THERMODYNAMICS OF SOLUTIONS WITH
LIQUID
CRYSTALSOLVENTS
1127
Thermodynamics of Solutions with Liquid Crystal Solvents. 11. Surface Effects with Nematogenic Compounds by Laurence C. Chow and Daniel E. Martire Department of Chemistry, Georgetown Uniuerstty, Washington, I). C.
,80007 (Received November I , 1 9 6 8 )
Differential scanning calorimetry and gas-liquid chromatography are used in a thermodynamic investigation of surface effects with two nematogenic liquid crystals (p-azoxyanisole and 4,4'-dihexyloxyazoxybenzene) . Studied are bulk samples (film thickness of about 4 X cm) and samples where the compound is spread on various common chromatographic support materials (film thicknesses ranging from 40 to 2200 8). I t is found that surface effects are absent at the solid-liquid crystal interface. However, at the gas-liquid crystal interface small surface effects are sometimes present with two-component systems (Le., when nonmesomorphic solute is added), but may be neglected provided that the film thickness is at least 1000 8.
Introduction Differential scanning ~alorimetryl-~(dsc) and gasliquid chromatography4,6 (glc) have been used recently to obtain thermodynamic information about liquid crystals. The former technique provides temperatures and enthalpies of solid -+ mesophase, mesophase + mesophase, and mesophase + liquid transitions; the latter method yields infinite dilution solute activity coefficients and partial molar excess enthalpies and entropies of solution in the anisotropic mesophase (s) and the isotropic liquid phase of liquid crystal solvents. The most recent glc work,6 which dealt with the cholesteric type substance cholesteryl myristate, was reported in paper I of this series. We now direct our attention to nematic type liquid crystals. An important problem that has not yet been directly approached is the effect of the contacting solid surface and the thickness of the liquid crystal layer on the determination of bulk thermodynamic properties for nematogenic compounds. It is not inconceivable that surface interactions could influence the molecular arrangement (and, hence, the thermodynamic properties) of the nematic and isotropic liquid phases through some sort of cooperative ordering or disordering phenomenon. If that were the case, the measured temperatures and heats of transition would depend on the nature of the solid surface in contact with the liquid crystal and/or the film thickness of the sample. Accordingly, by comparing the dsc values for a bulk sample with those for samples having film thicknesses orders of magnitude smaller and consisting of the substance in contact with several different types of surfaces (e.g., common chromatographic solid support materials), a useful study could be made. Furthermore, consider the glc experiment which involves a stationary phase (the liquid crystal) with a high surface area-to-bulk volume ratio (small film
thickness). The possibility exists that appreciable solute adsorption could occur at the carrier gas-liquid crystal interface and, hence, that the glc results could depend on the surface-to-volume ratio.6 Thus, before one can claim that thermodynamic information obtained from glc measurements reflects the behavior of the bulk liquid crystal, the absence of noticeable Gibbs solute surface excess concentrations and the validity of the usual assumption of a simple two-phase (gas/bulk liquid) glc partitioning process must be verified. This can be accomplished by establishing that for both nonpolar and polar solutes the specific retention volumes are independent of the weight per cent of stationary phase on the column packing (Le., the film thickness) and the nature of the solid support with both the nematic and isotropic liquid phases. The foregoing provides the rationale for the present study in which dsc and glc are used to determine what surface effects (in a thermodynamic sense), if any, are found with nematogenic compounds.
Preparation of Materials Liquid Crystals. The nematogenic compounds 4,4’dimethoxyaeoxybeneene or p-aeoxyanisole (PAA) and 4,4‘-dihexyloxyaeoxybeneene (DHAB) were obtained from Frinton Laboratories and were purified by the (1) E. M. Barrall. 11, R . 9. Porter, and J. F. Johnson, J . Phys. Chem., 7 1 , 895 (1967). (2) E . M. Barrall, 11, R . 5. Porter, and J. F. Johnson, ibid., 7 1 , 1224 (1967). (3) E. M. Barrall, 11, R . S. Porter, and J. F. Johnson, Mol. Cryst., 3 , 299 (1968). (4) H . Kelker and E. von Schiviahoffen, “Advances in Chromatography,’’J. C. Giddings and R . A. Keller, Ed., Vol. 6, Marcel Dekker, Inc., New York, N . Y . , 1968, pp 247-297. ( 5 ) D. E. Martire. P. A. Blasco, P. F. Carone, L. C. Chow, and H . Vicini, J. Phys. Chem., 7 2 , 3489 (1968). (6) D. E. Martire, “Progress in Gas Chromatography.” J. H . Purnell, Ed., Interscience Publishers, New York, N. Y . , 1968, pp 93-120. Volume Y9, Number 4
April 1969
LAURENCEC. CHOWAND DANIELE. MARTIRE
1128
Table I: Surface-Coated Liquid Crystals Liquid crystal PAA
DHAB
Solid support Chromosorb P (60-80 Mesh) Chromosorb W (60-80 Mesh) Tee Six (60-70 Mesh)
Chromosorb W (60-80 Mesh)
Surface area, A L , (m*/g of material)
Liquid crystal wt % 7.63 16.74 9.51 15.25 N3
Approximate fllm thickness, T (cm X 10a)c,d
Use
2.3" 1.90
0.7" 0.6" -7b
elf
elf
12 22
f e e
0.4
0.7" 0.7a
7.63
10.00
3
8
f
9 12
f
a Values interpolated from Figure 2 of ref 10. b Estimated from values of J. J. Kirkland, Anal. Chem., 35, 2003 (1963). Estimated by using a mean liquid crystal density of 1.15 g/ml (value reported in ref 11). Film thickness of bulk sample used in the dsc experiment was about 4X cm. Dsc experiment. Glc experiment.
hydrogen peroxide oxidation procedure described by Dewar and Goldberg: followed by triple recrystallization from hot ethanol. 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 3 nematic transition with the melting peak for indium, using the analytical procedure described in the manufacturer's literature.8 The dsc measurements (to be described later in more detail) yielded estimated purities of 99.7% for PAA and 99.6% for DHAB. Coating of Xupports. The solid support materials used in this study were: (a) Johns-Manville 60-80 mesh, acid washed and DMCS-treated Chromosorb W; (b) Johns-Manville 60-80 mesh, acid washed and DMCS-treated Chromosorb P; (c) Analabs 60-70 mesh Tee Six, made from specially processed tetrafluoroethylene. These supports mere coated with the liquid crystal compounds by using methylene chloride as the dissolving solvent, followed by gradual solvent elimination through either rotary vacuum evaporation under slight heat or natural evaporation from a large diameter pan. Summarized in Table I are the properties of the surface-coated liquid crystal samples (packings) that were prepared. An exact weight percentage was not determined for the Tee Six sample. For the others, the weight per cent of liquid crystal in each sample was found by taking the average of at least three gravimetric determinations, with about 1 g of sample, done both before and after combustion of the organic liquid crystal.9 The specific surface areas of the materials were estimated through interpolation of the measurements made by Pecsok, et aZ.,1° in the range 0-20 liquid wt %. The film thicknesses were then estimated by assuming a mean liquid crystal density of 1.15 g/ml. (the value reported for PAA a t the nematic-isotropic transition"). The .loumal o/ Physical Chemistry
Differential Scanning Calorimetry Experimental Section A Perkin-Elmer Model DSC-IB was used for the calorimetric studies. The dsc was purged continuously a t a slow and constant flow rate with dry nitrogen gas. Using a scanning rate of 1.25 deg/min, the temperature axis was carefully calibrated in the range 54 to 156" with the known triple points of the eight standards listed in Table 11. Each sample, containing from 5 to 10 mg of the standard, was weighed to the nearest 0.002 mg with a microbalance and then hermetically sealed in an aluminum pan.
Table 11: Calibration of Dsc Temperature Axis Std compound
Indium" Adipic acidb Dimethylterephthalateb Benzoic acidC Acetanilideb p-Dibromobenzeneb NaphthaleneC p-Dichlorobenzeneb
0
Triple point, OG 156.2 151.4 139.7 122.4 114.3 87.3 80.3 54.2
Temp correctiond, A T ("0) -0.2 -0.8 0.0
1.4 3.2 5.9 6.7 11.4
a Supplied by Perkin-Elmer. b Zone-refined, from J. Hinton. Fisher Certified Ther Metric Standard. AT = Taotual- Toba.
(7) M. J. S. (1966).
Dewar and R. 8. Goldberg, Tetrahedron Lett.,
24, 2717
(8) Thermal Analysis Newsletters, No. 1 to 7, Perkin-Elmer Gorp., Norwalk, Conn. (9) D. E. Martire and P. Riedl, J . P h y s . Chern., 7 2 , 3478 (1968). (10) R. L. Pecsok, A. de Yllana, and A. Abdul-Karim, Anal. Chem.,
3 6 , 452 (1964). (11) W. A. Hoyer
and A. W. Nolle, J . Chem. Phys., 24, 803 (1956).
THERMODYSAMICS OF SOLUTIONS WITH LIQUIDCRYSTAL SOLVENTS Liquid crystal samples for dsc were prepared by accurately weighing from 5 to 10 mg of bulk compound and from 10 to 15 mg of surface-coated compound. Each sample was melt-recrystallized a t least once before the three runs used in the determinations were carried out. The transition temperatures (7') were found with the aid of a calibration curve constructed from the results in Table 11. The heats of transition ( A H ) were determined by the usual method* of comparing the chart peak area per milligram of sample with that of a known weight of standard (indium). The areas were measured with a compensating polar planimeter (K &- E Model 62-0015) by taking the average value determined from three sweeps of each peak. The previously quoted purities were estimated from the thermograms of the bulk liquid crystal samples.
1129
Table IV: Heats of Transition for Bulk Samples-Complete Thermal Behaviop p-Asoxyanisole (PAA) Transition
Heat, cal/g
Solid I + nematicb Nematic -+ isotropicb Solid I1 -+ nematicb Nematic -+ solid 110 Solid I1 -+ solid Io
28.1 0.70 21.8 28d 4d
4,4'-Dihexyloxyaaoxybenaene (DHAB) Transition
Heat, cal/g
Solid -+ nematicb Nematic -P isotropicb Nematic + smectic0 Smectic + solid$
26.1 0.90 Small6 Large6
See Figure 1. b Scanning (heating) rate of 1.25 deg/min. Heats estimated to the Scanning (cooling) rate of 10.0 deg/min. nearest cal/g. 6 Heats could not be estimated because the two peaks overlapped. (I
Table 111: Comparison of Temperatures and Heats of Transition p-Aaoxyanisole (PAA) Solid + Nematic Transition
Reference 12 Reference 1 Bulka 7.63% on Chromosorb Pa 16.74% on Chromosorb Pa 15.25% on Chromosorb Wa
Temp, aC
Heat, cal/g
...
27.4 28.1 f 0.9 28.1f 0.3 27.9 f 0.9
117.6 117.5f 0.3 117.2f 0.3 117.3f 0.3 117.2f 0.3
... ...
Nematic + Isotropic Transition Temp, aC
Reference 12 Reference 1 Bulka 15.25% on Chromosorb -3% on Tee Sixb
... Fvb
133.9 134.2f 0.2 134.0 f 0.5c 134.6f 0.5C
Heat, cal/g
0.64 0.68 f 0.02 0.70 f 0.02
...
...
Results The calorimetric results are set out in Tables I11 and IV and in Figure 1. The agreement between our dsc bulk values and those of othersl97 is excellent; however, the dsc heats of transition are somewhat higher than those obtained from classical calorimetry.12 Some difficulty was encountered (due to the appearance of small broad peaks) in the AH measurements with the support-coated liquid crystal samples; however, one decent value was eventually obtained (see Table 111).
PAA:
\ 11 117.5'
SOLID I
4,4'-Dihexyloxyaaoxybenaene (DHAB) Solid + Nematic Transition Temp, "C
Reference 12 Reference 7 Bulka
... 81 81.0f0.4 Temp, OC
?TIC
24.9
...
Heat, cal/g
ISOTROPIC
104.4'
SOLID I1
Heat, cal/g
26.1f0.5
134.2' - 4
w:
81.08
SOLID
NEMRTIC- a
128.2'
ISOTROPIC
Nematic + Isotropic Transition Temp, OC
Reference 12 Reference 7 Bulka
... 128 128.2 f 0.3
Heat, cal/g SMECTIC
0.91
...
0.90 f 0.03
Scanning rate of 1.25 deg/min. b Scanning rate of 10.0 deg/min. Determined by comparison with the thermogram of a bulk sample run at a scanning rate of 10.0deg/min.
Figure 1. Thermal behavior of PAA and DHAB: superscript a, scanning (heating) rate of 1.25 deg/min; superscript b, scanning (cooling) rate of 10.0deg/min and temperatures estimated to the nearest degree, without any calibration correction. (12) H. Arnold, 2.Physdk. Chem.
(Leipsig),
226, 146 (1984).
Volume 78,Number 4 April 1960
LAURENCE C. CHOWAND DANIELE. MARTIRE
1130
Table V:
Composition of Glc Columns Column No.
1 2 3 4
5 5
Total wt
wt
Liquid crystal
Solid supporta
Total wt ( 9 ) of packing
of liquid crystal
liquid crystal
PAA PAA PAA DHAB DHAB
Chromosorb W Chromosorb P Chromosorb P Chromosorb W Chromosorb W
9.3224 10.1230 9.7866 6.7473 10.0072
0.8866 1.6946 0.7467 0.6750 0.7636
9.51 16.74 7.63 10.00 7.63
(9)
%of
All 60-80 mesh.
The detailed thermal behavior of bulk PAA and Table VII: Specific Retention Volumes (Veo)in DHAB bulk DHAB, summarized in Table 1V and Figure 1, (Nematic Phase Only) is worth further mention. We have deduced that the intermediate state obtained on cooling the nematic Column no.a------Solute 4 5 phase was solid for PAA and liquid-crystallinela for DHAB. This was because the AH'S are generally large n-Hep taneb 20.86 20.80 n-Octaneb 43.67 43.69 for liquid crystal --f solid transitions and small for 90.86 n-Nonaneb 90.73 liquid crystal -+liquid crystal transition^.'-^ All phase 65.17 2-Methyloctaneb 65.18 changes noted in Figure 1 were independent of the 42.64 2,2-Dimethylheptane* 42.69 heating or cooling rates. Despite several attempts, we n-Decanec 133.4 133.5 were unable to obtain a smectic + nematic transition TetrachloroethyleneC 62.03 62.20 112.2 112.1 in DHAB because of the short smectic range ( ~ 2 " ) ~p-Xylenec Cumenec 125.8 125.4 whereas for PAA the solid I1 --f nematic transition was always attainable a t 104.4 f 0.3". "SeeTableV. b T = 9 1 . 3 0 . O T = 102.3".
Gas-Liquid Chromatography Experimental Section The compositions of the five columns used in this study are given in Table V. All columns were made from 1/4-in. 0.d. copper tubing; they were condi-
Table VI:
Specific Retention Volumes (VEO) in PAA Yolutc
---CoIumn no.a 1
2
3
50.14 60.45 56.01 57.88 119.3
51.28 61.13 56.13 57.71 119.3
A. Nematic Phase (123.8') n-Undecane n-1-Undecene 1-Bromohexane o-Xylene n-Butylbenzene
49.41 59.80 56.00 57.83 119.4
B. Isotropic Phase (140.8') n-Undecane n-1-Undecene 1-Bromohexane o-Xylene n-Butylbenzene o-Dichlorobenzene See Table V. The JOUTnd of Ph~sicalChemistry
44.54 53.57 51.35 53.67 103.9 187.1
44 * 95 53.84 59.10 53.82 103.8 188.0
45.57 54.33 51.40 53.75 104.1 187.6
tioned, with a gentle flow of helium, a t about 150" for 24 hr. The glc unit employed was a dual column Perkin-Elmer Model 880 with dual flame ionization detectors. The detector temperature was maintained at about 200" and the injection port temperature a t about 300". The column temperature was determined at several points by using copper-constantan thermocouples attached to a Leeds and Northrup Type K-4 potentiometer which was equipped with a dc null detector and an Eppley standard cell. The maximum temperature difference along the column was found to be less than 0.3"; the average column temperature remained constant to within 0.1". The helium carrier gas flow was regulated with Negretti-Zambra precision pressure regulators, Model R/182. The flow rates, which ranged from 35 to 50 ml/min, were measured with a soap-film meter at the column outlet and were chosen to give convenient elution times and reasonable column efficiencies. The inlet pressures were measured to the nearest 0.05 psi with a calibrated Series 1400 U. S. Test Gauge (range, 0-30 psig). The outlet pressure (atmospheric) was determined from a barometric reading, A 1-mV Leeds and Korthrup (13) We have referred to this state as a smectic state because C. Weygand and R . Gabler, J. Prakt. Chem., 155, 332 (1940), have indicated that a monotropic nematic -ismectic transition occurs on cooling.
1131
THERMODYNAMICS OF SOLUTIONS WITH LIQUIDCRYSTAL SOLVENTS Table VIII: Surface and Bulk Contributions to Retention for Two Solutes in PAAa Percentage of surface contribution to the total retentionc------Column 3 Column 1 Column 2
7 -
Solute n-Undecane
n-1-Undecene
Phase
V,o(bulk)
Nematicd Isotropic6
48.8 44.3
1.2 0.5
2.7
Nematicd Isotropice
59.3 53.4
0.8 0.3
1.9 0.8
a All other systems gave no clear evidence of solute adsorption. T = 123.8'. e T = 140.8". eq 1 and Results in Table VI.
b
Results Specific retention volumes ( Vgo)were calculated from the corrected peak retention times and the column operating conditions by using the well-known expression of Littlewood, et all4 The results for several representative solutes in PAA and DHAB are set out in Tables VI and'VII. Only 72-undecane and n-1-undecene in PAA clearly exhibit the upward trend of V 2 with decreasing film thickness that is indicative of solute adsorption at the gas-liquid interface.6 To estimate the extent of this adsorption contribution to the total retention, the glc results for these two systems were further analyzed by the method proposed by Martids and successfully employed by others.'OJ6 Utilizing the values in Tables I, V, and VI and the suggested plotting procedure,ls we have determined the bulk phase contribution to the specific retention volume (Le., the value for the limit of infinite film thickness) and the approximate percentage of the surface phase contribution, i.e., the value for
The results are set out in Table VIII.
x
100
(1)
4.8 2.8 3.0
1.7
Determined by method outlined in ref 10, 15, and 16.
"Speedomax W" recorder, with a chart speed of 2 in./min, was utilized. The solutes studied were used without further purification ; the chromatograms indicated that no major impurities were present. Individual liquid samples were injected with a 1O-fi1Hamilton syringe, using the smallest detectable sample size. The solute peak retention times past methane were determined by taking the average of from 3 to 5 measurements. Early measurements were reproducible a t the end of the experiment to within the accuracy of the measurement; also, the use of an internal standard indicated that no bleeding loss of the stationary phase had occurred. It was established that the solute retention times were independent of sample size, hence that operation was taking place in the Henry's law region (i.e., infinite dilution).
V,o(measured) - V,O(bulk) Vgo(measured)
1.4
Calculated from
Discussion From the results in Table I11 it is evident that there are no discernible differences between the values for the bulk and support coated samples of PAA. Therefore, considering the wide range of film thickness covered in the dsc experiment ( a factor of about lo6), it is apparent that the support materials studied have little or no effect on the heats and temperatures of transition. The absence of solid surface effects does not, of course, preclude the possible presence of liquid surface effects with two-component systems (ie., when solute is added). A large Gibbs surface excess concentration for the solute component coupled with a high liquid surface-to-volume ratio for the solvent component can lead to appreciable solute adsorption at the gas-liquid interface.6 Such has been observed with glc for binary systems consisting of nonpolar solutes and polar liquid phases.10*16 It also takes place, but to a much smaller extent, with two solutes (the two least polar and polarizable ones) in PAA. The observed effects (see Table VIII) with the nematic phase are larger than (roughly double) those with the isotropic liquid phase, as would be expected for the more ordered (i-e., '(polar") phase.1°J6 However, for a high enough weight percentage (about 15%) of PAA on 60-80 mesh Chromosorb W, it appears that these effects may be neglected for all solutes, with minimal resulting error (less than 1%). The lack of any observable liquid surface effects for the paraffin solutes on DHAB could be due either to experimental error obscuring trends in the results or to the possibility that the ten additional methylene groups per molecule (compared with PAA) may lower the solvent surface free energy sufficiently to diminish the surface excess concentration of the solutes.6 In summary, one may conclude that, for all the solid supports and film thicknesses studied, surface effects are either absent or negligible at the solid-liquid crystal interface. Furthermore, at the gas-liquid crystal (14) A. B. Littlewood, 0.S. G . Phillips, and D. T. Price, J. Chem. Soc., 1480 (1955). (15) R . L. Martin, Anal. Chem., 33, 347 (1961). (16) D. E. Martire, i b i d . , 38, 244 (1966).
Volume 79,Number 4 April 1069
C. PXRKANYI, E. J. BAUM,J. WYATT,AND J. N. PITTS,JR.
1132
interface, small surface effects are present in some cases, but they may be neglected in the glc experiment provided that a large enough film thickness (greater than is used. These encouraging findings about 1000 now allow one to proceed with a detailed thermodynamic glc study of solutions involving these nem-
A)
atogenic solvents with confidence that the results will provide information about bulk liquid crystal behavior.
Acknowledgment. This work was supported through a basic research grant from the U. 8. Army Research Office, Durham, North Carolina.
Physical Properties and Chemical Reactivity of Alternant Hydrocarbons and Related Compounds. XVI. 1 Electronic Absorption and Phosphorescence Spectra of Aryl Phenyl Ketones
by C. Phrkbnyi,Z Institute of Physical Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia
E. J. Baum, Oregon Graduate Center for Study and Research,s Portland, Oregon
J. Wyatt, and J. N. Pitts, Jr. Department of Chemistry, University of California, Riverside, California
91502
(Received September 1 4 , 1 9 6 8 )
Spectral properties of eight aryl phenyl ketones were investigated by Pariser-Parr-Pople type LCI-SCF-MO calculations. Reasonable agreement is found between calculated (SO+ SI) excitation energies and wavelengths of absorption band maxima of the ketones, Phosphorescence emission spectra for six of the ketones were obtained, and reasonable agreement is found between calculated (So-+ T) excitation energy and wavelength of the 0-0 emission band for those compounds assigned to have a lowest-lying (n,n*) triplet state. The position of the 0-0 band was confirmed by phosphorescence excitation methods where possible. In all cases, the predicted transition energy was found to be too low. calculated values were found to deviate equally from observed values for both singlet and triplet transitions resulting in accurate predictions of S-T splittings for the lowest (a,n*) excited states. Major contributions to this error would be made by choice of calculation parameters for the keto group, deviations from planarity of the compounds studied, and solvent dependence of the spectral band positions.
Introduction Iln earlier paper in this series discussed the correlation of HMO characteristics with physicochemical properties of aryl phenyl ketones.' Among other results, linear correlations were found between calculated transition energies and the positions of maxima in the long wavelength electronic absorption bands of these ketones. The present paper reports the results of LC1-SCF-MO calculations of singlet-singlet transition energies and oscillator strengths for ketones I-VIII. The values are compared with those derived from experimental absorption curves. Furthermore, phosphorescence emission The Journal of Physical Chemistry
and phosphorescence excitation spectra are presented, and calculated values of triplet-singlet transition energies are compared with experimentally determined values. Prominent features of the emission and excitation spectra along with phosphorescence lifetimes are ( 1 ) Presented in part at the 2nd IUPAC International Symposium on Photochemistry. Enschede, The Wetherlands, July 16-22, 1967. Part X V : C. Pbrkbnyi, Z. Dolejiek, and R . ZahradnIk, Collect. Czech. Chem. Commun., 3 3 , 1211 (1968). (2) Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, Calif. 91109. (3) 9340 5. W.Barnes Road, Portland, Ore. 97225. (4) C. Pbrkbnyi, V. Horbk, J. Pecka, and R . Zahradnik, Collect. Czech. Chem. Commun., 31, 835 (1966).
