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Disperion in crystal growth rates: Palmitic acid-xylene system
Journal of Crystal Growth 102 (1990) 569 573 North-Holland
569
DISPERION IN CRYSTAL GROWTH RATES: PALMITIC ACID-XYLENE SYSTEM P.B.V. PRASAD Department of Physics, Government Polytechnic, Warangal 506 007, India
Received 31 January 1989; manuscript received in final form 26 January 1990
A study of the growth rates of different forms of some fatty acids has been taken up to throw some light on the influence of crystal structures on the growth rates. In this connection some experimental ssork has been carried out to obtain an insight into the dispersion in crystal growth rates, since it has relevance with the main object of the study. Both findings and conclusions are presented.
1. Introduction
purity of 99%; the solvent xylene (Glaxo made) is an analytical grade reagent.
A program of evaluation of growth rates of different forms of palmitic acid and pentadecanoic acid has been initiated to study the influence of the crystal structures on the growth rates. Investigations carried out on the crystallization of palmitic acid from xylene solutions showed unambiguous dispersion in growth rate data, and this problem is discussed in the present work, since there is an overall influence of the cell structure on the growth rates of crystals. Palmitic acid is known to have three distinct habits: (i) needle-like crystals (referred to as A form); (ii) lozenge shaped crystals (referred to as B and C forms). B and C forms can easily be distinguished basing upon the angle 4, which is defined as the acute angle between two consecutive <110) edges of the lozenge shaped crystals: ~ 56° and 74°in case of B and C forms respectively [1]. The A form crystals show triclinic symmetry, whilst B and C forms belong to monoclinic symmetry [11.
3. Experimental 3,1 Solubility of palmitic acid
Measurements on the solubility of palmitic acid in xylene have been made at temperature values ranging between 20 and 30°C. The palmitic acid xylene solutions taken in 10 ml standard flasks were suspended in a double wall beaker. Thermostated water circulated through the systern. The solutions were sufficiently cooled in order to crystallize the solute. Usually the entire solution was turned into a thick precipitate. Then the ternperature was raised step-wise and at each temperature setting the solution was allowed to stay for 1 h. During the whole process the solution in the standard flask and the thermostated water were continuously stirred with the help of a magnetic stirrer. The temperature at which the last trace of precipitate disappears was taken as the saturation point. The solubility data are shown in fig. 1.
2. Materials
3.2. Crystallization
The palmitic acid employed in the present work is Loba-Chemie (Indo-Austranol Co. made) with
A water circulation thermostat, which provides a 0.01°C constancy of temperature, was employed
0022-0248/90/$03.50
1990
Elsevier Science Publishers B.V. (North-Holland)
571)
P B.
.
Prasad
Di speri ion in crs Oal groit 16 rater pa/mew arid
i
/ene is stern
floating crystal. In all the experimental runs, the solution was cooled far below the saturation temperature. The crystals were spontaneously nucleated and then the crystallization temperature
25
~as raised causing the dissolution of the smaller crystals. The process was repeated until only one crystal survived. FinaIl~ the temperature was lowered to a pre-chosen crystallization temperalure. This method is similar to the one described by Human et a!. [2]. Either the crystals were photographed on 400 ASA film or their projections were recorded along with the time. The growth rates were measured from the variation of the crystal dimensions. The ad~ancement of faces ~as evaluated by
20
a E °-
15
measuring the total shift produced by the two couples of opposite faces. i.e.. (110). (110) and (110). (110). Then the average shift of the faces was evaluated. Therefore the shift taken into consideration shows the combined behaviour of a set of two (opposite) faces. The method gives the overall behaviour of the entire crystal: other meth 7
ods described in the literature, which consider the shifts produced on one face, are not suitable for the present purpose. as we are interested in the
____________________________________ 20
25
30
T
overall behaviour of a crystal.
5( C)
We only take into consideration the data of
Fig 1. Solubility curve of palmitic acid in xvlene.
in the present studies. Crystallization was carried out in a double-wall growth cell. An inverted microscope, fitted with very high quality surface reflecting mirrors, was employed for observing the crystals. A 500 W halogen lamp was used for illumination. A 10 cm thick water mass was used as infrared filter. The growing crystals settle on the bottom surface of the growth cell. We did not try other techniques, such as suspended or freely
crystals that gre~ in the central region of the growth cell’s bottom surface, in order to minimize the influence of the cell walls on the growth rates. ~•
Observations
One of the aspects observed was the fluctuations in the growth rates. Some of the representative curves selected among several curves are pre sented here. The crystallization data are shown in
Fable I C rvstallization data C rvstal No
Saturation temperature (°()
Crystallization temperature (°C)
Actual iniiial concentration C
K I, K 2 K 3, K 4 K 5. K 6
272 29.1 297
248 26.4 27 35
17.82 22 3~s
~a
g palmitic acid in 100 g xylene. [(C C~) Cl I
238)
Saturation
Supersaturation
concenirailon
a
129 16 1 17.4
0381 1) 388 0 369
PB. V. Prasad
/ Dispersion in ciystal growth rates
table 1. Fig. 2 shows the shift of the two sets of faces i.e., (110), (110) and (110), (110), belonging to two B form crystals (K-i and K-21 of palmitic acid versus time. In this case all the faces grow approximately with the same rate (except for a minor fluctuation) of about 4 x 10 ~ rn/s. In fig. 3, we see two pairs of curves belonging to two crystals (K-3 and K-4). The growth rates of the crystals show different behaviour. The growth rate of crystal K-3 changes from 1.0 x 10 6 to 6 x 10 m/s, whereas the growth rate of crystal K-4 is 3.7 x 10 ~ rn/s. In the case of crystal K-4, the
I
pa/mine acid xylene system
571
30
a 25 b
d E 20
C~P
301~ib Sri
25
10
10 Time (mm)
20
Fig. 3. Two crystals, K-3 and K-4. having different growth rates. (0) Crystal K 3: curve a ((110):(110)); curve b 10
~(110):(110)}. (.) Crystal K 4: curve c((110):(110)}: curve d {(110)t110)~.
(110) 5
(110) 0
10
20
growth behaviour of the entire set of lateral faces fluctuates. In fig. 4, one can see that the growth rates of the faces of the crystal K-S oscillate around a mean value of 2.7 x 10 ~ rn/s. In the case of crystal K-6, one set of faces advances with the same average velocity of 2.7 x 10 ~ m/s; however, the other two faces of crystal K-6 do not grow during an appreciable period of time.
Time (mm) Fig. 2. Growth rates of B form crystals of palmitic acids. The two crystals K-I and K-2 have almost equal growth rates. ~ii~ Crystal K-I: curve c belongs to the set of faces (110) and (110). briefly c((110):(110)}; curved ((II0):(110)). (.) Crystal K-2: curve a ((110):(I10)); curveb {(1l0):(I10)).
5. Discussion The growth curves (figs. 2 4) show that different situations occur. Crystals can have: (a) identi-
P B V. Pra~ad Dispersion in en stal grov.th rater: pulmitic acid vt/ene .s V stem
572
15-
10
a
C ~3’~ ,,y,___._
.
-
~
_a -
-
~_
~
-~
___
~
Time )min) Fig. 4 Uniform fluctuations in the growth rates of crystal K 5 and non uniform and erratic growth of crystal K-6. (a) Crystal K-5: curvec {(110))I10)}; curved ((110):(llO)}.(.) Crystal K—6. curve a {(I10):i110)}: curveb ((110):(110)}.
cal growth rates with slight fluctuations. (b) the gro~vthrates differing moderately and (c) severe differences in the growth rates. Case (c) is quite interesting. Because one finds not only fluctuations in the growth rates, but also that the faces stop growing for some time and then continue growing. Such a dispersion in the growth rates is most unfavourable when the aim of the experiment is to compare the growth rates of crystals belonging to different structures. In fact such a dispersion can mask the intrinsic growth rate differences: therefore an examination of the conditions that lead to such scattering was made. The role of dust particles, impurities and dislocations in influencing the growth rates is well known. It is also known that concentration gradients build up in case of growth from stagnant solution, which can influence the growth rate [3]. The method of stirring the solution to maintain even distribution of solute around a growing crystal has a definite advantage, as it can reduce the influence of concentration gradients. The growth rate curves obtained by Jetten [4] show negligible spread in case of stirred solution and fluctuations in case of unstirred solution. However, the method of stir-
ring the solution is not suitable in the present case. The reasons are that: (i) It is difficult to isolate the rotor of the magnetic stirrer, rotating on the bottorn surface, where the crystals are growing. (ii) Due to stirring, the position of a self-nucleated crystal will be continuously shifted from the field of observation; therefore measurements are not possible. (iii) The shifttng in position may damage the crystal, which can lead to generation of defects leading to enhanced growth rates. In spite of these drawbacks, we attempted stirring the solution and observed the following effect: A small cluster of crystals crystallized at higher supersaturations. The individual crystals have well-developed faces; the solution was completely clear. Then stirring was switched on at 600 RPM. Within a few seconds, the solution turned turbid and finally prectpitation filled the entire volume of the solution. Obviously. when supersaturation rises, the stirring increases the nucleation frequency enormously. Hence stirring was not suitable and was not employed. In the present case, it is therefore possible that both dislocations and fluctuations in the diffusion fields [3] influence the growth rates. Both supersaturations and crystallization temperatures (table
P.B. V. Prasad
/
Dispersion in crystal growth rates: palmitic acid xvlene system
573
1) employed in the present study do not differ greatly, so as to invoke different growth mechanisms and thus appreciable differences in the growth rates. The behaviour of crystal K-6 (fig. 4) is of some
rates, at least for those faces which belong to the same crystallographic form.
interest, because the set of faces stop growing for some time. Complete arrest of growth of the {011 } faces of ADP were reported by Chernov et al. [5]; a similar observation on a {100} face of potash alum crystal was reported by Van Enckevort [6]. Similar observations were also reported by Konak [7] and Ovsienko and Alfintsev [8]. Several reasons have been advanced to explain the observations [5 9].
The author is thankful to Dr. A. Purushotham Rao and Sri M. Vijaya Kumar for their helpful cooperation. The laboratory assistance given by P.B. Shashi Kanth is gratefully acknowledged. The author is thankful to the referee for his very useful comments.
A survey of the literature concerning the growth rate measurements shows that there is a tendency to present the growth rate data in the form of smooth curves and no indication of spread in the growth rates is given. Such a representation is probably not realistic.
6. Conclusions It is suggested here that treating the growth rate of a given crystal as a steady state growth, under a set of fixed parameters, is an ideal case. It may be interesting to remember that crystal growth is a highly non-stationary process, even if the external parameters which can influence the growth are kept constant [10]. Therefore it would be better to adopt the notion of average growth
Acknowledgements
Ref erencec [I] AR. Verma, Proc. Roy. Soc. (London) A228 (1955) 34.
[21 H.J. Human, J.P. van der Eerden, J.G.M. Odekerken, J. Crystal GrowthL.A.M.J. 51(1981)Jetten 589. and [3] H.J. Human, PhD Thesis, University of NiJmegen (1981) ch. IV. [41 L.A.M.J. Jetten. PhD Thesis. University of NiJmegen (1983) ch. III. [5] A.A., Chernov, IL. Smolski, V.F. Parov, Yu.G. Kuznetsoy and V.N. Rozhanskii. Soviet Phys.-Dokl. 24 (1979) 760. [61 W.J.P. van Enkevort, PhD Thesis. University of Nijmegen (1982) ch. IV. [7] AR. Konak, J. Crystal Growth 22 (1974) 67. [8] D.E. Ovsienko and Sirota, A. Alfintsev. in: and Crystallization Processes, Eds. N.N. F.K. Gorskii V.M. Vankash (Consultants Bureau. New York. 1966) p. 25. [9] S.J. Jan~im~,in: Industrial Crystallization 84, Eds. S.J. JanIié and E.J. de Jong (Elsevier, Amsterdam, 1984) p. 3. [10] B. Dam, E. Polman and W.J.P. van Enkevort, in: Industrial Crystallization, Eds. S.J. Jan~iéand E.J. de Jong (Elsevier, Amsterdam, 1984) p. 97 (and reference [8]quoted therein).
569
DISPERION IN CRYSTAL GROWTH RATES: PALMITIC ACID-XYLENE SYSTEM P.B.V. PRASAD Department of Physics, Government Polytechnic, Warangal 506 007, India
Received 31 January 1989; manuscript received in final form 26 January 1990
A study of the growth rates of different forms of some fatty acids has been taken up to throw some light on the influence of crystal structures on the growth rates. In this connection some experimental ssork has been carried out to obtain an insight into the dispersion in crystal growth rates, since it has relevance with the main object of the study. Both findings and conclusions are presented.
1. Introduction
purity of 99%; the solvent xylene (Glaxo made) is an analytical grade reagent.
A program of evaluation of growth rates of different forms of palmitic acid and pentadecanoic acid has been initiated to study the influence of the crystal structures on the growth rates. Investigations carried out on the crystallization of palmitic acid from xylene solutions showed unambiguous dispersion in growth rate data, and this problem is discussed in the present work, since there is an overall influence of the cell structure on the growth rates of crystals. Palmitic acid is known to have three distinct habits: (i) needle-like crystals (referred to as A form); (ii) lozenge shaped crystals (referred to as B and C forms). B and C forms can easily be distinguished basing upon the angle 4, which is defined as the acute angle between two consecutive <110) edges of the lozenge shaped crystals: ~ 56° and 74°in case of B and C forms respectively [1]. The A form crystals show triclinic symmetry, whilst B and C forms belong to monoclinic symmetry [11.
3. Experimental 3,1 Solubility of palmitic acid
Measurements on the solubility of palmitic acid in xylene have been made at temperature values ranging between 20 and 30°C. The palmitic acid xylene solutions taken in 10 ml standard flasks were suspended in a double wall beaker. Thermostated water circulated through the systern. The solutions were sufficiently cooled in order to crystallize the solute. Usually the entire solution was turned into a thick precipitate. Then the ternperature was raised step-wise and at each temperature setting the solution was allowed to stay for 1 h. During the whole process the solution in the standard flask and the thermostated water were continuously stirred with the help of a magnetic stirrer. The temperature at which the last trace of precipitate disappears was taken as the saturation point. The solubility data are shown in fig. 1.
2. Materials
3.2. Crystallization
The palmitic acid employed in the present work is Loba-Chemie (Indo-Austranol Co. made) with
A water circulation thermostat, which provides a 0.01°C constancy of temperature, was employed
0022-0248/90/$03.50
1990
Elsevier Science Publishers B.V. (North-Holland)
571)
P B.
.
Prasad
Di speri ion in crs Oal groit 16 rater pa/mew arid
i
/ene is stern
floating crystal. In all the experimental runs, the solution was cooled far below the saturation temperature. The crystals were spontaneously nucleated and then the crystallization temperature
25
~as raised causing the dissolution of the smaller crystals. The process was repeated until only one crystal survived. FinaIl~ the temperature was lowered to a pre-chosen crystallization temperalure. This method is similar to the one described by Human et a!. [2]. Either the crystals were photographed on 400 ASA film or their projections were recorded along with the time. The growth rates were measured from the variation of the crystal dimensions. The ad~ancement of faces ~as evaluated by
20
a E °-
15
measuring the total shift produced by the two couples of opposite faces. i.e.. (110). (110) and (110). (110). Then the average shift of the faces was evaluated. Therefore the shift taken into consideration shows the combined behaviour of a set of two (opposite) faces. The method gives the overall behaviour of the entire crystal: other meth 7
ods described in the literature, which consider the shifts produced on one face, are not suitable for the present purpose. as we are interested in the
____________________________________ 20
25
30
T
overall behaviour of a crystal.
5( C)
We only take into consideration the data of
Fig 1. Solubility curve of palmitic acid in xvlene.
in the present studies. Crystallization was carried out in a double-wall growth cell. An inverted microscope, fitted with very high quality surface reflecting mirrors, was employed for observing the crystals. A 500 W halogen lamp was used for illumination. A 10 cm thick water mass was used as infrared filter. The growing crystals settle on the bottom surface of the growth cell. We did not try other techniques, such as suspended or freely
crystals that gre~ in the central region of the growth cell’s bottom surface, in order to minimize the influence of the cell walls on the growth rates. ~•
Observations
One of the aspects observed was the fluctuations in the growth rates. Some of the representative curves selected among several curves are pre sented here. The crystallization data are shown in
Fable I C rvstallization data C rvstal No
Saturation temperature (°()
Crystallization temperature (°C)
Actual iniiial concentration C
K I, K 2 K 3, K 4 K 5. K 6
272 29.1 297
248 26.4 27 35
17.82 22 3~s
~a
g palmitic acid in 100 g xylene. [(C C~) Cl I
238)
Saturation
Supersaturation
concenirailon
a
129 16 1 17.4
0381 1) 388 0 369
PB. V. Prasad
/ Dispersion in ciystal growth rates
table 1. Fig. 2 shows the shift of the two sets of faces i.e., (110), (110) and (110), (110), belonging to two B form crystals (K-i and K-21 of palmitic acid versus time. In this case all the faces grow approximately with the same rate (except for a minor fluctuation) of about 4 x 10 ~ rn/s. In fig. 3, we see two pairs of curves belonging to two crystals (K-3 and K-4). The growth rates of the crystals show different behaviour. The growth rate of crystal K-3 changes from 1.0 x 10 6 to 6 x 10 m/s, whereas the growth rate of crystal K-4 is 3.7 x 10 ~ rn/s. In the case of crystal K-4, the
I
pa/mine acid xylene system
571
30
a 25 b
d E 20
C~P
301~ib Sri
25
10
10 Time (mm)
20
Fig. 3. Two crystals, K-3 and K-4. having different growth rates. (0) Crystal K 3: curve a ((110):(110)); curve b 10
~(110):(110)}. (.) Crystal K 4: curve c((110):(110)}: curve d {(110)t110)~.
(110) 5
(110) 0
10
20
growth behaviour of the entire set of lateral faces fluctuates. In fig. 4, one can see that the growth rates of the faces of the crystal K-S oscillate around a mean value of 2.7 x 10 ~ rn/s. In the case of crystal K-6, one set of faces advances with the same average velocity of 2.7 x 10 ~ m/s; however, the other two faces of crystal K-6 do not grow during an appreciable period of time.
Time (mm) Fig. 2. Growth rates of B form crystals of palmitic acids. The two crystals K-I and K-2 have almost equal growth rates. ~ii~ Crystal K-I: curve c belongs to the set of faces (110) and (110). briefly c((110):(110)}; curved ((II0):(110)). (.) Crystal K-2: curve a ((110):(I10)); curveb {(1l0):(I10)).
5. Discussion The growth curves (figs. 2 4) show that different situations occur. Crystals can have: (a) identi-
P B V. Pra~ad Dispersion in en stal grov.th rater: pulmitic acid vt/ene .s V stem
572
15-
10
a
C ~3’~ ,,y,___._
.
-
~
_a -
-
~_
~
-~
___
~
Time )min) Fig. 4 Uniform fluctuations in the growth rates of crystal K 5 and non uniform and erratic growth of crystal K-6. (a) Crystal K-5: curvec {(110))I10)}; curved ((110):(llO)}.(.) Crystal K—6. curve a {(I10):i110)}: curveb ((110):(110)}.
cal growth rates with slight fluctuations. (b) the gro~vthrates differing moderately and (c) severe differences in the growth rates. Case (c) is quite interesting. Because one finds not only fluctuations in the growth rates, but also that the faces stop growing for some time and then continue growing. Such a dispersion in the growth rates is most unfavourable when the aim of the experiment is to compare the growth rates of crystals belonging to different structures. In fact such a dispersion can mask the intrinsic growth rate differences: therefore an examination of the conditions that lead to such scattering was made. The role of dust particles, impurities and dislocations in influencing the growth rates is well known. It is also known that concentration gradients build up in case of growth from stagnant solution, which can influence the growth rate [3]. The method of stirring the solution to maintain even distribution of solute around a growing crystal has a definite advantage, as it can reduce the influence of concentration gradients. The growth rate curves obtained by Jetten [4] show negligible spread in case of stirred solution and fluctuations in case of unstirred solution. However, the method of stir-
ring the solution is not suitable in the present case. The reasons are that: (i) It is difficult to isolate the rotor of the magnetic stirrer, rotating on the bottorn surface, where the crystals are growing. (ii) Due to stirring, the position of a self-nucleated crystal will be continuously shifted from the field of observation; therefore measurements are not possible. (iii) The shifttng in position may damage the crystal, which can lead to generation of defects leading to enhanced growth rates. In spite of these drawbacks, we attempted stirring the solution and observed the following effect: A small cluster of crystals crystallized at higher supersaturations. The individual crystals have well-developed faces; the solution was completely clear. Then stirring was switched on at 600 RPM. Within a few seconds, the solution turned turbid and finally prectpitation filled the entire volume of the solution. Obviously. when supersaturation rises, the stirring increases the nucleation frequency enormously. Hence stirring was not suitable and was not employed. In the present case, it is therefore possible that both dislocations and fluctuations in the diffusion fields [3] influence the growth rates. Both supersaturations and crystallization temperatures (table
P.B. V. Prasad
/
Dispersion in crystal growth rates: palmitic acid xvlene system
573
1) employed in the present study do not differ greatly, so as to invoke different growth mechanisms and thus appreciable differences in the growth rates. The behaviour of crystal K-6 (fig. 4) is of some
rates, at least for those faces which belong to the same crystallographic form.
interest, because the set of faces stop growing for some time. Complete arrest of growth of the {011 } faces of ADP were reported by Chernov et al. [5]; a similar observation on a {100} face of potash alum crystal was reported by Van Enckevort [6]. Similar observations were also reported by Konak [7] and Ovsienko and Alfintsev [8]. Several reasons have been advanced to explain the observations [5 9].
The author is thankful to Dr. A. Purushotham Rao and Sri M. Vijaya Kumar for their helpful cooperation. The laboratory assistance given by P.B. Shashi Kanth is gratefully acknowledged. The author is thankful to the referee for his very useful comments.
A survey of the literature concerning the growth rate measurements shows that there is a tendency to present the growth rate data in the form of smooth curves and no indication of spread in the growth rates is given. Such a representation is probably not realistic.
6. Conclusions It is suggested here that treating the growth rate of a given crystal as a steady state growth, under a set of fixed parameters, is an ideal case. It may be interesting to remember that crystal growth is a highly non-stationary process, even if the external parameters which can influence the growth are kept constant [10]. Therefore it would be better to adopt the notion of average growth
Acknowledgements
Ref erencec [I] AR. Verma, Proc. Roy. Soc. (London) A228 (1955) 34.
[21 H.J. Human, J.P. van der Eerden, J.G.M. Odekerken, J. Crystal GrowthL.A.M.J. 51(1981)Jetten 589. and [3] H.J. Human, PhD Thesis, University of NiJmegen (1981) ch. IV. [41 L.A.M.J. Jetten. PhD Thesis. University of NiJmegen (1983) ch. III. [5] A.A., Chernov, IL. Smolski, V.F. Parov, Yu.G. Kuznetsoy and V.N. Rozhanskii. Soviet Phys.-Dokl. 24 (1979) 760. [61 W.J.P. van Enkevort, PhD Thesis. University of Nijmegen (1982) ch. IV. [7] AR. Konak, J. Crystal Growth 22 (1974) 67. [8] D.E. Ovsienko and Sirota, A. Alfintsev. in: and Crystallization Processes, Eds. N.N. F.K. Gorskii V.M. Vankash (Consultants Bureau. New York. 1966) p. 25. [9] S.J. Jan~im~,in: Industrial Crystallization 84, Eds. S.J. JanIié and E.J. de Jong (Elsevier, Amsterdam, 1984) p. 3. [10] B. Dam, E. Polman and W.J.P. van Enkevort, in: Industrial Crystallization, Eds. S.J. Jan~iéand E.J. de Jong (Elsevier, Amsterdam, 1984) p. 97 (and reference [8]quoted therein).