Langmuir 1993,9, 2084-2088
2084
Factors Influencing the Formation of Thermally Reversible Gels Comprised of Cholesteryl4-(2-Anthryloxy)butanoatein Hexadecane, LOctanol, or Their Mixtures1 Inna Furmant and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057 Received April 19, 1993 Gels containingsmall amounts of cholesteryl4-(2-anthryloxy)butanoate(CAB)and a liquid component (hexadecane, 1-octanol, or their mixtures) have been investigated during the process of their formation upon cooling from the respective isotropicphases by absorption,fluorescence,and excitationspectra. The spectra are compared to those of CAB in benzene, a solutionwhich is not gelled. The results indicate that bulk solvent properties,especially polarity, are more important than specificsolvent-CABintermolecular interactionsin determining the nature of the gel phases formed, but even the dependence on bulk polarity is complex. When 80-86/2&16 (wt/wt) l-odanol/hexadecane compositions are employed with CAB, two different gel types can be isolated depending upon the protocol for cooling the precursor isotropic phases. In addition, cells whose wall separations are smaller than the diameters of the colloidal unita which are assembled from CAB strands in the gels inhibit formation. The process of gelation in these systems is shown to be very complex, but manipulable by a surprising variety of factors.
Introduction Small concentrations of cholesteryl 4-(2-anthryloxy)butanoate (CAB)and some structurally related molecules have been shown previously to act with a wide variety of organic liquids to form thermally-reversible geh2 Since statistically more than 700 molecules of the liquid component can be immobilized by one molecule of CAB, direct intermolecular interactions cannot be responsible for the gels. In fact,we have demonstrated that three-dimensional networks of strands of CAB molecules, each ca. 10 nm by 20 nm in cross section, hold the isotropic liquid component in place primarily by surface tension.3
The nature of the gelator strands and the stability of the gels, as measured by the temperature of the transition between the isotropic and gel phases, T,,or the lifetime of a gel at room temperature, can differ enormously depending upon the concentration of CAB and the properties of the molecules constituting the liquid component. Some correlations between the spectroscopic properties and the stabilities of the gels have been made.2 However, they are empiricala t present and do not address the question of whether the processleadingto gel formation affects stability also. ~
+ Present address: Department of
Rochester, Rochester, NY 14627.
~~~
Chemistry, University of
(1)Part 49 in our series, Liquid-Crystdine Solvents aa Mechaniitic Probes. For Part 48,see Vilalta, P. M.; Hammond, G. S.; Weies, R. G. Langmuir l993,9,1910. (2)(a)Lin, Y.-c.; W e b , R. G. Macromolecules 1987,20,414.(b)Lin, Y.-c. Ph.D. Thesis,Georgetown University, Washington, DC, 1987. (3) Lin, Y.-c.;Kachar, B.;Weies, R. G. J. Am. Chem. SOC.1989,111,
5542.
Here, we exploit the rather large differences in the properties of CAB gels with 1-octanol and hexadecane to probe the dependence on the composition of the liquid component, the thermal history of the precursor isotropic phase samples, and the dimensions of the vessel in which the gels are formed. The experimental parameters measured include the process of gel formation, gel stability (as presented by Tg), and electronicabsorption, emission, and excitation spectra. Our results indicate that the type and stability of the gelator strands depend much more upon the bulk properties of the liquid component than upon discrete CAB-solvent molecular interactions. Furthermore, the nature of the strands is determined at the moment of CAB nucleation (and strand precipitation) rather than by an equilibration between strands and the liquid. However, when the required aggregationof strands into colloidal unita is inhibited by spatial constraints, gel formation can be inhibited. In the isotropic phases of samples which lead to gels a t lower temperatures, there is clear evidence for CAB aggregation.
Experimental Section The synthesis, purification, and characterization of CAB has been reported previously.2 The excitation and emission spectra of the isotropic and gel phases of ca. 1.5 w t % CAB in mixtures of 1-octanol (Mann Research Labs, 99%) and hexadecane (Aldrich, 99%) were examined using a Spex Fluorolog spectrofluorometer (150-Whigh-pressure xenon lamp; 0.5-mm slits) interfaced to a 386SX computer. Samples of appropriate compoeitionawere transferred to Kimax flattenedglass capillaries (i.d. 8 mm X 0.4 mm, 4 cm high, Vitro Dynamics)and degassed in the isotropic phase by three freeze-pump-thaw cyclesat (3-4) X 1W Torr and flame-sealed. Typically, emission spectra were recorded from 380to 540nm with the excitation monochromator fixed at 346 nm; excitation spectrawere recorded from 300 to 420 nm with the emission monochromatorset routinely at 436 or 440 nm. Samples were aligned at ca. 4 5 O to the incident radiation and emission was detected at a right angle from the back face of the cell. In many cases, excitation spectra were obtained at a variety of emission wavelengths and spectralranges to ensure reproducibility. Excitation and emission spectra for isotropic benzene (Baker,photorex)solutionswere obtained using 1.0 mm path length quartz cuvettes or 0.4 mm flattenedglass capillaries. Temperaturecontrol was achievedby incubatingthe capillaries in a hexadecane-filled quartz cuvette (1cm, 4 mL) which was
0743-746319312409-2084$04.00/0 0 1993 American Chemical Society
Langmuir, Vol. 9, No.8,1993 2086
Gels Containing CAB Table I. Gel and Isotropic Phase Emission (em) and Excitation (ex) Maxima. Various Solvent Systems emission maxima (nm) wt % l-octanol &%CAB in hexadecane gel isotropic 1.52 0 421 416 1.40 1 421 416 421 1.57 3 417 1.46 5 421 416 7 421 1.50 418 422 1.50 10 419 1.44 422 15 417 422 1.50 30 417 1.52 422 50 419 421 1.50 75 420 422d 1.56 80 419 427e 419 1.38 422d 85 419 427e 419 427 89 1.60 421 421 427 1.50 100 1.51
benzene
and Gelation Temperaturesb (Tg) for CAB in excitation maxima (nm) gelc isotropicC 407 403
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315,331,349 372,392 0 All wavelengths, fl nm. * f3 O C . c Only the most intense band of each excitation spectrum is listed for the l-octanol/hexadecanesyetems. d Cooling rate -8 deg/min. e Cooling rate -0.5 deg/min.
thermostated using a circulating water/ethylene glycol bath. For gelation temperature measurements, the temperature changes were monitored and recorded using a calibrated thermocouple (immerseddirectly into the hexadecane-filledcuvette) connected to a calibrated Linear 1200 chart recorder operating at 5 mV with a 1 cm/min chart speed. Sample phases were identified visually. Isotropic and gel phase spectra were recorded at 85and ca. 23 "C, respectively. Absorbance spectra in either SpectrocellO.05 mm path length quartz cells or 0.4mm Kimax flattened Pyrex capillaries were measured using a Hewlett-Packard 8514A diode array spectrophotometer. Gel phase absorbance spectra were recorded at room temperature and isotropic phase spectra were recorded immediately following heating gelled samples to their isotropic phase (with the sample phase c o n f i i e d visually in both cases after the spectrum was obtained).
Results and Discussion Fluorescence Spectroscopy. The position, shape,and intensity of emission from the anthryloxy fluorophore of CAB have been shown previously to exhibit large variations between the gel and isotropic In some instances, they can be used to distinguish the types of gelation strands formed, and the changes in fluorescence intensity as a function of sample temperature provide a means to determine a "spectroscopic" TP3 To allow comparisons among samples containing different compositions of the liquid component, the CAB concentrations were usually kept as near to 1.5 wt % as possible. The emission maxima obtained from the isotropic and gel phases of solutions containing ca. 1.5 wt % CAB are summarized in Table I. Emission and excitation spectra obtained for samples whose liquid composition is 575 wt % l-octanol (and 125 wt % hexadecane) are practically identical to those containing only hexadecane. Perhaps the most convenient indicator of the environment of the anthryloxy portion of CAB in the isotropic and gel phases is the position of the highest energy emission maximum: in the more polar l-octanol solvent, it is red-shifted by 5 nm with respect to that found in hexadecane. The 0, 50, and 75 wt % l-octanol gel samples display highest energy emission maxima at 422 nm. The emission spectra of the 80 and 85 wt % l-octanol gel samples, on the other hand, can be made to exhibit either a 422- or 427-nm emission maximum (but not bothsimultaneously), depending on the cooling method used to form the gel,
while the isotropic phase maxima are always the same (Table I). Gels obtained by allowing hot isotropic 80 or 85 wt % l-octanol samples to cool to room temperature in the air ("fast-cooledgels") displayed a 422-nm emission maximum; 427-nm maxima were observed for gels when the same samples in their isotropic phases were placed in a ca. 75 OC water bath and allowed to cool slowly to room temperature with the water ("slow-cooled gels"). The highest energy emission maxima of gels containing at least 89 wt % of l-octanol, regardless of the cooling protocol, are invariably at 427 nm. The excitation spectra of all of the isotropic solutions and gels containing various proportions of l-octanol and hexadecane display an intense lowest energy band above 400 nm whose position is invariant to change of the emission wavelength being monitored. This band is not present in the excitation spectra of 'normal" solutions of anthryl compound^.^ In fact, the shapes and positions of the emission and excitation spectra of CAB in both the gel and isotropic phases of the l-octanol/hexadecane solvent systemsare abnormal. Figures 1and 2 show representative emission and excitation spectra from the gel and isotropic phases of CAB in neat hexadecane and l-octanol. They should be compared to excitation and emission spectra of 1.5 wt % CAB in benzene (a nongelled solvent) in Figure 3. We assign the intense lowest excitation energy bands in Figures 1and 2 to excitation dipole coupling between anthryl groups stacked face-to-face in molecular aggregates." The absence of this band in Figure 3 provides evidence for a lack of aggregation;S ita presence above Tg in the hexadecane and l-octanol samples indicates that (4) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic Prese: New York, 1971. (5)Chandrose, E.A.;Fergueon,J.; McRae, E. G. J. Chem. Phys. 1966, 45,3546. (6)Chandrocls, E.A.; Fergueon, J. J. Chem. Phys. 1966,45,3554. (7)Fergueon, J.; Mau, A. W.H.;Morris, J. M.Aust. J . Chem. 1979, 26,91,103. (8) In one experiment with a benzene solution of 1.5 w t % CAE in a 0.4 m m path length flattened glass capillary cell, an intenee baud above 400 nm wae observed in the excitation spectrum. However, when the same solution wan decantad into a l-mm quartz cuvette, typical spectra l i e those shown in Figure 3 were obtained. Subsequent experimentsdid not duplicate the spurious observation and resulted only in spectra like those shown in Figure 3 regardleea of whether quartz or glass sample holden, were used. Thus, we believe that the unusualexcitationepectrum from the benzene solution resulted from some deposition of solid CAB onto the surface of the glass cuvette.
Furman and Weiss
2086 Langmuir, Vol. 9, No. 8,1993
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some components of the eventual gelator strands are already assembled. The fact that the emission and excitation spectra below Tgare simpler in appearance than those above it also suggests the presence of a distribution of aggregate types in the fluid samples. Once age1is formed, the environment of the CAB molecules may become more homogeneous or emission may occur preferentially at defect sites (energy holes) after exciton transfer along a ~ t r a n d .The ~ red shift in the vibronic progressions of the gel excitation spectra (compare Figure l a vs Figure lb, Figure 2a vs Figure 2b) is consistent with either hypothesis. It should be noted that no emission ascribable to an excimer of CABlO was (9) Markovitsi, D.;Lacuyer, I.; Simon, J. J. Phys. Chem. 1991, 96,
3620. (10) Birke, J. B . P h o t o p h y s i c s of A r o m a t i c Molecules; Wiley-Interscience: London, 1970; Chapter 7.
Figure 3. Normalized emission (X, = 346 nm) and excitation (X, = 440 nm) spectra of 1.5 wt % CAB and absorption spectra of 0.7 wt % CAB in benzene. Cell thickness = 0.4mm.
evident in any of our samplesin spite of the demonstrated molecular aggregation and our previous report that irradiation of isotropic or gel phases or CAB leads to stable photodimers.ll Since the geometric requirements for anthracene excimer emission are rather stringent,12Jswe suggestthat the restricted environmentsof anthryl groups in gel strands and in fluid-phase aggregates, combined with molecular steric factors, may inhibit attainment of the necessary bimolecular orientations. Determination of Gelation Temperaturesby Fluorescence. We define the gelation temperature, T,,to coincide with the initial precipitous increase in fluorescence intensity which occurs as isotropic solutions containing CAB are cooled; the fluorescence intensity of a gel phase can be more than 1order of magnitude larger than that of its isotropic phase, and the onset of intensity increase occurs reproducibly at one temperature. The reason for the increase is not completely clear at this time. We suspect that it is due, at least in part,to the formation and linking of the microcrystalline strands of CAB molecules which refract excitation radiation (leading to an enhanced probability for absorption throughout the gelled samples) and which attenuate the depolarization of fluorescence (leading to greater emission intensities in selected directions). In fact, small movements of a gel within the fluorometer sample compartment result in very large changes in the observed emission intensity. For that reason, gel containers were immobilized throughout an experiment. A "fast-cooling" rate of 1.4 deg/min was used to determine the T,values reported in Table I. Repetitive measurements on samples indicate a precision error of f 3 O . Typical plots of fluorescence intensity at one wavelength versus temperature have been published previ~usly.~ The observation that samples with l-octanol/hexadecane ratios as high as 75/25 retain the Tgvalue of neat hexadecane sampleseliminateshydrogen bonding as being the dominant factor upon which gel strand formation depends; at this weight composition, more than five of every six solvent molecules are l-octanol. Surprisingly, CAB in 80/20 and 86/15 l-octanol/hexadecane mixtures of liquid exhibit Tgand spectral characteristics which are those of gels with either neat hexadecane or l-octanol, depending upon the sample history: slow cooling (ca. 0.5 deg/min)results in l-octanol8 deg/min) yields hexadecanelike gels and fast cooling (a (11) Lin, Y.-c.;Weies, R. G. Lig. Cryst. 1989, 4, 367. (12) Cowan, D.0.;Drieko, R.L. Elements of Organic PhOtOCheMiStry; Plenum Preee: New York, 1976. (13) Bouae-Laurent,H.; Casteh,A.;Deevergne, J. Pure Appl. Chem. 1980, 52,2633.
Gels Containing CAB
Langmuir, Vol. 9, No. 8, 1993 2087
like gels. Regardless of the coolingrate, samples containing more l-octanol(Z89wt % of liquid component)form gels which are thermally and spectroscopicallylike those whose liquid is neat l-octanol. Thus, there appear to be two distinct CAB aggregate types in the gel phases whose formation is composition dependent, but in a "stepwise" fashion, and, in the critical liquid composition regions, is also dependentupon the rate of sample cooling. Previously reported freeze-fracture scanning electron micrographs of CAB/1-octanol and CAB/hexadecane gels show that the strands are of different morphologies.3 From data in the literature for l-octanol/heptane mixtures,lC we can estimate the polarity of mixtures of l-octan~l/hexadecane~~~ a t the critical compositions and temperatures where the CAB gel strands change their morphology. The polarities, as taken from dielectric constants (e), decrease monotonically with increasing temperature at one composition and with increasing weight percent of hexadecane at one temperature. Between 40 and 60 "C at 80/20 l-octanol/hexadecane, the t values change from about 6 to 5; at 89/11 l-octanol/hexadecane, the composition beyond which l-octanol-like gel strands are always observed, the t values are about 7 and 6 at the same temperatures. From these observations,we conclude that polarity is partially (but not solely) responsible for the type of gel formed and that the dependenceon polarity cannot be a continuous function since only one type of CAB strand is detected in each run. We suspect that temperature, another important variable, manifests itself by altering the equilibrium (Le., competing rates) for CAB molecules adding to and dissolving from the "crystaUinen faces of the two types of nucleating strands. In the critical composition region, it would appear that the longer the sample is kept above the Tg of the hexadecane-like gels (ca.40 "C) and below the Tgof the l-octanol-likegels (ca. 60 "C),the greater is the probability that the higher temperature modification will form. Since the lower temperature modification is found when the critical composition samples are brought rapidly to temperatures below 40 "C,where either form of the CAB strands should be possible, it appears that the lower temperature strand type is kinetically favored (i.e., ita aggregates assemble more rapidly) and the higher temperature one is thermodynamicallyfavored (Le., its higher Tgtranslates to a higher free energy for dissociation or melting). Absorption Spectra. At the 1.5 w t % concentration of gelator used in most fluorescence experiments, sample absorbances in the flattened capillaries were very high (>3) at the excitation wavelengths. Hence, lower CAB concentrations were used to record the absorption spectra for the 'Laand 'Lb regions of the anthryl chromophores15 (Figures 1-31, The positions and shapes of the emission and excitation spectra are the same in the more and less concentrated samples. Absorption spectra (along with their emission and excitation spectra) for gels of 0.8 wt % CAB in 80/20 l-octanol/hexadecane from fast and slow cooling are shown in Figure 4. They are not as clear a diagnostic of the gel type as are the excitation and fluorescence spectra. However, the progressions of vibronic bands in the excitationspectra are clearly red-shifted ~~
(14) (a) Smyth, C. P.; Stoops,W. N.J. Am. C h e m x c . 1929,61,3312. (b) We take the molecular volume of hexadecane to be twice that of heptane and the dielectric constante of the two liquidsto be equal. From the densities of the two paraffine, the former assumption appears to be almost exactly correct;the latter is ale0 valid since the reported dielectric constants at one temperature differ by lesa than 5%: see Physical Properties of Chemical Compounde; American Chemical Sociew. Washington,DC, 1959. (15) Klevens, H. B.; Platt, J. R.J. Chem.Phys. 1949,17,470.
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with respect to the analogous bands in absorption. Such shifta have been detected previously in anthracene samples and are attributed to aggregation, polaritone, and defect sites.3116 Although the absorption spectra do not display an intense band above 400 nm like that observed in the excitation spectra, hints of the presence of exciton coupling or a distribution of anthryl environments are evident in the broadeningof the lowest energy band in the gel phase spectra (Figures 1, 2 and 4). The shape and position of the emission spectrum from any of these gels is the same when excitation is in the region of the exciton coupling band (X,ca. 410 nm) or in another vibronic band (X,346 nm) of the same progression. The difference between the absorption and excitation spectra is a consequence of what is being sampled fluorescencereports the properties of only those molecules which are in an environmentwhich allows emission while absorption reflects the nature of all CAB molecules in a sample and the distribution of their environments. From the gross differences between both the shape and position of the peaks in the absorption and excitation spectra, it is evident that although there are several CAB environmenta, only a few lead directly to emission. Aggregation of CAB molecules may allow preferential emission from trap sites which are populated via both direct excitation and especially exciton hoppinggamong CAB molecules in "normal" sites. The appearance of the exciton coupling band above Tgindicated that some aggregation of CAB molecules must persist even in the isotropic phase. In order to record absorption spectra at a CAB concentration of 1.5 w t % while maintaining reasonably low optical densities, narrower sample cells were utilized. Figure 5 displays absorption, excitation, and emission spectra of 1.5wt % CAB in neat hexadecane and l-octanol in 0.05 mm path length quartz cuvettes. Comparison between these and spectra from 0.4-mm cells (Figures 1 and 2) indicates that the CAB aggregates and the properties of the gels are significantly affected by the width of the sample holder. For instance, the emission maxima of the gel phases in the 0.05-mm cell move to 430 and 426 nm in l-octanol and hexadecane, respectively. Interestingly, a sample of 1.5 w t % CAB in 80/20 l-octanol/hexadecane in the 0.05-mm cell did not yield a hexadecane-like gel even when the isotropic phase is rapidly cooled; only a l-octanol-like gel was formed. Another aliquot of the same sample, but in a 0.4 mm path length cell, did however produce a hexadecane-type gel (16) (a) Broth, T.Ph.D. Thesis, Universita de Bordeaux I, Talence, France, 1990. (b) Fergueon, J. 2.Phys. Chem. (Munich) 1976,101,45. (c) Ferguaon, J.; Miller, S. E. H. Chem. Phys. Lett. 1976,36, 635. (d) Ferguson, J. Chem. Phys. Lett. 1975,36, 316.
Furman and Weiss
2088 Langmuir, Vol. 9, No. 8,1993 1
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when a fast-coolingprotocol was followed. Previously, we have found that nearly spherical colloidal domains (i.e., integral units of CAB strands) of the hexadecane-typegel are ca. 200 pm in diameter while those of the l-octanol type gel are only 6-8 pm in diameter.3 Thus, it appears that the narrowness of the 0.05-mm (50-pm) cells inhibit hexadecane-type gel formation due to the constraints placed upon the growth of the colloidal domains! Even though individual strands with organization like that of hexadecane-like gels may be present initially in cooled samples, they cannot aggregate further into the colloidal network necessary to entrap the liquid component. As a result, either the slower-forming l-octanol-type or no gel develops.
Conclusions We have examined the process leading to formation of thermally-reversible gels with CAB as gelator and hexadecane, l-octanol, or their mixtures as the liquid component. Using absorption and emission spectroscopies, it has been possible to probe mechanical (spatial), thermal, and physical (polarity) parameters of gel formation. 1. Physical: Bulk polarity is a more important solvent property than specific solvent-solute intermolecular interactions in controlling the mode of gelation. consistent with this conclusion, we have found that the strands of CAB in gels with long-chainedn-alkanols resemble strands from alkane-type gels rather than those from shorter alcohols." 2. Thermal: The kinetics of CAB aggregation (as controlled by the rate of sample cooling) can lead to different gel morphologies in one solvent systemof critical polarity and composition. 3. Mechanical: Due to the disparate dimensions of colloidal units associated with different gel types, it is possible to inhibit gel formation or to force a less kinetically-favored gel to develop when microdimensional constraints (imposed by the walls of narrow cells) do not allow the unita to attain their normal size. These observations,taken together, indicate that CABbased gel formation is complex and surprisingly manipulable. We expect that many other systemswhich include low molecular weight gelators are also dependent upon the factors listed above.18 Efforts to verify this expectation and to ascertain the specific differences in molecular packing of CAB in strandsof the l-octanol and hexadecane types are ongoing.19
Acknowledgment. We thank Dr. Yih-Chyuan Lin for preparing the CAB used throughout this investigation. The National Science Foundation is gratefully acknowledged for ita support of this work. (17) Fee, C.;Kachar, B.; Weiss, R. G. Unpublished results. (18) See for instance: Terech, P. J. Phys. (Paris) 1989,50, 1967. (19) Terech, P.;Web, R. G. To be submitted for publication.
