LAWRENCE M. KUSHNERAND WILLARDD. HUBBARD
898
adsorption of silver ion alone than any of the values give above; substitution in the relation between capacity, charge and potential 20
x
96500x 10-6 = 0.1
gives x = 0.2 X 10-loequiv./cm.2 per 0.1 volt. Consequently adsorption is specific, involving attachment of both anions and cations to the surface, and not merely a process of charging the double layer. We may assume that silver ions are adsorbed first, but become a part of the metal lattice; the positive charge carried does not remain localized. To obtain the equivalent of monolayer adsorption, it would not be necessary that silver ions deposit over the whole surface but, rather, that anions become intimately attached in a monolayer. If the anions should be bound approximately as rigidly as in the salt lattice, further sorption might reasonably be expected to occur. The process would not continue indefinitely, because the effect of extra charge on the metal and the lattice forces would be balanced by thermal agitation and dissolution forces acting on the outer layer.
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One can understand why low temperatures might favor such multilayer sorption, but we have no explanation for the effect of nitric acid too dilute to etch the surface, or possibly the alcohol vapor treatment of Rudberg and v. Euler. Exchange experiments show that the surface atoms of a metal, to a depth of several atomic layers at least, have a great deal of mobility. The experiments of Table I11 show that 60 to 7701, of the silver ions originally in solution become part of the metal in some 20 hours, while an equal amount of the original metal dissolved. Exchange in depth may be aided by local cell action, but there is probably the equivalent of an abnormally large self diffusion coefficient in the surface layers, as postulated by Gerischer and Vielstich.la The mobility should be an aid in establishing adsorption equilibrium and especially in the displacement of less firmly adsorbed substances. The tremendously rapid exchange must serve to repair minor irregularities of the surface when the metal is immersed in a solution of its own ions. The true area would tend to become equal to the measured area unless the metal is actually porous or has major faults such as abrasion marks, corroded areas, etc.
ON THE DETERMINATION OF CRITICAL MICELLE CONCEKTRATIONS BY A BUBBLE PRESSURE METHOD BY LAWRENCE M. KUSHNER AND WILLARD D. HUBBARD Surface Chemistry Section, Division of Chemistry, National Bureau of Standards, Washington 26, D. C. Received March IS, 1966
Bubble pressure measurements have been made on aqueous solutions of pure and commercial grade sodium dodecyl sulfate. The curves obtained with the commercial material have considerable structure depending on the bubbling rate. The curves for the pure samples are what one might expect on the basis of the surface tension of the solutions. The interpretation of bubble pressure and conductance data for solutions of impure surface active materials is discussed.
Introduction In a recent publication,' Brown, et al., have described a bubble pressure method for the determination of the critical micelle concentration of surface active agents in aqueous solution. The method involves measurement of the air. pressure necessary to maintain a stream of bubbles from a small diameter tube immersed in the detergent solution. Plots of bubble pressure versus concentration of detergent, at different rates of bubbling, show distinct and reproducible irregularities at particular concentrations. The electrical conductances of the same solutions show sudden, small changes at some of these concentrations. Brown, et al., interpret these effects as indicating more than one critical micelle concentration and also feel that the data present a strong argument for the existence of more than one micellar species in these dilute solutions. They do however point out that all of the surface active compounds used were commercial grade, and suggest that some of the effects noted may be due to the presence of impurities. Since their apparatus is easily reproduced, it was decided to make bubble pressure (1) A. 8. Brown, R. U. Robinson, E. H. Sirois, H. G. Thibault, W. MoNeill and A. Toflas, TEXIS JOURNAL, 66, 701 (1962).
measurements of this type on a sample of commercial sodium dodecyl sulfate and a sample of pure sodium dodecyl sulfate which had been synthesized in this Laboratory for an earlier researchS2
Experimental Materials.-The commercial materials was used as received. The ure sodium dodecyl sulfate was synthesized, as described by j h e d l o ~ s k yfrom , ~ a vacuum-distilled sample of n-dodecyl alcohol. The chlorosulfonic acid for the synthesis was distilled immediately before use. All other reagents and subsequent purification conformed with American Chemical Society specifications. The final step in the purification procedure consisted of extracting the detergent crystals with diethyl ether for about 8 hours in a Soxhlet extractor. Apparatus and Procedure.-The apparatus design and procedure were the same as described by Brown, et al.1 A 20-gage hypodermic needle was used for all the measurements reported here.
Results and Discussion Typical plots of bubble pressure versus concentration of detergent for both pure and commercial grade sodium dodecyl sulfate are shown in Fig. 1 . (2) L. M. Kushner, B. C. Duncan and J. I. Hoffman, J . Research Natl. Bur. Standards. 49,No. 2, 85 (1952),RP 2346. (3) Obtained from the Fisher Scientifio Co., Silver Spring, Md., under the label, Sodium Lauryl Sulfate, U.S.P. (4) L. Shedlovsky, Ann. N. Y. Acad. Sci., 46, 427 (1946).
t
Dec., 1953
DETERMINATION OF CRITICAL MICELLECONCENTRATION
899
For both samples the upper curve corresponds to a bubbling rate of about 10 bubbles in 4 seconds, the lower t o 10 bubbles in 10 seconds. It is t o be noted that whereas the commercial material shows two regions of peculiar behavior (the effects being larger as the bubbling rate is increased, as observed by Brown, et aZ.), the pure sample shows only a rather sudden leveling off of the curve in the region 0.008 t o 0.009 M . This, it is true, is quite close to the region of micelle formation for sodium dodecyl sulfate, but it is also the region in which the surface tension of these solutions becomes essentially ~ o n s t a n t . ~ Since, at very low bubbling rates, there is a direct proportionality between surface tension and the maximum bubble pressure during bubble formation and CO MME RClAL detachment, one would expect an interdependence between the two phenomena even a t higher rates of bubbling. Thus, the observed behavior with the pure sample is reasonable without considering the formation of micelles. Further, the behavior of the commercial material in the region around 0.008 M may also be associated with the surface tension rather than micellar effects. Shedlovsky4 has observed, in this region, a minimum in the surface tension versus concentraL 1 tion curve for solutions of sodium dodecyl sulfate which have been deliberately contaminated with -2 dodecanol, an expected impurity in the commercial 16' 10 16' product. MOLARITY. The break in the curve at 0.03 to 0.04 M for the Fig. 1.-Comparison of the concentration dependence of commercial sodium dodecyl sulfate must also, on bubble pressure for pure and commercial grade sodium the basis of the data presented here, be ascribed dodecyl sulfate. For each sample, the upper curve corresponds to a bubbling rate of 10 bubbles in 4 seconds, the to the presence of an impurity. The breaks in the molar conductance versus lower, 10 bubbles in 10 seconds. r\.M curves observed by Brown, et al., a t con- to the conductance of the solution, such h process centrations higher than that expected for micelle could explain the conductivity data. Additional work should clarify these points and formation, are probably due to the nature of the impurity. I n the case of lauryl pyridinium chlo- aid in the evaluation of a bubble pressure technique ride and Santomerse No. 3 (used by Brown, et al.), for the determination of critical micelle concenthe presence of a lower homolog of the major con- trations. Of particular interest would be bubble stituent would probably result in the appearance pressure measurements on a pure and impure of a second region of micelle formation a t a con- sample of a surface active material which does not centration somewhat higher than that of the major form micelles. It would appear however that the constituent. The effect a t the higher concentra- interpretation of a surface effect, as is bubble prestion, however, may be the result of the solubiliza- sure, is risky in the presence of even a small amount tion of an impurity by the micelles of the major of impurity. Further, one must be extremely cauconstituent. Such behavior has been observed tious in interpreting a surface effect in terms of any in this Laboratory with commercial grade sodium bulk property of a solution, such as micelle formadodecyl sulfate. If the impurity were to contribute tion.
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