Sept., 1955
WETTABILITY OF CARBON BLACKS
any investigation where the size and/or molecular weight of pepsin must be considered. Acknowledgment.-We gratefully acknowledge the generosity and help of Dr. M. Halwer and Dr. B. A. Brice of the Eastern Utilization Research Branch during various phases of this investigation, and the efforts of Mi-. R. Troisi and Mr. C. Wood in their construction of the light scattering photo-
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meter, differential refractometer and associated equipment. We also wish to thank Mr. H. Levin and Mr. R. Townend for carrying out the determinations of proteolytic activity and the extinction coefficient of bovine serum albumin. It is a pleasure to acknowledge the aid of the Research Corporation for a grant which partially supported this investigation.
THE INFLUENCE OF ULTIMATE COMPOSITION UPON THE WETTABILITY OF CARBON BLACKS’ BY MERTONL. STUDEBAKER AND CARLW. SNOW^ Phillips Chemical Company, Akron, Ohio Received March 84, 1966
Few quantitative data have been published upon the wettability of carbon black. This is due largely to the difficulty of obtaining quantitative measurements by methods which apply to other solids. A simple method has been developed for determining contact angles of carbon black by a modification of Wolkowa’s method.3 Data are listed for a large number of carbon black samples, and the values have been correlated with their ultimate analyses.
The wetting characteristics of a finely divided solid are of considerable practical and theoretical importance. However, most methods of determining wettability have been disappointing when they were applied t o carbon black. In the work which is reported here, the procedure used was quite similar to one developed by Wolk ~ w a . This ~ method is based on an equation derived by W a ~ h b u r n . ~It can be obtained from Poiseuille’s equation for viscous flow through a capillary tube and Young’s equation for the determination of surface tension in a capillary. The Washburn relationship is
where 1 is the length of flow in time t seconds, and the viscosity is q , the surface tension of the liquid is y , r is the radius of the capillary tube and 8 is the contact angle. A discussion of the meaning of this contact angle, e, is beyond the scope of this paper; it is sufficient to note that it is a measure of the wettability-the larger the contact angle the more difficult it is to wet a given solid with the particular liquid. For perfectly wetting liquids, 0 is Oo, and cos e equals one. The relative wettability of solids is frequently reported in terms of cos 0 rather than in terms of the contact angle itself. If a powder is packed into a glass tube, the resulting plug will act like a series of capillaries. The length of flow in the capillaries will be greater than the flow measured relative to the glass tube by a constant factor. If the packing is reproducible, values of r will be equal in different, identically prepared tubes. Values of I will be constant if the times of flow are measured a t equal distances in the plugs, say a t 0.50 cm. Now if a liquid can be (1) This work was carried out prior to the middle of 1942 in the laboratories of the General Atlas Carbon Co., presently owned b y Godfrey L. Cabot, Inc. Presented before the Division of Colloid Chemistry, American Chemical Society, September 16, 1952. (2) United Carbon Conlgany, Borger, Texas. (3) 2. E. Wolkowa. KoZloid-Z., 67, 280 (1934). (4) E. W. Washburn, Phys. Reu., 17, 273 (1921).
found which wets the solid with a contact angle of
Oo,equation 2 applies when 6 = 0” 12
=7’rt‘/2vf
(2)
For the case of a liquid with a finite contact angle, we have when e is finite. Dividing (2) by (3) and transposing, equation 4 is obtained COS e” = Kt’/tK (4) Hence, to determine the contact angle of any liquid against a powdered solid, we merely measure the times required to flow a given distance through the plugs by (1) a perfectly wetting liquid and (2) an imperfectly wetting liquid and substitute these values in the equation COS
e
= Ktf/tb
(5)
K is a constant which can be calculated from the surface tensions and viscosities of the two liquids using the formula In this paper, the following values mere used for I
