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in a cut of wide boiling range, an increase in the charge size is desirable to sharpen the analytical tool. Samples of 300 to 2000 ml. have been used for this type of work, depending on the time interval available and the complexity of the separation. 3. Where it is desired t o separate two close-boiling groups and consistent results cannot be obtained using the standard charge, a larger charge may be required to decrease the effect of column hold-up, even though speed must be sacrificed.
Test Results Table I gives the results of analysis of several synthetic mixtures, using the still and analytical methods described herein. The degree of accuracy shown in this table has proved to be dependable, in the main, under routine control laboratory conditions during a period of more than a year. The accuracy of this method cannot be compared to that of
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inorganic quantitative analysis, but where approximations of the order shown are sufficient, it has proved of value in getting results in a quick and relatively simple way.
Acknowledgment The author wishes to acknowledge the helpful information and suggestions furnished by Edward H. Smoker, the work of Mark D. Snyder, Louis T. Lazzarini, and others of the Ugite Sales Corporation laboratory, and the cooperation of Floyd Hughes and others of Ace Glass, Inc., in fabrication of the apparatus. The photograph of the apparatus (Figure 3) by Theodore Shmanda, and the drawing of Figure 2, by Robert Kauffman, both of the Ugite Sales Corporation, are acknowledged with thanks.
The Film Balance As an Analytical Tool for Biological and Food Research GEORGE E. BOYD AND WILLI.i>I D. HARKINS University of Chicago and Universal Oil Products Company, Chicago, Ill. Surface films may be used to detect minute amounts of certain insoluble organic substances, and to identify unknown compounds when some knowledge of the homogeneity of the material is available. Data obtained from film pressurearea measurements are often of decisiFe value in the proof of the structure of complex organic molecules. Recent refinements in the technique appear to afford a method for determining the molecular weight of large molecules such as proteins, polymers, etc. The most widely used tool in the study of insoluble films is the combination of the surface trough and balance. The vertical and horizontal balance types of film balances are in use in this laboratory and a detailed description and theoretical discussion of the former are given, with a discussion of the technique of their use and the limitations and precautions involved. Accessory tools, such as the dark-field ultramicroscope with a cardiod condenser and apparatus for the determination of surface potential and surface viscosity, are described. Measurements of the permeability of films may he made. Temperature control is of great importance. To illustrate the nature of the data obtained, application of the method of surface films to a number of recent diverse scientific problems is described.
A
N OIL may be spread on water or mercury as a monomolecular or polymolecular film, but all films of the
latter type, commonly designated as duplex, are unstable and change spontaneously into a monolayer and a lens (6). Since films and membranes play a predominant role in biology, and are of fundamental importance in many lines of industry, it is essential to know not only what types of apparatus may be used to reveal their characteristics, but also what techniques are best suited. One of the most remarkable features of the film balance is that it is possible by its
use to determine the length, breadth, and approximate molecular weight of certain types of large molecules by only an hour’s work. The volume of any pure oil which covers 1 sq. cm. of the surface of water is given by the thickness of the film. If the thickest monolayer of a pure organic gubstance which can be obtained by compression is about 4.5 A,, the molecules lie flat on the surface. With thicker films the interpretation should be based upon various types of data. Often the position of a polar group in molecules of compounds such as vitamin D (calciferol), or in other sterols, can be shown by the film balance either to disagree or t o agree with that given by organic chemists who have studied their structure. In cases of disagreement it has been necessary to change the earlier structural formulas.
The Film Balance The most valuable of all the types of apparatus for the study of oil films is known as the film balance, or filmometer, and this is used to determine the film pressure, s,a t any given molecular area, u. The pressure of any film is, by definition, the difference between the surface tension of the clean surface of the subphase, yo, and that of the subphase when covered by the film, y,, or .rr = YO
- Y/
(1)
The justification for the designation of the difference between two tensions as a pressure lies in the fact that two-. dimensional T U and TUTdiagrams (Figure 1) resemble very closely three-dimensional P-V and P-V-T diagrams, respectively. Thus, in the two-dimensional gas law, T U = nkT, the value of k is that of the Boltzman constant of a threeerg deg.-’) which plays dimensional gas ( k = 1.371 X an important part in the theory of two-dimensional systems. Thus, from Equation 1, any apparatus, by means of which both of these surface tensions may be determined, could be. considered as a film balance. The first of these was used in 1891 by Pockels ( 1 2 ) . The apparatus consisted of a long rectangular trough, filled with water to the brim. The surface of the water was made clean by “sweeping” or “scraping” the surface by the use of a barriw which consisted of a
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determine the surface tension (y) itself, hut the principle of the vertical pull is used in the development of a balance which measures T directly. The vertical pull may he applied in two ways, one of which is described below, and is illustrated in Figure 2. A clean slide of glass, q u a ~ a or , platinum, or any other material which gives a aero contact angle, is suspended in the water in the trough from one arm of an ordinary balance as shown. The BUTface of the water is swept carefully both before and after the slide is dipped into the water. If the film is insoluble it is then spread on the surface, and the film pressure, r, is given directly hy the deflection of a beam of light as read on a scale at a distance of several meters. Area per Molecule C Sq.i.l PHASE DIAGRAM FOR MONOLAYERS ON :FIGURE 1. GENERAL
LIQUIDSUBPHASE
A
C. Gas A.
Transformation of liquid expanded (LI) phase hegins
LO. Two-phase region
The vertical 6lm balance exhibits the following advantages over the horizontal type com-mly used:
[t is much cheaper for any given ac!:uracy. [t is much simpler and may he in8tailed in a few minutes, since an ordinary balance and a micro'pe slide give results of a high pee of accurwy. It is specially useful in studies the effects of metal ions, since the slide, wuugn, ULIUuariieil~can all consist of quartz, 60 that the solution and its surface come in contact with quartz alone. In a simple form it will give the film pressure of a soluble film, while the use of the hcrizontal types involves the very elaborate movahle partition of the PLAWN trough of McBain.~
Transformation to LIoompieted BC or LI. Liquid expanded CD or I . Intermediate phase DE or LI. Liquid condensed phhase EF or S . Solid phase _.... oium ions, when the film consists of B long-chain parafin acid, sueh 8 8 stearic .acid. extends FE to very low pressure where the transition. So. IS between low-pressure vap or and solid B.
____ ._""
111
film.
transverse sheet of tension of the
1.5
wide. In the work the clean and the film-eoverd
water was obtained by the determination, by means Of an ordinary balance, of the force necessary to pull from the surface either a small disk (6 mm. in diameter) or a large ring (114 mm. in circumference). The measurements of film pressure may he made more exact if the difference between the two surface tensions is determined directly,'as in the film balance of Lord Rayleigh (fs),whichmakes use of the Wilhelmy slide. This apparatus gave excellent results, since with a monolayer of stearic acid the area at-which the 61m pressure begins to rise corresponded to 22.1 A.s at 15" C., while the most recent value, as determined in this laboratory, is about 21 A,%, when a minute amount of calcium ion is present in neutral water. This is presumably the kind of water used by Miss Pockels. '
Modern Film Balances A modern film balance is a device which measures directly the difference between two surface tensions, yl - yz, commonly to determine yo - y,, which (Equation 1) is defined as the film pressure, T . For such a balance the surface must be divided into at least two parts. The vessel which contains the liquid is usually designated as the "trough", though two separate vessels or troughs may be used if the film balance is of a tvue which allows such a seoaration. VERTICALFILMBALANCE.The simplest, the most generallv annlicahle. and Drohahlv the m i s t accurate film balance is thA'in which the'surface tension exerts a downward pull upon the vertical face of a sheet of glass or other suitable material, as in the surface tension method of Wilhelmy. On account of its inherent difficulties this surface tension method .~nu5 :~. 1 . ~.~I/ .1 r . I L . u^.,_:__ ^__1 h-_l^_^^I", 1s in ~ use,~ uu~i~b w w LVULW uy u a i u i m auu ~ U U C L W I I that these difficulties disappear if no attempt is made to "I
~~~1
4k FIGURE 2. ARRANGEMENT OF VER_TICAIITYPEOF SURFACE BALANCE FOR l
