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Monolayer Coverage–Sorbate Area Relationship for Mixtures of Sorbents
Journal of Colloid and Interface Science 219, 149 –154 (1999) Article ID jcis.1999.6456, available online at http://www.idealibrary.com on
Monolayer Coverage–Sorbate Area Relationship for Mixtures of Sorbents A. K. Helmy, 1 S. G. de Bussetti, and E. A. Ferreiro Universidad Nacional del Sur, 8000 Bahı´a Blanca, Argentina Received March 8, 1999; accepted July 23, 1999
The adsorption of molecules of different sizes, at monolayer coverages, was determined for binary mixtures of montmorillonite and hydroxy-Al montmorillonite in order to test the applicability of the general equation (NA z 5 k) relating the number of sorbate units at monolayer coverage (N) with their areas (A). The effect of the proportions of the components in the mixture on the value of the exponent z was also evaluated. The data obtained confirm the validity of the equation and indicate that the value of z for the mixtures assumes intermediate values between the two z values of the components, depending on their proportions. It is also demonstrated that the z exponent could have a value of unity (a value supposed to indicate surface uniformity) for certain mixtures of sorbents having z values different from unity, i.e., possessing nonuniform surfaces. © 1999 Academic Press Key Words: binary mixtures; hydroxy-Al montmorillonite; monolayer coverage; montmorillonite; sorbate areas.
tion mentioned above. For example, when the ratio A 2 /A 1 5 2, N 2 /N 1 5 0.62 for z 5 0.7 and N 2 /N 1 5 0.44 for z 5 1.2 (2). The z values, doubtless, reflect energetic rather than geometric characteristics of sorbent surfaces (2). No studies have been published so far about the applicability of Eq. [1] to mixtures of sorbents, hence the investigation reported below, where binary mixtures of montmorillonite and hydroxy-Al montmorillonite were used to test the applicability of Eq. [1] to mixtures of two sorbents possessing different z values. MATERIALS AND METHODS
INTRODUCTION
The following materials were used as sorbents. A local montmorillonite from Cerro Bandera, Argentina (3), was selected for the study. It has an exchange capacity of 0.91 meq/g for the fraction ,2 mm. A sample of this clay was interlayered with hydroxide-Al species. It was prepared by treating a clay sample with partially neutralized AlCl 3 solution as described by Helmy et al. (4). The hydroxy-Al montmorillonite had 41.9 mg Al per g (7.93% Al 2O 3); it had an exchange capacity of 0.56 meq/g and produced a basal spacing (001) of 18.0 Å in the X-ray diagram. The adsorbates were
It has been shown, recently, that the general equation relating the number of adsorbate units at monolayer coverage (N) and the area per adsorbate unit ( A) is given by (1, 2) NA z 5 k,
[1]
1. 2. 3. 4. 5.
where z and k are constants characteristic of each sorbent. Equation [1] classifies pure sorbents in three categories according to the z value being equal or less than or greater than unity (1). When z 5 1 a pure sorbent has a uniform surface and k 5 S, the true surface area of the sorbent. By true it is meant that the product NA is independent of the size of the adsorbate. Furthermore, sorbents with z , 1 show some preference toward larger sized sorbates while the reverse is true for sorbents with z . 1. This point is even made clearer by the plots in Fig. 1, which depicts how the value of z for a given ratio of sorbate areas ( A 2 /A 1 ) affects the ratios at monolayer coverages (N 2 /N 1 ), thus producing the preferences in adsorp-
1,10-phenanthroline (OP), (C 12H 8N 2 z H 2O), Merck, 2,29-bipyridine (BP), (C 10H 8N 2), Merck, quinoline (Q), (C 9H 7N), BDH, glycerol, [HOCH 2CH(OH)CH 2OH], Anedra, and ethylene glycol (EG), (HOCH 2CH 2OH), Sintorgan.
1
For the adsorption experiments the mixtures of the two clays were prepared so that the weight fraction of montmorillonite in the sorbents varied from 0 –1. Adsorption of OP, BP, and Q by the sorbents was carried out by adding to 0.2-g material samples in 40-mL bottles 25 mL of aqueous solutions of different concentrations of the above mentioned compounds. The bottles were shaken for 1 h then left for 24 h at 28°C with occasional shaking. Filtrates were analyzed by the spectrophotometric methods described in the cited references for each compound (5– 8). The amounts ad-
To whom correspondence should be addressed. E-mail: akhelmi@criba. edu.ar.
149
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
150
HELMY, DE BUSSETTI, AND FERREIRO
face coverage for OP, BP, and Q were obtained by the application of the usual Langmuir treatment, and for glycerol by heating a clay glycerol mixture in the presence of a partially saturated atmosphere of glycerol vapor until adsorption equilibrium was attained (9). For the EG the equilibrium procedure described by Eltantawy and Arnold (10, 11) was adopted, using an evacuated system containing a free liquid surface and dry CaCl 2 as a separate phase. The molecular areas used in the calculations, in units of Å 2, are OP, 60 (6), BP, 71.5 (7), quinoline, 58.5 (8), glycerol, 29.3 (9), and EG, 22.4 (10). FIG. 1. The ratio N 2 /N 1 plotted as a function of z for the ratios of sorbate areas ( A 2 /A 1 ) given in the rectangle.
sorbed were calculated as the differences between initial and equilibrium concentrations. The adsorption values corresponding to unimolecular sur-
RESULTS AND DISCUSSION
The adsorption isotherms of 2,2-bipyridine, orthophenanthroline, and quinoline on montmorillonite, hydroxy-Al montmorillonite, and their mixtures are shown in Fig. 2. All isotherms are of the high affinity L type (6 – 8). From the data,
FIG. 2. Adsorption isotherms of 2,29-bipyridine (a), 1,10-phenanthroline (b), and quinoline (c) by mixtures of montmorillonite and hydroxy-Al montmorillonite. The weight fractions of montmorillonite in the mixtures are given in the figures.
SURFACE AREAS OF MIXTURES
151
FIG. 3. Amounts adsorbed of various adsorbates by mixtures of montmorillonite and hydroxy-Al montmorillonite as a function of the weight fractions of montmorillonite in the mixtures.
152
HELMY, DE BUSSETTI, AND FERREIRO
FIG. 4. Log–log plots of the numbers of adsorbate units per g of mixtures of montmorillonite and hydroxy-Al montmorillonite. Mixture compositions are given in the figures. Symbols represent 2,29-bipyridine (■), 1,10-phenanthroline (‚), quinoline (}), glycerol (F), and ethylene glycol (E).
153
SURFACE AREAS OF MIXTURES
We end this section by stating that though Eq. [1] is valid for mixtures and the z values obtained have apparently the same significance as for pure sorbents, that the value of z equals unity for a mixture does not indicate uniformity of surface for the mixture as it conveys for pure sorbents. Surface uniformity is not only the property of the surface area of a sorbent that is independent of the size of the sorbate. In other words having the value of z in Eq. [1] equal unity is a necessary but not a sufficient condition for surface uniformity. APPENDIX
FIG. 5. The variation of the z value of mixtures of montmorillonite and hydroxy-Al montmorillonite as a function of the weight fractions of montmorillonite in the mixtures.
monolayer coverages on each sorbent were obtained by the application of the usual Langmuir treatment. For glycerol and ethylene glycol, monolayer coverages were obtained as mentioned in Materials and Methods, from equilibrium procedures. The numbers of adsorbates at monolayer coverages for the mixtures are plotted in Fig. 3, according to Eq. [2] (see Appendix), as a function of the weight fraction of montmorillonite in each mixture. Straight lines were obtained in agreement with the equation, thus indicating that conservation of the number of sorbate units at monolayer coverages in the mixtures is obeyed. The number of sorbates at monolayer coverages for the sorbents studied is plotted on a log–log scale as a function of sorbate area. The plots are shown in Fig. 4. Straight lines with different slopes were obtained in agreement with Eq. [1], thus validating its application for the mixtures. The z values obtained from the slopes varied from 1.002 for montmorillonite to 0.817 for the pillared clay. The mixtures of the two sorbents had values intermediate between the two mentioned values, increasing in value as the weight fraction of montmorillonite in the mixtures increased (see Fig. 5). The fact that the data for the mixtures obey Eq. [1] with z values different from unity indicates that the product NA for the mixtures represents accessible surface area for each mixture, and no single common value for the surface area of each mixture could be obtained by using sorbates of different sizes. The fact that the z value of a mixture assumes an intermediate value between the z values of the components offers the possibility of obtaining a mixture with z 5 1, by mixing sorbents with z values different from unity (sorbents with nonuniform surfaces). In the Appendix we give the procedure by which the mentioned z value of unity could be obtained.
Calculation of the Proportions of Components for a Binary Mixture of Sorbents with z 5 1 If we make up a mixture of x g of sorbent 1 and (1 2 x) g of sorbent 2, then conservation of the adsorbate numbers in the mixtures gives N 3 5 xN 1 1 ~1 2 x! N 2 ,
[2]
where N 3 , N 1 , and N 2 are the numbers of adsorbate units at monolayer coverage per gram of mixture and per g of sorbents 1 and 2, respectively. For the mixture, N 3 is given by N 3A z3 5 k 3,
[3]
where A is the area of an adsorbate unit and z 3 and k 3 are constants. Multiplication of both sides of [2] by A 1z 3 and/or A 2z 3 and arrangement gives N 3 A z13 5 xA z13~N 1 2 N 2 ! 1 A z13N 2
[4]
N93 A z23 5 xA z23~N91 2 N92 ! 1 A z23N92 ,
[5]
where A 1 and A 2 are two adsorbates of different sizes, N 1 and N91 are the numbers of adsorbate units at monolayer coverage of A 1 and A 2 , respectively, per g of sorbent 1. N 2 and N92 are the same for sorbent 2. N 3 and N93 are the number of adsorbate units at monolayer coverage of A 1 and A 2 per g of mixture. Since according to [3], Eqs. [4] and [5] are equal, xA z13~N 1 2 N 2 ! 1 A z13N 2 5 xA z23~N91 2 N92 ! 1 A z23N92 .
[6]
For example, in order to obtain one g of a mixture having a z 3 value of unity, Eq. [6] requires that x 5 0.633 g for a sorbent with z 5 0.9 and (1 2 x) 5 0.367 g for a sorbent with z 5 1.4, using ethylene glycol and ethylene glycol mono-ethyl ether as adsorbates (1).
154
HELMY, DE BUSSETTI, AND FERREIRO
REFERENCES 1. Helmy, A. K., Ferreiro, E. A., de Bussetti, S. G., and Peinemann, N., Colloid Polym. Sci. 276, 534 (1998). 2. Helmy, A. K., Ferreiro, E. A., and de Bussetti, S. G., J. Colloid Interface Sci. 210, 167 (1999). 3. Peinemann, N., Ferreiro, E. A., and Helmy, A. K., Rev. Asoc. Geol. Argentina 27, 399 (1972). 4. Helmy, A. K., Ferreiro, E. A., and de Bussetti, S. G., Clays Clay Miner. 42, 444 (1994).
5. Lawrie, D. C., Soil Sci. 92, 188 (1961). 6. de Bussetti, S. G., Ferreiro, E. A., and Helmy, A. K., Clays Clay Miner. 28, 149 (1980). 7. Ferreiro, E. A., de Bussetti, S. G., and Helmy, A. K., Z. Pflanzenernaehr. Bodenk. 156, 369 (1983). 8. Helmy, A. K., de Bussetti, S. G., and Ferreiro, E. A., Clays Clay Miner. 31, 29 (1983). 9. Woodside, K. H., and Ormsby, W. C., J. Am. Ceram. Soc. 43, 671 (1960). 10. Eltantawy, M., and Arnold, P. W., J. Soil Sci. 24, 232 (1973). 11. Eltantawy, M., and Arnold, P. W., J. Soil Sci. 25, 99 (1974).
Monolayer Coverage–Sorbate Area Relationship for Mixtures of Sorbents A. K. Helmy, 1 S. G. de Bussetti, and E. A. Ferreiro Universidad Nacional del Sur, 8000 Bahı´a Blanca, Argentina Received March 8, 1999; accepted July 23, 1999
The adsorption of molecules of different sizes, at monolayer coverages, was determined for binary mixtures of montmorillonite and hydroxy-Al montmorillonite in order to test the applicability of the general equation (NA z 5 k) relating the number of sorbate units at monolayer coverage (N) with their areas (A). The effect of the proportions of the components in the mixture on the value of the exponent z was also evaluated. The data obtained confirm the validity of the equation and indicate that the value of z for the mixtures assumes intermediate values between the two z values of the components, depending on their proportions. It is also demonstrated that the z exponent could have a value of unity (a value supposed to indicate surface uniformity) for certain mixtures of sorbents having z values different from unity, i.e., possessing nonuniform surfaces. © 1999 Academic Press Key Words: binary mixtures; hydroxy-Al montmorillonite; monolayer coverage; montmorillonite; sorbate areas.
tion mentioned above. For example, when the ratio A 2 /A 1 5 2, N 2 /N 1 5 0.62 for z 5 0.7 and N 2 /N 1 5 0.44 for z 5 1.2 (2). The z values, doubtless, reflect energetic rather than geometric characteristics of sorbent surfaces (2). No studies have been published so far about the applicability of Eq. [1] to mixtures of sorbents, hence the investigation reported below, where binary mixtures of montmorillonite and hydroxy-Al montmorillonite were used to test the applicability of Eq. [1] to mixtures of two sorbents possessing different z values. MATERIALS AND METHODS
INTRODUCTION
The following materials were used as sorbents. A local montmorillonite from Cerro Bandera, Argentina (3), was selected for the study. It has an exchange capacity of 0.91 meq/g for the fraction ,2 mm. A sample of this clay was interlayered with hydroxide-Al species. It was prepared by treating a clay sample with partially neutralized AlCl 3 solution as described by Helmy et al. (4). The hydroxy-Al montmorillonite had 41.9 mg Al per g (7.93% Al 2O 3); it had an exchange capacity of 0.56 meq/g and produced a basal spacing (001) of 18.0 Å in the X-ray diagram. The adsorbates were
It has been shown, recently, that the general equation relating the number of adsorbate units at monolayer coverage (N) and the area per adsorbate unit ( A) is given by (1, 2) NA z 5 k,
[1]
1. 2. 3. 4. 5.
where z and k are constants characteristic of each sorbent. Equation [1] classifies pure sorbents in three categories according to the z value being equal or less than or greater than unity (1). When z 5 1 a pure sorbent has a uniform surface and k 5 S, the true surface area of the sorbent. By true it is meant that the product NA is independent of the size of the adsorbate. Furthermore, sorbents with z , 1 show some preference toward larger sized sorbates while the reverse is true for sorbents with z . 1. This point is even made clearer by the plots in Fig. 1, which depicts how the value of z for a given ratio of sorbate areas ( A 2 /A 1 ) affects the ratios at monolayer coverages (N 2 /N 1 ), thus producing the preferences in adsorp-
1,10-phenanthroline (OP), (C 12H 8N 2 z H 2O), Merck, 2,29-bipyridine (BP), (C 10H 8N 2), Merck, quinoline (Q), (C 9H 7N), BDH, glycerol, [HOCH 2CH(OH)CH 2OH], Anedra, and ethylene glycol (EG), (HOCH 2CH 2OH), Sintorgan.
1
For the adsorption experiments the mixtures of the two clays were prepared so that the weight fraction of montmorillonite in the sorbents varied from 0 –1. Adsorption of OP, BP, and Q by the sorbents was carried out by adding to 0.2-g material samples in 40-mL bottles 25 mL of aqueous solutions of different concentrations of the above mentioned compounds. The bottles were shaken for 1 h then left for 24 h at 28°C with occasional shaking. Filtrates were analyzed by the spectrophotometric methods described in the cited references for each compound (5– 8). The amounts ad-
To whom correspondence should be addressed. E-mail: akhelmi@criba. edu.ar.
149
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
150
HELMY, DE BUSSETTI, AND FERREIRO
face coverage for OP, BP, and Q were obtained by the application of the usual Langmuir treatment, and for glycerol by heating a clay glycerol mixture in the presence of a partially saturated atmosphere of glycerol vapor until adsorption equilibrium was attained (9). For the EG the equilibrium procedure described by Eltantawy and Arnold (10, 11) was adopted, using an evacuated system containing a free liquid surface and dry CaCl 2 as a separate phase. The molecular areas used in the calculations, in units of Å 2, are OP, 60 (6), BP, 71.5 (7), quinoline, 58.5 (8), glycerol, 29.3 (9), and EG, 22.4 (10). FIG. 1. The ratio N 2 /N 1 plotted as a function of z for the ratios of sorbate areas ( A 2 /A 1 ) given in the rectangle.
sorbed were calculated as the differences between initial and equilibrium concentrations. The adsorption values corresponding to unimolecular sur-
RESULTS AND DISCUSSION
The adsorption isotherms of 2,2-bipyridine, orthophenanthroline, and quinoline on montmorillonite, hydroxy-Al montmorillonite, and their mixtures are shown in Fig. 2. All isotherms are of the high affinity L type (6 – 8). From the data,
FIG. 2. Adsorption isotherms of 2,29-bipyridine (a), 1,10-phenanthroline (b), and quinoline (c) by mixtures of montmorillonite and hydroxy-Al montmorillonite. The weight fractions of montmorillonite in the mixtures are given in the figures.
SURFACE AREAS OF MIXTURES
151
FIG. 3. Amounts adsorbed of various adsorbates by mixtures of montmorillonite and hydroxy-Al montmorillonite as a function of the weight fractions of montmorillonite in the mixtures.
152
HELMY, DE BUSSETTI, AND FERREIRO
FIG. 4. Log–log plots of the numbers of adsorbate units per g of mixtures of montmorillonite and hydroxy-Al montmorillonite. Mixture compositions are given in the figures. Symbols represent 2,29-bipyridine (■), 1,10-phenanthroline (‚), quinoline (}), glycerol (F), and ethylene glycol (E).
153
SURFACE AREAS OF MIXTURES
We end this section by stating that though Eq. [1] is valid for mixtures and the z values obtained have apparently the same significance as for pure sorbents, that the value of z equals unity for a mixture does not indicate uniformity of surface for the mixture as it conveys for pure sorbents. Surface uniformity is not only the property of the surface area of a sorbent that is independent of the size of the sorbate. In other words having the value of z in Eq. [1] equal unity is a necessary but not a sufficient condition for surface uniformity. APPENDIX
FIG. 5. The variation of the z value of mixtures of montmorillonite and hydroxy-Al montmorillonite as a function of the weight fractions of montmorillonite in the mixtures.
monolayer coverages on each sorbent were obtained by the application of the usual Langmuir treatment. For glycerol and ethylene glycol, monolayer coverages were obtained as mentioned in Materials and Methods, from equilibrium procedures. The numbers of adsorbates at monolayer coverages for the mixtures are plotted in Fig. 3, according to Eq. [2] (see Appendix), as a function of the weight fraction of montmorillonite in each mixture. Straight lines were obtained in agreement with the equation, thus indicating that conservation of the number of sorbate units at monolayer coverages in the mixtures is obeyed. The number of sorbates at monolayer coverages for the sorbents studied is plotted on a log–log scale as a function of sorbate area. The plots are shown in Fig. 4. Straight lines with different slopes were obtained in agreement with Eq. [1], thus validating its application for the mixtures. The z values obtained from the slopes varied from 1.002 for montmorillonite to 0.817 for the pillared clay. The mixtures of the two sorbents had values intermediate between the two mentioned values, increasing in value as the weight fraction of montmorillonite in the mixtures increased (see Fig. 5). The fact that the data for the mixtures obey Eq. [1] with z values different from unity indicates that the product NA for the mixtures represents accessible surface area for each mixture, and no single common value for the surface area of each mixture could be obtained by using sorbates of different sizes. The fact that the z value of a mixture assumes an intermediate value between the z values of the components offers the possibility of obtaining a mixture with z 5 1, by mixing sorbents with z values different from unity (sorbents with nonuniform surfaces). In the Appendix we give the procedure by which the mentioned z value of unity could be obtained.
Calculation of the Proportions of Components for a Binary Mixture of Sorbents with z 5 1 If we make up a mixture of x g of sorbent 1 and (1 2 x) g of sorbent 2, then conservation of the adsorbate numbers in the mixtures gives N 3 5 xN 1 1 ~1 2 x! N 2 ,
[2]
where N 3 , N 1 , and N 2 are the numbers of adsorbate units at monolayer coverage per gram of mixture and per g of sorbents 1 and 2, respectively. For the mixture, N 3 is given by N 3A z3 5 k 3,
[3]
where A is the area of an adsorbate unit and z 3 and k 3 are constants. Multiplication of both sides of [2] by A 1z 3 and/or A 2z 3 and arrangement gives N 3 A z13 5 xA z13~N 1 2 N 2 ! 1 A z13N 2
[4]
N93 A z23 5 xA z23~N91 2 N92 ! 1 A z23N92 ,
[5]
where A 1 and A 2 are two adsorbates of different sizes, N 1 and N91 are the numbers of adsorbate units at monolayer coverage of A 1 and A 2 , respectively, per g of sorbent 1. N 2 and N92 are the same for sorbent 2. N 3 and N93 are the number of adsorbate units at monolayer coverage of A 1 and A 2 per g of mixture. Since according to [3], Eqs. [4] and [5] are equal, xA z13~N 1 2 N 2 ! 1 A z13N 2 5 xA z23~N91 2 N92 ! 1 A z23N92 .
[6]
For example, in order to obtain one g of a mixture having a z 3 value of unity, Eq. [6] requires that x 5 0.633 g for a sorbent with z 5 0.9 and (1 2 x) 5 0.367 g for a sorbent with z 5 1.4, using ethylene glycol and ethylene glycol mono-ethyl ether as adsorbates (1).
154
HELMY, DE BUSSETTI, AND FERREIRO
REFERENCES 1. Helmy, A. K., Ferreiro, E. A., de Bussetti, S. G., and Peinemann, N., Colloid Polym. Sci. 276, 534 (1998). 2. Helmy, A. K., Ferreiro, E. A., and de Bussetti, S. G., J. Colloid Interface Sci. 210, 167 (1999). 3. Peinemann, N., Ferreiro, E. A., and Helmy, A. K., Rev. Asoc. Geol. Argentina 27, 399 (1972). 4. Helmy, A. K., Ferreiro, E. A., and de Bussetti, S. G., Clays Clay Miner. 42, 444 (1994).
5. Lawrie, D. C., Soil Sci. 92, 188 (1961). 6. de Bussetti, S. G., Ferreiro, E. A., and Helmy, A. K., Clays Clay Miner. 28, 149 (1980). 7. Ferreiro, E. A., de Bussetti, S. G., and Helmy, A. K., Z. Pflanzenernaehr. Bodenk. 156, 369 (1983). 8. Helmy, A. K., de Bussetti, S. G., and Ferreiro, E. A., Clays Clay Miner. 31, 29 (1983). 9. Woodside, K. H., and Ormsby, W. C., J. Am. Ceram. Soc. 43, 671 (1960). 10. Eltantawy, M., and Arnold, P. W., J. Soil Sci. 24, 232 (1973). 11. Eltantawy, M., and Arnold, P. W., J. Soil Sci. 25, 99 (1974).