NOTES
3345
for a further alloy also with a composition (confirmed by chemical analysis) of 40% Ag-GO% Pd. Measurements of lattice parameter and of specific electrical ohm cm-1 a t 25") of this resistance (33.36 X newer alloy were in keeping with those reported for this composition by previous investigator^.^*^^^ Furthermore, the form of the R/Ro against hydrogen content relationship which may be derived from the E against time plot (assuming the absorption of hydrogen to be governed by transport of hydrogen molecules through solution*) is similar in form to relationships obtained earlier by Rosenhallg for a 40% Ag alloy. Finally, the form of the initial changes (for times up to -20 min) of the R/Ro against time curve for the "original" alloy, in Figure la, also seem in keeping with the surface being richer in silver than the interior, since here these changes reflect relationships between hydrogen content and R/Ro characteristic of homogeneous alloys containing - 5 0 4 5 % Ag.9,10 Nevertheless, however, despite the obvious differences in detailed results between the two specimens, the hydrogen content of the more recently prepared alloy [H/Me (atomic ratio of hydrogen to total number 0.241 in equilibrium a t 25" with hyof metal atoms) drogen a t 1 atm pressure ( E = 0) was the same, within experimental error, as the value quoted earlier1 for the inhomogeneous alloy. I n a recent account of a related experimental study, Makrides" has reported that he was unable to find evidence to suggest that it was possible for Pd and Ag alloys containing 240% Ag to form ,&phase hydrides a t 25". However, the plot of E against time for the homogeneous alloy in Figure l b illustrates a fairly distinct change of slope followed by a shoulder occurring 120 mv. Indeed, in a further experia t and after E ment at 0" the E against time curve exhibited a brief minimum, over potentials close to E 120, which could perhaps be associated" with "supersaturation" prior to nucleation of a second b-phase. Even if not indicative of the onset of a region of coexistence of aand b-phase hydrides, the "quasi-arrest" at 25" seems still related in general form to the more pronounced but still rather imperfectly invariant arrest citedl as a potential plateau for a 26% Ag alloy, results for which together with a complementary R/Ro against time measurements also are illustrated in Figure lb. Pressurecomposition isotherms a t 25" derived from E against time curves for these alloys have shapes suggestive of isotherms not too far removed in temperature from critical isotherms defined with respect to coexistence of a- and P-phase hydrides.
-
-
-
Acknowledgment. We are indebted to the Inter-
national Nickel Co. (Mond) Ltd. for the preparation of the alloys and to Johnson Matthey & Co. Ltd., for the award of a research grant to A. W. C. (7) F. Kruger and G. Gehm, A n n . Physik, [5]16, 174 (1933). (8) See, e.g., J. C. Barton, W. F. N. Leitch, and F. A. Lewis, Trans. Faraday Soc., 59, 1208 (1963). (9) G. Rosenhall, Ann. Physik, [5]24,4 (1935). (10) Results to be published. (11) A. C. Makrides, J . Phys. Chem. 68, 2160 (1964).
The Self-Diffusion Coefficient of Sodium Dodecyl Sulfate Micelles by J. Clifford and B. A. Pethica Unileuer Research Laboratory, Port Sunlight, Cheshire, England (Received M a y $2, 1966)
I n a recent paper1 Maeo derived an expression for the concentration dependence of the self-diffusion coefficient of micelles and compared it with experimental results obtained by Stigter, Williams, and Mysels2 for sodium dodecyl sulfate micelles. Agreement is good except for solutions containing only surfactant and water with no added salt. The measurements of Stigter, Williams, and Mysels have been criticized on the grounds that the extensive glass surface of their apparatus catalyzes the hydrolysis of the sulfate to the alcoh01.~ The presence of dodecyl alcohol affects both the critical micelle concentration (cmc) and the properties of the micelles. The over-all effect would be expected to be greatest at low concentrations of dodecyl sulfate and in the absence of added salt. As data on micellar self-diffusion were required for studies of counterion mobility in these systemsI4 we measured the self-diffusion of sodium dodecyl sulfate in aqueous solution without added salt by a technique not subject to the above objection. The Anderson and Saddingtons capillary method was employed. The apparatus and the materials used have been de~cribed.~Capillaries of 0.1-em bore and 1.5-cm length were used. Aeobenzene was used as a (1) R. M. Mazo, J . Chem. Phys., 43, 2873 (1965). (2) D.Stigter, R. J. Williams, and K. J. Mysels, J . Phys. Chem., 59, 330 (1955). (3) K.J. Mysels and L. H. Princen, ibid., 63, 1699 (1959). (4) J. Clifford and B. A. Pethica, Trans. Faraday Soc., 60, 216 (1964). (5) J. S. Anderson and K. Saddington, J . Chem. Soc., 5381 (1949).
Volume 70, Number 10 October 1966
NOTES
3346
the results of Stigter, Williams, and Mysels due to hydrolysis are only important at low dodecyl sulfate concentrations. Under these conditions our measurements of micellar diffusion coefficients agree more closely with the theoretical predictions of RIazo.
0
DY.05
A Correlative Treatment of the Heat of Adsorption with Coverage on the PO
Monolayer Sidela
by S. P. Mouliklb MOLAR
COWENTRATICU
Figure 1. T h e self-diffusion coefficients of sodium dodecyl sulfate micelles in water: 0, our results; 0 , t h e results of Stigter, Williams, and Mysels.
tracer and concentrations were determined by optical density measurements at 315 mp with a Cary spectrophotometer. For the more dilute solutions smallvolume optical cells were employed. The labeled solution was made up in the manner described by Stigter, Williams, and R;Iysels.2 To determine if the nature of the solubilized tracer material affected results, measurements were also made, at two sodium dodecyl sulfate concentrations, using 14C-labeled decane as a tracer. Concentrations were determined by liquid scintillation counting. The concentration of tracer material, both in the systems containing azobenzene and in those containing decane, was such that there were approximately 25 molecules of micellized dodecyl sulfate to each molecule of tracer. The results of the measurements using azobenzene are compared with the results of Stigter, Williams, and Mysels in Figure 1. Each of our results is the mean of 4-12 measurements. The precision of the measurement increases with increasing concentration and is determined largely by errors in determining azobenzene concentrations. The maximum observed deviation from the mean value decreases from 0.06 X cm2 sec-l at 0.017 AI to 0.02 X lod6cm2sec-I at, 0.510 M. Results obtained using l*C-labeled decane as a tracer a t dodecyl sulfate concentrations of 0.035 and 0.028 &I agreed with these results within the limit of error but were about 3y0lower. Figure 1 shows that our results agree with those of Stigter, Williams, and Mysels a t concentrations above 0.06 M . At lower concentration our results are consistently higher. Thus, it would appear that errors in The Journal
of
Physical Chemistry
Department of Chemistry, UnBersity of Arizona, Tucson, Arizona (Receined November 8, 1966)
The adsorption of gases on solid surfaces is governed by surface characteristics of the solid2-s as well as the interaction among the molecules of the adsorbate.+l0 Much work (qualitative and quantitative) on surface heterogeneity and lateral interact,ion has been done. Recently, Aston, et uZ.,ll Joyner and Emmett,12 Kington, et ul.,13 and Beebe, et ul.,14 have demonstrated and indicated the presence of lateral interaction on some surfaces. Aston, et al., have given an equation for lateral interaction energy. (1) (a) This work has been supported by the National Science Foundation in the laboratory of Dr. M. L. Corrin, University of Arizona. (b) Correspondence should be addressed to Research Service, Veterans Administration Hospital, Tucson, Ariz. (2) H. Freundlich, “Colloid and Capillary Chemistry,” Dutton, New York, N. Y., 1926. (3) G. Halsey and H. S. Taylor, J . Chem. Phys., 15, 624 (1947). (4) S. Brunauer, K. S. Love, and R. G. Keenan, J . Am. Chem. SOC., 64,751 (1942). (5) D. b l . Young and A. D. Crowell, “Physical Adsorption of Gases,” Butterworth and Co. Ltd., London, 1942, Chapter 7, p 247. (6) M. Volmer and G. Adhikari, 2. Physik. Chem., 119, 46 (1926). (7) J. W. Gibbs, “The Scientific Papers of Willard Gibbs,” Vol. I, Thermodynamics, Dover Publications, New York, N. Y., 1961. (8) hl. Volmer, 2.Physik. Chem., 115, 253 (1925). (9) J. H. de Boer, “The Dynamical Character of Adsorption,” Clarendon Press, Oxford, 1953. (10) S. Ross and J. P. Olivier, J . Phys. Chem., 65, 608 (1961). (11) (a) J. G. Aston, E. S. J. Tomzsko, and H. Chon, “Solid Surfaces and Gas-Solid Interface,” Advances in Chemistry Series, No. 33, American Chemical Society, Washington, D. C., 1961, p 325; (b) J. G. Aston and H. Chon, J . Phys. Chem., 65, 1015 (1961); (c) J. G. Aston, R. J. Tokadi, and W. A. Steele, ibid., 59, 1055 (1955). (12) L. G. Joyner and P. Emmett, J . Am. Chem. Soc., 70, 2353 (1948). (13) G. L. Kington, R. A. Beebe, M. H. Polley, and W. R. Smith, ibid., 72, 1755 (1950). (14) R. A. Beebe, J. Biscoe, W. R. Smith, and C . B. Wendell, ibid., 6 9 , 95 (1947).
