3528
Langmuir 2006, 22, 3528-3532
Interfacial Composition and Structural Parameters of Water/ C12-s-C12‚2Br/n-Hexanol/n-Heptane Microemulsions Studied by the Dilution Method Ou Zheng, Jian-Xi Zhao,* and Xian-Ming Fu Department of Applied Chemistry, College of Chemistry and Chemical Engineering, Fuzhou UniVersity, Fuzhou, 350002, People’s Republic of China ReceiVed October 13, 2005. In Final Form: December 26, 2005 The interfacial composition of the stable water/C12-s-C12‚2Br/n-hexanol/n-heptane microemulsions has been studied in detail by dilution method. The results showed a marked maximum amount of the n-hexanol populating on the surfaces of droplets (represented as a ) nia/ns, where nia and ns are respectively the moles of n-hexanol and gemini surfactant on the surface of droplets) with increasing water content. At a constant level of water addition (the molar ratio of water to surfactant W0 ) 20), a decreased with increasing the spacer length in the C12-s-C12‚2Br molecule. The structural parameters of a w/o microemulsion were also estimated by analyzing the data of dilution experiments, and we found that the radius of the water pool was very sensitive to the increment of water content. The radius of the water pool varied from 0.74 to 5.35 nm with increasing W0 from 10 to 50. The variation extent reached 4.61 nm. In the cases of water/CPC/n-butanol/isopropyl myristate and water/CTAB/n-butanol/isopropyl myristate, however, the corresponding variation extents were only 1.22 and 1.68 nm, respectively, when increasing comparable water content. The ratio of N h a/N h 2C, where N h a and N h 2C are respectively the average numbers of n-hexanol and the total average numbers of alkyl chains of gemini surfactant populating on per droplet surface, decreased obviously with increasing water content at W0 > 15. This indicated that C12-2-C12‚2Br favored to form large droplets that were suitable to solubilize more water.
1. Introduction Preparation and properties of water-in-oil microemulsions have been the focus of extensive investigations in many laboratories1-3 since they are important templates for preparing metal nanoparticles.1 Aerosol OT (AOT), i.e., sodium bis(2-ethylhexyl)sulfosuccinate, is a surfactant investigated extensively and forms easily water-in-oil microemulsion in apolar solvents containing a small content of water.1 Alkyltrimethylammonium salts or alkylpyridinium salts are also able to form reverse microemulsions but generally require the assistance of alcohol.4-18 Addition of alcohol effectively changes the originally unfavorable packing geometry of the surfactant molecules and produces a stable surfactant/alcohol mixed interfacial film. This brings an additional * Corresponding author. Telephone: +86-591-87892035. Fax: +86591-87892035 E-mail:
[email protected]. (1) De, T. K.; Maritra, A. AdV. Colloid Interface Sci. 1995, 59, 95-193. (2) Moulik, S. P.; Paul, B. K. AdV. Colloid Interface Sci. 1998, 78, 99-195. (3) Capek, I. AdV. Colloid Interface Sci. 2004, 110, 49-74. (4) Giustini, M.; Palazzo, G.; Colafemmina, G.; Monica, M. D.; Giomini, M.; Ceglie, A. J. Phys. Chem. 1996, 100, 3190-3198. (5) Boussaha, A.; Ache, H. J. J. Phys. Chem. 1981, 85, 1693-1697. (6) Bisal, S.; Bhattacharya, K. P.; Moulik, S. P. J. Phys. Chem. 1990, 94, 350-355. (7) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1990, 94, 381-387. (8) Jada, A.; Lang, J.; Zana, R. Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387-395. (9) Lang, J.; Lalem, N.; Zana, R. J. Phys. Chem. 1991, 95, 9533-9541. (10) Yao, J.-H.; Romsted, L. S. J. Am. Chem. Soc. 1994, 116, 11779-11786. (11) Li, F.; Li, G.-Z.; Wang, H.-Q.; Xue, Q.-J. Colloids Surf. A 1997, 127, 89-96. (12) Mehta, S. K.; Kawaljit. Colloids Surf. A 1998, 136, 35-41. (13) Hait, S. K.; Moulik, S. P. Langmuir 2002, 18, 6736-6744. (14) Guo, X.; Liu, Y.; Guo, R. Colloids Surf. A 2002, 196, 71-78. (15) Garcia-Rio, L.; Leis, J. R. J. Phys. Chem. B 2002, 104, 6618-6625. (16) Palazzo, G.; Lopez, F.; Giustini, M.; Colafemmina, G.; Ceglie, A. J. Phys. Chem. B 2003, 107, 1924-1931. (17) Lopez, F.; Cinelli, G.; Ambrosone, L.; Colafemmina, G.; Ceglie, A.; Palazzo, G. Colloids Surf. A 2004, 237, 49-59. (18) Abuin, E.; Lissi, E.; Olivares, K. J. Colloid Interface Sci. 2004, 276, 208-211.
advantage to modulate the dimension of water pools and the surface dynamics by changing alcohol content besides the general method of changing water content.4 Such a kind of adjustable template provides a more effective and convenient way for synthesizing metal nanoparticles with different sizes.1,19,20 Alkanediyl-R,ω-bis(dimethylalkylammonium bromides) generally referred to as Cm-s-Cm‚2Br are a series of important gemini surfactants, and their self-assembly in aqueous solution has been extensively investigated in many laboratories.21-23 Cm-s-Cm‚2Br can be considered as a dimeric molecule of the corresponding alkyltrimethylammonium bromide linked by a spacer of variable length at the heads.24 Such a link by the spacer obviously changes the charge densities of the heads and the molecular geometry in comparison with those of the corresponding single-tail surfactant. The former leads certainly to the variation of the electric dipole moment on the heads of Cm-s-Cm‚2Br in apolar solvent. Therefore, it is possible to affect the formation and properties of microemulsions. However, only one paper relates to the formation of oil-in-water rather than water-in-oil microemulsions with Cm-s-Cm‚2Br so far, where the authors studied the polymerization of styrene in such oil drops.25 Recently, we have been interested in investigating the selfassembly of quaternary ammonium gemini surfactants in various apolar solvents, including the microemulsion formation upon the addition of water. We previously reported the apparent (19) Curri, M. L.; Agostiano, A.; Manna, L.; Monica, M. D.; Catalano, M.; Chiavarone, L.; Spagnolo, V.; Lugara, M. J. Phys. Chem. B 2000, 104, 83918397. (20) Chen, D.-H.; Wu, S.-H. Chem. Mater. 2000, 12, 1354-1360. (21) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205-253 and references therein. (22) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906-1920 and references therein. (23) Zana, R.; Xia, J.-D. Gemini Surfactants; Marcel Dekker: New York, 2004. (24) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 113, 1451-1452. (25) Dreja, M.; Tieke, B. Langmuir 1998, 14, 800-807.
10.1021/la052772k CCC: $33.50 © 2006 American Chemical Society Published on Web 03/18/2006
Composition and Parameters of Microemulsions
regularity of C12-s-C12‚2Br dissolution in n-heptane with the assistance of n-hexanol and found the critical moles (na) of n-hexanol required to dissolve given moles (ns) of C12-s-C12‚2Br in n0 moles of n-heptane obeying the equation na ) an0 + bns.26 The roles of alcohol on the stabilizing droplet surface have been stressed in many laboratories,4,13,16,27 which relates to the composition of the droplet surface. To estimate the distribution of alcohol between the interfacial plane and the bulk oil phase, SANS,28,29 conductivity,30-32 interfacial tension,29,33 and SAXS and DLS29 methods have been used. Besides these, a simple but elegant method of dilution, accomplished by adding oil at constant water and surfactant levels to destabilize an otherwise stable w/o microemulsion and then restabilizing it by adding a requisite amount of alcohol, has been recommended by a number of authors.13,27,29,30,34-36 Though Gu et al.37 raised doubts about the method of dilution, it has been recently again confirmed to be correct by Moulik 27 and Palazzo.38 In the present paper, we investigated the surface composition of the w/o droplets by dilution method, in which C12-s-C12‚2Br is used as the surfactant, n-hexanol as the cosurfactant, and n-heptane as the oil with addition of different amounts of water. The structural parameters of the w/o microemulsion are then estimated by analyzing the experimental data. The effect of the spacer length on the interfacial composition is discussed so as to realize the significance of the special molecular structure of the gemini surfactant on microemulsion formation.
Langmuir, Vol. 22, No. 8, 2006 3529 volume of oil was added in the system to destabilize it, and the solution appeared cloudy again. The n-hexanol was further added carefully until the solution became again just clear, and its volume was noted. The procedure was repeated several times to get the volumes of oil and n-hexanol at each step. The error of the dilution experiments was estimated to be ca. (3%.
3. Theoretical Consideration 3.1. Principle of the Dilution Method. For a stable w/o microemulsion, a critical amount of n-hexanol (cosurfactant) distributes between the bulk oil phase and the droplet surface, whereas the cationic surfactant almost solely resides on the droplet surface since it is unsolvable in n-heptane.16 Supposing the number of moles of n-hexanol at the interface and in bulk oil phase being nia and noa , respectively, the total number of moles of n-hexanol na equals to the sum of both, where the number of moles of n-hexanol in water, nwa , is negligible because of its very small solubility.
na ) nia + noa
Defining a ) nia/ns and b ) noa /no, where ns and no are the total number of moles of surfactant and n-heptane, respectively, eq 1 becomes
()
no na )a+b ns ns
2. Experimental Section 2.1. Materials. Alkanediyl-R,ω-bis(dimethyldodecylammonium bromides) C12-s-C12‚2Br were synthesized in our laboratory.37 The surfactants are represented as follows:
(26) Fu. X.-M.; Zheng, O.; Zhao, J.-X. Chin. Chem. Bull. (Chin.) 2005, 68, w121. (27) Moulik, S. P.; Digout, L. G.; Aylward, W. M.; Palepu, R. Langmuir 2000, 16, 3101-3106. (28) Caponetti, E.; Lizzio, A.; Triolo, R. Langmuir 1990, 6, 1628-1634. (29) Caponetti, E.; Lizzio, A.; Triolo, R.; Griffin, W. L.; Johnson, J. S., Jr. Langmuir 1992, 8, 1554-1562. (30) Petit, C.; Bommarius, A. S.; Pileni, M. P.; Hatton, T. A. J. Phys. Chem. 1992, 96, 4653-4658. (31) Lagues, M.; Sauterey, C. J. Phys. Chem. 1980, 84, 3503-3508. (32) Bisal, S. R.; Bhattacharya, P. K.; Moulik, S. P. J. Phys. Chem. 1990, 94, 350-355. (33) Kegel, W. K.; van Aken, G. A.; Bouts, M. N.; Lekkerkerker, H. N. W.; Overbeek, J. Th. G.; de Bruyn, P. L. Langmuir 1993, 9, 252-256. (34) Bowcott, J. E.; Schulman, J. H. Z. Elektrochem. 1955, 59, 283. (35) Bansal, V. K.; Chinnaswamy, K.; Ramachandran, C.; Shah, D. O. J. Colloid Interface Sci. 1979, 72, 524-537. (36) Kumar, S.; Singh, S.; Singh, H. J. Surf. Sci. Technol. 1986, 2, 85-91. (37) Gu, G.; Wang, W.; Yan, H. J. Therm. Anal. 1998, 51, 115-123. (38) Giustini, M., Murgia, S., Palazzo, G. Langmuir 2004, 20, 7381-7384.
(2)
In the dilution experiment at fixed ns, na and no are varied so as to get a series of na/ns and no/ns values, whose graphical plotting according to eq 2 yields the values of a and b from the intercept and the slope, respectively. Then the corresponding molar fraction of n-hexanol at the interface (Xia) and in the bulk oil phase (Xoa ) is obtained:
Xia Trace water was first removed from n-heptane (SCR, AR quality) by addition of calcium chloride and then was distilled before use. n-Hexanol (SCR, AR quality) was purified by reduced pressure distillation. The water used was Milli-Q grade. 2.2. Dilution Experiments. In the dilution experiment, a fixed amount of surfactant (C12-s-C12‚2Br) was placed in a series of dry test tubes and mixed with constant amounts of oil (n-heptane) and water. The samples were placed in the thermostatic water bath at 30 ( 0.1 °C, and then n-hexanol was added slowly by a buret under stirring using a magnetic stirrer until the original viscous and turbid solution became just clear. The test tubes were then stoppered and kept in the thermostatic water bath for 24 h to make sure the systems reached equilibrium. The critical volume of n-hexanol required for the solution becoming just clear was noted. Then, a small but accurate
(1)
)
Xia )
nia nia
+ ns noa
noa
+ ns
)
)
nia/ns nia/ns
+1
noa /no noa /no
+1
)
a a+1
(3)
)
b b+1
(4)
The distribution constant K can be related to the intercept (a) and the slope (b) of eq 2 as
K)
Xia Xoa
)
a(b + 1) b(a + 1)
(5)
This gives the standard Gibbs energy of transfer (∆Goofi) of n-hexanol from the oil to the interface by the relation o ) -RT ln K ∆Gofi
(6)
3.2. Estimation of the Structural Parameters of the Microemulsion. The droplets of the w/o microemulsion are assumed to be spherical and monodisperse and with a surface monolayer comprised of surfactant and n-hexanol molecules. The following relation represents the total volume of the droplets (Vd):
4 Vd ) πRe3Nd 3
(7)
where Re and Nd are the effective radius (including the interface) and the total number of droplets in the solution, respectively.
3530 Langmuir, Vol. 22, No. 8, 2006
Zheng et al.
The Vd can be also calculated from another point of view; that is, it is equal to the sum of the volumes of water (VH2O), of surfactant (Vs), and of n-hexanol (Via) at the interface
Vd ) VH2O + Vs + Via
(8)
Via can be obtained from the dilution experiment (parameter a)
Via )
niaMa ansMa ) Fa Fa
(9)
where Ma and Fa are the molar mass and the density of n-hexanol, respectively. The Vs is estimated in terms of the contribution of various groups (the volumes of tail (Vtail) and of head (Vhead) per surfactant molecule) as suggested by Hirata and his colleagues40
Vs ) (Vtail + Vhead)nsNA
(10)
Vtail ) 2[VCH3 + (nc - 1)VCH2]
(11)
Vhead ) 2(2VCH3 + VN) + sVCH2 + 2VBr
(12)
where NA is the Avogadro constant and VCH3, VCH2, VN, and VBr are 0.0426, 0.0282, -0.0305, and 0.0500 nm3, respectively.40 When the size of droplets is relative large, those molecules of the surfactant and n-hexanol are considered to pack closely on the surface of the water pool, and thus, the total sectional area of the surfactant and n-hexanol occupied equals approximately the total surface area of the water pool, i.e.
(nsAs + niaAa)NA ) (As + aAa)nsNA ) 4πRw2Nd (13) or
Rw ) 3VH2O/(As + aAa)nsNA
(14)
where As and Aa are the sectional areas of the headgroup of the surfactant and n-hexanol, respectively. Rw is the radius of water pool. The effective values of As can be estimated from Vhead, which are 0.50 (s ) 2), 0.53 (s ) 3), 0.57 (s ) 4), 0.60 (s ) 5), and 0.63 nm2 (s ) 6), respectively. The value of Aa of n-hexanol is 0.16 nm2.41 Thus, Re is obtained from the relation 3
Re ) Rw
x
Vd VH2O
(15)
Putting the values of Re and Vd in eq 7, we get the value of Nd. Thus, the effective thickness of the interfacial layer of droplet dI also follows
d I ) R e - Rw
(16)
The average aggregation numbers of surfactant (N h s) and n-hexanol (N h a) on the droplet surface are further estimated by the following relations:
N h s ) nsNA/Nd
(17)
N h a ) niaNA/Nd ) ansNA/Nd
(18)
(39) You, Y.; Zheng, O.; Qiu, Y.; Zheng, Y. H.; Zhao, J. X. Han, G. B. Acta Physico-Chim. Sinica (Chin.) 2001, 17, 74-78. (40) Hirata, H.; Hattori, N.; Ishida, M.; Okabayashi, H.; Frusaka, M.; Zana, R. J. Phys. Chem. 1995, 99, 17778-17784.
Figure 1. Plots of na/ns vs no/ns for C12-2-C12‚2Br (0.3 mmol)/ n-heptane/n-hexanol systems at different W0 ()[H2O]/[surfactant]).
Figure 2. Plots of na/ns vs no/ns for C12-s-C12‚2Br (0.3 mmol)/ n-heptane/n-hexanol systems at W0 ) 20 Table 1. Distribution of n-Hexanol at the Droplet Surface and in the n-Heptane Solution at a Fixed Amount (0.3 mmol) of C12-2-C12‚2Br but at Different W0 W0 2.5 5.0 7.5 10 15 20 30 40 50
a)
nia/ns
2.58 3.18 3.58 3.60 3.52 2.50 1.87 1.84 1.70
b)
noa /no
0.0180 0.0215 0.0267 0.0273 0.0428 0.0483 0.0346 0.0269 0.0239
Xa,m
Xa,o
K
0 , ∆Gofm kJ/mol
0.721 0.761 0.781 0.783 0.779 0.714 0.652 0.648 0.629
0.0177 0.0211 0.0260 0.0265 0.0411 0.0461 0.0334 0.0262 0.0233
40.7 36.1 30.0 29.5 19.0 15.5 19.5 24.7 27.0
-9.34 -9.04 -8.57 -8.53 -7.42 -6.91 -7.49 -8.09 -8.31
4. Results and Discussion 4.1. Interfacial Composition of the Microemulsion. Figure 1 shows the plots of na/ns vs no/ns at a fixed amount (0.3 mmol) of C12-2-C12‚2Br, in which each plot corresponds to a constant level of water addition. A similar graph at the same amount (0.3 mmol) of C12-s-C12‚2Br homologues and a fixed W0 (the molar ratio of water to surfactant) is shown in Figure 2. From the two o are obtained graphs, the values of a, b, Xia, Xoa , K, and ∆Gofi according to eqs 2-6 and are reported in Tables 1 and 2. In terms of the dilution method, the quantities of n-hexanol listed in Tables 1 and 2 corresponds to each minimum requirement yielding stable w/o microemulsion at constant contents of water and surfactant. Lowering of the n-hexanol concentrations by the addition of oil will destabilize the microemulsion. It can be seen from the data in Table 1 that at a constant content of C12-2-C12‚2Br, the initial increase in water content requires a increasing, where a represents the population of n-hexanol on the droplet surface, but at ca. W0 > 10 further increasing water content leads rather to a rapid reduction in a. This phenomenon
Composition and Parameters of Microemulsions
Langmuir, Vol. 22, No. 8, 2006 3531
Table 2. Effect of Spacer Length on the Distribution of n-Hexanol at the Droplet Surface and in n-Heptane Solution at Fixed Amount (0.3 mmol) of C12-s-C12‚2Br and at W0 ) 20 a)
s 2 3 4 5 6
nia/ns
2.50 3.33 3.93 4.42 4.60
b)
noa /no
0.0483 0.0494 0.0596 0.0720 0.0820
Xa,m
Xa,o
K
o ∆Gofm , kJ/mol
0.714 0.769 0.797 0.815 0.821
0.0461 0.0471 0.0562 0.0671 0.0758
15.5 16.3 14.2 12.1 10.8
-6.91 -7.04 -6.68 -6.30 -6.01
Table 3. Packing Parameters of C12-s-C12‚2Br Homologuesa s
2
3
4
5
6
CPC
CTAB
P ) V/(a0l)b Peff at W0 ) 20
0.58 1.06
0.45 1.07
0.43 1.09
0.37 1.10
0.33 1.09
0.40
0.28
a
The P values of CPC and CATB are also listed for comparison. b The values of V and l are calculated by Tanford formulas44 and the average coefficient of 0.85 is adopted to revise the calculated values of l as suggested by Mitchell and Ninham.43 The values of a0 are from the literature.41,49,50
is similar to the case with CPC (cetyl pyridinium chloride) as the surfactant but different from the case with CTAB (cetyl trimethylammonium bromide) as the surfactant; in both, isopropyl myristate and n-butanol were used as the oil phase and the cosurfactant, respectively.13 Even if in the CPC case, however, the maximum in the population of n-butanol on the droplet surface was not marked, the reduction in n-butanol quantity was considerably smaller than that of the present case beyond the maximum. This may be due to the different molecular geometries of the two surfactants, which has been demonstrated to be a main factor determining the size and shape of the aggregates.40 Israelachvili et al. suggested the dimensionless packing parameter P of the molecular geometry as an index to predict the size and shape of the micelles.42 P was defined as V/(a0l), where V is the hydrocarbon chain volume, a0 is the optimum headgroup area per molecule, and l is the hydrocarbon chain length that is taken to be ca. 80-90% of the fully extended chain length.43 The overall prediction was concluded as follows:42,43 spherical micelles P < 1/3 cylindrical micelles 1/3 < P < 1/2 bilayers (or vesicles) 1/2 < P < 1 inverted structures P > 1 In general, V and l are calculated by Tanford formulas44 and a0 is determined from the aggregation number when assuming spherical micelle45 or is approximately estimated by the minimum adsorbed area at the interface.46 For gemini surfactants, it is difficult for calculating a0 in terms of its aggregation number since the micelles continuously grow with increasing the concentration beyond the cmc,47 and thus, we adopt an approximate estimation in terms of the minimum adsorbed area at the interface. Although all of the values of P for the C12-s-C12‚2Br homologues are not larger than 1 (see Table 3), the fact that these surfactants form inverted structures just emphasizes the role of added n-hexanol that fills in the gaps among the surfactant on the droplet surface and leads to the formation of stable w/o microemulsions. (41) Biais, J.; Bodet, J. F.; Clin, B.; Lalanne, P.; Roux, D. J. Phys. Chem. 1986, 90, 5835-5841. (42) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1525-1568. (43) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601-629. (44) Tanford, C. J. Phys. Chem. 1972, 76, 3020-3024. (45) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774-783. (46) Dupont, A.; Eastoe, J.; Murray, M.; Martin, L.; Guittard, F.; de Givenchy, E. T. Langmuir 2004, 20, 9953-9959. (47) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448-1456.
Table 4. Structural Parameters of the w/o Microemulsions at a Constant Amount (0.3 mmol) of C12-2-C12‚2Br but at Different W0 W0
Rw/nm
Re/nm
dI/nm
N h s(N h 2C)
N ha
N h a(N h 2C)
10-18Nd
10 15 20 30 40 50
0.83 1.27 1.99 3.37 4.52 5.82
1.57 2.13 3.02 4.58 5.81 7.19
0.74 0.87 1.03 1.21 1.29 1.37
8 (16) 19 (38) 55 (110) 178 (356) 323 (646) 553 (1106)
29 67 139 333 594 934
1.8 1.8 1.3 0.9 0.9 0.8
22.2 9.54 3.25 1.01 0.56 0.33
Table 5. Structural Parameters of the w/o Microemulsions at Constant Amounts of C12-s-C12‚2Br Homologues (0.3 mmol) and of Water (W0 ) 20) s
Rw/nm
Re/nm
dI/nm
N h s(N h 2C)
N ha
N h a(N h 2C)
10-18Nd
2 3 4 5 6
1.99 1.68 1.50 1.37 1.31
3.02 2.63 2.40 2.23 2.15
1.03 0.95 0.90 0.86 0.84
56 (112) 33 (66) 24 (48) 18 (36) 16 (32)
139 111 93 80 73
1.3 1.7 2.0 2.2 2.3
3.25 5.43 7.66 10.0 11.1
For the mixed film consisting of C12-s-C12‚2Br and n-hexanol, the effective packing parameter Peff can be calculated by the following formula48
Peff ) (V/a0l)eff )
(XV/a0l)surfant + (XV/a0l)hexanol ) Xsurfant + Xhexanol (V/a0l)surfant + (aV/a0l)hexanol (19) 1+a
where X is the mole fraction of the corresponding species at the mixed film and a comes from the dilution experiment (see Table 1). Table 3 shows all Peffs at W0 ) 20 being indeed larger than 1, which explains well the reverse micelle formation. In comparison with CPC, C12-2-C12‚2Br has a considerably larger P (see Table 3) and favors the formation of large aggregates. This means it is suitable to constructing a large water pool that requires only a small amount of n-hexanol to fill the gaps on the surface, which interprets a rapid reduction in a with increasing W0 beyond the maximum, and thus, a marked maximum occurs in the present case compared with that of CPC. Table 3 shows that the P of individual C12-s-C12‚2Br decreases with increasing s, but the Peffs have almost identical values, which indicates the amount of n-hexanol populating on the droplet surface constructed by the gemini surfactants should be different with changing the spacer length (s). From the theoretical analysis, the smaller P requires the larger amount of n-hexanol populating on the droplet surface. This agrees well with the experimental results shown in Table 2. All of the values of ∆Goofi are negative, and these absolute values are comparable with those in the cases of CPC/n-hexanol/ alkane (C6 and C7)28 and of CPC or CTAB/n-butanol/isopropyl myristate.13 4.2. Structural Parameters of the Microemulsions. The structural parameters of the microemulsions at a constant amount of C12-2-C12‚2Br but with varied amounts of water are presented in Table 4 and at fixed amounts of C12-s-C12‚2Br homologues and water are given in Table 5 in order to compare the spacer length effect. The radii of the present droplets are more sensitive to the increment of water content in comparison with those droplets formed by CPC or CTAB in isopropyl myristate oil (48) Evans, D. F.; Ninhyam, B. W. J. Phys. Chem. 1986, 90, 226-234. (49) Wetting, S. D.; Verrall, R. E. J. Colloid Interface Sci. 2001, 235, 310316. (50) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John Wiley & Sons Inc.: New York, 1989.
3532 Langmuir, Vol. 22, No. 8, 2006
with n-butanol as the cosurfactant.13 In the CPC case, when W0 ) 12.6 and 50.3, Rw and Re are 2.50 and 3.57 nm and 3.72 and 4.67 nm, respectively, and in the CTAB case, when W0 ) 13.5 and 54, Rw and Re are 3.46 and 4.91 nm and 5.14 and 6.24 nm, respectively. Whereas in the present case, when W0 ) 10 and 50, the corresponding values of Rw and Re are 0.74 and 1.39 nm and 5.35 and 6.61 nm, respectively, in which the variation extent of the radius of the water pool reaches 4.6 nm. The variation extents of the radius of the water pool are only 1.22 nm for CPC and 1.68 nm for CTAB corresponding to an increasing comparable water content. The adjustable radius of the water pool is useful for preparing metal nanoparticles with different sizes using the w/o microemulsion as a template. Relatively, the variation of Rw with the spacer length (s) from 2 to 6 is not distinct at fixed water content (W0 ) 20), though the Rw of C12-2-C12‚2Br is ca. 1.5folds as large as that of C12-6-C12‚2Br. In fact, C12-2-C12‚2Br does not display its advantage for the solubilization of less water, where the Rw is smaller than that with CPC or CTAB as the surfactant, respectively, but it is obviously suitable to solubilizing more water as favoring to form large droplets. As mentioned in section 4.1, the surfaces of large droplets constructed by C12-2-C12‚2Br need only a small amount of n-hexanol to fill the gaps among the heads of the surfactant. This agrees well with the obvious decrease of the ratio of N h a/N h 2C
Zheng et al.
with increasing water content at W0 > 15 as shown in Table 4, where N h 2C is the total average numbers of alkyl chains of the gemini surfactant populating on each droplet surface. Another reasonable result appears in the increase in N h a/N h 2C with increasing the spacer length (s) in the C12-s-C12‚2Br homologues at W0 ) 20 (see Table 5) since their packing parameters reduce gradually with s. In the case of CPC with n-butanol as the cosurfactant, the decrease in N h a/N h s appears at W0 > 30.13 This supports the above view of C12-2-C12‚2Br favoring the formation of large droplets that are suitable to solubilize more water. The trend of the effective thickness (dI) of the interfacial layer varying with s is contrary to that of N h a/N h 2C. Obviously, the increase in the content of the n-hexanol at the interfacial layer will reduce dI since its length is much shorter than that of C12-s-C12‚2Br. The Nd values obviously decrease with increasing water content so as to satisfy the thermodynamic requirement of stability at the constant content of surfactant. In the case of C12-s-C12‚2Br homologues, the Nd values increase gradually with increasing s from 2 to 6, which follows the varying tendency of Rw and Re. Acknowledgment. The work was supported by National Natural Science Foundation of China (20373012). LA052772K
