DIELECTRIC POLARIZATION O F SOME PURE ORGANIC COMPOUNDS I N T H E DISSOLVED, LIQUID, A S D SOLID STATES BY S. 0 . MORGAN AND H. H. L O W R Y
The ultimate goal of an investigation of this sort is to be able to assign to each atom or group of atoms an electric moment which is a vector quantity and capable of addition so that the moment of the whole molecule may be calculated from the moment and position of its parts. Considerable success has been achieved along this line with the substituted benzene compounds but no such regularity appears to exist among the paraffins. The subject of vector representation of the moment is treated in papers by \Tilliams,l Hojendahl,? and Eucken and Meyer3 but it, has not been developed to a stage where it can be used to predict the moments of other compounds. The theory of dielectrics proposed by Debye satisfactorily explains the dielectric properties of liquids and gases and provides a means of relating these properties to the structure of the molecule. S o detailed discussion will be given of the theory except’ as is necessary to define the quantities used. Complete treatments of the theory may be found in monographs by P. Debye4 and J. Errera: and there are recent reviews of the subject by C. P. Smyth6 and J. IT. Williams7 in which references to the literature on the subject are given. hccording to Debye the polarization of a dielectric by an alternating field is made up of two parts as given by the following equation:
P’ the polarization due t o the induced dipoles is proportional to the polarizability y and P“, the polarization due to permanent dipoles, is proportional to the square of the electric moment and inversely proportional to the absolute temperature T. S is Avogadro’s number, k is the Roltzmann constant and 4n /’3 is the constant of the inner field. The derivation of this equation is rigid only for gases where the mutual attraction of the molecules is negligible but it has been amply proven that the same equation holds for dilute solutions in non-polar solvents. ‘Williams: J. .4m.Chem. Soc., 50, 23jo (1928). Hojendahl: Thesis. Copenhagen (1928). 3 Eucken and hleyer: Physik. Z., 30, 397 (1929). P. D. Debye: “Polar Molecule” (1929). 6 J. Errera: “Polarization di6lectrique” (1928). Smyth: Chem. Rev., 6, 549 (1929). Williams: Chem. Rev., 6 , 589 (1929).
2386
S. 0 . RlORGAPi AXD H. H. LOWRY
The polarization due to the induced dipoles may be divided into two parts, one due to the displacement of electrons, PE, and the other due to the displacement of atoms or groups of atoms, PA. This last term is not treated separately by Debye but has been considered by Ebert,‘ Errera? and S m ~ t h . ~ I n this paper the total polarization will be considered to be made up of three parts and to be given by the equation p =
PE
+ + PO P A
(2)
where the PAterm has the same form as the P’ term of equation ( I ) and Po is the orientation polarization represented by P” of equation ( I ) . From measurements of the dielectric constant and density of a gas or of dilute solutions the molar dielectric polarization may be calculated by the Clausius-Mosotti law € - - I
IT
x - = P d
where B is the dielectric constant, I1 the molecular weight and d the density. It is assumed that the total dielectric polarization of the solution of a polar substance in a non-polar solvent may be expressed by the additive law, namely PI2 = CiP, C*P? (4)
+
where C1 and C z and Pl and Pp are the respective mol fractions and molar polarizations of components I and 2. Thus the polarization of the polar substance, P2,may be determined from measurements on dilute solutions where it is not associated, if the polarization of the solvent, PI, does not change with concentration of polar solute. The polarization calculated in this way from the dielectric constant of dilute solutions when extrapolated t o infinite dilution is called the polarization at infinite dilution, P?-, and is the sum of the three terms as given in equation (2). If, however, the dielectric constant and density of the solid be substituted in equation (3) the resulting dielectric polarization will include only the Y E and PAterms since the dipoles are no longer able to rotate and hence do not contribute t o the polarization. The difference between the polarization in dilute solutions and in the solid state will then be the orientation polarization. The PE term, which is also called the molar refraction, may be calculated from refractive index measurements by the Lorentz-Lorenz Law n? - I 51 X-=PpE n2+z d where n is the refractive index. The refractive index is dependent upon the frequency and for calculating the PEterm the optical refractive index extra1 2
Ebert: Z. physik. Chem., 113, I (1924). Errera: Op. cit. Smyth: J. Am. Chem. SOC., 51, 2OjI (1929).
DIELECTRIC POLARIZATION O F ORGASIC C O Y P O U S D S
2387
polated to infinitely long wave length should be used. This may be determined with sufficient accuracy by means of the simplified Cauchy equation n
=
nso
+ a/X'
(6)
which may be solved graphically by plotting the refractive index for t x o or more lines in the visible region'against I/A', n p being the intercept at I/X'*= 0 . I t has been shown by Fajans' that the refraction is practically independent of the state of aggregation for non-ionic substances and hence the PAterm may be found from the difference between the polarization of the solid and the refraction. There are as yet insufficient data to tell whether or not P A is dependent upon the state of aggregation. PA is usually a small term and where POis large there will be no appreciable error in the calculated moment if it be neglected. It is not permissible however to neglect this term for slightly polar or non-polar substances as has been done by Williams and Ogg? whereby they calculate.for naphthalene a moment of 0.7. This will be discussed further in a later paper on naphthalene and halogen-substituted naphthalenes. Equation ( I ) may be written in the form
P
=
A
+ BIT
(i)
4?r1\'p? 4 a where -A = S y and B = -' If this equation is transposed to the 3 9k form PT=AT B (8)
+
it is apparent that the curve of PT vs T will be a straight line whose slope is A and whose intercept is B. Thus from data on the temperature variation of the total polarization the electric moment may be determined without, knowing the value of PE or PA. If the total polarization is given by equation ( 2 ) then the constant X should include both the PE and PAterms and knowing PE from refraction measurements P.4 may be calculated. This is the method used by Smyth. Still another check upon these quantities is possible if the frequency used in the dielectric constant measurement can be increased sufficiently so that the dipoles no longer have time to orientate in the field and hence do not contribute to the polarization. The difference between the polarization calculated from high frequency and low frequency measurements then will give the value of PO and by subtracting the value of PE obtained from refraction measurements from the polarization calculated from the high frequency dielectric constant measurements a value of PA in the liquid state may be obtained. This is now being done for a series of glycols where the viscosity is sufficiently high that the characteristic frequency of the dipole comes within the range of frequencies available. Fajans: Z. Elektrochemie, 34, j I j (1928). *Williams and Ogg: J. Am. Chem. SOC.,50, 96 (1928).
2388
S. 0 . MORGAN AND H. H. LOWRY
In the work reported in this paper, dilute solution measurements of dielectric constant and density were made on CH8C1, CH2C12,CH3Br and CHJ. The data were first plotted against temperature and the values at even 10' intervals read off from the curves. These values in turn were plotted against concentration and the values at 0.05 mol fraction intervals were determined from the curves. This step was unnecessary where the dielectric constant and density were determined upon the same set of solutions. From these values of dielectric constant and density the polarization was calculated by the equation E
-
~
e + z
I
x
AI,, = d
P12
(9)
where Ill?is the molecular Fyeight of the solution as given by the equation 1,112 = CIA11
+ C2II2
(10)
where C1and C? and I I l and 312 are the respective concentrations, expressed as mol fraction, and niolecular weights of the two components. The values of the total polarization thus determined were plotted against temperature and from these curves the values given in Tables VI t o S were read off. From these data the values of P2 were calculated by means of equation (4) and the P? values were plotted against, concenrration and extrapolated to infinite dilution. The values of P?, determined in this way represent the dielectric polarization of the polar substance at infinite dilution where there is no association. Another method of calculating the polarization at infinite dilution has been suggested by Hedestrand.' This method of calculation was applied to all of the dilute solution measurements but it did not simplify the calculations nor were the results obtained in this way any more concordant. It was considered desirable to determine the dielectric constants of the pure liquids and solids, in addition to making dilute solution measurements, because of the widely different values which have been found by different investigators. The substances studied in the pure state were CH3C1,CH3Br, CHJ, CH2C12,CHCls, CCI?, C2H513r,p-C16H4C12,CsH6 and C6Hl1.
Measuring Condensers During the course of this work six different condensers were designed in an attempt to determine the best form for various purposes. X condenser consisting of three concentric gold-plated cylinders similar to that used by Smyth and Morgan,? was used for comparative measurements and for dilute solutions. The cylinders, however, instead of being insulated from each other by mica blocks, were insulated by beads of glass fused onto stiff platinum wires which were welded t o the cylinders, each cylinder having two wires welded to it at opposite sides of each end. It was impossible to get reproHedestrand: Z. physik. Chem., 135, 36 (1928). Morgan: J. Am. Chem SOC., 50, 1536 (1928)
* Smyth and
DIELECTRIC POLARIZATION O F ORGASIC COMPOUSDS
2389
ducible values for the conductance with mica insulators. This condenser was used for grounded capacity measurements, the electrode consisting of the first and third cylinders being grounded. The total capacity consisted of two parts, a fixed capacity, C1, due to leads and a variable capacity, C p , which is directly proportional to the dielectric constant of the liquid. Two measurements, one with air and one with a standard liquid, are necessary to determine the constants of the condenser. Benzene having a value of e = 2 . 2 8 2 at 2ocC and de 'dt = 0.002, according to Hartshorn and Oliver,' was used as the calibrating liquid. The total capacity of this condenser was 93 MMF and the fixed capacity varied from 4.8 to 6.0 MMF. There are a number of condensers which are designed for absolute measurements of dielectric constant. One similar t o that suggested by R. Darbord,' was built but did not prove satisfactory. This condenser, shown in Fig. I , has a maximum capacity at one setting and a minimum when the armature is rotated 18o"from the maximum setting. This capacity difference is independent of conditions in the upper part of the condenser such as the liquid level or length of leads. The capacity difference of this condenser FIG.I was only 33 hIhlF and the settings were not sufficiently reproducible. Condenser for Measurement of Absolute Capacity suggested by Darbord This condenser can, moreover, be used only for liquids. The remaining condensers were designed so that they could be used for liquids and solids and while they were in general satisfactory for liquids they were only partially so for solids. The condenser shown in Fig. z had a fixed electrode A surrounded by a guard ring C and a parallel electrode D capable of continuously variable separation from A and C. A micrometer scale made it possible to read off the separation. This condenser was intended for use only for direct capacitance measurements, a description of which will be given in connection with a condenser of later design. A true absolute measurement of dielectric constant of liquids may be made with this condenser by making direct capacity readings at two separations. For use in measuring the dielectric constant of solids this condenser was not satisfactory. If solidification occurred when there was enough liquid in the condenser to cover the upper electrode then it was impossible to tighten this electrode down further except for soft waxy substances. Examination of the layer of dielectric formed by fusion in this way showed the presence of Hartshorn and Oliver: Proc. Roy. SOC.,123A,664 (1929) 183, 1193 (192;).
ZR.Darbord: Compt. rend.,
2390
S. 0. MORGAN A S D H. H. LOWRY
voids and air bubbles. If when solidification occurred the upper electrode were above the level of the liquid and then tightened down, complete contact with the upper electrode was obtained only if the solid were soft and wax-like. I t was impossible by any pressures which could be used on a condenser having insulating materials at critical places to press down a crystalline solid sufficiently to get good contact. If the condenser were not perfectly level when solidification occurred the difficulty was still greater and there was danger of the electrode being bent out of line if pressure mere applied under such circumstances. Surface tension usually caused the solid layer to creep up the sides of the guard ring so that the upper electrode made contact with the solid only at the edges, but widening of the guard ring would remove this difficulty. Some of these difficulties would be overcome if the top electrode were capable of being uniformly heated to temperatures above the melting points of the solids being studied. I t was observed, however, that even with waxy solids a particle would occasionally stick to the top electrode and as the electrode was rotated it would cut a groove in the surface of the dielectric. A further difficulty was Athat the mica insulation, bI, became saturated with the substance being studied and a thorough boiling with a Cvolatile solvent was necessary before FIG.2 proceeding to the measurement of a Parallel Plate Condenser for Direct Capacitance Measurements on Solids different substance. I t was found also and Liquids. that a t temDeratures much removed from room temperature, the part of the condenser containing the dielectric was usuallyat some temperaturebetween that of the bathandthat of the room. The condenser shown in Fig. 3 was designed with conical electrodes because of the greater electrode area which could be secured with a given diameter and because of the greater ease in keeping a small condenser at the temperature of the thermostat. Xore accurate settings were possible since for one revolution the electrode separation was changed by p X sin a12 instead of by p, where p is the pitch of the thread and a is the total angle of the cone. In this c,ondenser sin d 2 = 0.37. I t was expected furthermore that it would be easier to tighten up conical electrodes against the solid because of the
. .
DIELECTRIC POLARIZATIO?i O F O R C A S I C COMPOL'SDS
2391
/D
FIG. 3 Conical Electrode Condenser for Direct Capacitance &leasurernents on Solids and Liquids
FIG4 Conical Electrode Condenser for Dlrect Capacitance Measurements on Sol& and Liquids
possibility of shearing of the dielectrlc. There was, however, no noticeable improvement in this respect nor in the temperature control. The condenser shown in Fig. 4 was designed for simpliclty in taking apart t o clean and to solve the problem of keeping the dielectric at the temperature of the bath at temperatures much removed from room temperature. The upper part T was made of hard rubber to reduce heat conduction and the whole condenser fitted tightly into a thin-walled metal case S which was set
2392
S. 0. MORGAN AND
H. H.LOWRY
in the thermostat bath, and which served both to retain the liquid and as a shield. I t was found that the temperature of the condenser and the thermostat bath were the same I 4 to I 2 hour after a change of temperature of the bath. The plate A was insulated from the guard ring C by small micalyx bushings M and the rotating cone which formed electrode D was capable of continuously variable separation. Contact with the 3 plates was made by small fahnstock clips fastened to the hard-rubber top. While the condensers so far described were satisfactory for measurements with liquids it was not possible to get satisfactory measurements of solids with any of them. The condenser shown in Fig. 5 was intended primarily for solids and was designed for direct capaclty measurements. The brass one-piece container D served as one electrode and was grounded. The guard-ring and other electrode, insulated from each other by two micalyx bushings, M, fitted into the container to a position determined by the setting of the glass disc G which could be moved up or down on the shaft C and was D held in place by lock nuts. A layer of liquid of the desired M G thickness was pouredinto the container and allowed to solidify. The upper elecC trodes were then heated to a temperature just above the melting point of 4 the solid and pressed down until the glass disc seated into place. If the FIG.5 upper electrodes were too hot then Parallel Plate Condenser for Direct there was a layer of liquid of appreciaCapacitance Measurements on Solids ble thickness present when the glass and Liquids disc had seated and on solidifying ~- this left a void. If the upper electrodes were not hot enough it was not possible t o press them down sufficiently for the glass disc to seat properly. By adjusting the amount of solid and the excess temperature of the upper electrodes it was possible to get the plates set in their proper place and having a homogeneous layer of solid dielectric between them. This condenser was set in a thin-walled brass container which was immersed in the bath. The method of making direct capacity measurements will be illustrated by means of this condenser and the diagrams in Figs, 6a and 6b. The entire condenser may be considered as made up of a number of capacities in parallel, each represented by a condenser shunted by a resistance. For the first read-
DIELECTRIC POLARIZATION OF ORGANIC COMPOUNDS
2393
ing when electrodes A and C are both connected to the C terminal of the bridge these capacities are, as shown in Fig. 6b, all in the C-D arm, of the bridge or short-circuited. Thus, I , 2, 5, 6, 7 and 8 are in the C-D arm and 3 and 4 are short-circuited. When the A terminal of the condenser is transferred from the C to the A terminal of the bridge, by changing the switch from position SI to Spin Fig. 6b, then I and 8 are transferred into the A-D arm of the bridge, 2 , 5 , 6 and 7 remain in the C-D arm and 3 and 4 are across A
c
D
0
FIG.6 Schematic Diagram of Condenser and Measuring Circuit used for Making Direct Capacitance and Conductance Measurements
the detector A-C where they do not affect the balance. I is the capacity being measured and 8 is the air capacity to grolind of the contact tip. The capacity 8 is less than 0.2 A1.Xl.F. which is the limit of accuracy of the measurements. Thus the micalyx insulators cannot affect the balance and the glass plate always stays in the C-D arm of the bridge. Hence the only capacity and conductance change produced by this switching is that represented by I , that is by the dielectric being studied, if 8 be neglected. Electrical Measurements Measurements of dielectric constant and conductance were made on a capacitance bridge described by Shackelton and Ferguson.' The capacitance was measured to 0.1 M.M.F. and the conductance t o 1 0 - l ~ mhos. The 1
Shackelton and Ferguson: Bell System, Tech. J., 7, 70 (1928).
S. 0 . MORGAN AKD H. H. LOWRY
2394
oscillator and detector were standard Western Electric equipment and measurements were made over the frequency range from I to I O O kilocycles. The current supplied by the oscillator ranged from 5 to I O volts, peak, depending upon the frequency. h telephone was used as a detector in conjunction with an amplifier for audio frequencies and a heterodyne detector at higher frequencies. Temperature Control The temperatures mere controlled by a thermostat similar to that suggested by Walters and Loomis,' using petroleum ether as the bath liquid for low temperatures and toluene or acto oil for high temperatures. Toluene is the most suitable because of the large temperature range in which it is liquid but unless it is kept free from water it gets too milky below -2;' to permit visual observations. The thermostat was provided with a wooden cover which supported the heater and cooling coil and into which the various condensers fitted interchangeably. The method of controlling the supply of liquid air used for cooling differed from that of Walters and Loomis. Instead of the mercury column of variable height, a variable leak was provided by using a piece of capillary tubing in the exhaust air line and mounting it in a telegraph relay in such a way that the armature, provided with a soft rubber pad, fitted up against the end of the tube when the relay was actuated. The current through the relay, and thus the pressure in the liquid air container was controlled by means of rheostats in series with the relay coils. Temperatures were maintained to o.r"C with hand regulation. h platinum resistance thermometer with compensating leads was used for temperature measurements. Two such thermometers, of one ohm inter+ j 3 O (11.P. p-C6H4C12); 32.3S3 (SanY04. val were calibated at +IOO'; IoH20-+Na2S04 I o H ~ O )0;' ; - 2 2 . 8 j 0 (M,P.CCllj; - 4 j . 1 ' (11.P.CeH6C11 and -9j.o" (M.P. C6H5CH3). One of these thermometers was used as a working thermometer and the other kept as a standard.
+
+
Density Measurements The density of liquids was measured by means of a dilatometer shown in Fig. 7(a). Four dilatometers of this type were made up out of quartz capillary. The volume of the large bulbs was from 2 to 3 C.C.and that of the small bulbs ranged from 0 . 0 ; C.C. to 0.18 C.C. The average volume of the capillary was 0.001 C . C . per m.m. and there was a quartz to pyrex graded seal of 1/4" tubing at the top. A pipette having a capillary delivery tube fine enough to fit easily inside the capillary of the dilatometer was used to fill the dilatometer bulb. The dilatometer was filled t o the desired level at room temperature, stoppered with a small cork and then cooled until all of the liquid had contracted into the main bulb. I t was then frozen with liquid air and while a t liquid air temperature evacuated and sealed off under vacuum. The weight of the liquid was determined by weighing the dilatometer empty 1
Walters and Loomia: J. Am. Chem. SOC.,47,
2302 (1925).
DIELECTRIC POLARIZ.ATIOS O F ORGASIC COYPOUSDS
239s
and after sealing off. The dilatometer was then immersed in the thermostat bath and the temperature varied until the liquid level was brought successively to each of the stretches of capillary. Here it was held constant and a measurement macle of the distance of the liquid level, above or below the marker. The dilatometer was calibrated by filling with mercury to each marker and the volume of each stretch of capillary determined by varying the temperature of the Hg-filled dilatometer. The capillary was of sufficiently constant volume that an average value could be used for distances of * I O m.m. from the markers. The position of the meniscus was determined by a cathetometer to 0.01 m.m. and thus the volume determined with a precision of I part in IOO,OOO. For such accuracy however the dilatometer and cathetometer must be very carefully lined up. They were usually only lined up approximately since it is apparent that this correction is of very little importance when it is not the total height of the liquid column that is being measured but merely a very small distance from a marker. Density measurements are however given only to four significant figures since the precision of the dielectric constant measurements was o . j c and impurities of the dielectric were in some cases of the same order. 0 A -1correction was made for the vapor in FIG.7 the dilatometer when this correction was Dilatometers. A: For Liquids large enough to account for a difference in B. For Solids density of I in the fourth significant figure. The density of solids was measured in either of two mays. Solids which were insoluble in water were measured by the displacement of water in a z j C . C . pycnometer. Solids which were soluble in water or were low melting were measured in a special dilatometer shown in Fig. 7-b. This dilatometer, of Pyrex glass, had a volume of about 1 2 C.C.and the volume of the capillary was 0.01 c.c per division. I t was calibrated with water at on and z j" and with ether at -97' and t z j ' . The dilatometer was filled with liquid and the bottom immersed in the freezing bath. The dilatometer was lowered into the bath in stages allowing time for the liquid in the part below the cooling bath to completely solidify before further immersing it. I t was solidified to some point on the capillary and the excess liquid withdrawn with a capillary pipette. It was customary t o first solidify rapidly at the same time applying a vacuum to remove dissolved gases and then to gradually solidify by the above method. This method is only partially satisfactory but the values thus determined can hardly be too large.
2396
S . 0 . MORGAS AND H. H. LOWRY
Refractive Index Measurements Refractive index measurements, where they were not available in the literature, were made with a Pulfrich refractometer and were made on the pure liquids when possible. Otherwise, at least three solutions were measured and from density determinations the refraction for each solution calculated. I t has been shown by Smyth, Engel and Wilson1 that the mixture law holds for refraction in solutions even when one or both of the components is polar and thus the refraction of the polar substance may be calculated from solution measurements. Purification of Materials Benzene. Merck's crystallizable benzene was used as starting material. It was shaken with concentrated H z S 0 4several times and was then shaken with water until clear and dried with sodium wire. Hexane. Eastman Kodak Company's hexane from petroleum B.P. 6 j0-7oo, was used. I t was shaken several times with concentrated H 2 S 0 4 ,then with X / I O K&lnOl in 10% H2S01 and then with 3\10 K >In 0 , in 10% Na OH. It was next washed with water several times until clear and dried with sodium wire. I t was fractionated and the fraction boiling between 65' and 70" used. Carbon Tetrachloride. Xerck's CC1, was the starting material. I t was washed in conc. HSSO,, with water, then with conc. KOH and again with water. I t was dried over Na. Methyl Chloride. A tank of methyl chloride was obtained from the Mathieson .Alkali Works. I t is stated to be 9 j % pure. The gas from the tank was passed through wash bottles containing respectively conc. H2S0, and 10% KOH solution and then dried over PsOs. I t then passed into a train of 3 glass bulbs sealed together and provided with a stop-cock a t either end of the system, in which it was liquified by surrounding the first bulb with frozen acetone. When the bulb was filled the acetone was removed and the CH3Cl allowed to warm up. I t was boiled in the system until the air had all been flushed out and until the first few C.C.had boiled off. The system was then closed and frozen acetone put on the next bulb and all except 5 C.C. of the CHaC1 distilled over. The system was then opened and dry air was blown thru in a direction opposite to that of the distillation and the residue boiled out of the system. This fractionation was repeated several times and then the CH3Cl was distilled into the apparatus in which it was to beused. Methyl Bromide. Eastman Kodak Company's practical methyl bromide was used. It was purified by fractionation. The distilling column was cooled with ice water and after refluxing to allow the HBr to boil off the CH3Br was distilled over into a closed receiver immersed in an ice-salt mixture, leaving the last 2 5 C.C.behind. This process was repeated 3 times. Chloroform. Stock chloroform was washed several times with water and then with concentrated HzS04. I t was next washed with XaOH solution and then with ice water and dried over pure anhydrous K2C03. I t was stored over K2C03 in completely filled, tightly-st,oppered brown bottles and fractionated just before using. Smyth, Engel and Wilson: J. Am. Chem. SOC.,51, 1736 (1929).
DIELECTRIC POL.4RIZATIOK O F ORGANIC C O J I P O C S D S
2397
Methylene Chloride. Eastman Kodak Company’s pure methylene chloride was used. I t was fractionally distilled four times, four fractions being collected each time. The purification process was followed by measuring the conductance of each fraction The conductance of the two middle fractions did not change after the second distillation Ethyl bromide was prepared by the method given in Gattcrmann: “Practical Methods of Organic Chemistry ” I t was washed with ice-water several times, dried over CaClZ, and fractionated p-CsH4C1, was recrystallized twice from ethyl alcohol and dried for several months over CaC1, in a desiccator Discussion of Experimental Data The experimental values of dielectric constant, density and refractive index for the dilute solutions are given in Tables I to 1- and the values of total polarization and the polarization of the p o l x substance calculated from them are given in Tables VI to S inclusive. Measurements of dielectric constant were usually made at lon frequency a5 a matter of convenience However, the effect of frequency over the range from I to I O O kilocycles was inLestigated for each pure substance and solution and in no case was there a change of dielectric constant with frequency -1s mas pointed out earlier if the product of P?_Tbe plotted against T, a straight line should result according to equntion [ 8 ) , the slope being .A and the intercept at T = o being B These curies for the methyl halides, both in dilute solution and in the pure state, ale shonn in Fig 8, the upper set
0
IZC
160 200 TEMPERATURE T’A
240
280
320
360
FIG 8 (Upper Set) Polarization of Methyl Halides a t Infinite Dilution X Temperature vs Temperature (P2,T vs T) (Lower Set) Polarization of Methyl Halides in Pure State X Temperature (PzT vs T)
S. 0. MORGAN A S D H.
2398
CI i
0
c:
9 vi
0 vi
N
N
N
mcc
N
ic
vi C O N N I
v0 1 1
0 U 4
-
* C O
d
P P
N
N
N
-
E
.-x
c
3.13 N
r,
10 N
N
? 0 4
N
N
W
I
I
- *-
*
N
N
c
w
N
N
m
*
N
c.
? . I I
c. N
? W N
I
H. LOWRY
DIELECTRIC POLARIZATIOS O F ORGANIC COYPOCNDS
10
. v.i w.
d N
N
N
. -. a. m.
13 N
N
N
N
2399
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m
m
h. n . * ,- w . -. v . ) .N
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3
l
c
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l
l
* v i - m
l
l
l
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cc
d l l
s .-
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a
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N
N
N
3 0
N -
N
3 0
N N
n 3
I
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I
e
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w. a.
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S. 0 . MORGAN AND H. H. LOWRY
2400
y,
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2402
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2 403
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2406
S. 0 . MORGAN A S D H. H. LOWRY
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DIELECTRIC POLARIZATION OF ORGASIC COMPOUSDS
2411
of curves being for dilute solutions and the lower for the pure substances. From the values of the constant B the electric moment may be calculated by the equation p =
0.0127
4s x
10-18
(11)
the value of p being in electra-static units. The values of the constants X and B and of the moment are given in Table S I . In the last column the values of the moment as determined by others for these substances are given. The agreement with those determined in the gaseous state by Pircar, and by hlahanti and Sen Gupta is as good as can be expected. The value I .8j for C H K 1 given by Sanger is considerably higher than the average of 1.6 found for solutions in Hexane and in C C14. Panger’s value for CHzC’12 however is in excellent agreement, I n a later paper by Xahanti and Das Gupta the value of 1.61 is given for CH?C12as measured in benzene solutions. The agreement here is still to be considered good in view of the large possible error in such measurements. The refractive index of the CH2C12used by bfahanti and Das Gupta is given as 1.42 j at “room temperature.” This agrees with the value for the material we used if “room temperature” was 2 o T . However the value e = 2 . 2 6 used by Alahanti and Das Gupta for benzene corresponds to a “room temperature” of 30’. The density of their C”zC12 is given as 1.378 whereas at 30’ our value is 1.309, In view of such large differences in the constants of the materials used it is surprising that the agreement is as good as it is. -In interesting point in the paper by Mahanti and Das Gupta is that the moments of the dichloro, dibromo and diiodo methanes increase in that order and similarly for the halogenated ethylidene compounds. These are the only cases reported where the order is not the reverse, thus for the hydrogen, methyl, ethyl, cis-ethylene, mono-substituted benzene and the o and m
TABLE XI Values of the Constants and of Electric Moment Substance
A
CH3Cl CH2C12 CH3Br
pXIo’*
B
\
15100
1 29 .. 31 1 1
16900,
23.2
13300
35.0
13000
::$ .48 1.45 I
33.9 11300 1.35 Sircar: Ind. J. Phys., 3, 197 (1928). 2 Mahanti and Das Gupta: J. Ind. Chem. SOC., 5 , 673 (1928). Sanger: Physik. Z., 31, 306 (1930). Mahanti and Das Gupta: J. Ind. Chem. SOC., 6,411 (1929). 5 Sanger: Physik. Z., 27, 556 (1926).
C”3I
by others
pX10’8
I
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I . jZs5
I
.8s3
I
.64
I
.31*
disubstituted benzene halides the order is C1> B r > I. It should be pointed out that Mahanti and Das Gupta made no correction for the P A term. This may be rather large in such compounds but probably not large enough, however, to reverse the order of the moments.
2412
S. 0 . MORGAK
AKD H . H. LOWRY
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DIELECTRIC POLARIZATION O F ORGhNIC COMPOUXDS
2413
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2414
S. 0 . MORGAN AND H. H. LOWRY
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DIELECTRIC POLARIZATIOS OF ORGASIC COMPOUSDS
2415
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2416
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DIELECTRIC POLARIZATION OF ORGANIC COMPOUNDS
2417
m X N
0
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W
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2418
S . 0 . LIORGAS .4KD H. H . LOWRY
DIELECTRIC POLARIZATION O F ORGANIC COMPOUSDS
2419
e:
+
f.p,)
6 -
(13)
dr, P. I
.13;
I .j 3 2
,644 1,677 1.964 2.520 I ,732 I
0.893 -
6'
o 001687 001;40
00183 001943 002jI
oo28jj
0019jj
0 OOIOij
00087 j
The density of these substances is nearly a linear function of temperature for the liquid state and it has been assumed to be so for the solid state for the purposes of interpolating the values of density for the solids given in Tables XI1 to XXI. The density of the liquids may be expressed by the linear equation d = d m.p. - 6' (T - T m.p.) (16) where d m.p. is the density of the liquid at the melting point, T m.p., and 6' is the temperature coefficient of density determined from the slope of the straight line. The values of d m.p. and 6' are given in Table XXIII. Fig. I I shows the curves of density plotted against temperature for the methyl halides. Conductivity and Power Factor The specific conductivity and power factor of these liquids have been calculated from the A.C. measurements in order to determine the relationship betxeen power loss in dielectrics and polarity. During the course of the measurements it was observed that the conductance sometimes increased, due probably to a small amount of decomposition. This decomposition was too small to make a measurable change in the dielectric constant. However, this increase of conductance together with the increase resulting from the presence of small amounts of moisture due to handling makes the conductance results somewhat uncertain. The values for CH3C1 and CH2Clzare however given in Tables XYIV and S S V . The values for the remaining polar substances are less concordant and are not given a t this time. The conductance of the CeH6and C6Hlawas too small to be measured with our equipment. The power-factor is given by the equation
P.F. =
G
-
WC
DIELECTRIC POLARIZATION O F ORGANIC COMPOUSDS
2427
where G is the equivalent parallel conductance, in mhos, C the equivalent parallel capacitance in Farads and w = 2 lI X frequency. For the values of G and C measured in this work this expression is equivalent, to within I Y ~ , of the complex power-factor.
FIG.11 Density vs Temperature for Methyl Halides
The specific conductivity is given by the equation Sp. G =
0.088j X
E
X G
(18)
C
where G is the conductance in mhos, C the capacitance in 31.31.F., t: the dielectric constant, and 0.0885 = I ‘4II X the conversion factor 1 . 1 1 .
S. 0. MORGAN AND H. H . LOWRY
2428
12
GX D
DIELECTRIC POLARIZATION OF ORGANIC COMPOUNDS
-. .. -. -.
.
_ _ .N .P I .N .N N . N.
3
i
N
2429
S. 0 . MORGAN AND H. H. LOWRY
2430
The data in Tables SSIS' and XSV when plotted as log G vs log frequency in Fig. 1 2 show a regular and interesting behavior. The conductance is nearly independent of frequency for low frequencies but begins to increase with increasing frequency a t about I O K.C. and is increasing rapidly at IOO K.C. The isotherms cross as the frequency increases which indicates that there are two, at least, causes for the conductance, the first, probably ionic conduction, being nearly independent of frequency and becoming greater a t high temperatures where more ionization is to be expected or when the ionic mobilities are greater, and the second, probably associated with the energy dissipated by the rotation of the dipoles, which becomes larger
4 U
8 0 10."
0
z
0
c
2
2 s
u 2
0
0
4 . a
8 8 10..
1.0
2
4
b
o
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FREQUENCY K C
FIG.1 2
Log of Specific Conductances v8 Log Frequency for CH&1 and CH2Cl2at Several Temperatures
as the frequency is increased and is greater at low temperatures, where the viscosity and thus resistance to rotation is greater, than at high temperatures. The power factor plotted against frequency for these two substances in Fig. 1 3 shows a rapid decrease with frequency for low frequencies and apparently passes thru a minimum and then increases somewhat at the highest frequencies. summary The dielectric constants, densities and refractive indices of CH3C1, CH,Cl,, CH3Br and CHJ have been determined in dilute solutions of either hexane or carbon tetrachloride or both. The values of the moment calculated from these data are found to agree reasonably well with the values determined by others for these substances in the gaseous state. The dielectric constants and densities of CHSCl, CHzC12, CHCl,, CCL, CH,Br, CHJ, C2H5Br,p-CeH1 C12, C6H6 and CeHll have been determined in the pure state for the liquid and solid and the components of the polarization for these substances have been calculated.
DIELECTRIC POLARIZATION O F O R G A S I C COMPOUSDS
I
10
U
I
>
5
s
20
0
30 FUWCR TACTOR
5
40 X 103
U
x
” w 0 w
e
0
ID
20
30 R W E R FACTOR
-&-
40
50
60
X 10’
FIG.1 3 Power Factor
v6
Log Frequency for CH,CI and CHICI2a t several Temperatures
2432
S. 0 . MORGAN A N D H. H. LOWRY
A number of condensers have been designed for measurements of the dielectric constants of liquids and solids and a discussion of the use of these for direct capacity measurements is given. X simple equation is given which satisfactorily represents the dependence of dielectric constant of the methyl halides upon temperature. The densities of the methyl halides are nearly linear functions of temperature. The conductance and power factor of CHaC'l and CHzC1, are given as a function of temperature and frequency. Bell Telephotie Laboratories New York. A'. Y.
