Langmuir 1995,11, 3705-3711
3705
Effects of Introduction of Mesogenic Units in Ammonium Amphiphiles on the Aggregation Behavior in Water Marcel D. Everaars, Antonius T. M. Marcelis, Alma J. Kuijpers, Emilie Laverdure, Janna Koronova, Arie Koudijs, and Ernst J . R. Sudholter" Laboratory of Organic Chemistry, Wageningen Agricultural University, Drecenplein 8, 6703 HB Wageningen, The Netherlands Received March 22, 1995. I n Final Form: May 31, 1995@ A number of novel single- and double-chained ammonium amphiphiles with biphenyl, azobenzene, and stilbene mesogenic moieties at the terminus of one of the hydrophobic tails have been synthesized. The single-chainedcompounds form less developed bilayers or micellelike structures. The aggregate stability increases from biphenyl- to stilbene-to azobenzene-containingamphiphiles. This increased stability does not correlate with increased hydrophobicity or dipole moment, but correlates with increasing enthalpic contributions. These are attributed to favorable n-n stacking interactions between the mesogenic units. The double-chained compounds form stable bilayer vesicles upon dispersion in water. In the bilayers the mesogens from moleculesofoppositeleaflets ofthe bilayer are interdigitated. Bilayers from the azobenzene containing double-chained amphiphiles exhibit a gel-to-liquid crystalline phase transition. Also a blueshifted UV-absorptionmaximum is observed,indicating that the mesogens are stacked into H-aggregates. The single-chained amphiphiles readily form aggregates with sodium dodecyl sulfate. These so-called ion-pair amphiphiles form unstable bilayer vesicles.
Introduction In the early 1980s a new class of compounds was introduced which combined the structural features of amphiphiles and thermotropic liquid crystals. These molecules are composed of a polar head group, one or more hydrophobic chains, and a mesogenic unit incorporated in the hydrophobic part. The mesogenic unit usually consists of a rigid aromatic group and owes its name to its capability to induce thermotropic liquid crystalline phases or mesophases. Also being amphiphiles, these compounds show lyotropic mesomorphism when dispersed in water. The unique combination of amphiphilic and thermotropic properties is expressed in their class name, amphotropic compounds. The lyotropic behavior of a series of amphotropes was first investigated by Kunitake et al.1-8 They focused on single-chained compounds of the type head-spacermesogen-tail. Later, many researchers applied amphotropic compounds in the field of Langmuir-Blodgett t e c h n ~ l o g y . ~ -Thin l ~ films of these compounds are prom@
ising photochromic materials for optical information storage, light-induced switches, and nonlinear optical devices. Since then, many amphotropic monomers and polymers17J8have been designed and synthesized and their physical properties investigated. However, to date little is known about the effect of the nature of the mesogenic units on the lyotropic mesomorphism of these compounds. We have therefore synthesized a series of novel single- and double-chained ammonium amphiphiles with different chain lengths and attached different mesogenic units at the terminus of one of the alkyl chains. Critical aggregation concentrations were measured in water and thermodynamic parameters for the aggregation process were calculated. Aggregate morphology was investigated with electron microscopy and light microscopy. The thermal phase behavior of the lyotropic phases was investigated with differential scanning calorimetry. Finally, also the formation of ion-pair amphiphiles using sodium dodecyl sulfate as the second component was investigated by U V spectroscopy and light microscopy.
Abstract published in Advance ACS Abstracts, September 15,
Experimental Section
1995. (1)Nishimi,T.; Ishikawa,Y.;Ando,R.;Kunitake,T.Recl. Trau. Chim.
Pays Bas 1994,113, 201. (2) Okahata, Y.; Kunitake, T. Ber. Bunsenges. Phys. Chem. 1980,84, 550. (3)Shimomura, Y.; Kunitake, T. J . Am. Chem. SOC. 1982,104,1757. (4) Shimomura,Y.; Ando,R.; Kunitake,Ber.Bunsenges.Phys. Chem. 1983,87, 1134. (5) Nishimi, T.; Ishikawa, Y.; Ando, R.; Kunitake, T.; Sekita, M.; Xu, G.; Okujama, K. Chem. Lett. 1983, 295. (6) Kunitake, T. Angew. Chem. 1992, 104, 692. (7) Kimizuka, N.; Kunitake, T. Chem. Lett. 1988, 829. (8)Shimomura, Y.; Kunitake, T. Chem. Lett. 1981, 1001. (9) Everaars, M. D.; Marcelis, A. T. M.; Sudholter, E. J. R. Langmuir 1993, 9, 1986. (10) Everaars, M. D.; Marcelis, A. T. M.; Sudholter, E. J. R. Thin Solid Films 1994,242, 78. ( 1 l ) a ) Heesemann, J. J. Am. Chem. SOC. 1980, 101, 2167. b) Heesemann, J. J . Am. Chem. SOC. 1980, 101, 2176. (12) Fukuda, K.; Nakahara, H. J. Colloid Interface Sci. 1984, 98, 555. (13)Nakahara, H.; Fukuda, K. J . Colloid Interface Sci. 1983, 93, 530. (14) Ashwell, G. J.; Jackson, P. D.; Crossland, W. A. Nature 1994, 368, 438. (15) Ou, S. H.; Percec, V.; Mann, J. A,; Lando, J. B. Langmuir 1994, 10, 905.
Synthesis. 4'-Cyano-4-hydroxybiphenyl(a)was obtained from Merck. 4-Hydroxy-4'-nitrobiphenyl (b)was synthesizedas described before.15J6 4'-Cyano-4-hydroxyazobenzene (c) and 4-hydroxy-4'-nitroazobenzene(d)were prepared by reaction of the diazonium salts of 4-nitroanilineor 4-aminobenzonitrilewith
phen01.l~ 4'-Cyano-4-hydroxystilbene(e) was synthesizedin a two-step reaction. A 5 g (20 mmol) portion of p-bromophenylacetic acid and 2.5 g (20 mmol) ofp-hydroxybenzaldehydetogether with 3 mL of piperidine were heated at 110 "C for 3 h followed by heating at 160 "C for 4 h. The reaction mixture was treated with water and the precipitatewas collected. The reaction product &-bromo4-hydroxystilbene was purified by column chromatography on silica gel using dichloromethane as eluent (yield 55%). ( 16) Ou, S.H.; Percec, V.; Mann, J. A.; Lando, J. B.; Zhou, L.; Singer, K. D. Macromolecules 1993,26, 7263. (17) Adams, J.; Rettig, W.; Duran, R. S.;Naciri, J.; Shashidhar, R. J.Phys. Chem. 1993,97, 2021. (18)Menzel, H.; Weichart, B.; Schmidt, A,; Paul, S.; Knoll, W.; Stumpe, J.; Fisher, T. Langmuir 1994, 10,1926. (19) Vogel, A. I. Textbook ofPractical Organic Chemistry; Longman Scientific & Technical Harlow, 1989, p 949.
0743-746319512411-3705$09.00/0 0 1995 American Chemical Society
Everaars et al.
3706 Langmuir, Vol. 11, No. 10, 1995 A mixture of 3 g (11mmol) of 4'-bromo-4-hydroxystilbene and 2.2 g of CuCN was refluxed for 15 h in 10 mL of dry DMF. The mixture was allowed to cool and a solution of 4 g of F e C k 6 H z 0 and 2.5 mL of concentrated HC1 solution in 10 mL of water was added. This mixture was heated at 70 "C for 25 min. The reaction mixture was then extractedwith CHC13. The CHC13 layers were washed with 5 M HCl solution and with NaHC03 solution and dried on MgS04. The solvent was removed under reduced pressure and water was added to the residue. The crystalline precipitate was collected and purified by column chromatography on silica gel using methanollCHCl3 (1:lOO) as eluent (yield 73%). 1-Bromo-w-(substitutedphenoxy)alkane (Ia-e). A mixture of 25 mmol of the appropriate phenol (a-e), 50 mmol of 1,w-dibromoalkane, and 50 mmol KzCO3 in 100 mL of 2-butanone was refluxed for 16 h. The salt was removed by filtration and the filtrate was concentrated by evaporation of the solvent. The product was purified by column chromatography on silica gel using petroleum ether (bp 40-60 "C)/CHZClz, 1 : l v/v as eluent (yield 60%): 'H-NMR (CDC13, TMS, 6, ppm) 1.40 (m, 2n - 8 H, - ( C H Z ) ~ - ~ - 1.80 ) , (m, 4 H , BrCHZCHz, ROCHzCHd, 3.40 (t, 2 H, BrCHz), 4.00 (t, 2 H , ROCHz), 7.00-8.30 (m, 8 H, Ar-HI.
N-(w(Substitutedphenoxy)alkyl)-N,N,N-trimethylammonium Bromide (IIa-e). A solution of 0.8 mmol of the bromide (Ia-e) in 6 mL of a 20% (w/w)solution oftrimethylamine in ethanol was heated at 100 "C for 20 h in a closed reaction vessel. The mixture was allowed to cool to room temperature and was then thoroughly cooled in ice. The precipitate was collected by filtration and washed with diethyl ether (yield 90%): 'H-NMR (CDC13, TMS, 6, ppm) 1.40 (m, 2n - 8 H, -(CHZ)n-4-), 1.80 (m, 4 H, NCHzCHz, ROCHzCHz), 3.4 (m, 11H, N(CHd3, CH2N),4.00 (t, 2 H, ROCHz), 7.00-8.30 (m, 8 H, Ar-H).
N-(w-(Substitutedphenoxy)dodecyl)-N,N-dimethyl& dodecylammoniumBromide (IIIa-e). A solution of 0.8 mmol of the bromide (la-e) and a 4-fold excess of NAN-dimethyl-Ndodecylamine in 2-butanone was refluxed for 16 h. The mixture was allowed to cool and the solvent was removed under vacuum. The residue was dissolved in a small amount ofdichloromethane and precipitated by addition of diethyl ether. The precipitate was collected and recrystallized from acetone (yield 50%): 'HNMR (CDC13, TMS, 6, ppm) 0.90 (t, 3 H, CH3), 1.40 (m, 34 H, -(CHz)s-, -(CHz)g-, 1.80 (m, 6 H, NCHZCHZ,ROCHZCHZ), 3.40 (s, 6 H , CH~N(CH~)ZCHZ-), 3.50 (m, 4 H, -CHZN(CH~)ZCHZ-), 4.00 (t, 2 H, ROCHz), 7.00-8.30 (m, 8 H, Ar-H). Methods. Critical aggregation concentrations were determined by measuring the specific conductivity of an amphotrope solution as a function of the concentration. A Philips Digital Conductivity meter PW 9527 with a Philips PW9550/60 electrode was used. The measuring compartment was thermostated with a Haake FE 2 thermostat. Electron microscopy (JEOL 1200 EX I1 electron microscope) was carried out for samples that were negatively stained with a 1% aqueous uranyl acetate solution. The sample solutions for the differential scanning calorimetry (DSC) measurements (Perkin-Elmer DSC 7) consisted of a 1% (w/w) vesicle dispersion prepared by sonication. Giant unilamellar liposomes were prepared by allowing 0.1-0.2 mg of crystals of the compounds to hydrate for several minutes in 200 ,uL ofwater at 50-60 "C. X-ray reflectivity patterns were obtained from films which were prepared by casting a vesicle solution onto silicon wafers. The measurements were carried out with a Philips PW1710 diffractometer equipped with a Cu LFF X-ray tube a t 40 kV and 55 mA. Semiempirical calculations were performed by using MOPAC 6.0 with a PM3 hamiltonian (QCPE program no. 455).20
Results and Discussion The synthesis of the amphiphiles is represented in Scheme 1. The composition and purity of the compounds was checked by 200 MHz lH-NMR, thin layer chromatography, and elemental analyses. Most compounds contained 0.5 or 1mol of crystal water. The thermotropic phase behavior of these compounds is very complicated and often not univocal. This is caused by the presence of different crystal morphologies, exhibiting different melting points. The additional presence of mesophases further (20) Steward, J. J. P. J . Comput. Chem. 1989,10, 209.
Scheme 1. Synthetic Pathways and Structures of Compounds IIa-e and IIIa-e
BrC"H2,Br
HOR
ROC,H2,Br
.
/
II a-e
111 a-e HOR
e
complicates elucidation of the thermotropic phase behavior. Also, the bound crystal water is believed to influence the phase behavior.
Aggregation Behavior of the Single-Chained Compounds. Compounds IIa-e readily dissolve in water upon heating. The aggregate morphology of aqueous dispersions of IIa-e was studied by light microscopy. None of the compounds IIa-d form giant bilayer vesicles. This in contrast to the compounds IIIa-e which all form giant bilayer vesicles (vide infra). Compounds IIa-d are thus assumed to form less developed bilayer aggregates or (rodlike) micelles. This is in line with the results found by Shimomura et al.4 in their investigation of molecules of the type head group-decyl spacer-azobenzene-tail. They found that when the tail was shorter than eight carbon atoms no stable bilayer structures were formed. The stilbene-containing compounds IIe ( n = 10,12), however, formed giant bilayer vesicles. This shows that very subtle changes in the molecular structure can result in dramatic changes of the aggregate architecture. The Krafft temperatures of compounds IIa-e are all found in the range of45-65 "C. At room temperature the stability of the undercooled aqueous dispersions varies from several minutes to several hours before crystallization is observed. Although the dispersions are metastable a t room temperature, it is possible to determine critical aggregation concentrations (cad a t room temperature. The cac values a t 20 "C are given in Table 1. From the critical aggregation concentration the change of standard Gibbs energy for aggregation (AGO,,) can be deduced. According to the "phase separation m ~ d e l " , ~which l - ~ ~assumes 100% of counterion binding, the AGOaggof a monovalent ionic surfactant, in the absence of added electrolyte is given by
where R is the gas constant and T the absolute temperature. The cac values are expressed in dimensionless mole fraction units. Figure 1 shows the plot of AGO,, versus the number ofmethylene units in the spacer for compounds (21)van Os,N.M.; Daane, G. J.; Bolsman, T. A. B. M. J . Colloid Interface Sci. 1987,115, 402. (22)van Os,N.M.; Smit, B.; Karaborni, S. Red. Trav. Chim. Pays Bas 1994,113, 181. (23)Bijma, K.; Engberts, J. F. B. N.; Haandrikman, G.; van Os, N. M.; Blandamer, M. J.; Butt, M. D.; Cullis, P. M. Langmuir 1994,10, 2578.
Effects of Mesogenic Units in Ammonium Amphiphiles Table 1. Critical Aggregation Concentrations for IIa-e and Alkyltrimethylammonium Bromides (C,+1TAB) at 293 K in Water compd na cac (mM) C,+iTABb 9 65 11 15.3 13 3.60 IIa 10 0.70 12 0.28 IIb 10 0.64 12 0.27 IIC 10 0.27 12 0.05 IId 10 0.26 12 IIe 10 0.53 12 0.14 a n is the number of methylene units in the alkyl chains. Data from ref 24.
I I...
s8
.......".......
-.....-.............
.....
-454
"1
4
-60 -65
-I
9
A
10
11
12
13
number of methylene units (n)
Figure 1. Plot of AGOagg(kJ/mol)at 293 Kversus the number of methylene units in the alkyl spacer (n)for compounds IIa (O), IIb (+), IIc (O), IId (x), IIe (A), and alkyltrimethylammonium bromides (W).
IIa-e. Data of n-alkyltrimethylammoniumbromides are clearly seen that substitution of also d i ~ p l a y e d . ~It * ,is ~~ a terminal methyl group with a terminal mesogenic unit lowers AGO,,, drastically. When Figure 1 is examined in more detail it is seen that of all mesogens the azobenzenes give rise to the lowest cac values and the most negative AGO,, values. The biphenyl mesogens give rise to the highest cac values. Furthermore, it is seen that substitution ofthe cyano group by a nitro group in both the biphenyl and the azobenzene mesogens hardly influences the AGO,,,. AGOaggcan be considered a summation of the contributions from the various units which make up the amphiphile. Using this method the contribution to the change of standard Gibbs free energy for aggregation of the mesogenic units (AGOmes)can be calculated. If we take AGOCH~and AG"t,eadgroupto be identical for both the mesogen-containing compound and the alkyltrimethylammonium bromide, the difference in AGO,,, between a mesogen-containing compound and the alkyltrimethylammonium bromide with the same number of methylene units can be expressed as (AGOme8)- (AGOCHJ. AG0m3is the change of free energy associated with the transfer of the methyl group from the aqueous phase to the interior of the aggregate. The value of AGOcH3has been reported to be -9.6 The values of AGO,,, for the different mesogens can thus be calculated and are given in Table 2. (24) Lianos, P.; Zana, R. J . Colloid Interface Sci. 1981, 84, 100. (25) Barry,B.W.; Russell, G. F. I. J . Colloid Interface Sci. 1972,40,
174.
Langmuir, Vol. 11, No. 10, 1995 3707 Table 2. Calculated Dipole Moments, Hydrophobicity Values and Calculated Free Energy Change of Aggregation (AGO,.) of the Mesogens a-e mesogen dipole moment (D) AGO,,, (kJ/mol) a 4.11 2.828 -27 b 6.04 2.955 -27 C 4.44 -0.344 -33 d 6.76 -0.217 -33 e 4.43 3.298 -28
u,
We have tried to correlate these results with the hydrophobicity of the mesogens. The hydrophobicity of each of the mesogens can be estimated using Rekker's hydrophobic fragmental constants and the values are given in Table 2. This value is expected to predict the partitioning of a molecule in a water-octanol system. It turns out that the hydrophobicity increases from azobenzene to biphenyl to stilbene. This does not correlate with our observations. It is therefore concluded that hydrophobicity is not the only driving force for this aggregation process. As all the investigated mesogens are donor-n-acceptor systems, they have a permanent dipole. In order to investigate if electrostatic dipole-dipole interactions could dominate the observed aggregate stability, the dipole moment values of the mesogenic units a-e (in vacuum) were calculated by semiempirical computer calculations, and the results are given in Table 2. It is clear that substitution of a cyano group for a nitro group increases the dipole moment drastically. This is due to the fact that the nitro group is a much stronger electron acceptor than the cyano group. When going from the biphenyl system to the stilbene system, while the same donoracceptor pair is maintained, the dipole moment increases slightly because of the increased distance between donor and acceptor group. The stilbene unit and the azobenzene unit have practically the same dipole moment because they have almost the same geometry. Experimentally, it is found that substitution of a cyano group by a nitro group only slightly affects the cac. Substitution of the stilbene unit by a n azobenzene unit, while the same donor-acceptor pair is kept, has a large influence on the cac, although the magnitude of the dipole moment remains the same. It is therefore concluded that the dipole moment ofthe mesogens does not dominate the aggregate stability. As neither the hydrophobicity of the mesogens nor the dipole-dipole interactions between the mesogens could account for the observed aggregate stabilities, additional experiments have been performed to elucidate the type of interactions that dominate the aggregation behavior. In principle, determination of AG",, from cac values as a function of temperature leads to the enthalpy ( W a g , ) and the entropy (AS",) of aggregation. These latter terms can provide insight into the physical nature of the aggregation process. Hydrophobic aggregation is often associated with a large positive entropy change due to solvent disordering effects. On the other hand, physical attraction between molecules results in a favorable, negative enthalpy change upon a g g r e g a t i ~ n . ~ ~ Therefore, the cac values ofthe compounds IIb,c,ewere measured in the temperature range between 20 and 45 "C. For all measured compounds a n increase in cac was found with increasing temperature (Table 3). Equation
(uL),28
(26) Evans, D. F. Langmuir 1988,4, 12.
(27) Tanford, C. The Hydrophobic Effect, 2nd ed.;Wiley; New York, 1980. (28) Rekker, R. F. The Hydrophobic Fragmental Constant., 1st ed.; Elsevier Scientific Publishing Co.: Amsterdam, 1977.
Everaars et al.
3708 Langmuir, Vol. 11, No. 20, 1995 -23
I. 3-
-180
i I
-
1
-15
I
1
I
I
3.10 3.15 3.20 3.25 ,3.30 1/T (K-)
I
I
3.40~10'~
Figure 2. Plot of 2R ln(cac)versus UT for compounds IIb (01, IIc (A), and IIe ( 0 )with n = 12 and for dodecyltrimethylammonium bromide ( 0 )and hexadecyltrimethylammonium bromide (+I. The values of the slopes are given in the plot in kJ/
mol.
Table 3. Critical Aggregation Concentrations of Compounds IIb,c,e (n = 12) and DodecyltrimethylammoniumBromide (C12TAB) and HexadecyltrimethylammoniumBromide (C16TAB) at Different Temperatures compound temr, (K) cac (mM) 0.27 IIb (n = 12) 293
IIC (n = 12)
IIe f n = 12)
ClzTAB'
C16TAB"
298 303 308 313 318 293 298 303 308 313 318 293 298 305 313 318 29 1 298 303 3 13 323 298 303 313 323
0.31 0.35 0.39 0.43 0.48 0.05 0.07 0.09 0.13 0.18 0.30 0.14 0.17 0.28 0.37 0.40 5.40 5.27 5.33 6.40 8.28 0.82 0.87 0.95 1.05
Data from ref 25. See also ref 33.
1can be rewritten to give eq ZZ9
AGoagJT= 2R ln(cac) = (AH"a,$T)-AS"a, This first-order model neglects heat capacity changes (AC, = dAH/dT = 0)25327 and gives only a rough estimate of mag,. AHo,,, has been obtained from the slope of a plot of 2R ln(cac) versus 1/T (Figure 2). Some literature data of dodecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide are also displayed.25 It is clearly observed that the slopes for the compounds IIb,c,e are steeper than for the alkyltrimethylammonium bromide compounds, reflecting a more favorable Wage Furthermore, Moa,, becomes more favorable upon going from the biphenyl-containing to the stilbene-containing to the azobenzene-containing compound, corresponding with the (29) Stauffer, D. A.; Barraus Jr. R. E.; Daughtery, D. A. J . Org. Chem.
1990,55, 2762.
decrease in cac. It is therefore concluded that favorable enthalpic interactions, which are stronger for the azobenzene mesogens than for the stilbene or biphenyl units, play a n important role in determining the aggregate stability. Possibly, n-n stacking of the mesogens in the formed aggregates is an important driving force for aggregate formation. Such stacking interactions have been described as a form of dispersion interactions between aromatic units.30 When aromatic units are depicted as positively charged o-skeletons sandwiched between negatively charged n-electron clouds, mutual attraction between the a-skeleton of one molecule with the z-electron cloud of an adjacent molecule can compensate the repulsion between the n-electron clouds. Therefore, the aggregated state represents a free energy minimum.31 This means that azobenzene mesogens give better n-n stacking than biphenyl or stilbene mesogens,resulting in a more negative stacking enthalpy and consequently in a more negative AG",,,. This result agrees with calculations performed by Hunter,32who found that heteroatoms in an aromatic compound can cause large partial atomic charges which can lead to additional intermolecular electrostatic interactions. This means that local dipoles within the aromatic system can favor n-;t. stacking, whereas the net dipole moment of the whole aromatic system does not dramatically affect the stacking behavior. The biphenyl mesogens have a AGO,,, of -27 kJ/mol (Table 2). For an alkyl segment with the same calculated hydrophobicity as the biphenyl unit using Rekker's hydrophobic fragmental constants, a AGO,,, value of approximately -22 kJ/mol can be deduced. This means that the aggregate stabilizing effect ofbiphenyl can mainly be ascribed to the hydrophobicity of this mesogen and that stacking interactions play only a minor role. Similar results are found for the stilbene mesogen. The azobenzene mesogens have a AGO,,, of -33 kJ/ mol. This mesogenic unit, however, is not hydrophobic but slightly hydrophilic < 0). Therefore, hydrophobic interactions are not expected to contribute to the aggregate-stabilizing effect of azobenzenes. This shows that stacking interactions play a dominant role in the aggregation of azobenzenes.
(uL
AggregateMorphology and Phase Behavior of the Double-Chained Compounds. Compounds IIIa-e, which possess two hydrophobic tails, form small unilamellar bilayer vesicles upon sonication. These can be visualized by electron microscopy (Figure 3b). Giant unilamellar vesicle^^^-^^ of compounds IIIa-e are formed when crystals are immersed in water a t 60 "Cand allowed to hydrate. Hydration begins instantly, as is seen by the formation ofwormlike tubules a t the surface of the crystals (Figure 3a). It is important that the sample of the hydrating crystals is not mechanically agitated, because that would enhance the formation of small vesicles. After about 10 min, the formation of giant liposomes is observed which are formed by budding ofthe bilayer tubules. These (30) Hunter, C. A.; Sanders, K. M. J . Am. Chem. SOC.1990, 112,
5525.
(31)Zhao, X. M.; Perlstein, J.; Whitten, D. G. J . Am. Chem. SOC. 1994. llfi. ~~. ,- - ,10463. ~
(32) Hunter, C. A. Chem. SOC.Reu. 1994, 101. (33)The cac values for dodecyltrimethylammonium bromide at room temperature in Table 1and Table 3 are considerably different. These data are obtained from refs 24 and 25, respectively. We believe that the value from ref 24 is more reliable because of the good semilogarithmic relationship between the cac and the number of methylene units in the alkyl chain. (34) Mueller, P.; Chien, T. F.; Rudy, B. Biophys. J . 1983, 44,375. (35)Lasic, D. D. J . Colloid Interface Sci. 1990, 140,302. (36)Menger, F. M.; Gabrielson, K. J . Am. Chem. SOC.1994, 116, 1567.
Effectsof Mesogenic Units in Ammonium Amphiphiles 9y-m
Langmuir, Vol. 11, No. IO, 1995 3709
1
. 200
250
300
350
400
450
500
550
wavelength (nm)
Figure 5. W absorption spectra of compound IIIc a s monomers in ethanol (dashed line) and as vesicles in water M and 4 x (solid line). The concentrations are 3 x M, respectively.
Figure 3. (a) Optical micrograph of a hydrated crystal of IIIc in water a t 60 "C. (b)Electron micrograph of vesicles from IIId prepared by sonication and stained with uranyl acetate.
I
-10
0
I
I
I
30 temperature (OC) 10
20
I
i
40
50
Figure 4. DSC thermograms of vesicle dispersions of IIIc (solid line) and IIId (dashed line). The upper traces are heating curves and the lower traces are cooling curves.
giant liposomes can reach diameters of 10 pm and can easily be observed by light microscopy. Because the Krafft temperatures of compounds I11 lie around 40-50 "C, the vesicles are not thermodynamically stable a t room temperature. In the undercooled state vesicles are usually stable for 6-8 h before precipitation occurs. Figure 4 shows the DSC thermograms of vesicle dispersions from IIIc and IIId in water. Phase transitions are seen at 39 and 27 "C, respectively. These transitions are reversible upon cooling and are attributed to the gelto-liquid crystalline phase transition of the bilayers. For IIIc this phase transition has been confirmed by temperature dependent 'H-NMR measurements.37
When vesicle dispersions of IIIc and IIId are frozen and subsequently allowed to melt, turbid dispersions of hydrated microcrystals are obtained. DSC measurements of these dispersions show phase transitions at 52 and 42 "C, respectively. These are attributed to the crystal to bilayer transition (which corresponds to the Krafft point).38 Upon subsequent cooling to 5 "C, followed by heating, the original vesicle thermograms are restored. For the vesicle dispersions of compounds IIIa,b,e no gel-to-liquid crystalline phase transitions are observed by DSC. The transition temperatures are expected to lie below those of the azobenzene compounds IIIc and IIId because of the lower aggregate stabilizing effect of the biphenyl and stilbene mesogens with respect to the azobenzene mesogens. This will cause the bilayers to be less ordered and the phase transitions, if they occur at all, will probably be accompanied by a very small heat effect. These results agree well with the results for the dialkyldimethylammonium bromide series.39 Didodecyldimethylammonium bromide also has no measurable gelto-liquid crystalline phase transition whereas compounds with longer chains do have measurable phase transitions. For the shorter chains the driving force for aggregation is reduced, resulting in less ordered bilayers. This suggests that a phase transition is only observed when very ordered bilayers go to a less ordered phase. Figure 5 gives the W spectrum of an aqueous vesicle dispersion of IIIc together with the monomer spectrum observed in ethanol. The absorption maximum for the monomeric species lies at 355 nm and for the vesicle dispersion a t 335 nm. Similar blue shifts of the absorption maximum were observed for vesicles from IIId and IIIe. The main absorption band for the azobenzene and stilbene chromophore is the n-n* band for which the transition dipole is directed along the long axis of the aromatic moiety.40 According to the molecular exciton model proposed by McRae and K a ~ h a the , ~ ~blue shift of the absorption maximum for the vesicle dispersion is indicative of linear chromophoreaggregates with their transition moments parallel to each other and ordered perpendicular to the stacking direction (so-called H-aggregate12J3).For (37) Below the phase transitiontemperature hardly any N M R signals are observed. Above the phase transition temperature the spectrum is sharpenedconsiderably due to the increased mobility of the molecules in the bilayers. (38) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978,82, 1710. (39) Okahata,Y.;Ando, R.; Kunitake,T.Ber.Bunsenges.Phys. Chem. 1981,85, 789. (40) Uznanski, P.;Kryszewski,M.; Thulstrup, E. W. Spectrochimicu Acta 1990, 46, 23. (41) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology; Academic Press: New York, 1977.
3710 Langmuir, Vol. II, No. IO, 1995
Everaars et al.
Figure 6. Possible bilayer structure of IIIc as deduced from X-ray reflectivity measurements.
vesicle dispersions from IIIa and IIIb, no blue-shifted absorption maxima are observed. This may be due to the relatively high cac values of these compounds, so that at W-measuring concentrations ( M) predominantly the molecularly dispersed species is present.42 In order to obtain more information about the structure of the bilayer, the bilayer thickness of vesicles from IIIc was measured. X-ray reflectivity measurements' of cast elms of vesicles from IIIc gave a bilayer thickness of 40.1 A. It is assumed that upon casting the vesicles collapse and form stacked bilayer sheet^.^:^ Taking into consideration that the calculated length of a molecule is 32 this probably means that in the bilayer structure interdigitation of the mesogens occurs accompanied by tilting of the alkyl chains. For compound IIIc, which has both a C12 spacer and a C12 tail, we can obtain a perfectly ordered bilayer structure in which the mesogens of oppositely positioned molecules are interdigitated. From the blue shift in the W spectra of the vesicles it is known that the mesogens are more or less parallelly stacked. It is therefore assumed that the alkyl chains are tilted with respect to the mesogens and the plane of the bilayer. When the mesogens are interdigitated over their full length of 13 a tilt angle of the alkyl chains of ca. 45" is required to obtain a bilayer thickness of 40.1 as illustrated in Figure 6. The fact that the aggregate peak in the W spectrum is very narrow (wln = 30 nm) and bell-shaped indicates that there is a high degree of order in these bilayers. Interdigitation of mesogens is very commonly encountered for thermotropic liquid. crystals both in their crystalline state and in their me so phase^.^^^^^ By interdigitation, unfavorable interactions between the dipoles of the mesogens can be minimized. When the C12 tail of compound IIIc is replaced with a C16 tail, the W absorption maximum of the vesicle dispersion is also blue-shifted, but the peak'is broader = 65 nm) and less symmetrical. This shows that if the tail is longer than the spacer, this hampers the formation ofordered structures. DSC also shows no phase transition for vesicles of this compound. This could mean that the phase transition, if it occurs at all, has a very low transition enthalpy. When vesicles of IIIe are irradiated with W light of 366 nm, the absorption maximum at 295 nm disappears. This is attributed to a [2 21-photocycloaddition of the double bonds of two adjacent stilbene mesogens (Figure 7). This results in the loss of the conjugation between the two phenyl rings of the stilbene moiety and in the
A,
A,
A,
+
(42)An indication for this is the fact that blue shifts of a few nanometers are observed upon increasing the concentrations of these compounds. (43)Wakayama, Y.; Kunitake, T. Chem. Lett. 1993, 1425. (44)Vertogen, G.; De Jeu, W. H. Thermotropic Liquid Crystals, Fundamentals; Springer-Verlag: Berlin, 1988; p 36. (45)Zugenmaier, P.; Heiske, A. Liquid Cryst. 1993, 15, 835. (46)Nishimi, T.; Tachikawa, M.; Maeda, T.; Ishikawa, Y.; Kunitake, T. Chem. Lett. 1994, 331.
200
250
300
350
400
wavelength (nm)
Figure 7. Time dependent changes in the UV absorption spectrum of vesicles from IIIe upon irradiation with UV light of 366 nm (dashed line is the monomer spectrum in ethanol). [IIIe] = 5 x M.
formation of a cyclobutane ring. In this way it is possible to covalently connect both sides of the bilayer. Ion-Pair Amphiphiles. As it turned out that mesogenic units could stabilize aggregates of both singlechained and double-chained amphotropes, the question whether they could also stabilize aggregates of ion-pair amphiphiles (IPA's) arose. Single-chained cations that are paired with single-chained anions represent a novel class of bilayer-formingsurfactants. Ion-pair amphiphiles have been proposed as interesting materials for both theoretical investigations and practical device applicat i o n ~ Several . ~ ~ studies have been performed on IPA's of alkyltrimethylammonium bromides with sodium dodecyl sulfate (SDS)47-49and with fatty acids.50*51 The geometry of a surfactant molecule is believed to play a crucial role in defining its aggregation properties. Conical-shaped molecules pack most comfortably into spherical aggregates. In contrast cylindrical-shaped surfactants prefer to assemble into bilayers. It is reasoned that the effective interfacial head group area of each partner of a n IPA should be substantially smaller than for each individual surfactant, due to electrostatic attraction between the head groups and due to a reduction of hydration. The overall effect is an increase in the cylindrical character of each ion. This contributes to the formation of bilayer structures. For the single-chained compounds IIa-e, the formation of a 1:1complex with SDS can be followed very nicely with W spectroscopy. Figure 8 shows the spectral changes upon addition of SDS to a solution of IIc. The first spectrum shows monomerically dispersed IIc, under its cac. Upon addition of SDS the absorption maximum shifts to lower wavelengths due to stacking interaction of the chromophores in the newly formed aggregates. The spectrum continues to change until a 1:l composition is reached. Upon addition of a large excess of SDS, no further spectral changes occur. This means that the formed aggregates are not solubilizedby the excess of SDS,which presumably forms separate micelles. Upon hydration of IIa and IIb in a SDS solution at 60 "C, the formation of giant liposomes was observed by optical microscopy. These liposomes are not stable a t room (47)Kaler, E. W.; Kamalakara Murthy, A.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 1371. (48) Herrington, K. L.; Kaler, E. W.; Muller, D. D.; Zasadzinski, J. A.; Chiruvolu, S. J . Phys. Chem. 1993,97, 13792. (49)N. Filipovic-Vincekovic, N.; Skrtic, D.; Tomasic, V. Ber. Bunsenges. Phys. Chem. 1991,95, 1646. (50)Chung, Y.; Regen, S. L.; Fukuda, H.; Hirano, H. Lungmuir 1992, 8, 2843. (51)Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S. L. J . A m . Chem. SOC.1990, 112, 1635.
Langmuir, Vol. 11, No. 10, 1995 3711
Effects of Mesogenic Units in Ammonium Amphiphiles
Ilc : SDS
1.2
8 1.0c
e0 0.8-
0.6-
m 0.4-
0.20.0 200
I
I
I
250
300
350
I
I
400 4 5 0
I
I
500
550
wavelength (nm) Figure 8. Changes in the UV absorption spectrum of an aqueous solution of IIc upon addition of sodium dodecyl sulfate (SDS).[IICI = 5 x 10-5 M. temperature and tend to crystallize very soon after preparation. Upon hydration of IIc,d,ein an SDS solution, no formation of giant liposomes was observed.
Conclusions A series of novel amphotropes has been synthesized and their aggregation behavior in water has been investigated. The single-chained compounds IIa-d form less developed bilayer or micellelike structures, in contrast to IIe (n = 10 or 12),which forms bilayer vesicles. From the decrease of the critical aggregation concentration it was deduced that the aggregate stability increases from biphenyl- to stilbene- to azobenzene-containing amphotropes. This sequence does not correlate with the hydrophobicity as quantified by Rekker’s hydrophobic fragmental constants, or the dipole moments of the mesogens. From temperature dependent cac determinations it was
concluded that the aggregate stability is determined by hydrophobic interactions and favorable enthalpic interactions, which are attributed to n-n stacking interactions between the mesogens in the aggregate. The n-n stacking interaction increases from biphenyl to stilbene to azobenzene, which explains the excellent aggregate stabilizing properties of azobenzene. The double-chained compounds IIIa-e all form bilayer vesicles. Only bilayer vesicles from the azobenzene amphotropes IIIc and IIId show a measurable gel-to-liquid crystalline phase transition by DSC. For these liposomes a blue-shifted absorption maximum is observed in the UV spectrum with respect to the monomer spectrum, This is caused by parallel n-n stacking of the mesogens in the bilayer (so-called Haggregation). Layer spacings of 40.1 A are observed for cast vesicle films and indicate that the mesogens from opposite sides of the bilayer are interdigitated and that the methylene spacers are tilted with respect to the bilayer plane. Addition of SDS to the single-chained amphotropes IIa and IIb induces the formation of bilayer vesicles by the formation of ion-pair amphiphiles. However, the bilayer structures are less stable than the structures obtained from double-chained amphotropes.
Acknowledgment. We are indebted to Mr. A. van Veldhuizen for recording the 200 MHz lH-NMR spectra and Mr. M. van Dijk and Mr. H. Jongejan for performing the elemental analyses. Thanks are also due to Ir. R. Schrijvers for assistance with the computer calculations and to Drs. N. Sommerdijk of the Catholic University of Nijmegen for performing the X-ray reflectivity measurements. LA950226T
