Dec., 1963
IDEAL
SURFACE BEHAVIOR
hydrophobic ion with the water-water interactions, is greater, the larger the ion, for then the greater the interference. The oither reason is due primarily to an ion-solvent interaction ; the electrostatic free energy opposing the transfer of an ion from a high dieledric constant medium such as water into a low dielectric constant solvmt such as benzene will be less the larger the ion. T h k can be seen most simply from the Born expressionlgfor the electrostatic free energy of transfer of ions of cliarge Z+ and 2- and size r+ and r- from water into an organic solvent
The larger the cationic radius, r+, the smaller is the positive value of AFeleot. Although this positive free energy is parbially compensated for by ion-pairing of the cation and anion in low dielectric constant media, the compensation is only partial, and the effect remains. The magnitude of the increase in extraction into benzene with the addition of four -CH2 groups in going from TPAH to THAH found in this work is about 100, in agreement with the factor 3 per -CH2 group found by Collander for the extraction of a wide variety of organic species from water into diethyl ether.20 On the other hand, the present results for the extraction into nitrobenzene media show a smaller effect per (19) RI. Born, Z. Physik, 11, 45 (1920). (20) R. Collandor, Acta Chem. Scand., 8 , 717 (1949).
2793
O F hIIXED >\IONOLAYEKS
-CHz group. The difference between TPrAH and TBAH is about a, factor of 13, or a factor of 2 per -CHz group. However, this smaller effect is to be expected from eq. 5 , as the much larger dielectric constant of nitrobenzene, E 3,5, makes the magnitude of AJjlelecr and the effect of a chaiigc in ion size about 30 times smaller than for benzene, E 2.3. The great reduction in the electrostatic free energy of transfer of ions from water into nitrobenzene rather than into benzene also is the reason for the much better extraction of the bases into nitrobenzene than into benzene solution. In fact, the bases of the larger cations, THAH and TPAH, extract well enough into pure nitrobenzene, without amy alcohol present, to make difficult the study of their extraction by the method used in this report; Ihe necessary blank correction for their extraction into the pure solvent becomes too large. But as has been pointed out before,8such high dielectric constant media make very good extraction solvents for large ion systems. In summary it can be said that the present work has established the tricoordinate nature of the hydroxide ion, similar to that of the hydronium ion. It remains for further studies to determine whether a trihydrated species can actually be isolated in the extraction. Acknowledgments.-B. R. A. wishes t o thank the United States National Academy of Sciences for his appointment under the Tisiting Research Scientists Program which enabled him t o conduct this research.
THE IDEAL STJRFACE BEHAVIOR OF MIXED, MOSOIAYERS OF LONG-CHAIN n-PARAFFINIC ALCOHOLS BY VICTORE(.LA MER,L. A. G. AYLMORE, AND THOMAS ITT, HEALY Chemistry Department, Columbia University, New York, New York Received July 19, 1963 The rate of evaporation of water through mixed monolayers of high purity 1-alcohols of alkyl chain length from 16 to 20 carbons has been used to verify the ideal surface behavior of these mixed monolayers. The ideal behavior exists a t all surface pressures of the monolayers. It has been established that deviations from ideality observed a t low surface pressures by previous workers are due to impurities in the alcohols or in the water subphase.
Introduction Certain monomolecular films will markedly retard the evaporation of water through the water-air interface. The theoreticnl and practical aspects of this phenomenon have been discussed in detail recently in a monograph by La Mer1 and need not be repeated. To investigate quantitatively the rate of evaporation as a function of parameters of the monolayer such as chain length, surface compression, etc., the reciprocal of the rate of upi,ake of water by a desiccant suspended a few millimeteia above the water surface may be expressed as a specific evaporation resistance ( r ) by the equation
r
u(ww - wd)(t/mf - tlm,)
(1) where a is the area of water surface under the desiccant; ww and wa are the concentrations of water vapor in equilibrium with water and desiccant, respectively; tlm is the reciprocal evaporation rate with the sub(1) V. K. La Mor, Ed., ”Retardation of Evaporation by Rlonolayers,” -.
Academic Press, Inr-., New York, Tu’. Y., 1962.
scripts f and w referring to the surface with and without monolayer, respectively. The resistance, r, is a property of the monolayer alone, and is expressed in absolute units (sec.,/cm.). Previous work by Barnes and La Mer2showed that at high surface pressures ( > l 5 dynes crn.-l), mixtures of long-chain alcohols, Le., 1-hexadecanol and l-octadecanol, behaved as ideal surface layers, obeying a mixture lam that can be derived from the additivity of the free energies of activation for the kinetic process of transporting a molecule of water through the monolayer, Vi%
In
=
x1 In rl
+ x2 In r2
(2) Here rl and r2 are specific resistance values of pure components a t a given pressure, r12is the specific resistance of a binary mixture of the pure component at the same pressure, and XI and x2 are the mole fractions of the pure components in the mixture. In the low pressure region (
