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Aggregation of Sodium 1-(n-Alkyl)naphthalene-4-sulfonates in Aqueous Solution: Micellization and Microenvironment Characteristics Xiao-Li Tan,† Lu Zhang,† Sui Zhao,† Wen Li,† Jian-Ping Ye,‡ Jia-Yong Yu,† and Jing-Yi An*,† Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China, and Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 10080, People’s Republic of China Received April 14, 2004. In Final Form: June 8, 2004 In aqueous solution, the micellization and microenvironment characteristics of the micelle assemblies of three anionic surfactants, sodium 1-(n-alkyl)naphthalene-4-sulfonates (SANS), have been investigated by steady-state fluorescence and time-resolved fluorescence decay techniques using pyrene, Ru(bpy)32+, and 1,6-diphenyl-1,3,5-hexatriene as fluorescence probes. The critical micelle concentrations (cmc’s), effective carbon atom numbers (neff’s), hydrophilic-lipophilic balances (HLBs), mean micelle aggregation numbers, micropolarities, and microviscosities of these surfactant micelles have been determined. The logarithmic cmc of the alkylnaphthalene sulfonates decreases linearly with an increase in the neff. The logarithmic aggregation number of the alkylnaphthalene sulfonates increases linearly with an increase in the neff. However, in contrast to the alkylsufonates and the alkylbenzene sulfonates, the aggregation for these alkylnaphthalene sulfonate molecules is less sensitive to the increase in the neff. The micropolarity of these alkylnaphthalene sulfonate micelles is less sensitive to the increase in the alkyl chain length and is lower than that of sodium dodecyl sulfate (SDS). The microviscosity of these alkylnaphthalene sulfonate micelles increases with an increase in the alkyl chain length and is lower than those of nonionic surfactants and zwitterionic surfactants. These results suggest that naphthyl rings have a notable effect on the micellization of SANS.
* Corresponding author. Phone: +86 10 64888165. Fax: +86 10 64879375. E-mail:
[email protected]. † Technical Institute of Physics and Chemistry. ‡ Institute of Chemistry.
ing from two to six units. The salts of these condensates are excellent dispersants for finely divided solids and are used as pigment dispersants, cement plasticizers, and dispersants in wettable powders and flowable pesticide formulations.3 In our previous works,4-6 we described the synthesis of the pure long-chain alkylnaphthalene sulfonates, the measurement of some surfactant parameters, and the micellization of decylnaphthalene sulfonate by 1 H NMR. In this paper, the micellization of these surfactants in aqueous solution and the microenvironment characteristics (e.g., micropolarity and microviscosity) of the micelle assemblies were investigated by steady-state fluorescence, time-resolved fluorescence decay techniques, and the fluorescence depolarization technique. The effective carbon atom numbers (neff’s) were obtained from the critical micelle concentrations (cmc’s) of sodium 1-(n-alkyl)naphthalene-4-sulfonates (SANS), and values of the hydrophilic-lipophilic balance (HLB) of SANS were calculated from the Davies equation. We have attempted to illustrate the micelle structure in aqueous solution, the dependence of the cmc and the mean aggregation number on the neff, and the dependence of the micropolarity and the microviscosity on the alkyl chain length. Our findings demonstrate naphthyl rings, as a part of the hydrophobic moiety, have a notable effect on the micellization of SANS.
(1) (a) Lindman, B.; Wennerstrom, H. Top. Curr. Chem. 1980, 87, 1. (b) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: London, 1985. (c) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (d) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218. (2) (a) Burns, R. L. J. Surfactants Deterg. 1999, 2, 13. (b) Burns, R. L. J. Surfactants Deterg. 1999, 2, 483. (c) Burns, R. L.; Duliba, E. P. J. Surfactants Deterg. 2000, 3, 3. (d) Valint, P. L.; Bock, J.; Kim, M. W.; Robbins M. L. Colloids Surf. 1987, 26, 191. (e) Cayias, J. L.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1977, 59, 31.
(3) Rasheed, K. In Surfactants: A Practical Handbook; Lange, R. K., Ed.; Hanser Gardner: Munich, Germany, 1999. (4) Tan, X.-L.; Zhang, L.; An, J.-Y.; Zhao, S.; Yu, J.-Y. J. Surfactants Deterg. 2004, 7, 135. (5) Yuan, H.-Z.; Tan, X.-L.; Cheng, G.-Z.; Zhao, S.; Zhang, L.; Mao, S.-Z.; An, J.-Y.; Yu, J.-Y.; Du, Y.-R. J. Phys. Chem. B 2003, 107, 3644. (6) Yang, X.-Y.; Gao, H.-C.; Tan, X.-L.; Yuan, H.-Z.; Mao, S.-Z.; Zhao, S.; Zhang, L.; An, J.-Y.; Yu, J.-Y.; Du, Y.-R. Colloid Polym. Sci. 2004, 282, 280.
Introduction In aqueous solution, surfactant aggregation is predominated by the balance between hydrophobic and hydrophilic interactions. These interactions depend on the surfactant molecular structure (hydrophobic moiety and headgroup) and the environment factors.1 Many of these effects have been documented to investigate the micellization and microenvironment characteristics, and theories have been proposed to interpret the observations. Most of the previous investigations have been focused on cationic, anionic, nonionic, and zwitterionic surfactants, while the pure alkylnaphthalene sulfonates, especially the homologous ones, have received less attention. The alkylnaphthalene sulfonates were extensively applied in the printing, dyeing, and oil extraction industries.2 The first surfactant derived from coal was diisopropyl naphthalene sulfonate, synthesized by Cunther at BASF in 1917. These alkylnaphthalene sulfonates continue to be used as wetting agents under the general designation of Nekals. Recent study reveals that they are also efficient hydrotropes. Naphthalenesulfonic acids can be condensed with formaldehyde to oligomers contain-
10.1021/la049055v CCC: $27.50 © 2004 American Chemical Society Published on Web 07/23/2004
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Method Description Davies7
who has given a Previous work was done by structural definition of the HLB in which a summation of the effects of the molecule’s component groups is involved. Such effects are expressed as the “group numbers” characteristic of each functional group. These numbers are quite empirical, as they appear to have been obtained from several empirically defined values of the HLB. The Davies equation is
HLB )
∑(hydrophilic group numbers) ∑(hydrophobic group numbers) + 7
(1)
∑(hydrophilic group numbers) -
n(group number per -CH2-) + 7 (2)
Also it is well-known8 that
log(cmc) ) A - Bn
(3)
where cmc is the critical micelle concentration and A and B are empirical, experimentally determinable constants. Combining eq 2 with eq 3 leads to
log(cmc) ) a + b(HLB)
(4)
where a and b are empirical constants that can be calculated from A and B.9 For straight-chain compounds, n is the actual number of carbon atoms in the alkyl chain, and for branched compounds, n is assumed to be neff. For a homologous series of branch-chain or aromaticring hydrocarbon surfactants, eq 1 may be written as
HLB )
∑(hydrophilic group numbers) -
neff(-CH2-) + 7 (5)
Equation 3 may be written as
log(cmc) ) A - Bneff
(6)
Of the methods employed for aggregation number determination, time-resolved fluorescence quenching has been used preferentially.10 Fluorescence decay curves can be recorded using the single photon counting technique and fitted to the Infelta-Tachiya equation:11,12
I ) I0 exp{-A2t - A3[1 - exp(-A4t)]}
(7)
where the Ai parameters are explicitly
A 2 ) k0 +
A3 )
kqk-n (kq + k-) nkq2
(kq + k-)2
A 4 ) kq + k-
A2 ) k0; A3 ) n; A4 ) kq
(8a)
(8b) (8c)
where I0 is the intensity at time t ) 0, k0 stands for the deactivation rate constant of the probe fluorescence in the absence of the quenchers, kq is the first order rate constant of the intramicellar quenching, k- is the exit
(8d)
The n value leads to the mean aggregation number (N) as follows:
N)
For a homologous series of straight-chain hydrocarbon surfactants with n(-CH2-) groups, eq 1 may be written as
HLB )
rate constant for quenchers from the micelle, and n the average number of quenchers per micelle. If k- is much less than kq and k0, the parameters A2-A4 reduce to
n([SANS]m - [SANS]free) [Qm]
(9)
where [Qm] and [SANS]m are the quencher and surfactant concentrations in micelles and [SANS]free is the concentration of surfactant free as monomers in solution; the latter is often equal to the cmc. The experimental error was 10% for N value determination. Microviscosity determination can be performed by the fluorescence depolarization method. When a fluorescent molecule is excited by polarized light, its emission will be polarized, as long as the probe does not change its orientation during its excited-state lifetime. However, the Brownian motions will offset the orientation imposed by the polarized light. Because the tumbling motion of a molecule in a certain medium is related to viscosity, the higher the medium viscosity, the more difficult it is for a molecule to tumble. The correlation of the measured fluorescence anisotropy (r) and the medium viscosity can be shown by the Perrin equation:13
r0/robs ) 1 + KTτ/ηV0
(10)
in which robs, r0, τ, K, η, and V0 are the measured fluorescence anisotropy, limiting fluorescence anisotropy, fluorescence lifetime of the probe, Boltzmann constant, viscosity of the medium, and effective volume of the tumbling sphere, respectively. 1,6-Diphenyl-1,3,5-hexatriene (DPH) is chosen as a probe for fluorescence depolarization because of its well-known r0 value (0.362)14 and its strong absorption. This probe employed for the evaluation of microviscosity has been outlined previously in conjunction with studies on the hydrocarbon region of a surface membrane lipid layer,15 liposomes,16 and biological membranes17 and is available for microviscosity determination of micelle interiors. Experimental Section The sodium 1-alkylnaphthalene-4-sulfonate surfactants [CnH2n+1C10H6SO3Na; n ) 6 (SHNS), n ) 8 (SONS), n ) 10 (SDNS)] were synthesized as reported previously,4 and their purity was identified by 1H NMR, electrospray ionization mass spectrometry (ESI-MS), and IR. Pyrene and Ru(bpy)32+Cl2 were from Aldrich and recrystallized before use. Sodium dodecyl sulfate (SDS; 99% pure, Acros), 1,6-diphenyl-1,3,5-hexatriene (DPH; TCI), and 9,10-dimethylanthracene (DMA; Aldrich) were used as received. Distilled water by a sub-boiling quartz apparatus (7) Davies, J. T. Proc. Int. Congr. Surf. Act., 2nd 1957, 1, 426. (8) Klevens, H. B. J. Phys. Colloid. Chem. 1948, 52, 130. (9) Lin, I. T. Trans. Am. Inst. Min., Metall. Pet. Eng. 1971, 250, 225. (10) Zana, R., Ed. Surfactant Solutions: New Methods of Investigation; Marcel Dekker: New York, 1987; Chapter 5. (11) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (12) Infelta, P. P.; Gratzel, M.; Thomas, K. K. J. Phys. Chem. 1974, 78, 190. (13) Hamai, S.; Kokubun, H. Bull. Chem. Soc. Jpn. 1973, 46, 100. (14) Dale, R. E. In Time-Resolved Fluorescence Spectroscopy in Biochemistry and Biology; Cundall, R. B., Dale, R. E., Eds.; Plenum Press: New York, 1983; p 462. (15) Shinitzky, M.; Inbar, M. J. Mol. Biol. 1974, 85, 603. (16) Cogan, U.; Shinitzky, M.; Weber, G.; Nishida, T. Biochemistry 1973, 12, 521. (17) Rudy, B.; Gitler, C. Biochim. Biophys. Acta 1972, 288, 231.
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Figure 1. Representative fluorescence emission spectrum of pyrene solubilized in micelles. Excitation wavelength: 335 nm. [SDNS]: 2 × 10-3M. was available for all measurements. The concentrations of each surfactant were kept at ∼3 times the cmc to avoid possible micelle polydispersion, which would cause serious errors in the aggregation number determination. Steady-state fluorescence emission of pyrene (concentration of 2.0 × 10-6 M for each micelle solution) was performed on a Hitachi F4500 spectrophotometer using a 335 nm excitation wavelength. The fluorescence decay and the depolarization of DPH (the concentration in the tested micelle solution was kept at 6 × 10-6 M) were performed with a 343 nm excitation wavelength and a 435 nm emission wavelength on Edinburgh F900 analytical instruments and were fitted to give the fluorescence lifetimes of DPH in tested micelles. The fluorescence decay of Ru(bpy)32+Cl2 was performed on a Horiba NAES 1100 nanosecond fluorometer and was fitted according to eq 7 using the DECAN 1.0 software. The tested solution samples were degassed by high purity nitrogen for 30 min. The χ2 values of these fittings were kept as close to 1 as possible (0.990-1.100).
Results and Discussions
Tan et al.
Figure 2. I1/I3 values of pyrene fluorescence emission spectra as a function of the concentration of SHNS, SONS, and SDNS. Table 1. cmc’s of Tested Surfactants at 30 °C
surface tension measurements fluorescence measurements
SHNS (mM)
SONS (mM)
SDNS (mM)
10.0 10.0
2.36 4.00
0.80 1.10
Table 2. cmc’s of Some Anionic Surfactants in Aqueous Solution compound
solvent
T (°C)
cmc (M)
C8H17SO3Na C10H21SO3Na C12H25SO3Na C14H29SO3Na C16H33SO3Na p-n-C8H17C6H4SO3Na p-n-C10H21C6H4SO3Na p-n-C12H25C6H4SO3Na 1-(n-C6H13)-4-C10H6SO3Na 1-(n-C8H17)-4-C10H6SO3Na 1-(n-C10H21)-4-C10H6SO3Na
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
40 40 40 40 50 35 50 60 30 30 30
1.6 × 10-1 8 4.1 × 10-2 8 9.7 × 10-3 22 2.5 × 10-3 8 7.0 × 10-4 8 1.5 × 10-2 23 3.1 × 10-3 23 1.2 × 10-3 23 1 × 10-2 2.4 × 10-3 8.0 × 10-4
Self-Aggregation in Aqueous Solution. These surfactants in aqueous solution show high surface activity.4 Pyrene monomer fluorescence emission is available for monitoring their self-aggregation in aqueous solution. Because the fluorescence intensities for various vibronic bands in the pyrene monomer fluorescence show a strong polarity dependence, pyrene exhibits a characteristic fluorescence emission spectrum consisting of five bands (Figure 1). In polar media, the 0-0 band for pyrene molecules is enhanced by a mechanism involving vibronic coupling similar to the Ham effect in the absorption spectra of benzene;18 thus, the intensity ratio of the first to the third band (I1/I3) can be taken as a measure for the polarity of the environment. When surfactant molecule association takes place, pyrene molecules in water will be preferentially solubilized in the interior hydrophobic region of micelles to cause an abrupt change of the I1/I3 ratio.19 The absorption and fluorescence of the SANS had no effect on the steady-state fluorescence of pyrene when pyrene was excited at 335 nm. Figure 2 illustrates the I1/I3 ratio variation with the surfactant concentration, which is available for obtaining cmc values. These cmc values are in accordance with those obtained from surface tension measurements,4 indicating the reliability of this technique
(Table 1). As shown in Table 1, the cmc’s of SANS decrease with an increase in the alkyl chain length. neff and HLB in Aqueous Solution. The neff values for the alkylnaphthalene sulfonates can be determined through the same methods as those for the alkylbenzene sulfonates.20 It is well-known21 that the (sCHdCHs) bonds in a phenyl ring are considerably less hydrophobic (or more hydrophilic) than the (sCH2sCH2s) bonds. Further illustration of the intermediate nature of the carbon-carbon bonds in a phenyl ring comes from experimental results that a phenyl ring is the hydrophobic equivalent of 3.5 (sCH2s) groups in its effect on the cmc.20 Thus, the naphthyl ring can be determined through the same method. As shown in Table 2, the naphthyl ring is equivalent to about six (sCH2s) groups in its effect on the cmc. Also, there is supported evidence5 that the alkylnaphthalene sulfonates can more easily form micelles compared with the alkylbenzene sulfonates and alkylsulfonates with the same number of alkyl carbon atoms. Table 3 gives the neff values and the values of the HLB for SANS studied by eq 5. Figure 3 gives the linear
(18) Koyanagi, M. J. Mol. Spectrosc. 1968, 25, 273. (19) Bohne, C.; Rednond, R. W.; Scaiano, J. C. In Photochemistry in Organized and Constrained Media; Ramammurthy, V., Ed.; VCH: New York, 1991; Chapter 3.
(20) Paquette, R. G.; Lingafelter, E. C.; Tartar, H. V. J. Am. Chem. Soc. 1943, 65, 686. (21) Pomerantz, P.; Clinton, W. C.; Zisman, W. A. J. Colloid Interface Sci. 1967, 24, 16.
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Table 3. neff and HLB Values of Sodium 1-(n-Alkyl)naphthalene-4-sulfonates surfactant
T (°C)
neff
HLB
SHNS SONS SDNS
30 30 30
12 14 16
12.30 11.35 10.40
Figure 4. Correlation of the aggregation number and the effective carbon number. log(N) ) 0.8983 + 0.0275neff (r ) 0.9986) for SANS (1), log(N) ) 1.1987 + 0.0425neff (r ) 0.9959) for p-n-alkylbenzene sulfonates (2), and log(N) ) 0.761 + 0.0815neff (r ) 0.9969) for n-alkylsulfonates (3). Table 5. Aggregation Numbers (N’s) of Alkylsulfonates and Alkylbenzene Sulfonates in Aqueous Solution
Figure 3. Linear relationship of log(cmc) (mM) and the effective carbon number (neff) of SANS. Regression analysis: -log(cmc) ) -1.2833 + 0.2750n (r ) 0.9986). Table 4. Aggregation Numbers and the Values of Microviscosity of SANS fluorescence decay parameters, aggregation numbers, and microviscosity (30 °C)
SHNS SONS SDNS
DMA (µM)
1/A2 (ns)
A3
A4 (ns-1)
N
η (cP)
F (ns)
150 80 20
538.7766 522.9398 513.1894
0.1798 0.2014 0.3691
0.0101 0.0043 0.0024
17 19 22
20 22 28
2.30 2.53 3.22
relationship of log(cmc) and the neff values of SANS studied according to eq 6. Micelle Aggregation Number. Time-resolved fluorescence of Ru(bpy)32+ in the presence of DMA (quencher) is employed for determining the aggregation numbers of these surfactant micelles. For each micelle solution, the concentration of Ru(bpy)32+ was kept at 5 × 10-5 M and the quencher (DMA) concentration was comparable to the micelle concentration. Equations 7-9 can be used for calculating the aggregation number (Table 4), since the fluorescence lifetime of Ru(bpy)32+ in the absence of quencher for each micelle sample is comparable to the reciprocal of A2. A plot of logarithmic aggregation numbers versus neff gives a straight line (Figure 4). For a comprehensive illustration of this dependence, similar plots for the alkylsulfonate and alkylbenzene sulfonate micelle solutions from Table 5 are also presented in Figure 4. The smaller slope observed implies that the aggregation for these alkylnaphthalene sulfonate molecules is less sensitive to an increase in the alkyl chain length than that of alkylsulfonates and alkylbenzene sulfonates. This is due to the fact that the naphthyl group is bulkier than the alkyl group and phenyl groups. Moreover, in forming micelles, the naphthyl rings are packed radiating outward with the part connected to alkyl chains closer than the part substituted by the sulfonate group so that the distances among the negatively charged sulfonate groups are longer than those of the alkylsulfonates and alkylbenzene sulfonates and the repulsion is weakened.5
surfactant
T (°C)
neff
N
n-C8H17SO3Na n-C10H21SO3Na n-C12H25SO3Na n-C14H29SO3Na p-n-C8H17C6H4SO3Na p-n-C10H21C6H4SO3Na p-n-C12H25C6H4SO3Na
23 30 40 60 25 45 65
8 10 12 14 11.5 13.5 15.5
25 24 40 24 54 24 80 24 56 25 68 26 88 27
Table 6. I1/I3 Values of Pyrene Solubilized in Micellesa medium (aqueous solution)
T (°C)
I1/I3b
SHNS SONS SDNS SDS
30 30 30 25 30
1.12 1.09 1.13 1.18 1.20
a The I /I values determined in the present study were found 1 3 to be ∼6% higher than those reported by other scientists (Kalyanasundaram, K.; Thomas, J. J. Am. Chem. Soc. 1977, 99, 2039). However, since the I1/I3 values were compared only in different micelles, such a systematic difference does not affect the conclusion inferred from the above data. b The experimental uncertainty on the I1/I3 values is (2%.
Micropolarity. As mentioned above, when pyrene molecules are solublized in micelles, the I1/I3 values can be taken as a measure for the micelle micropolarity.28 From Table 6, we can see that the I1/I3 values for micelles of SANS at the same temperature are nearly the same. These low I1/I3 values demonstrate that, when pyrene is incorporated into a micelle, its solubilization site is in the palisade layer near the polar headgroups in all of the tested surfactant micelles. I1/I3 values lie in the range 1.131.09, which is smaller than those in anionic sodium dodecyl (22) Bujake, J. E.; Goddard, E. D. Trans. Faraday Soc. 1965, 61, 190. (23) Gershman, J. W. J. Phys. Chem. 1957, 61, 581. (24) Tartar, H. V.; Lelong A. J. Phys. Chem. 1955, 59, 1185. (25) Lindman, B.; Puyal, M. C.; Kamenka, N.; Burn, B.; Gunnarsson, G. J. Phys. Chem. 1982, 86, 1702. (26) Binana-Limbele´, W.; Van OS, N. M.; Rupert, L. A. M.; Zana, R. J. Colloid Interface Sci. 1991, 141, 157. (27) Caponetti, E.; Triolo, R.; Patience, C. HO; Johnson, J. S., Jr.; Magid, L. J.; Butler, P.; Payne, K. A. J. Colloid Interface Sci. 1987, 116, 200. (28) Kalyansundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
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sulfate (SDS) micelles (Table 6). This demonstrates the palisade layer of SANS is packed more tightly than that of SDS. The possible reason may be the difference in the chemical structures of the surfactants.5,6 The alkyl chains of SANS are snaky, and the first and second methylene groups next to the naphthyl rings are packed more tightly among the naphthyl rings in the palisade layer. Microviscosity. Microviscosity values represent the harmonic mean of the effective environment viscosities opposing the rotation of probe molecules. In this present study, fluorescence depolarization of DPH is used for evaluating the microviscosity in the hydrocarbon region of each kind of surfactant micelle. When DPH is excited at 343 nm, its degree of fluorescence depolarization relates to the viscosity that opposes the rotation of the DPH molecule in all possible directions. From eq 10, upon determination of the lifetimes (10.20-8.42 ns) of DPH in each sample, the microviscosity values of each micelle sample can be given when a K/V0 value of 8.6 × 105 p/deg‚ s was employed,15 as listed in Table 4. Table 4 shows that the microviscosity values increase with increasing alkyl chain length. As the alkyl chain length increases, the hydrophobic effect makes surfactant molecules aggregate tightly to provide DPH molecules with a more rigid environment. To illustrate the fluidity difference of DPH molecules in these micelles, rotation relaxation times (F’s) are also calculated from F ) 3V0η/ KT,17 as listed in Table 4. Because of the sensitivity of F to the viscosity of the medium, the F values of DPH molecules in each micelle sample could correspond to different microviscosities. It is worth mentioning that the microviscosities of these micelles are close to those of ionic (SDS) micelles (on the order of 30 cP)29 but are smaller than those of nonionic (Triton X-100) micelles (on the order of 160 cP)30 and betaine zwitterionic micelles (on the order of 120 cP).31 (29) Shinitzky, M.; Dianoux, A.-C.; Gitler, C.; Weber, G. Biochemistry 1971, 10, 2106. (30) Hertz, R.; Barenholz, Y. J. Colloid Interface Sci. 1977, 60, 188. (31) Guan, J.-Q.; Tung, C.-H. Langmuir 1999, 15, 1011.
Tan et al.
This indicates that the interior of alkylnaphthalene sulfonate micelles is considered to be less tightly packed than that of nonionic surfactant micelles and zwitterionic surfactant micelles due to the existence of electrostatic repulsive forces between the surfactant anions. Conclusions Micellization and microenvironment characteristics of three pure alkylnaphthalene sulfonates have been studied by steady-state fluorescence and time-resolved fluorescence techniques. The critical micelles concentrations (cmc’s), effective carbon atom numbers (neff’s), hydrophilic-lipophilic balances (HLBs), mean micelle aggregation numbers, micropolarities, and microviscosities of these surfactant micelles have been determined. The critical micelle concentrations of these surfactants in aqueous solution decrease with increasing neff, and the aggregation numbers of these homologous surfactants increase with increasing neff. Moreover, the aggregation for these alkylnaphthalene sulfonate molecules is less sensitive to an increase in the alkyl chain length than that for alkylsulfonates and alkylbenzene sulfonates due to the special structure of SANS. The micropolarity of these surfactant micelles is nearly constant as the alkyl chain length increases. Furthermore, the micropolarity of these micelles is relatively low compared with that of SDS micelles because the palisade layer of SANS is packed more tightly than that of SDS. The microviscosity values increase with increasing alkyl chain length and are relatively low compared with those of nonionic surfactant micelles and zwitterionic surfactant micelles due to the existence of electrostatic repulsive forces between SANS. Acknowledgment. The authors thank Prof. YongCai Jiang and Dr. Si-Guang Jiang for helpful discussions and critical reading of the manuscript. Financial support by the Fundamental National Key Basic Research Development Program “Studies of the Extensively Enhanced Petroleum Recovery” (Project Grant G1999022504) is gratefully acknowledged. LA049055V
