934
XOTES
Vol. 67
A detailed iceberg model should permit evaluation of these terms independently. The small differences in the solute-dependent term will then serve as a sensitive test of the model. It has been suggestedlO that a cell theory approach should be useful for this purpose. Solubility isotope effects with oxygen and nitrogen, for example, indicate a slightly larger free volume for the oxygen molecule, properly reflecting its smaller “hardsphere” diameter and consistent with the entropies of solution reported here. A further correlation between solute force constants and thermodynamic data of the present type has been noted previ~usly.~ The present data also illustrate unambiguously that a previous notion1’ of a unique water-oxygen complex is untenable. A recent communication12 suggesting a more general interaction of the contact charge transfer type is reasonable. The existence of a shifting equilibrium among more than one “kind” of iceberg is, however, not excluded and may be indicated by LIir temperature-dependent absorption spectrum.l1 (10) C. E. Klots and B. B. Benson, J . Chem. Phue., in press. (11) L. J. Heidt and A. 31.Johnson, J . Am. Chern. Soc., 79, 5587 (1957). (12) J. Jortner and U. Sokolov, J . Phus. Chem., 65, 1633 (1961).
IP
I 3.30
I
335
I
3.40
I
I
3.45
3.50
I
355
I
3.60
SOLUTIOK STRUCTURE IN CONCENTRI1IIED SON-IONIC SURFACTSST SYSTEMS BY J. M. CORKILL
L 36
( V T ) x IO3.
Fig. 1.-Experimental determinations of the ratio of the solubility coefficients cf argon and nitrogen as a function of temperature: open circles, mass spectrometric ratio measurements: closed circles, manometric measurements.
Baszc Research Dept., Procter &. Gamble Ltd., Newcastle-upon-Tune, England AND
K. W , HERRMANN
Miami T’alley Laboratories, The Procter & Gamble Companu, Cincinnati, Ohio
Received August $1, 1068
.310
The X-ray diffraction patterns obtained from moderately concentrated anionic surfactant solutions led Mattoon, Stearns, and Harkins’ to suggest that in addition to micelle formation, a second structural transition took place in these solutions. The existence of this change has also been demonstrated by other techniques, notably viscosity and diffusion coefficient2C3 measurements. The interpretation of the X-ray patterns is still a matter of discussion4; however, it is generally agreed that an increase in order in the solution takes place above the second transition yegion. I n this paper, the results of light scattering and X-ray measurements on aqueous solutions of the dimethyloctyl and dimethyldodecylamine oxides (GAO and C12AO) are described. These results are discussed in terms of the structural changes that occur above the critical micelle concentration (c.m.c.), but below the concentration at which a mesomorphic (middle) phase separates. Although the amine oxides can show cationic character below pH 7, the surfactant is completely non-ionic under the conditions e m p l ~ y e d . ~ ,287-
/
I
I
I
I
I
5
Experimental Light Scattering Measurements .-Turbidities were determined with a commercial apparatus (Phoenix Precision Instrument Company) similar t o that described by Brice, et ~ 1 . 8 The narrow
I/T x IO’
Fig. 2.-Experimental determinations of the ratio of the sohbility coefficients of oxygen and nitrogen as a function of temperature: open circles, mass spectrometric ratio measurements; closed circlos, manometric measurements.
into its cavity and is probably relatively insensitive to temperature.
(1) R. W. Mattoon, R. S. Stearns, and W. D. Harkins, J . Chem. Phys., 15, 209 (1947). (2) R . J. Vetter, J . Phus. Chem., 51, 262 (1947). (3) X. Tyuzyo, Koll. Z., 176, 40 (1961). (4) G. W. Brady, J . Chem. Phus., 12,1547 (1951). ( 5 ) K. VC’. Herrmann. J . PAys. Chem., 66, 295 (1962). (6) B. A. Brice, M. Halwer, and R. Speiser, J. Opt. SOC.Am., 40, 788
(1950).
KOTES
April, 1963 slits provided with this instrument were used along with a cylindrical cell which was painted black except for the window. Measurements were made a t rgom temperature (27') using the blue line of mercury (X 4358 A.). It was found that in these systems, the turbidities were practically independent of temperature in the 25-35' range. The N,N-dimethylalkylamine oxide solutions were prepared using doubly distilled water. All solutions were clarified by nitrogen pressure filtration through a 0.45 p pore size Millipore filter. The cell and the bottles in which the solutions were kept were made dust free using the technique described by Thurmond.' Scattering measurements were made a t e = 90' and also as a function of angle over the range 30-135". KO turbidity corrections were made for disnymmetry and depolarization effects. X-Ray Data.-The X-ray diffraction patterns were obtained from solutions sealed in '/z mm. internal diameter capillary tubes A General Electric XRD-1 diffraction unit was used for these measurements. Nickel filtered copper radiation was used, generated a t 45 kv. and 19 ma. The films (Kodak No-Screen) were conventionally processed and optical density measurements were made with a Jocyl-Lobe1 microdensitometer. Materials.-The dimethylalkylamine oxides [alkyl N( CH3)20] were prepared by the hydrogen peroxide oxidation of the corresponding dimethylalkylamines. After the oxidation was complete, the excess hydrogen peroxide was catalytically decomposed by the addition of platinium black. Unreacted amine was removed by repeated extraction with petroleum ether. The solution was then freeze-drfed and the residue repeatedly recrystallized from dried CaS04 and redistilled acetone. The dimethylalkylamines were prepared by treating the nalkyl bromides with dimethylamine. Vapor phase chromatography showed that after fractional distillation tlie dimethylalkylamines contained less than 0.5y0of homologous contaminants. Solutions.-All solutions were prepared on a weight basis in doubly distilled water. The concentration units employed in this paper are g. of solute per 100 g. of solution, unless otherwise noted. All solutions were of greater than p H 7 , thus, only non-ionic surfactant wah present.6
Results Light Scattering Data.-The turbidity-concentration results for CsAO and C12A0 are given in Fig. 1. The CI2i10 results show a clear maximum in the region of 9 g./100 g. of solution; the CsAO, in addition to the c.m.c. break a t 3.23 g./100 g., shows a plateau beginning a t 12 g./100 g. The dissymmetries observed for the CsAO solutions remaiined below 1.1 over the concentration range studied; however, for C12A0, both dissymmetry and depolarization began to rise in the region of the turbidity maximum (Fig. 2 ) . The micellar properties for CsAO and CIAO in the concentration region of their c.m.c 's have already been reported.s These results are summarized in Table I and are not inconsistent with the view that the micelles are small and sphere-like near the c.m.c. TABLE I SUMMARY OIF hIICELLAR PROPERTIES
c.m.c'.
Surfactant
C,$O CIAO
(g./lOO ml.)
3 23 0 048
MMU'
2,600 17,300
Monomers per micelle;
15 76
Estimates of a particle size parameter a t concentrations above the second transition region were made from low angle scattering data shown in Fig. 3. The radii of gyration of the micellar aggregates, estimated from the extrapolated intercepts, the approximate slopes at low a9gles, and Zimm's equation,8 ranged from 880 to 1040 A. in thle more concentrated solutions. Calculations were not made for concentrations below (7) C.D. Thurmond, J . Polymer Sci., 8, 607 (1952). (8) B. H. Zimm, J . Chem. Fhus., 16, 1099 (1948).
6.
935
1 ?'
24.0
2
1 /
X 18.0
o
w+ C M C : 3 , 2 3 1 m ~ / i O O m ~ 6. C
12.0
18.0
24.0
30.0
Dimethylalkylamine oxide concn. (g./lOO g. of soln.).
Fig. 1.-Turbidity-concentration
data for CsAO and C12A0 /
1.6
10.07
6 1.3
-3
8k 1.2
4 1.1
4I 0.01
7-
1.0
1 0.0
1
0.9
0 6.0 12.0 18.0 24.0 30.0 Dimethyldodeoylamine oxide concn. (g./lOO g. of soln.).
Fig. 2.-Depolarization 260
and dissymmetry data for C12A0.
1
220 180
2 2
140 100 60 20
4 0
0.2
Fig. 3.-Angular
0.4 0.6 sin2 8 1 2 .
0.8
1.0
scattering data for Cl2AO solutions.
the concentration at which the turbidity, dissymmetry, and depolarization breaks occur because of the low dissymmetries and the greater inaccuracies involved. X-Ray Data.-Some evidence for the existence of a long spacing in the G 2 A 0 solutions has been obtained as low as 5 g./100 g. from densitometer tracings. These diffraction rings are diffuse and do not become very distinct until solution concentrations of 10-12 g./100 g. are reached. Furthermore, the minimum concentration at which these rings can be detected depends somewhat on the experimental conditions employed, particularly exposure time. As the concentration is further increased, the rings become more intense and the spacings decrease (Table 11). The spacings given in Table I1 were calculated from the relationship d = X/2 sin 6. Middle phase can first be detected at room temperature at 34% C12A0 by
936
XOTES
TABLE I1 X-RAI
~ I F F R A C T I O XD A T A FOR
Concn. (e./lOO g. s o h )
5 0 10 0 15 0 20 0 25 0 30 0 41 0
7-40SCILLTIONS
cj
d Spacing
30$efinite band 5 1 A . w m k , diffuw 53 2 1 . s ~ [ o n gtliniisc , 51 $.
47 44 A. 40 4 23
1
strong, dittnse strong, less diffuse strong, less diffuse middle phase; strong, sharp
polarized light microscopy methods. The detailed interpretation of the X-ray results is still uncertain. As in the case of CIAO, distinct diffraction bands are observed with CsAO solutions from the middle phase boundary (53% C8AO) down to the 20% CsAO concentration region. Below this concentration there is less certainty as to the origin of the density differences observed on the X-ray films.
Discussion The light scattering results for ClzAO indicate a considerable growth in micelle size, relative to the species at the c.m.c., by the time the concentration region has been reached where the turbidity maximum and the breaks in the dissymmetry and depolarization curves occur. The appearance of definite X-ray bands in the same concentration region suggests that the same structural change in solution is responsible for all these phenomena. I n view of the absence of ionic forces in this system, the turbidity decrease may well result from steric interference between the micelles. The crowding together of these large micelles could lead to regions of order in the solution, and the appearance of the X-ray long spacings would be a consequence of the existence of these ordered regions. The word “order” as used here must remain somewhat ambiguous because existing theory does not allow a precise interpretation of the experimental data. The authors prefer to think of “increased order” as meaning an increase in the alignment of rod-like micellar aggregates within localized regions of solution and an increase in the size of these regions. Some specific points supporting this general concept, which a t present must be considered as being a hypothesis, are given below. (1) The first mesomorphic phase formed by the ClnAO-water system as the surfactant concentration is increased (34% w.w., 27’) has the microscopic appearance of a typical middle phase.9 Middle phase, in surfactant-water systems, has been shown by Luzzati. Nustacchi, and SkoulioslO to be formed by the hexagonal arrangement of cylinders of indefinite length, t&e diameters of these cylinders being of the order of 30 A. for the Clz soaps. The cylinders themselves are thought to be similar in structure to micelles, with a hydrocarbon chain interior and a surface composed of the hydrated hydrophilic groups. We consider that steric interference and ordering between rods, similar to those found in the middle phase, might account for our results. (2) The existence of a maximum in the turbidity us. concentration plot has not previously been reported for surfactant systems but has been observed for poly(9) F. B. Rosevear, J . dm.0 2 2 Chem Soc., 31, 628 (1954).
(IO) V. Luseati, H. Mustacohi, and A. Skoulios, Trans. Faraday Xne., 25, 43 (1958),
VOI.
67
mer systems. Debye and Buechell l 2 have attempted a theoretical treatment of tfhe maxima in polymer systems where the particle molecular weight is large and fixed, and where the configuration of the polymer is taken to be similar to a flexible string. The theoretical coiisidrmt ions risrd in l h t l tlcrivntion of t h c w ruprrs.;ions are not applicable to surfactant systems. Ziiiirn’” has considered the effect of steric interference OR thc colligative properties of certain solutions. The use of Zimm’s expression for the second virial coefficient in a system of rods leads to the conclusioii that an elongating rod system will exhibit an experimental turbidity maximum. For rods of fixed length, the turbidity will approach a limit asymptotically. It should be recognized that these treatments include rough approximations (e.g., neglect of higher virial coefficients), however, they do indicate that increased aggregation with increased concentration can lead, theoretically, to the turbidity maxima observed, (3) Onsagerl* has proposed that there exists a region of instability of isotropic phase relative to nematic phase (a mesomorphic phase). If rod-like micellar aggregates are assumed and if the rod-length ( I ) is much greater than rod-diameter (d), then according to Onsager
where +i is the concentration of instability in terms of volume fraction. The experimental data suggest that the volume fraction where marked deviatioiis in turbidity, dissymmetry, and depolarization occur, i.e., ClzAO volume fraction G 0.09, may be taken as 6. In the case of aggregates having rod-diameters of 30.4., the particle length according to this interpretation of Onsager’s term would be about 1000 A. This value is consistent with the large dimensions calculated for the micellar aggregates in the more concentrated solutions using the low angle scattering data. (4) Some further support is given to the concept of increased micellar aggregation and increased particle ordering by preliminary measurements of the relative vapor pressure of both the CsAO and ClzAO solutions using a thermistor technique. The degree of vapor pressure lowering (Aplpo) remains relatively constant from AJp -
pso1vent
- psolutlan
Po Psolvent the c.m.c. to the second transition region but increases with increased surfactant concentration from this point and 3.5 =k 0.5 on. Increases in A p / p o of 12 f 0.5 X X were observed on increasing the surfactant concentration from 14 to 30 g. of CsAO/lOO g. and 12 to 30 g. of ClzAO/lOO g., respectively. This behavior is indicative of a large departure from solution ideality and deserves further study. It is known that water forms an integral part of the ordered structure in the mesomorphic phase. The vapor pressure depressions above the second transition point may, therefore, be a consequence of the incorporation of water into the ordered arrangement of micellar aggregates. X-Ray and light scattering methods indicate that the (11) P. Debye J Chem Phys , 31, 680 (1959). (12) P Debve and A. X. Bueche, %bid 18,I423 (1S‘iO) (13) B. H Zimm, h d , 14, 164 (1946)
(14) L,Onsagel, Rnn, J,Y , dead, Scz,, 53, 627 (1949).
XOTES
April, 1963 second transition region is less distinct in the case of CJO. The low dissymmetry in the transition region is evidence for a reduced particle size (relative to C l d O ) , and the weakness of the long spacing diffraction bands suggests that the micelles are less ordered than in the G 2 A 0 syRtem of thP same concentration. Conclusions Although it is difficult to interpret the results individually in terms of' changes in micellar structure with increasing surfactant concentration, the combined results have led us to propose the following hypothesis in explanation of the observed behavior. Soniewhare above the c.m.c. but below the turbidity maximum the initially spherical micelles begin to elongate. I n the transition region, steric interference is accompanied by increasing alignment of the elongated micelles. As the concentration further increases, the degree of orientation increases until, finally, a well-ordered phase (mesomorphic) appears. The assignment of a definite second transition concentmtion for these systems does not seem possible. It would appear that the structural changes in these solutions are gradual rather than sharp transitions. It is hoped that these observations will stimulate further work in this relatively neglected region of surfactantwater systems and will encourage theoretical treatments of the structural changes that occur with increased surfactant concentration. TEMPERATURE COEFFICIENT OF T H E MERCUROUS ACETATE ELECTRODE BY W. D. LARSOX Department of Chemistrv, College of St. Thomas. St. Paul 1 , iMinn. Received Septembe!. 1.5, 1962
m
E = EO - RT/F, In U K U O A ~=
- RT/P, In mK(1 - a ) y ,
(1)
where m is the molality of the acetic acid, K is its thermodynamic dissociation constant, a is its degree of ionization, and yu is its activity coefficient for undissociated molecules. Values of K and of Q may be obtained from the work of Harned and Ehlers.3 Experimental methods and materials used were substantially the same as in the earlier work. Agreement between replicate cells was within 0.2 mv. Table I shows the average values of the e.m.f. of the cells a t various molalities and temperatures. At least four cells were measured at each concentration; since cells were measured only in the range 5-25' or 25-37.5', each figure a t 25' represents the average of a t least eight cells while the fjgures for the other temperatures (1) W.D. Lsrson and F. B. nIacDougrtl1, J . Phus. Chem., 41, 493 (1937). (2) W.D. Larson and W. J. Tomsicek. J . A m . Chem. Sac., 61,65 (1939). (3) H. S. Harned and R. W. Ehbrs, J . A m . Chem. Soc,, 64, 1350 (1932).
4 T VARIOI-S TEMl'ER.4TlTRE:S Eoa~o.
Bobs.
A(mv.1
5 O
0.4057 0.5217 0.7730 1.039
0 80745 $01 39 7!1 I!)0 78484
0.4057 0,5217 0.7736 1,039
0 81093 80470 79501 78772
0.4057 0.5217 0,7730 1.039
0 81391 80753 79756 70010
0.40517 0.5217 0.7736 1.039
0 81625 81013 79962 70203
0.4057 0.5217 0.7736 1.039
0 81786 81127 80095 .79323
0 80749 80130 7!)I SG 78488
-0.04 - b .09 .0-i - .04
0 81095 80481 79501 78774
-0.02 - ,11 . 00 - .02
0 81412 80778 79777 79026
-0.21 - .25
0 81602 81011 79940 79207
+0.23 .02 .22 - .04
0 81777 81128 80087 79314
$0,09 - .01
0 82009 81332 80276 79492
-0.11 - .09 - .08 - .16
+
12 5O
20"
-
. 10
.16
25
+ +
30"
+ +
.os .09
37 5" 0.4057 0.5217 0.7736 1.039
0 81998 81323 80268 .79476
are the average of a t least four cells. The electromotive forces a t various temperatures were fitted by the method of least squares to an equation of the form
E
were measured over a temperature range from 5 to 37.5' and a concentration range of 0.4 to 1.0 molal. Previous work1%gave the standard e.m.f. of the mercury, mercurous acetate electrode a t 25' only. The e.m.f. of cells I is given by the equation
I
TABLE l
