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Langmuir 1996, 12, 1117-1123
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Articles Thermotropic and Lyotropic Mesophase Behavior of Amphitropic Diammonium Surfactants Stuart Fuller,* Neeta N. Shinde, and Gordon J. T. Tiddy* Division of Chemical Sciences, Science Research Institute, University of Salford, Salford M5 4WT, U.K.
George S. Attard and Owen Howell Department of Chemistry, University of Southampton, Southampton SO9 5NH, U.K. Received August 21, 1995. In Final Form: October 27, 1995X We have synthesized two series of gemini diammonium surfactants with chain terminating poxycyanobiphenyl (OCB) groups (I) or conventional structures (II). The thermotropic mesomorphic behavior of series II has been examined using optical microscopy, differential scanning calorimetry, and X-ray diffraction. All the compounds form one or more smectic phases, at least one of which resembles the smectic T (ST) phase reported by Alami et al. (Liq. Cryst., 1993, 13, 201), where the ionic headgroups form an ordered array, but the chains have some disorder. The exact chain packing differs from that of the ST structure. The lyotropic mesophases formed with water have been investigated for both I and II by optical microscopy and X-ray diffraction (I only). Series II compounds form hexagonal (H1), intermediate (INT) and/or cubic (V1) and lamellar (LR) phases; the size of the spacer group (m) has little influence on this. The OCB compounds all form LR as the only lyotropic mesophase. These results show that the insertion of the rigid terminal OCB moiety has a much larger influence on mesophase structure (and by implication on micelle shape) than the spacer group for m e 6.
Conventional surfactants consist of a polar headgroup attached to a large hydrophobic chain. The molecules aggregate in solution (usually aqueous) due to the hydrophobic effect to form micelles. At higher concentrations the micelles become ordered, forming lyotropic liquid crystals (mesophases).1 Conventional thermotropic mesophases occur on heating rigid, rod or disc shaped aromatic compounds which often have attached alkyl chains.2 (Surfactants also form thermotropic mesophases, but since these swell in water they are usually considered along with the lyotropic phases.) Thermotropic liquid crystals arise from anisotropic interactions between rigid or semirigid elongated cores. In previous papers3,4 we have examined systems where both types of liquid crystal could occur. First, we studied mixtures of conventional thermotropic and lyotropic mesogens, where we found that the mesophases were immiscible.3 Then we looked at mesophase formation by a compound where thermotropic and lyotropic mesogenic groups were chemically linked via an alkyl chain (spacer).4 Our intention was to seek mesophases where both thermotropic and lyotropic order occurs. We term such mesophases as amphitropic. Although we did not identify any truly amphitropic mesophases, the compound studied (N,N′-bis(5-(4′-cyano4-biphenyloxy)pentyl)-N,N,N′,N′-tetramethylhexanediammonium dibromide, compound I, m ) 6, n ) 5) did exhibit unusual lyotropic phase behavior, in that two separate lamellar phases were observed. * Authors to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1954. (2) Gray, G. Molecular Structure and the Properties of Liquid Crystals; Academic Press: London and New York, 1962.
I
Here we report on the behavior of further compounds having the same general structure. Compound I contains two midchain polar groups and R,ω-terminal p-oxycyanobiphenyl groups. We have examined a series of compounds where m ) 2-6 and n ) 4-6. In order to understand the effect of the cyanobiphenyl moieties on the behavior, a second series of compounds containing just C15 alkyl chains and spacer groups of varying lengths has been synthesized (compound II, n ) 15, m ) 1-6).
II
The lyotropic liquid crystal phases formed by surfactants which are miscible with water are dependent on micelle shapes.5,6 These can be related to surfactant molecular structure by packing constraint concepts.7 Our aim is to (3) Corcoran, J.; Fuller, S.; Rahman, A.; Shinde, N. N.; Tiddy, G. J. T.; Attard, G. S. J. Mater. Chem. 1992, 2, 695. (4) Fuller, S.; Hopwood, J.; Rahman, A.; Shinde, N. N.; Tiddy, G. J. T.; Attard, G.S.; Howell, O.; Sproston, S. Liq. Cryst. 1992, 12, 521. (5) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975.
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consider how these concepts apply to the unusual surfactants, I and II. We recall that for tetraalkylammonium compounds the positive charge is located on the carbons attached to the nitrogen.8,9 Hence we expect these groups to be in an aqueous environment. Thus the hydrophobic core of the surfactant begins at C-2. It is likely that the rigid OCB moiety will impose conformational restrictions on the alkyl chains of the micelle interior. The possibility also exists that the OCB group might loop around into the micelle surface. Both of these aspects will be considered in the evaluation of any mesophase formation. Related studies on II where n ) 12 and m ) 2-16 have been reported by Zana, Talmon, and co-workers.10-14 They find that for m ) 2 and 3, the surfactants form long, threadlike and entangled micelles and for m ) 4, spheroidal micelles.11 They also report cylindrical (hexagonal) and lamellar mesophases.12 Furthermore, a number of compounds in the range 10 e n e 18 and 4 e m e 16 were found to show no thermotropic phases. This was surprising as the same group found that the dialkylmonoammonium surfactants (CnCm/2NMe2Br, 12 e n e 18, 6 e m/2 e 9) do show thermotropic behavior.14 The results reported here, together with a previous survey of cationic surfactant phase behavior,6 help to clarify these complex observations. In this study we have investigated both thermotropic and lyotropic behavior using polarized light microscopy, X-ray diffraction, and differential scanning calorimetry (DSC). Experimental Section A Carl Zeiss Jenaval polarizing microscope fitted with a Linkam THM600 hot stage and TMS 90 control unit was used with a range of heating rates between 0.1 and 99 °C min-1. Temperature accuracy is to (0.5 °C. X-ray diffraction studies were carried out at the EPSRC Daresbury synchrotron radiation laboratory on crystallography stations 2.1, 7.2., and 8.2. At the time the measurements were made, distances in the range 3-70 Å on 7.2 and 2-20 Å on 2.1 could be measured with the diffraction patterns being recorded on photographic film. The instruments were calibrated using standards such as silica, graphite powder, and sodium dodecyl sulfate, all of which give known reflections. On station 8.2, used for low-angle measurements, the data are recorded on 2-D detectors constructed at Daresbury. Distances were calibrated using rat-tail collagen. Differential scanning calorimetry studies were carried out using a Mettler TA3000 thermal analysis system which comprises a TC10 TA processor and DSC30 measuring cell and furnace. Scanning rates of 5-10 °C min-1 were employed . The temperature accuracy is estimated to be (1 °C. The compounds were synthesized according to the procedure described in ref 4 for 5-6-5 OCB. For series II the same method was employed, except that the alkyl-ω-OCB bromide was replaced by pentadecyl bromide. The purity was established by a single spot on TLC/HPLC and 1H high-resolution NMR at 300 Hz.
Resultssn-m-n Compounds (i) Optical Microscopy. First we report on the thermal studies on the n-m-n compounds. Zana et al.12 (6) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1115. (7) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (8) Jorgensen, W. L.; Gao, J. J. Phys. Chem. 1986, 90, 2174. (9) Adolph, D. B.; Tildesley, D. J.; Pincheo, M. R. S.; Kingdom, J. B.; Madden, T; Clark, A. Langmuir 1995, 11, 237. (10) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (11) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (12) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940. (13) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (14) Alami, E.; Levy, H.; Zana, R.; Weber, P.; Skoulios, A. Liq. Cryst. 1993, 13, 201.
Fuller et al. Table 1. Transition Temperatures (°C, (3°) Determined by Optical Microscopy for n-m-n Compoundsa (S f VN) (°C) 15-1-15 15-2-15 15-3-15 15-6-15
95 107 170 120
(VN f SA) (°C)
isotropicb (°C)
206 230
180 220 260 244
a S ) Solid phase; VN ) viscous neat phase; S ) smectic A A phase. b Isotropic liquid forms with some decomposition.
Table 2. Transition Temperatures (T) and ∆H Values Obtained from DSC Curves for n-m-n Compounds compound
T °C
15-1-15
88 170 90 192, 205 86 95
15-2-15 15-3-15
∆H ∆H (kJ mol-1) compound T (°C) (kJ mol-1) 34 9 35 5 6 14
15-6-15
100 108 154 205 225
10 9 4 4 1.3
a Temperatures are accurate to ca. 3 °C or better, while ∆H values are accurate to ca. (10%.
have already discussed in some detail difficulties resulting from the thermal decomposition of the materials when they are heated above 200 °C. This certainly does hinder the study of thermotropic mesophases because it is not possible to fully melt the compounds and subsequently cool them to obtain the characteristic mesophase textures. However, below 200 °C the compounds are stable for hours, hence thermotropic mesophases can be studied, but the transition temperatures cannot be determined with the same degree of accuracy as for stable compounds. On initial heating of the solids, softening was observed above 100 °C, the exact temperature being dependent on the particular compound. The “softened” materials are birefringent, having ill-defined “cloudy” textures that are similar to the viscous neat textures of Rosevear.16 These phases are extremely viscous. As the only way to detect the crystal/“viscous neat” (VN) transition is by poking the sample using a blunt implement, this transition is difficult to measure accurately. At higher temperatures the viscous phase transforms to an isotropic liquid or to a classical smectic A (or lamellar, LR) phase which subsequently melts to the liquid (Table 1). On cooling, the same mesophase sequence was observed, but with ca. 10 °C lower transition temperatures. These results differ from those of the previous workers12 who were unable to detect any mesophase formation. We emphasize that our results are reproducible since they have been carried out independently on several occasions in different laboratories by at least four of the authors. It is not clear from ref 12 what range of n-m-n derivatives was examined, but the bulk of the data appears to apply to the 12-m-12 series. (ii) DSC Studies. To examine thermotropic mesophase formation in more detail, we carried out DSC studies on the compounds. All of the results were taken from the (several) first heating runs of each material because the thermal instability certainly did induce large changes in subsequent scans. The initial heating cycle was always reproducible, although the “lumpy” nature of the solid material occasionally resulted in a slight broadening of the transitions due to poor sample/pan thermal contact. Table 2 lists the DSC data derived from curves illustrated in Figure 1. For all the compounds a major transition occurs at ca. 100 °C, with further much smaller transitions at higher temperatures. This clearly confirms the oc(15) Danino, D.; Talmon, Y.; Zana. R. Langmuir 1995, 11, 1448. (16) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628.
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Langmuir, Vol. 12, No. 5, 1996 1119 Table 3. Diffraction Data (spacings, Å) from Viscous Neat Phase of 15-2-15 175°C
195°C
159°C (cooling)
intensitya
order
31.5 15.7 10.5 7.9 6.49 6.36 6.06 5.50 5.35
32.4 16.2
32.4 16.0
8.0
8.0
n)1 n)2 n)3 n)4
6.46 6.09 5.52 5.38 4.97 4.86 4.79 4.66
6.4 6.06 5.54 5.36 4.95 4.88 4.81 4.64
v.v.s s v.w m v.w m v.v.w v.v.w m v.w s s m
4.95 4.88 4.7
n)5 n)6
a v.v.w, very very weak line; v.w, very weak; w, weak; m, medium; s, strong; v.s, very strong; v.v.s, very very strong.
Figure 1. DSC traces for 15-m-15 compounds: (a) (top) 151-15; (b) 15-2-15; (c) 15-3-15; (d) (bottom) 15-6-15.
currence of the mesophases observed by optical microscopy. It is obvious that the polymorphism is rather complex for 15-3-15 and 15-6-15, which show two transitions in this region. However, both 15-1-15 and 15-2-15 show two transitions which approximately correspond to those observed by microscopy. All of the compounds show broad, small exothermic transitions above ca. 170 °C, indicative of the onset of decomposition. Unless DSC features were reproducible above this temperature and were present with fast scan rates, we have not included them in the results. Hence, for 15-3-15 we are unable to document the high-temperature transitions. The ∆H values for the S f VN transition are much larger than those of the higher temperature transitions, as expected. However, even the S f VN transition has a much smaller ∆H than that expected for the melting of a ca. C36 hydrocarbon (∆H ) ca. 89 kJ mol-1),17 the rough equivalent of the compounds studied here. This implies that the n-m-n solids have significant residual disorder or that significant conformational restrictions occur even in the liquid state. The ∆H values do suggest that most of the molecule (i.e. the alkyl chains) is in a molten state above ca. 100 °C. The complex polymorphism of 15-3-15 and 15-6-15 has not been investigated in detail because of the decomposi(17) Small, D. M. Handbook of Lipid Research Plenum Press: New York and London, 1986; Vol. 4.
tion problems. However, both form more than one mesophase. For 15-3-15 the major melting peaks occur at a much lower temperature than the “softening” observed by microscopy. With 15-6-15 we observe several DSC peaks that do not correspond to optical transitions. These presumably correspond to transitions between different viscous neat phases. Such a transition is clearly visible in the DSC trace for 15-6-15 at 154 °C, although no change in the optical morphology was detected. The DSC peak at 225 °C may correspond to the VN/SA transition. (iii) X-ray Studies. To investigate these phenomena further, X-ray measurements were carried out on 15-2-15 and 15-6-15 using beam line 7.2 at the EPSRC Daresbury synchrotron radiation center. Because of the experimental arrangements we were unable to collect diffraction data for distances smaller than ca. 4.65 Å. Hence we were unable to detect the lines below 4 Å which Alami et al.14 observed for the novel ordered “tetragonal” smectic phase (labeled ST) of dialkyldimethylammonium bromides. The synchrotron source was employed to give short exposure times (typically 4 min) so that sample degradation was minimized. For 15-2-15 three temperatures were examined in the sequence 175, 195, and 159 °C, all within the viscous neat region. The data are located in Table 3. All of the diffraction patterns are very similar, confirming that chemical degradation is not a problem. There is a major repeat at ca. 32 Å with up to the sixth order reflection being observed. These show arcs indicative of partial macroscopic orientation and are attributed to the layer spacings. The other reflections are of much more uniform intensity. These we attribute to additional order within the ionic headgroup/counterion array. The distances, ca. 4.7-6.5 Å, are exactly what we expect given the dimensions of the polar groups. The major repeat distance is much smaller than the length of the all-trans molecule (46.5 Å) or than two extended C15 chains (42.6 Å). Hence we picture the structure as a bilayer one, with disordered alkyl chains filling the space between ordered headgroup layers where the spacer (CH2)2 group is within the ordered array. The large number of orders observed for the bilayer repeat indicates that part of the molecule is much more ordered than with a normal lamellar phase. As Table 4 shows, we observe six orders of the layer repeat spacing (34.2 Å) for 15-6-15 at 180 °C. Here there are no visible orientation effects with all the rings being of uniform intensity. Only one reflection (at 4.61 Å) is not attributable to the layer orders. This diffraction pattern corresponds to the mesophase above 154 °C detected by DSC. Clearly there is much less order in the polar group region than with 15-2-15, but the existence of a mesophase is confirmed. At higher temperature (229.3 °C) we observe
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Table 4. Diffraction Data for 15-6-15 at 180 and 229.3 °C (a) 180 °C
Table 7. Layer Spacings and Areas Per Headgroup in Thermotropic Phases of n-m-n Compound.
d-spacing
magnitude
order
d-spacing
intensitya
order
compound
temp (°C)
layer spacing d0 (Å)
area per headgroup a (Å2)
34.2 17.1 11.4 8.58
v.s s v.w s
n)1 n)2 n)3 n)4
6.84 5.68 4.61
s w s
n)5 n)6
15-1-15
120 150 195,159 175 190 180 229.3
32.4 33.5 32.4 31.5 33.2 34.2 34.2
74.0 71.5 75.1 77.2 75.0 78.0 78.0
15-2-15 15-3-15 15-6-15
(b) 229.3 °C.
a
d-spacing
intensitya
order
34.2
v.s
n)1
As for Table 3.
Table 5. Diffraction Data for Viscous Neat Phase of 15-1-15
a
120°C
150°C
intensitya
order
16.5 10.8 8.03 6.39 5.31 4.54 3.48 2.85 2.12 1.67
17.4 11.2 8.24 6.66 5.49 4.67 3.56 2.90 2.09 1.67
w v.w w w w m m w v.w v.w
n)2 n)3 n)4 n)5 n)6
As for Table 3. Table 6. Diffraction Data for 15-3-15
190 °C viscous neat 8.24 2.12 1.67 a
220 °C smectic A
intensitya
order n)4
2.09 1.67
w m m
As for Table 3.
only a single very strong reflection at 34.2 Å, and at even higher temperature still (251 °C) no lines are visible. At 229.3 °C we believe that the phase present is the smectic A phase observed by microscopy. Although the optical transition is at 230 °C, we believe that this slightly overestimates the temperature. The single order observed at 229.3 °C is consistent with the occurrence of a much less well-ordered material, i.e. a smectic A phase. Toward the completion of this study we obtained X-ray diffraction data for 15-1-15 and 15-3-15 using station 2.1. The detection was again with photographic film using exposure times of ca. 8 min. On this occasion the instrument setting allowed detection of distances below 4 Å but not above ca. 25 Å. The data are listed in Tables 5 and 6. No preferred orientation of the sample was observed. For 15-1-15 two temperatures of the viscous neat phase were studied, 120 and 150 °C. In both cases we observe five higher orders of a fundamental layer repeat distance together with additional lines at higher angles. For 15-3-15 one pattern was recorded for the viscous neat phase (190 °C) and a second in a smectic A phase (220 °C). For the VN phase we again observe a higher order layer reflection with two wide angle lines. At 220 °C no layer lines are observed, only spacings at higher angles. Clearly, all of these VN phase data are similar to those of 15-6-15; hence we attribute the lines at ca. 4.6-4.8 Å and below to headgroup and/or alkyl chain reflections. The only difference is that the correlations between layers appear to be much smaller for 15-3-15 at 190 °C. For this compound we observe no layer reflection at 220 °C, but if only one reflection at ca. 33 Å were present (as with 15-2-15), it would not be observed with the experimental arrangement that was employed on station 2.1. The presence of high-angle peaks at 2.09 and 1.67 Å suggests that some residual order within the headgroups
remains. From the difference in the high-angle X-ray reflections between the various compounds, it appears that a number of different bilayer phases occur, with varying degrees of residual order in the polar group regions which impose different degrees of order between neighboring layers. For all the mesophases the d0 values are much smaller than the length of a fully extended molecule; hence a layer or bilayer structure appears likely. Using known values for the polar group and chain densities, the area per molecule in the various smectic phases has been calculated (Table 7). All of the areas are much larger than that expected for two all-trans alkyl chains (ca. 46 Å2). They suggest that there could be considerable disorder in the alkyl groups that are some distance from the headgroups, but this does not agree with the observations of high-angle reflections in the region 2-6.5 Å. An alternative is that some type of interdigitated structure is present, with the alkyl chains of one surfactant molecule being located on top of the spacer chain of the opposing one, still within the alkyl chain region. We estimate a layer thickness of ca. 25-29 Å for this structure, but the observed d0 spacings (31-35 Å) are still larger. Hence we conclude that the alkyl chain structure may be some tilted bilayer with different detailed arrangements for the different compounds. Clearly, these materials could be subject to intensive study to elucidate the exact structure and the nature of the order/disorder balance. However, it is likely that similar phenomena occur with less thermally labile compounds that would be less challenging to study. This is our preferred next step. The observation of mesophases with ordered headgroups and molten chains is not new. Skoulios, Luzatti, and coworkers have reported similar phases for soaps many years ago,18-21 while more recently such phases have been observed even in the presence of a nonpolar solvent.22 It appears that a wide range of mesophases can occur, comprising various smectic arrangements as observed here and previously by Alami et al.,14 or ordered rod/globular aggregate structures as observed with various soaps.18-23 This area merits a systematic investigation, where headgroup structure and alkyl chain shape (branching) are varied. Perhaps the most surprising result is that Alami et al. did not observe thermotropic mesophases in their previous studies of the n-m-n compounds. We note that shorter chain alkylammonium surfactants with a high alkyl group content do not form mesophases readily, particularly at very high concentrations, either with water or on heating (see below). This is probably why mesophases were not observed. Lyotropic Mesophases. The lyotropic behavior of the compounds synthesized has been surveyed using the (18) Skoulios, A. E.; Luzzati, V. Acta Crystallogr. 1961, 14, 278. (19) Gallot, B.; Skoulios, A. E. Kolloid Z. Z. Polym. 1966, 213, 143. (20) Luzzati, V.; Tardieu, A.; Gulik-Krzywicki, T. Nature (London) 1968, 217, 1028. (21) Skoulios, A. E. Acta. Crystallogr. 1961, 14, 419. (22) Harrison, W. J.; McDonald, M. P.; Tiddy, G. J. T. J. Phys. Chem. 1991, 95, 4136. (23) Narayan, K. S.; Shinde, N. N.; Tiddy, G. J. T.; Holmes, M. C. Liq. Cryst. 1994, 17, 617.
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Langmuir, Vol. 12, No. 5, 1996 1121
Table 8. Lyotropic Behavior of Straight-Chain 15-m-15 Diammonium Compoundsa N 15-1-15
42-54c
15-2-15 2 -
