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(11) MILLIGAN, WEISER, AND SIMPSON: Results presented before the Division of Colloid Chemistry at the 105th Meeting of the American Chemical Society, which was held in Detroit, Michigan, April 12-16, 1943. (12) PATRICK: Colloid Symposium Monograph 7. 129 (1930). Can. J. Research 10, 713 (1934). (13) PIDQEON: (14) RAO:J. Phya. Chem. 46, 500 (1941). (15) TAYLOR: Ind. Eng. Chem. 37, 649 (1945). (16) TROMSON: Phil. Mag. [4]42, 448 (1871). J. Textile Ind. 20, 117 (1929). (17) CRQUHART: AND COPPOC: J . Phys. Chem. 43,1109 (1939). (18) WEISER,MILLIGAN, (19) WEISER,MILLIGAN, AND HOLMES: J . Phys. Chem. 46,586 (1942). (20) WEISER, MILLIQAN, AND SIMPBOX: J. Phys. Chem. 46, 1051 (1942).
COMMUNICATION TO THE EDITOR THE EQUILIBRIUM SPREADING COEFFICIENT OF AMPHIPATHIC ORGANIC LIQUIDS ON WATER' H. L. Cupples (.J, Phys. Chem. 60,283 (1946)) has again taken exception to our reasoning concerning the equilibrium spreading coefficient (.J. Phys. Chem. 49,239 (1945)). In his opinion we err in assuming that W ; is the work of adhesion between the organic bulk phase and the interfacial layer of the amphipathic compound. In an earlier communication Cupples (J.Phys. Chem. 48,75 (1944)) has agreed with us that
F: = y. - y o - Two= w: - w; 1
1
)
However, since WL = Zy:, it follows that
where 76,is the interfacial tension between the mutually saturated phases, 7: the surface tension of the ,bulk phase of the amphipathic organic compound saturated with water, and ywthe surface tension of the aqueous phase, saturated with, and having a surfaceplm of, the amphipathic compound with the hydrocarbon chains turned upward and a pressure corresponding t o the equilibrium surface pressure. W: is thus the change in free surface energy which occurs on bringing the two mutually saturated phases in contact, Le., on establishing contact between the organic bulk phase and the monolayer in the surface of the aqueous phase. For this reason the cross-section BB' in Fig. 1 of our paper is assumed to be above the oriented layer; (we have not shown in Fig. 1 the oriented layer which, in the equilibrium state, also extends over the water surface). H. L. Cupples has apparently overlooked in his criticism that, in the evaluation of W;, we are concerned with an aqueous phase which is fully saturated with the 'Rereived September 17,1946.
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amphipathic compound and therefore has a surface film of the latter a t a pressure corresponding to the equilibrium pressure. A similar reasoning is implied by W. D. Harkins, discussing the “final” spreading coefficient of heptyl alcohol on water in his article, “General Thermodynamic Theory of Spreading” (J. Chem. Phys. 9,559 (1941)),to which reference was made in our paper. It is clear from the discussion that Harkins envisages separation in a plane corresponding to that of cross-section BB’ in Fig. 1 of our paper. Finally, the choice of cross-section BB’ as the plane of separation above the oriented interfacial layer, and beyond the range of influence of the water molecules in the aqueous phase, is indicated by the actual behavior of the systems involved. When a drop of oleic acid is brought on a water surface, it will spread to a duplex film first, but soon contraction occurs to lenses whilst a monolayer is left in the water surface. Moreover, if the water surface is covered by a monolayer of oleic acid and a drop of oleic acid is placed on it, the drop will not spread but remain as a lens. This behavior is obviously due to the fact that the work of cohesion of the oleic acid bulk phase is greater than the work of adhesion between the bulk phase and the oleic acid monolayer in the water surface. Cupples has also objected to the derivation of our equation 4. NOWW: is the work involved in separating the organic bulk phase from the monolayer in the water surface which is oriented. This means that W: is equivalent to the work of cohesion
w’, = w: + w:,
(24
except that, on separation, it is necessary only to orient the layer of molecules at the surface of the bulk phase, since the layer on the water surface is already oriented. Therefore the work of orientation included in W: is about one-half of that included in Wk, though this is only approximate for reasons set out in our previous publication. Thus
wl,s w’,- W:, 2 and by equation 2a we obtain
w: E wc + W’ -$
(4)
The notation in this communication is the same as that used previously (J. Phys. Chem. 49,239 (1945)). E. HEYMANN. Chemistry Department, A. YOFFE. Melbourne University Melbourne, Australia and Trinity College, Cambridge, England
