J. Phys. Chem. 1984,88, 4593-4596 smaller or larger values of 8,which are not improbable in a model system, are unstable against perturbations due to the surface effects which are always present in the z direction in a real experimental cell. The scattering method is analogous to that recently used to study magnetic colloidal particles (ferrofluids) aligned in an applied magnetic field.24 In the case of interacting anisotropic (24) Pynn, R.;Hayter, J. B.;Charles, S. W. Phys. Rev. Lett. 1983,51, 710.
4593
micelles, the technique opens the way to, a direct measurement of the parallel fluid structure factor S,(Q) and should stimulate the development of an appropriate theory for this quantity.
Acknowledgment. We are indebted to S. Hess for many stimulating discussions about shear alignment and to P. Cummins, A. Muddle, A. Smith, and G. Tiddy of Unilever Port Sunlight Laboratories for several discussions and for the gift of the TDPS. We also thank P. Lindner and R. Oberthur for the loan of a cylindrical shear cell and for help with some of the experiments.
Glass-Formlng Microemulslons C. A. Angel],* R. K. Kadiyala, and D. R. MacFarlane’ Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: July 3, 1984)
Pseudoternary microemulsions, in which the alcohol is polyhydric and is probably contained almost wholly within the aqueous phase, are described. Separate O/W and W/O clear-phase regions exist. These, like the O/W macroemulsions in this system which were reported earlier, are capable of being cooled without separation or crystallization into the vitreous state. To determine the extent to which microscopic droplet systems retain the bulk liquid characteristics, glass transition temperatures Tg,which are determined by the achievement of liquid structural relaxation times of 100 s, have been measured during reheating of the vitrified samples. T, for the dispersed droplet microemulsion state of o-xylene is almost the same as for the macroemulsion or bulk liquid phase. This is interesting since each individual system in the microdispersed state is no larger than one of the “clusters” often imagined to make up the subunits of the vitreous-phase structure. Vitrified microemulsions, in which the dispersed phase has a lower T, than that of the matrix, offer a means of preparing noncommunicating, truly ultramicroscopic liquid systems, for which a number of interesting possible applications exist. As one example, we have been able to vitrifyp-xylene which according to the Tb/T,,,> 2 rule for glass-forming ability should be almost impossible to vitrify (Tb/Tm< 1.43). This implies that, in microemulsion form, crystallization of molecular liquids during normal cooling will tend to be an unusual phenomenon.
-
Introduction In a recent report from this laboratory’ brief mention was made of the existence of microemulsions which, in contrast to the usual case of limited temperature range stability, could be cooled continuously into the glassy state. No characterization of the microscopic structures within this unusual microemulsion was made at that time. The present report explores the composition range in which microemulsions of the dispersed oil-in-water type occur in this system and examines the question of the extent to which specific characteristics of the oil (in this case o-xylene) are retained in the microemulsion state. The system in which these microemulsions occur is pseudoternary in character. Although there is an alcohol component present in addition to the oil, water, and surfactant, the alcohol is not of the medium chain length n-alcohol type but rather is highly hydrophilic in character and may be assumed to remain almost wholly associated with the aqueous phase. Indeed, it is the presence of this alcohol constituent which is responsible for the suppression of crystallization in the aqueous phase during cooling. It is not clear, on the other hand, that the alcohol has a real surface active effect, as do the more normal cosurfactants in reducing the oil/water interfacial tension; however, attempts to form a microemulsion in this system without the alcohol have uniformly failed. The significance of this work is severalfold. In the first place the ability to supercool into the vitreous state implies that an enormous range of liquid-state relaxation times, within a truly microscopic dispersed phase, is now available for study by appropriate techniques (nuclear spin lattice relaxation, dielectric relaxation, etc.). Thus, “system-size” dependence studies of various ?Present address: Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia.
types may now be comtemplated, using 2-10-nm-diameter samples suspended in a rigid continuous matrix. It means also that the microemulsion structure can be frozen for subsequent microscopic and/or X-ray examination. It even raises the possibility that the dispersed phase, or the matrix, could be subjected to controlled polymerization thereby trapping the microemulsion state (or alternatively specific size droplets, depending on which phase is polymerized) in a permanent condition. This has been attempted on liquid-state microemulsions but not on microdroplets dispersed in the liquid state within a vitrified matrix. Its success could open a new way to studying the structure of these systems by leaching out the unpolymerized part and characterizing the residue. Electron microscope studies of the vitrified microemulsion structure are currently in progress.2
Experimental Section Phase Diagram Determinations. The ambient-temperature phase diagram for the pseudoternary system has been mapped out by making several “cuts” across the pseudoternary diagram at chosen water/oil and surfactant/oil volume ratios of which the 3:1, 2:1, and 1:l ratios (see Figure 1) were the most carefully investigated. In all this work the aqueous “component” was maintained at constant composition, viz., 3 mol of water per mole of propylene glycol (hereafter referred to as PG-3H20). A solution of this inole ratio is very resistant to crystallization during cooling and is only moderately viscous and hence easy to use for sample preparation by syringe volumetrics. The “oil” phase is always o-xylene. For clarity, it will be designated X. The surfactant is always Tween 80. A mixture of 3 volumes of surfactant to 1 (1) D. R. MacFarlane and C. A. Angell, J. Phys. Chem., 86, 1927 (1982). (2) Dubochet, J. Teixeira, and C. A. Angell, to be submitted for publication.
0022-3654/84/2088-4593$01 .50/0 0 1984 American Chemical Society
4594
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
Letters
-
009
679 I
Weight % Aqueous Phose
Figure 1. Ambient-temperaturephase relations in the pseudoternary system PG.3H20 (aqueous phase) o-xylene (oil phase) Tween 80
+
+
(surfactant). Dispersed oil-in-water microemulsions are believed to occur in the ”finger” region. Note that continuous variations in oil phase content between 0% and 50% can be made along the appropriate join (dotted line). All compositions in this region are easily vitrifiable. volume of o-xylene will be described as a 3S:lX mixture. Each cut was examined at roughly 5 vol % intervals. In most cases the state obtained by brief manual agitation of a l-cm3 mixture of aqueous oil and surfactant oil solution in a small glass vial was the same state observable 24 h later; Le., microemulsion states formed quickly. Intermediate two-phase regions (see Results) were turbid in appearance on initial mixing and in most cases slowly separated into two clear coexistent phases on standing 24 h. The resultant glass vials were arranged on a triangular diagram for easy visualization of the character of the ternary system. Additional samples near the microemulsion phase boundaries were made to define these boundaries precisely. Glass Transition Temperatures. The thermal characteristics of dispersed and matrix phases, of which the dispersed-phase glass transition temperature was of principal interest, were determined initially by differential thermal analysis (DTA) studies at 10 deg min-’ heating rates on -0.1 cm3samples contained in -2-mm i.d. glass tubes. These were quenched in liquid N2 outside thc twin-bore aluminum DTA block both to increase the cooling rate and to permit visual vertification that the microemulsion samples remained optically clear in the glassy state. This is an important point since microemulsions frequently become unstable and separate on cooling. Whether the stability of our samples is thermodynamic or merely kinetic in nature has not yet been established. For more sensitive and quantitative comparisons, a series of upscans along the join AB in Figure 1 were performed using a Perkin-Elmer Model DSC-4 differential scanning calorimeter. Approximately 20-mg samples were contained in hermetically sealed A1 pans and scanned at 10 deg min-’. Results Clear-phase regions (solution or microemulsion in nature) are shown in Figure 1. We use a volume percent scale for all components for simplicity but include a weight percent scale for comparison purposes. Warm-up traces for normal emulsion, microemulsion, intermediate turbid (two phase) region, and second clear-phase region, taken along the pseudobinary line AB in Figure 1, are shown in Figure 2. The compositions are marked in volume percent of the pseudocomponent B, which is a 33.3% o-xylene/66.7% surfactant (Tween 80) solution, and their positions in the ternary diagram are marked along the line AB in Figure 1. Moving along the line, we find that AB corresponds to dispersing immiscible oil + aqueous phase (2:l volume ratio) using increasing amounts
+
-
489
460, 424 -
rn
357
210,
-
-i Figure 2. Warm-up traces for several compositions along the join AB of Figure 1. Note the clearly resolved glass transition for o-xylene in the 42% (clear microemulsion) sample. Significant increases in the glass transition temperature of the aqueous matrix phase occur with increasing surfactant content. Compositions are marked in volume percent of the respective components. The numbers in brackets correspond to the
corresponding mass fractions. of a 2: 1 surfactant-to-oil solution. The volume fraction of o-xylene (X) in the ternary mixtures remains constant along AB. We refer to this join as the 2W:lX 2S:lX join. Across the normal emulsion region, and in the beginning of the microemulsion region, the glass transition temperature Tg shows a small upward drift from the value attributed to the pure o-xylene phase (see Figure 3). There is, however, no sudden variation as the microdispersed region is entered, and the total drift is less than the difference in Tgbetween the ortho and meta isomers. There is, however, a distinct change in form of the glass transition as the surfactant content increases. The “overshoot” typical of the bulk glass transition, which is also seen in the 21% B emulsion (scan 1, Figure 2), disappears for the samples with more surfactant. The overshoot is due to relaxation effects, and its disappearance, along with the general “smearing out” of the transition region, implies a broadening in “distribution of relaxation times” for, or nonexponentiality of, the relaxation process. It could, however, also be influenced by heat flux factors in the dispersed
+
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4595
Letters 1
7
0
1
,(tween80)= 204 K Tg for PG 3HzO
150-
Tg (IO
IC
1 .o I+
1
macroemulsion
IE
turbid
I S I ‘E
140-
See fig
and ten
P
I
t
.
_ --
I
1
m-xylene IO
20
i
-
I
1
30
40
8 0
6(
1
80
70
VOI ‘106
I
IO
20
30
40
50
90
,
60
1
Vol Surfactant Figure 3. Variation of glass transition temperature of oil and aqueous phases with surfactant content at constant volume fraction of o-xylene, along the join AB. The upper composition scale is the volume percent of pseudobinary component B (2S:lX) in the pseudobinary mixture A(2W:lX) B. The lower composition scale is the volume fraction of surfactant in the pseudoternary mixture.
+
medium, mechanical strain relaxation, etc. and will need to be carefully evaluated in the future. The matrix-phase PG.3H20 exhibits a more pronounced glass transition at a temperature (163-166 K) close to the bulk PG3H20 value despite the surfactant content. This transition is pronounced both because of the preponderance of aqueous phase in each emulsion and because of the unusually large change in heat capacity observed with polyalcohol and aqueous sample^.^
Discussion The form of the ambient-temperature phase diagram is rather similar to that seen in earlier work on quaternary systems, such as the system H20/K01/3M2B/C6H6 studied by Clausse and colleague^.^ In the latter system electrical conductivity studies on mixtures roughly equivalent to the join studied in detail in the present work showed that the structure could most probably be described as dispersed droplets of oil in a continuous aqueous phase in the “finger” region of Figure 1. O/W microemulsions tend to a phase inversion to W/O microemulsions halfway along the join AB of Figure 1, but the progressive changeover is interrupted by the two-phase region (hatched in Figure 1). The effect of temperature decrease on this diagram will be determined in future work. The extent to which the microdispersed oil phase retains the characteristics of the bulk phase can be tested by measurements which are sensitive to the relaxation time of the liquid. In general, the relaxation time of a liquid is a more sensitive indicator of its structural state than any structural measurement which can be simply performed. The glass transition temperature reflects the arrival, with changing temperature, of the system at a relaxation time commensurate with the time scale of the experiment used (3) C. A. Angell and D. L. Smith, J . Phys. Chem., 86, 3845 (1982). (4) C. A. Angell, “Proceedings of a Workshop on Relaxation Processes”, in press. (5) J. Peyrelasse, C . Boned, J. Heil, and M. Clausse, J. Phys. C, 15, 7099 (1982). See in particular ’Figure 2.
to detect it; hence, the glass transition temperature Tgmay serve well as an indicator of the extent to which the character of o-xylene is retained in truly microscopic samples. Since cluster or paracrystallite models of the glassy state commonly suppose that the jammed subsystems or ”amorphons” that make up the glass superstructure (with tissue material filling the interstices) are of the order of 20-30 %r. in diameter, it could be imagined that the glass transition temperature determined for a microemulsion might differ dramatically from that of the bulk glassy state. Figure 3 shows that, at least on the water-rich side of the microemulsion region, the state of the substance, judged by Tg, is essentially unchanged from that in the normal macroemulsion (although in both high-surfactant macroemulsion and microemulsion cases the transition appears smeared out). This is a very interesting result since it implies that the properties of molecular liquids which can be obtained in microemulsion form in systems of the present type can be investigated at temperatures below the glass transition temperature of the PG.3H20 matrix, in isolated noncommunicating microsystems containing of the order of 1000 molecules more or less. The dimensions of such microsystems are of special interest since they fall in the “gray” range between localized and collective phenomena, for each of which a distinct body of physics has been developed. Certain properties sensitive to long-wavelength perturbations such as crystal nucleation might be expected to show strong deviations from macroscopic behavior, while other more microscopic properties would presumably be normal. Certain problems in working with glassy microemulsion systems will have to be carefully evaluated; e.g., when the matrix phase has the higher glass transition temperature, the dispersed phase will be thrown into tension (negative pressure) as T falls below the matrix Tgdue to the continued attempt of the dispersed phase to contract within a now-rigid matrix. In our case we estimate a 100-bar pressure differential could develop between the matrix Tgof 160 K and the o-xylene T of 127 K, a difference which could postpone the glass transition 0%the latter by some 1-2 K.6 Also, the importance of the nature of the microsystem’s boundaries will need careful evaluation. It is reasonable, though, to expect the study of these systems to lead to new insights on microstructure-related phenomena though initially, at least, this may come more from the identification of features (such as relaxation times and “secondary” relaxations) which are common to bulk and microscopic systems than from the converse. Returning to Figures 2 and 3, it is noteworthy that while neither emulsions 2 and 3 nor microemulsion 4 showed the presence of crystallization for xylene in their warm-up scans, there is, at >46% B along the line AB (Le. at 2W:lX 2S:lX), a rapid exothermic effect following immediately the glass transition for the aqueous phase. Since the aqueous phase is resistant to crystallization and since the subsequent fusion proved to be that of c-xylene, it appears that the nucleation probability for xylene in the turbid two-phase region of Figure 1 must become high at about the temperature that diffusion becomes easy in the aqueous phase. Presumably at this point separation of the admixed phases can proceed, enhancing the probability of heterogeneous nucleation of the oil “infecting” the whole system. Beyond the edge of the middle two-phase region the Tgbehavior changes. At 66.7% of B along the line AB, only one glass transition is seen, and it occurs between the PG.3H20 and o-xylene values. This clear phase still scatters light from a laser beam very strongly so is still microheterogeneous. Evaluation of its nature will require further work. The behavior of the pure 100% B (Le., 2S:lX) ”solution” during warm-up is unfamiliar. The large endothermic effect preceding crystallization has the correct direction for a glass transition, but the similarity ends there. Its interpretation is, however, not of importance to the main point of this paper which has been the identification and characterization of the oil-in-water microdroplet “finger” region. A more detailed study of this system and the properties of the microdispersed phase will be presented at a later time.
+
( 6 ) T. Atake and C. A. Angell, J . Phys. Chem., 83, 3218 (1979)
J. Phys. Chem. 1984, 88, 4596-4599
4596
An example of the interest potential of such microemulsions is the following. Molecular liquids may usually not be obtained in the glassy state unless the melting point is low relative to the boiling point, Tb/T,,, > 2.738 0-Xylene, with Tb/T,,, = 1.7, therefore, usually does not vitrify but can be vitrified in ordinary emulsion form both because of the suppression of the heterogeneous nucleation phenomenonlggand of the reduction in homogeneous nucleation fluctuation probability associated with the smaller system size?+10 The microemulsion state takes this latter
factor close to the limit since critical nuclei, which must at least exceed the crystal unit cell in dimensions, are of the order 10-20 A in diameter. Thus, vitrification of a much wider range of liquids should be observable in microemulsified form. As a first example we have been able to vitrify p-xylene which has a very unfavorable T b / T mratio, viz. 1.43. Further work along these lines will be reported in a future articlee2
Acknowledgment. The support of this work by a National Science Foundation under Grant No. DMR 8007053 is gratefully acknowledged. (9) B. J. Mason, Ado. Phys., 7, 22 (1958); 17, 71 (1945). (10) D. Turnbull and J. C. Fisher, J. Chem. Phys., 17, 71 (1949).
The Proton-Transfer Laser. Galn Spectrum and Ampllficatlon of Spontaneous Emission of 3-Hydroxyflavone P.Chou, D. McMorrow, T.J. Aartsma, and M. Kasha* Institute of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee, Florida 32306 (Received: July 27, 1984)
The efficient generation of amplified spontaneous emission (ASE) in 3-hydroxyflavonein methylcyclohexane and p-dioxane solutionsat 293 K is reported. This application of excited-state proton-transfer tautomerization approaches an ideal four-level laser system involving four different molecular electronic species in separate electronic states and constitutes a photoinduced chemical laser. The gain coefficient (a)for the ASE (530 nm) of 3-hydroxyflavone in methylcyclohexane (293 K) is calculated to be 10-15. Under similar conditions in our apparatus, a is observed to be in the range 7-9 for a proprietary coumarin laser dye (Molectron 70371-4 C485) and for rhodamine-6G. The tunable range for 3-hydroxyflavone is observed to be 518-545 nm. The peak laser power is comparable with that observed for the coumarin dye.
The Four-Level Proton-Transfer Laser The efficient generation of amplified spontaneous emission (ASE) in 3-hydroxyflavone (2-phenyl-3-hydroxychromone)via intramolecular proton transfer is reported here. This phenomenon was singled out in a recent discussion1 of four-level molecular electronic systems as an especially good candidate to act as an efficient laser “dye”. The rapidity of the excited-state protontransfer tautomerization (picoseconds),* the ease of achieving population inversion because the four levels each represent discrete molecular species with separate electronic states (Figure 1) (with the tautomer ground state at zero initial concentration), and the 10 000-cm-I band gap of the tautomer fluorescence band from the excitation band all serve to offer advantages over known dye lasers. The absence of primary interfering phenomena and the achievement of photostability in degassed systems in the 3hydroxyflavone case make it especially attractive as a molecular laser material. The first UV absorption band has maxima at 354 and 340 nm, suitable for excitation by a nitrogen laser, or by the third harmonic of the Nd:YAG laser, or by an excimer laser. The fluorescence of the tautomer at ca. 526 nm is the only emission observed at room temperature in liquid hydrocarbon solvent^.^.^ The intrinsic intramolecular excited-state proton-transfer rate SI (T, n*) SI’(n, T * ) (tautomer) has been measured to be less than 8 ps at room temperature in dry hydrocarbon solution.2 The tautomer excited state is populated with high quantum yield, and
-
(1) A. U.Khan and M. Kasha, Proc. Nutl. Acud. Sci. U.S.A., 80, 1767 (1983). (2) D. McMorrow, T. P. Dzugan, and T.J. Aartsma, Chem. Phys. Lett., 103, 492 (1984). (3) P. K. Sengupta and M. Kasha, Chem. Phys. Lert., 68, 382 (1979). (4) D. McMorrow and M. Kasha, J . Am. Chem. SOC.,105, 5133 (1983); J . Phys. Chem., 88, 2235 (1984).
0022-3654/84/2088-4596$01.50/0
this, combined with the zero population of the tautomer ground state Sd, leads to a relatively high gain factor of ASE. A schematic diagram for proton transfer in 3-hydroxyflavone in a four-level laser scheme is given in Figure 1, The lack of a temperature dependence for this proton transfer2v4in the temperature range 298-77 K indicates the absence of a significant energy barrier for excited-state proton transfer. The reverse proton transfer from the tautomer to the normal molecule ground state is found by our study also to be very rapid, with a barrier at 298 K of certainly less than k T . Experimental Section 3-Hydroxyflavone (Aldrich Chemicals) was recrystallized twice from methanol and then once from methylcyclohexane. Purity was confirmed by fluorescence excitation spectra. The spectral grade methylcyclohexane (J. T. Baker) and p-dioxane (Matheson Coleman and Bell) were used as received. A schematic diagram for the experimental arrangement is shown in Figure 2. The 3-hydroxyflavone was excited with the 337.1-nm output from a pulsed nitrogen laser (Molectron, UV-24) with up to 10 mJ/pulse and a pulse duration of 10 ns. The output of the nitrogen laser was focused to a narrow line in the dye cell in a transverse geometry. A laser dye cell of 0.6-cm optical length (Molectron DL 25 1) was used, which is designed to prevent optical feedback from the windows. The dye solution was stirred to prevent secondary processes (local heating, triplet population, etc.) from interfering with the experiments. The amplified spontaneous emission (ASE) was detected by an optical multichannel analyzer (OMA) system consisting of an intensified silicon photodiode array (EG&G/PAR, Model 1420), coupled to a 0.25-m polychromator, and analyzed by a system processor (EG&G/PAR, Model 1215). To reduce the noise, the detector was operated in a gated mode synchronously with the 0 1984 American Chemical Society
