Thermodynamics of Solutions with Liquid Crystal Solvents Surface Effects with Cholesteric Compounds1 J o e l M. S c h n u r and D a n i e l E. M a r t i r e Department nf‘ Chemistry, Georgetown Unioersity, Wushington, D. C. 20007 Differential scanning calorimeter (DSC) and gasliquid chromatography (GLC) are used in a thermodynamic investigation of surface effects with cholesteryl chloride, cholesteryl myristate, and a 1.743: 1.000 (weight:weight) mixture of the chloride and myristate. Studied are bulk samples (film thickness of about 4 x 10-2cm) and samples where the compound is spread on common chromatographic support materials (film thicknesses from about 300 to 1900 A). Surface effects are negligible at both the solid supportliquid crystal and liquid crystal-carrier gas interfaces for the support materials and solute-solvent systems studied, i.e., the DSC transition temperatures and heats and the GLC retention data are independent of the liquid crystal film thickness.
THE POTENTIAL of gas-liquid chromatography (GLC) for the definitive study of solution thermodynamics in liquid crystals was demonstrated in the first paper (1) of this series. This was followed by a differential scanning calorimetry (DSC) and GLC study of possible thermodynamic surface effects with nematogenic compounds (2). Most recently, through a n analysis of infinite dilution solute activity coefficients and heats of solution, a molecular interpretation of solubility in nematogenic liquid crystals (3) and a G L C method for obtaining the degree of order in a nematic mesophase ( 4 ) were proposed. We now report some findings o n surface effects with cholesteryl myristate, cholesteryl chloride, and a “compensated mixture” thereof (5-7). This present study is the necessary precursor to a thermodynamic G L C investigation of solubility in these cholesteric compounds. Furthermore, with the increasing interest in the use of liquid crystals for GLC stationary phases (8, 9), the extent of possible surface contributions (i.e., solute adsorption at the solid support-liquid crystal and/or liquid crystalcarrier gas interfaces) to solute retention needs t o be examined. The approach used to establish the conditions under which the retention parameters in these cholesteric compounds are independent of complicating surface effect is essentially identical t o the one used previously for nematogenic substances (2). We report here only the notable differences in the apparatus and procedure, the salient features of our experiment, and the results. The first four papers of this series appeared in references I - 4 . (1) D. E. Martire, P. A. Blasco, P. F. Carone, L. C. Chow, and H. Vicini, J . P l i y ~ Chem., . 72, 3489 (1968). (2) L. C. Chow and D. E. Martire, ihid., 73, 1127 (1969). (3) Ihid., 75, in press. (4) L. C. Chow and D. E. Martire, Mol. Cryst. Liq. Cryst., in press.
(5) E. Sackmann, S. Meiboom, and L. C. Snyder, J. Amer. Chem. Soc., 89, 598 1 (1967). ( 6 ) E. Sackmann, S. Meiboom, and L. C . Snyder, ibid., 90, 2183 (1 968). (7) E. Sackrnann, S. Meiboom, L. C. Snyder, A. E. Meixner, and R. E. Deitz, ibid.. p 3567. (8) H. Kelker and E. von Schivizhoffen, “Advances in Chromatography”, Vol. 6, J. C. Giddings and R. A. Keller, Ed., Marcel Dekker, New York, N. Y . , 1968, p 247. (9) W. L. Zielinski, Jr., D. H. Freeman, D. E. Martire, and L. C . Chow, ANAL.CHEM., 42, 176 (1970).
PREPARATION OF MATERIALS Liquid Crystals. The cholesteric compounds cholesteryl myristate (CM) and cholesteryl chloride (CC) were obtained from Steraloids, Inc., Pawling, N. Y., and were purified by multiple recrystallization from ethanol and acetone, respectively. A Perkin-Elmer differential scanning calorimeter, Model DSC-IB, was employed to determine the purity of the compounds by comparing the peak for the solid + smectic isotropic transition for C M and the peak for the solid transition for CC with the melting peak for indium, using the analytical procedure described in the manufacturer’s literature (2, 10). The DSC measurements yielded estimated purities of 99.2 % for C M and 99.5 for CC. Coating of Supports. The solid support materials used in this study were Johns-Manville 60-80 mesh, acid washed, and DMCS-treated Chromosorb W ; and Johns-Manville 60-80 mesh, acid washed, and DMCS-treated Chromosorb P. The procedure for coating the liquid crystal onto the support and the method of determining the exact weight percentage of liquid crystal in the total packing material are described elsewhere (2). Summarized in Table I are the properties of the surface-coated liquid crystal samples prepared. -+
DIFFERENTIAL SCANNING CALORIMETRY Experimental. The procedure used for determining the transition temperatures and enthalpies for the bulk and support-coated liquid crystals is described elsewhere (2). All runs were made at a scanning (heating) rate of 1.25 “C/min. Results. The calorimetric results are summarized in Table 11. The cholesteric phase of CC was observed (optically) to occur only upon cooling of the isotropic liquid. The cholesteric isotropic transition for the compensated CC/CM mixture was not observed by DSC. However, for a 1.75 :1.00 mixture, a value of 61 “C has been reported for this transition, with the “nematic point” being given as 44 “C (11). -+
GAS-LIQUID CHROMATOGRAPHY Experimental. The G L C apparatus employed is identical to that described elsewhere (12), with the sole exception of a slightly modified (but standard) system for controlling the temperature (to *0.05 “C) of the silicone oil bath used here. T h e procedure used for obtaining the G L C retention data has been described before (2). Results. Instead of reporting specific retention volumes as done previously (2), we report relative retention volumes (or times). F o r purposes of investigating possible liquid and/or solid surface effects, the relative retention value with different column loadings and o n different types of solid support is a n index more convenient than and as definitive as the specific retention volume (13). Therefore, listed in Tables (10) Thermal Analysis Newsletters, Perkin-Elmer Corp., Norwalk Conn. (11) H. Baessler, R. B. Beard, and M. M. Labes, J. Cliem. Phys., 52, 2292 (1970). (12) Y. B. Tewari, D. E. Martire, and J. P. Sheridan, J. Phys. Cliem., 74, 2345 (1970). (13) D. E. Martire, “Progress in Gas Chromatography,” J. H. Purnell, Ed., John Wiley & Sons, New York, N. Y., 1968, p 93.
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Table I. Surface-Coated Liquid Crystals
Liquid crystal CM
Liquid crystal, wt 2 4.83 12.12 6.63 19.73 5.83 10.59 6.97
Solid support Chromosorb W Chromosorb P
cc
Chromosorb W Chromosorb P
CCjCM Mixture'
Surface area, m2/g. of materiala 0.80
0.65 2.35
Approximate film thickness, cm X 108h8C 6 19 3
1.80
11 8
0.75 0.70 2.30
15 3
Use d
d, e d, e d, e d, e d, e
e
Chromosorb W
7.00 0.70 10 d 11.08 0.70 16 d a Values interpolated to nearest 0.05 mz/g from Figure 2 of R. L. Pecsok, A. de Yllana, and A. Adbul-Karim, ANAL. CHEM.,36, 452 (1964). * Estimated using a mean liquid crystal density of 1.O g/ml. c Film thickness of bulk samples used in the DSC experiment was about 4 X cm. d GLC experiment. e DSC experiment. f 1.743:1.000 by weight CC:CM.
Table 11. Comparison of Temperatures and Heats of Transition Cholesterol myristate (CM) Solid e smectic Smectic Ft cholesteric Cholesteric e isotropic Temp, "C Heat, cal/g Temp, "C Heat, cal/g Temp, "C Heat, cal/g 73.6 18.7 79.7 0.52 85.5 0.41 Ref. a 70.5 18.6 17.8 0.56 83.2 0.41 Ref. b Ref. c 69.8 ... 79.0 ... 84.2 , . . Bulk 70.0 f 0 . 3 18.9 i 0 . 5 79.8 i 0 . 2 0.49 i 0.04 84.9 f 0 . 3 0.40 ZII 0.03 19.4 i 0 . 5 79.9 f 0 . 2 ... 84.8 i 0 . 3 ... 1 2 . 1 2 2 on W 69.7 i 0 . 3 19.73% on P 70.0 i 0 . 3 19.3 i 0 . 5 80.0 i 0 . 3 ... 85.0 2~ 0 . 2 ... 6.63% on P 69.8 rt 0 . 3 ... 80.0 i 0 . 2 ... 84.8 It 0 . 2 ... Cholesteryl chloride (CC) Solid -z isotropice Isotropic -,cholesteric. Cholesteric -, solide Temp, "C Heat, calxTemp, "C Temp, "C Heat, cal/g Opticald 92.5 i 1 . 0 ... 92.5 =k 1 . 0 78 i 1 Bulk 91.8 =t0 . 2 1 2 . 1 i 0.1 77 i. 1f 12.1 f 0.1' 10.59% on W 91.6 i 0 . 2 ... 5.83% on W 91.7 i 0 . 2 ... 6.97% on P 91.6 i 0 . 2 ... E. M. Barrall 11, R. S . Porter, and J. F. Johnson, J . Phys. Chem., 71, 1224 (1967). 6 G. J. Davis, R. S . Porter, and E. M. Barrall IT, Mol. Cryst. Liq. Cryst., 10, 1 (1970). c Bulk values from Ref. 1. d Apparatus for optical observation of transitions described in L. Goldberg and J. M. Schnur, Appl. Phys. Leu., 14, 306 (1969). e With DSC we were able to observe only the solid --t isotropic and cholesteric -+ solid transitions. This probably indicates that the isotropic -,cholesteric transition is calorically quite small (less than 0.05 cal/g). On the basis of optical observations, we assign the following transitions to CC: solid -+ isotropic \ J5 cholesteric 1 Determined at a scanning (cooling) rate of 5.0 "C/min. ~~
Column Solute n-Octane n-Nonane n- I-Octene n-1-Nonene o-Xylene m-Xylene Chlorobenzene
~~
Table 111. Retention Times Relative to o-Xylene. Values for Cholesteryl Myristate 81.0 "C(Cholesteric phase) 12.12% w 19.73% P 4.83% W 6,63% P 0.291 0.660 0,277 0.627 1 ,000" 0.814 0.720
0.293 0,662 0.278 0.624 1 .Ooo 0.817 0.719
0.293 0.658 0.278 0.622 1.Ooo 0.811 0.718 99.4 "C (Isotropic liquid phase) 4.83% W
0.293 0.658 0.277 0.628 1 .ooo 0.814 0.715
Mean value 0.293 0.660 0.278 0.625 1.000
0.814 0.718
Column 12.12% w 19.73% P 6.63% P Mean value Solute n-Octane 0.359 0.352 0,361 0.353 0.356 n-Nonane 0.754 0,747 0,749 0.753 0.751 ri-1-Octene 0,339 0.331 0.336 0.331 0.334 u-l-Nonene 0.700 0.700 0.706 0.703 0.702 1 .Ooo 1.000 1 .ooo 1 .Ooo 1 .ooo o-Xylene m-Xylene 0.833 0.829 0.831 0.838 0.833 Chlorobenzene 0,722 0.719 0.724 0,720 0.721 a We found a specific retention volume of 448 ml/g for o-xylene; this is in excellent agreement with the 81.0 "C value of 449 ml/g from Ref. I (determined on a 16.17% W column). 1202
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Table V. Retention Times Relative to +Xylene. Values for a 1.743 CC:l.OOO CM (Weight:Weight) Mixture 57.0 "C (Cholesteric phase) Column 11.08% W 7.W% W Mean value Solute n-Octane 0.299 0.302 0.301 n-Nonane 0.785 0.785 0,785 12- 1-Octene 0.274 0.277 0.276 if-1-Nonene 0.699 0.691 0.695 1 .Ooo 1 .Ooo o-Xylene 1 .Ooo m-X ylene 0.828 0.816 0.822 Chlorobenzene 0.643 0.635 0.639 66.0 "C (Isotropic liquid phase) Column 11.08z W 7.00z W Mean value Solute /!-Octane 0.318 0.316 0.317 ri-Nonane 0.758 0.770 0.764 ri-1-Octene 0.284 0.282 0.283 ri-1-Nonene 0.703 0,701 0.702 o-Xylene 1.000 1.000 1.000 m-Xylene 0.818 0.818 0.818 Chlorobenzene 0.642 0.632 0.637
Table IV. Retention Times Relative to o-Xylene. Values for Cholesteryl Chloride 85.0 "C (Cholesteric phase) Column 10.59% W 5.83zW Mean value Solute
n-Octane r:-Nonane it-1-Octene n-1-Nonene o-Xylene m-Xylene
Chlorobenzene Column Solute Ji-Octane ti-Nonane ri-1-Octene ri-1-Nonene o-Xylene m-Xylene Chlorobenzene
0.309 0.305 0.307 0.707 0.695 0.701 0.291 0.288 0.290 0.651 0.649 0.650 1 .Ooo 1 .ooo 1 .Ooo 0.823 0.823 0.823 0.663 0.661 0.662 99.4 "C (Isotropic liquid phase) 10.59z W 5,83% W Meanvalue 0.317 0.682 0.297 0.653 1.000 0.840 0.692
0.313 0.688 0.297 0.642 1 .Ooo
0.834 0.688
0.315 0.685 0.297 0.648 1 .Ooo 0.837 0.690
111, IV, and V are retention times relative t o o-xylene for several representative solutes. Values for a given solute are listed in order of decreasing liquid crystal film thickness (see Table I). DISCUSSION
From the results in Table 11, it is evident that there are n o discernible differences between the values for the bulk and support-coated samples. Therefore, considering the wide range of film thicknesses covered in the DSC experiment (a factor of about IO4), it is apparent that the support materials studied have little or no effect on the heats and temperatures of transition. This is an indication that the contacting surface does not measurably perturb the liquid crystal molecular arrangement through some sort of cooperative ordering or disordering phenomenon. This, of course, does not preclude the possible presence in the GLC experiment of solute adsorption at the solid support-liquid crystal and/or liquid crystal-carrier gas interfaces. F o r the types of solutes and solid supports being considered here (and t o be considered in our future studies), solute adsorption on the solid support has been shown to be negligible (14, 15). However, the possibility of solute adsorption on the liquid crystal surface could not be dismissed beforehand for the solute/solvent systems and
film thicknesses being studied ( 2 , 13). Fortunately, the results in Tables 111, IV, and V clearly indicate that this latter form of interfacial adsorption is negligible. The relative retention times reveal no discernible trends with decreasing film thickness (13), and are, in fact, within experimental error of the mean values for all column loadings considered. Therefore, we conclude that, for the solid supports and film thicknesses studied here, surface effects (in a thermodynamic sense) are negligible at both the solid supportliquid crystal and liquid crystal-carrier gas interfaces. These findings permit one to proceed with GLC investigations involving these cholesteric solvents with confidence that the results will provide information about bulk liquid crystal behavior.
RECEIVED for review March 15, 1971. Accepted April 26, 1971. Work supported through a basic research grant from the U. S. Army Research Office, Durham, N. C.
(14) R. L. Martin, ANAL.CHEM., 33, 347 (1961). (15) R. L. Pecsok, A. de Yllana, and A. Abdul-Karim, ANAL. CHEM., 36, 452 (1964).
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