TEMPERATURES OF LIQUIDCRYSTAL TRANSITIONS IN CHOLESTERYL ESTERS
385
Temperatures of Liquid Crystal Transitions in Cholesteryl Esters by Differential Thermal Analysis’
by Edward M. Barrall, 11, Roger S. Porter, and Julian F. Johnson Chevron Research Company, Richmond, California (Received July 8, 1966)
The first differential thermal analysis (d.t.a.) study of liquid crystal phases in cholesteryl esters is reported. Analysis was performed on a series of nine highly purified straightchain aliphatic esters from formate to stearate. The esters of cholesterol are especially interesting since between the true solid and isotropic liquid a t least two mesophases are possible, the smectic and cldesteric. I n this study the number of first-order transitions in each ester was found by d.t.a. heating curves. Mesophase transition temperatures observed as endothermal minima by d.t.a. are compared with those available from other techniques. Data obtained from different methods are in generally good agreement concerning transition temperatures and mesophase identification. Several previously unreported solid phases, depending on conditions of crystallization, have been found by d.t.a. I n addition, a transition reportedly formed only on cooling in the n-nonanoate ester has been found to be a reversible first-order transition. For several esters, speculation is offered concerning the path for mesophase transitions.
The temperature of liquid crystal, viz., mesophase, transitions for esters of cholesterol have been determined. Data have been obtained by controlled-heating studies using calibrated differential thermal analysis (d.t.a.). This technique has been recently applied to the nematic type of mesophase.2 The cholesteric mesophase is found in derivatives of cholesterol but not in cholesterol itself.a Despite their importance in biological systems, the cholesteric type of mesophase has not, until this time, been studied by d.t.a.4 in spite of the fact that the first established liquid crystal behavior was observed for a cholesteryl e ~ t e r . ~ ? ~ Trends with molecular weight in the number and relative magnitude of first-order transitions have been evaluated for a series of nine saturated straight-chain cholesteryl esters from formic to stearic. Relative d.t.a. peak areas are used to estimate relative heats of transition. Quantitative transition heats for cholesteryl esters are the subject of a future study. The lower molecular weight members of the series have been reported previously to show a single mesophase (cholesteric) ; whereas, the six highest molecular weight members of the series studied here are reported
to show, in addition, a monotropic smectic type of mesophase. A comparison is made with earlier studies of the mesophases in cholesteryl esters which have been made generally by optical means. The nomenclature for liquid crystal phase types has been adopted from the literature for cholesteryl esters with no specific structure determinations made in this study. The molecules in cholesteric liquid crystals are arranged in layers. The layers are reported to be thin with the long axes of the essentially flat molecules parallel to the plane of the layers.’ts
(1) Part V of a series on order and flow of liquid crystals. Presented in part before the American Physical Society, Kansas City, Mo., March 1965. (2) (a) E. M. Barrall, 11, R. S. Porter, and J. F. Johnson, J . Phy8. Chem., 68, 2810 (1964); (b) H.Martin and F. H. Mtlller, Kolloid-Z., 187, 107 (1963). (3) G. W.Gray, J . Chem. SOC.,3733 (1956). (4) G. W.Gray, “Molecular Structure and the Properties of Liquid Crystals,” Academic Press Inc., New York, N. Y.,1962,p. 114. (5) F. Reinitser, Momtsh., 9, 421 (1888). (6) G. A. Hulett, 2. Physik. Chem. (Leiprig), 28, 629 (1899) (7) J. L.Fergason, Sei. Am., 211, 76 (1964). (8) C. Robinson, Trans. Faraday SOC.,52, 571 (1956).
Volume 70, Number B
February 1966
E. M. BARRALL, 11, R. S. PORTER, AND J. F. JOHNSON
386
Experimental Section D.t.a. Method. The basic differential thermograph and electronic system employed in these measurements have been described p r e v i o ~ s l ylo. ~ ~The cell design, a modification of an earlier method,1° is shown in Figure 1. The present design represents an improvement over previous designs in that temperatures of transition may be read directly from the same thermogram that records the heats of transition. The previously discussed problem of thermal lag has essentially been The improved sharpness of the endotherms and accuracy of the temperature axis have been effected by making the sample and sample container as small as possible. This lowers the specific heat of the apparatus, improves its sensitivity, and permits the use of very thin samples to avoid heat-transf er problems. The differential thermograph cell compartment was operated after evacuation to a pressure of 1 torr, the residual gas being air. Sample Preparation. Samples of the cholesteryl esters, the n-propionate, myristate, stearate, and palmitate were obtained from Applied Science Laboratories, Inc., State College, Pa. Samples of some of these and other esters were obtained from Columbia Organic Chemicals Co., Inc., Columbia, S. C. These samples were recrystallized three times from boiling ethanol. Carbon-hydrogen analyses are shown in Table I.
Table I : Carbon-Hydrogen Analysis of Cholesteryl Esters -%
ca1cd.-
foundaH
7 %
Cholesteryl eeter
C
H
C
Formate Acetate n-Propionate n-Heptylate n-Nonanoate n-Decanoate Myristate Palmitate Stearate
81.2 81.3 81.45 81.9 82.1 82.2 82.55 82.7 82.8
11.1 11.2 11.3 11.65 11.75 11.85 12.1 12.2 12.3
81.16 81.24 81.50 82.00 82.11 82.21 82.60 82.70 82.78
11.08 11.20 11.31 11.70 11.79 11.90 12.10 12.18 12.28
Figure 1. Calorimetric differential thermal analysis cell: A, copper sample (S) and reference ( R ) cells 0.3 in. by 0.1 in. diameter attached to copper-constantan thermocouples; B, heat shield 1.5 in. in diameter by 2 in. in height; C, program thermocouple attached to B; D, nickel base 1.5 in. in diameter; E, ceramic insulator 0.8 in. high; F, furnace-centering ring; G, furnace 2.5 in. in diameter by 4 in. in height; H, furnace cover; I, transite base; entire apparatus is enclosed in an evacuated bell jar; J, leads to programmer,
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The Journal of Physical Chemistry
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Table 11: CalibTation Standards for Differential Thermograms -Transition
Average of three determinations.
These analyses indicate a, relatively high purity for these compounds and compare with analyses published previously by Gray.3 A 2-mg. portion of the triply recrystallized sample was packed into 1 em. X 1 mm. i d . glass capillary tubes and sealed with the flame of a microtorch. Care was exercised not to heat samples with the flame. All weighings were made on a semimicrobalance of 0.01-mg. sensitivity. The samples were melted and remelted in place in the thermograph.
~
temp., "C.-
Material
Ref. 11-14
Found
n-Octadecane n-Dotriacontane Ammonium nitrate
28.18 69.7 125 169.0 156.4
28.21 69.00 125.60 169.55 156.0
Indium
(9) E. M. Barrall, 11, J. F. Gernert, R. S. Porter, and J. F. Johnson, Anal. Chem., 35, 1837 (1963). (10) E. M. Barrall, 11, R. S. Porter, and J. F. Johnson, ibid., 36, 2172 (1964).
TEMPERATURES OF LIQUIDCRYSTAL TRANSITIONS IN CHOLESTERYL ESTERS
387
Table I11 : Transition Temperatures for Cholesteryl Esters" ester
Ti
D.t.a.--Tz
Formate Acetate n-Propionate n-Heptylate n-Nonanoate n-Decanoate Myristate Palmitate Stearate
...
...
44 99 i 1
81-87 1101
Cholesteryl
74.0 73.6
80.8 85.7 79.7 79.7
-Other Ts
97.3 118.4 115.3 114.1 93.0 91.2 85.5
TI
... 78 72
85.1
workers16-1-T2
(-90) (80-90) 93
,
Ts
Grays-,
S
96.5 112.8 107.2
...
...
79 82.2 78 77 74.5
90.5 90.6 83 81 82
(
