102
J. PhyS. Chem. 1981, 85, 102-106
Diffusion in Organic Liquids. 3. The Isotope Mass Effect and Diffusivlty of Cyclohexane Diffusing in a Range of Solvents Robert Freert and John N. Sherwood”
Department of Pure and Applied Chemistfy, University of Strathclyde, Glasgow GI IXL, Scotland (Received: May 18, 1960)
A gel-sectioning technique was used to examine the diffusivity and isotope mass effect for diffusion of cyclohexane tracers diffusing as trace solutes in the series of solvents: benzene, perfluorobenzene, n-hexane, n-heptane, n-octane,and n-dodecane. The measured diffusion coefficients show a parallel variation with solvent type and viscosity to that observed previously for other systems. The isotope mass effects vary considerably,being normal for perfluorobenzene (i.e., with the diffusion coefficient for the lighter species (DL)being greater than that for zero for benzene, n-dodecane, and n-octane, and inverse (DH> DL)for the two solvents the heavier species (DH), of lowest viscosity. The last confirms the observation of similar behavior for self-diffusionin cyclohexane but is different to the situation noted for benzene (DL< DH for all solvents examined). There is currently no theoretical model which predicts this distinction.
Introduction In recent years, the search for isotope mass effects in self-diffusion in organic liquids has concentrated largely on systems containing benzene.’ Studies of self-diffusion in this liquid carried out to date have yielded a variety of mass-effect differences ranging from zero to an inverse root-mass dependence.‘ In spite of this wide variation for self-diffusion, there is general agreement that, for trace amounts of benzene diffusing as a dilute solute in a range of solvents, there exists a small isotope mass effect. Here, the differential has been noted to increase with decreasing molar volume and molar mass of the solventsa2 In all cases (self- and tracer diffusion) the isotope mass effect is of the expected sense, with the diffusion coefficient of the lighter species (DL) being greater than that for the heavier species (DH).In contrast, recent studies of self-diffusion in cyclohexane have produced the unusual result of a small, but distinct, mass effect of the inverse sense (DH > DL).173,4 There is, as yet, no obvious physical explanation why benzene and cyclohexane should behave in such different ways. Additional to the definition of mass effects, the examination of the diffusivity of a single species in a series of solvents is also of theoretical and practical i n t e r e ~ t .Up ~ to the present time, such examinations have been limited to only a few solute/solvent sytem~.~-’There have been comparitively few studies of cyclohexane. In view of the unusual isotope mass effect observed for self-diffusion in this liquid, coupled with the lack of data on its diffusivity in other solvents, radiotracer, isotope-maw effect experiments have been carried out for cyclohexane diffusing in a series of liquids at 298 K by use of a gel-sectioning technique.s The majority of previous diffusion studies have involved the use of the diaphragm cell9or optical techniquedo with a single tracer or species diffusing in each experiment. Evaluation of the isotope mass effect is then based on the comparison of the results from two successive experiments. The disadvantage of this procedure is that cumulative errors may encompass a possible mass-effect difference.’ Simultaneous diffusion of two tracers eliminates some experimental errors and allows the direct appraisal of any distinction. Recently, this approach has been successfully ‘Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW. 0022-3654/81/2085-0102$01 .OO/O
employed in both diaphragm cell and layer-analysis experiments.lJl In the present experiments, protonated and deuterated species were singly labeled with 14Cand 3H, respectively, to provide the maximum mass difference, and both tracers were diffused simultaneously. The results of the experiments are compared with those for benzene diffusing in the same solvents and, where possible, with the results of other previous studies of tracer solute diffusion.
Experimental Section The solvents used were reagent grade benzene, perfluorobenzene, n-hexane, n-heptane, n-octane, and n-dodecane (199% purity). With the exception of benzene they were not purified. The purification of benzene and techniques for the preparation of the gels and the tracers 14C112C51H12 and 12C62H113H1 have been described previously.lP8 After standardization, the tracers were mixed in the ratio 3H/14C 1-4 for use. Diffusion experiments were performed by the gel-sectioning procedure.s Briefly, a “syringe” (ref 8) filled with a Cab-o-Si1 gel (Cabot Corp.) of a specific solvent was equilibrated for several hours at the experimental temperature, and the mixed tracer applied to a freshly cut gel surface. The “syringe” was enclosed in a large glass tube and immersed in a thermostat bath (298.18 f 0.01 K) for the duration of the diffusion experiment (-24 h). After this, the gel was extruded and rapidly sectioned, and the separate distribution of both tracers was determined by liquid scintillation counting pulse height analysis tech-
-
(1) R. Freer and J. N. Sherwood, J. Chem. SOC.,Faraday Trans. I, 76, 1030 (1980). (2) I. R. Shankland, P. S. Arora, and P. J. Dunlop, J . Phys. Chem., 81, 1518 (1977). (3) R. Mills, J. Phys. Chem., 79, 852 (1975). (4) R. Mills, J. Phys. Chem., 80, 888 (1976). (5) K. Nakanishi, Bull. Chem. SOC.Jpn., 51, 713 (1978). (6) R. H. Stokes, P. J. Dunlop, and J. R. Hall, Trans. Faraday SOC., 49, 886 (1953). (7) B. R. Hammond and R. H. Stokes, Trans. Faraday SOC.,51,1641 (1955). (8) R. Freer and J. N. Sherwood,J. Chem. SOC.,Faraday Trans. 1,76, 1021 (1980). (9) R. Mills and L. A. Woolf, “The Diaphragm Cell”, A.N.U. Press, Canberra, 1968. (10) I. R. Shankland, P. J. Dunlop, and L. W. Barr,Phys. Reu. B , 12, 2249 (1975). (11) L. W. Barr and M. A. M. I. Elmessiery, Nature (London),281,553 (1979).
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 1, 1981 103
Diffusion in Organic Liquids
TABLE I : Tracer Diffusion Coefficients for the Species (DH)and 14C112C31H12 ( D L ) in a Series 12C,aHl,3H, of Solvents at 298.18 K Measured by the Gel-Sectioning Technique, and Selected Literature Values for the Same Systems
DH/IO-’ m as - ’
n-H EXANE
solvent 4
-C
4-
2-
-
-
I
,
0
I
4
8
I
16
12
20
Xi(crn2)
N
lit. values
n-dodecane 1.142(028) cycle1.453( 0 4 2 y hexane perfluoro- 1.563( 035) benzene benzene 2.103(031)
1.147(030) 1.441(045)a 1.44gb
n-octane 2.41 7( 03 5) n-heptane 2.995( 040) n-hexane 4.004( 090)
2.419( 033) 2.988(050) 3.896(085)
1.570( 038) 2.095( 037)
2.104,‘ 2.090,d 2.084e 2.09,f 2.082g 3.883g
‘
Reference 1. Average value, see ref 8. Reference Reference 14. e Reference 15. f Reference 16. Reference 17. a
Flgure 1. Seif-dlffusion profiles plotted as In A against x 2 for 14C1’2C51H12 in liquid n-hexane and liquid ndodecane ( T = 298.18 K,
t
DLi10-9 m 25 - l
13. g
75000 s).
niques.12 Sections were dissolved in liquid scintillator and counted several times to 0.1% statistical accuracy against well-defined standards to test reproducibility. Tests applied which used standard mixtures of the two tracers of various activities in the experimental range showed that the 3Hand 14Cactivities could be reproducibly separated to much better than 1% accuracy and that this separation was independent of count rate. The overall maximum cumulative errors on the isotope effect experiments are shown in Figure 3 as error bars. Tests, noted elsewhere,’ were applied to assess the degree of interaction of the gelling agent with the liquid. In all cases, such effects were found to be negligible; the liquid in the gel behaved as the pure liquid.
Results As the application of a thin layer of radiotracer (-0.3 pm) to a column of gel 15 cm long resulted in diffusion depths of (typically) 5 cm in 24 h, the experimental arrangement may be approximated to the situation of diffusion from a thin source of total activity Q into a semiinfinite medium. The specific activity A of the diffusing species at a depth 3c into the material after a time t may be defined by A = Q/(rDt)’/2 exp(-x2/4Dt) (1) where D is the diffusion coefficient. In all gel runs, plots of In A against x 2 were linear over the full range, confirming the applicability of eq 1to the variety of gels used. Typical plots of In A against x 2 for the diffusion of 14Cl-cyclohexanein n-hexane and n-dodecane are shown in Figure 1 as examples of the extreme cases. Calculated tracer diffusion coefficients for cyclohexane in the series of solvents are listed in Table I. The numbers in parentheses record the standard deviation for these experiments. Results for the self-diffusion of cyclohexane’ are included for completeness. The reliability ~~
(12) R. Fox and J. N. Sherwood, “Atomic Transport in Solids and Liquids”, A. Lodding and T. Lagerwall, Ed., Verlag der Z. Naturforsch, Tubingen, 1971, p 258; Trans. Faraday SOC.,67, 1 (1971). (13) R. Mills, J. Phys. Chem., 69, 3116 (1965). (14) L. Rodwin, J. A. Harpst, and P. A. Lyons, J. Phys. Chem., 69, 2783 (1965). (15) K. R. Harris, C. K. N. Pua, and P. J. Dunlop, J. Phys. Chem., 74, 3518 (1970). (16) S. A.Sanni, C. J. D. Fell, and H. P. Hutchison, J. Chem. Eng. Data, 16,424 (1971). (17) K. Aoyagi and J. G. Albright, J . Phys. Chem., 76, 2572 (1972).
n - DODECANE
-1.14
1 In A t
4
5
7
6
n -OCTANE
InAL
5
-0.12
6
7
1
-0,20[ 4
8
ri
- HEXANE
, InAL 5
6
7
Figure 2. IME plots of In (ALIA,) against In A L for the diffusion of 14C112C51Hl, and 12CtH113Hltracers in ndodecane, n-octane, and n-hexane.
of individual runs was typically f0.4% but the reproducibility was generally closer to f 2 %. This can be compared with *0.2% claimed for the diaphragm cell in particular laboratories. Even for this technique, however, the universal error is *2%. Results of the present study are in satisfactory agreement with the available literature values (see Table I). This agreement confirms the reliability of the technique. Due to accumulated errors the isotope mass effect cannot be assessed accurately by direct comparison of DL and DH. When the two tracers are diffused simultaneously, however, the progress of each will be described by eq 1. Compounding the two forms of this equation for each isotope yields In (AL/AH) = (DL/DH - 1) In A, + constant (2)
104
Freer and Sherwood
The Journal of Physical Chemistty, Val. 85,No. 1, 1981
‘26
1 0
1
I
I
2
3
t
1
5
4
0
50
1O3/q2‘P a s )
100 Molar Volume V,
Figure 3. Plot of limltlng tracer diffusion coefficients ( D i )of solutes benzene (0)and cyclohexane (0)against the reciprocal vlscoslty (1/v2) of the host solvent at 298.18 K. nAlkanes are indicated by C,;carbon tetachloride, C,; cyclohexane, Cy;perfluorobenzene, C,;chlorobenzene, C,,;benzene, B;, toluene, B,; for data see ref 2 and Table 11.
The determination of ratio DL/DH(which corresponds to the ratio of the diffusion coefficients for the 14C-and 3Hlabeled species, respectively) then depends only on the comparison of the activities of the two species at different penetration depths into the sample. Figure 2 shows typical relative activity plots for the diffusion of the cyclohexane tracers in three of the n-alkanes. The error bars on individual points indicate accumulated errors in the evaluation of the activities and their ratios. For n-dodecane and n-octane the gradients are zero, indicating a zero mass effect. For n-hexane there is a significant positive slope confirming the existence of a mass effect of the inverse sense. The full results are summarized in Table 111; the parenthesized numbers represent the standard deviations on the final result.
Discussion Diffusiuity.Ghai, Ertl, and Dullienls have noted that the diffusion coefficient D1 of a particle of radius rl diffusing in a solvent of viscosity q2 may be described by the simple Stokes-Einstein r e l a t i ~ n s h i p ’ ~ ~ ~ ~ (3) where X = 6 when the coefficient of sliding friction = a,and X = 4 when pF = 0. This equation has been applied to self, mutual, and intradiffusion with varying degrees of suuccess.18 For intra- (or impurity) diffusion of one species, the diffusion coefficient (DJ should be inversely proportional to the viscosity of the host (i.e., the solvent) v2. Plots of D1 against q2 (Figure 3) demonstrate the degree of applicability of this relationship to benzene and cyclohexane a t 298.18 K. All data for cyclohexane as solute lie close to the line of best fit. The data for benzene diffusing in benzene and chlorobenzene deviate significantly from that line. By selecting the appropriate form of the Stokes-Einstein equation (eq 3, X = 4 or 6) several authorsla have attempted to calculate the size of the diffusing particle from diffusion and viscosity data. Conversely, by assuming a value for rl, the value of the constant X may be deduced. McLaughlin, for example,21examined self-diffusion data (18) R. K. Ghai, H. Ertl, and F. A. L. Dullien, AZChE J., 19, 881 (1973). (19) A. Einstein, “Investigations on the Theory of Brownian Movement”, 1905, Reprinted, Dover, New York, 1956. (20) H. Lamb, “Hydrodynamics”, Dover, New York, 1945, p 602.
200
150 (cm’
250
m01.’
Figure 4. Hammond-Stokes plot (see text) of Diq2against V , for the diffusion of cyclohexane in various solvents at 298.18 K. The Hammond-Stokes llmlt Is denoted by A.
TABLE 11: Limiting Tracer Diffusion Coefficients (D, ) for Cyclohexane in a Series of Solvents and Associated Physical Parameters Required for Hammond-Stokes Plots solvent n-dodecane cyclohexane perfluorobenzene benzene n-octane n-hep tane n-hexane
cc1,
toluene
D,/10-9 7 1 , / l O - ~ D,q,/ m2 s-’ Pasa 10-”Nb
V,,c cm3 mol-’
1.148 1.439 1.571
1.353d 0.89Bd 0.874e
1.55, 1.29, 1.37,
227.5 108.1 115.Se
2.094 2.419 2.987 3.876 1.275c 2.42OC
0.608f 0.514d 0.390d 0.292d 0.906f 0.558f
1.27, 1.24, 1.16, 1.13; 1.16 1.35
88.9 162.6 146.6 130.5 96.5 106.3
a 1 poise = l o - , Pa s throughout this work. lo-’ N throughout this work. Reference 25.
1 dyne =
Reference 26. e J. Robertson, private communication. f Reference 27 (value interpolated where necessary).
for five liquids and obtained values ranging from X = 3.54 (for CC14)to X = 5.24 (for methanol). This variation may well arise from the simplicity of eq 3 since it is believed that the Stokes-Einstein equation fails to account adequately for frictional forces.ls Many attempts have been made to improve the relationship with no great general success. These attempts have been discussed by Tyrrel122 and Ghai, Ertl, and D ~ l l i e n . ~ ~ In the present case the gradients of the diffusivity/reciprocal viscosity plots (Figure 3) are significantly different for the two solutes: 1.031, cyclohexane; 1.258, benzene. A number of techniques are available for estimating molecular sizes including hard sphere (HS), van der Waals (VW),24and molar volume ( M W calculations. Values of X based on the different estimates of rl lie in the sequence XMv< Xw C XHSand average 3.7 f 0.5 for benzene and 4.2 f 0.5 for cyclohexane. These are well within the range cited by McLaughlin.21 Some of the first detailed studies of the diffusion of a single species (CC14and iodine) in a range of solvents were made by Stokes and his co-~orkers.~~’ Hammond and Stokes‘ found that plots of the viscosity reduced diffusion coefficient (D1v2)against the molar volume (V2)of the solvent yielded three distant linear relationships. Each (21) E. McLaughlin, Trans. Faraday Soc., 66,28 (1959). (22) H. J. V. Tyrrell, “Diffusion and Heat Flow in Liquids”, Butter-
worth, London, 1961. (23) R. K. Ghai, H. Ertl, and F. A. L. Dullien, AIChE J.,2 0 , l (1974). (24) K.J. Czworniak, H. C. Andersen, and R. Pecora, Chern. Phys., 11, 451 (1975).
The Journal of Physical Chemistry, Vol. 85, No. 1, 198 1
Diffusion in Organic Liquids
105
TABLE 111: Value of t h e I s o t o p e Mass Effect R a t i o (DL/DH) a n d Constant ( A / D , )f o r t h e Diffusion of Cyclohexane, a n d Constant ( A l D , ) for t h e Diffusion of Benzene in a Series of Solvents cyclohexane diffusion
solvent b
(DL/DH)
dodecane cyclohexane CP, benzene octane heptane hexane
1.003(003) 0.992(006) 1.007(006) l.OOO(003) 1.001(001) 0.996(003) 0.980(004)
benzene 1Oa diffusiona ( A I D , ) / 103 ( A I D ,11 mol g-’ mol g-l
0.00 -0.74 0.36 0.00 0.00 -0.18 -1.6
0.52 0.98, 1.1 0.53, 0.00 1.1 1.1,1.5 1.2, 1.4
T h e sola Based on t h e compilation given in ref 2. vents are arranged in order of decreasing viscosity.
0
100
Molar
20 0 Volume
300 V2 ( c m 3 mol-’)
Figure 5. Hammond-Stokes plots for the d w b n of solutes C,H, (O), C6H6 (0),iodine (W), CCI, (O),C!,1 i6 (A),and C& (A)in various n-alkanes (C,) at 298.18 K. Data are from this study and ref 5.
corresponded to a specific type of solvent, namely, the spherical molecules, the alcohols, and the homologous n-paraffins. Each plot had a positive slope and converged to a common point when extrapolated to V2 = 0. This is defined to be the Stokes-Einstein limit where large particles diffuse in an essentially continuous medium! Harris, Pua, and Dunlop15 have confirmed that the diffusion coefficients of benzene in n-hexane, n-heptane, and n-octane follow the Hammond-Stokes relationship. Figure 4 shows the corresponding plot for the limiting tracer diffusion coefficients for cyclohexane diffusing in eight liquids. Relevant parameters are given in Table 11. The data divide into the n-alkanes and spherical molecules. The Stokes-Einstein limit for a solute is usually calculated5 from eq 3 with X = 6 and rl = ( ~ V , / ~ T N ‘ )where )~/~ NO is the Avogadro number and V, the molar volume. In view of the preceeding comments on the constant X , it is probable that a lower value might be more appropriate. For comparison with previous data, however, calculations based on this interpretation (X= 6) yield values of 0.665 X 10-l2Nfor benzene5and 0.623 X 10-l2Nfor cyclohexane. Finally, Nakanishi5 has proposed that, whenever a constant slope of a Dlqz against V2plot can be established for the same group of solvents, it is almost independent of the solute used. The degree to which this is obeyed for the n-alkanes as solvents is shown in Figure 5. Most of the data for the larger, similarly sized, molecules including cyclohexane do group together well. Those for the smaller ethane molecules diverge significantly. In conclusion, it can be claimed that the present data for cyclohexane diffusivity follows a closely similar pattern to that observed previously for other systems. (25) J. A. Sanni, C. J. D. Fell, and H. P. Hutchison, J. Chem. Eng. Data, 16, 424 (1971). (26) A. Weissberger, “Techniques of Organic Chemistry”, Vol. VII, Interscience, New York, 1967. (27) “Handbook of Physics and Chemistry”, 53 ed, Chemical Rubber Co., Cleveland, Ohio, 1972.
OL----J800
I
1
1000
1200
I
-
14 00
AH, ( J o u l e )
Flgure 6. Plot of the normalized IME constant ( A I D , )against excess heat of mixing (AHE)of the solute and solvent, for the diffusion of benzene in various organic liquids. Data are from ref 2 and ref 29.
Isotope Mass Effects. The ratios of the diffusion coefficients for the light (DL,14C labeled) and heavy (DH, 3H labeled) species are noted in Table 111. For n-dodecane, n-octane, and benzene solvents, there is no significant mass effect. Perfluorobenzene shows a potential, but small, normal mass effect (DL > DH). n-Hexane and n-heptane give well-defined inverse mass effects (DH < DL). Shankland, Dunlop, and BarrlO have reported that the tracer diffusion coefficients of variously labeled benzenes in a number of organic liquids may be represented by an equation of the form (4) where D1 and Mo are, respectively, the limiting tracer diffusion coefficient and molecular weight of the solute. DT and MT the diffusion coefficient and molecular weight of the tracer, and A, a constant for a particular system. They suggest that the values of the normalized constant (AID,) are a measure of the isotope mass effect and have compared the published results for benzene tracer diffusion in a series of solvents.2 Adopting a similar procedure we present the results for cyclohexane in Table I11 and, where possible, compare them with those for benzene. The latter show a simple relationship between (AID,) and the reciprocal viscosity (see Table 11); the mass effect is of the normal sense (DL < DHand AID1 positive) for all solvents and decreases with increasing viscosity. In contrast, there is no simple systematic trend for the cyclohexane diffusion data. The constant (AID,) is positive for perfluorobenzene, zero for n-octane, n-dodecane, and benzene, but
108
J. Phys. Chem. 1981, 85, 106-112
negative for n-heptane and n-hexane. This inverse sense mass effect was first observed for cyclohexane self-diffusi~n.l~~p~ The largest isotope mass effect is therefore observed in the solvent of lowest viscosity, and the magnitude of (AID,) is similar to that obtained for benzene diffusing in that l i q ~ i d . 2 ~The 7 ~ variation of the cyclohexane results cannot be rationalized so easily in terms of a simple viscosity dependence, and there is currently no obvious physical explanation of why the mass effects should be in the inverse sense to those observed for benzene. We can only speculate that the noted variation must reflect the detailed and specific changes in the intermolecular force system between solute and solvent. In this context, we note (Figure 6) that for benzene there is an equally good correlation between the isotope mass effect (AID,) and the excess heat of mixing for benzene
with the solvent. We have made a similar proposal to account for the distinction between benzene and cyclohexane in self-diffusion and noted that, in this case, the small amount of available experimental data (excess heats and volumes of mixing of C6H6/C6D6against those for C6H12/C6D12)30 do provide a reasonable basis for the speculation. In view of this, it would be most interesting to have values of equivalent parameters for the mixing of C6H12 and C6D12 with the solvents used in this experiment. Quite large distinctions might be expected between, for example, the heats of mixing of C6H12 and C&, with perfluorobenzene and n-hexane. Unfortunately, this data is not available. Acknowledgment. We thank the S.R.C. for their financial support of this work. (29) J. H. Dymond, personal communication. (30) M. La1 and F. L. Swinton, Physica, 40, 446 (1969).
(28) J. G. Albright and K. Aoyagi, J . Chem. Phys., 64, 81 (1976).
Rodlike Micelles of Sodium Dodecyl Sulfate in Concentrated Sodium Halide Solutions Sholchl I k e d a , ” Shojl Hayashl, and Toyoko I m a e Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya 464, Japan (Received: June 6, 1980)
Light scattering from aqueous solutions of sodium dodecyl sulfate has been measured in the presence of 0.80 M sodium halides at 35 “C. Sodium dodecyl sulfate forms micelles having a molecular weight of -50000 or less, at the critical micelle concentration, 0.013 g dL-l, in solutions of all sodium halides except for NaSCN, in which the micelle size is even smaller. The average micelle size increases with increasing concentration of sodium dodecyl sulfate further, and it reaches a maximum around 1g dL-l. The molecular weight of the large micelle formed at the high concentrations is in the order: NaSCN < NaF < NaCl < NaBr < NaI, and it is 185000 in NaSCN and 357 000 in NaI solutions. The angular dependence of light scattering indicates that the large micelles are rodlike and have a common structure in NaCl, NaBr, and NaI solutions. It is concluded that the salt-induced sphere-rod transition of ionic micelles is not caused by the change in the structure of water, since the ionic strength for transition is independent of co-ion species of added salt. The dissymmetry and the angular dependence of light scattering in NaF solutions are anomalous at the concentration range just above the critical micelle concentration, and this can be attributed to the formation of trace amounts of microgel particles consisting of spherical micelles, possibly linked by the contaminant polyvalent cations in NaF.
Introduction In a previous paper1 we have shown by means of light scattering that the micelle of sodium dodecyl sulfate (SDS) changes its shape from spherical to rodlike when the concentration of added NaCl exceeds 0.45 M and the micelle concentration is -1 g dL-’. Such an effect of added NaCl on the SDS micelle was also observed by Mazer and his c o - w o r k e r ~by ~ ~the ~ measurements of quasi-elastic and total-intensity light scattering. Corti and Degiorgi0~7~ also observed large values of aggregation number and hydrodynamically equivalent radius of the SDS micelle in 0.6 M NaCl by using the same techniques, but they reserved judgment on the formation of rodlike micelles and condisered other possibilities as well. (1) S.Hayashi and S. Ikeda, J. Phys. Chem., 84, 744 (1980). (2) N. A. Mazer, G . B. Benedek, and M. C. Carey, J. Phys. Chem., 80, 1076 (1976). ( 3 ) C. Y. Young, P. J. Missel, N. A. Mazer, G . B. Benedek, and M. C. Carey, J . Phys. Chern.,82, 1375 (1978). (4) M. Corti and V. Degiorgio, Ann. Phys. (Paris),3, 303 (1978). (5) M. Corti and V. DeGorgio in “Solution Chemistry of Surfactants”, Vol. 1, K. L. Mittal, Ed., Plenum, New York, 1979, p 377.
In a recent work6 we have also demonstrated that the micelle of dodecyldimethylammoniumchloride changes its shape from spherical to rodlike when the NaCl concentration is higher than 0.80 M and the micelle concentration is finite. With some other cationic surfactants we can similarly observe definite evidence for the occurrence of sphere-rod transition of micelle shape induced by the change in concentration of added salt. There are at least two factors responsible for determining such a transition of micelle shape. One is the electrostatic effect of simple salt due to the counterion binding on ionic micelles, and the other is the hydrophobic interaction between surfactant molecules or ions caused by the change in the hydrogen-bonded structure of water. In order to distinguish these two factors for the transition of micelle shape, we have undertaken to measure light scattering from SDS micelles in concentrated salt solutions simply by changing the co-ion species of added salt and to determine the micelle size and shape. (6) S. Ikeda, S.Ozeki, and M. Tsunoda, J. Colloid Interface Sci., 73, 27 (1980).
0022-3654/81/2085-0106$01.00/00 1981 American Chemical Society
