1986
Langmuir 1993,9, 1986-1989
Aggregation Behavior of Double-Chained Ammonium Amphiphiles Containing (Cyanobiphenyly1)oxy Units Marcel D. Everaars, Antonius T. M. Marcelis, and Ernst J. R. Sudholter’ Laboratory of Organic Chemistry, Wageningen Agricultural University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Received February 1 , 1993. In Final Form: June 1, 1993 A seriesof amphotropicbis[w- [4(4’-cyanobiphenylyl)oxy]alkyl]dimethylammoniumbromideswith alkyl spacers of 6-12 methylene units were synthesized. All these new compounds show monotropic liquidcrystalline behavior. UV absorption maxima of vesicles of these compounds are strongly blue shifted relative to the monomer absorption. Upon increasingthe temperature the “monomernabsorptionreappears. Differential scanning calorimetry measurements clearly show a phase transition. This phase behavior is associated with a transition from a “stacked” gel phase to a more mobile liquid-crystalline phase of the vesicle bilayers. Upon spreadingthese moleculesat the water-air interface of a Langmuir-Blodgetttrough, an unusually high lift-off area of about 200-250 A2/molecule is observed in the surface pressure-area isotherms. This is associated with a situation in which the molecules are completely stretched on the surface. When the available area is reduced to about 40 AZ/molecule, a situation is obtained in which the tails in a close-packed crystallike arrangement.
Introduction Amphiphiles in water can exist in various forms such as molecular dispersions, micelles, bilayers, and microcrystals. Furthermore, they may form monolayers at the water-air interface. Usually amphiphiles containing one alkyl chain form micelles, whereas those with two alkyl chains form bilayer aggregates. Shimomura’ and Okahata2 have shown that singlechained amphiphiles which possess a rigid aromatic group are able to form bilayers. For good bilayer-forming properties these amphiphiles must have the aromatic moiety somewhere in the middle of the alkyl chain or near the head group, and a sufficiently long alkyl tail is required. Until now no study has been performed on the aggregation behavior of double-chained amphiphiles with rigid aromatic groups at the terminus of both alkyl chains. Therefore, we have initiated a study on the aggregation Experimental Section behavior of a series of novel compounds having this structural characteristic. Synthesis. The synthesis of the amphiphiles is outlined in As a rigid segment the 4-(4’-cyanobiphenyly1)oxy moiety was synthesized Scheme I.6 4-Hydroxy-4/-cyanobiphenyl(l) was chosen because of ita special spectroscopic properties. as described before.’ Consistingof a donowr-acceptor system, this chromophore 4’-Cyano-4-(o-bromoalkoxy)biphenyl(2). 4-Hydroxy-4‘shows extensive solvatochromic shifts in its fluorescence cyanobiphenyl(30“01) was dissolved in a stirred solution of and UV absorption which make it useful for intrinsically 30 mmol of sodium ethanolate in 200 mL of ethanol. To this probing bilayer properties. Furthermore, the (cyanobisolution was added 50 mmol of the appropriate 1,o-dibromoalphenyly1)oxy unit is a conventional thermotropic moiety kane. This mixture was stirred for 2 h under reflux. The solvent was evaporated, the residue was treated with diethyl ether, and (mesogenic unit). These compounds may therefore comNaBr was removed by fiitration. After evaporation of the solvent, bine lyotropic and thermotropic mesomorphism. Possibly, Langmuir-Blodgett layers of these new a m p h o t r ~ p i c , ~ ~ ~the residue was purified by repeated crystallizationfrom ethanol or by column chromatography (silica gel, petroleum ether (bp compounds can be oriented by external electric or magnetic 40-60 OC)/CHC&,2:3 v/v; yield 50%). fields without loss of the layered structure, resulting in [o-[4 44'-C yanobiphenylyl)oxy]alkyl]dimet hylamine monodomain films.s (3). Dimethylamine(15mmol)wasaddedasa20%(w/v)solution In this paper the first resulta on the synthesis and the of dimethylamine in chloroform to a solution of 2 (3 mmol) in aggregationbehavior in water and at the water-air interface 5 mL of chloroform. After 24 hat room temperaturethe solvent of novel double-chained ammonium amphiphiles with was evaporated. The substancewas dissolved in 50 mL of diethyl ether and extractedwith 2 X 100mL of 1M aqueous HC1solution. incorporated (cyanobiphenyly1)oxyunits is presented. The acidic water layers were made alkaline by addition of a concentrated NaOH solution. The water layers were extracted (1) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. with 3 x 100 mL of diethyl ether, the collected organic layers Chem. 1983,87,1134. were dried on MgSOd and filtrated, and the diethyl ether was ( 2 ) Okahata, Y.; Kunitake, T. Ber. Bunsen-Ges.Phys. Chem. 1980,84, 650. evaporated. The residue was recrystallized from hexane (-20 ( 3 ) Ringsdorf, H.;Schlarb, B.; Venzmer, J. Angew. Chem. 1988,100, “C;yield 40-50%). 117. (4) Chandrasekhar, 5.Liquid Crystals: Cambridge University Press: Cambridge, 1977; Chapter 1. (6)Penner, T. L.; Schildkraut, J. S.; Ringsdorf, H.; Schuster, A. Macromolecules 1991,24, 1041.
(6) All newly Synthesized compounds gave satisfactory elemental analyaes and 200-MHz 1H NMR spectra. (7) Percec, V.; Lee, M. Macromolecules 1991,24, 1017.
0743-746319312409-1986$04.00/0 0 1993 American Chemical Society
Langmuir, Vol. 9, No.8, 1993 1987
Letters Table I. Melting Points (mp) and Isotropic to Liquid-Crystalline Phase Transitions
4(6,6) 4(8,8) 4(10,10) 4(12,12) a
197 145 117 114
107 110 111 108
4(6,12) 4(8,12) 4(10,12)
169 137 120
107 107 109
See Scheme I.
Bis[e[ 444'-cyanobiphenylyl)oxy]alkyl]dimethyla"oium Bromide (4). Compound 3 (0.5 mmol) and compound 2 (0.7 mmol) were dissolved in 10 mL of acetone. This solution was refluxed for 20 h (65 "C). A white precipitate was formed which was collected and washed with diethyl ether (yield 6070%). Methods. Vesicle solutions were prepared by sonication of the insolublecompoundsin ultrapure water. Electron microscopy (Jeol1200EX I1 electron microscope)was carried out for samples that were prepared by freezefracturing. Polarization microscopy was performed using an Olympus BH-2 microscope equipped with a Mettler FP82HT hot stage and a FPSOHT temperature controller. The sample solutions for the differential scanning calorimetry (DSC) measurements (Perkin-Elmer, DSC 7) consisted of a 0.5 % (w/w)vesiclesuspension prepared by sonication. A scan rate of 10 "C/min was used. The F A isotherms were recorded on a Lauda Filmwaage,type FW2, which was thermostated at 20 "C. The water used for the subphase was purified by filtration through a Seralpur pro 9OC purification system. The amphiphiles were spread from chloroform solutions (1mg/mL) onto the aqueous subphase by use of a Hamilton syringe. After spreading, the monolayer was allowed to equilibrate for 10 min before compression started. The area was reduced at aspeed of 15cm2/min. All measurements were performed at least in duplicate.
Figure 1. Freeze fracture electron micrograph of a vesicle from 4(10,12).
'.'A,
Results and Discussion Liquid-Crystalline Properties. The thermotropic phase transition temperatures of these compounds were investigated by hot-stage polarization microscopy (Table I). All compounds exhibit a monotropic liquid-crystalline (LC) phase. From the observed textures the LC phase is assigned to be smectic. It can be seen from Table I that an increase of the alkyl spacer length lowers the melting point of these substances. The extra flexibility of the longer spacer probably destabilizes the crystallinepacking. The isotropicLC transition temperature8 on the contrary is hardly affected by the length of the spacer. This transition seems to be dominated by the mesogenic group. Formation of Vesicles. When compounds 4(6,6) and 4(8,8) are sonicated in water, no clear dispersions are obtained. Compounds 4(6,12) and 4(8,12) do give translucent dispersions by this method, but crystallization is observed within a few hours standing at room temperature. Only 4(10,10), 4(10,12), and 4(12,12) give stable, clear dispersions upon sonication. The presence of bilayer vesicles in these dispersions has been observed by electron microscopy (Figure 1). The UV absorption spectra of vesicles from 4(12,12) at different temperatures are shown in Figure 2. It is remarkable that at room temperature the absorption maximum is blue shifted by 30 nm relative to the absorption maximum at higher temperatures. Because the absorption maximum of the (cyanobiphenyly1)oxy chromophore is not affected to this extent by the polarity of the medium? this indicates that the observed blue shift
Figure 2. Spectral changes observed upon heating of a vesicle dispersion of 4( 12,12).
(8) Gray, W. G.; Moaley, A. J. Chem. SOC.1976,97. (9)4(n,m) in CHClS: A, = 297 nm. [0-[4-(4'-Cyanobiphenylyl)oxy]alkylltrimethylammoniumbromide in water: A, = 292 nm.
(10)Heeaemann, J. J. Am. Chem. SOC.1980,102,2167. (11)Fukuda, K.;Nakahara, H. Proc.7th Int. C o w . Surf.Act. Subst. 1976,B-I-2, 186.
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is caused by stacking of the (cyanobiphenyly1)oxyunits in the bilayer. This is often called H aggregation and is explained by parallel orientation of the transition dipole moments.' Similar blue sWitshave been observed in other systems. Heesemannlo and Fukuda and Nakaharall independently reported blue shifts for azobenzene chromophores aligned in a surface monolayer. Shimomura et al.' observed the samephenomenon for aggregatesof azobenzene containing amphiphiles in water. Although the compounds contain the same chromophore, different blue shifts are observed for vesicles from 4(10,10) and 4(12,12): 22 and 30 nm, respectively. This spectral difference is probably caused by a different ~
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1988 Langmuir, Vol. 9, No. 8, 1993 35.0
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Figure 3. DSC thermogramsof vesicle dispersionsfrom 4(10,lO) (dashed lines) and 4(12,12) (solid lines). The upper traces correspond to the heating process and the lower traces to the cooling process. interchromophore distance, which is probably smaller for vesicles from 4(12,12) than for vesicles from 4(10,10). Increasing the temperature of an aqueous vesicle dispersion does not affect the the measured ionic conductivity considerably. From this observation it is concluded that at higher temperatures the vesicles do not dissociate, but remain intact. From the temperature dependence of the UV absorption (Figure 2) a phase transition can be deduced in the interval of 40-60 "C (midpoint Tc = 50 "C) for vesicles from 4(10,10) and 4(10,12) and in the temperature interval of 50-70 "C (midpoint Tc = 60 "C) for vesicles from 4(12,12). The phase transitions are confirmed by DSC measurements (Figure 3) by the observation of endothermic peaks at 55,49, and 65 "C upon heating vesicles from 4(10,10), 4(10,12), and 4(12,12), respectively. The transitions are reversible since upon cooling exothermic transitions are observed at 19, 20, and 42 "C, respectively. Additional evidence for the formation of bilayer vesicles was obtained from the followingexperiments. When clear vesicle dispersion from 4(10,10) and 4(10,12) are frozen to -20 OC and subsequently melted, the dispersions become very turbid and the presence of microcrystalline material can be readily observed by light microscopy. When these turbid suspensions are subsequently heated to 55 and 50 "C, respectively,they become translucent again and remain clear even after cooling to room temperature. This phenomenon had already been reported for bilayerforming amphiphiles.12 What probably happens is that by freezing the vesicle dispersion the bilayer structure is disrupted by the formation of ice crystals, causing crystallization of the amphiphilic material. Upon heating the bilayer aggregates are reconstituted. Also from the DSC experiments it is learned that upon heating frozen dispersions of 4(10,10) and 4(10,12) the endothermic peaks at 55 and 50 "C (turbid to translucent) show an increased enthalpy change, with respect to an unfrozen vesicle dispersion. Subsequent cooling and heating experiments on the same sample (temperature not below zero) show a restoration of the original thermogram. In conclusion, we statethat sonication of the investigated compounds (except 4(6,6) and 4(8,8)) in water results in the formation of bilayer vesicles. The change in UV absorption upon heating the solution is attributed to a reversible phase transition of the bilayer molecules, presumably from a rigid ("stacked") gel phase to a more mobile liquid-crystalline phase. (12)Okahata,Y.;Ando,R.;Kunitake,T.Ber. Bunsen-Ges.Phys.Chem.
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