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Effect of LB monolayers on the mixed crystals of lead and barium sulfate
Colloids and Surfaces A: Physicochemical and Engineering Aspects 175 (2000) 161 – 164 www.elsevier.nl/locate/colsurfa
Effect of LB monolayers on the mixed crystals of lead and barium sulfate Lu Lehui *, Wang Liying, Zeng Guangfu, Cui Haining, Xi Shiquan Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, PR China
Abstract The effect of LB monolayers on the mixed crystal was investigated by using X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), and Inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results show that LB monolayer has selectivity for the different nucleation ions with equal charge numbers and about the same ion radius. The selectivity is dependent on the head groups of monolayer. The monolayer and the doped ions have also an effect on the crystal morphology. © 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Mixed crystal; LB monolayers; Selectivity
1. Introduction Recently, there is much interest in the application of highly organized organic assemblies, such as LB monolayers, as potential templates for directing the nucleation and growth of crystals [1– 3]. These studies have shown that LB monolayer have high selectivity for nucleation ions, due to specific interaction between ions and the head groups of monolayers, and the effect of electrostatic binding, geometric matching and stereochemical correspondence [4,5]. Therefore, we wonder whether LB monolayer can provide a significant implication for the understanding of the mechanism about the mixed crystals. For this purpose, we studied the crystallization of supersaturation barium sulfate solution with a small * Corresponding author.
amount of lead ions under monolayers. A series of interesting results have been obtained. 2. Experiments The supersaturated barium sulfate solution was prepared by mixing equimolar solutions of barium chloride and sodium sulfate (3.10 × 10 − 4 M). Lead nitrate was added to barium chloride solution before mixing. The solution (Ba2 + 3.10× 10 − 4 M, Pb2 + 1× 10 − 6 M, pH 6.0) was used for subphase. A calculated quantity of benhenic acid or sodium sulfopalmitate (1 mg ml − 1 in chloroform) was spread on the surface of freshly prepared subphase in a Langmuir trough (KSV-5000 Finland). The solvent was allowed to evaporate for 10 min, and then the monolayer was compressed to the required surface pressure (25 mN m − 1).
0927-7757/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 5 1 5 - X
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L. Lehui et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 175 (2000) 161–164
Preparation of samples for TEM: after keeping the pressure for 50 min, crystals grown under monolayer were collected by slowly dipping 400mesh copper grids (formvar-coated, carbon-reinforced). A JEOL 2000-FX TEM was operated at 200 kev. Preparation of samples for XPS: similar to the above operation, after 30 min (sample I), 35 min (sample II), and 45 min (sample III), respectively crystals under benhenic acid monolayer were transferred to silicon wafer in the Y-tape deposition mode for XPS (VG ESCA MKII). The samples of crystals under sodium sulfopalmitate
monolayer were taken in the same procedure. Excess ions collected during transferring procedure were washed with fourthly distilled water. The model of crystals transferred onto the silicon wafer or under monolayers is showed in Fig. 1. Preparation of samples for ICP-AES: similarly, after 30 min, a small part of subphase solution (sample I) under benhenic acid and sodium sulfopalmitate monolayers was respectively collected. After 35 min the above procedure was repeated for the collection of sample II, respectively. Measurement was performed with ICP-AES (POEMS TJA Company).
3. Results and discussions
Fig. 1. The proposed model of crystals; (a) crystal model under monolayer, (b) model of the crystals transferred onto the silicon wafers.
Fig. 2. TEM micrograph of the crystals grown under monolayers; (a) TEM micrograph of barium sulphate under the sodium sulfopalmitate monolayer, (b) TEM image of Pb-doped crystals under the sodium sulfopalmitate monolayer, (c) TEM micrograph of Pb-doped crystals under the benhenic acid monolayer. Table 1 Data of XPS analysis (molar percentage) Monolayer
Sample
Ba2+
Pb2+
Benhenic acid
I II III I
19.29 19.93 23.25 43.78
80.71 80.07 76.75 56.22
Sodium sulfopalmitate
II III
46.81 58.62
53.19 41.38
In general, when sulfate ion was added to the solution containing Ba2 + with a small amount of Pb2 + , lead ion will replace some of the barium ion sites in the barium sulfate lattice, and the replacement tends to be rather uniform during the crystallization, therefore, the crystal morphology has hardly changed [6]. But in the presence of the monolayer the result is different. As shown in Fig. 2, under the same subphase conditions but the different monolayers (Fig. 2b, c), the crystal morphology is different. In particular, when Pb2 + ion was doped, the crystal morphology was obviously changed (Fig. 2a, b), even under the same monolayers. Interestingly, the crystal morphology of barium sulfate grown under the sodium sulfopalmitate monolayer is similar to that under n-eicosyl sulfate [7], suggesting that –SO3 head group is a dominant factor during the process of crystallization. XPS measurement shows two peaks, located at 780.5 and 139.3 ev, the data were consistent with the binding energy of barium sulfate (780.6 ev), lead sulfate (139.3 ev). In Table 1, the analysis of the composition on the crystal surface indicated that the majority of crystals grown under the benhenic acid monolayer are lead sulfate, and lead sulfate constitute about half of crystals grown under the sodium sulfopalmitate monolayer. The molar percentage of Pb2 + in crystals grown under monolayers decreases while the induction time becomes long.
L. Lehui et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 175 (2000) 161–164 Table 2 Summarized results of ICP–AES analysis (ng ml−1)a Monolayer
Sample
Ba2+
Benhenic acid
I
4.27×104
II
4.25×104
10.0
I II
4.21×104 4.10×104
45.0 27.6
Sodium sulfopalmitate
Pb2+ 9.34
a Initial solution: CBa = 4.28×104 ng ml−1; CPb =2.07×103 ng ml−1
In addition, during the crystal growth the change of Pb2 + and Ba2 + concentration in subphase was studied (Table 2), it is clear that, Pb2 + concentration in subphase under the benhenic acid monolayer decreased more quickly than Ba2 + . Under sodium sulfopalmitate monolayer, the reduction of Pb2 + and Ba2 + concentration was almost the same. The above experimental results indicate that the presence of LB monolayer had three major effects on crystallization. Firstly, the induction time of crystal was reduced from 2.5 h to below 1 h. Secondly, the morphology of the crystal grown under different monolayers is different. Thirdly the LB monolayer can preferentially select different ions with equal charge numbers and about the same ion radius. The head groups of monolayer are dominant contributors to selectivity. These results can be rationalized in terms of the thermodynamic theory [8] for crystallization, DGn = 16p(DGi )3/3(DGb)2
(1)
DGb = kT loge S
(2)
In Eq. (1), where DGn is the activation energy for nucleation, DGi is the energy required to form new interface and DGb is the energy released in the formation of bonds in the bulk of the aggregate. In Eq. (2), where k is the Boltzmann constant, T is temperature and S is the relative supersaturation.
163
Evidently, DGn can be reduced by lowering DGi or increasing S. According to the principles of coordination, the head group of benhenic acid monolayer is a preferable ligand for lead ions, the lead ions are selectively transferred to the surface of monolayer, the local Pb2 + concentration under monolayer is increased and ion exclusion make Ba2 + concentration lower. Interface energy(DGi ), on the other hand, is lowered by the presence of the monolayer. As for sodium sulfopalmitate monolayer with –SO3 and –COOH head groups, the difference of specific interactions between the head groups of monolayer and two kinds of ions (Ba2 + and Pb2 + ) is little. So that the monolayer cannot show obviously preferential selectivity for certain mixed crystal ions (Ba2 + or Pb2 + ). Moreover, the molar percentage of Ba2 + in crystals grown under monolayers increases while the induction time becomes long. Because [Pb2 + ] is much lower than [Ba2 + ] in initial solution, and the ability of monolayer to direct further crystallization becomes weaker and weaker during the process of crystallization [4].
4. Conclusion Here we have shown that LB monolayer has the ability to preferentially select certain mixed crystal ions. Selectivity for ion is closely associated with head group of monolayer and the monolayer has an effect of the morphology of crystal. It is well known that the mixed crystal has been one of many challenges facing crystal science. The present study has provided an important guideline for the understanding of the mechanism of mixed crystal, purification of crystals and enriching of ions. Additionally, some practical application might be obtained by synthesizing proper film materials in the future.
Acknowledgements We thank the National Science Foundation of China for financial assistance.
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L. Lehui et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 175 (2000) 161–164
References [1] A.H. Hewer, D.J. Fink, Science 255 (1992) 1098. [2] S.J. Cooper, R.B. Session, J. Am. Chem. Soc. 120 (1998) 2090. [3] B.C. Bunker, P.C. Rieke, Science 264 (1994) 48.
.
[4] S. Mann, D.D. Archibald, Science 261 (1993) 1286. [5] S. Mann, J. Mater. Chem. 5 (1995) 935. [6] J.W. Meller, A Comprehensive Treatise on Inorganic and theoretical Chemistry, vol. 1, 590. [7] B.R. Heywood, S. Mann, J. Am. Chem. Soc. 114 (1992) 4681. [8] S. Mann, Nature 332 (1988) 119.
Effect of LB monolayers on the mixed crystals of lead and barium sulfate Lu Lehui *, Wang Liying, Zeng Guangfu, Cui Haining, Xi Shiquan Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, PR China
Abstract The effect of LB monolayers on the mixed crystal was investigated by using X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM), and Inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results show that LB monolayer has selectivity for the different nucleation ions with equal charge numbers and about the same ion radius. The selectivity is dependent on the head groups of monolayer. The monolayer and the doped ions have also an effect on the crystal morphology. © 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Mixed crystal; LB monolayers; Selectivity
1. Introduction Recently, there is much interest in the application of highly organized organic assemblies, such as LB monolayers, as potential templates for directing the nucleation and growth of crystals [1– 3]. These studies have shown that LB monolayer have high selectivity for nucleation ions, due to specific interaction between ions and the head groups of monolayers, and the effect of electrostatic binding, geometric matching and stereochemical correspondence [4,5]. Therefore, we wonder whether LB monolayer can provide a significant implication for the understanding of the mechanism about the mixed crystals. For this purpose, we studied the crystallization of supersaturation barium sulfate solution with a small * Corresponding author.
amount of lead ions under monolayers. A series of interesting results have been obtained. 2. Experiments The supersaturated barium sulfate solution was prepared by mixing equimolar solutions of barium chloride and sodium sulfate (3.10 × 10 − 4 M). Lead nitrate was added to barium chloride solution before mixing. The solution (Ba2 + 3.10× 10 − 4 M, Pb2 + 1× 10 − 6 M, pH 6.0) was used for subphase. A calculated quantity of benhenic acid or sodium sulfopalmitate (1 mg ml − 1 in chloroform) was spread on the surface of freshly prepared subphase in a Langmuir trough (KSV-5000 Finland). The solvent was allowed to evaporate for 10 min, and then the monolayer was compressed to the required surface pressure (25 mN m − 1).
0927-7757/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 5 1 5 - X
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L. Lehui et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 175 (2000) 161–164
Preparation of samples for TEM: after keeping the pressure for 50 min, crystals grown under monolayer were collected by slowly dipping 400mesh copper grids (formvar-coated, carbon-reinforced). A JEOL 2000-FX TEM was operated at 200 kev. Preparation of samples for XPS: similar to the above operation, after 30 min (sample I), 35 min (sample II), and 45 min (sample III), respectively crystals under benhenic acid monolayer were transferred to silicon wafer in the Y-tape deposition mode for XPS (VG ESCA MKII). The samples of crystals under sodium sulfopalmitate
monolayer were taken in the same procedure. Excess ions collected during transferring procedure were washed with fourthly distilled water. The model of crystals transferred onto the silicon wafer or under monolayers is showed in Fig. 1. Preparation of samples for ICP-AES: similarly, after 30 min, a small part of subphase solution (sample I) under benhenic acid and sodium sulfopalmitate monolayers was respectively collected. After 35 min the above procedure was repeated for the collection of sample II, respectively. Measurement was performed with ICP-AES (POEMS TJA Company).
3. Results and discussions
Fig. 1. The proposed model of crystals; (a) crystal model under monolayer, (b) model of the crystals transferred onto the silicon wafers.
Fig. 2. TEM micrograph of the crystals grown under monolayers; (a) TEM micrograph of barium sulphate under the sodium sulfopalmitate monolayer, (b) TEM image of Pb-doped crystals under the sodium sulfopalmitate monolayer, (c) TEM micrograph of Pb-doped crystals under the benhenic acid monolayer. Table 1 Data of XPS analysis (molar percentage) Monolayer
Sample
Ba2+
Pb2+
Benhenic acid
I II III I
19.29 19.93 23.25 43.78
80.71 80.07 76.75 56.22
Sodium sulfopalmitate
II III
46.81 58.62
53.19 41.38
In general, when sulfate ion was added to the solution containing Ba2 + with a small amount of Pb2 + , lead ion will replace some of the barium ion sites in the barium sulfate lattice, and the replacement tends to be rather uniform during the crystallization, therefore, the crystal morphology has hardly changed [6]. But in the presence of the monolayer the result is different. As shown in Fig. 2, under the same subphase conditions but the different monolayers (Fig. 2b, c), the crystal morphology is different. In particular, when Pb2 + ion was doped, the crystal morphology was obviously changed (Fig. 2a, b), even under the same monolayers. Interestingly, the crystal morphology of barium sulfate grown under the sodium sulfopalmitate monolayer is similar to that under n-eicosyl sulfate [7], suggesting that –SO3 head group is a dominant factor during the process of crystallization. XPS measurement shows two peaks, located at 780.5 and 139.3 ev, the data were consistent with the binding energy of barium sulfate (780.6 ev), lead sulfate (139.3 ev). In Table 1, the analysis of the composition on the crystal surface indicated that the majority of crystals grown under the benhenic acid monolayer are lead sulfate, and lead sulfate constitute about half of crystals grown under the sodium sulfopalmitate monolayer. The molar percentage of Pb2 + in crystals grown under monolayers decreases while the induction time becomes long.
L. Lehui et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 175 (2000) 161–164 Table 2 Summarized results of ICP–AES analysis (ng ml−1)a Monolayer
Sample
Ba2+
Benhenic acid
I
4.27×104
II
4.25×104
10.0
I II
4.21×104 4.10×104
45.0 27.6
Sodium sulfopalmitate
Pb2+ 9.34
a Initial solution: CBa = 4.28×104 ng ml−1; CPb =2.07×103 ng ml−1
In addition, during the crystal growth the change of Pb2 + and Ba2 + concentration in subphase was studied (Table 2), it is clear that, Pb2 + concentration in subphase under the benhenic acid monolayer decreased more quickly than Ba2 + . Under sodium sulfopalmitate monolayer, the reduction of Pb2 + and Ba2 + concentration was almost the same. The above experimental results indicate that the presence of LB monolayer had three major effects on crystallization. Firstly, the induction time of crystal was reduced from 2.5 h to below 1 h. Secondly, the morphology of the crystal grown under different monolayers is different. Thirdly the LB monolayer can preferentially select different ions with equal charge numbers and about the same ion radius. The head groups of monolayer are dominant contributors to selectivity. These results can be rationalized in terms of the thermodynamic theory [8] for crystallization, DGn = 16p(DGi )3/3(DGb)2
(1)
DGb = kT loge S
(2)
In Eq. (1), where DGn is the activation energy for nucleation, DGi is the energy required to form new interface and DGb is the energy released in the formation of bonds in the bulk of the aggregate. In Eq. (2), where k is the Boltzmann constant, T is temperature and S is the relative supersaturation.
163
Evidently, DGn can be reduced by lowering DGi or increasing S. According to the principles of coordination, the head group of benhenic acid monolayer is a preferable ligand for lead ions, the lead ions are selectively transferred to the surface of monolayer, the local Pb2 + concentration under monolayer is increased and ion exclusion make Ba2 + concentration lower. Interface energy(DGi ), on the other hand, is lowered by the presence of the monolayer. As for sodium sulfopalmitate monolayer with –SO3 and –COOH head groups, the difference of specific interactions between the head groups of monolayer and two kinds of ions (Ba2 + and Pb2 + ) is little. So that the monolayer cannot show obviously preferential selectivity for certain mixed crystal ions (Ba2 + or Pb2 + ). Moreover, the molar percentage of Ba2 + in crystals grown under monolayers increases while the induction time becomes long. Because [Pb2 + ] is much lower than [Ba2 + ] in initial solution, and the ability of monolayer to direct further crystallization becomes weaker and weaker during the process of crystallization [4].
4. Conclusion Here we have shown that LB monolayer has the ability to preferentially select certain mixed crystal ions. Selectivity for ion is closely associated with head group of monolayer and the monolayer has an effect of the morphology of crystal. It is well known that the mixed crystal has been one of many challenges facing crystal science. The present study has provided an important guideline for the understanding of the mechanism of mixed crystal, purification of crystals and enriching of ions. Additionally, some practical application might be obtained by synthesizing proper film materials in the future.
Acknowledgements We thank the National Science Foundation of China for financial assistance.
164
L. Lehui et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 175 (2000) 161–164
References [1] A.H. Hewer, D.J. Fink, Science 255 (1992) 1098. [2] S.J. Cooper, R.B. Session, J. Am. Chem. Soc. 120 (1998) 2090. [3] B.C. Bunker, P.C. Rieke, Science 264 (1994) 48.
.
[4] S. Mann, D.D. Archibald, Science 261 (1993) 1286. [5] S. Mann, J. Mater. Chem. 5 (1995) 935. [6] J.W. Meller, A Comprehensive Treatise on Inorganic and theoretical Chemistry, vol. 1, 590. [7] B.R. Heywood, S. Mann, J. Am. Chem. Soc. 114 (1992) 4681. [8] S. Mann, Nature 332 (1988) 119.