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Langmuir 1999, 15, 646-648
Aqueous Properties of Cationic Biphenyl Type Surfactants Tetsuo Takemura,*,† Noriaki Shiina,† Masako Izumi,† Kanae Nakamura,† Munetaka Miyazaki,‡ Kanjiro Torigoe,‡ and Kunio Esumi*,‡ Departments of Chemistry and Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Figure 1. Chemical Structures of C1Ph2, C6Ph2, and C12Ph2. Received August 6, 1998. In Final Form: October 19, 1998
Introduction Recently, many novel surfactants have been synthesized to study the relationship between their physicochemical properties and chemical structures.1 For example, calix[4]resorcinarenes each bearing four hydrophobic side chains showed a unique solubilization behavior; the aromatic compound solubility decreased in proportion to the compound molecular size and the solubility of a longalkyl-chain alcohol was considerably dependent on their chain length.2 Interestingly, when asymmetric surfactant molecules form micelles, it is expected that they recognize the chirality of other compounds or provide reaction media for asymmetric reactions. For that purpose it is important to characterize the micellar properties of chiral surfactants. The objectives of this study were to synthesize new cationic surfactants consisting of a hydrophobic chain of chiral biphenyl and hydrophilic group of trimethylammonium and to characterize their physicochemical properties in an aqueous solution.
Figure 2. Variation of the surface tension with the concentration of C1Ph2, C6Ph2, and C12Ph2 at 25 °C.
Experimental Section Materials. The structures of three surfactants synthesized are given in Figure 1. These cationic surfactants were synthesized as follows. Oxidative coupling of 3,4,5-trimethylphenol was carried out with horseradish peroxidase/H2O2. After Williamson alkylation of the hydroxyl groups, samples obtained by chloromethylation at C-3 and C-3′ positions were finally treated with excess trimethylamine gas. Crude solid products were recrystallized several times from ethanol/hexane or acetonitrile. These surfactants were characterized by mass and NMR spectra. Each mass spectrum showed the molecular ion peak losing a chloride ion. 1H NMR data are shown as follows: C1Ph2: 1H NMR (TMS) δ 1.97, 2.19, 2.40 (Ph-CH3), 3.05 (N-CH3), 3.15 (O-CH3), 4.55∼4.69 (N-CH2-Ph). C6Ph2: 1H NMR (TMS) δ 0.73 (-CH3), 0.86∼1.04 (-(CH2)2-CH3), 1.32 (-CH2-(CH2)2-CH3), 1.97, 2.19, 2.40 (Ph-CH3), 2.86 (-CH2-CH2-), 3.05 (N-CH3), 3.48 (OCH2-), 4.55∼4.69 (N-CH2-Ph). C12Ph2: 1H NMR (TMS) δ 0.73 (-CH3), 0.86∼1.25 (-(CH2)8-CH3), 1.32 (-CH2-(CH2)8-CH3), 1.97, 2.19, 2.40 (Ph-CH3), 2.86 (O-CH2-CH2-), 3.05 (N-CH3), 3.48 (O-CH2-), 4.55∼4.69 (N-CH2-Ph). The water used in this study was purified through a Milli-Q system. The other chemicals were of analytical grade. Measurements. The surface tensions of aqueous surfactant solutions were measured by the Wilhelmy plate method (Kruss K12 tensiometer). Static light-scattering measurements were * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Applied Chemistry and Institute of Colloid and Interface Science. (1) Esumi, K; Ueno, M. Structure-Performance Relationships in Surfactants; Marcel Dekker: New York, 1997. (2) Koide, Y.; Li, B.; Kawaguchi, Y.; Shosenji, H.; Esumi, K. J. Jpn. Oil Chem. Soc. 1998, 47, 57.
performed on a light-scattering spectrometer, model DLS-700Ar (Otsuka Electronics Co., Ltd). The light source used was an argon ion laser. The electrical conductivities of the surfactant solutions were also measured with a TOA electrical conductivity meter. Fluorescence spectra of pyrene were obtained using a fluorescence spectrophotometer (Hitachi 650-10S), where the concentration of pyrene was 1 × 10-5 mol dm-3. The excitation wavelength was 342 nm. All the measurements were carried out at 25 °C.
Results and Discussion The surface tension curves as a function of the logarithm of the concentration for the aqueous solutions of C1Ph2, C6Ph2, and C12Ph2 are shown in Figure 2. The surface tension decreased gradually with an increasing surfactant concentration and then showed a break point, which was taken as the critical micelle concentration (cmc). The cmc values of C1Ph2, C6Ph2, and C12Ph2 were 72.3, 9.4, and 0.19 mmol dm-3, respectively. The surface tension values at the respective cmcs (γcmc) were 46.1 mN m-1 for C1Ph2, 43.6 mN m-1 for C6Ph2, and 41.1 mN m-1 for C12Ph2, respectively. These results indicate that an increase from methyl to hexyl and dodecyl in the hydrophobic chain length renders the surfactants more surface-active. In general, an empirical equation relating the cmc to the various surfactant structures can be expressed in the form3 log cmc ) A - BN, where A is a constant for a particular ionic head at given temperature, B is a constant, and N is the number of carbon atoms in the hydrophobic chain. It is customary to take N as one-half of the total number of carbon atoms in the surfactants consisting of two heads
10.1021/la9810004 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/24/1998
Notes
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Table 1. Physicochemical Properties of C1Ph2, C6Ph2, and C12Ph2 surfactant
cmc (mmol dm-3)
γcmc (mN m-1)
A (nm2/molecule)
R
C1Ph2 C6Ph2 C12Ph2
72.3 9.4 0.19
46.1 43.6 41.1
3.45 3.25 2.29
0.73 0.56
and two tails, when comparing to surfactants of single heads/single tails.4 As a result, B was obtained as 0.23 which is smaller compared to those of ionic single-chained surfactants (B ) 0.3) and nonionic and zwitterer ionic surfactants (B ) 0.5).5 According to Rosen,6 a way of measuring the relative effects of some structural or microenvironmental factors on micellization and on adsorption is to determine its effect on the cmc/C20 ratio, where C20 is the concentration of the surfactant in the bulk phase that produces a reduction of 20 mNm-1 in the surface tension of the solvent. In this study, the cmc/C20 ratios were obtained to be 1.8 for C1Ph2, 9.3 for C6Ph2, and 10.6 for C12Ph2, respectively. Generally, the cmc/C20 ratio is not increased substantially by an increase in the length of the alkyl chain of the hydrophobic group in single-alkyl-chain ionic surfactants, where they show low cmc/C20 ratios of 3 or less. An increase in the cmc/C20 ratio with an increasing chain length from C1Ph2 to C6Ph2 and C12Ph2 may indicate that micellization is inhibited more than adsorption at the air/water interface or adsorption is facilitated more than micellization. The cmc’s were also determined by the electrical conductivity measurements. It was found that the cmc values of C6Ph2 and C12Ph2 determined by the electrical conductivity measurements are consistent with those by the surface tension measurements. The micelle ionization degree (R) at the cmc was taken as the ratio of the values of dκ/dC above and below the cmc: 0.73 for C6Ph2; 0.56 for C12Ph2. In the case of C1Ph2, the micelle ionization degree was not obtained because no distinct break points were observed in the conductivity. Values of the cmc, γcmc, and R are given in Table 1. The degree of micelle ionization of C6Ph2 is large compared to that of C12Ph2, suggesting that C6Ph2 forms micelles of a fairly small aggregation number. The surface excess Γ at the air-water interface can be calculated by applying the Gibbs adsorption isotherm equation
Γ ) -(1/iRT) (dγ/d ln C) where γ is the surface tension and C is the surfactant concentration. i ) 3 for the three surfactants. The occupied area (A) per surfactant molecule is calculated from A ) 1/NΓ, where N is Avogadro’s number. The calculated occupied areas were 3.45 nm2/molecule for C1Ph2, 3.25 nm2/molecule for C6Ph2, and 2.29 nm2/molecule for C12Ph2, respectively. These values are considerably greater than that of dodecyltrimethylammonium bromide (DTAB) (0.49 nm2/molecule)7 which is a typical cationic surfactant. Further, in the case of eicosane-1,20-bis(triethylammonium bromide),8 its cmc is 6 mmol dm-3 and the calculated occupied area is about 1.17 nm2/molecule. This surfactant (3) Klevens, H.B. J. Am. Oil Chem. Soc. 1953, 30, 74. (4) Zana, R.; Benrraou, M; R. Rueff, R. Langmuir 1991, 7, 1072. (5) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989; p 136. (6) Rosen, M. J. Surfactant and Interfacial Phenomena, 2nd ed.; WileyInterscience: New York, 1989; p 143. (7) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039. (8) Ikeda, K.; Yasuda, M.; Ishikawa, M.; Esumi, K.; Meguro, K.; Binana-Limbele, W.; Zana, R. Colloid Polym. Sci. 1989, 267, 825.
Figure 3. Debye plots of C6Ph2 and C12Ph2 obtained from static light scattering measurements at 25 °C.
Figure 4. Variation of the I1/I3 ratio of pyrene with the concentration of C1Ph2, C6Ph2, and C12Ph2 at 25 °C.
has a hydrocarbon chain length similar to that of C6Ph2 if the phenyl groups correspond to the three and half methylene groups. These differences in the cmc and the occupied area suggest that the C6Ph2 surfactant is less oriented at the air/water interface because of some steric hindrance of the hydrophobic portion of the surfactant compared to eicosane-1,20-bis(trimethylammonium bromide. Static light-scattering measurements were carried out to estimate the aggregation number of the micelles. The scattering intensity can be related to the average micellar weight with the following equation:
K(C - C0)/(R90 - R090) ) 1/Mw + B2(C - C0) where K is the optical constant as K ) 4π2n02 (dn/dc)2/ NAλ04, R90 and R090 are the Rayleigh ratios at a 90° scattering angle for solutions of surfactant concentration C and critical micelle concentration C0, respectively, λ0 is the wavelength of the laser (488 nm), Mw is the weightaverage micellar weight, and B2 is the second virial coefficient. From the Debye plots (Figure 3) the aggregation numbers of C6Ph2 and C12Ph2 at their cmc’s were
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determined to be about 14 ( 5 and 53, respectively. However, we could not obtain the aggregation number of C1Ph2, as is to be expected, because the intensity of light scattering was very low, even above the cmc. The fluorescence spectrum of micelle-bound pyrene is very sensitive to the polarity of the microenvironment at the site of solubilization of the fluorophore.9 In this study, the polarity of the micelles was evaluated by the intensity ratio I1/I3 of the first and third vibronic bands of monomeric pyrene. Figure 4 shows the variations of I1/I3 as a function of the surfactant concentration. The I1/I3 values of these three surfactants decreased with the concentration and attained constant values above their cmcs where the order of the constant values was as follows: C1Ph2 > C6Ph2 > C12Ph2. This result is probably related to the fact that the (9) Kalynasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
Notes
aggregation number of the surfactants increases from C1Ph2 to C6Ph2 and C12Ph2, resulting in less contact between water and pyrene as the carbon chain length of the surfactants increases. In the case of C1Ph2, there is a possibility that C1Ph2 is not a true surfactant but a hydrotrope. It can be concluded from the above results that the novelsynthesized cationic surfactants of biphenyl-type having different hydrocarbon chain lengths provide a linear relationship between log cmc and the carbon number in the hydrophobic group in which the longer the chain length, the larger the aggregation number. In addition, the micropolarity sensed by pyrene decreased with an increasing chain length of the surfactants. Studies on dynamic properties of these surfactants using NMR are in progress. LA9810004
