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(5) GRONOYER, A . , A N D WOHNLICH, E.: 2. Untersuch. Lebensm. 63, 392 (1927). (6) HARNED, H. S., AND OWEN,B. B.: T h e Physical Chemistry of Electrolytic Solutions. Reinhold Publishing Corporation, New York (1943). (7) HEDVALL, J. A . , BOSTROM, N., COLLIANDER, B., AND HAMMARSON, i.: Z. anorg. allgem. Chem. 243, 231 (1940). (8) JABLCZYNSKI, K., AND PIERSZCHALSKI, T . : Z. anorg. allgem. Chem. 217, 298 (1934). (9) JABLCZYNSKI, K., AND WAJCHSELFISZ, H.: Roceniki Chem. 9, 340 (1929). (10) JABLCZYNSKI, K., HEWNOWICZ,E., AND WAJCHSELFICS, W.: 2. anorg. allgem. Chem. 180, 184 (1904). (11) KIMBALL, G. E . : J. Chem. Phys. 8, 199 (1940). (12) KIMBALL, G. E., AND GLASSNER, A . : J. Chem. Phys. 8, 820 (1940). (13) PLENETEV, S. A , , AND SOSUNOV, S.: J. Phys. Chem. (U.S.S.R.) 13,901 (1939). (14) ROETHELLI, B., FRANZ,C. J., AND MCKCSICK, B. L. Metal Ind. (N.Y.) 30,361 (1932). (15) WALPERT,G.: Z . physik. Chem. A161, 219 (1930). (16) WARNER,J. C . : Trans. Electrochem. SOC.83, 319 (1943). See also BROWN,R. H., ROETHELI,B. E., AND FORREST, H. 0.: Ind. Eng. Chem. 23, 350 (1931).
T H E LONGITUDINAL DISPERSION OF INFRARED RAYS I N POLYSTYREKES W. W: LEPESCHKIN U . S. A r m y M i l i t a r y , Agricultural and Technical school, Weihenstephan, Germany Received J u n e 9, 1946 I. INTRODUCTION
In some papers published by the author in German journals during the war, it was stated that the so-called longitudinal dispersion of light (discovered by Plotnikov in 1928), in spite of doubts expressed by some physicists, can be easily observed for infrared rays penetrating through substances having carbon chains in their molecules. That the longitudinal dispersion of light can be observed especially easily in this case had been pointed out by previous authors, but their results mere not convincing because they did not consider the simultaneously produced Tyndall effect (Splait, 1933; Coban, 1935; Gjuric, 1933, 1939; Jorg, 1937, 1939). Reeent experiment8 by the author on proteins and polystyrenes showed that the Tyndall effect observed in these substances and in their solutions is either absent, or can be made imperceptible by filtration repeated many times, without ally effect on the longitudinal dispersion of infrared rays. They proved, moreover, that this dispersion depends upon the molecular weight of the suhstanres. In this paper the latest apparatus used by the author and some results obtained on polystyrenes are described. 11. METHOD AND APPARATUS
In the apparatus used (see figure 1)a low-voltage bulb (6 volts; exactly5 amp.) with a small incandescent body was the light source. This bulb mas enclosed in a
870
W. W. LWESCHKIN
rected back to the opening of the diaphidgm Ds, but for the most part it was absorbed by the strip, nhile the rays dispersed fell above the strip on a photographir plate which was sensitive to infrared rays (Agfa, 800, rapid) and was placed behind the cuvette in the plate case. After development of the plate a dark semicircle appeared above the clear space corresponding to the brass strip. The surface of the semicircle expressed in mm.2was measured and corresponded to the relative strength of the dispersion and the Tyndall effect. In the case of polystyrenes the Tyndall effect for dark red rays was not perceptible either in solutions or in solid substances. Thus this light dispersion was in this case and also for infrared rays a pure longitudinal dispersion, the Tyndall effect being inversely proportional to the fourth power of the wave length. In solid polystyrene plates no Tyndall effect was visible even in white light. It should be emphasized that the photographic plates used for comparing the strength of the longitudinal dispersion in different polystyrenes or their solutions were taken from the same box (these plates gradually lose their sensitivity to infrared rays), and the photographing of the dispersion took place on the same day. Also, the plates were developed in the same bath. In some experiments the Tyndall effect observed in solutions of polystyrenes in white light was compared with that of rosin suspensions. Two cuvettes of the same shape and size were placed close to each other, perpendicular to the direction of the rays, between the lens and the diaphragm Dz of the apparatus described above, behind the black wall of a stand having two rectangular openings corresponding to both cuvettes (the distance between the openings was 0.5 mm ) and 1 mm. above the level of both liquids. The Tyndall effect was observed from above through a tube the length of which corresponded to clear vision. The standard suspension of rolophony (rosin) was obtained by dropping 1 cc. of a 5 per cent solution of rosin in alcohol into 10 cc. of a 0.5 per cent solution of gelatin warmed to 50°C. and then cooling to room temperatuie. The suspension was added by means of a micropipet to water in the cuvette until the Tyndall effect in both cuvettes became the same. The concentration of rosin in per cent expressed the strength of the Tyndall effect. Some peculiarities of the method in the case of photographing the longitudinal dispersion of light in solid polystyrenes should be mentioned. In this case the cuvette in the plate case was replaced by an equally large glass plate (1 mm. thick), on the lower edge of which the same brass strip \!as pasted as on the lower edge of the cuvette. On this glass plate a plate made of solid polystyrenes was placed in such a way as to allow the light beam to penetrate the latter exactly 2 mm. below the upper edge of the brass strip. The polystyrene plates being even enough but nbt polished nere separated from the glass plate by a thin layer of olive oil and fastened by another glass plate of the same size and thickness, which was pressed on by the front wall of the plate case. The photographing of the dispersion and the observation of the Tyndall effect were made in a dark room. The black semicircles on negatives nere measured in the following manner. As they are really not semicircles but segments (the
LONGITUDINAL DISPERSION
OF INFRARED RAYS IN POLYSTYRENES
871
center of the circle lies 2 mm. below the upper edge of the brass strip), the surface of the semicircles is:
F = 0.39B2
+ 6.3 mm.2
where B is the width of the segments a t the brass strip (in millimeters). The blackness of the segments disappears gradually a t their rims; thus B mas measured as follows: In each of two equal-sized rectangular pieces of black cardboard ( 2 cm. x 1 cm.) a slit (5 mm. x 0.6 mm.) was cut beginning from the short edge of the pieces. One of the latter was glued onto a developed, fixed, and slightly veiled photographic plate of the same size. The plate thus obtained mas placed on the negative, whereas the slit was put below the black semicircle and perpendicular to its large axis. The other cardboard piece was also placed on the negative, but the slit was put on the black semicircle itself a t its large axis and perpendicular to it. This piece was moved along the axis until the blackness in both TABLE 1 Molecular weights of the polystyrenes POLYSTYXENE
YOLECLLAX WEIGHT
EF
600,000 100 ooo 210,000 55,000
I11 I\ L 1
102,300
2 3
152,000 143 000
slits became alike. The position of the slit at the axis was marked; after the same manipulation was repeated at the other side of the semicircle, the nidth B was measured. 111. MATERIAL USED
The molecular weights of the polystyrenes* used mere determined according to the Staudinger method (see table 1). Polystyrenes 1 and 2 were delivered in the form of quite transparent plates (thicknesses 1 mm. and 0.G mm.), which could be used directly for the determination of the longitudinal dispersion of infrared rays. The light beam went either through tn-o plates (polystyrene 1) or through three plates (polystyrene 2 ) ; the whole layer of the substance was therefore in both cases 2 mm. thick. The 1.3-mm. t'hick, quite transparent plates of polystyrenes EF and IV were
* Some of the polystyrenes were obtained from the I. G. Colour Company, Frankfurt am Main; the others from Prof. Dr. Jenkel, Institute for Physical Chemistry of the Technical Academy in Aachen. The author wishes to espress his hearty thanks for these samples.
872
W. JV. LEPESCHKIK
obtained by the evaporation of their concentrated solutions in benzene. A quite transparent plate of polystyrene 111 could not be obtained. Other polystyrenes mere used only in their solutions in benzene. IV. RESULTS
As already mentioned. the solid polystyrenes did not show any Tyndall effect either in dark red or in white light. The light dispersion in their plates was thus a pure longitudinal dispersion. The dispersion measured was as given in table 2 (the exposure to rays lasted 5 min.). Table 3 shows that the values obtained for the longitudinal dispersion ( F ) are proportional to the cube roots of the molecular neights of the polystyrenes. TABLE 2 Dispersion by polystyrenes
1
POLYSrnNE
EF. ..........I IV. ......... 1
..........
2. .........
1..........
THICKNESS OF POLYSTYRENE LAYER
1
B
.
mm.
mm
1.3 1.3 1.3 2 2
31 26 22.5 42 32.5
I
1 1
1 ~
F mm.’
380 269 203 693 418
TABLE 3 Relation between longitudinal dispersion and molectalav weight
1
IV 2
EF
I ,
210,000 452,000 102’300 600,000
1 1.32 1.69 1.77
1 1.27 1.64 1.80
F,/F1 is the ratio between the longitundinal dispersions of polystyrenes Is’, 2 , and EF and that of polystyrene 1. Solutions of polystyrenes in benzene, toluene, or carbon disulfide did not show any Tyndall effect in dark red rays. It must therefore be concluded that they do not shorv any Tyndall effectin infrared rays either, this effect being inversely proportional to the fourth power of the wave length. However they show, as mentioned above, a very weak Tyndall effect in white light, but it has no relation to the molecular weight and must probably be ascribed to some impurity present in different polystyrenes in a different amount, as is shown in table 4. The solutions of polystyrenes must evidently be equally concentrated to demonstrate the dependence of the longitudinal dispersion of infrared rays upon the molecular weight. In order to obtain such solutions it should be considered that polystyrenes have thread-like molecules which in the solid state are entangled
LONGITUDINAL DISPCRSIOS
OF ISFRARED
8T3
RAYS IK POLYSTYRENES
with one another, similarly to threads in felt, and stick together. I t is therefore comprehensible that the longer the thread-like molecules of polystyrenes (that is, the greater their molecular weight), the more time is necessary to dissolve them. Dissolving polystyrene L requires, for instance, some hours, while dissolving polystyrene EF requires’some weeks. It should be pointed out that solutions of polystyrenes can be quite transparent in spite of their containing still solid and swollen particles of the solute. The longitudinal dispersion increases with the concentration, but the greater the latter, the less is the increase of the dispersion. Thus it is smaller in a solution containing 5 per cent of polystyrene only partly dissolved than in a 5 per cent solution in which the polystyrene is dissolved completely. Correspondingly, in the experiments in which the longitudinal dispersion of infrared rays by polystyrenes IV, 111, and L was compared, the solutions of polystyrenes IV and ILI were investigated two months TABLE 4 Tyndall eflect shown b y polystyrenes i n whtte lzght POLYSNPENE.
...........................................
L
1
--__--
1
I11
IV
EF
Tyndall effect expressed in per cent concentration of rosin suspension . . . . . . . . . . . . . . . 0.00015 0.00062,O.OOO41 0.00062 0.00062 Molecular weight.. . . . . . . . . . . . . . . . . . . . . . . . .
1-
55,000 102,3001100,000 210,000 600,000
-~MI/M~
YOLECULAP W?XGXT CAICCU T E D PROM THE POPYUL.4
LONGITJDMAL DISPERSION m L Y s m E m
IV . . . . . . . 111.; . . . L. . . . . . .
2385 21 18.5
F
1
221 178 139
YOLECGLAR WEIGXT
210,000 100,000 55,000
FIIF~
~
I
109,000 53,200
after the beginning of the dissolution, while the solution of polystyrene L was photographed 3 days after it was put into benzene, but both solutions were photographed on the mme day. The concentration of polystyrene was 5 g. in 100 cc. of solution. The exposure to light lasted 5 min. The results are given in table 5. The rule of the proportionality between the longitudinal dispersion and the cube root of the molecular weight of the polystyrene is thus valid for solutions of polystyrenes also. V. INDEX O F REFRACTION OF SOLVENT A N D LOSGITCDINAL DISPERSION
According to Neugebauer (l),who postulated the theory of the longitudinal dispersion of light, this dispersion results as a deflection of light rays from their direction in consequence of a total internal reflection from the surfaces of bun-
874
W. W. LEPESCHKIN
dles of molecules. In the case of polystyrenes this reflection is to be considered as a reflection from the interior surfaces of the molecules, because the latter do not form bundles. The calculations of Ncugebauer showed that such deflection of rays must increase with the length of the bundles or, in our case, with the length or weight of the molecules and be especially great for infrared rays. Therefore, the index of refraction of the solvent must be important for the strength of the longitudinal dispersion. This is shown indeed by the experiment in which polystyrene I11 was mixed with benzene, toluene, and carbon disulfide, and, after 2 weeks, the solutions 30 obtained were used for the determination of the longitudinal dispersion of infrared rays. The exposure to light lasted 2 min. The results are given in table 6. 4 s the index of refraction of bennene is greater than that of toluene and smrtller than that of carbon disulfide, the results given in table 6 show that the longitudinal dispersion increases with decrease in the index of refraction of the solvent. This fact confirms the theory of Xeugebauer. This theory is also confirmed by TABLE 6 Relation between longitudinal dispersion of infrared rays and refractive index of solvent SOLVENT
Benzene.. .....................
,
I ,
I
i :,
CONCENTRATIONOF POLYSTYRENE Ill
p.
ycr
iw cc. 5
i
10
Toluene. . . . . . . . . . . . . . . . . . . . ., I
5 10
I
Carbon disulfide
i)
I
10
I,
,
LONGITUDINAL DISPERSION
B
I ~
I I
1
P
1
~
6.5
10 i.5
22
I
i ~
45 25 53
~
I
6
9
I
20 36
the dependence of the longitudinal dispersion upon the orientation of the molecules of polystyrene, as shown by the following experiment: A 5 per cent solution of polystyrene EF in benzene showed h' = 17.5 and F = 125. The cuvette containing this solution was rotated for 10 min. around its axis so that, after installation in the plate case, it would have the same direction as the light bundle. The polystyrene molecules were thereby oriented in the direction perpendicular to the light bundle. Photographing the longitudinal dispersion gave B = 23.5 and F = 221. When the cuvette was rotated in the opposite direction, the longitudinal dispersion was R = 16 and F = 105. VI. S U M U R Y
After describing the latest apparatus used for the determination of the longitudinal dispersion of infrared rays in polystyrenes, the methods of photographing this dispersion in the case of solid polystyrenes and their solutions and of observing the Tyndall effect were described. The solid polystyrenes used in the
LONGJTCDIS.iL D I S P E R S I O S O F INFRARED RAYS
875
shape of transparent plates did not show any Tyndall effect either in dark red or in white light. The solutions of polystyrenes in benzene, tolucne, and carbon disulfide did not show any Tyndall effect in dark red and infrared rays but showed a very weak effect in white light, a result which has no connection with the molecular weights of the polystyrenes and must be ascribed to an impurity which is contained in different amounts in different polystyrenes. The dispersion of infrared rays in solid polystyrenes as well as in their solutions is a purely longitudinal dispersion. This dispersion proved to be proportional to the cube root of the molecular weight of the polystyrene in both cases. I t also depends upon the index of refraction of the solvent: the greater is the latter, the weaker is the dispersion. The orientation of the polystyrene molecules is important too: the dispersion is stronger if they are oriented perpendicular to the direction of the light bundle than if theii orientation is parallel to the same. Both these facts confirm thr Neugebauer theory of the longitudinal dispersion of light which considers it as a total internal reflection from thc interior surfaces of bundles of molerules or, in our case, from the surfaces of the moleciiles themselves. REFERENCE
(1) NEUGEBAUER, TH.:Physik. Z. 41, 55 (1940).
LOXGITCDIXAL DISPERSIOS O F INFRARED RAYS I X OPTICALLY EMPTY A S D T P R B I D MEDI-4, AND AIOLECTLAR WEIGHTS OF SUBSTASCES W. W. LEPESCHKIS C . S. A r m y Militaru, Agricultural and Technical school, Weihenstephan, Germany
Received J u n e 5 , 1046 I. INTROD'CCTION
I n a series of papers published in German journals during the last years of the war, the author showed that the so-called longitudinal dispersion of light (discovered by Plotnikov in 1928), in spite of some doubts espressed by physicists, can be easily established if infrared rays and substances with long carbon chains in their molecules are used for experiments. In a paper published recently ( 5 ) some esperiments on polystyrenes mere described. These substances are especially suited for use in proving the dispersion mentioned, because they do not, show any Tyndall effect either in dark red or in ivhite light if they are in the solid state, and they show only a very weak effect in Tf-hitelight but no effect in dark red rays if they are dissolved in benzene. An important peculiarity of the longitudinal dispersion of infrared raps is its dependence upon the molecular weight of the sulistance producing the dispersion. I t is (within the limits of t h c esactness of the method) inversely proportional to the cube root
