ARTICLE pubs.acs.org/JPCA
Isothermal Evaporation of Ethanol in a Dynamic Gas Atmosphere Adriyan S. Milev,*,† Michael A. Wilson,†,‡ G. S. Kamali Kannangara,† Hai Feng,† and Phillip A. Newman† † ‡
School of Natural Sciences, University of Western Sydney, Penrith, NSW 2751, Australia CSIRO Earth Sciences and Resource Engineering, Riverside Corporate Park, 11 Julius Avenue, North Ryde, NSW 2113, Australia ABSTRACT: Optimization of evaporation and pyrolysis conditions for ethanol are important in carbon nanotube (CNT) synthesis. The activation enthalpy (ΔHq), the activation entropy (ΔSq), and the free energy barrier (ΔGq) to evaporation have been determined by measuring the molar coefficient of evaporation, kevap, at nine different temperatures (3070 °C) and four gas flow rates (25200 mL/min) using nitrogen and argon as carrier gases. At 70 °C in argon, the effect of the gas flow rate on kevap and ΔGq is small. However, this is not true at temperatures as low as 30 °C, where the increase of the gas flow rate from 25 to 200 mL/min results in a nearly 6 times increase of kevap and decrease of ΔGq by ∼5 kJ/mol. Therefore, at 30 °C, the effect of the gas flow rate on the ethanol evaporation rate is attributed to interactions of ethanol with argon molecules. This is supported by simultaneous infrared spectroscopic analysis of the evolved vapors, which demonstrates the presence of different amounts of linear and cyclic hydrogen bonded ethanol aggregates. While the amount of these aggregates at 30 °C depends upon the gas flow rate, no such dependence was observed during evaporation at 70 °C. When the evaporation was carried out in nitrogen, ΔGq was almost independent of the evaporation temperature (3070 °C) and the gas flow rate (25200 mL/min). Thus the evaporation of ethanol in a dynamic gas atmosphere at different temperatures may go via different mechanisms depending on the nature of the carrier gas.
rate, k, to temperature, ΔG q , ΔS q , and ΔH q : ! kB T ΔGq exp k ¼ ktrans h RT ! ! kB T ΔSq ΔH q exp ¼ ktrans exp h R RT
1. INTRODUCTION Evaporation is a universal phenomenon important in a wide range of physical and chemical processes. The evaporation of ethanol in a carrier gas is important in many chemical reactions and specifically of interest to us in preparing new forms of synthetic carbon such as nanotubes.1,2 Evaporation can be carried out in the absence of other gases or in the presence of a carrier gas, the rate of flow of which can be varied. It is an activation process in that the molecules must gain energy to evaporate,35 and they must have a source of energy for this process. The amount of this energy may alter the nature of species in the evaporated state and thus the mechanism. The evaporation mechanism can be understood within the frameworks of transition state theory (TST), which is based on a quasi-equilibrium steady state process. The transition state (or activated complex) is an unstable transitory combination of reactant molecules that occurs at a saddle point on a multidimensional potential surface. The saddle point can be described as a maximum in only one dimension (along the reaction coordinate) and minimum in all the others. 5 The reaction rate k is defined as the rate at which the activated complex crosses the saddle point defined by ΔG q in order to form products. 35 The entropy ΔS q and enthalpy ΔH q of activation can also be determined to elucidate the mechanism since the energy barrier, ΔG q , reflects both enthalpic and entropic contributions. TST leads to the Eyring equation, eq 1, which relates the reaction r 2011 American Chemical Society
ð1Þ
where kB is the Boltzmann constant, h is the Planck constant, and ktrans is a dimensionless transmission coefficient (the probability that the activated complex will pass through the transition state to form product). The parameter kBT/h represents the “speed” of barrier crossing (barrier-crossing frequency). During evaporation, the reaction rate kevap can be measured by thermogravimetry and should be of zero-order kinetics mechanism provided that the surface area of evaporation remains unchanged.68 However, it is possible that the liquid ethanol temperature, nature of the carrier gas, and gas flow rate might affect ΔGq, ΔSq, and ΔHq if the nature of the activation complex can be dependent on weak interactions of ethanol molecules with each other. In this work, the evaporation rates were measured directly from the weight loss in the linear region of the mass-loss profiles at a set of isothermal temperatures under regulated argon Received: June 5, 2011 Revised: November 23, 2011 Published: December 02, 2011 150
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Table 1. Total Mass Losses during the Last 30 min of 1 h Isothermal Heating total mass loss (mg) over 30 min period 25 mL/min
50 mL/min
100 mL/min
200 mL/min
temp (°C)
temp (K)
Ar
N2
Ar
N2
Ar
N2
Ar
N2
30
303
11.1735
128.2708
16.2588
154.3065
19.9505
179.214
64.0352
196.885
35
308
17.2127
158.3029
24.4888
205.3043
28.9756
216.2523
96.8624
220.8044
40
313
31.1277
195.0729
37.2227
263.4974
46.3181
237.1079
127.7379
276.3512
45
318
55.5169
253.0352
61.4268
307.7701
76.0615
297.6471
169.1088
329.6462
50
323
90.3991
265.4497
99.9568
351.5502
142.8564
372.273
222.4417
382.3608
55
328
141.0388
266.1647
159.0726
476.7338
201.8625
433.321
306.2821
462.3263
60
333
237.9753
352.2809
260.5931
476.7384
327.7869
536.2284
409.8148
554.5681
65 70
338 343
349.324 502.2519
444.8608 591.118
360.8857 509.1365
567.3665 635.4745
433.7815 625.8624
628.424 703.8459
504.8699 716.2241
641.5765 702.1541
the reaction rate k to temperature.3,5,9 kevap kB ΔH q R ln ln lnðktrans Þ ¼ ΔSq T h T
or nitrogen as carrier gases. These measurements were supplemented by spectroscopic analysis of the ethanol molecules in the vapor state, and the results are interpreted in terms of collisions of ethanol vapors with carrier gas molecules.
ð3Þ
where ktrans is a transmission coefficient usually assigned to unity; i.e., it is assumed that the transition state always proceeds to products and never reverts back to reactants; kB is the Boltzmann constant, h is Planck’s constant, and R is the gas constant. Values of both entropic ΔSq and enthalpic ΔHq contributions to the activation barrier of evaporation, ΔGq, were determined by linear regression of the left-hand side of eq 3 on 1/T, which gives ΔHq from the slope and ΔSq as the ordinate intercept. The determination of the entropy of activation by eq 3 could be sometimes unreliable because it is calculated from the ordinate intercept by a large extrapolation to 1/T = 0; i.e., T = ∞.10 Therefore, if the linear fit of [ln ktrans + ln(kB/T) + ΔSq/R] vs 1/T is not very good, the error in ΔSq can be quite large. We propose another linear form of eq 3, which allows the determination of ΔSq from the slope rather than from the ordinate. It is derived simply by multiplying both sides of eq 3 by the temperature T: kevap kB RT ln ln lnðktrans Þ ¼ TΔSq ΔH q T h
2. EXPERIMENTAL DETAILS Isothermal Mass-Loss Measurements. The apparatus used for evaporation rate measurement was a NETZSCH Jupiter 449C thermoanalyzer equipped with a 107 g resolution balance. A round-bottom cylindrical Al2O3 crucible with a volume of ∼3.5 mL and an internal diameter of 1.6 cm was used for the evaporation rate measurements. Argon with purity >99.999% or nitrogen with purity >99.999% was used as the carrier gas and the gas flow rate was maintained within (1 mL/min for all experiments. The buoyancy effect was minimized by collecting and subtracting the mass changes of the empty crucible under the same temperature, gas type, and gas flow conditions. The evaporation rate measurements of ethanol (>99.9% purity) were carried out in the isothermal mode having experimental temperatures of the liquid ethanol varied from 30 to 70 °C in 5 °C increments. The ethanol samples were heated from 25 °C to the selected temperature, and then the sample was held at the respective temperature, gas type, and gas flow rate for 60 min. The experiments were repeated at different gas flow rates of 25, 50, 100, and 200 mL/min, respectively. For each experiment, the initial mass of the ethanol sample was about 2.5 g. The measurement conditions and the mass losses are summarized in Table 1. Data Analysis. The mass-loss data were analyzed in terms of weight m plotted against time t at constant temperature. The isothermal evaporation rate kevap was determined from the isothermal mass-loss data according to 1 dm ð2Þ kevap ¼ aM dt T
ð4Þ where the slope of the linear fit in coordinates RT[ln(k/T) ln(kB/T) ln(ktrans)] vs T gives ΔSq, and the ordinate intercept gives ΔHq. Infrared (IR) Spectroscopy of Vapor Phase. To investigate the molecular state of the evolving vapors produced during the isothermal heating experiments, the thermoanalyzer was connected to a temperature-regulated IR gas cell (120 mm long with an internal diameter of 8 mm) via a 0.8 m long, heated transfer line with an external diameter of ∼3.5 cm and an internal diameter of 2 mm. The IR spectra were collected by a Bruker Vertex 70 IR spectrometer through a 1 mm aperture by a liquid nitrogen cooled IR detector. The spectral resolution was 2 cm1. To maintain a constant temperature of the ethanol vapors during IR spectra collection, the temperature of the tubular crucible containing liquid ethanol was measured by a thermocouple attached to the bottom of the crucible. Also, the temperature of the vertical tube of the thermogravimeter was measured and manipulated with the help of another thermocouple. Three more
where dm/dt is the rate mass loss (g s1) with the time per unit area, a (cm2), and M is the molecular weight (g mol1). The isothermal molar evaporation rate kevap (mol s1 cm2) for each temperature and gas flow rate was calculated by dividing the experimental mass loss of the sample by the molecular weight of ethanol, 46.068 g mol1 (assumed to be a monomer). The kevap data were used to estimate the enthalpies and entropies of activation in coordinates given by eq 3, which relates 151
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As is evident from Figure 1 and Table 1, the nature of the purging gas has a strong effect on the evaporation process. At 30 °C kevap in nitrogen is approximately 10 times higher than that in argon, but the difference decreases with the temperature, and at 70 °C the gas type has minimal effect on the evaporation rates. The gas flow rate also modulates the evaporation process, though its effect is more significant for the evaporation in argon. Lower argon flow rates (25 and 50 mL/min) have little effect on kevap in the whole temperature interval, but increase of the gas flow rate to 200 mL/min increases kevap significantly. While a similar tendency is observed for evaporation in nitrogen, the effect is much smaller compared to the evaporation dynamics in argon. This suggests that ethanol evaporation kinetics is modulated not only by the temperature and gas flow rate but also by the nature of the carrier gas. Values of ΔHq and ΔSq from kevap. The kevap data were used to estimate the enthalpies and entropies of activation by linear regression in coordinates given by eqs 3 and 4. The temperature/ gas flow rate dependences of the activation parameters are shown in Figure 2 AC, and the results are summarized in Table 2. The comparison between the activation parameters derived according to these two equations demonstrates small differences between ΔSq and ΔHq values; therefore, we conclude that the errors related to the determination of kevap from the isothermal mass-loss data are small. The activation enthalpy of evaporation in argon values show that at 30 °C the increase of the gas flow rate from 25 to 200 mL/ min decreases ΔHq from ∼82 to ∼48 kJ/mol, respectively. Simultaneously, the ΔSq at 25 mL/min gas flow rate is negative (113 J/(mol K)) and becomes more negative (209 J/(mol K)) with the increase of the gas flow rate to 200 mL/min. Unlike in argon, the activation parameters of evaporation in nitrogen show no significant variations on the gas flow rate (Table 2 and Figure 2 C,D). The enthalpy barrier to evaporation in nitrogen is much lower than that in argon and is ∼27 kJ/mol, whereas ΔSq is about 270 J/(mol K). The 3 times lower activation enthalpy to evaporation, ΔHq, could explain the much higher rates of evaporation in nitrogen at 30 °C than in a dynamic argon atmosphere. The large negative entropies of activation suggest a more ordered structure in the transition state than in the ground state for both nitrogen and argon and also more so at high gas flow rate in argon. In other words, the system loses degrees of freedom with the increase of gas flow rate possibly due to restriction of the motion of the ethanol molecules in the transition state. This rather surprising result is understandable if, in the transition state, two or more molecules are strongly bound to each other via intermolecular interactions differently from in the liquid state. Energy Barrier to Evaporation, ΔGq. The free energies of activation, ΔGq, in the temperature interval 3070 °C were determined according to ΔGq = ΔHq TΔSq. In nitrogen, the gas flow rate has little effect on the energy barrier to evaporation (Figure 3 A). At 30 °C the increase of the nitrogen rate from 25 to 200 mL/min slightly decreases ΔGq from ∼109.6 to ∼108.6 kJ/mol. However, under the same conditions the argon purging rate modulates the energy barrier to evaporation significantly. At 30 °C, the decrease of ΔHq (favorable) is almost fully compensated by negative and decreasing ΔSq (unfavorable) for purging rates up to 100 mL/min (Figure 3 B). That is, the contribution of ΔSq to ΔGq effectively cancels out the energy benefit from the decreasing activation enthalpy. The increase of the argon gas flow rate to 200 mL/min, however, reduces ΔGq due to significant
Figure 1. Temperature and gas flow rate dependence of the evaporation coefficient, kevap, in (A) argon and (B) nitrogen.
thermocouples were used to monitor and regulate the temperature along the length of the gas transfer line and the IR gas cell. During each of the isothermal heating steps the evolving ethanol vapor was collected by the stream of the regulated gas and transferred to the IR gas cell. To stabilize the ethanol evaporation rate, the samples were maintained at the respective temperature and gas flow rate for 20 min and then the IR spectra were collected. For example, when the evaporation was carried out at 30 °C, that temperature was maintained along the pathway of the evalved vapors, from the cylindrical crucible through the transfer line and during IR spectra acquisition. Similarly, when the evaporation temperature was 70 °C, the temperature along the pathway and in the IR gas cell was kept at 70 °C.
3. RESULTS AND DISCUSSION Isothermal Evaporation Rates as a Function of Temperature, Gas Type, and Gas Flow Rate. Table 1 shows the mass-
loss data collected during isothermal heating at nine different temperatures ranging from 30 to 70 at 5 °C increments and four gas flow rates ranging from 25 to 200 mL/min and with two carrier gases: nitrogen and argon. The isothermal measurements showed that after initial temperature and mass-loss fluctuations, which typically lasted less than 25 min, the sample temperature remained constant ((0.1 °C) and the weight of the sample decreased linearly at each temperature and gas flow rate. The linear mass loss indicates that the evaporation of ethanol follows expected zero-order kinetics during the final stages (3035 min) of the isothermal experiments. The temperature and gas flow rate dependences of the respective molar evaporation rates, kevap, in units of mol s1 cm2, calculated according to eq 2, are shown in Figure 1. 152
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Figure 2. Determination of activation parameters of evaporation as a function of temperature and gas flow rate in argon (A, B) and in nitrogen (C, D). (A) ΔHq was determined from the slope, whereas ΔSq was determined from the Y-intercept according to eq 3. (B) Activation entropy of evaporation, ΔSq, was determined from the slope, whereas ΔHq was determined from the Y-intercept according to eq 4. (C) Activation entropy of evaporation in nitrogen, ΔSq, was determined from the slope, whereas ΔHq was determined from the Y-intercept according to eq 3. (D) Activation entropy of evaporation in nitrogen, ΔSq, was determined from the slope, whereas ΔHq was determined from the Y-intercept according to eq 4.
Table 2. Activation Parameters, ΔHq and ΔSq, for Ethanol Evaporation as a Function of the Gas Flow Rate gas flow rate (mL/min) 25 activation parameter
Ar
50 N2
Ar
100 N2
Ar
200 N2
Ar
N2
According to eq 3 ΔHq (kJ/mol)
81.7 ( 0.6
27.3 ( 1.0
74.3 ( 0.7
27.2 ( 0.8
74.6 ( 0.9
28.1 ( 0.4
47.9 ( 0.4
26.2 ( 0.35
ΔSq (J/(mol K))
112.7 ( 2.0
272 ( 3.5
134.6 ( 2.0
270 ( 2.5
131.6 ( 2.9
266 ( 1.5
209.4 ( 1.2
272 ( 1.0
ΔHq (kJ/mol)
81.6 ( 0.6
27.5 ( 1.1
74.3 ( 0.6
27.0 ( 0.8
74.5 ( 0.9
28.1 ( 0.4
47.9 ( 0.4
26.2 ( 0.3
ΔSq (J/(mol K))
113.1 ( 2.0
271 ( 3.5
134.5 ( 2.0
270 ( 2.5
131.9 ( 2.9
267 ( 1.3
209.3 ( 1.2
272 ( 1.0
According to eq 4
decrease of ΔHq, which offsets the increase of the barrier to evaporation due to the large negative ΔSq. Thus, the height of the energy barrier to evaporation at 30 °C drops from ∼116 kJ/mol at 25 mL/min to ∼111 kJ/mol at 200 mL/min. Under these conditions the evaporation rate increases nearly 6 times: from 0.3724 to 2.1345 mg/min (Table 1). The effect of the gas flow rate on ΔGq diminishes with temperature, and at 70 °C the increase of the argon gas flow rate from 25 to 200 mL/min increases the rate of evaporation by 50%: from 16.74 to 23.87 mg/min (Table 1). The same holds for evaporation in nitrogen, where the evaporation rate increases from 19.70 to 23.42 mg/min with the increase of the
gas flow rate from 25 to 200 mL/min. Also, at 70 °C the nature of the carrier gas and the gas flow rate have minor effects on the energy barrier to evaporation: at 25 mL/min in argon ΔGq is ∼120.4 kJ/mol whereas in nitrogen ΔGq is ∼120.5 kJ/mol, at 200 mL/min ΔGq is ∼119.7 and ∼119.5 kJ/mol in argon and nitrogen, respectively. EntropyEnthalpy Compensation in Argon. Figure 4 A shows an excellent linear correlation of ΔHq on ΔSq with the argon gas flow rate. Such linear correlations have been observed for many reactions and are referred to as the enthalpyentropy compensation effect or isokinetic or isoequilibrium relationships.11,12 However, these 153
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Figure 3. Free energy barrier to evaporation, ΔGq, as a function of gas flow rate (25200 mL/min) and temperature (3070 °C), (A) in nitrogen and (B) in argon.
Figure 4. Enthalpyentropy compensation in argon. (A) Relationship between ΔHq and ΔSq. Compensation temperature, Tc, determined from the slope is 0.5 K. (B) Relationship between ΔHq and ΔGq at two temperatures and four argon gas flow rates.
relationships need to be treated with caution since it has been argued that this is an artifact of correlated errors in a measurement because these quantities are mutually dependent, with both being derived from the same kinetic data.1317 Indeed, many examples have been found to be statistical artifacts,15,16,1820 but some are genuine signatures of the perturbations in molecular characteristics.21 Moreover, near ambient temperature enthalpyentropy compensations are considered a typical feature of bimolecular associations arising from weak intermolecular interactions,12,22 particularly hydrogen bonding.23,24 Krug et al.15,16 proposed two tests to determine whether statistical factors or thermodynamic factors are responsible for the entropyenthalpy compensation effect. They argued that (i) if there is a substantial difference between the mean experimental temperature and the correlation temperature Tc determined from the ΔH on ΔS slope and (ii) if there is a linear relationship between ΔH and ΔG, then the compensation effect has a physicalchemical origin. Figure 4 A shows that the first of these two criteria is met (Tc = 0.5 K and average experimental temperature 323 K). While the second (i.e., linear dependence of ΔHq on ΔGq) is also met (Figure 4 B), the changes in the slope indicate different compensation mechanisms as a function of temperature and argon gas flow rate. It seems that the most probable basis for the decreasing ΔHq and negative ΔSq shown in Figure 4 A is the formation of a tightly bound transition state consisting of two or more ethanol molecules. However, there is a limit to the cost in entropy, which is reached when the bond
strength in the transition state is large compared to thermal energies, and at this point the slope of the ΔHq versus ΔGq should increase as the bond strength in the transition state increases, which it does (Figure 4 B). This also suggests that the most likely result of the lowering of the free energy barrier to evaporation in argon is due to changes in the transitional state configuration resulted because of interactions with argon molecules. Molecular State of Ethanol Vapors. Gas-phase analysis of ethanol vapors by infrared spectroscopy (IR) is useful to characterize the interactions between the evolving ethanol molecules and the carrier gas molecules. While supersonic expansions could be a way of studying this25,26 here we use a thermogravimetricevolving gas analysis method because we want to investigate the molecular state of the ethanol vapors under evaporation. Supersonic measurements would not give this information. The information is relevant in order to optimize conditions for carbon nanotube (CNT) formation from ethanol,1,2 whereas studies under supersonic conditions will provide information on possible aggregates present at conditions not applicable to CNT synthesis. The IR gas cell, the transfer line, and the thermogravimeter output line were equipped with individual tubular heating elements and temperature controllers. This setup allowed the entire path from the thermogravimeter to the spectrometer gas cell, which is 0.8 m long, to be maintained at the same temperature at which the evaporation took place. Figure 5 shows the IR absorption spectra 154
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Figure 5. Infrared spectra of evolving ethanol vapors as a function of gas type, gas flow rate, and temperature. For comparison, the respective vibrational features of liquid ethanol are also shown. (A) OH stretches at 30 °C in argon. (B) OH stretches at 30 °C in nitrogen. (C) OH stretches at 70 °C in argon. (D) OH stretches at 70 °C in nitrogen.
of evolving species in the wavenumber interval 38003100 cm1 which is usually interpreted in terms of alcohol cluster size and isomerism. Four main contributors appear in the hydroxyl stretching region of vapors produced at 30 °C: a narrow band centered at 3676 cm1 consisting of multiple absorptions, a sharp weak band at ∼3568 cm1, broader bands at approximately 3474, 3402, and 3253 cm1, and a shoulder at ∼3225 cm1 (Figure 5 A). Comparison with investigations by supersonic jet expansion of noble gasalcohol vapor mixtures allowed the assignment of these bands to ethanol monomers and small-sized and large-sized ethanol clusters.25,27,28 By analogy, we assign the band centered at ∼3670 cm1 to conformation isomers. These conformers are designated trans-ethanol and two equal gauche-ethanol conformers distinguished by the orientation of the hydroxyl group relative to the methylene group.2931 The narrow band centered at 3568 cm1 is assigned to hydrogen-bonded dimer, while the broad contours centered about 3474, 3402, and 3253 cm1 are assigned to higher hydrogen-bonded linear aggregates, whereas the broad shoulder at 3225 cm1 is assigned to large ethanol aggregates consisting of cyclic tetramers. The increase of the gas flow rate from 25 to 200 mL/min has a strong influence on the intensity of the OH stretches. While the bands corresponding to ethanol monomers (centered about 3670 cm1) are unchanged, the intensity and the area of the bands characteristic of the hydrogen-bonded species are
significantly increased. In particular, the intensity of the vibrations centered about 3253 cm1 nearly triples when the gas flow rate is increased from 25 to 200 mL/min. With the increase of the temperature to 70 °C, however, the intensity of the broad contours attributed to the hydrogen-bonded aggregates significantly decreases, but unlike the ethanol vapors produced at 30 °C, the change of the argon gas flow rate from 25 to 200 mL/min has no measurable effect on their intensities (Figure 5 C). The IR spectra of the ethanol species evaporated in flowing nitrogen at either 30 or 70 °C are dominated by the monomer species (Figure 5 B,D). While some hydrogen-bonded aggregates are also present in the vapor phase, the increase of the gas flow rate or temperature causes no measurable change in the vibrations related to the aggregate species. Therefore, IR spectroscopy shows that at 30 °C not all evolving ethanol molecules in argon are monomers; some of them make new intermolecular bonds with other ethanol molecules and form various hydrogen-bonded aggregates. However, this is not the case during evaporation in nitrogen, where the amount of hydrogen-bonded aggregates is small and does not depend on the gas flow rate. At 70 °C, neither the gas flow rate nor the nature of the carrier gas has a major effect on the molecular structure of the evaporated ethanol molecules. EthanolCarrier Gas Interactions. The data reported here represent the energy barrier to the evaporation, ΔGq, of a 155
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The Journal of Physical Chemistry A system operating under gas flow conditions, that is, under conditions of kinetic control, not a classic thermodynamic equilibrium. Within the framework of TST there are three major factors that determine the rate of reaction: (i) the concentration of the activated complex (the species of the transition state), (ii) the rate at which the activated complex breaks apart, and (iii) the way in which the activated complex breaks apart;whether it breaks apart to re-form the reactants or whether it breaks apart to form a new complex, the products. The outcome depends on factors such as the relative kinetic energy, relative orientation, and internal energy of the molecules. From the kinetic and spectroscopic measurements we can deduce several characteristics of the transition state through which the ethanol molecules must pass. During evaporation in argon, both the gasethanol interactions and the temperature are evaporation rate determining. At 30 °C the negative ΔSq (unfavorable) indicates a transition state that is more ordered than the reactants due to association of two or more ethanol molecules. In turn, the restriction of the relative motion of the molecules is accompanied by increased hydrogen bond strength; hence ΔHq decreases (which is favorable). The stronger the intermolecular bonds that hold the activated complex together, the larger should be the restriction of overall motion. Thus, the two activation parameters, ΔHq and ΔSq, can be traded off against each other because the strength of hydrogen bonds, near room temperature, is comparable to the thermal energies that oppose them. When the argon gas flow rate is increased from 25 to 200 mL/min, the favorable decrease of the enthalpy brought about by structural tightening is only partially compensated for by the unfavorable entropy, with a net result of a more favorable free energy barrier to evaporation. It seems therefore that the argonethanol interactions stabilize the transition state and thus a significant amount of ethanol is evaporated as hydrogen-bonded ethanol aggregates. Clearly at 30 °C under nitrogen, energy from collisions can more readily be transferred than from argon and also the activated complex in nitrogen more readily breaks apart to form the products. The latter is supported by the much higher evaporation rates at 30 °C in nitrogen and the fact that the ethanol vapors in nitrogen consist of mostly single ethanol molecules. The enthalpy barrier to evaporation in nitrogen is also much lower than in argon, which is supportive. However, it would be speculative here to discuss whether this was due to the nature of the bonding in nitrogen but not in argon, due to collision geometry, or due to differences due to differing transfer of vibrational, rotational, or translational energy. In contrast, during evaporation at 70 °C the hydrogen bond strength is not comparable to the thermal energy; i.e., the restraint is effectively opposed by the motion in both nitrogen and argon. At that temperature, the gas flow and the nature of the carrier gas have little effect on the energy barrier to evaporation. One area where our results may be useful is in CNT synthesis, Successful CNT synthesis requires optimization of catalyst, growth temperature, residence time in the reaction zone, and gas flow conditions.32 The latter is of particular interest, since a change of the evaporation conditions of ethanol may influence the amount of ethanol vapors and thus carbon flux during CNT formation and hence type of nanotubes.33 Therefore the results reported here may be useful for the optimization of pyrolysis conditions of ethanol vapors for CNT synthesis where similar conditions are used.
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4. CONCLUSIONS 1. The activation parameters for ethanol evaporation in dynamic gas atmosphere as a function of the gas flow rate have been determined experimentally from the temperature dependence of the evaporation rate constants in the temperature interval 3070 °C. At 30 °C the evaporation rate in nitrogen is 10 times higher than in argon. At 70 °C the nature of the gas has no significant effect on the evaporation rates. 2. The changes of the argon gas flow rate can manipulate the ΔGq barrier to evaporation in terms of the relative contributions of the enthalpy and entropy of activation to ΔGq. In nitrogen, the changes of the gas flow rate have minimal effect of the energy barrier to evaporation. 3. In argon at different temperatures, the change in thermodynamic functions at different flow rates is attributed to different evaporation mechanisms due to interactions of ethanol with argon molecules. This is supported by simultaneous gas infrared spectroscopy, which demonstrated changes in concentrations of linear and cyclic hydrogen bonded ethanolic aggregates under the different evaporation conditions. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: +61 2 9685 9945. Fax: +61 2 9685 9915. E-mail:
[email protected].
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