FUMIKATSU TOKIWA
1214
Solubilization Behavior of Sodium Dodecylpolyoxyethylene Sulfates in Relation to Their Polyoxyethylene Chain Lengths
by Fumikatsu Tokiwa Research Laboratories, Kao Soap Co., Wakayama-$hi, Japan
(Received September 26, 1967)
Solubilization behavior of a series of sodium dodecylpolyoxyethylene sulfates (SDPS) with oxyethylene units from 0 to 10 has been studied, in comparison with the behavior of dodecylpolyoxyethyleneethers (DPOE) and sodium alkyl sulfates (SAS),to understand the nature of the polyoxyethylene chain in SDPS. The two surfactants with a polyoxyethylene chain, SDPS aud DPOE, have a much greater solubilizing power than SAS. The absorption spectra of a water-insoluble dye, Yellow OB, in the aqueous solutions of SDPS and DPOE suggest that solubilization occurs in both the hydrocarbon and polyoxyethylene regions of the micelle. With respect to the effect of the chain length of polyoxyethylene part, a different solubilization behavior is observed between SDPS and DPOE, which could be explained by different micellar structures between these two types of surfactants. The nature of the polyoxyethylenepart in SDPS is in a sense similar to that of the hydrocarbon part; the solubilizing power increases and the cmc value decreases with increasing oxyethylene content. The numbers of solubilized Yellow OB molecules per micelle at saturation have been calculated by combining the solubilization data with previous data on micellar molecular weights, and they are greater than unity.
micellar properties of SDPS are highly dependent on the chain length of the polyoxyethylene part, as are those of DPOE.2-4 With respect to the effect of polyoxyethylene chain lengths on the properties of their micelles in solutions, however, some dissimilarities have been found between these two types of surfactants. For instance, different solubilization behaviors are observed between them when the numbers of oxyethylene units in the polyoxyethylene parts are increased. I n general, the solubilization capacity of a nonionic surfactant of polyoxyethylene type for water-insoluble materials is much greater than that of an ionic surfactant having the same hydrocarbon chain length as that of the nonionic surfactant; for example, the solubilizing power of DPOE for a water-insoluble dye, Yellow OB, is about 10 times greater than that of sodium dodecyl sulfate. This fact suggests that the polyoxyethylene part in the molecule plays an important role in solubilization. I n the present work, in order to understand the nature of the polyoxyethylene chain of anionic SDPS, the solubilization behavior has been studied in comparison with the behavior of nonionic DPOE and anionic sodium alkyl sulfates (SAS) with changing chain length of the polyoxyethylene or hydrocarbon part. I n addition, the hypothesis reThe Journal of Physical Chemistry
Materials. Sodium dodecylpolyoxyethylene sulfates, ClzHz6(OCHzCHJ ,0S03Na, were the same samples as those used in a previous work.' Dodecyl polyoxyethylene ethers, ClzH25(OCH&H2),0Hwith p of 7, 10, 13, 15, and 20 were prepared from dodecyl alcohol of a high purity by addition of ethylene oxide using sodium hydroxide as a catalyst. Polyethylene glycol, a by-product of the reaction, was removed by the solvent extraction method' using 1-butanol-saturated water and water-saturated 1-butanol. Paper chromatography showed the purified samples to be free of polyethylene glycol.8 The value of p for each sample of DPOE was determined from its hydroxyl value. Sodium alkyl sulfates, CnH2,+' o s o 3 x a , were prepared from respective alcohols by sulfation with chlorosulfonic acid (1) F. Tokiwa and K. Ohki, J . Phya. Chem., 71, 1343 (1967).
(2) F. Tokiwa and T. Isemura, Bull. Chem. SOC.Jap., 35, 1737 (1962). (3) F. Tokiwa, ibid., 36, 222 (1963). (4) F. Tokiwa, ibid., 37, 1837 (1964). (5) H. Sohott, J . Phys. Chem., 68, 3618 (1964). (6) H. Schott, ibid., 70, 2966 (1966). (7) K. Nagase and K. Sakaguchi, Kogyo Kagaku Zasshi, 64, 635 (1961). (8) K. Hattori and K. Konishi, ibid., 64, 1195 (1961).
1215
SOLUBILIZATION' BEHAVIOR OF SODIUM DODECYLPOLYOXYETHYLENE SULFATES according to the method of Dreger, et aL9 The samples were purified by repeated recrystallization from isopropyl alcohol, followed by extraction with petroleum ether. Yellow OB (li-o-tolyl-azo-2-naphthylamine), the water-insoluble dye used for solubilization experiments, was a particularly good commercial product obtained from Wako Pure Chemicals Co. The sample was purified by repeated recrystallization from a mixed solvent of ethanol-water and was dried under vacuum. Dodecane was of analytical reagent grade. Polyethylene glycol, the molecular weight of which was approximately 400, was obtained from Sanyo Yushi Kogyo Co. Both dodecane and polyethylene glycol were redistilled before use. Solubilization. Solubilization runs were made in a water bath at 25" for 48 hr to attain equilibrium, the method described in a previous paper being employed.a Excess Yellow OB was added either to surfactant solutions of the correct concentration or to solutions which were several times too concentrated, which were later diluted with water or with sodium chloride solution. Agreement between solubilization data obtained from the side of supersaturation and those obtained by starting with the solid dye indicates that both represent equilibrium values. Excess dye was removed by filtration through a glass filter of Shibata Kagaku G-4. Removal of suspended dye was complete because raising the surfactant concentration of filtered samples by adding solid surfactant did not increase the absorbancy. The amounts of the solubilized dye were determined by optical density measurements at a wavelength of 445 mP. Absorption Spectra. Spectral measurements were performed with a Shimadzu Model QV-50 spectrophotometer at room temperature (about 25").
Results and Discussion Typical results for solubilization in aqueous solutions of SDPS with p from 0 to 10 are shown in Figure 1, in which the amounts of solubilized Yellow OB, S , are plotted against the concentrations of the surfactant, C. The S os. C curves for DPOE and SAS are similar to those for SDPS Eihown in Figure 1and are not illustrated here. For all OF the surfactants examined, the S vs. C curves are linear in the region of relatively low concentrations, although they are slightly curved at higher concentrations. The solubilizing power of each surfactant can, therefore, be obtained from the slope of the linear portion of the curve. I n Figure 2, the solubilizing powers of SDPS in water and 0.1 M NaCl solution and of DPOE and SAS in water are plotted against the number of oxyethylene or methylene units. The solubilization of the dye per mole of DPOE is large and almost constant over the range of p examined, while the amounts solubilized by SDPS and SAS are smaller and increase with increasing number of oxyethylene
-
SDPS IO
0
0.6
2.0
1.0 1.5 10%C, M .
Figure 1. The amount of solubilized Yellow OB, S, plotted against the concentration, C, of SDPS with different numbers of oxyethylene units in water at 25'. The number written after SDPS represents the approximate average number of oxyethylene units per molecule.
0
5
10
15
20
No. of oxyethylene or methylene units.
.Figure 2. Solubilizing powers of SDPS, DPOE, and SAS for Yellow OB plotted against the number of oxyethylene or methylene units: 0, SDPS in water; 6, SDPS in 0.1 M NaCI; 0, DPOE in water; 8, SAS in water.
units or methylene units in the molecule. Both SDPS and DPOE compounds exhibit a greater solubilization than SAS compounds with no polyoxyethylene chain, indicating that the polyoxyethylene portion in the molecule plays an important role in solubilization. Figure 3 shows the spectra of Yellow OB in dodecane and polyethylene glycol, as well as in the aqueous solutions of SDPS with a p of 5, DPOE with a p of 10, and SAS (sodium dodecyl sulfate). Aside from the shape of the spectra, the wavelength of the maximum absorpin the solution of SAS cortion of Yellow OB, A, responds closely to the Am, in dodecane, suggesting that the solubilization of Yellow OB occurs mainly in the hydrocarbon region of the SAS micelle. On the other hand, the spectra of Yellow OB in the solutions of SDPS and DPOE are rather similar to the spectrum in polyethylene glycol. This suggests that in the case of SDPS and DPOE the solubilization of the dye also occurs in the polyoxyethylene region of the micelle and, further, that the portion of the dye solubilized in the (9) E. E. Dreger, G. I. Keim, G. D. Miles, L. Shedlovsky, and J. Ross, Ind. Eng. Chem., 36, 610 (1944).
Volume 78, Number Q April 1968
1216
FUMIKATSU TOKIWA I
I
I
I
1
1
I 0
I
I
f
5 10 No. of oxyethylene units,
Figure 4. The number of solubilized dye molecules per micelle (0)and the aggregation number of the micelle of SDPS ( 0 )in 0.1 M NaCl solution plotted against the number of oxyethylene units. The aggregation numbers were taken from Table I of ref 1. 20
75 400
425
450 475 Wavelength, mp.
500
525
9 *-
50
Figure 3. Absorption spectra of Yellow OB in dodecane (U) and polyethylene glycol (---@-), and in the aqueous solutions of: SDPS with a p of 5 (--U-), DPOE with a p of 10 (-X-), and sodium dodecyl sulfate (-A-). 5
polyoxyethylene region is larger than that in the hydrocarbon region. Now, one has to explain the reason why the solubilization by DPOE is almost constant while that by SDPS increases with increasing oxyethylene units. The following explanation seems plausible for this. The polyoxyethylene chains of DPOE in the micelle are not highly extended, as compared with those of SDPS, but the locus of solubilization is the hydrocarbon interior of the micelle and the portion of the polyoxyethylene shell near that hydrocarbon core. Then, after a certain number of oxyethylene units are present, further increase in p will have no further effect on the solubilization process. The much smaller solubilization shown by SDPS micelles of equal p would then be a result of the extension of the polyoxyethylene chains because of the repulsion of their attached charges. This extension would make the oxyethylene units less available for interaction with the polar part of the dye molecule. The extension may be expected to relax somewhat as the number of oxyethylene units is increased. The less extended polyoxyethylene chains of SDPS will favor a greater solubilization and, therefore, in this case the degree of solubilization increases with increasing p . Addition of sodium chloride will cause reduction of the repulsion between charged heads at the surface of the SDPS micelle, which permits compaction of the polyoxyethylene portion of the micelle and makes more oxyethylene units available to the dye, and thus promote solubilization, as seen in Figure 2. Solubilization is an effect which begins to be noticeable at the critical micelle concentration (cmc). I n The Journal of Physical Chemistry
10 15 20 No. of oxyethylene units.
Figure 5. The number of solubilized dye molecules per micelle (0)and the aggregation number of the micelle of DPOE (46) in water plotted against the number of oxyethylene .units. The aggregation numbers were taken from Table I of ref 2. Table I : Critical Micelle Concentration Values of SDPS from Solubilization Data at 25’ ---cmo
Samples
SDPS-0 SDPS-1 SDPS-3 SDPS-5 SDPS-10
values, 1O-amo1/1.----
In water
7.8 4.6
2.8 1.9 1.3
In 0.1 M NaCl
1.6 0.60 0.25 0.20 0.15
Table I are summarized the cmc values of SDPS with different p in water and in 0.1 M NaCl solution at 25” which were taken from sharp breaks in the S vs. C curves. The cmc value of SDPS decreases with increasing oxyethylene content. This result is in contrast to the result for DPOE in which the cmc value increases with oxyethylene ~ontent.~JOLengthening of the polyoxyethylene chain in the SDPS molecule corresponds in a sense to that of the hydrocarbon chain, although the former effect is not so marked as the latter one. This fact is probably related to the interesting (10) K. Shinoda, et al., “Colloidal Surfactants,” Academic Press Inc., New York, N. Y., 1963,pp 98-112.
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A2BzAROMATIC PROTON NMRSPECTRA solubi1iz:btion behavior of SDPS, that is, the solubilizing capacity increases with increasing polyoxyethylene chain length. Combining previous data on the mmw of SDPS’ and DPOE2 with their solubilizing capacities for Yellow OB, we obtain the numbers of solubilized dye molecules per micelle, R, by the following relation, under the assumption that the solubilized dye does not change the size of the micelles.6
X
rnmw
R=-
s
= A (1) C - cmc where m is the rnolecular weight of the surfactant,A is the aggregation number of the micelle, and the quantity of S/(C - cmc) iei equivalent to the slope of the linear portion of the X vs. C curve. I n Figures 4 and 5, the values of R for SDPS and DPOE are plotted against the number of oxyethylene units, together with their aggregation numbers. Similar plots for SAS will also be obtained if solubilization data are combined with available mmw values of SAS.11r12 Recently, Schott5s6has postulated that only one dye molecule of Orange OT issolubilized per m
C
- cmc
micelle. If thissolubilization limit is general, the R value of about unity would be expected in the present experiment, since Yellow OB is almost equivalent to Orange OT (1-o-tolyl-azo-2-naphthol) in molecular weight and chemical structure. The R values obtained from eq 1 are 1.0-3.5 for SDPS with p from 0 to 10, and 1.2-5 for DPOE with p from 7 to 20. These results would suggest that the hypothesis that one dye molecule saturates a micelle is not general but only valid for some limited surfactants.6
Acknowledgments. The author expresses his thanks to Professor E. Hutchinson of Stanford University for his helpful discussion, to Dr. P. Becher of Atlas Chemical Industries for his valuable suggestion, and to Dr. H. Kita, the Director of the Research Laboratories, for his encouragement and permission to publish this paper. (11) H. V. Tarter and A. L. M. Lelong, J . Phys. Chem., 59, 1185 (1955). (12) E. Hutchinson and J. C. Melrose, 2.Phys. Chem. (Frankfurt), 2, 363 (1954).
The A2B2Aromatic Proton Nuclear Magnetic Resonance Spectra of para-Substituted Anilines, Diphenylamines, and Triphenylaminesl by Robert D. Allendoerfer,2Griffith Smith,2and Robert I. Walter3 Department of Chemistry, Haverford College, Haverford, Pennsylvania
19041
(Received September 6 , 1967)
The aromatic proton nmr spectra of 4-substituted anilines, 4,4’-disubstituted diphenylamines, and 4,4’,4”trisubstituted triphenylamines with the substituents I, Br, COCH,, SO3-, CHs, and OCHZ have been analyzed to determine optimum values for the chemical shifts and coupling constants. These parameters display the regularities to be expected with increasing arylation of the amine nitrogen and provide evidence for anisotropic ring field effects exerted on the ortho protons by adjacent rings. They also suggest that the electron density measured by the nmr experiment is higher at the protons meta to the amino group than at those ortho to it, in both di-p-anisylamine and tri-p-anisylamine. Some problems in the precise analysis of spectra of this type are discussed. The AtBz proton nrnr spectra of a large number of para-disubstitu.ted benzene derivatives have been reported, together with the chemical shifts assigned by analysis of these ~ p e c t r a . ~The effects on the AzB& spectrum of accumulation of phenyl groups on one central atom have been investigated for the diphenylcarbonium and! triphenylcarbonium ions, 5 but no systems in which substituents are present in the aromatic rings have been studied. We report here the results of an nmr study of a series of 4-substituted anilines, 4,4’-disubstitutjed diphenylamines, and 4,4’,4’’-tri-
substituted triphenylamines in which the same substituents are carried through the series. This work was originally undertaken in order to determine the struc(1) This work has been supported in part by Grant GM-10605 from the National Institutes of Health to Haverford College. (2) Summer research student supported by the National Science Foundation Undergraduate Research Participation Program. (3) Inquiries should be directed to this author. (4) Data on these systems have been summarized by G, W. Smith, J. M o l . Spectry., 12, 146 (1946). ( 5 ) G. A. Olah, J . Am. Chem. Soc., 86, 932 (1964); D.G.Farnum, ibid., 86,934 (1964).
Volume 72, Number 4 April 1968
