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J. Phys. Chem. 1981, 85, 3545-3546
anisms of some heterogeneously catalyzed reactions.
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Editor’s Note: Fmfesw M o w has examined this comment and consklers lt to be an interesting interpretation of his work.
(14) Bianchi, D.; Gardes, G. E. E.; Pajonk, G. M.; Teichner, S. J. J. Catal. 1974,33, 145. (15) Hoang-Van, C.; Mazabrard, A. R.; Michel, C.; Pajonk, G. M.; Teichner, S. J. C.R. Acad. Sci., Ser. C 1975,281, 24. (16) Repellin, M.; Perrier, R.; Lamartine, R.; Bertholon, G.; Pajonk, G. M., C. R. Acad. Sci., Ser. C 1977, 285, 335. (17) Lacroix, M., Ph.D. Thesis, University Claude Bernard, Lyon, France, 1980.
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M. J. D. Low
Department of Chemistry New York University New York, New York 10003
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Received: June 1, 1981; In Final Form: June 18, 1981
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Effect of Impuritles on Partial Molal Volume and Critlcal Micelle Concentration of Sodium Dodecyl Sulfate. Correction of Micelle Aggregation Numbert
Sir: In a recent paper published in this journal, micelle aggregation numbers, N , were reported for the surfactant sodium dodecyl sulfate (SDDS) in various NaCl(aq) background solutions.’ These values were obtained by using equilibium ultracentrifugation in conjunction with isopiestic distillation experiments used to determine the prefential interactions which occur in multicomponent electrolyte systems. A commercial SDDS was used in the experiments, and, on the basis of good agreement between the measured values of the critical micelle concentrations (crnc) and partial molal volumes with those in the literature, this lot of SDDS was considered to be acceptably pure. The values obtained for the aggregation numbers using this SDDS have been shown to be in excellent agreement with the best available values.2 Current investigations of SDDS in higher NaCl(aq) concentrations, using a second sample of SDDS from the same supplier, indicated the molal volume values were anomalously high for this SDDS sample. A direct comparison of the two samples, by density measurements in HzO, showed that the second sample had higher values of the partial molal volume than the original sample. Because the purity of either sample of SDS was now in doubt, an investigation of the effects of possible impurities in SDDS on the molal volume and cmc properties was begun. If the original SDDS sample were found to be impure, the reported values for the aggregation numbers could possibly be corrected for the presence of the impurity. Four varieties of SDDS were available to be investigated: a specially prepared SDDS of high purity (RLB);3Lot C6A from Eastman Organic Chemicals (the same SDDS used in the previus investigation); Lot A7C from Eastman Organic Chemicals; and Lot 6550452 from BDH Biochemicals. In addition, two “impure SDDS” samples were prepared by using Lot A7C SDDS to which was added, respectively, 1.908% n-dodecyl alcohol (DDA) by weight (A7C*) or 1.930% NaCl by weight (A7C**). Densities of the aqueous solutions of these six SDDS samples were measured at 25.00 “C with a Picker densimetera4 Surface tension measurements were made at 25 “C (1) D. A. Doughty, J. Phys. Chem., 83,2621 (1979).
(2) J. P. Kratohvil, J. Colloid Interface Sci., 75, 271 (1980). (3) R. L. Berg, “Thermodynamicsof Aqueous Sodium Dodecylsdfate”,
Energy Research and Development Administration, BETC/TPR-77/3, 1Q77 --. ..
(4) P. Picker, E. Tremblay, and C. Jolicoeur, J.Solution Chem., 3,377 (1974). 0022-3654/81/2085-3545$01.25/0
Flgure 1. Variation of vfor SDDS as a function of square root of molality: (A)A7C‘ *; (0)RLB; (0)C6A; (A)A7C; (B) BDH; (0) A7C’.
on three of the samples, RLB, A7C, and BDH, with a du Nouy tensiometer. The results of the density measurements for the six SDDS samples are summarized in Figure 1in which the partial molal volumes are plotted as a function of square root of the molality. The cmc’s were obtained by extrapolating the trends in density vs. molality plots from above and from below to the point of intersection. Five of the samples (excepting A7C*) had essentially identical cmc’s, averaging (8.28 f 0.17) X 10” m. Sample A7C* had a cmc of (7.45 f 0.43) X m. AT, the change in partial molal volume upon micellization, was obtained by extrapolating the trends in V from above and from below to the cmc and taking the difference a t the cmc. The six samples had essentially the same Avaveraging 11.51 f 0.11 cm3mol-l (sample A7C had a lower value, 11.03 f 0.78 cm3 mol-’, but the large standard deviation negates the significance of the difference). The results of the surface tension measurements were as follows: RLB, no detectable minimum; A7C, substantial minimum of 9 mN m-*, concentration at minimum 6.2 X m; BDH, shallow minimum of 1 mN m-l, concentration at minimum 5 X m. The most striking feature of Figure 1is the parallelism of the curves, including the curves of the two “impure SDDS” samples. The presence of impurities obviously affects the absolute value of the partial molal volumes but in a very regular manner. The invariance of A B indicates that AV values reported in the literature can apparently be accepted without much concern about the absolute purity of the surfactant used. Another important feature is the essentially insignificant effect the impurities have had on the cmc. This differs from the results of others who have reported very substantial effects caused by impurities on the cmc of SDDS as determined by conductometric or surface tension method^.^^^ My surface tension data on two of the commercial SDDS samples confirms this marked effect. The apparent discrepancy can be resolved by considering that density measurements, reported as partial molal volumes, represent a bulk property of the solution, so that the effects of the modest amounts of impurities are averaged over the entire solution volume. In contrast, if the impurity, such as DDA, is itself surface-active, it can have a substantial effect on those types of measurements that “act” where the impurity is concentrated, e.g., a t the surface of the solution or on the surface charge of the micelles. (5) B. R. Vijayendran, J. Colloid Interface Sci., 60, 418 (1977). (6) M. J. Rosen, J. Colloid Interface Sci., 79, 587 (1981).
0 1981 American Chemical Society
3546
Additions and Corrections
The Journal of Physical Chemistry, Vol. 85, No. 23, 1981
If the RLB sample of SDDS can be considered the most nearly pure variety, a designation supported by the lack of a minima in the surface tension data,5 then all the commercial SDDS samples behave as if th_e impurity were DDA. Using the known displacement in V of A7C caused by the addition of 1.908% DDA, the amount of DDA equivalent impurity can be calculated for each of the commercial SDDS samples. The following results were obtained: C6A, 1.18 f 0.38% DDA equivalent; A7C, 1.90 f 0.45%; BDH, 3.84 f 0.68%. This calculation was made assuming that the displacement in F' is proportional to the amount of impurity present. Investigations of the partial molal volumes of primary alkanols in surfactant solut i o n ~ , or ~ ~of* the molal volume of NaCl(aq)gand the effect of NaCl on the cmc of SDDS,' could be used to correct the value of the measured density increment, Ad, for the presence of a known impurity. This corrected Ad would then be used to calculate the apparent molal volume from which F' is derived. This correction process was applied to the density data for the A7C* and A7C** samples. V and AP values which were essentially identical with the values for A7C SDDS were obtained. Two assumptions describing the possible behavior of the impurity in solution with the surfactant will be considered in calculating the effect of the impurity on the aggregation numbers of SDDS in NaCl(aq): (1)The impurity behaves as a typical inorganic material remaining in the bulk solution together with the NaCl(aq) and is not incorporated into the micelle. Therefore the micelle is totally SDDS and the only effect of the impurity is to increase the apparent concentration of SDDS in solution. In correcting the value of N , the effect of this concentration increment on each parameter used in calculating N must be considered. For example, the partial molal volumes of SDDS in NaCl(aq) reported previously are assumed to decrease by the same increment as was found for the C6A SDDS in H20. The preferential interaction parameter, (dg!3/dg2)0,3; and S = (d In m2*)/d?, will be increased by the correction. (7)M.Manabe, K.Shirahama, and M. Koda, Bull. Chen. Soc. Jpn., 49,2904 (1976). (8) E. Vikingstad, J . Colloid Interface Sci., 72,75 (1979). (9)F.Vaslow, J . Phys. Chem., 70,2287 (1966).
(2) The impurity behaves as a long-chain alcohol and is totally incorporated into the micelle.' An assumption is made that N , representing the number of surfactant monomers in the micelle, is not changed by the incorporation of a small amount of this type of impurity. This assumption is supported by current models picturing micelles as loosely bound aggregates of surfactant monomers having appreciable room for the incorporation of other hydrophobic molecules such as DDA.lOJ1 The micelle can then be considered as being composed of monomers having an effective molecular weight determined by the relative mole fractions of SDDS and impurity (assumed to be DDA). The partial molal volume will then be decreased but in proportion to the effective molecular weight so the partial specific volume will remain the same. There will be no change in (dg3/dg2)O,+and no change in S. These two assumptions were used to calculate corrected values for N based on the calculated amount of impurity in the C6A SDDS sample. Both assumptions led to the same corrected values, at each NaCl(aq) concentration, and the corrected values were lower by essentially the percentage amount of impurity calculated to be present in the C6A SDDS sample. Compared to the uncertainty in the original values of N , this correction could be considered insignificant. However, the effect is probably real and should be taken into account in evaluating the previously reported values of N . Assumption (2) probably represents the actual behavior of the impurity based on the results of this investigation.
Acknowledgment. I thank T. E. Burchfield and L. A. No11 of this Center for supplying the RLB and BDH SDDS samples for my investigations. (10) F. M. Menger, Acc. Chem. Res., 12, 111 (1979). (11) H.Wennerstrom and B. Lindman, Phys. Rep., 52, 1 (1979). Contribution No. 253 from the thermodynamic laboratory at the Bartlesvllle Energy Technology Center.
Department of Energy Bartlesville Energy Technology Center Barflesville, Oklahoma 74003
Daryl A. Doughty
Recelved: July 20, 1981; In Flnal Form: September 23, 1981
ADDITIONS AND CORRECTIONS 1981, Volume 85 Anthony K.Rappi5, Terry A. Smedley, and William A. Goddard, III*: The Shape and Hamiltonian Consistent (SHC) Effective Potentials. Four digits in Table I and one digit in Table I11 are incorrect. The correct values are as follows: Page 1664 (Table I). Under aifor Si (p) 0.3550 should be changed to 0.3350. Under ci for Si (p) -0.052555 should be changed to -0.0525555. Under aifor P (s) 1.3539857 should be changed to 1.3534857. Under ci for P (s) 2.3081419 should be changed to 1.3081419. Page 1665 (Table 11). Under aifor S (p-d) 1.28475 should be changed to 1.23475.
