Adsorption on Monolayers. VI. Adsorption of the Isomeric Hexanes on

Kenneth E. Hayes and Robert B. Dean. Vol. 57 ... It was found by Dean and McBain7 that many ... by Dean and Li8 who, using a simplified form of the...
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VOl. 57


The adsorption of the isomeric hexanes on condensed stearic acid monolayers (at 8.0 Gibbs) and on clean water surfaces has been investigated using the vertical type surface balance described in paper I11 of this series. On water the adsorption follows type I11 isotherms, the amount adsorbed a t any one partial pressure being greatest for n-hexane and decreasing in the order of the boiling points. On the condensed monolayers the adsorption follows type V isotherms. At low pressures the adsorption is independent of the nature of the hexane, while a t high pressures, where the isotherms level o f f ,the amount adsorbed appears to depend on the ability of the molecules to ack into a well ordered monolayer. The heats of adsorption on clean water are not significantly different from the heats ofvaporization of the respective hexanes. On the monolayers the heats of adsorption increase with increasing adsorption, approaching the heat of vaporization asymptotically. The results are found to be incompatible with the BET theory.

It was found by Dean and, McBain' that many organic vapors would cause an increase in the area of sodium stearate monolayers which were held a t constant surface pressure. Vapors of liquids, which have a spreading pressure greater than the surface pressure of the monolayer, would be expected to displace the monolayer and thus increase the total film area. It was observed however that vapors of some liquids having a lower spreading pressure would also cause expansion. This indicates that the vapors are adsorbed by, and form mixed films with, the stearic acid monolayer. A semi-quantitative investigation of this phenomenon was made by Dean and Lis who, using a simplified form of the Gibbs' adsorption equation, calculated the amount of certain selected organic vapors on stearic acid monolayers, a t various concentrations of the stearic acid. More,recently, Dean and ha ye^^-^ have investigated the system stearic acid-n-hexane a t 20 and 30' using more accurate experimental techniques,* and have calculated the adsorption isotherms for n-hexane, using the complete form of the Gibbs equation. In this way it may be shown5that the amount of vapor adsorbed, rl,is given by

rl =

[(g)- r@)]

where the subscript 1 refers to hexane and 2 to stearic acid. T is the surface pressure and P the relative partial pressure of the hexane. Koenig9 (I) Presented before the twenty-sixth National Colloid Symposium which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society in Los Angeles. California, June 1618. 1952.

(2) This work was supported in part by a grant from the Frederick Cardner Cottrell Fund of the Research Corporation, and in part by a grant from the U. 9. Institute of Public Health. This work, together with the contents of papers 111, IV and Vd-6 in this series is reported more extensively in the Ph.D. thesis of K. E. Hayes, Oregon, June, 1952. Now Department of Chemistry, University of Princeton, New Jersey. (3) Chemical Division, The Borden Co., Bainbridge, N. Y. (4) R. B. Dean and K. E. Hayea, J . A m . Chem. SOC.,73, 5583 (1951). ( 5 ) R. B. Dean and K . E. Hayes, ibid., 7 5 , 5584 (1951). (6) R. B. Dean and K. E. Hayes, ibid.. 74, 5982 (1952).

(7) R . B. Dean and J. W. McBain, J . Colloid Sei., 1, 383 (1947). (8) R. B. Dean and Fa-Si Li, J . A m . Chsm. Soc., 79, 3979 (1950). (9) F. 0. Koenig, "Computation of Surface Concentrations froin

Surface Tension Data," Academic Preas, Inc.. New York, N. Y., in print.

has shown how to compute dp2/dP from spreading pressures. From the data at 20 and 30' it was possible to compute the isosterii: heats of adsorption of nhexane on stearic acid monolayers a t 25' and t o assign probable structures to the mixed films. At low stearic acid concentrations it appears that hexane dissolves in the hydrocarbon chains of the monolayer to form "Duplex" films. However, a t 8.0 Gibbs, 8 X 10-lO moles/cm.2,10where the stearic acid molecules in the monolayer are close packed, there is very little room for hexane to penetrate between the chains. n-Hexane is adsorbed on the top of the stearic acid monolayer, as a second monolayer with its molecules oriented almost vertically. This investigation of the adsorption of the isomeric hexanes on closed packed stearic acid monolayers and on clean water surfaces, was undertaken with a view to substantiating the postulated structures ascribed to the mixed monolayers. Experimental Materials.-Benzene was Baker and Adanison "B and A" quality free from thiophene and was twice distilled, the fraction boiling between 79.5 and 80" being collected. The hexanes were all Phillips Petroleum Companjr 99% grade. They were all twice distilled just before use, that fraction boiling within one half of one degree of the boiling point as given in National Bureau of Standard tables being collected. The hexanes and benzene were tested and found to be free from surface active impurities. Stearic acid was from Armour and Company and was a sample from the same lot used by Ralston" in his investigations, it was recrystallized from ethanol, m.p. 89.4". Benzene solutions of stearic acid were prepared by weighing the stearic acid and dissolving in the redistilled benzene in a volumetric flask. The concentrations used were 0.0004 and 0.00072 111. Phosphoric acid solutions (0.01 N ) were prepared froin Baker and Adamson Reagent Grade phosphoric acid and this was used as the liquid phase throughout the entire investigation. Apparatus and Methods.-The apparatus and technique are described in detail elsewhere.'

Results Figure 1 shows the results of partial pressuresurface pressure data for n-hexane on stearic acid monolayers a t r2 = 8.00 and I'3 = 0.00 Gibbs, that is, on close packed monolayers and on clean (10) R. B. Dean, THIEJOURNAL, 66, 611 (1951). (11) A. W. Ralston, J . O w . Chsm., 7 , 546 (1942).



Jan., 1953

water. These curves are typical of the behavior of all of the hexanes, the data for the other hydrocarbons being omitted for the sake of clarity.

r 22





in the presence of the other hexanes. It is probable that this assumption is very near the truth. 2,2Dimethylbutane does not appear, a t first sight, to belong to the same series as the other hexanes (Fig. 3), it was thought that the anomalous behavior of this hydrocarbon might be ascribed to a different standard state.6 Experiments were made however, which show that the spreading pressure of stearic acid under 2,2-dimethylbutane vapors at 30' follows the same pattern as n-hexane.6 Typical adsorption isotherms for the isomeric hexanes on clean water surfaces are shown on Fig. 2 and on condensed monolayers in Fig. 3. The complete results are tabulated in Tables I and 11.



..... 20%












( m m H g 1.

Fig. 1.-Surface pressure-relative pressure curves for nhexane on clean water surfaces and on,condensed stearic acid monolayers a t 20 and 30".









i m m . HI.).

Fig. 2.-Typical adsorption isotherms for the isomeric hexanes on clean water surfaces a t 20 and 30'. (The curves shown are for: 1, n-hexane; 2, 2,2-dimethylbutane. The other hexanes fit between these two extremes in the same order as their respective boiling points.)

From these curves values of the function ba/dP were obtained, which mere then inserted into a simplified form of equation 1, namely

r l = - P(



RT ~ I P and values of rl calculated. The second term in equation 1, rz(bp2/bP)is obviously equal to zero when I'z = 0.00 Gibbs, i.e., - 30-C for the adsorption on clean water, and equation 2 2O.C is then the same as that used by Micheli12 and Cassel and Formstecher'a in their investigations of the adsorption of insoluble vapors on water surfaces. I t is interesting to note that the a-P data for the MTIAL PRESSURE ( m m HI). adsorption of the isomeric hexanes on water, may Fig. 3.-Adsorption isotherms for the isomeric he$anes on be fitted to a degenerate cubic equation condensed stearic acid monolayers a t 20 and 30 : 1, nhexane; 2, 3-methylpentane; 3, Zmethylpentane; 4, 2,3P = aP bPa dimethylbutane; 5, 2,2-dimethylbutane. from which values of rl may be obtained analytiThe surface pressure data are believed t o be accally. Values of rlobtained in this way are in good agreement with the values obtained by graphical curate to better than h0.05 dyne per cm. and that relative partial pressure measurements to about differentiation of the r-P curves. It was shown in paper IV of this series5for n- 10.005. By assuming maximum errors in the T-P hexane on stearic acid, that bp2/bP is zero or neg- curves it was possible to ascribe confidence limits ligible for monolayers of stearic acid a t 8.0 Gibbs, to the adsorption isotherms. We are confident and we have tacitly assumed that dpi/bP is also that the reported values of are accurate to better negligible for stearic acid monolayers at 8.0 Gibbs than 5y0. Heats of adsorption for the systems studied were (12) L.I. A. hlicheli, Phil. Mag., 3, 895 (1927). calculated using the Clausius-Clapeyron equation. (13) H. M. Cassel and M. Formstecher, Kolloid-Z., 61, 18 (1932).




VOl. 57


TABLE I layer is diffuse to a depth of 2-3 A hexane ADSORPTION OF THE ISOMERIC HEXANESON CLEANWATER molecule, on being adsorbed a t a pure water sur-

face, becomes imbedded in a soft matrix of water molecules which surround i t on all sides except the top. The attractive forces between a water mole+ cule and a hexane molecule would be expected to be of the same order of magnitude as those between 200 30 two hexane molecules. The heat of adsorption of a b 4 b hexane on water should therefore not differ greatly n-Hexane 0.42 8 . 3 8 0.63 7.15 from the heat of vaporization of liquid hexane. 3-Methylpentane .53 12.64 .29 13.46 Further evidence that the hexane molecules are 2-Methylpentane .;