Article pubs.acs.org/JPCC
Photophysical and Photochemical Studies of Liquids Below Their Freezing Points J. K. Thomas* Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556, United States ABSTRACT: Photophysical and photochemical methods are used to investigate liquids below their melting point. In benzene, immediately below the freezing point the rate of decay of pyrene fluorescence increases dramatically. The k increases to some 220- to 230-fold compared to that of fluid solution. With pyrene, the increased rate of decay is associated with excimer formation and with dimethyl aniline with exiplex formation. Decreasing the temperatures from the freezing point to 77 K initially gives rise to an increase in k which then decreases gradually to 77 K. Spectroscopy shows that in the frozen medium the pyrene environment is that in liquid benzene. The data point to the fact that on freezing all the benzene does not freeze, a small 0.2% remaining fluid. On freezing the pyrene is forced from the bulk crystalline phase to the liquid zones, thus increasing its concentration and rate of excimer formation. Cyclohexane initially shows similar behavior, but further cooling rapidly decreases the rate of fluorescence decay and increases the rate of excimer formation. In the case of water, the sodium salt of sulfuric acid of pyrene (PSA) was used due to the low solubility of pyrene in this liquid. Freezing this system immediately removed the PSA. This is attributed to the low solubility even of the acid in water. Addition of small (mM) amounts of ethanol to the system produced data similar to those in benzene. It is suggested that, on freezing water, pools of ethanol are formed and that the PSA is forced from the water to the ethanol. The formation of an epoxy polymer from its constituents is very slow when diluted in benzene at 22 °C. However, the same system polymerizes rapidly at 0 °C, and a variety of particle structures in the micrometer region are obtained.
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in porous glass7 are in agreement with eq 1. However, other studies in porous silica8 indicate that an alternative formulation, eq 2, gives a better fit of theory and experiment.
INTRODUCTION There are several reports1−5 stating that the rates of certain chemical reactions show a marked increase on freezing the solvent containing the reactants. The general conclusion is that, on freezing, the reactants are confined to a smaller liquid-like portion of the frozen solid. The increased reactant concentration thus promotes more rapid reaction. It is a well-known colligative property that solutes lower the melting point. As the solvent freezes the solute is ejected into a smaller volume of solvent, thus increasing the local solute concentration and leading to pockets of liquid within the solid frozen structure. An alternative suggestion is that on freezing a small portion of the solvent does not freeze but forms microliquid domains. The reason for the small droplets or domains is discussed below. In terms of the Gibbs−Thomson6 treatment, small droplets have a lower freezing point than bulk liquid. The melting point depression ΔTm of the liquid is then given by eq 1. ΔTm = Tm − Tm(d) =
4αTm ΔHf (p)d
ΔTm =
(2)
where r is the radius of the droplets and A and B are constants. In both studies porous solids are used, and the droplet diameters are assumed to be synonymous with those of the pores. It is stressed that the works quoted are taken from a large body of work of similar vein by the two research groups. A formulation of eq 2 is given by Quirke,9 who took into account the variation of the free energy with droplet size as Δγ, defined as γs − γe. Here γs and γe are the surface free energies of the solid and liquid phase, respectively. He showed the decrease in melting point with size is given by eq 3. ΔT = A′N −1/3 − B′N −2/3
(3)
where A′ and B′ are constants and N is the number of molecules in the droplet. As the radius of the droplet r varies with N1/3, an equation similar to 3 is obtained.
(1)
Here Tm is the normal melting point of bulk material; Tm(d) is the melting point of the small droplets; ΔHf is the bulk enthalpy of fusion; ρ is the density of the solid; α is the solid interface energy; and d is the droplet diameter. Studies of the melting points of benzene and other solvents as small droplets © 2013 American Chemical Society
A B − 2 r r
Special Issue: Michael Grätzel Festschrift Received: September 25, 2013 Revised: December 5, 2013 Published: December 12, 2013 16380
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In this study, photophysical and photochemical studies are used to investigate frozen solvents and link the lowering of the melting point in small droplets to increased reactivity. The study is akin to similar studies in micelles10 and polymer systems.11 The unique difference between those studies and the earlier work references1−5 is that only small amounts (10−6 m/ L) of solute are required compared to the much larger amounts (10−2−0.1 m) used previously. In this study, this rules out a simple colligative solute depression of the freezing point.
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EXPERIMENTAL SECTION The experimental procedures are similar to those used previously.10 Cyclohexane (Chromosolv Plus ≥99.9% purity) and benzene (Chromosolv Plus ≥99.9% purity) were obtained from Sigma Aldrich. Ethanol, 200 proof, was obtained from KopTec. Water was deionized to a resistance of 15.5 MΩ. Ultradry nitrogen (99.99%) was obtained from Praxair. The epoxy polymer was a Hardman product and is called Double/ Bubble Epoxy, Extra Fast Setting. It is a kit made by Royal Adhesives and Sealants, LLC (Wilmington, CA). Sample Preparation. Samples with various solute concentrations were from solvents of the highest purity, and further purification by distillation did not change the results. Solutes were obtained from Aldrich and were of the highest purity available. The samples were contained in quartz or silica tubing which was in the shape of a closed cylinder of 3 mm diameter. The size of the sample was 0.25 cc. The samples were deoxygenated by prolonged (15 min) bubbling with ultrapure nitrogen.11,12 The top of the cylinder terminated in a ground glass joint which was closed by a greased ground glass stopper. It is pertinent to note that no fluorescence was observed from the pure solvents. Pulsed Studies. Samples were excited with a 1 ns pulse of 347.1 (1 mJ) radiation from a PTI PL230A N2 laser. The emission from the sample was directed through an Oriel monochromator and detected with a Hamamatsu channel plate detector. The signal from the detector was captured by a Techtronix digitizer. The whole response time of the system (10−90%) was measured by reflected laser light to be 2 ns. The samples, first chilled to 77 K, were maintained at a selected temperature in a Dewar, through which cold N2 was passed. Steady State Fluorescence Measurements. Steady state fluorescence spectra were recorded on a SLM-Aminco spectrofluorimeter, with an excitation wavelength of 337 nm and with a band-pass of 2 nm on the excitation channel and 1 nm on the emission channel. Analysis of Data. Data were fitted by a program for first- or second-order plots, or if this was not a good fit of data and experiment, a Gaussian program was used.13 This treatment has been successfully employed for time-resolved emission studies of pyrene on silica14 and alumina.13,14 The data are fitted with eq 4. A = π −1/2 A0
+∞
∫−∞
exp( −x 2)exp[−kt exp(γx)]dx
Figure 1. Gaussian and first-order fits of the pyrene fluorescence decay in cyclohexane at 77 K. Pyrene concentration was 4.25 × 10−6 M.
Gaussian fit is far better than that of a single first-order process. The rate constants are similar, 4.08 × 10−6 s−1 and 6.67 × 10−6 s−1. Either kinetics was selected according to the quality of the calculated fit. The deviation from simple kinetics increased at lower temperatures in the solids. Photophysical Studies of Pyrene in Frozen Benzene. Both the fluorescence fine structure and the kinetics of excited pyrene have been studied in benzene at low (typically 10−6 M) solute concentrations. Figure 2 shows the fluorescence spectra of 10−6 and 3 × 10−5 M pyrene in benzene liquid at 22 °C and in the solid at at 77 K and 0 °C. At low concentration only singlet emission of P is observed at 22 °C, and the fine structure ratio, III/I, of the third peak to that of the first is 0.92 in the liquid at 22 °C and 0.7115 in the solid at 0 °C.13 The fine structure ratio is temperature dependent,16 and using the correction of this reference, the extrapolated III/I ratio is 0.71 in benzene at 0 °C and in good agreement with that at 22 °C. This shows that in frozen benzene the environment of pyrene is identical at 22° and 0 °C, while some P2* is observed at 0 °C. At high pyrene concentration the rate of P* decreases more rapidly due to reactions 5, 6, and 7. P + P* → P2*
(5)
P2* → P + P*
(6)
P2* → P + hv
(7)
−5
At 3 × 10 M pyrene the fluorescence at 0 °C also contains marked excimer emission. At this concentration a little excimer emission is also observed at 25 °C in the liquid benzene. Figure 3 illustrates the time dependence of both P* and P2* at 0 °C and at 3 × 10−5 M. The rate of decay of P* is much faster at 0 °C compared to 22 °C, a fact linked to the concomitant growth of P2* at 0 °C. The excimer is formed after formation of the monomer fluorescence and after the end of the pulse. These data show that P2* is formed dynamically at 0 °C, apparently in a liquid state of benzene. This indicated that benzene does not completely solidify on freezing but forms small liquid regions. The pyrene is ejected from the solid benzene phase to these small liquid phases, thus enhancing its local concentration.
(4)
where k is the average rate constant and γ is width of the distribution. The treatment indicated that the pyrene in the pools experiences a variety of conditions, in particular size. Figure 1 illustrates the two kinetic fits in 4.25 × 10−6 M cyclohexane at −196 K. It is immediately obvious that the 16381
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Figure 4. Change in the rate of fluorescence decay versus temperature for 10−5 M pyrene in benzene.
rate of decay of P* is due to the concentration effect indicated earlier, where the pyrene is forced into small pools of benzene. At about −20 °C the decay rate slows and exhibits a continuous decrease to −100 °C; thereafter, the decay rate is reasonably constant. These data indicate that below the freezing point the effective pyrene concentration increases, while data in Figure 2 show that pyrene is still in solution in benzene. It is concluded that as the benzene progressively freezes with lowering temperature a variety of small liquid zones are formed. Further decrease of the temperature leads to a freezing of the larger liquid benzene zones and to a further increase in the effective pyrene concentration. Below −100 °C the whole sample is frozen, and quenching of excited pyrene by its ground state ceases. Quenching Studies. Figure 5 shows quenching studies of pyrene fluorescence by pyrene, and similar studies are found for dimethyl aniline in benzene at 0 °C. The reactions occur in the small liquid pools in the solid benzene. The kinetics follow those observed in fluids; i.e., the rate of decay of pyrene fluorescence increases linearly with quencher concentration.
Figure 2. Fluorescence spectrum of pyrene in benzene, bubbled with N2, at 0 °C, 22 °C, and 77 K. (a) 3 × 10−5 M pyrene and (b) 10−6 M pyrene.
Figure 3. Fluorescence and excimer emission versus time in 3 × 10−5 M pyrene in benzene. The solution was bubbled with N2.
Figure 4 shows the variation of the decay of P* (to give P2*) with temperature. In the liquid the rate of decay of P* decreases with decreasing temperature, as expected from reactions 5, 6, and 7. At the freezing point of benzene the rate of decay of P* increases dramatically, and P2* is formed again via reactions 5, 6, and 7. So in the liquid phase down to 5 °C the decrease in the rate of decay of P* is due to Arrhenius behavior of reactions 5, 6, and 7. Below the freezing point, the rapid increase in the
Figure 5. Rate of decay of pyrene fluorescence in various solutions of pyrene in benzene plotted versus pyrene concentration at 0 °C. 16382
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The data show a variety of liquid sites in frozen benzene when the polymerization proceeds. Structures as small as 1 μm were observed; smaller structures could not be identified on our microscope. The important point is that concentration enhancement at lower temperatures is necessary for epoxy polymerization. Events in Cyclohexane. Figure 7 shows the rate constant, k, of excited pyrene in cyclohexane versus temperature in a
The low laser power produces excited pyrene at a concentration that is much less than that of the quencher, and pseudo firstorder kinetics are observed; i.e., the rate of decay of P* increases linearly with quencher concentration. The slopes of the quenching plots in Figure 4, together with the bulk solute concentration, give bimolecular rate constants which are 3.3 × 1012 L M−1 s−1 for pyrene and 2.2 × 1012 L M−1 s−1 for quenching by dimethyl aniline. In liquid benzene at 20 °C, the bimolecular quenching rate constant is 7.2 × 109 L M−1 s−1 for pyrene and 6.0 × 109 L M−1 s−1 for dimethyl aniline. To equate the data in frozen benzene at 0 °C with that in the liquid state at 0 °C, it is necessary to state that the solute concentrations at 0 °C are increased by a factor of 300 for dimethyl aniline and 450 for pyrene compared to those in the liquid at 20 °C. Overview of Benzene Data. The above studies show that only 99.75% of liquid benzene freezes at its freezing point, while 0.25% remains liquid. On freezing any solutes in the liquid benzene, the crystalline benzene ejects the solute to the small liquid zones. This leads to an effective increase in the solute concentration and to any bimolecular reactions of the solute. Decreasing the temperature to well below the natural freezing point initially leads to a further increase in the bimolecular rate as larger pools freeze, ejecting their solute into the smaller pools. At still lower temperatures the small liquid droplets also freeze, and bimolecular reaction ceases, eventually disappearing at 77 K. The spectroscopy of pyrene shows that the liquid droplet zones of low temperature are indeed “benzene like”. The products of reaction of excited pyrene with ground-state pyrene are the excimer of pyrene and the exiplex in the case of dimethyl aniline. Polymerization in the Liquid Center. In an attempt to “image” the liquid centers, epoxy resin Hardman was polymerized in benzene, at 0.1% concentration. At room temperature, in liquid benzene down to its freezing point no polymerization was observed over 24 h. In solid benzene at 0 °C and −20 °C significant polymerization was observed over 24 h. Here, at −20 °C the white epoxy resin separated out as a fine heterogeneous mixture. Figure 6 shows a picture of an
Figure 7. Fluorescence decay of 10−5 M pyrene in benzene, bubbled with N2, versus temperature.
system that was deoxygenated with N2. The rate constant for decay decreases with temperature to the freezing point. At temperatures immediately below the freezing point (5 °C) a marked increase in k is observed followed by a sharp drop at 2.5 °C. Thereafter, the decay constant, calculated via the Gaussian formulation, is reasonably constant to 77 K, with a constant for the width of the Gaussian of 1.7, which also is constant in this temperature range. The small rise in k at about −90 °C correlates with a reported phase change in cyclohexane. This system is similar to benzene, i.e., a sharp rise in k at lower temperatures, and the rate constant reaches a higher value in benzene, 9.3 × 10−6 s−1, compared to that in cyclohexane, 3.5 × 106 s−1. The data suggest that small liquid pools of solvent containing pyrene are found just below the freezing point of cyclohexane. Further cooling rapidly freezes these liquid domains. Events in Water. The probe selected for water is the sodium salt of pyrene sulfonic acid, PSA. This probe, in contrast to pyrene, has a significant solubility in water. Low concentrations of the probe (10−6 M) in water at 20 °C exhibit fluorescence typical of pyrene derivatives. On freezing the solution, the pyrene-like fluorescence is lost and replaced by a broad, weak emission associated with solid PSA. These data indicate that water freezes completely at 0 °C and that NaPS crystallizes out of solution. However, data similar to that in benzene may be produced by including small amounts (3 × 10−3 to 2 × 10−2 M) of ethanol into the benzene. Freezing at 0 °C then produces ice and small packets of liquid containing PSA. The fluorescence of PSA is then clearly observed in the ice−ethanol system. Figure 8 shows the quenching of NaPSA fluorescence by nitrobenzene in the ethanol pools at 3 × 10−3 and 2 × 10−2 M
Figure 6. Microsope picture of epoxy polymer at 200× magnification: (a) polymer bulk and (b) teased portion.
optical microscope at 200 magnification of the solid polymer with both fibrous and threadlike structures. Teasing the polymer with a fine probe separated out some cylinder-like structures with dimensions in the tens to micrometer range. Microscopy with polarizers showed that these structures had a high degree of order or crystallinity. Polymerization of the pure epoxy components at 20 °C produces a bland solid of no structure. 16383
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Figure 8. Decay rate of fluorescence in sodium salt of 10−6 M pyrene sulfonic acid in water, bubbled with N2, versus concentration of nitrobenzene.
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1. Established in three quite different liquids the conditions where small zones of liquid are maintained in the frozen system. 2. It is clearly shown that in benzene and cyclohexane this is due to imperfections of the crystal. Structures that lead to small voids where the liquid has a lower freezing point. 3. In water it is necessary to include a solute that reduces the freezing point via a colligative effect. Increasing the solute does not enlarge the liquid zones but increases their frequency. 4. However, in benzene no added solute at high concentration is needed to produce the effect. 5. The studies comment on the amount of benzene frozen and provide an initial picture of the unique liquid zones of the frozen solid. 6. Further temperature studies show the heterogeneous nature of the freezing process by Gaussian kinetics and via the slow freeing of the smaller zones at lower temperature.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
ethanol. The rate constant at 0 °C for the above quenching is 1.12 × 10 L M−1 s−1 in 3 × 10−3 M ethanol. If the reactants are confined to the ethanol pools then the true bimolecular rate constant is 1.7 × 107 L M−1 s−1 in 3 × 10−3 M ethanol and 1.8 × 107 L M−1 s−1 in 2 × 10−2 M ethanol. This is considerably smaller than the quenching rate constant in water at 20 °C, which is 1010 L M−1 s−1. This lowered rate constant in the frozen system suggests considerable obstruction to movement in this system. The true quenching rates at the two ethanol concentrations in Figure 7 are in agreement which suggests that on freezing ethanol−water mixtures increasing the alcohol content increases the extent of the alcohol zones and does not significantly alter their configuration or size. An increase in size of the ethanol pools would change their structure, making them less viscous and therefore increasing the quenching rate constants. Unlike benzene, the much lower rate constant for quenching in the frozen water/ethanol mixture can only be explained by a significant restriction placed on the reactants by the artificially created liquid centers.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author would like to thank the Chemistry Department at Notre Dame for support of this work and Dr. D. Miller for much technical support and Dr. A. Oliver for microscopy support. Professor T. Fehlner is also thanked for drawing the attention of the author to these effects, and Dr. G. Hartland and Dr. M. Prorok for much help with the computer.
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REFERENCES
(1) Butler, A. R.; Bruice, T. C. Catalysis in Water and Ice. A Comparison of the Kinetics of Hydrolysis of Acetic Anhydride, βPropiolactone, and p-Nitrophenyl Acetate, and the Dehydration of 5hydrol-6-hydroxy-deoxyuridine in Water and in Ice. J. Am. Chem. Soc. 1964, 86, 313−319. (2) Bruice, T. C.; Butler, A. R. Catalysis in Water and Ice II. The Reactions of Thiolactones with Morpholine in Frozen Solution. J. Am. Chem. Soc. 1964, 86, 4104−4108. (3) Pincock, R. E.; Kiovsky, T. E. Bimolecular Reactions in Frozen Organic Solution. J. Am. Chem. Soc. 1965, 87, 2072−2073. (4) Pincock, R. E.; Kiovsky, T. E. Reactions in Frozen Solution II. Base Catalyzed Decomposition of t-Butylperoxy Formate in Frozen pXylene. J. Am. Chem. Soc. 1965, 87, 4100−4107. (5) Pincock, R. E.; Kivosky, T. E. Reactions in Frozen Solution III. Methyl Iodide with Triethylamine in Frozen Benzene Solutions. J. Am. Chem. Soc. 1966, 88, 51−55. (6) Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; J. Wiley: New York, 1966. (7) Jackson, C. L.; McKenna, G. B. The Melting Behavior of Organic Molecules Confined in Porous Solids. J. Chem. Phys. 1990, 93, 9002− 9011. (8) Dutta, D.; Pujari, P. K.; Sudashan, K.; Sharma, S. K. Effect of Confinement on the Phase Transition of Benzene in Nanoporous Silica. A Positronium Annihilation Study. J. Phys. Chem. 2008, 112, 19055−19060. (9) Quirke, N. The Microcrystal Melting Transitions. Mol. Simul. 1988, 1, 249−270. (10) Gratzel, M.; Thomas, J. K. The Application of Fluorescence Techniques to the Study of Micellar Systems. Mod. Fluoresc. Spectrosc. 1976, 2, 169−213.
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CONCLUSION The studies show that in benzene, and to some extent in cyclohexane, but not in water, freezing leads to incomplete solid formation with a small percent (0.2%) of the system remaining liquid. This effect agrees with the concept of Gibbs and Thomson that small pools of liquid exhibit lower freezing points that their large bulk counterparts. Water can be made to emulate “benzene” effects if a low-freezing component, e.g., ethanol, is included in the system. The data fully illustrate earlier work of increases in the rates of chemical reaction in solvent as they freeze. The varied behavior of benzene to water is suggested to be due to the different crystal structures in these liquids. Further studies are underway, on other liquids, to find a correlation of the Gibbs−Thomson effect with crystal structure. Using phototechniques, the outcomes of this work are as follows: 16384
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(11) Chu, D. Y.; Thomas, J. K. Characterization of Polymers by Excited States; CRC Press: Boca Raton, 1991; Vol. 3, pp 49−102. (12) Thomas, J. K. The Chemistry of Excitation at Interfaces; ACS Monograph: Washington, D.C., 1984; p 181. (13) Krasnansky, R.; Koike, K.; Thomas, J. K. Gaussian Approximation to the Unique Heterogeneous Langmuir-Hinshelwood Fluorescence Quenching at the Silica Gel/Gas/Solid Interface. J. Phys. Chem. 1990, 94, 4521−4528. (14) Pankasem, S.; Thomas, J. K. Pyrene, Pyrene Derivatives and 1,1′-Binaphthyl as Luminescent Probes for Photophysical Studies of Alumina Surfaces. J. Phys. Chem. 1991, 95, 7385−7393. (15) Kalyanasundaram, K.; Thomas, J. K. Environmental Effects on Vibronic Band Intensities in Pyrene Monomer Fluorescence and the Application in Studies of Micellar Systems. J. Am. Chem. Soc. 1977, 99, 2039−2044. (16) Kavanagh, R. PhD Thesis, University of Notre Dame, 1997.
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