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_eimpurity 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.
