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Mechanism of Butanol Adsorption from Solution. 2. Further Evidence of Association Importance Andrzej L. Dawidowicz,* Andrzej Patrykiejew, and Dorota Wianowska Faculty of Chemistry, Maria Curie Sklodowska University, 20-031 Lublin, Pl. Marii Curie Sklodowskiej 3, Poland Received June 15, 2000. In Final Form: October 23, 2000 The influence of temperature on the calorimetric effects associated with adsorption and desorption of alcohols from hydrocarbon solutions on siliceous materials is studied. The results provide further support for the concept of a strong effect of alcohol association on the results of calorimetric measurements.
1. Introduction 1,2 we
have considered calorimetric In our recent papers effects of adsorption and desorption of alcohols from hydrocarbon solution on materials of different chemical and structural properties. We have mainly used active carbon and siliceous materials covered with a polymer layer. The most striking finding was a clear demonstration that in those systems the heat effects due to adsorption and desorption recorded by the flow microcalorimeter were different. In particular, it was found that the heat of adsorptions (QADS) appeared to be considerably lower than the corresponding heat of desorption (QDES). We have considered several possible sources of that rather strange behavior of adsorption and desorption heats. First, we have attempted to explain the experimental results by assuming that the adsorption process occurs in two steps, characterized by different kinetics.1 Some experimental data clearly supported such a model. We have tried to connect the presence of a slow second step of adsorption with either swelling of the polymer layer deposited on the surface of siliceous materials1 or with a slow penetration of solute molecules into micropores.2 More detailed study2 has shown, however, that the discrepancies between the heats of adsorption and desorption may result from another source. Thus, we have considered a possibility that the alcohol association in hydrocarbon solution may be responsible for the effects observed. This hypothesis has been supported by the measured changes in the adsorption and desorption heats with the alcohol concentration in solution as well as by the measurements of the heat effects of adsorption of alcohols from other solvents. In particular, it was shown that in the case of polar solvents, which prevent alcohol association, the recorded heats of adsorption and desorption were the same. We have also proposed2 a crude theoretical model of adsorption influenced by solute association, which appeared to be quite consistent with experimental results. Nevertheless, our simple model appeared not to be capable of explaining all the observed changes in the calorimetric behavior of the adsorption processes studied. First of all, the model does not allow one to take into account possible changes in the structure of the deposited polymer layer following the adsorption of alcohol molecules. Therefore, to support further the hypothesis of strong influence of * Corresponding author. E-mail:
[email protected]. (1) Dawidowicz, A. L.; Patrykiejew, A.; Wianowska, D. J. Colloid Interface Sci. 1999, 214, 362. (2) Dawidowicz, A. L.; Patrykiejew, A.; Wianowska, D. Langmuir 2000, 16, 3433.
association on the calorimetric properties of adsorption we have decided to use materials of rigid structure. For this purpose siliceous materials such as silica gel and controlled porosity glass have been chosen. It was found2 that such rigid materials also exhibit large differences between the heat effects due to adsorption and desorption of alcohols from non polar solvents. It has occurred to us that one possible way to verify our model of association affected adsorption is to study the temperature dependencies of calorimetric effects. It is known that both adsorption and association processes depend quite strongly on temperature. The increase of temperature leads to gradual decrease of adsorption.3,4 Also, the degree of association in bulk solution is known to decrease rapidly with the increase of the temperature.5-7 Thus, it is conceivable that measurements of temperature changes in adsorption and desorption heats may allow to get a more detailed information concerning the mechanism of adsorption process and support the hypothesis that it is strongly influenced by molecular association in solution. The primary goal of this paper is to present the results of such investigations. The paper is organized as follows: In section 2, we provide the necessary information about the experimental procedures used. In section 3 we present and discuss the results of our measurements. Finally, section 4 contains a short summary of our findings and final conclusions. 2. Experimental Section Materials. Two types of nonmicroporous siliceous materials, controlled porosity glass (CPG) and silica gel (Si), were used in microcalorimetric investigations. Controlled porosity glass was prepared using Vycor type glass composed of 10% Na2O, 35% B2O3, and 55% SiO2 (fraction 200250 µm) according to the procedure described elsewhere.8,9 Macroporous silica gel was prepared by hydrothermal treatment of silica gel Si-100 (fraction 200-250 µm) (Merck, Darmstadt, Germany) by following the procedure described in refs 10 and 11. (3) Brown, C. E.; Everett, D. H.; Morgan, C. J. J. Chem. Soc., Faraday Trans. 1 1975, 71, 833. (4) Findenegg, G. H.; Koch, C.; Liphard, M. In Symposium on Adsorption from Solution; School of Chemistry, Univeristy of Bristol; Academic Press: London, 1982; pp 81-91. (5) Roberts, J. D.; Caserio, M. C. Chemia organiczna (Basic Principles of Organic Chemistry); PWN: Warsaw, 1969; pp 392-397. (6) Verrall, R. E.; Jacobson, D. K.; Hersberger, M. V.; Lumry, R. W. J. Chem. Eng. Data 1979, 24, 289. (7) Hofman, T.; Barbes, B.; Casanova, C. J. Chem. Soc., Faraday Trans. 1996, 92, 3565. (8) Haller, W. J. Phys. Chem. 1965, 42, 686. (9) Dawidowicz, A. L. Pol. J. Chem. 1992, 66, 313.
10.1021/la000842b CCC: $20.00 © 2001 American Chemical Society Published on Web 12/28/2000
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Table 1. Physicochemical Properties of the Adsorbents Used adsorbent CPG Si
specific surf area mean pore diameter pore vol (SBET) (m2/g) (D) (nm) (Vp) (cm3/g) 38.2 25.6
69.5 117.0
1.24 1.02
Physicochemical properties of both materials are summarized in Table 1. Methods. BET Measurement. Specific surface areas of CPG’s and silica gel were estimated by means of low-temperature adsorption of nitrogen. Nitrogen adsorption/desorption isotherms at 77 K were determined volumetrically using an ASAP 2405N analyzer (Micrometrics Corp., Norcross, GA). Mercury Porosimetry. Mercury porosimetry was used for the characterization of mean pore diameters and pore volumes of CPG’s and silica gel. The porosimetric measurements were carried out by means of the mercury porosimeter type 4000 (Carlo Erba, Milano, Italy). Flow Microcalorimetry Measurements. To determine the heats of adsorption and desorption the flow microcalorimeter, type 3Vi (Microscal, Ltd., London, U.K.), was used. The heats of adsorption/desorption of n-butanol were measured from n-hexane at 25 °C and for methanol, n-butanol, and n-octanol solutions in octane over a temperature range between 20 and 80 °C. The solute concentrations ranged between 0.5 and 4% v/v (in most cases 1% v/v), depending on the type of the investigation. Before calorimetric measurements each material placed in the measuring cell was carefully degassed under vacuum. The adsorption measurements followed the immersion of the sorbents in the pure solvents which were percolated through the beds of the sorbents placed in the calorimetric cell. Heat evolution during adsorption/desorption processes was indicated by the changes in the potential (imbalanced in the thermistor bridge in which two thermistors detect temperature changes in the adsorbent bed) recorded in microvolts by an analogue recorder and in digital form. Next, after calibration, the heat effects were integrated. The data obtained are presented in Tables 2-5 and Figures 1-4 (mean values for n ) 5). All the data reported here were obtained using the same flow rate of the solvents and solutions equal to 3 mL/h.
3. Result and Discussion The first measurements have aimed at the determination of adsorption and desorption heats from solutions of different alcohol concentration. This was done in order to find whether there are similar differences between QADS and QDES as observed in the systems previously studied.1,2 The results of our measurements are presented in Figure1a,b. It is quite clear that in both systems studied, the heat of desorption rapidly increases with alcohol concentration. On the other hand, the heat of adsorption initially slightly increases with alcohol concentration and then starts to decrease. Anyway, in both cases the differences between the desorption and adsorption heats increase with alcohol concentration, in quite the same way as that observed for active carbon and siliceous materials covered with a polymer layer.2 Thus, at concentrations above 1% v/v of butanol in hexane the calorimetric effect of desorption exceeds the effect of adsorption, irrespective on the sorbent used. This suggests that the observed effects are connected with the specific properties of the alcohol hydrocarbon solution rather then with adsorption properties of the sorbent. In our previous work2 we have postulated that molecular association of alcohol considerably influences heat effects of adsorption (10) Unger, K. K. Porous Silica (Its Properties and Use as Support in Column Liquid Chromatography); Journal of Chromatography Library Vol. 16; Elsevier Scientific Publishing Co.: New York, 1979; pp 47-49. (11) Dawidowicz, A. L.; Mendyk, E. Mater. Chem. Phys. 1989, 21, 463.
Figure 1. Adsorption heats (filled symbols) and desorption heats (open symbols) of n-butanol from n-hexsane vs solute concentration on CPG (a) and silica gel (b).
and leads to the appearance of large difference between QDES and QADS. We have assumed that the heat effect connected with the adsorption process is lowered due to the destruction of alcohol aglomerates, which is an endothermic process. In consequence, the net heat effect of adsorption is lower than the corresponding heat of desorption. It should be emphasized that the desorption heat is not likely to be significantly affected by recombination of alcohol aglomerates as it takes place in the region of the cell that is already not detected by the sensors. The results shown in Figure 1a,b seem to support our previous hypothesis that the molecular association may dominate the behavior of adsorption systems studied. Of course one cannot exclude other sources of the discrepancies between QADS and QDES in the system. For example, the heats of mixing are expected to increase the heat of desorption and to decrease the heat of adsorption. The mixing is always an endothermic process for associating systems.12 To estimate the magnitude of that effect we have measured the heats of mixing for n-butanol solution in hexane with pure hexane at different concentration of
Mechanism of Butanol Absorption from Solution
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Table 2. Results of Mixing Heats for n-Butanol Solution in Hexane with a Pure Hexane at Different Concentration of Alcohol Solution (Mean Values ( SD) (n ) 5), Obtained during the “Adsorption” (QADS*) and “Desorption” (QDES*) Cycles (Nonporous Glass Beads in the Measuring Cell) n-butanol concn (v/v %)
QADS* (mJ/g)
QDES* (mJ/g)
1 2 3 4 5 6
-54 ( 4 -65 ( 7 -77 ( 7 -79 ( 7 -144 ( 11 -243 ( 18
-81 ( 6 -88 ( 6 -96 ( 7 -125 ( 10 -109 ( 11 -261 ( 19
Table 3. Differences between the Heat of Desorption and the Heat of Adsorption (Mean Values ( SD) (n ) 5) of n-Butanol from n-Hexane (∆Q) vs Solute Concentration Obtained on Materials CPG and Si (see Figure 1a,b) and the Corresponding Values (∆Q*) Corrected for the Mixing Effect
n-butanol concn (v/v %) 1 2 3 4
∆Q ) |QDES| QADS (mJ/g) obtained on material CPG Si 1094 1360 2535 2689
∆Q* ) ∆Q - QADS* QDES* (mJ/g) obtained on material CPG Si
789 1022 1388 2699
959 1207 2362 2485
654 869 1215 2495
Table 4. Adsorption/Desorption Heats (Mean Values ( SD) (n ) 5) of n-Butanol from Hexane and Octane (1% v/v Solution) Obtained on CPG heat of solvent
adsorption (QADS) (mJ/g)
desorption (QDES) (mJ/g)
hexane octane
4758 ( 42 4809 ( 45
-5852 ( 43 -5855 ( 41
alcohol solution. The measurements of the heat of mixing were done in the same way as the measurements of adsorption and desorption heats but with the measuring cell filled with nonporous glass beads.13 The results of those measurements are summarized in Table 2. As it can be safely assumed that the adsorption effects in such systems are negligible, the obtained heats of “adsorption” (QADS*) and “desorption” (QDES*) are assumed to represent the heats of mixing. As indicated by the data summarized in Table 2, the heats of mixing increase with alcohol concentration but are too small to explain the observed values of ∆Q. Table 3 lists the values of ∆Q for the system from Figure 1 and the values of ∆Q*, corrected for the heat of mixing [∆Q* ) ∆Q - (QADS* + QDES*)]. It is clear that the corrected values of ∆Q* are still quite large. Therefore the data still support our concept that the molecular association may be the main source of the discrepancies between QADS and QDES. To obtain further evidence for this concept, we have measured the temperature dependencies of QADS and QDES. It was clear, however, that hexane becomes too volatile above room temperatures. Therefore we have decided to change the solvent and perform the measurements from octane solution. At first, however, it was necessary to check how the change of the solvent influences the recorded heat effects. From the data shown in Table 4 it follows that both adsorption and desorption heats are practically the same from the hexane and octane solutions. Figure 2 presents the temperature changes of QADS and QDES obtained for adsorption and desorption of 1% v/v (12) Bich, E.; Papaioannou, D.; Heintz, A.; Tusel-Langer, E.; Lichtenthaler, R. N. Fluid Phase Equilib. 1999, 156, 115. (13) Groszek, A. J. Thermochim. Acta 1998, 312, 133.
Figure 2. Adsorption heats (black diamonds) and desorption heats (white diamonds) of n-butanol from n-octane (1% v/v solution) vs temperature on CPG.
solution of butanol in octane on CPG. We observe that the heat of desorption decreases monotonically with temperature, as expected, and may be connected with a gradual decrease of adsorption with temperature. On the other hand, the heat of adsorption shows a rather strange behavior. In particular, it exhibits a rather large increase over a narrow temperature range and then remains nearly constant. A question arises concerning the origin of such behavior. For example, it may result from a sudden increase of specific adsorption connected with activation of chemical interaction between alcohol molecules and the surface active centers (e.g. hydroxyl groups). On the other hand, it is also possible that it is connected with a lowering of the degree of association at elevated temperatures. We have attempted to resolve that question by additional measurements of temperature changes in QADS and QDES obtained for methanol and octanol adsorbed from octane solution. Methanol is expected to show the strongest tendency toward specific adsorption, while specific interaction of octanol should be the weakest. On the other hand, methanol exhibits also the strongest association compared with butanol and octanol. Thus, if the specific adsorption were to be responsible for a sudden increase of QADS with temperature, then the observed anomaly should occur at the lowest temperature for methanol and at the highest temperature for octanol. Butanol should fall between those two cases. From the data presented in Figures 3 and 4 it rather follows that the temperature region over which QADS increases shows an opposite tendency. We have marked the inflection point at the QADS vs T curve by vertical dotted lines. One finds that in the case of methanol the inflection point is located at T ≈ 42 °C and for butanol at T about 32 °C, while for octanol it is probably located below the lowest temperature used, i.e., below 20 °C. The above sequence again supports the association dominated mechanism. Tendency toward association increases with the increase of polarity.14 Thus, strongly associating methanol molecules require the highest temperature to diminish strong association as compared with the longer and less polar molecules of butanol and, of course, octanol. (14) Marcus, Y. Chem. Soc. Rev. 1993, 22, 409.
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Dawidowicz et al. Table 5. Adsorption/Desorption Heats (Mean Values ( SD) (n ) 5) of n-Butanol from n-Hexane (1% v/v Solution) Obtained on the Same Sample of CPG in Subsequent Measurements heat of type of sorbent
no. of subsequent measment
adsorption (QADS) (mJ/g)
desorption (QDES) (mJ/g)
∆Q ) |QDES| - QADS (mJ/g)
1 2 3 4 5 6
4758 ( 42 4002 ( 36 3991 ( 33 4047 ( 36 4069 ( 34 3970 ( 32
-5852 ( 43 -5693 ( 40 -5612 ( 41 -5590 ( 41 -5641 ( 39 -5574 ( 42
1094 1691 1621 1543 1572 1604
CPG
Figure 3. Adsorption heats (filled symbols) and desorption heats (open symbols) of methanol (diamonds) and octanol (circles) from n-octane (1% v/v solution) vs temperature on CPG.
increase of QADS, should induce a sharp drop of QDES in the same temperature region. This is surely not the case here. Of course, specific adsorption is present in our system. Table 5 presents the results of repeated cycles of adsorption and desorption heat measurements for 1% v/v butanol solution in hexane at 25 °C on the same sample of CPG. Irreversible specific adsorption manifests itself by a difference at QADS obtained in the first and the second adsorption cycles. Indeed, QADS in the first measurement is higher by about 750 mJ/g than the values obtained in a series of subsequent measurements. One should note that the values of ∆Q in the first cycle are considerably lower than in the subsequent cycles. Thus, specific adsorption effect cannot be used as an alternative explanation for the observed discrepancies between QADS and QDES. The increase of ∆Q, after the first cycle, is an additional argument for our prediction of strong association effects. Also, the constancy of ∆Q in subsequent cycles is consistent with our model. 4. Final Conclusions
Figure 4. Adsorption heats (filled symbols) and desorption heats (open symbols) of butanol from n-octane (2% v/v solution) vs temperature on CPG.
Further support for the strong influence of association is provided by the measurements of temperature changes of QADS for still more concentrated (2% v/v) butanol solution (see Figure 4). In this case we observe the same general behavior of QADS and QDES, as in the case of 1% v/v solutions, but the temperature region at which QADS rapidly increases is wider and the inflection point is shifted toward the higher temperature of about 38 °C. This again, is consistent with the prediction of stronger association in the more concentrated solutions. We have not performed any extensive measurements aiming at the determination of specific adsorption effects involved. However, one can anticipate that any increase of strong specific adsorption, which would result in an
This paper aimed at finding a further support for the previously proposed2 concept of strong influence of molecular association on the heat effects of adsorption and desorption processes recorded by the flow microcalorimetry. In particular, we have studied the changes of the measured heats of adsorption and desorption with temperature. The measurements performed for different alcohols (CH3OH, C4H9OH, C8H17OH) from octane have shown that the temperature at which the adsorption heat exhibits a rapid increase depends on the ability of alcohol molecules to associate. In particular, the increase of QADS for methanol, which associates most strongly, occurs at the highest temperature. It is quite consistent with the assumed model of adsorption. Additional support for our model has been obtained by the measurements of temperature changes of QADS in more concentrated (2% v/v) butanol solution. The results presented here, as well as those reported in ref 2, demonstrate that the flow microcalorimetry may be a very useful tool that allows one to study the processes accompanying adsorption. The apparent discrepancies between the measured values of QADS and QDES have prompted us to look for the origin of that effect for the alcohol-hydrocarbon system and finally led us to the proposed model of adsorption. The results of more detailed study, including adsorption isoterm measurements, which provide even stronger evidence of the influence of the molecular association on adsorption of alcohols from nonpolar solvents will be reported in the next paper of this series. LA000842B
