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Comparative Studies of Structurally Similar Surfactants: Sodium Bis(2-ethylhexyl)phosphate and Sodium Bis(2-ethylhexyl)sulfosuccinate Yuxia Luan, Guiying Xu,* Shiling Yuan, Li Xiao, and Zhiqing Zhang Key Laboratory for Colloid & Interface Chemistry of Education Ministry, Shandong University, Jinan, 250100, People’s Republic of China Received November 20, 2001. In Final Form: June 12, 2002 Sodium bis(2-ethylhexyl)phosphate (NaDEHP) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) are studied by atomic-level molecular modeling, using UFF force field and surface tension measurements. Five representative geometries of NaDEHP are shown, and the corresponding structural parameters such as the energy, area, and volume are analyzed. The effects of additives such as NaCl, water-soluble polymer, and alcohol on the surface activity of NaDEHP and AOT solutions are investigated by surface tension measurements. The results suggest the following: (i) The bond angle of the hydrophobic chains of NaDEHP is smaller than that of AOT. (ii) AOT is more sensitive to NaCl than NaDEHP, that is, the surface tension of AOT is obviously decreased as a small amount of NaCl is added, but it has only a slight influence on the NaDEHP system. (iii) Two transition points appear in the surface tension curves of AOT with addition of 0.5 wt % PVP (poly(vinylpyrrolidone)), but in the NaDEHP surface tension curves two transition points appear only when the PVP concentration is high enough, for example, 2.0 wt % in this experiment, which suggests that the interaction between AOT and PVP is stronger than that between NaDEHP and PVP. (iv) The surface tension and critical micelle concentration are decreased with the increase of the carbon number and the concentration of the alcohol. (v) The mean squared end-to-end distance (〈r2〉) of the polymer chain obtained from the simulation is reduced by adding NaDEHP and AOT.
Introduction Sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and sodium bis(2-ethylhexyl)phosphate (NaDEHP) are structurally related double-tail anionic surfactants and can easily aggregate in apolar media. They are widely studied because of their properties.1-4 AOT is not only an excellent wetting agent but also an important material to simulate biological membranes. However, most of the studies about AOT have been focused on the properties in nonaqueous polar solvents.5-18 Bis(2-ethylhexyl)phosphate (HDEHP) is an important material in metal extraction,19,20 and the microemulsions composed of its sodium salt, NaDEHP, * To whom correspondence should be addressed. Fax: +86-5318564464. E-mail:
[email protected]. (1) Kurumada, K.; Shioi, A.; Harada, M. J. Phys. Chem. 1995, 99, 16982. (2) Shioi, A.; Harada, M.; Tanabe, M. J. Phys. Chem. 1993, 97, 8281. (3) Shioi, A.; Harada, M.; Tanabe, M. J. Phys. Chem. 1995, 99, 4750. (4) Shioi, A.; Harada, M.; Tanabe, M. Langmuir 1996, 12, 3201. (5) De Tapas, K.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (6) Cassin, G.; Badiali, J. P.; Pileni, M. P. J. Phys. Chem. 1995, 99, 12941. (7) Bangar, B.; Raju, M.; Costa, S. B. Phys. Chem. Chem. Phys. 1999, 1, 5029. (8) Kotlarchyk, M.; Chen, S. H. J. Chem. Phys. 1983, 79, 2461. (9) Kotlarchyk, M.; Chen, S. H.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054. (10) Sheu, E. Y.; Chen, S. H.; Huang, J. S. J. Chem. Phys. Rev. A 1989, 39, 5867. (11) Kotlarchyk, M.; Sheu, E. Y.; Capel, M. Phys. Rev. A 1992, 46, 928. (12) Hayes, D.; Gulari, G. Biotechnol. Bioeng. 1990, 35, 793. (13) Hayes, D. G.; Gulari, E. Biotechnol. Bioeng. 1991, 38, 507. (14) Hayes, D. G.; Gulari, E. Biotechnol. Bioeng. 1992, 40, 110. (15) Hayes, D. G.; Gulari, E. Biocatalysis 1994, 11, 223. (16) Hayes, D. G.; Gulari, E. Langmuir 1995, 11, 4695. (17) Ray, S.; Moulik, S. P. Langmuir 1994, 10, 2511. (18) Laia, A. C. T.; Lo´pez-Cornejo, P.; Costa, S. M. B.; d’Oliveira, J.; Martinho, J. M. G. Langmuir 1998, 14, 3531. (19) Mapara, P. M.; Godbole, A. G.; Swarup, R.; Thakur, N. V. Hydrometallurgy 1998, 49, 197. (20) Yang, C. F.; Cussler, E. L. J. Membr. Sci. 2000, 166, 229.
have been used in the hydrometallurgical industry because of some unique characteristics, such as a short phase separation time, a very high recovery rate, and the easy recycle. Although the hydrophobic chains of NaDEHP and AOT are similar (Figure 1), the aggregation behaviors for them in apolar media are different, even reverse.21-23 For example, in the case of AOT, the reversed micelles are spherical in shape and the size is increased with the increase of W0 (the molar ratio of water to surfactant in the oil),24 while NaDEHP in heptane could form giant rodlike micelles under extremely dry conditions.25-27 Later, K. I. Feng et al.28,29 found that it was crystalline rods with a permanent dipole moment but not giant rodlike micelles that NaDEHP in benzene formed under extremely dry conditions. The crystalline rods dissolve upon increasing water content of the solution, and above a critical water content (W0 ≈ 3) the proper reverse micelles form. All these findings are in violent contrast with those for AOT. Although there have been a number of investigations concerning the aggregation behaviors of AOT and NaDEHP in apolar media,30-33 no attention has ever been (21) Kurumada, K.; Shioi, A.; Harada, M. J. Phys. Chem. 1994, 98, 12382. (22) Li, Q.; Weng, S. F.; Wu, J. G.; Zhou, N. F. J. Phys. Chem. 1998, 102, 3168. (23) Li, Q.; Li, T.; Wu, J. G.; Zhou, N. F. J. Colloid Interface Sci. 2000, 229, 298. (24) Ruckenstein, E.; Nagarajan, R. J. Phys. Chem. 1980, 84, 1349. (25) Yu, Z. J.; Zhou, N. F.; Neuman, R. D. Langmuir 1992, 8, 1885. (26) Yu, Z. J.; Neuman, R. D. J. Am. Chem. Soc. 1994, 116, 4075. (27) Yu, Z. J.; Neuman, R. D. Langmuir 1994, 10, 2553. (28) Feng, K. I.; Schelly, Z. A. J. Phys. Chem. 1995, 99, 17207. (29) Feng, K. I.; Schelly, Z. A. J. Phys. Chem. 1995, 99, 17212. (30) Eicke, H. F.; Christen, H. J. Colloid Interface Sci. 1974, 48, 281. (31) Faure, A.; Tistchenko, A. M.; Zemb, T.; Chachaty, C. J. Phys. Chem. 1985, 89, 3373. (32) Faure, A.; Tistchenko, A. M.; Chachaty, C. J. Phys. Chem. 1987, 91, 1827.
10.1021/la0116995 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/02/2002
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Figure 1. The schematic structures of NaDEHP and AOT.
paid to the difference of the two surfactants in aqueous solution and the effects of additives on their surface tension. The molecular dynamics simulation can give some useful information about the aggregate structure and the behaviors of the surfactants at the oil-water interface from the atomic level.34-38 Recently, it has been more and more used in the study of micelle formation and the behaviors of surfactants at the oil-water interface.38-42 The present paper aims to carry out comparative studies of NaDEHP and AOT in aqueous solution by molecular simulation and surface tension measurements. The surface activity dependence of these two structurally related surfactants on the environment is also investigated. Experimental Section Materials. AOT was purchased from Fluka. NaCl (A.R.), NaOH (A.R.), HDEHP (C.R.), and all alcohols (A.R.) used in this experiment were purchased from Shanghai Chemical Corp. (China). NaDEHP was obtained by the following method: HDEHP and an equivalent molar NaOH solution (5 M) were mixed with stirring for several hours and placed for several days in order to react completely. Poly(vinylpyrrolidone) (PVP), of which the average molecular weight is 38 000, was bought from Beijing Chemical Reagent Co. The water used to prepare solutions was all triply distilled. Methods. The surface tension was measured on a Processor Tensiometer-K12 (Kru¨ss Corp.; the precise degree of the measurement is 0.01 mN m-1) by a Wilhelmy plate. The concentration of PVP changed from 0.5 to 3.0 wt % of the solution. All of the experiments were carried out at 30.0 ( 0.1 °C.
Results and Discussion 1. The Structures of NaDEHP and AOT Molecules in a Vacuum. The three-dimensional shape and size of surfactant molecules play a crucial role in their packing in the aggregates and indirectly determine the aggregation number and diameter of the aggregates formed.34 The optimized structures of NaDEHP and AOT obtained by (33) Shioi, A.; Narada, M.; Matsumoto, K. J. Phys. Chem. 1991, 95, 7495. (34) Derecskei, B.; Dereskei-Kovacs, A.; Schelly, Z. A. Langmuir 1999, 15, 1981. (35) Palmer, B. J.; Liu, J. Langmuir 1996, 12, 746. (36) Alper, H. E.; Stouch, T. R. J. Phys. Chem. 1995, 99, 5724. (37) Alper, H. E.; Bassolino-Klimas, D.; Stouch, T. R. J. Chem. Phys. 1993, 99, 5547. (38) Smit, B.; Hilbers, P. A. J.; Esselink, K.; Rupert, L. A. M.; van Os, N. M.; Schlijper, A. G. Nature 1990, 348, 624. (39) Smit, B.; Hilbers, P. A. J.; Esselink, K.; Rupert, L. A. M.; van Os, N. M.; Schlijper, A. G. J. Phys. Chem. 1991, 95, 6361. (40) Karaborni, S.; van Os, N. M.; Esselink, K.; Hilbers, P. A. J. Langmuir 1993, 9, 1175. (41) Smit, B.; Esselink, K.; Hilbers, P. A. J.; van Os, N. M.; Rupert, L. A. M.; Szleifer, I. Langmuir 1993, 9, 9. (42) Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.; Karthauser, J.; van Os, N. M.; Zana, R. Science 1994, 266, 254.
Figure 2. The optimized structures of NaDEHP and AOT.
minimizing energy of the surfactant molecules using the MOPAC program are shown in Figure 2. The bond angles composed of different atoms in AOT and NaDEHP molecules are listed in Table 1. From the table, we can see that the polar group of NaDEHP is a deformed tetrahedron centered around the phosphorus atom. The bond angle of two hydrophobic chains composed of O(1)-P(19)-O(8) is 102.60°, while that of AOT composed of C(2)-C(1)-C(10) is 109.90° and the angle composed of C(1)-C(10)-C(11) is 110.00°, which suggests that the hydrophobic chains of the AOT molecule are more stretched than those of the NaDEHP molecule. Because the structure of AOT is similar to that of NaDEHP, only five representative structures of NaDEHP (in Figure 3) are chosen for detailed analysis in a vacuum after the optimization has been finished using a UFF force field within the Cerius2 environment.34 Two types of structures are selected in the five representative structures of NaDEHP: one is the “closed tail”,34 such as conf 1, conf 2, and conf 3 in Figure 3, searched by random sampling; the other is the “open tail”, conf 4 and conf 5 in Figure 3, also searched by random sampling. These classes can be distinguished according to the distance between the hydrogens in the end CH3 group of two hydrophobic chains
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Table 1. Bond Angles of the Optimized AOT and NaDEHP Structures by Use of the MOPAC Program AOT
actuala (deg)
optimal (deg)
NaDEHP
actual (deg)
optimal (deg)
C(2b)-C(1)-C(10) C(2)-C(1)-S(25) C(10)-C(1)-S(25) C(1)-C(2)-O(3) C(1)-C(2)-O(23) O(3)-C(2)-O(23) C(2)-O(3)-C(4) O(3)-C(4)-C(5) C(4)-C(5)-C(6) C(4)-C(5)-C(19) C(6)-C(5)-C(19) C(5)-C(6)-C(7) C(6)-C(7)-C(8) C(7)-C(8)-C(9) C(1)-C(10)-C(11) C(10)-C(11)-O(12) C(10)-C(11)-O(24) O(12)-C(11)-O(24) C(11)-O(12)-C(13) O(12)-C(13)-C(14) C(13)-C(14)-C(15) C(13)-C(14)-C(21) C(15)-C(14)-C(21) C(14)-C(15)-C(16) C(15)-C(16)-C(17) C(16)-C(17)-C(18) C(5)-C(19)-C(20) C(14)-C(21)-C(22) C(1)-S(25)-O(26) C(1)-S(25)-O(27) C(1)-S(25)-O(28) O(26)-S(25)-O(27) O(26)-S(25)-O(28) O(27)-S(25)-O(28)
112.909 118.753 111.348 113.604 129.783 116.525 117.108 106.522 109.404 109.794 112.214 111.785 111.285 111.476 117.971 116.106 125.714 118.181 117.414 107.657 109.002 109.729 112.513 112.040 111.299 111.581 112.562 112.885 106.522 105.483 101.418 113.462 111.341 117.106
109.900
C(2)-O(1)-P(19) O(1)-C(2)-C(3) C(2)-C(3)-C(4) C(2)-C(3)-C(15) C(4)-C(3)-C(15) C(3)-C(4)-C(5) C(4)-C(5)-C(6) C(5)-C(6)-C(7) C(9)-O(8)-P(19) O(8)-C(9)-C(10) C(9)-C(10)-C(11) C(9)-C(10)-C(17) C(11)-C(10)-C(17) C(10)-C(11)-C(12) C(11)-C(12)-C(13) C(12)-C(13)-C(14) C(3)-C(15)-C(16) C(10)-C(17)-C(18) O(1)-P(19)-O(8) O(1)-P(19)-O(20) O(1)-P(19)-O(21) O(8)-P(19)-O(20) O(8)-P(19)-O(21) O(20)-P(19)-O(21)
113.967 109.739 108.174 110.563 112.614 112.854 111.220 111.542 113.922 109.680 108.199 110.549 112.513 112.840 111.217 111.544 112.574 112.548 100.446 110.967 103.980 109.942 105.471 123.488
116.000 107.400 109.510 109.510 109.510 109.500 109.500 109.500 116.000 107.400 109.510 109.510 109.510 109.500 109.500 109.500 109.500 109.500 102.600
102.000 107.100 122.500 122.000 109.900 107.400 109.510 109.510 109.510 109.500 109.500 109.500 110.000 107.100 122.500 122.000 109.900 107.400 109.510 109.510 109.510 109.500 109.500 109.500 109.500 109.500 107.700 107.700
108.200 108.200
116.600
a The actual bond angles indicate the bond angles of the molecules originally given. b The number corresponding to the atom is in accord with that in Figure 1.
Figure 3. Five representative geometries of the NaDEHP molecule in a vacuum calculated with the UFF force field. The corresponding energy and area are listed in Table 2.
in one molecule. The structure parameters of AOT and NaDEHP molecules are shown in Table 2 and Table 3. From the molecular structures and the structure param-
eters, some useful information at the molecular level can be obtained: the NaDEHP surfactant molecules may have many geometrical changes corresponding to different energies, which means that the NaDEHP molecules may have different surface areas and volumes in the aggregates formed or at the interface of water and oil. These different energetic fluctuations may indirectly affect the shape of the micelle and the aggregation number of the aggregates. 2. Effects of NaCl on Surface Activity of NaDEHP and AOT. NaDEHP used as an extractant often contains some inorganic electrolytes, especially NaCl, so it has much meaning to investigate the effects of salt on its surface activity. Figure 4 shows the effects of NaCl on the surface activity of NaDEHP and AOT. The minimum surface tension of AOT is about 27 mN m-1 without NaCl. It is in accord with the results obtained by dynamic surface tension measurements.45 It is clear that the surface tension of AOT is obviously decreased by adding 0.005-0.01 M concentrations of NaCl, while that of the NaDEHP system is not markedly decreased even in the solution with 0.01-0.08 M NaCl. Both the critical micelle concentration (cmc) and γcmc for the AOT system are decreased with the increase of NaCl concentration, while that for NaDEHP is not so. The γcmc value for AOT without NaCl is larger than that for NaDEHP although they have the same hydrophobic chains. Because the hydrophilic group of AOT has stronger acidity and the electrical double layer of the polar headgroup of AOT is greatly compressed with NaCl added, the AOT molecules in the surface layer arrange more tightly. But the acid corresponding to NaDEHP is a weak acid and may be hydrolyzed in the solution, giving HDEHP molecules. The HDEHP molecules serve as cosurfactant and screen the electrostatic repulsive in(43) Connolly, M. L. Science 1983, 221, 709. (44) Connolly, M. L. J. Appl. Crystallogr. 1983, 16, 548. (45) Kragh, A. M. Trans. Faraday Soc. 1964, 60, 225.
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Table 2. Energy, Area, and Volume for the Five Representative Structures of NaDEHP Molecules Depicted in Figure 3a parameter Connolly
surfaceb
contact saddle concave total
volume energy
conf 1
conf 2
conf 3
conf 4
conf 5
151.232 159.714 63.157 374.103 435.418 -365.49
158.568 166.224 66.207 391.000 440.293 -362.89
162.648 162.312 65.982 390.942 432.914 -364.66
156.800 163.523 65.530 377.653 443.782 -365.54
151.575 153.875 60.332 365.782 417.061 -342.48
a Energy in kcal/mol, area in Å2, and volume in Å3. b The Connolly surface is the boundary surface around the molecule that is available for interaction with solvent using the algorithms developed by Connolly (refs 43 and 44).
Table 3. Energy, Area, and Volume for the Five Representative Structures of AOT Moleculesa parameter Connolly
volume energy
surfaceb
contact saddle concave total
conf 1
conf 2
conf 3
conf 4
conf 5
178.051 183.609 70.501 432.161 532.967 -97.4454
182.305 182.294 77.280 441.879 528.005 -127.312
178.995 192.094 84.059 455.147 543.136 -118.609
205.915 208.174 84.962 499.052 526.306 -120.978
201.202 201.326 91.177 493.704 537.542 -120.145
a Energy in kcal/mol, area in Å2, and volume in Å3. b The Connolly surface is the boundary surface around the molecule that is available for interaction with solvent using the algorithms developed by Connolly (refs 43 and 44).
Figure 4. Effects of NaCl concentration on the surface tension of NaDEHP and AOT solutions.
teraction between the headgroups of NaDEHP molecules, so the NaDEHP-HDEHP molecules in the surface layer without salt have arranged tightly and are less influenced by NaCl. This is also the reason that the γcmc for NaDEHP aqueous solution is lower than that for AOT. 3. Effects of PVP on the Surface Activity of NaDEHP and AOT. Figure 5 shows the effects of PVP concentration on surface tension of NaDEHP and AOT.
Figure 5. Effects of PVP on the surface tension of NaDEHP and AOT solutions.
Obviously, there are differences in the interactions between the NaDEHP-PVP and AOT-PVP. Generally, surfactant molecules associate with polymers to form surfactant-polymer aggregates (complexes) as the concentration of surfactant is higher than a given concentration. There are two transition points (T1 and T2) in the curve of surface tension versus surfactant concentration if the polymer and surfactant form a complex.46 T1, also named cac (critical aggregate concentration), is usually
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Figure 6. Effects of different alcohols on the surface tension of NaDEHP and AOT.
less than cmc, corresponding to the concentration at which the association between surfactant and polymer starts; T2 is usually more than cmc, corresponding to the concentration at which regular micelles start to form. For the NaDEHP-PVP system, there are no two significant transition points appearing when the concentration of PVP (CP) is less than 1.0 wt %, and T1 and T2 appear when the PVP concentration is higher than 2.0 wt % but they are not so obvious as in the AOT system, which suggests that the interaction between AOT and PVP is stronger than that between NaDEHP and PVP. When CP is less than 1 wt%, the PVP has almost no effect on the surface tension of NaDEHP. The surface tension is decreased with the increase of CP at the same NaDEHP concentration when the NaDEHP concentration is below a certain value (about 4.6 × 10-3 M), while the case for the NaDEHP concentration over that value is the contrary. This is because the NaDEHP molecules are too small to bind with the polymer initially and they exist in the form of free monomers; in this case, they do not influence the surface tension of NaDEHP. The decrease of surface tension in the NaDEHP-PVP system is attributed to the surface activity of PVP itself as the surfactant solution is dilute. This phenomenon agrees well with the results of our previous study.47 No NaDEHP molecule cluster is formed below (46) Xu, G. Y.; Sui, W. P.; Li, G. Z. Acta Chim. Sinica 1997, 55, 1179. (47) Li, F.; Li, G. Z.; Xu, G. Y.; Wang, H. Q.; Wang, M. Colloid Polym. Sci. 1998, 276, 1.
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Figure 7. Effects of pentanol concentration on the surface tension of NaDEHP and AOT.
cmc when the concentration of PVP is 0.5 wt % in the experiment. Therefore, the formation of NaDEHP micelles is not influenced by addition of PVP. The different length of the PVP chain can be obtained by the simulation in the simulated cells.48 The mean squared end-to-end distance (〈r2〉) of polymer chain in the absent of surfactant is 214.23 ( 5.12 Å, and in the presence of AOT and NaDEHP surfactant, the 〈r2〉 values are 201.24 ( 4.71 Å and 174.89 ( 9.65 Å, respectively. At the same surfactant concentration, the 〈r2〉 of polymer is more reduced by NaDEHP than by AOT. There are two reverse actions in the solution as the surfactant is added: (a) the Na+ destroys the hydration layer of PVP molecules, which makes the molecular chains of PVP shrink; (b) the PVP chain is stretched due to the electrostatic interaction caused by the binding of surfactants onto the polymer. The 〈r2〉 of the polymer is decreased only by 13 Å with addition of AOT because these two actions are competitive. However, the 〈r2〉 of PVP is reduced by 40 Å when NaDEHP is added, suggesting that the first action is dominant. The surfactant molecules bound to PVP are fewer, resulting in the weak repulsive interaction, which is also consistent with our surface tension measurements. 4. Effects of Alcohol on the Surface Activity of NaDEHP and AOT. Effects of alcohols with different carbon numbers on the surface tension of NaDEHP and AOT solutions are shown in Figure 6. From the curves, (48) Yuan, S. L.; Liu, C. P.; Xu, G. Y.; Jiang, Y. S. J. Phys. Chem., submitted.
Comparative Studies of Similar Surfactants
we can see that the surface tension decreases in the order of methanol, ethanol, n-propanol, n-butanol, and npentanol in the two surfactant systems. The alcohol has no significant influence on the surface tension when the carbon number of the alcohol is less than five. However, the surface tension is obviously decreased in the presence of 0.1 M n-pentanol. The alcohol added serves as a cosurfactant and can insert between the surfactant molecules, shielding the repulsive interaction between the polar group of the surfactants, so the surfactant molecules could be more easily aggregated; at the same time, the solubility of the two surfactants is increased when the alcohol is added. The hydrophilicity of the alcohol is dominant when the number of alcohol carbons is less than five and most of the alcohol molecules exist in the monomer state, so the efficiency in decreasing the surface tension is lowered. With the number of alcohol carbons increasing, the hydrophobicity of the alcohol molecules is intensified and they can be easily inserted into the surfactant molecules. To obtain detailed information about the effects of alcohol on the surface activity of double-chain surfactants, the surface tension dependence of NaDEHP and AOT on n-pentanol concentration is shown in Figure 7. From the curves, it is seen that the surface tensions are decreased for these two surfactants with the concentration of n-pentanol increasing. This can be easily understood if the cosurfactant action of the alcohol is considered. Conclusions The microstructures of NaDEHP and AOT are investigated by molecular dynamics simulation and surface
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tension measurements. Five representative structures and the parameters such as energy, area, and volume corresponding to them are given. Effects of additives such as NaCl, PVP, and alcohol on the surface tension of NaDEHP and AOT are also investigated. The following results are obtained: 1. The bond angle of the two hydrophobic chains of AOT is larger than that of NaDEHP molecules. 2. A small amount of NaCl (0.005 M) can obviously decrease the surface tension of AOT, but it is “inert” to the NaDEHP solution, even with a higher concentration. 3. Two transition points in the AOT solution appear when PVP is added, but the transition points in the NaDEHP solution are not so obvious even the PVP concentration is higher, which suggests that the interaction between AOT and PVP is stronger than that between NaDEHP and PVP. This phenomenon is consistent with our simulated results that the 〈r2〉 of PVP is more reduced by NaDEHP than by AOT. 4. The surface tensions of the two surfactant solutions are decreased not only with the increase of the carbon number in alcohols but also with the concentration of n-pentanol increasing. Acknowledgment. The authors gratefully acknowledge financial support from the National Natural Science Foundation (29973023) and the Natural Science Foundation (Y2001B08) of Shandong Province in China. LA0116995
