Langmuir 1992,8, 2147-2151
2147
Micelle Formation by Pure Nonionic Surfactants and Their Mixtures Masahiko Abe,*ftJHirotaka Uchiyama,t Tomohiro Yamaguchi,? Tomoko Suzuki,t and Keizo OginotJ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan, and Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan
John F. Scamehorn and Sherril D. Christian Institute for Applied Surfactant Research, The University of Oklahoma, Norman, Oklahoma 73019 Received December 5, 1991. In Final Form: May 27,1992 Micelle formation by pure nonionic surfactants and their mixtures has been investigated with static and dynamic light scattering and with fluorescence probe measurements. The surfactants used in this study are alkyl poly(oxyethy1ene)ethers (C,POE,; where m = 12,14, and 16, at n = 20; n = 10,20,30, and 40, at m = 16). In the single component nonionic surfactant system,the hydrodynamic micellar size increases with an increase in alkyl or poly(oxyethy1ene)chain lengths. The aggregationnumber of single component surfactant micelles increases with increasing alkyl chain length or with decreasing number of oxyethylene groups. Moreover, the micropolarity in the micelle decreases as the length of the alkyl group increases or that of the oxyethylene group decreases. In the case of the C190E10419OE40 mixed surfactant system, at 0.5 mole fraction C19OEa at fixed concentration above the critical micelle concentration, the molecular weight of the mixed micelle shows a maximum and the concentration of micelles exhibits a minimum. The hydrodynamic micellar size in the mixed system increases with an increase in the mole fraction of C1aOEa and remains constant above a mole fraction of 0.5. The micropolarity in the micelle increases with an increase in the mole fraction of C16POE40. The hydrophilic part of the mixed micelle seems to be compact due to the penetration of ClsPOElo and C18OEN monomers into the poly(oxyethylene) shell of the micelle. Introduction Nonionic surfactants are widely used as solubilizers and emulsifiers. Almost all commercial nonionic surfactants are polydisperse preparations containing a distribution of poly(oxyethy1ene) chain lengths. Only recently have synthetic, monodisperse substances, incorporating a single poly(oxyethy1ene) chain, become readily available. However, these are only available for relatively short poly(oxyethylene) hydrophilic groups. There have been few studies of micelle formation by either monodisperse or polydisperse nonionic surfactants having long poly(oxyethylene) chain lengths. Most surfactants used in practical applications are mixtures. Hence, understanding both the structure and properties of mixed micelles containing nonionic surfactants is essential for many industrial uses of surfactants. Mixed surfactant systems are also of great theoretical interest in their own right. In solutions containing mixtures of surfactants, the tendency to form aggregates is substantially different from that in solutions of pure surfactants. Many recent papers and several books1t2have been published regarding the solution properties of mixed surfactant systems. In our own previous studies of mixed micelle formation in the anionic-nonionic mixed surfactant systems:-' we
* To whom correspondenceshould be addressed at the Faculty of
Science and Technology. t Faculty of Science and Technology. t Institute of Colloid and Interface Science.
(1)Ogiuo,K., Abe,M., Ede.MimdSurfactant Systems;MarcelDekker:
New YGk, in press. (2) Scamehorn,J. F.,Ed. Phenomena in Mixed Surfactant Systems; ACS Symposium Series 311;American Chemical Society: Washington, DC. 1986.
0743-746319212408-2147$03.0010
have found that mixed micelle structures and properties can vary substantially with changes in the molecular characteristics of the nonionic surfactant; particularly large variations in behavior occur as a function of poly(oxyethylene) chain length. In this paper, we describe the solution properties of several poly(oxyethy1ene) ethers (C,POE,; m = 12, 14, and 16, a t n = 20; n = 10,20,30, and 40, a t m = 16). We report information about the micellar size, aggregation number, and micropolarity of these pure surfactants and their mixtures. Experimental Section Materials. Nonionic surfactants (C,POE,; m = 12,14, and 16, at n = 20; n = 10,20, 30, and 40, at m = 16) were supplied by Nihon Surfactant Industries Co., Ltd.,Tokyo. These compounds have a narrow molecular weight distribution;their purity was determined by surfacetension measurements. The measured critical micelle concentration (cmc) value^^^^ are ClzPOEm, 1.5 X lo-' M; ClJ?OEm, 2.3 X lo+ M; CisPOElo, 7.5 X 1od M; CisPOEN,1.7 X lo-" M; CizOEm, 1.2 X 1V M and CiePOEa, 7.0 X 1o-BM. Pyrene used as a fluorescenceprobe was purchased from Wako Pure Chemical IndustriesCo., Ltd.,Tokyo. It was recrystallized three times from ethanol, dissolved in cyclohexane, and passed (3)Abe, M.; Kakihara, T.; Uchiyama, H.; Ogino, K. J. Jpn. Oil Chem.
Soc. 1987. - - - - ,36. 31. - - I
(4)Ogino, K.; Kakiiara, T.; Uchiyama, H.; Abe, M. J. Am. Oil Chem. SOC.1988,65,405. ( 5 ) Ogino, K.; Uchiyama, H.; Kakihara, T.; Abe, M. In Surfactant in Solution; Mittal, K. L., Ed.;Plenum Preea: New York, 1989;Vol. 7, pp .. 413-429. (6) Abe, M.; Tsubaki, N.; Ogino, K. J. Colloid Interface Sci. 1985,107,
---. SnR
(7) Ogino, K.; Tsubaki, N.;Abe, M. J. Colloid Interface Sci. 1986,107,
509.
0 1992 American Chemical Society
Abe et al.
2148 Langmuir, Vol. 8, No. 9, 1992 through silica gel. Dodecylpyridinium chloride ((DP)Cl), used as a fluorescence quencher, was obtained from Tokyo Kasei Kogyo Co., Ltd., Tokyo;the compound was recrystallizedthree times from ethyl acetate. Water used in this experiment was distilled water for injection JP (Japanese Pharmacopoeia), obtained from Otsuka Pharmacy Co., Ltd., Tokyo. Measurements. All experiments were performed at 30 OC. The micelle molecular weights, aggregation numbers of micelles, and hydrodynamic sizes of nonionic surfactanta were measured with a submicron particle analyzer (4700 type of Malvern Instruments Ltd., Worcestershire, England). The optical source on the light scattering apparatus was an argon ion laser operating at 488.0 nm with an output power of 5 W maximum (Innova 90 of Coherent Co.). The average scattered intensity at a scattering angle of 90° was measured to determine the micelle molecular weight and aggregation number for micelles(static light scattering method). The measurement of the time-dependent correlation function of the scattered intensity at severalangleswas performed to determine the change of micellar diameter with concentration of surfactant (dynamic light scattering method). The data analysis was performed by the combined use of the cumulant method and the model free algorithm? Before measurement, the aqueous solution of surfactant as a sample was passed three times through the membrane filter of O.l-clm pore size (cellulose nitrate type of Toyo Roehi Co., Ltd., Tokyo) for optical purification. The refractive index measurements required in the micelle molecular weight and aggregation number determinations were obtained with a differential refractometer (RM-102 of Otauka Electronics Co., Ltd., Osaka, Japan). The reduced intensity of light scattered, R,is given by
where IO and I are the measured intensities of incident and scattered light, and no and nb the refractive indices of water and benzene, respectively. The calibration constant of the apparatus for benzene, 4b, was determined by the value of the reduced intensity of light scattered from benzene: 3.259 X 1od cm-l. Light scattered from a dilute micellar solution at the given concentration, C , is described by Debye's equation
!zz?l 00
0.5
1.0
1.5
Concentration of C,,POE,
2.0
(g~cm') x10'
Figure 1. Mutual diffusion coefficient as a function of the concentration of nonionic surfactant. (6)
where kB is the Boltzmann constant, 5" the absolute temperature, and qo the viscosity of water. To determine the aggregation number of micelles by the fluorescence probe technique, the fluorescenceemission spectra of pyrene monomers in the surfactant solution were measured with a fluorescence spectrophotometer (Shimadzu Co., model RF-540; the excitation wavelength is 335 nm). Each spectrum haa five predominant vibronic peaks, numbered 1-5 from the shorter to the longer wavelength. The fluorescence intensities of pyrene were monitored at 395 nm (the f i peak). Before measurement, all the solutions were deoxygenated by freezepumpthaw cycles. According to Turro and Yakta,'O the ratio of the fluorescence intensity with or without (DP)Cl can be expressed by the very simple equation (7) where I and IOare the fluorescence intensities with (DP)Cl and without (DPIC1, respectively. C e is the (DP)Cl concentration, and CMis the surfactant concentration in micellar form existing in the solution. The aggregation number is calculated from the following formula:
N=- c-c,, "
(8)
where M is the average molecular weight of micelles and B is the second virial coefficient. Ro is the reduced scattering intensity for the solution at the cmc, CO,which is substantially equal to the reduced scattering intensity of water. K is the opticalconstant derived from eq 3, where anlac is the specific refractive index
As the ratio Il/Is, which is the ratio of the intensities of the first (376 nm) to the third (386nm) peaks, is well known to be almost proportional to the polarity in the region near the pyrene molecule solubilized in the micelles, in other words, the ratio 11/13decreases with an increase in the hydrophobic environment, the micropolarity in the micelle was monitored by measuring the ratio 11/Z3.11J2
(3)
Results and Discussion SolutionProperties of Single Surfactant Systems. Figure 1 shows the relationship between the diffusion coefficient of the nonionic micelle and surfactant concentration. The diffusion coefficient of the ClsPOElo micelle is nearly independent of surfactant concentration. On the other hand, that of the CISPOEMmicelle increases slightly with an increase in surfactant concentration. Even ClsPOElo and ClePOEa micelles, which have no electrical charge, have positive hydrodynamic virial coefficients. That is, an excluded volume effect can contribute to the diffusion coefficient increase, because increasing the concentration of surfactant increases the volume of micelles in the solution. The effect of excluded volume on the diffusion coefficient increases with increasing poly-
increment of a solution, N Ais Avogadro's number, and A is the wavelength of incident light. The aggregation number is calculated with the following equation: c
N = M/M,
(4)
where M, is the molecular weight of the surfactant. The mutual diffusion coefficient, D , for a dilute micellar solution at the given concentration can be expressed as D = Do(l + k,(C - C,))
(5)
where DOis the translational diffusion coefficient and kD is the hydrodynamic virial coefficient. The translational diffusion coefficient is related to the hydrodynamic radius, RH, by the Einstein-Stokes equation (8) Beme,B. J.;Pecora,R.DynumicLightScattering;Wiley-Interface: New York, 1976; p 195. (9) Gulari, E.; Chu, B. Biopolymers 1979,18, 2943.
(10) Two,N. J.; Yekta, A. J. Am. Chem. SOC.1978,100,6961. (11) Kalyanasundaram,K.;Thomas,J. K. J. Am. Chem. SOC.1977,99, 2039. (12) Two,N. J.; Kuo, P. L.; Somasundaran, P.; Wong, K. J. Phye. Chem. 1986,90, 288. ~~
Langmuir, Vol. 8, No. 9, 1992 2149
Micelle Formation by Nonionic Surfactants
I
POE chain length 10
20
1 ‘?
I 10
I
I
I
q
120
; 1.0x1o-”ol/L
~
(FPM)
20
10
30
40
1 ,
POE chain length
12
14
Figure 4. Effect of poly(oxyethy1ene) chain length on the
16
aggregation number for single surfactant (ClePOE,) systems.
Alkyl chain length
Figure 2. Effects of alkyl chain length and/or poly(oxyethy1ene) chain length on the micellar size at the cmc.
2
; I .Ox 10’2mol/L(FPM) 1 0 0 -. o : cmc(SLS)
5
od
2
4
6
8
1’0
1’2
Concentration of C,,POE, (molll) x103
40
t
1
12
13
14
15
16
Alkyl chain length
Figure 3. Effect of alkyl chain length on the aggregationnumber for single surfactant (C,POEm) systems.
(oxyethylene) chain length at the higher surfactant concentrations. Figure 2 shows the dependence of the hydrodynamic micellar size calculated by eq 5 on the alkyl and poly(oxyethy1ene) chain lengths. Those hydrodynamic micellar sizes were obtained by extrapolating to the cmc. The micellar size at the cmc increases with increasing alkyl and poly(oxyethy1ene) chain lengths. Figure 3 shows the dependence of micelle aggregation numbers on alkyl chain length in the nonionic surfactants. The aggregation number could not be measured by the static light scattering method (SLS) at the higher concentrations above the cmc, because the Debye plot based on eq 5 gives molecular weight from the intercept and virial coefficient from the slope. It was, however, possible to measure the aggregation number by the fluorescence probe method (FPM) at these concentrations. The open circules in Figure 3 indicate aggregation numbers obtained by the static light scattering method at the cmc. Values of the aggregation number obtained by the fluorescence probe method at 1.0 X mol/L (above the cmc) are plotted as solid circles. As can be seen in Figure 3,the aggregation numbers inferred at both concentrations increase with increasing alkyl chain length of the nonionic surfactants. Figure 4 depicts the effects of poly(oxyethy1ene) chain length on the aggregation of the nonionic Surfactants. Values of the aggregation numbers, obtained at the cmc
Figure 5. Relationship between the aggregation number and the concentration of ClaOE, for single surfactant systems. by the light scattering method and at 1.0 X mol/L by the fluorescenceprobe method, decrease with an increasing number of ethylene oxide groups in C,POE,. A similar variation of the aggregation number of nonionic surfactants was observed by Tanford et d.13and Imae.14J6 In summary, as the hydrophobic character of the nonionic surfactants increases, the micelle aggregation number increases. Either increasing the alkyl chain length or decreasing the poly(oxyethy1ene) chain length contributes to both an enhanced hydrophobicity and an increase in aggregation number. The aggregation numbers obtained with the fluorescence probe method at various concentrations are shown in Figure 5. The aggregation numbers of both the C16POE1o and C16POEm micelles increase with an increase in surfactant concentration. The aggregation number curves are extrapolated to the cmc, and the aggregation numbers of C16POElo and C16POE40 at the cmc are shown in Figure 4 (double open circle). As can be seen from Figure 4,the aggregation numbers inferred with the light scattering method are considerably larger than those obtained by the fluorescence method at the cmc. The discrepancy in the aggregation number at the cmc is larger for ClaPOElo than for C16POE40; Le., the greater the hydrophobicity, the larger the differences between the aggregationnumbers determined by the two methods. Here, the micelle aggregation number obtained by light scattering is based on a weight-average value, and that based on the fluorescence method is a number-average value. It seems reasonable to conclude that the polydispersity of the mi(13) Tanford, C.; Nozaki, Y.; Rohdo, M. F. J. Phys. Chem. 1977.81, 1555. (14) Imae, T. J. Phys. Chem. 1988,92, 5721. (15) Imae, T. J. Colloid Interface Sci. 1989, 127, 256.
Abe et al.
2150 Langmuir, Vol. 8, No.9, 1992 Alkyl chain length
q
1.20, +
f
l
l 101
-$ Y
'I f
" 1 /: 1 1.10
1.0s-
A ;
10
20
I
I
m
I1
I
9t
/.
8jj
6b
Oi2
014
016
0:s
/
Mole fraction of C, ,POE,,,
cmc I.oxIO~~~I/L.
30
l
Figure 8. Number of micelles as a function of the mole fraction of ClSpOE, in the C#OElo-Cl&'OEa mixed system at the concentration of 1.0 x 10-* mol/L.
40
18,
POE chain length
I
I
I
I
I
Figure 6. Effecta of alkyl chain length and/or poly(oxyethy1ene) chain length on the Il/Zs ratio for single surfadant systems.
i1
I
0
Mole fraction of C,,POE,,
Figure 9. Micellar size as a function of mole fraction of CISPOEa in the Cl&'OE&hPOEa mixed system.
1
0.2 0.4 0.6 0.8 Mole fraction of C,,POE,o
1
Figure 7. Micelle molecular weight veraue mole fraction of C I ~ PO& in the Cl&'OEldhPOE, mixed surfactant system.
celle aggregation number at the cmc increases with an increase in the hydrophobicity in the nonionic surfactant. Figure 6 demonstrates the 11/13ratio, both at a concentration of 1.0 X lom2mol/L and at the cmc, from the pyrene monomer fluorescence spectrum. As can be seen in Figure 6, the Id&ratios at a concentration of 1.0 X 1W2 mol/L were lower than those a t cmc, and the IdIs ratio decreases with increasing alkyl chain length and/or with decreasing poly(oxyethy1ene) chain length. Inother words, the micropolarity in the micelle decreases at higher concentration and decreases with an increase in the alkyl chain length and with a decrease in the poly(oxyethy1ene) chainlength. Thisindicatesthatamorerigidmicelleforms at higher concentration and with increasing hydrophobicity of the nonionic surfactant and/or a decrease in the number of water molecules hydrating the hydrophilic chain. SolutionProperties of Mixed SurfactantSystems. Next, mixed micelle formation and micellar properties were investigated for the ClSpOE1,,-Cl@OE, system, in which the poly(oxyethy1ene) chain lengths are quite different for the two surfactants. Values of the micellar molecular weight obtained by the static light scattering method are plotted against the mole fraction of C1@OEa in Figure 7. The micellar molecular weight at the cmc in the mixture reaches a maximum for an equimolar mixture of the two surfactants. Figure 8 depicts the relationship between the concentration of micelles by the fluorescence probe method at 1.0 X mol/L and the mole fraction of ClSpO&. The concentration of micelles shows a minimum for an equimolar mixture of the two surfactants.
The cmc values of the pure nonionic surfactant are approximately 10-8-10-5mol/L, and those of the ClSpOElr C l S p O b mixed surfactant system are ale0 about 1W6mol/ L. The monomer concentration is therefore negligible compared to the surfactant qoncentration in micelles at a total concentration of 1.0 X 1W2 mol& well above the cmc. Therefore, the decrease in the micelle concentration is not caused by an increase in concentration of the surfactant monomer but by an increase in the aggregation number of the surfactants in the mixed micelle. Figure 9 represents the relationehip between the micelle diameter and the mole fraction of ClSpOEL). At the cmc, the micellar diameter initially increaaes with increasing ClSpOEh mole fraction up to 0.5 and slightly decreaees at mole fractions greater than 0.5. The diameter of the mixed micelle will increase with increasing mole fraction of ClSpO& penetrating into the micelle up to a mole fraction of 0.5. Above a mole fraction of 0.6, the mixed micelle is primarily composed of ClSpOEm, so that the diameter of the mixed micelle is nsarly constant at the value for a pure ClSpOEa micelle. The I& ratio for the mixed surfactant systems is plotted against the mole fraction of ClSpOEa in Figure 10. As is shown in Figure 10,I1/I3 increases with an increase in the ClSpOEh mole fraction at both concentrations. The &/I3 ratios a t a concentration of 1.0 X 1W2moUL were lower than those at the cmc. In other words, the micropolarity in a mixed micelle decreases, and a more rigid mixed micelle forms a t higher concentration. If ClePOElo and ClSpO& were to form ideal mixed micelles, the values of would be given by the b h e d line in Figure 10. The observed values of Id&in the mixed surfactant system are much smaller, and the polarity is significantly less than would be predicted for an ideal mixed micellar system. The decrease in the polarity in
Langmuir, Vol. 8, No. 9, 1992 2161
Micelle Formation by Nonionic Surfactants I
I
1
1
1
1.200
1.181.161.1
&
C16PoE10
micelle
1.0
C16POE.30
micelle
Figure 11. Schematic micellizationmodels of single component and mixed micelles.
1.121.10-
mixed micelle
, , , w.4----
$‘--
A / ’.;
0.2
0.4
1
I
0.6
0.8
1
Mole fraction of C,,POE,,
Figure 10. I1/Is ratio as a function of mole fraction of Cid‘O& in the C1SPOE104190E~mixed system.
the mixed system can be attributed to the compactness in the mixed micelle. The structure of the mixed micelle is apparently rigid enough to reduce water penetration from the bulk into the mixed micelle. This effect may reflect the fact that the aggregation number of the mixed micelle is generally larger than that of each individual micelle. The results presented here support the schematic micellization model for the C~~POE~&&’OE~Omixed surfactant system shown in Figure 11. In the pure CIBPOE~O micelle, the hydrophilic region is relatively loose or expanded, owing to the random structure of the long poly-
(oxyethylene) chains, so that the polarity within the micelle is higher than that in the ClsPOElo pure micelle. In the mixed system, the aggregating condition seems to lead to an increased compactness in the ethylene oxide region 80 that the aggregation number is greater than in individual micelles. Thus, the long poly(oxyethy1ene)chain may act as an amphiphilicfunctional group which has hydrophobic properties because of the many methylene groups in the long chain. As a result, the hydrophilic part of the mixed micelle may be penetrated by some additional Cl6POElo and C16POE40 monomers, producing a structure that is quite dense. Registry NO. CI~POE,9002-92-0; C1$’0E, 27306-79-2; CISPOE, 9004-95-9; pyrene, 129-00-0; dodecylpyridinium chloride, 104-74-5.
