MESOMORPHIC BEHAVIOR OF CHOLESTERYL S-ALKYLTHIOCARBONATES
1545
The Mesomorphic Behavior of Cholesteryl S-Alkyl Thiocarbonates by Wolfgang Elser and Reinhard D. Ennulat U.S . A r m y Electronics Command, Night Vision Laboratory, Fort Belvoir, Virginia 8.8060 (Received September 10, 1969)
The first 20 members of the cholesteryl S-alkyl thiocarbonates were investigated under the microscope and in the capillary to identify mesomorphic phases and with the differential scanning calorimeter to determine the temperature and the latent heat of each phase transition. All compounds exhibit a cholesteric and, with the exception of the first four members of the series, a smectic mesophase. However, some of these compounds did not show cholesteric colors. The plots of transition temperatures and transition heats as a function of alkyl chain length reveal relationships typical of sterols with cholesteric and smectic mesophases.
Valuable information about the requirements for the existence of cholesteric mesophases as a function of molecular features has been obtained by studying homologous series.1--6 These studies require extremely pure materials, so that the measurable effects cannot be linked to impurity content, and also require that all errors associated with the physical measurements be ascertained. This entails both a synthesis which yields a minimum amount of impurities and the complete removal of unavoidable side products.
I. Preparation Cholesteryl chloroformate6 (l), alkanethiols, and pyridine as a base reacted to form the corresponding cholesteryl S-alkyl thiocarbonates (2), in analogy to the synthesis of cholesteryl alkyl carbonatesS2 Cholesterol, 3p-chlorocholest-5-ene, cholesta-3,5-diene, and dicholesteryl carbonate were formed as side products, while the yield of S-alkyl thiocarbonate was lower than the one obtained for cholesteryl carbonate. An excess of pyridine led to a higher amount of dicholesteryl carbonate. Since some of the formed impurities, especially larger amounts of dicholesteryl carbonate, are difficult to remove by conventional purification method^,^ other tertiary amines were tried as the base. Because alkanethiols are more acidic than the corresponding alkanols, stronger bases were expected to improve the yields of cholesteryl S-alkyl thiocarbonates. The use of triethylamine resulted not only in a higher yield but also in a substantial lessening of cholesterol and dicholesteryl carbonate. Cholesteryl diethyl carbamate @), another expected side product in the reaction with triethylamine as a base, could not be detected. As reported for stigmasteryl chl~roformate,~ it is formed by reaction of 1 with either triethylamine or an excess of diethylamine. The presence of the more acidic alkanethiol apparently prevents this side reaction. For purification, the compounds were chromatographed on silica gel, which allowed a separation from cholesterol and dicholesteryl carbonate, as demonstrated by thin-layer chromatographic analysis (Figure
I1
c1-c-0O
H
'
1 N(C.H,),
R-S-C-0
I
R-SH
II 2
1). Because of the high effectiveness of this method in separating these impurities, all remaining contaminants must originate in the starting materials. This aspect, extensively covered in the Experimental Section, leads to the conclusion that the prepared series of cholesteryl S-alkyl thiocarbonates has a minimum purity of 98% with the exception of cholesteryl S-pentadecyl thiocarbonate, which may contain about up to 5% of the tetradecyl thiocarbonate. The physical properties, yields of analytically pure compounds, analytical (1) R. D. Ennulat, mol. Crust. Lip. Cryst., 8 , 247 (1969). (2) W. Elser, Mol. Crust., 2, 1 (1966). (3) J. L. W. Pohlmann, ibid., 2, 15 (1966). (4) J. L. W. Pohlmann and W. Elser, Mol. Crust. Lip. Cryst., 8 , 427 (1969). (5) W. Elser, ibid., 8, 219 (1969). (6) A . F. McKay and G. R. Vavasour, Can. J . Chem., 31, 688 (1953). (7) J. A . Campbell, J . Org. Chem., 22, 1259 (1957).
Volume 7 4 , Number 7 April 8 , 1970
W. ELSERAND R. D. ENNULAT
1546 Table I : Cholesteryl S-Alkyl Thiocarbonates
Anal. values, %
I
Yield,
MP,
S-Ch,a
Ch-I,‘
Mol
-Calod---
R (a1kyl)
%
OC
OC
OC
wt
C
H
S
------Found-C
H
S
Methyl Ethyl Propyl Butyl Pentyl Hexyl Heptyl Octyl Nonyl Decyl Undecyl Dodecyl Tridecyl Tetradecyl Pentadecyl Hexadecyl Heptadecyl Octadecyl Nonadecyl Eicosyl
84 79 77 78 75 75 82 78 77 74 75 74
101.2 114.7 97.9 97.5 92.9 94.1 61.0 80.9 75.2 78.1 72.5 60.6 55.2 59.4 62.7 47.1 68.3 58.8 73.6 64.8
103.8 97.1 90.3 93.3 87.3 85.4 84.2 82.3 79.3 77.9 74.8 71.6 69.3 67.6 67.6 63.6 62.4 63.5 60.6 60.4
460.7 474.8 488.8 502.8 516.8 530.8 544.8 558. 9 572.9 586.9 600.9 615.0 629.0 643.0 657.1 671.1 685.2 699.2 713.2 727.2
75.59 75.89 76.17 76.43 76.69 76.93 77.15 77.39 77.57 77.77 77.94 78.12 78.27 78.45 78.59 78.74 77.88 79.02 79.14 79.27
10.50 10.62 10.72 10.80 10.90 11.01 11.10 11.18 11.26 11.33 11.41 11.47 11.53 11.60 11.66 11.72 11.77 11.82 11.87 11.92
6.96 6.75 6.56 6.37 6.20 6.04 5.89 5.74 5.60 5.46 5.34 5.21 5.10 4.99 4.88 4.78 4.68 4.59 4.50 4.41
75.67 76.02 75.90 76.31 76.57 76.66 77.42 77.30 77.53 77.51 77.78 78.02 78.34 78.49 78.84 78.73 78.73 78.95 79.25 79.38
10.77 10.68 10.53 10.63 10.73 11.07 10.82 10.89 11.31 11.23 11.42 11.40 11.78 11.51 11.89 11.69 11.69 12.08 11.91 12.06
6.72 6.66 6.33 6.53 6.02 6.21 5.85 5.74 5.80 5.47 5.40 5.40 5.11 5.20 4.83 4.92 4.82 4.50 4.82 4.53
a
80 76 76 73 73 73 75
77
...
*..
...
... 40.3“ 58. Oc 68.2 69.8 71.8 71.3 69.7 65.2 63.6 60.8 61.9 57.2 55.8 57.3 54.1 54.2
Smectic,-cholesteric transition.
Cholesteric-isotropic transition.
0 W
- 1
I 2 3 4 5 6 7 Figure 1. Thin-layer chromatography of products on Silica Gel H R (Merck), benzene-hexane 5:95 (v/v). (1) Cholesteryl S-heptyl thiocarbonate, crude reaction product; (2) cholesteryl S-heptyl thiocarbonate; (3) cholesterol; (4) dicholesteryl carbonate; (5) 3p-chlorocholest-5-ene; (6) cholesta-3,fj-diene; cholesteryl diethyl carbamate.
data, and calorimetric determinations are summarized in Table I.
11. Mesomorphic Behavior For reasons discussed elsewhere,s we identified mesophases by optical means and determined temperatures of phase transitions by thermal analysis. The transition temperatures and transition heats were measured to determine their dependence on chain length. T y p e s of Mesophases. Using a polarizing microscope under conoscopic and orthoscopic operating conditions, The Journal of Physical Chemistry
Microscopical determination.
we studied samples contained between thin glass cover slips in transmitted and reflected light and heated on a microscope stage. The difference of the optical sign shown by homeotropic textures and the difference of the sign of elongation exhibited by focal conic bands (i.e.’ Friedel’s “oily streaks”)g permitted a clear distinction of the observed mesophases into smectic and cholesteric. However, several compounds (see Experimental Section) did not exhibit iridescent colors typical of cholesteric mesophases. This behavior may be caused by the failure of forces acting at the liquid crystal-substrate interface to favor the plane texture over coexisting birefringent textures or to align the plane texture sufficiently and thus reduce the diffuseness of selectively reflected light’. It is also possible that the temperature-dependent repeat distance required for the interference effect in the visible may not be realized within the temperature range in which the cholesteric mesophases are observed. For example, cholesteryl S-pentyl thiocarbonate displays cholesteric colors only when the homeotropic texture is rapidly and excessively undercooled. In summary) the optical evidence establishes the existence of cholesteric mesophases from cholesteryl S-methyl to 8-eicosyl thiocarbonate and a smectic mesophase, occurring at lower temperatures, for S-pentyl thiocarbonate and higher members of the series. Transition Temperatures. The transition temperatures of these compounds were measured with an espe(8) R. D. Ennulat in “Analytical Calorimetry,” R. S. Porter and J. F. Johnson, Ed., Plenum Publishing Corp., New York, N. Y., 1968, p 219. (9) G. Friedel, Ann. Phys. (Paris), 18, 273 (1922).
NESOMORPHIC BEHAVIOR OF CHOLESTERYL S-ALKYLTHIOCARBONATES
1547
K-CAL,
MOLE 14
I2 IO O '
i
a
50
6
40
4 4
_ I
2
cially modified calorimeters (DSC-1, Perkin-Elmer Corp.), with a maximum uncertainty of A0.4" for transitions in the melt and, because of the higher heat flux, a larger uncertainty of * l o for melting points. For details see Experimental Section. I n Figure 2 the transition temperatures of the cholesteryl S-alkyl thiocarbonates are plotted as a function of S-alkyl chain length. As observed in other sterol series some of these compounds exhibit a high melting point after storage at room temperature for hours or days and a low melting point when the sample is immediately measured after solidification obtained at a cooling rate of 10" per minute. Depending on the compound these melting points differ by as much as 18". We report only the high melting points, because they are presumably associated with the crystal modification stable at and above room temperature. To indicate the low-temperature limit of the melted state we also plotted the onset of freezing with a cooling rate of 10" per min. Both curves show the expected erratic behavior, while the curve of the clearing points and of the smecticcholesteric transition temperatures indicate a definite relation to chain length. Probably the presence of unknown impurities causes the few major deviations. Because of intervening freezing and because of instrumental limitations of the calorimeter, the monotropic smectic-cholesteric transition temperatures of S-pentyl and S-hexyl thiocarbonate could only be estimated by optical means on rapidly undercooled samples. Transition Heats. Since we were interested in the general trend of transition heats on chain length, we avoided the cumbersome high precision methods of adiabatic calorimetry and used the more expedient but less accurate differential scanning calorimeter. The latter permitted the determination of transition heats in the melt within less than j=2OOj, (for details see Ex-
20
16
A L K Y L CHAIN LENGTH
NO.OFC-ATOMS OF ALKYL CHAIN
Figure 2. Transition temperatures of cholesteryl S-alkyl thiocarbonates: - - -, melting points; -A-, cholestericisotropic transitions; -0-, smectic-cholesteric transitions; * * ., onset of freezing.
12
8
Figure 3. Heats of fusion.
2
4
6
IO
8
12
14
16
I8
20
NO.OFC-ATOMS O F A L K Y L C H A I N
Figure 4. Heats of transitions in the melt: -A-, cholesteric-isotropic transitions; -0--, smectic-cholesteric transitions.
-
perimental Section). The much larger heats of fusion were measured with an uncertainty of a few per cent. Figure 3 shows the dependence of the heat of fusion on the chain length. Although only the presumably more stable high melting phases were considered, tjhe scattered data indicate only a general upward trend with chain length. Since no attempt was made to establish thermal equilibrium, a condition which might not even be achievable with the differential scanning calorimeter,'O the data are unreliable and therefore should only be used qualitatively. In particular, Figure 4 shows that the heat of fusion is at least 40 times larger than the transition heats in the melt. However, because of the independence of thermal history, the transition heats in the melted state should correspond to thermal equilibrium data at least within the uncertainty of the measurement. This does not contradict the results of light-scattering experiments," which indicate a thermal equilibration time of up to 1 hr for a temperature change within a given mesophase. This time is needed t o achieve the average equilibrium size of the ordered regions required for the new tem(10) Even much simpler organic compounds attain thermal equilibrium only after many hours of slow crystallization; see J. P. McCullough and G. Waddington, Anal. Chim. Acta, 17, 80 (1957). (11) L.M. Cameron, Mol. Cryst. L i q . Cryst., 7, 235 (1969).
Volume 7 4 , Number 7
April 2 , 1970
1548 perature and not to change the type of the structure during a phase transition. The apparently low viscosity of our melted samples results in a high molecular mobility, which does not permit such structural differences and structural instabilities to exist for long.l 2 Although the transition heats in the melt exhibit the same dependence on chain length observed for other homologous sterols, the scatter of the data is larger than the uncertainty. We assume that unknown nonhomologous impurities are responsible for the marked deviation of the smectic-cholesteric transition heats of dodecyl and tetradecyl thiocarbonate because this deviation cannot be caused by the small amount of homologous impurities in our samples. In spite of the repeated attempts to remove these nonhomologous impurities, we were not able to reduce these discrepancies. As expected, the fact that the pentadecyl thiocarbonate might have contained up to 5% of lower homologs did not cause a noticeable deviation from the general trend of the data. Obviously major and minor components of this material are so close in their molecular properties that the pliable mesophase can accommodate both types of molecules without apparent effect. The extrapolation of the smetic-cholesteric curve of latent heats beyond C7 toward lower chain length indicates that smectic phases could be expected down to Cg. Performing the similar operation on the corresponding transition temperature curve (Figure 2 ) we find that the transitions for Ce and C5 lie in the freezing region. Optical tests verified the temporary existence of the smectic mesophase in rapidly undercooled samples. However, for shorter chain lengths we observed no smectic mesophase even in cases for which the extrapolated transition temperature is above the freezing curve.
111. Discussion Following Gray’s reasoning13 that the purer the compounds the smoother the relationships between chain length and transition temperatures in the melt-and we extend this idea to include transition heats-and considering that the scatter of our data is larger than the measurement uncertainty, we must conclude that the impurity content of our compounds is not negligible. I n spite of this shortcoming the extrapolation of our data indicates a definite relation between transition parameters and chain length. This justifies the comparison of our results with those obtained from other homologous series. Disregarding minor variations, the shape of the curves, shown in Figures 2 and 4, is typical for homologous series of sterols exhibiting smectic and cholesteric mesophases and showing no oddeven effects in their chain length dependence. The thermal stability of the cholesteric mesophase decreases with chain length while that of the smectic mesophase first increases and then decreases a t about the same The Journal of Physical Chemistry
W. ELSERAND R. D. ENXULAT rate. This results in an almost constant temperature interval for the cholesteric mesophase from about Clo on. The beginning of the curves for smectic-nematic systems shows the same trend; the nematic clearing points decrease with chain length,’* while the smectic mesophase, occurring from a certain chain length on, increases in thermal stability. However, with increasing chain length the temperature interval of the nematic mesophase steadily decreases until this phase does not exist. By considering the relative contributions of lateral and terminal forces to molecular interactions, Gray’3 was able to explain the behavior of smectic-nematic systems. However, for smectic-cholesteric series this approach fails. Of course, the increase of the thermal stability of the smectic mesophase can still be explained by the rise in polarizability of the alkyl chain and the resulting larger lateral attraction. But why does the smectic mesophase not displace the cholesteric mesophase? To answep this question we think that Gray’s approximation of the molecular force field should be extended by adding a tensorial interaction force. We speculate that the source of this addition may be due to the same asymmetric molecular force field, which is responsible for the optical activity and thus in part for the cholesteric behavior. The latter is supported by the following empirical facts. 1. No nematogenic compound is known which has optically active centers resulting in net optical activity. 2. No cholesteric compound is known that is optically inactive. 3. Certain mixtures of dextro- and laevo-rotatory cholesteric mesophases exhibit nematic behavior at a temperature for which the optical activity vanishes. 9,13 4. Minute additions of optically active compounds, which are not necessarily mesomorphic, to a pure nematic compound can induce cholesteric behavior of the mesophase.16 Unfortunately we have no evidence clarifying the features which create the molecular force field essential for the occurrence of cholesteric mesophases. We think that the introduction of the tensorial force implied by optical activity may make the existence of a helical structural plausible. We will attempt empirically to link optical activity data with the chain length dependence of cholesteric transition temperatures.
IV. Experimental Section ( a ) Preparation of Compounds. The general procedure is exemplified by the first synthesis as outlined (12) For example, the orientation relaxation time in nematic mesophases is about 10-6 sec; see ref 33, p 73. (13) G. W. Gray, “Molecular Structure and Properties of Liquid Crystals,” Academic Press, Inc., New York, N . Y., 1962. (14) Gr&y mentions a few compounds for which the clearing points rise with chain length, ref 33, p 236. Since we do not know of a counterpart for cholesteric mesophases, we restriot our discussion to homologous series with falling clearing points. (15) G. Friedel, C. R. H . Acad. Sci., 176, 475 (1923).
MESOMORPHIC BEHAVIOR OF CHOLESTERYL S-ALKYLTHIOCARBONATES below. The proportions of reactants and solvent were the same in all synthetic procedures. The reactions were carried out in a nitrogen atmosphere. Cholesteryl S-Ethyl Thiocarbonate (1). A solution of 1.01 g (0.01 mol) of triethylamine in 10 ml of absolute benzene is added to a stirred solution of 4.49 g (0.01 mol) of cholesteryl chloroformate and 0.62 g (0.01 mol) of ethanethiol in 70 ml of absolute benzene within 30 min at room temperature. Stirring is continued for another 2 hr under reflux. Then the cooled reaction mixture is filtered, the solvent distilled off, the residue dissolved in benzene-hexane, and chromatographed on silica gel (45 X 350 mm). Elution with approximately 1500 ml of benzene-hexane (30/70), combination of the fractions containing the S-ethyl thiocarbonate, evaporation of the solvent, and recrystallization of the residue from acetone yields 3.77 g (79%) of colorless needles, mp 113-115". Cholesteryl Diethyl Carbamate (2) (1). A solution of 4.49 g (0.01 mol) of cholesteryl chloroformate in 50 ml of absolute benzene and 1-01 g (0.01 mol) of triethylamine is boiled under reflux for 5 hr. The solution is filtered, the solvent evaporated, and the residue twice recrystallized from acetone. Yield : 4.05 g (83.5%); nip 143-144", not mesomorphic. A n a l . Calcd for C32Hh5N02: C, 79.11; H, 11.41; N, 2.88. Found: C, 79.33; H, 11.81, N, 2.62. RIol~t485.8. (2) A solution of 4.49 g (0.01 mol) of cholesteryl chloroformate in 50 ml of absolute benzene and 1.46 g (0.02 mol) of diethylamine is heated under reflux for 3 hr. The white precipitate is filtered off, the filtrate evaporated l o dryness, and the residue recrystallized twice from acetone. Yield: 4.0 g (82.5%) of material, melting at 143-144", and identical in all respects with that of reaction 1. ( b ) Properties, as Observed in the Capillary of a Biichi Melting Point Apparatus. (Temperatures are uncorrected and the readings were not adjusted for the thermal lag due to varying heating and cooling rates. These rates were chosen to facilitate observations rather than to obtain temperature equilibrium.) Cholesteryl S-methyl thiocarbonate melts at 101" and gives a blue color at 103", which disappears at 104". On cooling a blue color appears at 104", which changes to a light blue at l o l o , and disappears at 99", with the now turbid melt solidifying at room temperature. Cholesteryl S-ethyl thiocarbonate melts at 113-115". On cooling, a light blue color is exhibited between 96" and 86". Cholesteryl S-propyl and S-butyl thiocarbonate do not exhibit cholesteric colors on either heating or cooling. Cholesteryl S-pentyl thiocarbonate melts at 9192.5" and on cooling gives a blue color at 86", mhich disappears at 52". Further cooling gives violet a t
1549
44", blue at 43", green at 42.5", yellow at 42", and red at 41.~5"~ followed by solidification. Cholesteryl S-hexyl thiocarbonate melts at 91.593" and on cooling gives a blue color at 84", which disappears at 59". On further cooling, the complete solar spectrum is exhibited between 56" and 53' for an undercooled sample. Cholesteryl S-heptyl thiocarbonate melts at 62" and clears at 86". On cooling blue appears at 68", green at 67.5", and red at 67", followed by crystallization. Cholesteryl S-octyl thiocarbonate melts at 79-80 " and on cooling exhibits blue at 78", followed by a brief green and red at 71.5", which disappears at 71'. Cholesteryl S-nonyl through S-tetradecyl thiocarbonate do not exhibit any cholesteric colors on either heating or cooling. However, they do show a slight blue haze on cooling. Cholesteryl S-pentadecyl thiocarbonate melts at 62-65' and on cooling becomes turbid at 63", and crystallizes near 40" without exhibiting colors. On rapid cooling the phenomenon of "crystal colors" can be noticed, e.g., cholesteric colors in connection with cry~tallization.~ Cholesteryl S-hexadecyl thiocarbonate melts at 48" and clears at 58", exhibiting the solar spectrum between 56" and 55" on cooling. Cholesteryl S-heptadecyl thiocarbonate melts at 67-69" and on cooling turns turbid at 61" and crystallizes at 60". On rapid cooling, with an air stream blown against the oil bath of the melting point apparatus, cholesteric colors can be observed at 58-57'. Cholesteryl S-octadecyl thiocarbonate melts at 58.5" and clears at 62". On cooling the solar spectrum is exhibited around 57.5". Cholesteryl S-nonadecyl thiocarbonate melts at 71-73.5", becomes turbid on cooling to 65", and starts to crystallize at 62". On rapid cooling cholesteric colors appear a t 52-51'. Cholesteryl S-eicosyl thiocarbonate melts at 6466". On cooling it becomes t8urbid a t 63" with the solar spectrum appearing at 57-56". (e) Purification. The crude compounds, prepared on a 0.01-mol scale, were chromatographed on silica gel (45 X 350 mm) and eluted with mixtures of nhexane and benzene. Dicholesteryl carbonate was eluted first, followed by the thiocarbonate, with cholesterol being retained on the column. For the lower members (up to C,) a 70 :30 mixture of hexane-benzene was used, while an 85315 mixture was sufficient for the higher members. The fractions were monitored by thin-layer chromatography for the presence of impurities, and then the materials were recrystallized from acetone or acetone-butanone for the higher members. No change in the transition temperatures was observed after several additional recrystallizations and thin-layer chromatography gave only one spot. Volume 74, Number 7
A p r i l I, 1970
1550 ( d ) Detection of Impurities. (1) Thin-layer Chromatographic Methods. Ascending thin-layer chromatography was performed on 0.25-mm layers of Silica Gel H R (Merck) and Aluminum Oxide H (Merck). The plates were activated for 1 hr at 120” before use. As in the case of cholesteryl ester~,~8!~7 thin-layer chromatographic separation of the lower members is easily achieved. This technique, however, does not allow satisfactory separation of individual members from cholesteryl S-octyl through S-eicosyl thiocarbonate. By reversed-phase partition thin-layer chromatography cholesteryl esters of long-chain fatty acids can be separatedl7,l5on silica gel layers impregnated This method gives a with paraffinslg or silicone clear separation of cholesteryl hexadecanoate and cholesteryl octadecanoate, although the RF values are close together. However, cholesteryl heptadecanoate and cholesteryl octadecanoate are only slightly separated from each other. The same negligible separation was found for an attempted separation of cholesteryl S-nonyl and S-decyl thiocarbonate on silanized and tetradecane-impregnated layers of silica gel and aluminum oxide in the solvent systems benzene-hexane, acetonitrile-butanone, tetralin-hexane, and acetonitrile-acetic acid. Two-dimensional thin-layer chromatography, using reversed-phase separation in the first dimension, followed by impregnation with silver nitrate before the second development, was also not conclusive. This procedure works well in the separation of cholesteryl estersl0 and fatty acid methyl esters,21which are separated according to chain length, structure, and configuration. Gradient thin-layer c h r ~ m a t o g r a p h y with , ~ ~ ~gra~~ dients between silica gel and aluminum oxide, and silica gel with up to 50% silver nitrate, respectively, also did not give a clear separation between adjacent homologs. These results show that thin-layer chromatographic methods are of little value for the detection of minor contaminations of the higher members with close homologs. ( 2 ) Gas-Liquid Chromatography of Alkanethiols. Because of thermal instability of cholesteryl S-alkyl thiocarbonates, a direct gas chromatographic analysis is ruled out. An alternative solution is the analysis of the starting materials. The alkanethiols, partially obtained from commercial sources and redistilled under nitrogen, and partially prepared from high-purity alkanols (99+y0) via the bromide and the isothiuronium bromidelZ4were checked for purity by gas-liquid chromatography. The materials generally had a purity of about 98y0 with the following exceptions: pentadecanethiol contained about 5% tetradecanethiol, hexadecanethiol about 8% octadecanethiol, and octadecanethiol about 10% hexadecanethiol. The analyses were performed on 6-ft glass columns packed with The Journal of Physical Chemistry
W. ELSERAND R. D. ENNULAT 3% QF-1 and 15% Carbowax 2011, respectively, on silanized support. The instrument used was a HewlettPackard F & M Gas Chromatograph Model 5756B with electronic integrator. As already outlined, a thin-layer chromatographic separation of cholesteryl S-hexadecyl and 8-octadecyl thiocarbonate is achieved, and both compounds were uniform on thin-layer chromatography. The only remaining ambiguity is cholesteryl S-pentadecyl thiocarbonate, which may contain some of the tetradecyl thiocarbonate. This may, to some extent, explain the observed “crystal colors,” although the same phenomenon has been observed in several stigmasteryl S-alkyl thiocarbonate~.~ (e) Stability. The prepared cholesteryl S-alkyl thiocarbonates form a variety of products over a period of several months. The same behavior was also found in the series of cholesteryl alkyl carbonates. Only two of these products, cholesterol and dicholesteryl carbonate, could be identified by thin-layer chromatography and verified with authentic samples after chromatographic isolation. However, if the materials were kept in the cold (-35”) and in the dark, decomposition was not observed over a period of several months. ( f ) Infrared Spectra.26 Cholesteryl S-heptyl thiocarbonate shows infrared absorption at 1695 cm-‘ (C=O stretch) and 1160 cm-‘ (C-S stretch), in agreement with reported frequencies of thiol ester^.^^^^^ This is a distinct shift to longer wavelengths compared with those of cholesteryl alkyl carbon ates,2*28 which absorb a t 1733-1741 cm-’ and 1265-1270 cm-l. (9) Measurement Uncertainty of the hfod$ed Xcanning CalorimeterDSC-1. (1) Temperature. Using reference materials with melting points known within *0.1”, we obtained the calibration curve of the temperature scale, shown in Figure 5, for samples weighing (16) H . Weicker, Klin. Wochenschr., 37, 763 (1959). (17) H . P. Kaufmann, 2. Makus, and F. Deicke, Fette, Beifen, Anstrichm., 63, 235 (1961). (18) 6. Michalec, M . h l c , and J. MeStan, ,’future, 193, 63 (1962). (19) H . P. Kaufmann and Z, Makus, Fette, Seifen, Anstrichm., 62, 1014 (1960). (20) D. C. Malins and H. K. Mangold, J . Amer. Oil Chem. Soc., 37, 576 (1960). (21) L. D. Bergelson, E. V. Dyatlovitskaya, and V. V. Voronkova, J . Chromatogr., 15, 191 (1964). (22) E. Stahl, Angew. Chem., Int. Ed., Engl., 3, 784 (1964). (23) C. G. Honegger, Helv. Chim. Acta, 47, 2384 (1964). (24) G. G. Urquhart, J. W. Gates, Jr., and R. Connor in “Organic Syntheses,” Coll. Vol. 111, E. C. Homing, Ed., John Wiley and Sons, New York, N. Y., 1955, p 363. ( 2 5 ) The samples were examined in KBr disks and the spectra recorded on a Perkin-Elmer Model 421 double-beam grating spec-
trometer. (26) A. W.Baker and G. H. Harris, J . Amer. Chem. SOC.,82, 1923 (1960). (27) R . A . Nyquist and W. J. Potts, Spectrochim. Acta, 17, 769 (1961). (28) J. L. Hales, J. I . Jones, and W.Kynaston, J. Chem. SOC., 618 (1957).
MESOMORPHIC BEHAVIOR OF CHOLESTERYL S-ALKYLTHIOCARBOKATES
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as a function of the time during which the temperature linearly increases or decreases. At the beginning of the phase transition the heat flux deviates from a +I* constant value, rises almost linearly, and, at the end of the transition, drops back to a constant value. The 300 320 340 360 380 400 420 440 T E M P E R A T U R E ['K] area between this approximately triangular transition curve and the straight base line is proportional to Figure 5. Calibration curve for differential scanning the latent heat of the phase transition. By using calorimeter. Temperature is corrected by adding value of the known heat of fusion of pure indium as a reference ordinate to instrument reading. The following zone-refined materials were used-with increasing melting points: this instrument can be calibrated with a precision of a 2-methylnaphthalene; 4-nitrotoluene; cyclododecane; few per cente30 Based on the assumption that certain biphenyl; 1,4-diethoxybenxene ; naphthalene ; amplifiers in the instrument have a negative feedback xanthene; 4,4'-azoxyanisole; trans-stilbene; of about 99% we estimate an additional uncertainty benzamide; adipic acid; indium; benzanilide. of =k 1% of full scale. Since transitions in the melt yield transition curves less than 1 mg. Repeated calibration tests revealed with peak heights ranging from 0.1 to 0.4 of full scale that unknown changes of the instrument caused a even for the most sensitive instrument settings,31we unidirectional parallel shift of this curve accumulating obtain uncertainties of between % 4 and ~ l O O j ,with within several months to 0.2". Correcting for this respect to the peak height. In addition spurious shift and considering that the temperature indication fluctuations of the recording prevent the precise is reproducible with a standard deviation of %0.2"729 location of the base line. This error of judgment we estimate for measurements of transition temperais increased by the presence of sample impurities, tures of the melted state a maximum uncertainty of which reduce the peak height and widen the base *0.4". of the transition curve. We estimate a total uncerThe determination of melting points of samples tainty of less than *2OOj, for all compounds except weighing more than 1 mg requires an additional corfor cholesteryl S-nonadecyl thiocarbonate. Since the rection to allow for the temperature difference between smectic-cholesteric transition curve of the latter was the center of the sample and the temperature sensor superimposed on the beginning of the freezing curve, of the instrument30 caused by the much higher heat we could only estimate the lower limit of the latent flux occurring during melting. The correction ranged heat. from 0.6 to 0.9" and, due to variations of the thermal contact resistance between sample capsule and heating Acknowledgment. The authors are grateful to Mr. platform, had an uncertainty of =t50%',. ,4s a result A. J. Brown for obtaining the calorimetric data and the melting points of the homologous series were deterto Dr. L. 31.Cameron and Dr. J. L. W. Pohlmann mined within & l o .The sample weight was chosen for many valuable discussions. to be large enough for the detection of small latent heats and small enough to obtain instrument limited (29) Thermal Analysis Newsletter, No. 5 , Perkin-Elmer Carp., operating conditions and thus a definable transition Norwalk, Conn. temperature. These conditions are fulfilled for any (30) R. F. Schwenker, Jr., and J. C. Whitwell in "Analytical Calorimetry," R. 5.Porter and J. F. Johnson, Ed., Plenum Publishweight between 5 and 10 mg. ing Carp., New York, N. Y., 1968, p 249. (2) Transition Heats. The scanning calorimeter (31) This modified instrument is ten times more sensitive than the records the heat flux entering or leaving the sample standard version.
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Volume 74, Number 7 April 2 , 1970
