NOTES
4331
A comparison among the solvation numbers of alkali metal ions in this and other solvents13 shows that the smallest values for all of the cations are found in sulfolane and in nitrobenzene, solvents which have the greatest molecular volume.
Solubilization Behavior of a Polyoxyethylene Sulfate Type of Surfactant in Connection with the Micellar Charge
by Fumikatsu Tokiwa Table I11 : Solvation Numbers of Ions in Sulfolane at 30""
Li + Na +
K+ Rb + c s+ NHr
+
c1-
BrIclod-
4 . ;33 3.61 4.05 4.16 4.27 4.97 9.30 8.92 7.22 6.685
1.92 2.30 2.05 2.00 1.95 1.67 0.89 0.93 1.15 1.24
3.74 4.23 3.92 3.85 3.78 3.36 1.98 2.06 2.47 2.64
0.60 0.95 1.33 1.48 1.69 1.48 1.81 1.95 2.16 2.40
1.4 2.0 1.5 1.4 1.3 0.9
... ... ... . ..
r in A. The a XO in (international ohm)-' cm2 mol-'; crystallographic radii ra are taken from ref 10, p 461. The crystallographic radius of the c l o d - ion was reported by MadauleAubry.12
As far as the anions are concerned the extrapolation of the curve of Figure 2 for the calculation of the rc/rs factors seems more arbitrary than in the case of the cations, owing to the greater distance of the values of rs for anions from the tetraalkylammonium ions rS values. Nevertheless, it must be pointed out that the error due t o extrapolation probably has very little influence on the calculated value of the corrected radius which expresses interaction between the anions and the solvent. For C1-, Br-, I-, and Clod- ions this interaction seems to be very small and they may be assumed as practically bare, since the radii obtained by the Robinson and Stokes method are close to the crystallographic ones (Table 111). The absence of solvation of the anions in sulfolane agrees with the views of Parker14 concerning the solvation of anions in dipolar aprotic solvents. I n addition, the association constants K A for LiC1, =~ 13,860 M-l; LiBr, and LiI in sulfolane's ( K A , L I C K A , L i B r = 278 M-'; K A , L < ~ I10 M-l) agree with the view that the anions are unsolvated; in fact, the interaction between Li+ and C1- can be explained, assuming that the anions are bare, in terms of the charge density decrease from (21- to I-.14
(13) R. Gopal and M. M. Husain, J . I n d i a n Chem. Soc., 40, 981 (1963). (14) A. J. Parker, Quart. Rev. (London), 16, 163 (1962). (15) R. Fernandeli-Prini and J. E. Prue, Trans. Faraday Soc., 62, 1257 (1966).
Research Lahoralories, K a o S o a p Company, Wakayama-shi, J a p a n (Received J u n e PI, 1968)
Although the phenomenon of solubilization of surfactants has frequently been discussed in connection with the structure of micelles, it has not been considered from the point of view of their electrical nature. The electrical nature of a micelle is probably one of the important factors which governs solubilization behavior, because the structure of the micelle depends highly on this nature. I n the previous work' studying the solubilization behavior of a polyoxyethylene sulfate type of surfactants, it has been suggested that the potential or charge on the surface of the micelle has an important effect on the extension of the polyoxyethylene chains in the micelle and the solubilization capacity of the micelle. I n the present note, the solubilization behavior of sodium dodecylpolyoxyethylene sulfate^^^^ (SDPS), C12H2j0(CH2CH20),SOs?;a, with different numbers of oxyethylene units ( p ) has been discussed in connection with the [ potential and the charge of the micelle in order to understand the solubilization mechanism of this type of surfactants. Solubilization results of SDPS with p from 0 to 10 in water and solutions of NaCl and CaClz are shown in Figure 1, in which the solubilizing power toward Yellow OB is plotted against the number of oxyethylene unit^.'^^^^ It has been shown from the spectral study of the solubilization of Yellow OB that, in the case of surfactants having a polyoxyethylene chain, solubilization occurs in both the hydrocarbon core of the micelle and the outer shell of the polyoxyethylene chains.' This accounts for the increase in the solubilizing power of SDPS with increasing oxyethylene content (shown in Figure 1). However, the effectiveness of the polyoxyethylene part for solubilization, i.e., the degree of solubilization per oxyethylene unit, decreases with increasing oxyethylene content, as shown in Figure 2 . (The degree of solubilization per oxyethylene unit was calculated by subtracting the solubilizing power of SDPS-0 (sodium dodecyl sulfate) (1) F. Tokiwa, J . P h y s . Chem., 72, 1214 (1968). (2) The samples were the same as those used in a previous work.3 (3) F. Tokiwa and K. Ohki, J . P h y s . Chem., 71, 1343 (1967). (4) Measurements of solubilizing Dowers and electroDhoretic mobilities of micelles were carried o u t using the procedures previously reported.' 36 (5) F. Tokiwa and K. Ohki, Kollo.id-Z., 223, 38 (1968). V o l u m e 78, Number 18 November 1968
NOTES
4332 Table I: Some Properties of SDPS Micelles in Solutions of NaCl and CaCla
a
.
In 0.10 M NaCl
No. of
nI-
0.03 M CaC110-4#,
Samples"
oxyethylene units
1087, cm
n
cmP/V seo
esu/cm*
Q
cmz/V sec
SDPS-0 SDPS-1 SDPS-2 SDPS-3 SDPS-5 SDPS-9 SDPS-10
0 1.05 2.14 3.18 4.94 9.05 9.93
22.6 23.6 24.3 24.8 26.5 27.3 27.4
74.7 74.9 71.5 65.9 57.4 39.7 36.0
2.80 2.80 2.81 2.84 2.62 2.04 1.97
1.76 1.75 1.74 1.74 1.56 1.15 1.10
23.6 25.5 27.0 28.0 28.8 22.5 21.6
...
...
..,
1.25 1.39 1.59 1.49 1.33 1.24
0.85 0.96 1.12 1.01 0.88 0.80
12.4 14.8 18.1 18.6 17.3 15.7
1 0 4 ~ ~ 10-4a,
104un1,
esu/cmz
6l
The number written after SDPS represents the approximate number of oxyethylene units per molecule.
k
I
I
t
0
5
10
No. of oxyethylene units ( p ) .
0
Figure 1. The solubilizing power of SDPS toward Yellow OB plotted against the number of oxyethylene units (p) at 25': 0, in water; a, in 0.10 M NaC1; 8,in 0.033 M CaClg.
5
10
P.
Figure 3. The potential of the SUPS micelle plotted against p at 25": 0, in 0.10 M NaCl; @, in 0.033 M CaC12.
than in water, probably owing to the reduction of the repulsion among the charged heads which permits less extension of the polyoxyethylene chains. The electrical state of the micelle may be understood by the charge of the micelle through its { potential. Figure 3 shows the t potentials of the micelles of SDPS in solutions of NaCl and CaClz plotted against p . The { potentials were calculated from the electrophoretic mobilities4 ( U M ) ,given in Table I, by using Henry's approximatione { = 0
5
10
P.
Figure 2. The solubilization of Yellow OB per oxyethylene unit (left) and the degree of ionization of the micelle (right) in 0.10 M NaCl plotted against p at 25".
from that of each SDPS and dividing it by p . ) This could be a result of extension of the polyoxyethylene chains in the micelle because of the electrostatic repulsion of their attached charges, as will be described later. I n the presence of added salt, on the other hand, the SDPS micelle exhibits a greater solubilization T h e Journal of Physical Chemistry
6ar]U~/Df( K r )
where f(Kr) is Henry's function, r is the radius of the micelle, K is the Debye-Hiickel parameter, and 9 and D are the viscosity and dielectric constant, respectively, of the medium. (The use of this approximation does not introduce any significant error in discussing the result as long as the potential is relatively low as in the present case.') The radius of the micelle in NaCl solution, given in Table I, was evaluated from its hydrodynamic v ~ l u m e ,and ~ , ~the radius in CaClz Henry, Proc. Rou. SOC.,A133, 106 (1931). (7) P. H. Wiersema, A. L. Loeb, and J. Th. G. Overbeek, J. Colloid Interfac. Sci., 22, 78 (1966). (6) D. C.
NOTES
4333
solution was assumptively taken to be equal to that in NaCl solution. It is interesting to note that in NaCl solution the value of { is almost constant in the range of p 5 3 and then decreases with increasing p , while in CaCl2 solution there appears a small maximum around p = 3 in the {-p curve. The surface charge density, u, can be obtained using the data given in ref 8, where the relation between u and is given in tabulated form. Furthermore, the number of effective charges on the micelle, Q, is obtained by using the equation
Q = 4.rrr2u/e where e is the elementary charge. The values of CT and Q of the micelle in solutions of NaC1 and CaClz are given in Table I. The micellar charge can be also expressed as a degree of ionization, a = Q/n, where n is the number of molecules per m i ~ e l l e . ~In Figure 2 the value of a i n KaC1 solution is plotted as a function of p . A similar plot of a us. p in CaC12 solution would be obtained if we have available data of n in this solution. The results shown in Figure 2 indicate that the degree of effective charge on the micelle increases with increasing oxyethylene content, which could promote the repulsion among the charged heads of the polyoxyethylene chains in the micelle and the extension of the chains into the mater owing to this repulsion. This extension will make the oxyethylene units less available for interaction with the dye molecules and, therefore, in this case the degree of solubilization per oxyethylene unit decreases as the oxyethylene content is increased.
Acknowledgment. The author expresses his thanks to Dr. H. Kita, Director of the Research Laboratories, for encouragement and permission to publish this paper. (8) A. L. Loeb, F. H. Wiersema, and J. Th. G. Overbeek, “The Electrical Double Layer Around a Spherical Particle,” The M.I.T. Press, Massachusetts Institute of Technology, Cambridge, Mass., 1961. (9) D. Stigter and K. J. Mysels, J . Phus. Chem., 59, 45 (1955).
The Chloride Catalysis of the Np(II1)-Fe(II1) Reaction in Aqueous Acid Solutions1
by T. W. Kewton, Gloria E. RlcCrary, and W. G. Clark2 University of California, Los Alamos Scientific Laboratory, Los Alamos, New lMexico 87644 (Received June 10,1968)
Chloride ion usually increases the rates of aqueous oxidation-reduction reactions which involve the Fe(I1)Fe(II1) couple. The relative slowness of the equilibrium reaction
Fea+
+ C1-
FeC12+
=
(1)
has made it possible to specify the location of the chloride in the activated complexes for of the more rapid reactions. Thus for the Cr(I1)-Fe(II1) reaction, Dulz and S u b 3 used their stopped-flow technique to distinguish the terms k(Cr2+)(FeC12+)and k’(Cr2+)(Fe3+)(C1-) in the rate law. ;\lore recently, Carlyle and Espenson4 found analogous terms in the rate law for the Eu(I1)-Fe(II1) reaction. These results show the existence of two activated complexes of the same stoichiometric composition, one with chloride coordinated to iron and the other with the chloride elsewhere. I n the present work, the effect of chloride on the slower Kp(II1)-Fe(II1) reaction has been shown to be due primarily to the term k(Np3+)(FeCl2f) with a small contribution from k’(Np3+)(Fe3+)(Cl-). Related, preliminary experiments have shown that the oxidations of Fe(I1) by Ce(IV), V(V), and Np(V1) in the presence of C1- produce FeC12+ in greater than equilibrium concentrations.
ExDerimental Section The reagents and procedures used were essentially the same as previously de~cribed.~The spectrophotometric measurements were made in the range from 3350 to 3700 8 using the same stirred 10-cm absorption cells as beforea5 The hydrochloric acid used was prepared by diluting reagent grade acid to 6 ill and redistilling.
Results A t wavelengths where both Np3+ and FeC12+absorb appreciably, plots of absorbance us, time showed distinct minima when Fe(II1) was in excess. A typical result is result is shown in Figure 1. The simplest mechanism consistent with such behavior is Fe3+
+ C1-
=
FeC12+
+ C1Np3+ + FeC12+ = Np4+ + Fez+ + C1Np3+ + Fe3+ = Np4+ + Fez+ FeC12+ = Fe3+
kl
(1)
k2
(2)
k3
(3)
kd
(4)
This system leads to the simultaneous differential equations dz/dt
=
k3(A - X ) Y
+ ka(A - x ) ( B - x - y)
(5)
and (1) W o r k done under the auspices of the U. S. Atomic Energy Commission; presented in part a t the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968. (2) Student summer employee. (3) G. Dull and N. Sutin, J . Amer. Chem. Soc., 86, 829 (1964). (4) D. W.Carlyle and J. H. Espenson, ibid., 90, 2272 (1968). (5) T. W.Newton and N. A. Daugherty, J . P h y s . Chem., 71, 3768 (1967).
Volume 7.9, Number 1.9 November 1068
