2950 m-s 2921 vs 2852 s 1484 w-m 1467 m 1420 sh 1402 w 1383 w

3011 w-m 2950 m-s 2925 vs 2854 s 1485 w-m 1467 m 1413 w 1399 w 1378 w

νas(CH3-N+) νas(CH3) νas(CH2), νs(CH3-N+) νs(CH3), νs(CH2) δas(CH3-N+) δ(CH2) δ(CH3-N+) δs(CH3-N+) δ(CH3)

analysis, mass spectra, and no minima in the surface tension isotherms14,15 confirmed the high purity of prepared compounds. It is important to note that 12-10-14 was found to be hygroscopic, thus demanding special care in handling. Infrared Analysis. Fourier transform infrared (FTIR) spectra (4000-600 cm-1) were recorded on a Perkin-Elmer 2000 spectrophotometer using KBr pellets. NMR Analysis. 1H NMR spectra were obtained with a Varian broadband Gemini 300 spectrometer in CDCl3 and DMSO-d6 containing tetramethylsilane as an internal standard. Twodimensional COSY (1H-1H shift correlation) experiments were performed by standard pulse sequences, using Gemini Data System software, version 6.3. X-ray Diffraction (XRD). The phase structure was examined by means of X-ray diffraction analysis. XRD patterns at lower diffraction angles 2θ/° ) 1.5-5° were obtained with appropriate slit collimation (Philips PW 1050 diffractometer with monochromatic Cu KR radiation, proportional counter) at room temperature (RT); the overall diffraction angle region was 2θ/° ) 1.5-50. An automatic X-ray powder diffractometer having a high-temperature attachment (Philips MPD 1880, monochromatized Cu KR radiation, proportional counter) was used for analysis at higher temperatures in the Bragg’s angle region 2θ/° ) 4-40 (diffraction data at a lower angle could not be collected with this apparatus). Temperatures for recorded high-temperature XRD patterns were chosen in accordance with differential scanning calorimetry (DSC) curves. During heating runs, heating was stopped at chosen temperatures for 20 min in order to take diffraction patterns. Thermogravimetric Analysis. Thermogravimetry was performed from 293 to 673 K with a Mettler TA 4000 system. Differential Scanning Calorimetry. Calorimetry was carried out with a Perkin-Elmer DSC7 calorimeter. The solid sample was put into an aluminum capsule and heated at the rate of 1 K min-1. Measurements were made in the range T/K ) 298480. The transition enthalpy (∆H) was determined from the peak area of the DSC thermogram. The transition entropy (∆S) was calculated by using the equation ∆S ) ∆H/T, where T is the transition temperature corresponding to the DSC maximum. Due to thermal instability, all results were taken from the (several) first heating runs of each compound. The thermodynamic data were mean values of several independent measurements on different samples, the standard deviation being of the order of 2%. Polarizing Optical Microscopy. The microscopic textures were examined using a polarizing optical microscope (Leitz Orthoplan) equipped with a Linkam THM 600/S hot stage and a digital camera. Crystals were heated at the rate of 1 K min-1. Liquid crystal phases were identified by comparison of textures with literature examples.16

Results and Discussion Characterization of the 12-s-14. Infrared Analysis. The dimeric surfactants were characterized by IR and 1H NMR spectral analyses. The IR spectral data are summarized in Table 2. The results are similar to those obtained for the conventional quaternary alkylammonium bromide surfactants.17,18 The asymmetric methyl stretching vibration of CH3-N+ is observed between 3021 and 3005 cm-1, whereas its symmetric mode is superimposed on the vibration of the νas(CH2) absorption giving a very strong band around 2920 cm-1. The band at 2950 cm-1 is ascribed to the asymmetric stretching vibration of the terminal CH3 group, whereas the band near 2852 cm-1 arises from its symmetric mode overlapped by the sym-

a vs, very strong; s, strong; m, medium; w, weak; vw, very weak; br, broad; sh, shoulder, as, asymmetric; s, symmetric.

metric CH2 stretching vibration. The bending vibrations of both types of CH3 groups and of the CH2 groups are characterized by weak absorptions between 1500 and 1350 cm-1. NMR Analysis. The 1H NMR data of dimeric surfactants in CDCl3 and CDCl3/D2O are summarized in Table 3, according to the proton designation in Chart 1. The assignment of the spectra was performed on the basis of literature chemical shift data for similar compounds,19-21 splitting patterns, and signal intensities and is ascertained by connectivity in the COSY spectra. As a general remark, signals of the corresponding protons from both dodecyl and tetradecyl chains are equivalent. Resonances of the R, β, and γ protons in the tails are well resolved and give separated signals on the upfield side with respect to those of the corresponding R′, β′, and γ′ protons from the spacer. The difference in their chemical shifts depends on the spacer length and is more pronounced for the R and R′ protons. In the 12-2-14 gemini, this difference is about 1.1 ppm, while in the case of the 12-6-14 and 12-10-14 geminis, it is up to 0.2 ppm, similar to those observed between the β and β′ and between the γ and γ′ signals, respectively. Most of these protons show a typical surfactant aspect, broadened signals either with a singlet (β,β′) or a triplet (R,R′) pattern. As expected, nonsignificant differences in the spectra of all three surfactants could be seen for the methyl and the long-chain methylene protons. The terminal ω-CH3 protons showed a triplet around 0.88 ppm, whereas the methyl protons of the CH3-N+ groups gave a singlet between 3.5 and 3.30 ppm. The superposition of the long-chain methylene protons -(CH2)8- and -(CH2)10- from both surfactant tails, as well as of -(CH2)4- from the spacer in the case of the 12-10-14 gemini, gave an absorption near 1.24 ppm. When the results obtained in CDCl3 were compared with those when D2O was added, it could be observed that the chemical shifts of most signals were generally moved upfield in the presence of D2O, similar to the results obtained for some quaternary ammonium bromide surfactants.19 The greatest changes could be noticed for the methylene protons R and R′ from both sides of the CH3-N+ headgroups. More profound changes are observed for the spacer R′ protons amounting to 0.44, 0.23, and 0.11 ppm for the geminis 12-2-14, 12-6-14, and 12-10-14, respectively. (16) Demus, D.; Richter, L. Textures of Liquid Crystals, 2nd ed.; Verlag Chemie: Weinheim, 1980. (17) Scheuing, D. R.; Weers, J. G. Langmuir 1990, 6, 665. (18) Tomasˇic´, V.; Popovic´, S.; Tusˇek-Bozˇic´, Lj.; Pucic´, I.; Filipovic´Vincekovic´, N. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1942. (19) Mandal, B.; Wang, L.; Brown, K.; Verrall, R. E. J. Colloid Interface Sci. 1993, 161, 292. (20) Huc, I.; Oda, R. Chem. Commun. 1999, 2025. (21) Luchetti, L.; Mancini, G. Langmuir 2000, 16, 161.

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Langmuir, Vol. 19, No. 24, 2003 Table 3.

1H

NMR (d, ppm; J, Hz) Spectral Dataa of the 12-s-14 Surfactants

12-2-14 in the following solvents CDCl3 ω-CH3 (CH2)n γ-CH2 γ′-CH2 β-CH2 β’-CH2 CH3-N+ R-CH2 R′-CH2

Sikiric´ et al.

CDCl3/H2O

0.87 t 1.24b 1.37 s

0.88 t 1.26b 1.37 s

1.82 br s

1.79 br s

3.69 s 3.69 br s 4.75 s

3.40 s 3.64 br s 4.31 s

12-6-1 in the following solvents

12-10-14 in the following solvents

CDCl3

CDCl3/H2O

CDCl3

CDCl3/H2O

0.88 t 1.25b 1.35 s 1.57 br s 1.72 br s 1.99 br s 3.39 s 3.51 t 3.71 t

0.86 t 1.23b 1.32 s 1.50 s 1.68 br s 1.87 br s 3.27 s 3.40 t 3.48 t

0.88 t 1.25c 1.35 s 1.41 s 1.70 br sd 1.75 br sd 3.36 s 3.53 t 3.66 t

0.84 t 1.22c 1.29 s 1.37 s 1.70 br sd 1.73 br sd 3.30 s 3.44 t 3.56 t

a Multiplicities: s, singlet; t, triplet; br, broad signal. b n ) 18 [(CH ) + (CH ) ]. c n ) 22 [(CH ) + [CH ] + (CH ) ]. 2 8 2 10 2 8 2 4 2 10 β-CH2 and β′-CH2 resonances. Values were obtained from a COSY experiment.

Chart 1. Proton Designation for NMR Analysis

n ) 0, 12-6-14; n ) 4, 12-10-14.

d/Å

12-6-14 Irel

d/Å

12-10-14 Irel

28.2a

XRD at Room Temperature. Structural information about the phase type of investigated surfactants was obtained by X-ray diffraction studies. The diffraction patterns of all samples contain a number of Bragg reflections located at both small and wide angles indicating the formation of well-developed three-dimensional crystalline phases, which differ in their characteristics from the crystal structure of the parent surfactants, dodecyltrimethylammonium bromide (C12TAB) and tetradecyltrimethylammonium bromide (C14TAB).22 Diffractograms of 12-s-14 surfactants contain a series of diffraction lines having a detectable intensity identified as multiple orders of diffractions from the crystal planes (the long spacing), characteristic of lamellar arrangement and a number of reflections, which originate from the side-by-side distance (the side spacing) between neighboring chains. A list of characteristic parameters showing the interplanar spacings (d) and corresponding relative intensities (I/I0) is presented in Table 4. All compounds show the reciprocal spacings of the small-angle reflections in the ratio 1:2:3:4 (up to the 12th order reflection of the major repeat spacing being observed), typical for lamellar structure. These diffraction lines were indexed as 00l reflections and characterize the alkyl chain packing within a layer. Other diffraction lines show mainly smaller intensities (except for the intensive peak d ) 3.97 Å for 12-2-14). The average values of the long period, d001 (as calculated from the 00l lines), are presented in Figure 1 as a function of the spacer length, showing a curve with minimum at s ) 6. According to Tanford,23 the maximum length of one hydrocarbon chain with 12 C-atoms is 16.7 Å and with 14 C-atoms is 18 Å. Therefore, the value d001 ) 34.6(8) Å for the 12-2-14 surfactant, very close to the sum of the lengths of the molecules in the fully extended zigzag conformation, indicates the trans configuration of two alkyl chains with respect to the extended N-(CH2)2-N skeleton, that is, an almost orthogonal packing of the trans alkyl chains into crystal layers. This is in accordance with the crystal and molecular structure of the bis(quaternaryammonium bromide) surfactants with s ) 2 determined by X-ray analysis6 and Raman scattering spectra.24 Their (22) Powder Diffraction File; Joint Committee on Powder Diffraction Standards: Swarthorne, PA, 1987; Organic Volume, Card No. 30-1956. (23) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1973; p 52. (24) Hattori, N.; Hara, M.; Okabayashi, H.; O’Connor, C. J. Colloid. Polym. Sci. 1999, 277, 306.

Overlapped

Table 4. Interplanar Spacings (d) and Relative Intensities (Irel) of the 12-s-14 Surfactants at Room Temperature 12-2-14

a

d

21.8 17.1 (002) 11.3 (003) 8.46 8.07 7.66 6.67 (004) 6.49 6.21 5.89 (006) 5.77 5.52 5.28 5.03 (007) 4.90 4.82 4.54 4.25 4.13 3.97 3.79 3.58 3.53 3.43 (0010) 3.36 3.31 3.18 3.14 (0011) 3.10 3.06 2.97 2.90 (0012) 2.86 2.77 2.73 2.61 2.55 2.41 a

3 74 3 29 4 6 2 5 12 10 2 7 8 16 27 6 20 91 21 100 89 14 26 17 9 4 13 21 15 12 9 12 13 11 6 8 12 3

(001) 18.9 14.0 (002) 9.27 (003) 8.17 7.92 7.43 6.93 (004) 6.74 6.52 6.48 6.12 6.03 5.81 5.59 (005) 5.19 5.01 4.65 (006) 4.48 4.35 4.20 4.10 (007) 4.07 3.93 3.81 3.71 3.69 3.66 3.59 (008) 3.49 3.39 3.35 3.29 3.23 3.16 (009) 3.07 3.06

d/Å

Irel

32.1a

3 4 5 2 3 1 8 4 3 3 3 3 6 5 2 4 21 7 23 14 100 76 22 17 12 12 9 13 12 9 9 8 11 10 8 12

(001) 16.1a (002) 10.3 (003) 8.84 8.30 (004) 7.67 6.30 5.99 5.55 5.24 (006) 5.08 4.90 4.67 (007) 4.52 4.31 4.17 4.09 (008) 3.97 3.83 3.74 3.63 (009) 3.53 3.45 3.19 (0010) 3.00 2.98 (0011) 2.86 2.77 2.72 (0012) 2.64 2.55 2.54 2.48 2.41

52 10 3 16 9 7 5 8 7 11 52 39 31 43 100 57 21 14 14 13 6 26 15 16 26 31 9 6 30 26 5 2

Data taken at low angle.

two alkyl chains are in the trans configuration with respect to the extended N-(CH2)2-N skeleton (Figure 2a). Standing upright or slightly tilted and set side by side in a tail-to-tail configuration, the two alkyl chains are arranged in monolayers. On the basis of the present results, we are unable to provide more details on the arrangement of the dimethylammonium and bromide ions within the layer. The relatively short distance between layers of the 12-6-14 surfactant (d001 ) 28.2(5) Å) implies two possible arrangements: (i) one corresponding to the trans configuration of the two alkyl chains with respect to the extended spacer and tilted with respect to the layer normal


Figure 1. The interplanar spacings (d001) as a function of the spacer length (s) of the 12-s-14 surfactants.

Langmuir, Vol. 19, No. 24, 2003 10047

Figure 3. The DSC heating curves of the 12-s-14 surfactants during heating and the cooling curve of the 12-10-14 surfactant. Table 5. Transition Temperatures (T) and Associated Enthalpy Changes (∆H) and Entropy Changes (∆S) of the 12-s-14 Surfactants T/K

Figure 2. Schematic presentation of the two alkyl chain configurations with respect to the extended spacer, where R1 and R2 denote dodecyl and tetradecyl chains: (a) 12-2-14, (b) 12-6-14, and (c,d) 12-10-14.

or (ii) the other corresponding to the cis configuration of the two alkyl chains with respect to the spacer (Figure 2b). The cis configuration of the two alkyl chains with respect to the extended N-(CH2)6-N skeleton,6,24 the lower long spacings in comparison with those of the two other members, and the relatively low intensities of the reflections in the small-angle region are clear indications of the interdigitated bilayer structure.25 Such a conclusion can be additionally supported by almost the same value of bilayer thickness (28.2 Å) and the value obtained by

∆H/kJ mol-1

∆S/J K-1 mol-1

365 431 444

12-2-14 53.8 3.9 5.7

145 9 13

362 381 440 507

12-6-14 31.3 20.2 10.1 95

86 53 23 187

360 423 388

12-10-14 65.4 15.5 -9.4

182 35 24

summing the following: (i) the extended length of the hydrocarbon chain with 14 C-atoms (18 Å),23 (ii) the average length of the C-N bond (1.54 Å),6 (iii) the average length of the methyl groups attached to the nitrogen atoms (1.48 Å), (iv) the radius for the CH3 group (2.0 Å),26 and (v) the shortest distance of Br(1)‚‚‚N (4.21 Å)6 (the sum of the van der Waals radii of the ammonium cations and bromide anions (5.42 Å) may exceed their center-to-center distance because of appreciable penetration of bromine atoms into the equivalent sphere of tetramethylammonium groups).27 Bromide anions are located in two distinct planes within the crystalline layers;6 that is, the Br(1) ion is placed in an interdigitated hole, and Br(2) is placed in the vicinity of the methyl groups attached to the nitrogen atom. Considering a longer methylene spacer and the experimentally determined relatively long spacing value of d001 ) 32.1(9) Å, two arrangements of the two alkyl chains in the 12-10-14 surfactant lamellae are possible: the cis or trans configuration with respect to the N-(CH2)10-N skeleton (Figure 2c,d). The close similarity between the thermal behavior of 12-2-14 and 12-10-14 surfactants (as shown later) suggests a similar configuration of the two alkyl chains with respect to the spacer, that is, the trans configuration of the two alkyl chains in a monolayer (25) Terreros, A.; Galera-Gomez, P. A.; Lopez-Cabarcos, E. J. Therm. Anal. Calorim. 2000, 61, 341. (26) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: San Diego, 1985; Chapter 17. (27) Alami, E.; Levy, H.; Zana, R.; Weber, P.; Skoulios, A. Liq. Cryst. 1993, 13, 201.

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Figure 4. The micrographs of the 12-s-14 surfactants taken at different temperatures upon heating and cooling (crossed polarizers; magnification, ×250). The temperatures are as indicated. The 12-2-14 surfactant: (a) steeped drops observed in the melt and (b) the fan-shaped texture obtained on cooling the melt to room temperature. The 12-6-14 surfactant: (c) spherulitic domains in the melt and (d) the birefringent phase with Maltese crosses formed on cooling the melt to room temperature. The 12-10-14 surfactant: (e) the texture of the melt and (f) the fan-shaped texture formed on cooling from the melt to room temperature.

and the arrangement of headgroups and counterions in a single plane.6 In comparison with 12-2-14 with a possible upright packing of the trans configuration of the two alkyl chains in the monolayers, the relatively shorter long spacing value implies tilted alkyl chains. Thermal Behavior. On heating, 12-s-14 surfactants start degrading at temperatures depending on the length of the spacer. A thermogravimetric analysis revealed that the decomposition of 12-2-14 and 12-10-14 surfactants starts at almost the same temperatures (about 473 K), with peaks at 501 and 510 K, respectively. The thermal stability of 12-6-14 is higher; the decomposition starts at about 513 K, with a peak at 594 K. The fact that the melting point of 12-6-14 is highest in comparison with those of the two other members implies a stronger intermolecular interaction. Packing of surfactant molecules in the crystal is the result of a compromise between two counteracting tendencies of chains and the headgroup

packing.28 The polar headgroup at one end of an aliphatic chain poses an obstacle for the close packing of molecules, whereas parallel chain packing will maximize the van der Waals interaction of the hydrophobic alkyl chains. The interdigitated position of alkyl chains strengthens the intermolecular cohesion primarily due to van der Waals forces between the surfactant methylene groups and thus increases the melting point. DSC and optical polarizing microscopy were used to monitor the thermal phase transitions. Figure 3 displays thermograms of 12-s-14 surfactants, that is, sets of DSC heating curves of 12-s-14 surfactants and a cooling curve of the 12-10-14 surfactant. On heating, all samples exhibited several endothermic transitions, the number of which strongly depends on the spacer length. All first (28) Fo¨rster, G.; Meister, A.; Blume, A. Curr. Opin. Colloid Interface Sci. 2001, 6, 294.


endotherms are sharp and associated with relatively high enthalpy and entropy changes (Table 5). Microscopic observation showed that the melting point coincided with the last endothermic transition for all the members. This means that all transitions at lower temperatures subsume changes between solid crystalline (SC) phases, that is, solid to solid crystalline phase transitions. Members with s ) 2 or 6 start decomposing immediately above the melting point without having reached the isotropic liquid state, and because of that, only heating scans were analyzed (repeated heating-cooling cycles could not be completed). The heating curve of the 12-2-14 surfactant exhibits three endothermic transitions, which correspond to the SC1f SC2 transition associated with a high enthalpy change and two closely spaced endotherms associated with a small heat change (Table 5). The high ∆H value of the major transition indicates that most alkyl chains in the SC2 phase are in a molten state.8 The endotherm slightly below the melting point indicates the premelting or pretransition chain rotation.29 Observation of the liquid crystalline phase using an optical polarizing microscope was difficult due to the start of thermal decomposition; that is, microscopic observations were possible for a limited time above the melting point while textures were not satisfactorily developed. Nevertheless, we have been able to detect textures, which indicate that melted phases have a smectic structure. The appearance of steeped drops in the transition light (Figure 4a), spherulitic droplets, and small and weakly birefrigent batonnets indicates the 12-2-14 mesophase is of a smectic type.16 The smectic phases were additionally confirmed by the partial appearance of homeotropic texture without subjecting the sample to mechanical stress. It was not possible to obtain an isotropic liquid for this sample, but upon slow cooling of the melt to room temperature, a fan-shaped texture appeared (Figure 4b). This is an indication that the sample did not transform into the crystalline phase on cooling to room temperature, exhibiting paramorphism. On heating, the 12-6-14 surfactant exhibits four endothermic transitions up to the melting point (Table 5). The sequence of transitions below melting is SC1 f SC2 f SC3 f SC4. A visual examination on the hot stage of the polarizing microscope showed an increased softening after the SC2 f SC3 transition. The softened material shows birefringence, having textures that are similar to the viscous neat phases.30 On melting, the appearance of a few spherulitic droplets may imply the formation of liquid crystalline domains of small dimensions (Figure 4c). A birefringent texture retaining Maltese crosses at room temperature after cooling from the melt (Figure 4d) indicates the liquid crystalline phase of a smectic type. Only two transitions were observed for the 12-10-14 surfactant. The first, a rather strong peak, corresponds to the SC1 f SC2 phase transition, whereas the second, narrow peak corresponds to the very narrow temperature range characterized by SC2 f LC f IL transitions (LC denotes the liquid crystalline phase while IL denotes the isotropic liquid). The isotropic liquid phase is stable up to 473 K, when decomposition starts. A very narrow temperature range governing the second transition causes some problems in texture identification (Figure 4e). Upon cooling the isotropic liquid, the crystallization temperature is lower by about 40 K than the melting temperature indicating the existence of a supercooled state. This means that the crystallization occurred under nonequilibrium conditions. The separation of the mesophase from the isotropic phase at crystallization temperature took place with a very large hysteresis. There is no exotherm on the cooling line corresponding to the solid

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Figure 5. The total transition enthalpy (∆H/kJ mol-1) and entropy (∆S/J K-1 mol-1) changes with the spacer length (s) for the 12-s-14 surfactants.

to solid crystalline phase transition. Upon cooling to room temperature, a fan-shaped texture appeared (Figure 4f). The first transition temperatures corresponding to the SC1 f SC2 transformation (Table 5) slowly decrease with increasing spacer length, whereas at the same time the melting temperatures increase and go through a maximum at s ) 6. Similar behavior was observed for m-s-m surfactants,11 where melting temperatures go through a maximum at s close to 5, irrespective of the length of the alkyl chain. In Figure 5, the total enthalpy and entropy changes of all transitions at constant pressure are plotted versus the number of C-atoms in the spacer. The total enthalpy change displays a curve with the maximum indicating the strongest lattice forces between the molecules in the crystalline sample with s ) 6. The total enthalpy changes per one alkyl chain of the 12-2-14 (31.7 kJ mol-1) and 12-10-14 (40.5 kJ mol-1) surfactants are comparable to those of the parent molecules (42.3 kJ mol-1 for C12TAB and 46.9 kJ mol-1 for C14TAB)29 and the corresponding dimethyldialkylammonium bromides (22 kJ mol-1 for dimethyldidodecylammonium bromide and 31 kJ mol-1 for dimethylditetradecylammonium bromide31), whereas the total enthalpy change per chain of the 12-6-14 surfactant displays the highest value (78.3 kJ mol-1). This is due to the rigid structure of alkyl chains in the interdigitated position. The motions of alkyl chains in 12-6-14 are restricted due to higher cohesion forces between alkyl chains. The differences observed for the total ∆S reflect differences in the extent of the molecular motion changes during transitions. The observed dependence of the total entropy change with the number of C-atoms in the spacer arises from a substantial contribution of the conformational effect due to the flexible spacer. To obtain further information about solid to solid crystalline phase transitions, X-ray patterns were taken at elevated temperatures and immediately after cooling to room temperature. All diffraction patterns taken at higher temperatures differ from those taken at room temperature, indicating structural changes in all cases. Figure 6 shows X-ray diffraction patterns of the 12-2-14 surfactant recorded at 293, 403, and 473 K and after cooling from 473 K to RT. Upon heating from 293 to (29) Iwamoto, K.; Ohnuki, Y.; Sawada, K.; Seno, M. Mol. Cryst. Liq. Cryst. 1981, 73, 95. (30) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628. (31) Dynarowicz, P.; Godlewska, M.; Witko, W. Mol. Mater. 1997, 8, 309.

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Figure 6. The X-ray diffraction patterns of the 12-2-14 surfactant taken at different temperatures upon heating and cooling to room temperature. The temperatures are indicated.

473 K, diffractograms clearly show a constant decrease in intensities and a slight shifting of mainly 00l reflections toward lower angles indicating lower ordering of alkyl chains in layers. In the diffractogram taken at 473 K, all reflections below 2θ/° ) 22 and some reflections in the wide-angle range disappear. The rapid intensity decrease and the final disappearance of all 00l reflections (the residual reflection at the 0012 peak position probably originates from some overlapped reflections) can be explained by assuming disorder of alkyl chains in the layers and/or in the stacking of the layers. This is in accordance with literature data, which show that above 373 K almost all alkyl chains are in a molten state.8 The residual peaks in diffractograms taken at 473 K, remaining from some reflections in the wide-angle region of the virgin sample, indicate developed short-range organization in the headgroup and the counterion region located around the spacer. Although the exact arrangement of ammonium and bromide ions could not be estimated from their spacing ratio, the persistence of reflections at high angles, for instance, the reflections at d ) 2.65 and 2.55 Å indicative of some crystalline order, is indicative of some crystalline order in the region constituted by the headgroups and counterions. This residual crystallinity is not lost upon cooling from 473 K to RT. This phase could be defined as “presmectic” in the sense that it would differ from the true smectic phase mainly by the presence of crystalline phase in the regions constituted by the headgroups and counterion.32 Figure 7 shows the patterns of the 12-6-14 surfactant recorded at different temperatures. On heating, the peaks (32) Busico, V.; Corradini, P.; Vacatello, M. J. Phys. Chem. 1982, 86, 1033.

at higher angles gradually disappear (370 K) and alternatively a diffuse band appears around 2θ ) 20° (d = 4.4 Å) at 403 K. This diffuse diffraction maximum indicates an increased disordering of hydrocarbon chains. The most intensive peak at 2θ ) 21.6° (d ) 4.12 Å) in the diffractogram decreases by further heating (453 K), and the neighboring reflection at 2θ ) 22.4° (d ) 3.97 Å), superimposed onto a diffuse maximum, becomes the most intensive. Such changes in the diffraction pattern may indicate a structural change in lateral ordering and a possible change of bromide ion position. Although weaker, the 00l reflections still remain even at lower angles. When heated to 503 K, the 12-6-14 exhibits shifting of residual 00l reflections toward lower diffraction angles, more expressed than for other samples, implying a significant lamellar thickening (most probably caused by molecular translation or rearrangement in the headgroups and the counterion region). The obtained results are in accordance with the Raman scattering spectra24 taken at elevated temperatures, showing that a disordering of the cis-type skeletal structure includes a breakup of the structure and an increase in the randomness of the alkyl chains. Three kinds of solid to solid crystalline phase transitions indicate that the 12-6-14 surfactant may adopt different crystal structures because of a different balance between intermolecular interactions and a close packing of alkyl chains. The SC1 solid phase has a long spacing of 28.2 Å, whereas the SC2 phase has a slightly shortened long spacing of 27.8 Å. A possible explanation is that the SC1 f SC2 transition represents a “conformational melting” of the hydrocarbon portions of molecules. As the temperature increases, the molecules become more flexible and disordered on going from a highly ordered crystalline state


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to a more disordered mesophase or liquid crystalline phase. The SC2 f SC3 transition of the 12-6-14 surfactant involves structural changes, which involves an increase in the d-value. The SC3 phase shows a further decrease in 00l intensities and the appearance of a diffuse amorphous maximum centered at d = 4.4 Å indicating conformational disordering and an increase in randomness of the alkyl chains. The SC4 phase shows several intensive reflexes superimposed on a diffuse amorphous maximum. Detectable reflections are identified as multiple diffraction orders (2-7, 10) of major spacing d001 ) 31.0(4) Å indicating molecular ordering in the mesophase layers.

Only two reflections at 4.60 and 3.98 Å are not attributable to the layer orders. The diffractograms of the sample heated to 503 K and cooled to RT displayed close similarity. Several intensive reflexes superimposed on a diffuse amorphous maximum shifted to the higher angles are identified as multiple diffraction orders (1-9) of major spacing d ) 30.0(5) Å. The appearance of several reflections (denoted with arrows in the diffractogram of the sample cooled to RT) indicates that crystallization of the melted sample is kinetically controlled. X-ray diffraction patterns of the 12-10-14 surfactant taken at different temperatures are shown in Figure 8.

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The diffractogram taken at 403 K indicates a decrease in ordering of the alkyl chain of the SC2 phase similar to the observed disordering in the SC2 phase of the 12-2-14 surfactant. The diffractograms taken at 473 K and of the sample cooled from the melt to RT are identical, exhibiting close similarity to those for the 12-2-14 surfactant obtained under the same conditions. Although the aliphatic chains have been melted, the ionic layers are practically unaffected, being similar to those in the solid crystalline phase. The persistence of reflections at high angles, indicative of some crystalline order in the regions constituted by the molecules’ polar heads and counterions, implies a trans configuration of alkyl chains with respect to the spacer as in the solid crystalline 12-2-14 surfactant. Because the reflections below 3.14 Å were not collected, we were unable to detect a tetragonal arrangement of ionic dimethylammonium and bromide groups as shown in the literature for dialkylammonium bromides27 from the reflection series 4.27, 3.03, and 2.71 Å. A close similarity between X-ray scattering patterns of samples taken at different temperatures suggests a similarity in crystal structures between virgin crystals. Both compounds show major transitions at almost the same temperature (365 and 360 K, respectively) and high values of the enthalpy change. The ∆H values suggest that most of the alkyl chains are in a molten state in both SC2 phases. Since the 12-10-14 surfactant was found to be extremely hygroscopic, it was possible to follow the evolution of a lyotropic liquid crystalline phase on the sample heated to 473 K and then slowly cooled to room temperature. During the 30 min stay in the microscopic stage in the open air, the sorption of water induces the successive transformations from a fan-shaped texture to the giant vesicular phase (Figure 9), indicating amphitropic behavior. The X-ray diffractogram of the hydrated

sample shows two sharp reflections 001 at a low angle (d001 ) 32.1 Å and d002 ) 16.1 Å) and a diffuse halo centered around d = 4.4 Å confirming that the liquid crystalline phase is of a smectic type. In comparison with corresponding monomeric surfactants, alkylammonium bromides,29 dialkylammonium bromides,27,31 and their symmetric gemini counterparts,7,8 the 12-s-14 surfactants show some peculiar properties. Alkylammonium bromides do not display typical mesomorphic properties; although the aliphatic chains have been melted, the ionic layers are practically unaffected, being similar to those in the solid phase. At the temperature of isotropization, they change to a transparent liquid and subsequently decomposition occurs around 520 K. Dimethyldialkylammonium bromides show at least two endothermic transitions interpreted as a solid to liquid crystal transition and a liquid crystal to isotropic liquid transition. A novel smectic mesophase was identified in which the lateral packing of the molecules within the layers is ordered and tetragonal in symmetry.27 A detailed study of this compound revealed the appearance of a hightemperature phase identified as a highly ordered smectic B phase.31 Alami et al.7 were unable to detect any mesophase for 12-s-12 surfactants, while Fuller at al.8 have shown that 15-s-15 surfactants form one or more mesophases, at least one of which resembles the smectic phase, where the ionic headgroups with associated counterions form an ordered array, and the chains are disordered. It seems that the presence of a longer tetradecyldimethylammonium chain and dissymmetry contribute to the complex polymorphism and mesomorphism observed for 12-s-14 surfactants. On cooling, they display peculiar properties: an ordered mesophase and molten chains. Preliminary results revealed that crystallization from a melted sample


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Figure 9. The micrographs of the phases formed by sorption of water (12-10-14 surfactant; sample obtained on cooling from the melt to RT) by standing at the microscope stage in the open air (crossed polaroids; magnification, ×250). The X-ray diffractogram of the hydrated sample is shown at the bottom (right side).

is kinetically controlled. This is the subject of our further investigation. Conclusions Neither the structure nor the space groups of the 12-s-14 surfactants are known; however, it has been generally shown from the X-ray diffraction data that the alkyl chains are arranged in monolayers or interdigitated bilayers, whereas the dimethylammonium ions with associated bromide ions are arranged within an ionic array. The relative geometric structure of the two alkyl chains with respect to the spacer in 12-s-14 surfactants, the trans or cis configuration, strongly depends on the spacer length. Geminis with s ) 2 or 10 adopt the transtype structure, whereas that with s ) 6 adopts the cistype structure in the solid crystalline state. The presence of bromide ions implies electrostatic interactions that are usually much stronger (but less directional) than the van der Waals and hydrogen bonding interactions. The packing of the headgroups and counterions in a single plane or two separate planes depends on the trans or cis configuration of the alkyl chains with respect to the spacer. Differences between different polymorphs arise from positional and conformational changes in the alkyl chain region and the relative orientation of cations and anions

in the ionic region. The principal driving force toward mesophase formation is the segregation of the polar and apolar regions to form lamellar mesophases with a varying degree of intra- and interlamellar spatial ordering. Such segregation generally results from the melting of hydrocarbon chains in the molecules. The formation and stability of the mesophases in the investigated 12-s-14 surfactant series are very sensitive to the spacer length. The 12-6-14 molecules are generally more easily adaptable in the process of overall crystal energy minimization, offering many opportunities for a polymorphic formation. Interdigitated ordering of bilayers results in stronger interchain interactions and thereby in the highest melting point and consequently the highest sum of the heats of transitions. The maximum melting temperature indicates the lowest stability in the melted (liquid) state, showing a similarity between surfactants in the micellar state and in the melted state.4 It is thus evident that a variation in the spacer length changes the geometrical packing parameter which in turn influences the arrangement of a surfactant molecule in its micellar and mesomorphic state as well as in the crystalline state. LA034799E

Effect of the Spacer Length on the Solid Phase Transitions of

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