from the heated glass produced changes not only in the weight but also in the appearance of various relatively unstable samples. The above factors make necessary the use of B protective device that will completely block do\m\vard radiation and will lie inert, containing no reacting or interfering components. Copper discs sealed within a glass capsule form a satisfactory type of guaid. Gin. Pyres test tube lvhose outside diameter is of such calibre aH t,o fit smoot,hly inside the 22-mm. Pyres tube (A) is cleaned and filled t,o about 5 half inch in depth with copper discs (cut from copper foil with :in old cork-borer). The test tube is “necked down,” evacuated to the limits of a IIyvac pump’s capacity, and the neck sealed off to yield such a capsulr as illustrated. small glaes hook was added to the top of the capsule t,o facilitate its insertion and removal from the balance. The capsule is then inserted within the unsealed balance unit to rest, on the nirhrome wire spring-loop (F);t.he loop is made with protruding ends to insure its stability wit,hin the tube (A) REFERENCES (1) HIYTLER, S. S., FISCHER, E. A,, AND FREEMAX, I. R . : J. Am. Chem. Soc. 66.1009 (1943). (2) MCBAIX,J. W . , AND BAKR,A. M.: J. Am. Chem. SOC.48,690 (1926). ( 2 ) MCBAIN,J. W., AND LEE, W. W.: Oil & Soap 20, 17 (1943); Ind. Eng. Chem. 36, 784
(1943). (4) SHREVE, G.
W . : Ph.D. thesis, Stanford University, 1946.
ALUMINUM DILAURATE AS ASSOCIATION COLLOID I K BEXZENE’ JAMES W . McBAIS
AND
EARL B. WORKING
Ijeparlmenl of Chemistry, Stanford University, California Receiced March IS, 1947
Ordinary soaps in \vat,er are the best known esamples of association colloids. Jn them ions and molecules spontaneously associate to form colloidal particles or micelles, and these micelles are in true reversible equilibrium with the ions and molecules from which they form. It is characteristic of such colloids that the particle weight or apparent molecular weight is a function of concentration and temperature. The present investigation brings evidence for the existence of an associatioil colloid in a non-aqueous solvent, (1) : namely, aluminum dilaurate, ill(OH)L?, in benzene. This is sho\vn by the change in osmotic pressure with concentration, which contrasts strongly with the behavior of a polymeric colloid, and is confirmed by viscosity measurements. 1 Study conducted under Contract OEMsr-1057 between Stanford University and the Office of Emergency Management. recommended by Division 11.3 of the National Defense Research Committee and supervised by Professor J. W. McBain.
975
ALUMINUM DILAURATE AS ASSOCIATION COLLOID MATERIALS
The aluminum dilaurate was prepared by precipitation of aluminum trichloride by the addition of 1 mole of potassium laurate (made from Eastman Kodak Company lauric acid) to each mole of aluminum chloride in water at the boiling point, followed by extracting the dried precipitate with acetone dried over Drierite (calcium sulfate). The benzene was Kahlbaum's thiophene free. THE OSMOTIC PRESSURE O F ALUMIWUM DILAURATE IK BENZEKE
Direct evidence has been obtained by measuring the osmotic pressure of bolutions of aluminum dilaurate in various concentrations in benzene a t 20°C. and a t 50°C. The glass osmometers mere suppliea by the Scientific ilpparatus Co., Bloomfield, N . J., as described by R. H. Wagner (4). The ceIlophane membranes employed for most of the measurements were prepared by soaking cellophane in 62 per cent zinc chloride solution for from 15 to 30 min., washing thoroughly first in dilute hydrochloric acid and then in water, and then transferring through T4BLE 1 Loss of s o a j from anside to outsade of osmometer I
I
RESIDUE APTER EVAPORATION MEMBRANE
Inside
Collodion. . . . . . . . . . . . . . . . . . . . . . Collodion. . . . . . . . . . . . . . . . . . Cellophane, . . . . . . . . . . . . . . . . . . . . Cellophane . . . . . . . . . . . . . . . . . . . . . . . . . Cellophane . . . . . . . . . . . . . . . . . . . . . . . . .
Outside
weeks
per
