Dielectric Constant and Effective Dipole Moment of Drying Oils B. P. CALDWELL Polytechnic Institute, Brooklyn, N. Y.
H. F. PAYNE American Cyanamid Company, Stamford, Conn.
HE correlation betw-een the dielectric properties of a material and its molecular structure has been the subject of extensive investigation in recent years ( 2 , 6 , 8 ) . The dielectric,constant and dipole moment of a large number of pure organic compounds have been tabulated and related to their molecular structure. A large variety of commercial
substances have also been investigated (5, 11, 14). Morgan (6) studied the frequency and temperature variation of the dielectric properties of a variety of materials and found relations between the dielectric behavior and chemical composition of these materials. Wilson (18) in a critical theoretical discussion reviewed the most recent theories of the dielectric constants of polar liquids. Some isolated work has been done on the dielectric properties of the vegetable oils with the object of using the data for identification of the oil or determining its degree of purity or extent of processing. The purpose of the present investigation is ( a ) to determine to what extent the dielectric constant, molar polarization, or effective dipole moment can be employed as a means of identifying various drying oils and mixtures thereof, and (b) to investigate the effect of the degree of heat bodying, as determined by viscosity arid molecular weight measurements, upon the dielectric constant, molar polarization, and effective dipole moment of 'the oils. Heller and Clever (3) stressed the ease and rapidity with which the dielectric constant could be determined once the necessary apparatus had been set up. They found the values for the drying oils so close to one another that a high sensitivity of the apparatus is necessary to secure the requisite exactness. They found no relation between the dielectric constant and the other physical characteristics of the oils, but used the relative values for the different oils as criteiia for identification. They found the dielectric constant of linseed oil to decrease with heat bodying and to approach a constant value, but failed to state that their measurements were made a t high frequencies. Ender this condition the internal friction of the oil is sufficiently high so that the displacement of the polar complexes lags behind the applied field with a consequent lower value of the dielectric constant. The present paper shows that the dielectric constant of heat-bodied linseed oils increases with degree of heat bodying when measured on the Schering bridge (60 cycles) and decreases when measured by the resonance method a t higher frequency. The dielectric constant and dipole moment of linseed, tung, and castor oils were determined by Stoops (10) with the bridge method. The electronic and atomic polarizations were eliminated from the total polarization by making measurements over a temperature range sufficient to include the solid condition. The present work uses the difference between the molar polarization and refraction to give the orientation polariaation. This method provided close checks on the values obtained by Stoops, which indicate the reliability of both methods. Some substances contain polar molecules or groups which have some degree of rotational freedom in the solid state (14) so that the temperature method cannot be used to obtain the Orientation polarization of these compounds. It
T
The dielectric constants of raw and heatbodied linseed, perilla, tung, and oiticica oils were determined by two methods : Schering bridge at 60 cycles and resonance at 45-60 megacycles. The effective dipole moments were calculated by Debye's equation. The orientation polarization was obtained as the difference between the molar polarization and refraction. The values are plotted against viscosity and molecular w-eight Relatively small differences were obtained between linseed, perilla, and tung, although these oils contain nonconjugated and conjugated structures, but a large increase in polarity resulted from the ketonic group in oiticica. Mixtures of tung with linseed or perilla may be identified by refractive index measurements, but this method of identification is not satisfactory for tung with oiticica. Mixtures of the latter may, however, be identified by density and also by dielectric constant measurements. The rapid resonance method was applicable for mixtures of tung and oiticica, and the variation in dielectric constant with composition was found to be linear. The dielectric constant of the oils investigated varied considerably with frequency, the variation in general being greater, the higher the viscosity of the oil.
.
95 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1941
would appear that this condition does not exist in linseed and tung oils; otherwise the close checks referred to above would not have been obtained. Yakimetz (16) found the dielectric constant of refined and polymerized linseed oil to decrease with increasing frequency in accord with the theory of Debye (2). According to the Debye theory of polar molecules @), the total molar polarization is the sum of the electronic, atomic, and orientation polarizations. The first two are distortional polarizations due to the displacement of electrons and atoms in the molecules. The third is produced by the orientation of molecules or groups having permanent dipoles. These are known as polar molecules and are unsymmetrical with respect to the distribution of their positive and negative charges. Nonpolar molecules are symmetrical and have no orientation polarization. The optical refractive index is a measure of the electronic distortion, and Maxwell showed that the refractive index squared is equal to the dielectric constant when measured at the same frequency, provided absorption does not occur. The atomic distortion has been shown to be very small and is usually neglected. It will be seen, therefore, that the difference between the molar refraction and polarization is a measure of the orientation polarization. The effective dipole moment may be calculated from the orientation polarization as shown in the following equations : Dielectric- constant: c substance E = (1) c vacuum where c = capacitance Clausius-Mosotti equation : (2)
whered = density a = polarizability Lorentz-Lorenz equation : molar refraction R = where
q =
M
=
' 3 oz+2 d
(3)
molar polarization of solute and solvent, respectively mole fraction of solute and solvent, respectively MlMz = molecular weight of solute and solvent, respectively = dielectric constant of solution t12 dlz = density of solution PIPz
fifi
Equations 5 , 6, 7, and 8 are based upon the assumption that the intermolecular forces between polar molecules are negligible. This condition is fulfilled only in the case of molecules in the gaseous state. Quantitative results can be obtained for dilute solutions of polar molecules in nonpolar solvents if account is taken of the solvent effect. When dealing with pure polar liquids or mixtures of polar liquids, account must be taken of the molecular interaction if quantitative results are to be obtained. Nevertheless, experimental data obtained on viscous polar liquids are, usually a t least, in qualitative agreement with the Debye theory. I n such cases, however, the dipole moment obtained by means of Equation 6 is not the true dipole moment of the molecule but must be regarded as an effective value. Thus, in the case of drying oils, the effective dipole moment calculated in this manner represents the effective dipole moment of the large oil molecules or aggregates of molecules regarded as rigid structures and rotating as such.
Materials Four drying oils were studied-linseed , perilla, tung, and oiticica. They have certain characteristics in common, but they have structural differences which may affect their behavior in an electric field in such a manner as to produce variations in dielectric constant, molar polarization, or effective dipole moment values which could be used for identification, analysis, or control of processing. The oils are chiefly triglycerides of the 18-carbon-chain acids. Linseed and perilla are mixed unsaturated glycerides of similar composition; tung is mainly the triglyceride of eleostearic acid, and oiticica contains a large percentage of the ester of licanic acid. The acids and their structural relations are as follows:
Maxwell's relation: 7 2
= e (at
Debye's equation:
+
-
molar polarization P = PA. c+2 T =PE c l M - 4?rNcyO 4rrNp2 €+2 7 3 +9LT where k = Boltamann constant T = absolute temperature N = Avo adro number p = dipofe moment o0 = distortional polarizability
+ Po
(5)
Orientation polarization :
,U
=
= 0.01273
P - R; Po
I/KT X
47rNp2
=
-
wherei = dl w = 27r frequency y = relaxation time of dipoles
P2
3kT 1
+1i w y
Linoleic
Tung
Eleostearic
Oiticica
Licanic
Formula
CHa(CHz)rCH=CH(CHz)&OOH CHs(CHd&H=CH (CHd . .CH= CH(CHz)&OOH CHa(CHz)CH=CH(CHz)CH= CH (CHa)CH=CH(CHe) 7COOH CHa(CHz)aCH=CH-CH= CH-CH=(CHz)rCOOH CHa(CHzJsCH=CH-CH= CH-CH=CH(CHa)aCO COOH
(CHz)2-
Linolenic acid is an isomer of eleostearic, the difference being the conjugated structure of the latter; and licanic acid, with its ketonic group, may be expected to be more polar than the others. The materials used represent the standard grades of industry. No additional purification was made because it was desired to relate the resilts t o actual industrial practice. The linseed and perilla oils were alkali-refined varnish grade, and the tung and oiticica were the standard raw oils.
Experimental Procedure
electrostatic units
P=--&-2T=T(LYo+---) B 1M 47rN -
(6)
Aoid Oleic Linolenic
the same frequency, provided absorption does not occur) ( 4 )
Po
= =
Oil Linseed. perilla
refractive index molecular weight
955
(7)
The oils were heat-bodied in open aluminum kettles with a stream of carbon dioxide bubbling through the oil. Batches of 5000 grams were heated to top temperature in 1.5 hours for linseed and perilla and 1 hour for tune and oiticica. The oils were maintain6d at top heat and samples taken at the time intervals shown. The various physical tests were made according to the following procedures:
Vol. 33, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
956
PROPERTIES OF DRYINQ OILS TABLE I. PHYSICAL Temp. Oil No.
Oil
1
Linseed
1B 2A
Linseed
4
6
6 7 8
C.
F.
1A
Time H X
..
.. .
Oiticica
14
Perilla
Viscosity, Poises
Mol. W t .
Mol. Vol.
Refractive Index
Refractive Index Souared
0 0 30
2.2 3.9 7.8
0.9260 0.9500 0.9710
36.8 38.0 41.8
0.385 145 255
783 1520 1780
846 1600 1835
1.4792 1.4862 1.4917
2.190 2.209 2.225
560 560 560
293 293 293
2 8 12
30
2.89 3.5
0.9423 0.8595 0.9665
37.7 38.7 42.1
2.02 19 187
1015 1320 1915
1077 1375 1980
1.4825 1.4883 1.4903
2.208 2.215 2.221
450
232 232 232
0 0 0 1
0 10 45 10
7.1 7.0 6.8
7.0
0.936& 0.9476 0.9554 0.9594
38.5 39.0 39.9 45.5
2.2 7.3 47 340
640 790 1100 1620
673 833 1152 1687
1.5173 1.5130 1.5103 1.5090
2.302 2.289 2.281 2.277
254 254
0 3 3 3
0 10 20 30
4.9 6.4 6.6 7.3
0.8750 0.9830 0.9860 0.9923
37.7 37.8 38.5 41.2
7.8 22.4 47
153
712 905 1040 1282
730 921 1055 1292
1.5140 1.5110 1.5105 1.5078
2.292 2.283 2.282 2.273
0 2 3 4 4 4
0 0 45 0 15 30
0.5 3.5 4.2
0.9324 0.9565 0.9704 0.9720 0.9727 0.9735
37.3 38.6 41.2 42.0 43.4 44.5
3.6 70 108 187 233 285
774 1250 1710 1836 1872 1930
830 1308 1763 1893 1930 1981
1.4827 1.4890 1.4930 1.4936 1.4939 1.4941
2.1984 2.2171 2.2290 2.2309 2.2318 2.2324
.. . 490
.. . 254
...
. I .
575 575 575 575 575
16 17 18 19
Surface Tension Dynes/Cd.
0 1 3
490 490
15
Density, 25/4
316 316
,
450 450
Q
Acid No.
600 600
Tung
10 11 12 13
.
302 302 302 302 302
0
0
5.1
5.2 5.4
From 1 to 2 grams of oil were weighed into a 250-ml. Erlenmeyer flask for acid number determinations; 50 ml. of a 1 to 1 mixture of ethyl alcohol and benzene were added. This was titrated t o a faint permanent pink with 0.1 N aqueous sodium hydroxide using phenolphthalein. A pycnometer of 30 grams capacity was used for density measurements at 25' C. Refractive index was determined with an Abbe refractometer; a sodium lamp was the light source. Surface tension was measured with a Cenco-Du Nouy tensiometer which gives results directly in dynes per centimeter. All measurements were made at 25" C. and 50 per cent relative humidity. The viscosity was determined with a MacMichael viscometer a t 25" C. with the cup rotating at 24 r. p. m. Four standardized wires were used and the degrees MacMichael converted into poises. Molecular weight was measured by the Rast method. From 20 to 30 mg. of sample were melted with 12 to 15 times its weight of Eastman's camphor 587 (melting point, 177' C.). The difference in melting oints of the camphor and melt was determined and the molecuEr weight calculated as follows: Mol. wt. =
39.7 x
sx
VANTENNA I
3
I . ?
COILS- L( 3 4 TURNS, TAP A T Lp93 I+ 'I 'I
?+
1000
'i
TURNS I'
' TURNS I{ DIAM, NQ22TINNEDCOPPER
d X C
where S = grams of sample c = grams of camphor d = difference in melting points The dielectric constant was determined on the oils and also on three solutions of the oils in reagent-grade thiophene-free benzene. It was measured with a Schering bridge, using 500 volts at 60 cycles and a Berberich-type Monel metal cell maintained
.?.SV. 1.5A.
FIGURE 2. HETERODYKE FREQUENCY METER at 25' C. The dielectric constant was also measured by a resonance method, as described by Wyman (fS), over a frequency range of 45 to 60 megacycles. This method requires a resonating system and a meter for measuring the frequency of resonance. An inductance-capacitance cell placed in the liquid under investigation is tuned to resonance with an oscillator capable of emitting the desired frequencies. The frequency of resonance is then measured with a heterodyne frequency meter. The dielectric constant is given as
POW;yOSK ;,LT 1,2,3,4
2
6
Woz/Wz
where o0 = frequency in air w = frequency in sample I I
L
--------------_--__---------------------FIGURE 1. HIQH-FREQUENCY OSCILLATOR
Coil LI, ,one t u r n of la/,-inch diameter. a/~6-inoh 0. d. oopper tubing: coil
Lz,two turns; ooil Ls,four turns:
coil
Ld,seven turns
The wiring diagram for the oscillator is shown in Figure 1. I t is a typical radio-frequency source, using an acorn-type R. C . A. tube 955 with four coils t o cover the various ranges required. The wiring diagram for the frequency meter is shown in Figure 2. The meter uses a standard electron-coupled circuit with a separate detector tube. Two plug-in coils are used: No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1941
957
which covers the range of oscillator coils 2 and 2 on the second harmonic; and No. 2 which covers coil 1 on the fourth harmonic and coil 4 on the second harmonic. A short antenna is used because of the low power output of the oscillator. The resonator is suspended by a silk thread in the sample to be tested which should be maintained at a constant temperature. A suitable coil is chosen so that proper resonance will be obtained from the coupling of the resonator t o the oscillator. The oscillator is then tuned to maximum cou lin as shown by a minimum deflection on the milliammeter. %it% the oscillator set at this position, a suitable coil is chosen for the frequency meter by consulting the cdibration charts, and this meter tuned to the oscillator. Resonance is heard as a beat note in the headphones at the approximate setting shown in the tracking charts. The setting of the oscillator overns the range of the frequency meter to be used in searching b r resonance; otherwise a variety of odd harmonics would be heard leading to false results. The proper range is obtained from the charts on which the frequency meter numbers (tracking figures) are shown above the oscillator readings. The value of the resonance frequency is obtained from the charts and the dielectric constant can then be calculated according to the above equation.
Dielectric Data The values obtained for the physical characteristics in Table I agree within the experimental limits with those reported in the literature (1,4,9) for oils processed under similar conditions. The variation of the density and of the molecular weight of the drying oils with increase in viscosity resulting from heat bodying is shown graphically in Figure 3. The acid number increases with time of processing (except tung oil); since the free acidity may be expected to affect the values for the dielectric constant, a test was made as follows: Linseed acid was added to linseed oil in the proportions shown in the following table, and the acid number determined in the regular manner. The dielectric constant was determined by the Schering bridge method. Grams of Linseed
c
Oil
Acid
Acid Number
Dielectric Constant
100 97 95 93
0 3 5 7
2.2 8.7 12.1 16.8
3.197 3.190 3.183 3.172
,Io
do ,io ,I do Lo ,a VISCOSITY IN POISES 2L
FIUURE3. VARIATIONOF DENSITYAND MOLECULAR WEIQHT OF OILS WITH VISCOSITY The effect of heat bodying upon the dielectric constant of the drying oils is not very great. The dielectric constant of linseed oil a t 60 cycles increases somewhat with increasing viscosity and molecular weight. The dielectric constant of perilla oil a t 60 cycles appears to decrease a t first and then
The effect of the increase in acidity on the dielectric constant is so small in comparison to the effect produced by viscosity or molecular weight increase that it may beneglected. The effect of heat bodying upon the 60TABLE11. EFFECT OF HEATBODYING cycle and high-frequency dielectric constant of Dielectric Constantd linseed, tung, oiticica, and perilla oils and of Mol. Fraction of Oil in Bensenea A B C benzene solutions of these oils is recorded in Oil No. Bb C' 60 H 60 H 60 H Table 11. Figure 4 shows the effect of heat 1 0.0906 0,0029 3.197 3.173 2.725 2.807 2.312 2.349 bodying upon the dielectric constant and the lA 0.0488 0.0027 3.214 3.031 2.774 2.807 2.315 2.373 1B 0.0420 0.0023 3 432 2.88 2.811 ... 2.311 ... square of the refractive index of the bulk oils 2.765 2.82 2 0.0714 0,0040 3.253 3.10 2.311 2.37 in terms of the resulting increase in viscosity 2.796 2.84 0.0031 3.346 2.94 4 0.0558 2.312 2.47 and molecular weight, respectively. The di5 0.0391 0.0021 3.408 3.00 2.811 2.84 2.312 2.35 electric constant values for linseed, perilla, and 3.133 2.84 6 0.1080 0,0064 3,342 3.10 2.342 2.35 2.833 2.82 0,0052 3.329 3.03 7 0,0899 2.323 2.35 tung oils are so closely grouped together that it 2.780 2.82 0,0041 3.320 2.96 8 0.0717 2.315 2.35 would be exceedingly difficult and uncertain 2.771 2.79 0,0026 3.319 3.15 9 0,0459 2.310 2.36 to employ them as an index for purposes of 10 0.0987 0.0057 0.436 4.22 3.88 2.568 2.42 11 0,0793 0.0045 5.380 3.95 5.107 3.64 2.480 2.42 identification, Oiticica oil, however, can be 12 0.0098 0.0039 5.381 4.05 4.437 3.64 2.406 2.42 identified by its dielectric constant since it is 13 0,0573 0,0032 5.394 4.05 4.052 3.64 2.380 2.42 much higher than that of the other drying oils 14 0.0915 0,0064 3.538 3.23 6.204 2.83 2.546 2.37 15 0.0587 0.0032 3.298 2.96 3.084 2.80 2.784 2.37 investigated, both at low and high frequency. 16 .... .... 3.365 3.00 ... The refractive index provides a satisfactory 17 0,0407 o.0022 3.364 2.89 2:Sis 2:SO 21339 2.37 1s . . . . . . . . 3.378 3.05 criterion for differentiating between tung and 19 0,0388 0,0021 3.393 2.86 2:809 2.83 2:327 2:37 linseed or perilla oils, but tung and oiticica a Dielectric constant of benzene, 2.2675 at 60 05 oils cannot be differentiated by this method. However, oiticica oil can definitely be distinguished from tung oil by its dielectric constant. .
.
I
958
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
Vol. 33, No. 7
6,0-YzLD ..I;. ' ' 0 LINSEEO BOO-F.
0 LINSEED BOO'F. LINSEED 560.F. {OD
TUNG 8 OITICICA PERILLA
0
I-
5
A
CICLES
>:
5.0-
8
'
LINSEED
560.L
A TUNG
Q OITICICA
>:
-
PERILLA
-
- 5.0
RICH FREQUENCY
!k
?> &;E;;.-B;+;:*z
