J. Phys. Chem. 1985,89, 21 10-21 13
2110
Effects of Polyoxyethylene Chaln Length on Micellar Structure Yoshihiro Saitb and Takatoshi Sato* Department of Pharmacy, College of Science and Technology, Nihon University, 8, Kanda Surugadai, I - Chome, Chiyoda- ku, Tokyo, Japan (Received: September 11, 1984; In Final Form: November 26, 1984)
The correlation between micellar structure and micellar inner polarity of nonionic surfactants is investigated. The micellar structure has been determined with low-angle light-scattering and viscosity methods. The micellar inner polarity measurement is made by a comparison of keto-enol tautomerism using benzoylacetaanilide. It is shown that the aggregation number decreases while the hydration and micellar volume increase with an increase in the polyoxyethylene chain length. Also, a decrease in the enolic absorption occurs as the polyoxyethylene chain length increases. Interpretation of these results is discussed.
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Introduction Knowledge of the micellar structure is of fundamental importance in understanding catalysis in micellar systems.' In a recent paper,2 we have reported on the effects of micellar structure on the oxidation stability of p-isopropylbenzaldehyde in aqueous solutions of nonionic surfactants. The results showed that the oxidation stability of pisopropylbenzaldehyde is affected by the micellar aggregation numbers. Several papers on the micellar structure of nonionic surfactants including the size, shape, and hydration of the micelle in relation to the polyoxyethylene chain length have been presented and the polyoxyethylene chain lengths are considered to be one of the factors that affect the micellar s t r ~ c t u r e . ~ However, water-micelle interactions (i.e., the depth to which water penetrates into the hydrocarbon portion of the micelle) are still not clearly understood, especially in nonionic surfactant^.^^^ Here, we are reporting on the correlation between micellar structure and micellar inner polarity.
Experimental Section Materials. The nonionic surfactants polyoxyethylene cetyl ethers (PCE-n, n = 20, 30,40, when n is the average number of ethylene oxide groups) and polyoxyethylene monostearates (PMS-n, n = 25, 45, 55) were obtained from Nikko Chemicals Co., Ltd. (Tokyo, Japan). Purification of the nonionic surfactants was carried out by an extraction method6 using water-saturated 1-butanol and 1-butanol saturated water. It was confirmed by testing with thin-layer chromatography that the purified samples contained no detectable free polyethylene glycol. Benzoylacetoanilide (BZAA), obtained from Tokyo Kasei Co., Ltd. (Tokyo, Japan), was extrapure grade and was used without further purification. The water used in this experimental was double-distilled ionexchanged water. Method. Light scattering was measured with a low-angle laser (He-Ne, 633 nm) light-scattering photometer (LS), Model LS-8, from Toyo Soda Co., Ltd. (Tokyo, Japan). Specific refractive index increments were measured with a precision differential refractometer (RI), Model SE-11,from Showa Denko Co., Ltd. (Tokyo, Japan). The procedure for determining the micellar weight of the nonionic surfactants using LS and RI has been described in detail.* Viscosities were measured by a CannonFenske viscometer. Densities were determined by picnometry. The above experiments were measured at 25 f 0.01 OC. The absorption spectra of the keto-enol tautomers in aqueous solutions (1) J. H. Fendler, and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press,New York, 1975. (2) T. Sato, Y. Saito, and I. Anazawa, Yukaguku, 32, 215 (1983). (3) P. Becher, J. Colloid Sci., 16, 49 (1961). (4) F. M. Menger, J. M. Jerkunica, and J. C. Johnston, J. Am. Chem. Soc.,
100, 4676 (1978). ( 5 ) H. WennerstrBm and €3. Lindman, J . Phys. Chem., 83, 2931 (1979). (6) K. Nagase and K.Sakaguchi, Nippon Kagaku Kaishi, 64,635 (1961).
0022-3654/85/2089-2110%01.50/0
of nonionic surfactants were measured by the Shoji et al. method' using a recording spectrophotometer, Model UVIDEC- 1, from Nihon Bunkb, Ltd. (Tokyo, Japan). Solubility determinations were made by shaking excess BZAA in various concentrations of nonionic surfactants for 2 days a t a constant temperature, 25 f 0.01 OC. After sample solutions were pipetted through filters (0.22 pm), BZAA was extracted with chloroform and assayed spectrophotometrically.
Results and Discussion As described in a previous paper,2 eq 1 is applied to the
light-scattering data, where hEI and h i y are the outputs of the refractometer at the concentration of the nonionic surfactants and the cmc, respectively. h& and h C p are the outputs of the light-scattering photometer at the concentration of the nonionic surfactants and the cmc, respectively. The output from the light-scattering photometer was recorded on a dual-pen recorder together with that from the refractometer. The constant, K, is an instrument constant which can be determined from measurements using standard polymers of known molecular weight. (C - C,,,) is the micellar concentration in g/mL and B is the second virial coefficient as given by the slope of eq 1. The light-scattering results are shown in Figure 1 as plots of The micellar K(hgI - hcp)/(h& - h C e ) against (C - Ccmc). weights (M,) of the nonionic surfactants have been determined by extrapolating K(h& - hCe)/(h& - hCW) to zero micellar concentration, the intercept being equal to l/Mmaccording to eq 1. The second virial coefficient shows .a significant increase in value with an increase in the polyoxyethylene chain length. This shows that the water-micelle interaction is greater at a longer polyoxyethylene chain length. In case the micellar weight is below 100000, it generally is believed that the micellar shape is spherical.* We investigated the micellar shape and volume of the nonionic surfactants by the Guth and Simhas equation9 (eq 2 and 3), applying it to the qrel =
1 + 2.54
+ 14.1@
v = (#J/c
(2) (3)
dispersion system of spherical particles, where qrelis the relative viscosity, 4 is the volume fraction occupied by the particles, C is the concentration of the nonionic surfactants in g/mL, and V (7) N. Shoji, M. Ueno, and K. Meguro, J. Am. Oil Chem. SOC.,53, 165 (1976). (8) M. J. Schick, S. M. Atlas, and F. R. Eirich, J . Phys. Chem., 66, 1326 (1962). (9) D. C. Robins and I. L. Thomas, J. Colloid Interface Sci., 26, 415 (1968).
0 1985 American Chemical Society
Effects of Chain Length on Micellar Structure
X h
E
2'o-
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uv)
4 '
0 :P M S - 2 5
0 :P C E - 2 0
0:PMS-45
0 :PCE-30
--
TABLE I: Light-Scattering and Hydrodynamic Results
surfactants
v
7
m
\ 4
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 2111
2 -
, -
0 : PCE-20
0 : PMS-25
>
0 : PMS-45
0 : PCE-30
0 : PMS-55
0 : PCE-40
1
I
I
1
PMS-25 PMS-45 PMS-55 PCE-20 PCE-30 PCE-40
10-94, 122.5 112.2 85:2 121.5 114.3 113.8
A, 89 50 32 108 73 57
1o4vh,A'
W, g
44.8 52.2 63.1 42.8 51.6 64.3
1.20 1.80 2.78 1.11 1.71 2.39
where W is expressed in grams of water per gram of the nonionic surfactant, pw is the density of the solvent, and P is the specific volume of the nonionic surfactant. Table I shows light-scattering and hydrodynamic results. Light-scattering trends are those expected for nonionic surfactant^,^^^^^ Le., the longer the polyoxyethylene chain, the lower the micellar molecular weight and aggregation numbers (A"). Increasing the polyoxyethylene chain length increases the aqueous solubility of the monomer and, therefore, decreases the driving force leading to micellization.
From the hydrodynamic results, it was shown that the micellar volume and hydration increased with an increase in the polyoxyethylene chain length, since for spherical micelles the space available for trapping water increases with polyoxyethylene chain length.14 More recently, El Eini and @workers reported the same results for the micellar size, shape, and hydration of long-chain polyoxyethylene nonionic surfactant.15 From these results, it was thought that water-micelle interactions increased with the polyoxyethylene chain length of the nonionic surfactants. Watermicelle interactions have been studied By thermodynamic and spectroscopic t e c h n i q ~ e s . ~We , ~ applied them to determine the polarity of the microenvironment by a comparison of keto-enol tautomerism in the micelle using BZAA. It is known that BZAA dissolves in aqueous solutions of nonionic surfactants easily, enolizing in the core of the micelle without enolization in polyoxyethylene chain length on the micellar surface.' Therefore, BZAA is expected to offer information on the micellar inner polarity. Changes in the absorption spectra in aqueous solutions of nonionic surfactants containing 10 mg/L of BZAA are shown in Figure 3. The concentration of the nonionic surfactants is 5 X lo-' mol/L. The absorption maxima around 240 and 310 nm are assigned to the ketonic and enolic forms, respectively.16 A decrease in the enolic absorption occurs as the polyoxyethylene chain length increases. This phenomenon suggests an increase in the micellar inner polarity with an increase in the polyoxyethylene chain length.' Figures 4 and 5 show the solubilizing efficiency of the nonionic surfactants for BZAA. The solubilization of BZAA per mole of nonionic surfactants increases with an increase in the polyoxyethylene chain length. However, when the solubilizing power of the nonionic surfactants is expressed in terms of BZAA per ethylene oxide equivalent, the efficiency of solubilization decreases with an increase in polyoxyethylene chain length. From Figures 3-5, the degree of the enolic absorption is thought to be due to micellar inner polarity. From the results of this study, it was concluded that the micellar
(10) H. Schott, J. Colloid Interface Sci., 24, 193 (1967). (11) P. Becher and H. Arai, J. Colloid Interface Sei.,27, 634 (1968). (12) B. W. Barry and D. I. D. El Eini, J. Colloid Interface Sei.,54, 339 (1976). (13) D. I. D. El Eini, B. W. Barry, and C. T.Rhodes, J. Pharm.Pharmcol., 25, 166P (1973).
(14) P. H. Elworthy and C. B. Macfarlane, J. Chem.Soc.,537 (1962). (15) D. I. D. El Eini, B.W. Barry, and C. T. Rhodes,J. Colloid Interface Sei.,54, 348 (1976). (16) R.A. Morton, Ali Hassan, and T. C. Calloway, J. Chem.Soc.,883 (1934).
0
1
2
PMS-n c o n c n , %
0
1
2
PCE-n c o n c n , %
Figure 2. Plots of effective specific volume vs. concentration of nonionic
surfactants.
is the effective specific volume of the nonionic surfactants including hydrated water. From the Figure 2 results, the graphs are linear because all micelles of nonionic surfactants are probably ~ p h e r i c a l . Also, ~ Vincreased with an increase in the polyoxyethylene chain length. The relation between the intrinsic viscosity and the micellar volume is given bylo NVV h [VI = (4) 1OOM, where [v] is the intrinsic viscosity, N is Avo adro's number, V, is the micellar volume including hydration and u is a shape factor equal to 2.5 for spheres. Thus, the extent of hydration of the micelle can be given by"
(I3),
2112
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
Saita and Sate
0.6
*00.4 1 (r
0 J 0
n
0.2
0
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32 0
240
280
3 20
Wave l e n g t h , n m
2 80
240
Wave l e n g t h , n m
Figure 3. Absorption spectra of BZAA in aqueous solutions of nonionic surfactants.
0
0 : PMS-25
0 : PCE-20
@ : PMS-25
o
0 : PUS-45
0 : PCE-30
0 :PCE-30
0 : PUS-55
0 : PCE-40
0 : PMS-45 :P M S - 5 5
,-I 2
4
6
PMS-n concn, m o l e / l X103
0
2
4
6
Figure 4. Solubilization of BZAA in aqueous solutions of nonionic surfactants.
inner polarity increases with the polyoxyethylene chain length of nonionic surfactants. The factors affecting it could be as follows: ( I ) increase in water penetration within the micelles owing to a decrease in the aggregation number and increase in the micellar volume; (2) increase in the water-micelle interaction as showed by the increases of the hydration number and the second virial coefficient. That is to say, the increase in polyoxyethylenechain length leads to a decreased in the compactness of the micelle which causes an increase in the micellar inner polarity.
0.1
0
PCE-n c m c n , m o l e / l U03
0.2
0.3
Ethylene oxide e q u i v . / l
0
:PCE-20
0.1
0.2
0.:
Ethylene oxide e q u i v . / l
Figure 5. Solubilization of BZAA in aqueous solutions of nonionic surfactants. Summary
In conclusion, we have examined the correlation between micellar structure and micellar inner polarity. We found that the aggregation number decreased while the hydration and micellar volume increased with an increase in the polyoxyethylene chain length. Also, we showed that the micellar inner polarity increased with an increase in the polyoxyethylene chain length. Registry No. BZAA, 959-66-0; PCE-n,9004-95-9;PMS-n, 900499-3.
