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Stability, occurrence and step morphology of polymorphs and polytypes of stearic acid
236
Journal of Crystal (I~rowth87 ( 1988) 236—742 North—Holland, Amsterdam
STABILITY, OCCURRENCE AND STEP MORPHOLOGY OF POLYMORPHS AND POt YTYPES OF STEARIC ACID I. Stability and occurrence Kiyotaka SATO
*
72(1,Japa,i
Faculti’ of Applied Biological Science, Hiroshima Unii’ersiti’, Fukui’ama
and
Masamichi KOBAYASHI and Hirofumi MORISHITA
* *
I)epartment of Macromolecular Science, Faculir of .S’cience, Osaka Cnn‘c’rsitr. Toionah a 56Q,.Jupun Received 26 June 1987: manuscript received in final form 9 September 1987
The thermodvnamical stability and occurrence of three different structural modifications: monoclinic and pseudo-orthorhombie polytvpes of the B polymorph. called B(mon) and B(orth 11) respectivelx. and monoclinic (‘ polvmorph, CO on), of stearic acid haxe been examined. The solubility measurements, overgrowth and isothermal crystallization proved that B(crth II) and ((moo) arc most stable below and above 32°C. respectively, whereas B(mon) is always nietastahle. turning to he more stable than Cli on) below between 23 and 24°C. It was confirmed b~micro-probe Ranian spectroscopy that the polytvptc structure of the newly-overgrown crystal on the (001) faces of the seed crystal of BOon) changed from the original monoclinic to pseudo-orthorhombic. B(orth II). at ver~ small supersaturations above 23°C. This conversion was caused h~ the solubilits difference. The occurrence experiment indicated a slight tendency that B(mon) crystallized more than B(orth II) at lower temperatures.
1. Introduction Stearic acid, one of the most common bio-lipid constituents, reveals polymorphism and polytypism. The former is caused by different molecular conformations and packings, resulting tn different three-dimensional unit cell structures. The polytypism originates from alternative stacking modes of the long-chain lamellae normal to the lamellar surface with certain periodicity [1]. As for the polymorphism. four modifications are availaHe: triclinic A. monoclinic B, C and E [2—4]. Since E crystallizes below about 10°C, the other three polymorphs have recently attracted considerable attention, * * *
To whom all correspondence should he addressed. Present address: Faculty of Education. Nagasaki ljniversity. 1-~4.Bunkyo-cho. Nagasaki, Japan.
In case of the polvtvpic structure of the fatty acids, the first work was carried out three decades ago [5,61. An optical observatton of interlaced step patterns on the as-grown (001 ) faces of the crystal gave proof for the alternative lamellar stacktng sequences along the [001] axis. Very recently we have investigated two basic polyt~pes of the B polymorph of stearic acid by polartzed Raman and infra-red (IR) spectroscopy [7]. One polytype shows a monoclinic structure with P2~/a named B(mon) and another one belongs to a pseudo-orthorhombic structure with Pcab. named B(orth II). in which each lamella is turned by 1800 with respect to both adjacent layers. We found that different i nterlamel lar structures between the above two polytypes are convtncinglv detectable
either tn the methyl hendino0 IR hand or in the . low-frequency Rarnan spectra. Addittonall’v, Brtlbum scattering was applied to the single crystals
0022-0248/88/$03.50 31~Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
K. Sato et al.
/ Polymorphs and polvtypes of stearic acid.
237
1
of B(mon) and B(orth II) to measure elastic stiffnesses [8]. As for the C polymorph, the IR and Raman studies over several tens of single crystals have yielded a single pattern. This presumably
indicates that room C may not reveal multiple poiycrate typism crystallization around conditions temperature [9].and under modSo far the crystallization behavior of the polymorphs and of the polytypes of stearic acid has been explored in a separate way, or sometimes the polytypism has been rather disregarded. To get a full understanding, however, it is convenient to systematically clarify the crystallization processes of the two mentioned structural modifications having the same framework. In doing so, one has to examine thermodynamical stability, crystallization kinetics and mono-lamellar surface step morphology of all the possible modifications with multiple methods. We have reported some preliminary results very recently [8]. In the present papers, after a short display of the crystal structures, we report the stability and crystallization from solution of B(mon), B(orth I~ and C(mon) (paper I). In the following paper (paper II) we will report the mono-lamellar step morphology of each modification and microscopic mechanisms of composite polymorphic/polytypic transformation, which has singly been elucidated by the observation of the step pattern.
2. Structures of B(mon), B(orth II) and C(mon) Fig. I shows the crystal structures of C(mon) and B(orth II) and B(mon). The C polymorph reveals all trans conformation both of aliphatic chains and carboxyl groups [9]. Meanwhile, B shows a partially twisted structure due to gauche conformations adjacent to the carboxyl groups
/
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-c,-~ Cmon
a
Bmon Borth II
Fig. L Crystal structures of C(mon). B(mon) and B(orth II) of stearic acid as revealed (h-axis projection).
[10]. B(mon) is a single-layered polytype in which one lamella is a repetition period with the same stacking mode [10]. B(orth II) is a double-layered polytype formed by an alternative rotation of one lamella about normal to the lamellar interface [6]. Table 1 summarizes the lattice parameters of the three modifications.
3. Materials and methods The purity of stearic acid (Nippon Oil and Fats Co.) was guaranteed 99.8%. No further purification was done. The solvents employed were n-hexane (Yoneyama Pharm. > 97%) and decane (Tokyo Kasei Co. > 99%). The single crystals were grown from n-hexane solutions either by cooling or by iso-thermal methods in a glass-made growth cell thermostated with circulating water (±0.05 °C). In the former method, the solution saturated at T~was rapidly
Table I Lattice parameters of B(orth II). B(mon) and C(mon) of stearic acid
B(orth II) B(mon) C(mon)
Space group
a
(A)
b (A)
c (A)
$
P cab 2i/a P P2 1/a
5.58 5.587
7.34 7.386
87.58 49.33
90 L17.24
9.363
4.965
50.88
1.28.46
(deg)
238
KSato ci al. / /‘oliSfriorphs and poIrtrpe~sof siearic cicicl. /
cooled down to 1/ We could not examtne the effects of temperature and supersaturation mdcpendently. although it was possible for the relative occurrence of B and C [11.12]. This is because an tn-sttu determination of the relative occurrence of the two polytypes of B was not facilitated in the present method (see below). Instead, the relative frequency of each polytype was measured for all the precipitates taken out of the solution after the overall crystallization which might expertence varying temperature and supersaturation. Even so. we examined the effect of temperature on the occurrence of the two polytypes of B by changing
soluhility relationship between two polytypes and C. 15 cm~of solution were first saturated with respect to B, then a large amount of C’ crystals (approximately 0.5 g) were put into the solution. This was done so that the solution became nearly saturated with respect to C. After half an hour. a single crystal of B(rnon) or B(orth II) was further put itlto the solution, then the overgrowth was carried out over several hours. The overgrown seed crystals were taken out of the solution for the characterization by micro-probe Rarnan method. In all the experiments, the identification of the polymorphs was easily done by observing the
7~and 7~.. The initial supersaturation value corresponding to the difference (7/—I~)was calculated from the solubility data of B [13], e.g.. approximately 18%/°C. Additionally we fluctuated the solution temperature like this: cooled from 180 C (1~)to 16°C (T~)over one night, and raised to 18°C and kept there over one week, then again cooled to 16°C over one night. We checked a change in the concentration of each polytype tn the precipitates. The solution was gently stirred by hand to accelerate nucleation. after T~was reached, In case of the iso-thermal crystallization, the single crystals were grown in quiescent solutions vta solution-mediated transformation [14] at different temperatures. i.e.. the C(mon) crystals were put into the solution nearly saturated with respect
crystal shape: acute angles between { 110) faces were 55° for C and 75° for B. No macroscopic difference, however, was detectable in the crystal shapes of B(mon) and B(orth II). Therefore. polarized Raman spectroscopy was applied to all the precipttated B crystals. We employed a Jasco R500 monochromator using the 514.5 nrn line (Ar laser). The single crystal was mounted on the goniometer head and the scattered radiation at the right angle was measured. The spectra were recorded in the frequency range 2---400 cm i: yet the clearest difference was available below 50 cm [7]. The overgrowth feature was examined by a micro-probe Raman spectroscope consisting of an epi-illurnination optical microscope (Olympus BH-2) and a Jasco CT-1000D double monochrontator [15]. The inctdent polarized beam (514.5 nrn, Ar laser) was focused at a selected position of the crystal. The dtrnension of the laser spot was I ~tm and the depth of the focal point measured from the crystal surl’ace can he adjusted ±I p.m with an objective lens of x 100 magnification. The posttion subjected to the spectral measurement was monitored by a TV camera. The backward scattered light was collected by the same objective lens, passed through a beam splitter, and entered the monochromator through a telemeter lens. Before the entrance of the nionochromator. an analyser and a polarizatton scramber were put. as in the case of the ordinary Rarnan measurements. If necessary, an aperture was inserted in front of the telemeter lens in order to cut-off the light passing through the peripheral area of the objective lens. The Raman spectrum in the region helow 50 cm was recorded.
to the more stable form, then the iso-thermal crystallization was carried out over several days. The solution temperature was controlled within ±0.1°C. We measured the concentrations of B(orth II) and B(mon) out of the precipitates which occurred at the expense of C(mon).
The stability relationship of the three modiftcations was evaluated in more detail by measuring the solubility relationship in decane solutions. Since the soluhilities of C and B were already examined [13]. we first paid attention to B( mon) and B(orth II) using the same method as that of ref. [13]: a single crystal of each polytype was put in the solution, then we fluctuated the solution temperature to obtain the saturation temperature for each poI~type in the same solute concentration. Thereafter we studied overgrowth of the more stable polytype on the less stable one to get the
K. Sato et of
/ Polvmorphs and p01stvpes of stearic acid.
1
239
4. Results and discussion
0.2°C between B(orth II) and B(mon) leads to the conclusion that the B(mon)-saturated solution is
4.1. Stability
supersaturated with respect to B(orth II) by approximately 3.6% of a supersaturation value.
First, we report the thermodynamical stability which was evaluated by the solubility measure-
Previous works [13,14] show that C is less stable than B below 32°C. So, we next examined the
ments. Fig. 2 shows an evolution of the growth of B(orth II) at the expense of B(mon) in the nearly saturated decane solution at 30.0 ±0.05°C. The same results were seen at 20.0 and 15.0°C.From this we concluded that B(mon) has a higher soluhility. being less stable, than B(orth II) at 15 < T <30°C. Then, a difference in the saturation temperatures (7~)of B(orth II) and B(mon) in the decane solution was measured around 30 and 17°C. As a result, we found that T 5 differs by
solubility relationship among B(mon), B(orth II) and C(mon). This determination cannot he done by such a simple method as fig. 2. because of a possible conversion of the polytypic structure of B during growth at the expense of C(mon). Therefore the overgrowth was examined by micro-probe Raman method. The results at various temperatures are displayed in table 2. In this table, the results we
0.2 ±0.1°C at both temperatures.This means that
the solubilities of B(orth II) and B(mon) differ in an almost parallel way around room temperature. This difference in the saturation temperature by
obtained using the seed crystal of B(orth II) are not displayed because of no conversion. This means that B(orth II) has the lowest solubility at the temperatures examined, proving that the lowtemperature stable form, singly referred as B in the previous work [12,13], is actually the doublelayered polytype of B. On the other hand, B(mon) revealed a conversion of the polytypic structure 22°C. a normal overgrowth of B(mon) occurred,
rth]I on.
.
~‘.
crystal. but B(orth Raman II) overgrew spectra corresponding onto above to each 23°C. part and overgrowth of B(orth II) B(mon) on the same B(mon)
.
-. S.
—
-~
,
-.
.
3o~
into 3B(orth II) at higher temperatures. Below Fig. shows photographs of dissolution of B(mon)
•
-
are also shown. In fig. 3a, the typical dissolution hillocks are seen, whereas growth spiral steps appear in fig. 3b. These two patterns were observed on the opposite surfaces of the proved seed crystal. micro-probe Raman spectrum that The the overgrown spiral evidenced steps are ofbyB(orth II) polytype. This was clearly changing the focal point from the crystal surface denoted by
*
in fig.
Table 2 Overgrowth of B(orth It) or B(mon) on B(mon) in n-hexane solution
.,~
I
i~.
2. Dissolution of Blmon) and growth of B(orth II) in nearly saturated decane solution at 30.0°C.
T5 (° C)
Overgrowth
28 25
23
B(O)/B(m) B(O)/B(m)
22 21
B(m)/B(m) B(m)/B(m)
240
A~ sul’’ 1 1/
P/i
nis,i’~’/is and
p~~li iii’~ S sst Os
U/is
is
51 /
*
~pm~hll
44 pm~ 30
20
0
(crri’)
(a)
30
20
10
(b)
Fig. 3. Micro-probe Raman spectra and corresponding photographs taken on (a) dissolving B(mon) crystal surface and (h) overgrowing spiral steps of B(orth II) on the same seed crystal of B(mon) which was inserted in n-hexane solution containing an excess of C(mon) crystals. Raman spectra were taken at the positions denoted by * close to the crystal surface (4 jim) and inside (44 tim).
3b into an interior of crystal (depth 44 pm). A change is seen in the characteristic spectrum from B(orth II) to B(mon) when the focal point moved from the outer edge to the interior of the crystal along the normal to the basal plane. In contrast, the dissolving part of fig. 3a is evidenced as B(mon). We confirmed that the dissolving pattern of fig. 3a appeared on the crystal surface facing downward at the bottom of solution. Fig. 3 seems to demonstrate that the solubtlity of C(mon) is lower than that of B(mon) at 23°C. But this is actually not true, because an opposite conclusion was drawn from the isothermal crysta.Jlization experiment which proved that B(mon) also crystallizes at the expense of C(mon) at the same temperature (see below). This contradiction can he solved by taking into account both the mass transfer around the overgrowing seed of B(mon) in a quiescent solution and the fact that
T= 23°C is just below the crossing point of the solubilities of B(mon) and C(mon). Even if the B(mon) would first grow at the expense of C(mon) just after the crystal was put in the solution, further growth was soon replaced by the overgrowth of B(orth II). The solute molecules coming from C(mon) crystals might be incorporated into the overgrowing B(orth II) on the top of the seed crystal. Eventually, the solution around the seed crystal became almost saturated with repect to the B(orth II), leading a dissolution of B(mon) at the bottom side. The overgrowth experiments at 25 and 28°C revealed no contradiction with the isothermal crystallization. Additionally, the soluhility data and overgrowth experiments may he contradictory in determining the thermodynamical stability. because the overgrowth needs a formation of new growth layers, arising either from two-dimensional (2D)
K. Sato ci at.
/ Potymorphs and poivtypes of stearic acid.
Borth II
°c
30
241
B(mon)
GTI~~mon
20
1
Fig. 4. Tentative representation of Gibbs energy relationship among B(mon), B(orth II) and C(mon) of stearic acid.
nucleation or from spiral growth. In the present case, however, the former mechanism may not be plausible: in fact many growth spiral steps appeared on the (001) faces of B (see fig. 3, ref. [16]), and we may argue that the overgrowth may occur principally via a spiral growth mechanism which needs smaller driving force than 2D nucleation. Next, the lattice misfit between B(mon) and B(orth II) is negligible, since the structure within the lamella is identical in the two forms. Therefore the excess driving force for overgrowth due to the lattice misfitting between two polytypes can be disregarded. Consequently, we conclude that the solubility increases in the following order: B(orth II) <
tionship between B(mon) and B(orth II) was well consistent with a free energy calculation [8], which was based on frequency data of low-frequency Raman bands [7], and also on acoustic branches in Brillouin scattering bands [8] of the two forms. The Gibbs energy—temperature relationship of fig. 4 may be the first case concerning the thermodynamical stability among the polymorphic and polytypic modifications in a group of long-chain compounds, some of which have been reported to reveal similar phenomena, for instance, n-paraffin crystals [17—19]. 4.2. Occurrence from solution
The experimental occurrence frequencies of B(mon) and B(orth II) from n-hexane solutions are summarized in table 3. Actually, the C polymorph simultaneously appeared at higher L~and larger initial supersaturation. The triclinic A crystals also sometimes occurred. This tendency is
Table 3 Occurrence of B(orth II) and B(mon) polytypes from n-hexane at various temperatures and under different crystallization conditions; numerical values represent the amount of crystals examined Method
T
14 15 16 18
11 13 14 16
3 2 2 2
—
21 22
2 2
1/15 5/23 0/5 3/19 10/28 9/24
(s)~ (s) (s) (s)
23 24
14/15 18/23 5/5 16/19 11/28 13/24
7/28 2/24
(s) (s)
16
18
2
0/12
12/12
—
(s)
3/8 11/19 9/21 0/13 0/25 0/35
5/8 8/19 12/21 13/13 25/25 35/35
0
Cooling
Fluctuation Isothermal (B
(° C)
sIT (°C)
B(mon)
(° C)
~
C)
20 22
23 24 28 30
“tntermediate” is referred to crystals revealing diffuse Raman bands. and (q) stay for stirred and quiescent crystallization conditions, respectively.
hi(s)
B(orth 11)
Intermediate — — —
— — — — — —
0
(q) (q) (q) (q) (q) (q)
K. .Saio ci at. / Polmniorpli c asid poli’(ips’v HI sicarli wit. /
242
consistent with the occurrence of A. B and C tn decane solutions [12]. Although less straightforward. the data in table 3 indicate that B(mon) prevails at lower temperatures. This is true when we compare the results obtained by the cooling method at T~ 13 and 14°Cwith those at J~ 21 and 22°C. In all cases, 2°C whtch ts equivalent to the initial supersaturation of approximately 36%. The total percentage of B(mon) at 13 and 14°C is 82% (23/28), but it decreases to 46% (24/52) at 21 and =
=
—~
additton, the careful growth rate measurement is also determinative. In this experiment, however. the seed crystals must he characterized by spectro-
scoptc methods before and after the growth above 23°C,. because the overgrowth of B(orth II) onto B(mon) at very small supersatitration valites is confirmed tn the present study.
=
In the case of the isothermal crystallization,
B(orth II) exclusively appeared
Acknowledgement The authors are indebted to Professor M. Okada for his encouragement throughoLtt this work.
above 24°C’.
Whereas below 23°C. both polytypes concurred with more or less the same percentage. The results of the tsothermal crystalltzation reasonably came from the Gibbs energy relationship drawn in fig. 4
The effect of temperature fluctuation between 16 and 18°C is manifest. The simple cooling from 18 to 16°C yielded more B(mon) crystals. However, all the crystals converted to B(orth II) after the temperature fluctuatton. This must be caused by the disappearance of the less stable B(mon) and growth of B(orth II) during the above procedure. Finally we obtained the crystal called “inter-
medtate which reveals peculiar Raman spectra The low-frequency bands of the “intermediate” crystal are not a simple superposition of those relative to B(mon) and B(orth II). Instead, the spectra are more diffuse. The crystal was rather thin and also of less-defined shape. No clear interpretation can be done for these “intermediate”
References [1] A. R. Verma and P. Krishna, Polymorphism and Pal’s — typism in (‘rystals (Wiley, New York, 1966). [2] L ‘sot’ Sydm~,Arkiv Kemi 9 (1955) 231.
[31 R.F. Holland
[4] .A.V. Bailey. I). Mitcham. R.A. Pittman and (. Sumrcll. .1. Am. Oil (‘hem. Soc. 49 ( 1972) 419. [5] A R Verma. Prod’. Roy Soc (London) A228 )l955( 34 [6] S. ,Amclinckx .,Acta (rvst. 9 (1956) 16. [7] M. Kohayashi.
I. Kobayashi. Y. Itoh and K. S~ito. .1.
(‘hem. Phys. 80 (1984) 2897. [8] M. Kohayashi. F. Kohayashi, Y. Itoh and K. Sato, Bull. Mineral. 109 ) 1986) 171. [9) \ Malt i (, (i.lotto R / inmtti md A F M tits.lli I Cl,em. Soc. B (1971) 548.
[10] M.
Goto and F.. ,Asada. Bull. (hem. Soc. Japan 51 (1978)
2456. [II] K. Sato and R. Boistelle..l.Colloid
Ititerlame Sci, 94
[12] K. Sato and R Boistelle, J Crystal Growth 66 (1984) 441 [13] W. Beckmann,
crystals. hut it is possible to infer that the lamellar
R.
Boistelle and K. Sato. J. (‘hen,. l/ng.
Data 29 (1984) 215.
stacking may be disordered. The present study revealed the slight tendency of the preferable occurrence of B(mon) at lower
]14[ K.
temperature. However, this must he further dartfied, most convincingly by in-situ determination of the occurrence frequency with micro-probe Raman method at different temperatures and supersaturattons. In dotng so, a sharp separatton
[16]
arises between temperature and supersaturation effects on the occurrence of the polytypes. In
and JR. Nielsen .1. Mo). Spectrosc. 9(1962)
436.
Sato. K. Suzuki. M. Okada and N. (iartl. .1. (‘rystal
Gro’sst172 (1985) 699. [IS] H. Morishita. T. Ishioka. M. Kohavashi and K. S/ito. .J. ~ Ii~. 91(1987) 2273
K. S:ito,
Japan.
J.
AppI. Phvs. 19 (1980) 1257. 1829.
[17] R. Boistelle. B. Simon and 0. Pepe..Acta Cry’st. B32 (1976) 1240.
[18] R. Boistelle. in: Current Topids in Materi,ils Sciende, Vol. 4, Ed. F. Kaldis (North-Holland. ,Amsterdani. 198))) p. 413 [19] M. Kohayashi. ‘I. Kohayashi, Y. Itoh, Y. (‘liatani and H. Tadokoro. .1. (‘hem. Phys. 72 (1980( 2024.
Journal of Crystal (I~rowth87 ( 1988) 236—742 North—Holland, Amsterdam
STABILITY, OCCURRENCE AND STEP MORPHOLOGY OF POLYMORPHS AND POt YTYPES OF STEARIC ACID I. Stability and occurrence Kiyotaka SATO
*
72(1,Japa,i
Faculti’ of Applied Biological Science, Hiroshima Unii’ersiti’, Fukui’ama
and
Masamichi KOBAYASHI and Hirofumi MORISHITA
* *
I)epartment of Macromolecular Science, Faculir of .S’cience, Osaka Cnn‘c’rsitr. Toionah a 56Q,.Jupun Received 26 June 1987: manuscript received in final form 9 September 1987
The thermodvnamical stability and occurrence of three different structural modifications: monoclinic and pseudo-orthorhombie polytvpes of the B polymorph. called B(mon) and B(orth 11) respectivelx. and monoclinic (‘ polvmorph, CO on), of stearic acid haxe been examined. The solubility measurements, overgrowth and isothermal crystallization proved that B(crth II) and ((moo) arc most stable below and above 32°C. respectively, whereas B(mon) is always nietastahle. turning to he more stable than Cli on) below between 23 and 24°C. It was confirmed b~micro-probe Ranian spectroscopy that the polytvptc structure of the newly-overgrown crystal on the (001) faces of the seed crystal of BOon) changed from the original monoclinic to pseudo-orthorhombic. B(orth II). at ver~ small supersaturations above 23°C. This conversion was caused h~ the solubilits difference. The occurrence experiment indicated a slight tendency that B(mon) crystallized more than B(orth II) at lower temperatures.
1. Introduction Stearic acid, one of the most common bio-lipid constituents, reveals polymorphism and polytypism. The former is caused by different molecular conformations and packings, resulting tn different three-dimensional unit cell structures. The polytypism originates from alternative stacking modes of the long-chain lamellae normal to the lamellar surface with certain periodicity [1]. As for the polymorphism. four modifications are availaHe: triclinic A. monoclinic B, C and E [2—4]. Since E crystallizes below about 10°C, the other three polymorphs have recently attracted considerable attention, * * *
To whom all correspondence should he addressed. Present address: Faculty of Education. Nagasaki ljniversity. 1-~4.Bunkyo-cho. Nagasaki, Japan.
In case of the polvtvpic structure of the fatty acids, the first work was carried out three decades ago [5,61. An optical observatton of interlaced step patterns on the as-grown (001 ) faces of the crystal gave proof for the alternative lamellar stacktng sequences along the [001] axis. Very recently we have investigated two basic polyt~pes of the B polymorph of stearic acid by polartzed Raman and infra-red (IR) spectroscopy [7]. One polytype shows a monoclinic structure with P2~/a named B(mon) and another one belongs to a pseudo-orthorhombic structure with Pcab. named B(orth II). in which each lamella is turned by 1800 with respect to both adjacent layers. We found that different i nterlamel lar structures between the above two polytypes are convtncinglv detectable
either tn the methyl hendino0 IR hand or in the . low-frequency Rarnan spectra. Addittonall’v, Brtlbum scattering was applied to the single crystals
0022-0248/88/$03.50 31~Elsevier Science Publishers By. (North-Holland Physics Publishing Division)
K. Sato et al.
/ Polymorphs and polvtypes of stearic acid.
237
1
of B(mon) and B(orth II) to measure elastic stiffnesses [8]. As for the C polymorph, the IR and Raman studies over several tens of single crystals have yielded a single pattern. This presumably
indicates that room C may not reveal multiple poiycrate typism crystallization around conditions temperature [9].and under modSo far the crystallization behavior of the polymorphs and of the polytypes of stearic acid has been explored in a separate way, or sometimes the polytypism has been rather disregarded. To get a full understanding, however, it is convenient to systematically clarify the crystallization processes of the two mentioned structural modifications having the same framework. In doing so, one has to examine thermodynamical stability, crystallization kinetics and mono-lamellar surface step morphology of all the possible modifications with multiple methods. We have reported some preliminary results very recently [8]. In the present papers, after a short display of the crystal structures, we report the stability and crystallization from solution of B(mon), B(orth I~ and C(mon) (paper I). In the following paper (paper II) we will report the mono-lamellar step morphology of each modification and microscopic mechanisms of composite polymorphic/polytypic transformation, which has singly been elucidated by the observation of the step pattern.
2. Structures of B(mon), B(orth II) and C(mon) Fig. I shows the crystal structures of C(mon) and B(orth II) and B(mon). The C polymorph reveals all trans conformation both of aliphatic chains and carboxyl groups [9]. Meanwhile, B shows a partially twisted structure due to gauche conformations adjacent to the carboxyl groups
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.113?;:
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-c,-~ Cmon
a
Bmon Borth II
Fig. L Crystal structures of C(mon). B(mon) and B(orth II) of stearic acid as revealed (h-axis projection).
[10]. B(mon) is a single-layered polytype in which one lamella is a repetition period with the same stacking mode [10]. B(orth II) is a double-layered polytype formed by an alternative rotation of one lamella about normal to the lamellar interface [6]. Table 1 summarizes the lattice parameters of the three modifications.
3. Materials and methods The purity of stearic acid (Nippon Oil and Fats Co.) was guaranteed 99.8%. No further purification was done. The solvents employed were n-hexane (Yoneyama Pharm. > 97%) and decane (Tokyo Kasei Co. > 99%). The single crystals were grown from n-hexane solutions either by cooling or by iso-thermal methods in a glass-made growth cell thermostated with circulating water (±0.05 °C). In the former method, the solution saturated at T~was rapidly
Table I Lattice parameters of B(orth II). B(mon) and C(mon) of stearic acid
B(orth II) B(mon) C(mon)
Space group
a
(A)
b (A)
c (A)
$
P cab 2i/a P P2 1/a
5.58 5.587
7.34 7.386
87.58 49.33
90 L17.24
9.363
4.965
50.88
1.28.46
(deg)
238
KSato ci al. / /‘oliSfriorphs and poIrtrpe~sof siearic cicicl. /
cooled down to 1/ We could not examtne the effects of temperature and supersaturation mdcpendently. although it was possible for the relative occurrence of B and C [11.12]. This is because an tn-sttu determination of the relative occurrence of the two polytypes of B was not facilitated in the present method (see below). Instead, the relative frequency of each polytype was measured for all the precipitates taken out of the solution after the overall crystallization which might expertence varying temperature and supersaturation. Even so. we examined the effect of temperature on the occurrence of the two polytypes of B by changing
soluhility relationship between two polytypes and C. 15 cm~of solution were first saturated with respect to B, then a large amount of C’ crystals (approximately 0.5 g) were put into the solution. This was done so that the solution became nearly saturated with respect to C. After half an hour. a single crystal of B(rnon) or B(orth II) was further put itlto the solution, then the overgrowth was carried out over several hours. The overgrown seed crystals were taken out of the solution for the characterization by micro-probe Rarnan method. In all the experiments, the identification of the polymorphs was easily done by observing the
7~and 7~.. The initial supersaturation value corresponding to the difference (7/—I~)was calculated from the solubility data of B [13], e.g.. approximately 18%/°C. Additionally we fluctuated the solution temperature like this: cooled from 180 C (1~)to 16°C (T~)over one night, and raised to 18°C and kept there over one week, then again cooled to 16°C over one night. We checked a change in the concentration of each polytype tn the precipitates. The solution was gently stirred by hand to accelerate nucleation. after T~was reached, In case of the iso-thermal crystallization, the single crystals were grown in quiescent solutions vta solution-mediated transformation [14] at different temperatures. i.e.. the C(mon) crystals were put into the solution nearly saturated with respect
crystal shape: acute angles between { 110) faces were 55° for C and 75° for B. No macroscopic difference, however, was detectable in the crystal shapes of B(mon) and B(orth II). Therefore. polarized Raman spectroscopy was applied to all the precipttated B crystals. We employed a Jasco R500 monochromator using the 514.5 nrn line (Ar laser). The single crystal was mounted on the goniometer head and the scattered radiation at the right angle was measured. The spectra were recorded in the frequency range 2---400 cm i: yet the clearest difference was available below 50 cm [7]. The overgrowth feature was examined by a micro-probe Raman spectroscope consisting of an epi-illurnination optical microscope (Olympus BH-2) and a Jasco CT-1000D double monochrontator [15]. The inctdent polarized beam (514.5 nrn, Ar laser) was focused at a selected position of the crystal. The dtrnension of the laser spot was I ~tm and the depth of the focal point measured from the crystal surl’ace can he adjusted ±I p.m with an objective lens of x 100 magnification. The posttion subjected to the spectral measurement was monitored by a TV camera. The backward scattered light was collected by the same objective lens, passed through a beam splitter, and entered the monochromator through a telemeter lens. Before the entrance of the nionochromator. an analyser and a polarizatton scramber were put. as in the case of the ordinary Rarnan measurements. If necessary, an aperture was inserted in front of the telemeter lens in order to cut-off the light passing through the peripheral area of the objective lens. The Raman spectrum in the region helow 50 cm was recorded.
to the more stable form, then the iso-thermal crystallization was carried out over several days. The solution temperature was controlled within ±0.1°C. We measured the concentrations of B(orth II) and B(mon) out of the precipitates which occurred at the expense of C(mon).
The stability relationship of the three modiftcations was evaluated in more detail by measuring the solubility relationship in decane solutions. Since the soluhilities of C and B were already examined [13]. we first paid attention to B( mon) and B(orth II) using the same method as that of ref. [13]: a single crystal of each polytype was put in the solution, then we fluctuated the solution temperature to obtain the saturation temperature for each poI~type in the same solute concentration. Thereafter we studied overgrowth of the more stable polytype on the less stable one to get the
K. Sato et of
/ Polvmorphs and p01stvpes of stearic acid.
1
239
4. Results and discussion
0.2°C between B(orth II) and B(mon) leads to the conclusion that the B(mon)-saturated solution is
4.1. Stability
supersaturated with respect to B(orth II) by approximately 3.6% of a supersaturation value.
First, we report the thermodynamical stability which was evaluated by the solubility measure-
Previous works [13,14] show that C is less stable than B below 32°C. So, we next examined the
ments. Fig. 2 shows an evolution of the growth of B(orth II) at the expense of B(mon) in the nearly saturated decane solution at 30.0 ±0.05°C. The same results were seen at 20.0 and 15.0°C.From this we concluded that B(mon) has a higher soluhility. being less stable, than B(orth II) at 15 < T <30°C. Then, a difference in the saturation temperatures (7~)of B(orth II) and B(mon) in the decane solution was measured around 30 and 17°C. As a result, we found that T 5 differs by
solubility relationship among B(mon), B(orth II) and C(mon). This determination cannot he done by such a simple method as fig. 2. because of a possible conversion of the polytypic structure of B during growth at the expense of C(mon). Therefore the overgrowth was examined by micro-probe Raman method. The results at various temperatures are displayed in table 2. In this table, the results we
0.2 ±0.1°C at both temperatures.This means that
the solubilities of B(orth II) and B(mon) differ in an almost parallel way around room temperature. This difference in the saturation temperature by
obtained using the seed crystal of B(orth II) are not displayed because of no conversion. This means that B(orth II) has the lowest solubility at the temperatures examined, proving that the lowtemperature stable form, singly referred as B in the previous work [12,13], is actually the doublelayered polytype of B. On the other hand, B(mon) revealed a conversion of the polytypic structure 22°C. a normal overgrowth of B(mon) occurred,
rth]I on.
.
~‘.
crystal. but B(orth Raman II) overgrew spectra corresponding onto above to each 23°C. part and overgrowth of B(orth II) B(mon) on the same B(mon)
.
-. S.
—
-~
,
-.
.
3o~
into 3B(orth II) at higher temperatures. Below Fig. shows photographs of dissolution of B(mon)
•
-
are also shown. In fig. 3a, the typical dissolution hillocks are seen, whereas growth spiral steps appear in fig. 3b. These two patterns were observed on the opposite surfaces of the proved seed crystal. micro-probe Raman spectrum that The the overgrown spiral evidenced steps are ofbyB(orth II) polytype. This was clearly changing the focal point from the crystal surface denoted by
*
in fig.
Table 2 Overgrowth of B(orth It) or B(mon) on B(mon) in n-hexane solution
.,~
I
i~.
2. Dissolution of Blmon) and growth of B(orth II) in nearly saturated decane solution at 30.0°C.
T5 (° C)
Overgrowth
28 25
23
B(O)/B(m) B(O)/B(m)
22 21
B(m)/B(m) B(m)/B(m)
240
A~ sul’’ 1 1/
P/i
nis,i’~’/is and
p~~li iii’~ S sst Os
U/is
is
51 /
*
~pm~hll
44 pm~ 30
20
0
(crri’)
(a)
30
20
10
(b)
Fig. 3. Micro-probe Raman spectra and corresponding photographs taken on (a) dissolving B(mon) crystal surface and (h) overgrowing spiral steps of B(orth II) on the same seed crystal of B(mon) which was inserted in n-hexane solution containing an excess of C(mon) crystals. Raman spectra were taken at the positions denoted by * close to the crystal surface (4 jim) and inside (44 tim).
3b into an interior of crystal (depth 44 pm). A change is seen in the characteristic spectrum from B(orth II) to B(mon) when the focal point moved from the outer edge to the interior of the crystal along the normal to the basal plane. In contrast, the dissolving part of fig. 3a is evidenced as B(mon). We confirmed that the dissolving pattern of fig. 3a appeared on the crystal surface facing downward at the bottom of solution. Fig. 3 seems to demonstrate that the solubtlity of C(mon) is lower than that of B(mon) at 23°C. But this is actually not true, because an opposite conclusion was drawn from the isothermal crysta.Jlization experiment which proved that B(mon) also crystallizes at the expense of C(mon) at the same temperature (see below). This contradiction can he solved by taking into account both the mass transfer around the overgrowing seed of B(mon) in a quiescent solution and the fact that
T= 23°C is just below the crossing point of the solubilities of B(mon) and C(mon). Even if the B(mon) would first grow at the expense of C(mon) just after the crystal was put in the solution, further growth was soon replaced by the overgrowth of B(orth II). The solute molecules coming from C(mon) crystals might be incorporated into the overgrowing B(orth II) on the top of the seed crystal. Eventually, the solution around the seed crystal became almost saturated with repect to the B(orth II), leading a dissolution of B(mon) at the bottom side. The overgrowth experiments at 25 and 28°C revealed no contradiction with the isothermal crystallization. Additionally, the soluhility data and overgrowth experiments may he contradictory in determining the thermodynamical stability. because the overgrowth needs a formation of new growth layers, arising either from two-dimensional (2D)
K. Sato ci at.
/ Potymorphs and poivtypes of stearic acid.
Borth II
°c
30
241
B(mon)
GTI~~mon
20
1
Fig. 4. Tentative representation of Gibbs energy relationship among B(mon), B(orth II) and C(mon) of stearic acid.
nucleation or from spiral growth. In the present case, however, the former mechanism may not be plausible: in fact many growth spiral steps appeared on the (001) faces of B (see fig. 3, ref. [16]), and we may argue that the overgrowth may occur principally via a spiral growth mechanism which needs smaller driving force than 2D nucleation. Next, the lattice misfit between B(mon) and B(orth II) is negligible, since the structure within the lamella is identical in the two forms. Therefore the excess driving force for overgrowth due to the lattice misfitting between two polytypes can be disregarded. Consequently, we conclude that the solubility increases in the following order: B(orth II) <
tionship between B(mon) and B(orth II) was well consistent with a free energy calculation [8], which was based on frequency data of low-frequency Raman bands [7], and also on acoustic branches in Brillouin scattering bands [8] of the two forms. The Gibbs energy—temperature relationship of fig. 4 may be the first case concerning the thermodynamical stability among the polymorphic and polytypic modifications in a group of long-chain compounds, some of which have been reported to reveal similar phenomena, for instance, n-paraffin crystals [17—19]. 4.2. Occurrence from solution
The experimental occurrence frequencies of B(mon) and B(orth II) from n-hexane solutions are summarized in table 3. Actually, the C polymorph simultaneously appeared at higher L~and larger initial supersaturation. The triclinic A crystals also sometimes occurred. This tendency is
Table 3 Occurrence of B(orth II) and B(mon) polytypes from n-hexane at various temperatures and under different crystallization conditions; numerical values represent the amount of crystals examined Method
T
14 15 16 18
11 13 14 16
3 2 2 2
—
21 22
2 2
1/15 5/23 0/5 3/19 10/28 9/24
(s)~ (s) (s) (s)
23 24
14/15 18/23 5/5 16/19 11/28 13/24
7/28 2/24
(s) (s)
16
18
2
0/12
12/12
—
(s)
3/8 11/19 9/21 0/13 0/25 0/35
5/8 8/19 12/21 13/13 25/25 35/35
0
Cooling
Fluctuation Isothermal (B
(° C)
sIT (°C)
B(mon)
(° C)
~
C)
20 22
23 24 28 30
“tntermediate” is referred to crystals revealing diffuse Raman bands. and (q) stay for stirred and quiescent crystallization conditions, respectively.
hi(s)
B(orth 11)
Intermediate — — —
— — — — — —
0
(q) (q) (q) (q) (q) (q)
K. .Saio ci at. / Polmniorpli c asid poli’(ips’v HI sicarli wit. /
242
consistent with the occurrence of A. B and C tn decane solutions [12]. Although less straightforward. the data in table 3 indicate that B(mon) prevails at lower temperatures. This is true when we compare the results obtained by the cooling method at T~ 13 and 14°Cwith those at J~ 21 and 22°C. In all cases, 2°C whtch ts equivalent to the initial supersaturation of approximately 36%. The total percentage of B(mon) at 13 and 14°C is 82% (23/28), but it decreases to 46% (24/52) at 21 and =
=
—~
additton, the careful growth rate measurement is also determinative. In this experiment, however. the seed crystals must he characterized by spectro-
scoptc methods before and after the growth above 23°C,. because the overgrowth of B(orth II) onto B(mon) at very small supersatitration valites is confirmed tn the present study.
=
In the case of the isothermal crystallization,
B(orth II) exclusively appeared
Acknowledgement The authors are indebted to Professor M. Okada for his encouragement throughoLtt this work.
above 24°C’.
Whereas below 23°C. both polytypes concurred with more or less the same percentage. The results of the tsothermal crystalltzation reasonably came from the Gibbs energy relationship drawn in fig. 4
The effect of temperature fluctuation between 16 and 18°C is manifest. The simple cooling from 18 to 16°C yielded more B(mon) crystals. However, all the crystals converted to B(orth II) after the temperature fluctuatton. This must be caused by the disappearance of the less stable B(mon) and growth of B(orth II) during the above procedure. Finally we obtained the crystal called “inter-
medtate which reveals peculiar Raman spectra The low-frequency bands of the “intermediate” crystal are not a simple superposition of those relative to B(mon) and B(orth II). Instead, the spectra are more diffuse. The crystal was rather thin and also of less-defined shape. No clear interpretation can be done for these “intermediate”
References [1] A. R. Verma and P. Krishna, Polymorphism and Pal’s — typism in (‘rystals (Wiley, New York, 1966). [2] L ‘sot’ Sydm~,Arkiv Kemi 9 (1955) 231.
[31 R.F. Holland
[4] .A.V. Bailey. I). Mitcham. R.A. Pittman and (. Sumrcll. .1. Am. Oil (‘hem. Soc. 49 ( 1972) 419. [5] A R Verma. Prod’. Roy Soc (London) A228 )l955( 34 [6] S. ,Amclinckx .,Acta (rvst. 9 (1956) 16. [7] M. Kohayashi.
I. Kobayashi. Y. Itoh and K. S~ito. .1.
(‘hem. Phys. 80 (1984) 2897. [8] M. Kohayashi. F. Kohayashi, Y. Itoh and K. Sato, Bull. Mineral. 109 ) 1986) 171. [9) \ Malt i (, (i.lotto R / inmtti md A F M tits.lli I Cl,em. Soc. B (1971) 548.
[10] M.
Goto and F.. ,Asada. Bull. (hem. Soc. Japan 51 (1978)
2456. [II] K. Sato and R. Boistelle..l.Colloid
Ititerlame Sci, 94
[12] K. Sato and R Boistelle, J Crystal Growth 66 (1984) 441 [13] W. Beckmann,
crystals. hut it is possible to infer that the lamellar
R.
Boistelle and K. Sato. J. (‘hen,. l/ng.
Data 29 (1984) 215.
stacking may be disordered. The present study revealed the slight tendency of the preferable occurrence of B(mon) at lower
]14[ K.
temperature. However, this must he further dartfied, most convincingly by in-situ determination of the occurrence frequency with micro-probe Raman method at different temperatures and supersaturattons. In dotng so, a sharp separatton
[16]
arises between temperature and supersaturation effects on the occurrence of the polytypes. In
and JR. Nielsen .1. Mo). Spectrosc. 9(1962)
436.
Sato. K. Suzuki. M. Okada and N. (iartl. .1. (‘rystal
Gro’sst172 (1985) 699. [IS] H. Morishita. T. Ishioka. M. Kohavashi and K. S/ito. .J. ~ Ii~. 91(1987) 2273
K. S:ito,
Japan.
J.
AppI. Phvs. 19 (1980) 1257. 1829.
[17] R. Boistelle. B. Simon and 0. Pepe..Acta Cry’st. B32 (1976) 1240.
[18] R. Boistelle. in: Current Topids in Materi,ils Sciende, Vol. 4, Ed. F. Kaldis (North-Holland. ,Amsterdani. 198))) p. 413 [19] M. Kohayashi. ‘I. Kohayashi, Y. Itoh, Y. (‘liatani and H. Tadokoro. .1. (‘hem. Phys. 72 (1980( 2024.