1222
J. W. Y. HEYNIS AND L . MAASKANT
AN IMPROVED SPREADING APPARATUS FOR THE MEASUREMENT OF FILM PRESSURES J. W. Y. HEYNIS A N D L . MAASKANT Research Laboratmy of A . K .U .and Afilialed Companies, Amhem, Holland Received December 16, 1948 I . INTRODUCTION
The study of monomolecular layers has contributed to the solution of many problems of scientific, medical, and industrial importance. The apparatus constructed by Gorter and Seeder (3) for the measurement of film pressures has especially proved its merits in this field of investigation. In several details, however, this apparatus could be improved. The necessity for these improvements is apparent when the apparatus is used for routine work,’ where equipment of solid construction, offering simplicity of handling while retaining speed and accuracy of measurement, is required. 11. THE APPARATUS OF GORTER AND SEEDER
For further elucidation of the modifications which have been introduced by the authors a schematic drawing of the apparatus of Gorter and Seeder is given in figure 1. In this apparatus film pressures are transmitted to the balance by means of a freely movable, paraffin-waxed metal strip (called a swimmer (C)), which floats on the surface of a liquid in a shallow rectangular trough. The swimmer is connected by two thin, flexible, finely rolled platinum strips (PI, P2)to the edge of the trough. These strips allow the swimmer sufficient freedom to make it possible to measure the forces acting on it. When pressure is exerted by a film placed between the glass rod (Z) and the swimmer, the balance can be adjusted to its zero position by the torsion of a spring (Sp). The amount of torsion is read on a scale (S). The deviations of the balance are made visible by an optical system. For this purpose a mirror is fixed on the axis of the balance. The light beam first passes through a slit 0 and is reflected in a mirror M; the image of the slit is projected on an adjustable scale (SI). The use of this optical system implies that the measurements take place preferably in a semidark room. Moreover, the continual readjustment of the light beam caused by slight dislocations takes time and impedes continuous work. For a more detailed description of the apparatus of Gorter and Seeder, its handling, the spreading technique, etc., reference should be made to the papers by Gorter and Seeder (3) and van Ormondt (5). 111. MODIFIED APPARATUS
In the apparatus as modified by the authors the following principles are embodied: (a) The devices for zero adjustment of the balance and for measuring the film pressures on a scale are brought into one plane in front of the apparatus at a 1 Reference may be made to the application of the spreading method for microquantitative determination8 (1, 2, 4).
APPARATUS FOR YE.ISURINC FILM PRESSURE
1223
normal viewing distance. ( b ) A less vulnerable construction is achieved by encasing the balance, thus at the same time keeping it dust-free and to a certain extent humidity-free. (c) The devices for manipulation are made as easily ac-
FIG.1. Schematic drawing of the spreading apparatus of Gorter and Seeder
J--&
G
x
\
.C FIG.2. Schematic drawing of the balance and ita optical system \
cessible as possible. These improvements are illustrated in the schematic drawing (figure 2) and the photographs of the apparatus (figures 3, 4,and 5 ) .
A . The optical system The operation of the optical system (figure 2) is as follows: A bulb (D) with a strong convex lens (E), both assembled in a container, provides the lighting of
1224
J . W. Y. HEYNIS AND L. MAASKANT
the slit in the thin metal plate (R)fixed on the balance. Behind F1a double condensor (G) is mounted in such a way that the image of the slit in F1 is reflected via the mirror (J) through the window (F2) on the frosted glass (H). The distances FIG and GJH relate as 1:20, so that a small dislocation of F1 causes a considerable movement of the slit projection. On the frosted glass (H) arcs are scratched, and a dislocation of the light-line in the point of contact of the arcs of 0.14.2 mm. is very easily perceptible. As a consequence, the accuracy of adjustment of the swimmer (C) will be about 10 p.
B. Balance construction In our constlvction the same type of differential balance as in the apparatus of Gorter and Seeder was used. The following practical modifications were introduced (figure 2) : (I) The axes of the balance and of the knob (R) attached to the scale (S) have an independent bearing and are linked by two springs (Sp). As a consequence of the independent bearing, manual movements of the knob are transmitted only by the spring-linking and this guarantees smooth functioning of the balance. As in Gorter’s apparatus, for the sake of linearity two springs are mounted with the spiral windings in opposite directions. (2) To slow down the movements of the balance without diminishing the sensitivity an oil damper (L) is inserted. The damping effect may be varied by pushing the oil tube up or down. (3) For the calibration of the balance a small side arm (M) is inserted, the length of which is one-fifth of the distance between the swimmer (C) and the balance axis. The calibration can be carried out by placing weights on a small scale, which may be hung on the hook of M. It is important that the balance can be tested while the apparatus is in condition for use. The sensitivity of the balance is 0.02 dyne/cm./scale degree. (4) At the beginning of a measurement the zero index of the scale must correspond with the zero position of the balance. This can be effected by an independent turning of the scale with the aid of little knobs (SI)attached to the scale. (6) The stability of the balance can, if necessary, be increased by placing weights on the small pin (K) which passes through the hole of the weight. This pin is fixed on the horizontal arm of the balance. The fitting of the lower part of the balance in the trough is similar to that in the apparatus of Gorter and Seeder. PlPl are platinum strips connecting the swimmer (C) with small metal These metal blocks are attached by a spring to the frame of the balblocks (P2). ance in such a way that they are pressed against the edge of the trough when the apparatus is in condition for measurement (figure 3). C. The encasing of the balance and further particulars of the apparatus Figures 3 , 4 , and 5 show the apparatus in different positions. Figure 3 shows the balance box turned to the left. The balance with the inserted optical system is encased in the metal box N, which may be opened by lifting the lid (N1).
U
M0LECUL.W WEIGHTS O F POLYSTYREKE FRACTIONS
1227
IV. CONCLUSION
The apparatus which has been described combines among other things reliability, accuracy, protection of the essential parts, speed of measurement, and suitability for undisturbed continuous work. These improvements were achieved to a considerable extent by the following modifications: (a) The devices for zero adjustment of the balance and for measuring the film pressures were brought into one plane in front of the apparatus a t a normal viewing distance. Thus a general requirement for a measuring apparatus was fulfilled. (b) The optical system was inserted in a special and very convenient way. (c) The metal box protects the balance and hence increase its lifetime. Owing to the solid construction disturbances or breakdowns in the essential parts of the apparatus are reduced to a minimum. Experience has proved the necessity of this construction when comparatively unskilled laboratory assistants have to work with the apparatus. V. SUMMARY
A spreading apparatus has been constructed which shows practical advantages over the instruments described in the literature and meets every demand for undisturbed continuous work. REFERENCES (1) GORTER,E., A N D BLOKKER, P. C.: Proc. Acad. Sci. rlmsterdam 46, 151 (1942). (2) GORTER,E., AND HERMAM,J. J.: Proc. Acad. Sci. Amsterdam 46, 902 (1942).
(3) GORTER,E., A N D SEEDER.W.A , : J. Gen. Physiol. 18, 427 (1935). (4) MAASKANT, L.: “The Use of a Spreading-Method Technique in Research on Artificial Protein Fibers,” SOC. Dyers Colourists, Symposium on Fibrous Proteins 1946, 105. ( 5 ) ORMONDT, J. V A N : “Spreading of Proteins,” Symposium on Proteins which was held at Amsterdam in 1938, p. 78.
COMPARISON OF MOLECULAR WEIGHTS OF POLYSTYREXE BY THE LIGHT-SCATTERING AND OSMOMETRIC METHODS Q. A . TREhlENTOZZI Monsanlo Chemical Company, Dayton, Ohio
Receiued December 16, I949 INTRODUCTIOK
Since the pioneering work of Debye (3) established the light-scattering method as a means of determining molecular weights of polymers, there has been a great deal of published work (2, 7, 8, 12) on determining the usefulness and limitations of the method. Nearly all of this research has been concentrated on polymer fractions below 400,000 in molecular weight. In general, the agreement of the light-scattering method with the other direct methods has been satisfactory for molecular weights of 400,000 or less. For polystyrene fractions of higher molecu-
