Aggregation Behavior of Nitrophenoxy-Tailed Quaternary Ammonium

Also, the variations in 1H NMR signals at different surfactant concentrations provide the .... All of the measurements were performed at 25.0 ± 0.1 Â...
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J. Phys. Chem. B 2007, 111, 12439-12446

12439

Aggregation Behavior of Nitrophenoxy-Tailed Quaternary Ammonium Surfactants Xu Huang, Yuchun Han, Yingxiong Wang, and Yilin Wang* Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: April 23, 2007; In Final Form: August 25, 2007

Cationic surfactants N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (N10TAB) and N,N,N′,N′tetramethyl-N,N′-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide (N10-6-10N), bearing aromatic nitrophenoxy groups in the ends of their hydrophobic chains, have been synthesized, and their selfassembling properties in aqueous solutions have been studied by conductivity, isothermal titration microcalorimetry, 1H NMR spectroscopy, and dynamic light scattering. Below the critical micelle concentration, N10-6-10N can form premicelles with 2 or 3 surfactant molecules. Beyond the critical micelle concentration, the two surfactants have strong self-aggregation ability and can form micelles of rather small size and with small aggregation numbers N, which are 30 ( 3 for N10TAB and 20 ( 2 for N10-6-10N, respectively. Also, the variations in 1H NMR signals at different surfactant concentrations provide the information on the environmental change of the surfactants upon their micellization progress. The most prominent phenomenon is the shielding effect of the aromatic groups over the protons in the aliphatic chains, implying that the nitrophenoxy groups partially insert into the micelles and face the several middle methylenes of the hydrophobic side chains.

Introduction Cationic surfactants have been extensively utilized in a wide range of industrial, domestic, and medical applications. Numerous attempts have been made to modify and manipulate surface and solution properties of these surfactants by rationally tuning their molecular structures. In the past two decades, gemini surfactants, which have two hydrophilic headgroups and two hydrophobic tails connected by a linking spacer, have brought revolutionary improvement for the performance of cationic surfactants, because of their high surface activity, low critical micelle concentration, multifarious aggregate structures, and unusual viscosity behavior.1 To investigate the chemical and physical properties of surfactant solutions, a great deal of experimental methods have been employed. NMR spectroscopy, for instance, is a versatile and powerful technique to study the formation and properties of organized assemblies.2 It has been evidenced that 1H NMR chemical shifts of a surfactant can be influenced by its medium polarity and its molecular conformation.3 The variation of the chemical shifts versus the surfactant concentration can provide useful information on critical micelle concentration (CMC), aggregation number (N), and intermolecular interaction.4 Moreover, the ring current of aromatic group affects the chemical shifts of nearby protons and gives evidence on its locus in the micelle.5 Recently, it was reported that the surfactants containing a solvent-sensitive, hydrophobic chromophore of 4-nitrophenol can serve as “molecular rulers” to probe the thickness and roughness of liquid/liquid interfaces.6 Because the 4-nitrophenoxy group is a sensitive indicator to solvent polarity, it is competent for probing how local dielectric environment around a solute molecule changes across the interfaces. Moreover, the * To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn.

4-nitrophenoxy chromophore is ideal for being used as molecular rulers because of its photochemical stability and its large change in permanent dipole upon excitation. Its small size imparts finer spatial resolution than the probes with large chromophores, which have extensive and delocalized electronic structures. However, there are no further investigations on the selfassembling properties and aggregate structures of these molecular rulers. To further understand the aggregation behavior of this kind of surfactant, we synthesized two model molecules of such a kind of cationic molecular ruler, N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (N10TAB) and N,N,N′,N′tetramethyl-N,N′-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide (N10-6-10N), and then studied their aggregation behavior in aqueous solution by electrical conductivity, isothermal titration calorimetry, 1H NMR spectroscopy, and dynamic light scattering. The structures of these two surfactants are presented in Figure 1. One nitrophenoxy group exists at the end of each hydrophobic tail of the surfactants. The CMCs of the surfactants were determined by independent techniques, and they were in excellent agreement with each other. The thermodynamic parameters of micellization of the two surfactants illuminate that the aromatic group facilitates the aggregation process. The loci of the nitrophenoxy group in the micelles were derived from the NMR studies. Also, we obtained the aggregation numbers by fitting our NMR data with the model based on the mass action law. The DLS results indicate that both N10TAB and N10-6-10N can form small micelles with a diameter paralleling their extended molecular length, while the gemini surfactant N10-6-10N can form vesicles at high concentration. Experimental Section Materials. N,N,N′,N′-Tetramethyl-1,6-diaminohexane, 1,10dibromodecane, and 4-nitrophenol were from Aldrich. All of

10.1021/jp0731046 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007

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Figure 1. Chemical structures and 1H NMR signal assignments of the studied surfactants.

the inorganic salts were purchased from Beijing Chemical Co. and were of analytical grade. All of the organic solvents were dried and distilled. Triply distilled water was used in all experiments. Synthesis. Compounds N10TAB and N10-6-10N were synthesized according to Scheme 1 and characterized by 1H NMR, mass spectrum, and elemental analysis. 10-(4-Nitrophenoxy)decyl Bromide (N10Br).7 A mixture of 4-nitrophenol (1.39 g, 10 mmol), 1,10-dibromodecane (3.60 g, 12 mmol), potassium carbonate (1.38 g, 10 mmol), and tetrabutylammonium bromide (1.0 g, 0.33 mmol) in 25 mL of water was heated at 100 °C for 3 h with vigorous stirring. After being cooled, the mixture was extracted with chloroform (20 mL × 3). The combined extracts were dried over anhydrous potassium carbonate and filtered. The solvent was removed in vacuo, and the residue was column separated with hexane/ dichloromethane ) 2:1 to afford 2.76 g (77%) of N10Br as yellow crystals (mp 57-58 °C). 1H NMR (CDCl3, ppm): δ 1.1-1.2 (m, 12H, (CH2)6CH2CH2Br), 1.48 (m, 2H, CH2CH2Br), 1.52 (m, 2H, OCH2CH2), 3.44 (t, 2H, CH2Br), 3.96 (t, 2H, J ) 6.4, OCH2), 6.90 (d, 2H, J ) 9.2, ortho-NO2-ArH), 8.15 (d, 2H, J ) 9.2, meta-NO2-ArH). MS-EI (m/z): calcd, 357; found, 358 (M + 1). Anal. Calcd for C16H24BrNO3: C, 53.64; H, 6.75; N, 3.91. Found: C, 53.82; H, 6.88; N, 3.89. N,N,N-Trimethyl-10-(4-nitrophenoxy)decylammonium Bromide (N10TAB). N10Br (0.71 g, 2 mmol) was added to a large excess amount of trimethylamine (33% in ethanol solution, 10 mL), stirred, and heated at 40 °C for 48 h. Upon completion, the solvent was removed in vacuo, and the residue was repeatedly recrystallized from acetone/methanol (90:10 in volume) to afford 0.68 g (81%) of N10TAB as light yellow crystals. MS-ESI (m/z): calcd, 416; found, 337 (M - Br). Anal. Calcd for C19H33BrN2O3: C, 54.68; H, 7.97; N, 6.71. Found: C, 54.82; H, 8.01; N, 6.81. 1H NMR analyses of N10TAB and N10-6-10N are given in the Results and Discussion and Supporting Information. N,N,N′,N′-Tetramethyl-N,N′-bis[10-(4-nitrophenoxy)decyl]1,6-hexanediammonium Dibromide (N10-6-10N). N,N,N′,N′Tetramethyl-1,6-diaminohexane (0.24 g, 1.5 mmol) was added to a solution of N10Br (1.43 g, 4 mmol) in 10 mL of methanol and heated at 40-50 °C for 72 h. The solvent was then removed in vacuo, and the residue was repeatedly recrystallized from acetone/methanol (80:20 in volume) to afford 0.93 g (70%) of N10-6-10N as a white solid. MS-ESI (m/z): calcd, 886; found, 365 [(M - 2Br)/2]. Anal. Calcd for C42H72Br2N4O6: C, 56.75; H, 8.16; N, 6.30. Found: C, 56.87; H, 8.18; N, 6.19.

Huang et al. Electrical Conductivity. Electrical conductivity was used to determine the CMC and the micelle ionization degree (R) values of N10TAB and N10-6-10N. The conductivity of the surfactant solutions was measured as a function of concentration, using a JENWAY model 4320 conductivity meter. Measurements were performed in a temperature-controlled, double-walled glass container with a circulation of water. Sufficient time was allowed between successive additions to allow the system to equilibrate. During the conductivity run, the temperature of the solution was maintained at 25.0 ( 0.1 °C. Isothermal Titration Microcalorimetry. A TAM 2277-201 isothermal titration microcalorimeter (Thermometric AB, Ja¨rfa¨lla, Sweden) was used to measure the CMC values and the enthalpy changes for micelle formation of these surfactants. Both the sample cell and the reference cell of the microcalorimeter are 1 mL, which were initially loaded with 0.6 and 0.7 mL of pure water, respectively. Concentrated surfactant solution was injected consecutively into the stirred sample cell in each portion of 10 µL using a 500 µL Hamilton syringe controlled by a Thermometric 612 Lund pump until the desired concentration range had been covered. During the whole titration process, the system was stirred at 50 rpm with a gold propeller, and the interval between two injections was sufficiently long for the signal returning to the baseline. The observed enthalpies (∆Hobs) were obtained by integrating the area of the peaks in the plot of thermal power against time. The accuracy of the calorimeter was periodically calibrated electrically and verified by measuring the dilution enthalpy of concentrated sucrose solution. The reproducibility of experiments was within (4%. All of the measurements were performed at 25.00 ( 0.01 K. NMR. 1H NMR measurements were carried out at 20.7 ( 0.3 K on a Bruker AV400 FT-NMR spectrometer operating at 400.1 MHz. Deuterium oxide (99.9%) was purchased from CIL Cambridge Isotope Laboratories and used to prepare the stock solutions of the surfactants in D2O. About 0.7 mL of each of the solutions was transferred to a 5 mm NMR tube for the measurement. The center of the HDO signal (4.479 ppm) was used as the reference in D2O solutions. In all NMR experiments, the number of scans was adjusted to achieve good signal-tonoise ratios depending on the surfactant concentration and was recorded with a digital resolution of 0.04 Hz/data point. The signal assignments of the surfactants were achieved by the 2D NMR methods (1H-1H COSY). X-ray Diffraction (XRD). Self-supported cast films were prepared by dispersing the surfactant solutions onto pre-cleaned glass plates, and then air-dried at room temperature. Finally, the plates were kept under vacuum for 15 min. Reflection XRD studies were carried out with an X-ray diffractometer (Rigaku model D/MAX2500). The X-ray beam was generated with a Cu anode at 40 kV and 200 mA, and the wavelength of the KR1 beam was 1.5406 Å. The X-ray beam was directed to the edge of film, and the scanning 2θ was recorded from 1° to 15°, using a step width of 0.01°. Dynamic Light Scattering (DLS). Measurements were carried out using an LLS spectrometer (ALV/SP-125) with a multi-τ digital time correlater (ALV-5000). Light of λ ) 632.8 nm from a solid-state He-Ne laser (22 mW) was used as the incident beam. The measurement was conducted at a scattering angle of 90°. All of the solutions were filtered through a 0.45 µm membrane filter of hydrophilic PVDF before the measurements. The correlation function was analyzed from the scattering data via the CONTIN method to obtain the distribution of diffusion coefficients (D) of the solutes. The apparent hydrodynamic radius Rh was deduced from D by the Stokes-Einstein

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J. Phys. Chem. B, Vol. 111, No. 43, 2007 12441

Figure 2. Variation of the electrical conductivity (k) with the surfactant concentration (C) for (a) N10TAB and (b) N10-6-10N at 25.0 °C.

SCHEME 1: Synthetic Procedure of N10TAB and N10-6-10N

equation Rh ) kBT/(6πηD) for spherical particles, where kB represents the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. All of the measurements were performed at 25.0 ( 0.1 °C. Transmission Electron Microscopy (TEM). The TEM samples were prepared by negative-staining method.8 A carbon Formvar-coated copper grid (300 mesh) was laid on one drop of the sample solution for 10 min, and the excess solution was wiped away with filter paper. Next, the copper grid was put onto one drop of uranyl acetate solution (1%) as the staining agent. The excess liquid was also wiped with filter paper. After being dried, the samples were imaged under a JEOL-200CX electron microscope at a working voltage of 100 kV. Results and Discussion Electrical Conductivity. The electrical conductivity (k) of N10TAB and N10-6-10N is plotted against the surfactant concentration (C) in Figure 2. Each curve furnishes two straight lines, which intersect at the concentration corresponding to the micelle formation, which allows for the identification of the CMC. Also, the micelle ionization degree (R) can be obtained using the method demonstrated by Evans.9 Following Evans’

idea, the electrical conductivity data of N10TAB and N10-610N may be fitted to the following equation:

S2 )

(A - B)2 4/3

A

(S1 - ΛBr-) +

A-B ΛBrA

(1)

where S1 and S2 are the slopes of the conductivity curve below and above the CMC, respectively; A is the aggregation number of the surfactant chains in one micelle, which is the micelle aggregation number N for N10TAB (A ) N) and double the micelle aggregation number N for N10-6-10N (A ) 2N); B is the number of Br- counterions that are bound to the micelle; and ΛBr- is the equivalent conductivity of the Br- counterion, which can be taken as the value at infinite diluted solution (78.4 × 10-4 S‚m2/mol at 25.0 °C9b,c). Thus, R can be calculated by

R)

A-B A

(2)

Because the nitrophenoxy group can effectively quench the fluorescence of pyrene, time-resolved fluorescence quenching method10 could not be used to determine the micelle aggregation number of the present surfactants. Here, the aggregation numbers N of N10TAB and N10-6-10N are obtained by 1H NMR. The NMR data will be discussed in the following text. The CMC and R values are presented in Table 1.

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TABLE 1: Critical Micelle Concentration (CMC), Micelle Ionization Degree (r), and Thermodynamic Parameters for N10TAB and N10-6-10N at 25.0 °C CMC (mM)

N10TAB N10-6-10N

conductivity

calorimetry

8.41 ( 0.02 0.71 ( 0.02

9.6 ( 0.1 0.46 ( 0.01

H NMRa

R

∆Hmic (kJ mol-1)

∆Gmic (kJ mol-1)

T∆Smicd (kJ mol-1)

6.6 ( 0.2 0.72 ( 0.02

0.17 0.28

-14.4 ( 0.1 -28.1 ( 0.3

-21.0b -48.2c

6.8 20.1

1

a The CMC values are the average values from all resolved 1H chemical shifts in D2O. b Calculated from ∆Gmic ) (2 - R)RT ln(CMC). c Calculated from ∆Gmic ) (3 - 2R)RT ln(2CMC) - RT ln 2. d Calculated from ∆Gmic ) ∆Hmic - T∆Smic.

Figure 3. Variations of observed enthalpy (∆Hobs) of (a) N10TAB and (b) N10-6-10N with the final concentration (C) at 25.0 °C.

As shown in Table 1, the CMC value of N10-6-10N is about 1 order of magnitude smaller than that of N10TAB, which is one of the most characteristic properties of gemini surfactants.1a,b,e Also, the micelle ionization degree of N10-6-10N is larger than that of N10TAB. For the gemini surfactants with a spacer of medium length, it is believed that the increase in the number of headgroups could increase the solubility and hydration of the surfactant molecules,11 and the improved hydration of the headgroups should facilitate the ionization. Furthermore, the counterion condensation on micelle-water interfaces is also dependent on the size of micelles. A larger micelle has greater tendency to attract counterions than does a smaller one.12 Therefore, the larger R value of N10-6-10N than that of N10TAB should be due to the improved headgroup hydration and a decrease in micelle size. ITC. Isothermal titration calorimetry was used to study the intermolecular interaction on the thermodynamics of micellization for N10TAB and N10-6-10N. The observed enthalpies of dilution (∆Hobs) as a function of the final surfactant concentration are shown in Figure 3. The titration curves are all approximately sigmoidal in shape, and both CMC and the enthalpy changes for micellization (∆Hmic) can be derived by the method described previously.12b,13 The Gibbs free energies of micellization (∆Gmic) can be calculated from CMC and R following the standard procedure in the literature,14 and the entropies of micellization (∆Smic) can then be derived from ∆Hmic and ∆Gmic. There is an excellent agreement between CMC values obtained by conductivity and ITC methods, as can be seen from the complete set of the parameters listed in Table 1. The ∆Hmic values of micellization are negative for both N10TAB and N10-6-10N, because water molecules close to the aliphatic chain are less polarized in a hydrophobic environment and are probably in an unfavorable situation as compared to the water molecules close to the micelle surface. Thus, in the micellization process, the nitrophenoxy groups have great tendency to replace those water molecules merged more deeply in the micelles, which are not highly structured, providing a

strong exothermic effect to the surfactant aggregation. This phenomenon is consistent with previous works,8,13c where the micellization of sodium bis(4-phenylbutyl) sulfosuccinate, sodium bis(2-ethylheyl) sulfosuccinate, and a series of nonionic surfactants with different hydrophobic chains was investigated. The present calorimetric measurements indicate the exothermic effect of micellization for N10TAB is almost the same as that per chain in N10-6-10N, meaning that the C6H12 spacer of N106-10N has no contribution to the enthalpy change of the micelle formation. It implies that the spacer may lie on the watermicelle interface rather than immerse in the aggregates. This result agrees well with the following analysis of the NMR data. 1H NMR, DLS, and TEM. Figure 1 indicates the assignments of the hydrogen atoms on various carbons. The NMR spectra for N10TAB and N10-6-10N as a function of concentration are given in Figures 4 and 5, respectively. The COSY experimental results and the expanded spectra between 2.00 and 0.75 ppm of N10TAB and N10-6-10N can be found in the Supporting Information (Figures S1-S4). A summary of the assignments and chemical shifts of distinguishable protons at different concentrations is also supplied in the Supporting Information (Tables S1, S2). The following phenomena are observed. Below the CMC, the chemical shifts scarcely change for N10TAB and change gradually for N10-6-10N with the increase of the concentration. Above CMC, the resonances of the two surfactants undergo relative large chemical shifts upon micelle formation, and then approach constant values. With the increasing of the surfactant concentrations from well below the CMC to about 10 times the CMC, several sets of resonance changes are characteristic in the NMR spectra of both N10TAB and N10-6-10N. The aromatic protons ortho and meta to the -NO2 group (Ha and Hb) at 8.1 and 6.9 ppm, respectively, show significantly upfield shifts (0.27 ppm). The peak at 4.0 ppm (Hc) ascribed to -OCH2 methylene shifts upfield and gradually loses its triplet structure. The methylene peaks at 3.1 ppm are ascribed to the methylenes adjacent to the quaternary ammonium group (Hg for N10TAB, Hg and Hg′ for N10-6-10N). These peaks appear as a quintuple peak5f,15 and shift downfield

Nitrophenoxy-Tailed Quaternary Ammonium Surfactants

Figure 4. 1H NMR spectra and proton assignments of N10TAB in D2O at different concentrations.

Figure 5. 1H NMR spectra and proton assignments of N10-6-10N in D2O at different concentrations.

below the CMC, and then become featureless upon the micellization process. The large narrow N-methyl (headgroup, Hh) singlet at 2.9 ppm broadens and shifts slightly downfield with the micellization. For N10TAB, the multiplet at 1.6 ppm below CMC gradually splits into two clear multiple peaks. COSY spectra show that the significant upfield shifted peak corresponds to Hd signal, and the peak seldom moving should be attributed to Hf signal. The rest of the methylenes (He) appear as a large multiplet at 1.2 ppm with a small shoulder multiplet centered at 1.3 ppm at low N10TAB concentration. All of the peaks move upfield, and the small multiplet intergrades into the large one with the bisection of the latter upon micellization. The situation for N10-6-10N is more complicated. Upon micellization, the large multiplet at 1.6 ppm splits into three distinct broad peaks: the one moving downfield is the Hf′ signal in the spacer group, and the other two upfield shifted peaks correspond to the methylenes on the hydrocarbon side tails; the more obviously shifted one is for Hd and the less shifted one is for Hf. Interestingly, the peak centered at 1.27 ppm is divided. One part of the peak is broad and has a minute downfield shift, which is attributed to the middle two methylenes at the center

J. Phys. Chem. B, Vol. 111, No. 43, 2007 12443 of the surfactant spacer (He′). Another part is a multiplet merging into the large peaks upfield, which is one of the undistinguishable methylene signals on the hydrocarbon side chains (He). The rest of the He signals go through the same transformation as that of N10TAB. All of the changes in chemical shifts described above reflect the structure and solvation of the surfactant aggregates. Chemical shift changes with micelle formation have been discussed in terms of medium effects and conformation effects in literatures.16 The former is caused by the transfer of monomer surfactant from water circumstance into micelle, where the hydrophobic part is immersed into the oil-like core, and the hydrophilic headgroup is solvated by the interfacial water whose polarity is lower than that of the bulk water.17 The decrease of polarity in solvation commonly leads to a downshift of 1H NMR signal for alkyl group, as the phenomena observed on the present signals of Hg and Hh in the region of headgroup, and He′ and Hg′ in the spacer of the gemini surfactant N10-6-10N, because these groups can be transferred into the surface water pseudophase. Regarding the conformation effect, micelle formation is always accompanied by a partial changeover from gauche to trans conformation in the alkyl chains. Normally, this change also leads to a downfield shift, and its magnitude is the largest for the protons in the middle hydrophobic chain.5d,16a,18 Yet the opposite changes are observed on the present signals for the hydrophobic chain, because the anisotropy of the π electron system also gives rise to the chemical shifts on both chemically and spatially nearby atoms.5b,19 The upfield shifts of He and Hf in the alkyl chain indicate that each of these protons experiences, on average, the shielding cone portion of the aromatic ring current from nearby nitrophenoxy group. This ring current effect has quite precise geometrical requirements, and only those protons situated in the field of the ring are substantially shifted. From this, it can be inferred that the nitrophenoxy groups are preferentially located at the inner of micelles and intercalate with the several middle methylenes of the C10 chain. Besides, the changes in the chemical shifts of the aromatic protons reveal the average location of the nitrophenoxy group in the micelles. The aromatic protons move upfield in less polar media, because the less polar media reduce the deshielding effect on the protons, and their insertion in micelles has the same effect. So the upfield shifts of the peaks for the aromatic Ha and Hb, and Hc of the chemically connected OCH2 group, should be due to a less polar environment they sensed. That also suggests the overlapping of the nitrophenoxy groups at the end of the hydrophobic chains with the C10 chains at the inner of the micelles. Previous publications3a,16b,20 have shown that 1H NMR chemical shifts can be used to determine the CMC of surfactants. Under fast exchange in the NMR time scale, the observed chemical shift for the corresponding proton (δobs) can be expressed as the sum of the chemical shifts of the monomer (δmon) and the aggregated form (δmic). Each one is averaged with its respective molar fractions as

( ) ( )

δobs ) δmon

Cmon Cmic + δmic CT CT

(3)

where Cmon, Cmic, and CT are the concentrations of the surfactant molecules existing as monomers, in micelles, and in total in solution. It is assumed that the monomer concentration is constant above CMC, thus

δobs ) δmic -

( )

CMC (δmic - δmon) CT

(4)

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Figure 6. Variation of δobs versus 1/CT for the Hc signal of N10TAB in D2O.

Therefore, a plot of δobs versus 1/CT should yield two straight lines below and above its CMC, and the intersection of these lines corresponds to the CMC. δmon and δmic can be obtained by extrapolation of the plots of δobs versus CT and δobs versus 1/CT, respectively. The δobs values at different concentrations for different protons of N10TAB and N10-6-10N are listed in Tables S1 and S2 in the Supporting Information. An example of such plots is shown in Figure 6 for the Hc proton of N10TAB. The calculated δmon and δmic for the different protons of N10TAB and N10-6-10N are collected in Table 2. Because the CMCs determined by NMR are in D2O and D2O is a more structured liquid,21 the CMC in D2O is slightly lower than that in H2O. The 1H NMR data can also be employed to obtain the micelle aggregation number N.3b,4a,16b,20a Assuming the idealized situation where surfactant molecules may exist either as monomers or as a single type of micelle with a fixed N value, the aggregation equilibrium can be quantified as mass action law model K

NS y\z SN, K )

[SN] [S]N

(5)

where S and SN are the monomer and aggregated form of the surfactant, and K is the equilibrium constant. The expression for K can then be arranged to give

Figure 7. Variation of lg[CT(|δobs - δmon|)] versus lg[CT(|δmic - δobs|)] for N10TAB.

Figure 8. Variation of lg[CT(|δobs - δmon|)] versus lg[CT(|δmic - δobs|)] for N10-6-10N.

TABLE 2: Micellization Parameters of N10TAB and N10-6-10N Obtained by Analysis of the Chemical Shifts N10TAB

N10-6-10N

lg[CT(|δobs - δmon|)] ) N lg[CT(|δmic - δobs|)] + lg NK + (1 - n) lg(|δmic - δmon|) (6) Consequently, a plot of lg[CT(|δobs - δmon|)] versus lg[CT(|δmic - δobs|)] yields a straight line with the slope of N. The chemical shifts of all distinguishable protons in the surfactant can be used to obtain N as shown in Figure 7 for N10TAB and Figure 8 for N10-6-10N. The values of N thus calculated from the different protons of N10TAB and N10-6-10N are collected in Table 2. It should be noted that two sets of straight lines are obtained using the chemical shift data of N10-6-10N as presented in Figure 8, and the two lines intersect at the CMC of N10-610N. This pattern can be attributed to two different equilibria. Below the CMC, the variation of the NMR data gives a line with a slope of N ) 2-3. This may correspond to a premicelle formation process, which is especially typical in some gemini surfactant systems.3b,22 At high concentrations, the NMR data give an N value of around 20, corresponding to the micelle formed by N10-6-10N. For the single chain surfactant N10TAB, only one set of lines above the CMC is observed, giving the N value around 30.

a

Ha Hb Hc Hg Hh Ha Hb Hc Hg Hh

δmon/ppm

δmic/ppm

CMC/mM

Na

8.108 6.961 4.051 3.148 2.947 8.073 6.927 4.029 3.096 2.876

7.787 6.641 3.656 3.217 3.032 7.792 6.660 3.663 3.177 2.984

6.45 6.54 6.54 6.82 6.71 0.72 0.70 0.72 0.74 0.71

27 29 33 30 31 22 22 19 20 18

Expressed as the number of surfactant molecules per micelle.

With regard to the above discussion on the 1H NMR spectra, we are able to draw out the aggregation manner of these nitrophenoxy-tailed surfactants. A very schematic representation of the micelles formed by N10TAB and N10-6-10TAB is illustrated in Figure 9. The nitrophenoxy group may penetrate between the surface and the center of the micelle and overlap with the middle few methylenes in the aliphatic chains. This packing situation will make the radius of the micelle much smaller than the extended length of the molecule. According to the XRD data, the extended length of one hydrophobic chain plus the headgroup of N10TAB and N10-6-10N is around 1.65 nm (XRD spectrum is shown in Figure S5 of Supporting Information). As shown in Figure 10a, the mean radii for N10TAB and N10-6-10N micelles from DLS are 0.78 and 0.63 nm, respectively. The parallel results from XRD and DLS confirm our presented aggregation model.

Nitrophenoxy-Tailed Quaternary Ammonium Surfactants

J. Phys. Chem. B, Vol. 111, No. 43, 2007 12445 ments are normally quite insensitive to the molecule signals in large aggregates, and thus can only provide the situation of surfactant molecules in the micelles.16b,25 Conclusions

Figure 9. Schematic illustration of the proposed structures of micelles formed by (a) N10TAB and (b) N10-6-10N.

Nitrophenoxy-tailedquaternaryammoniumsurfactantsN10TAB and N10-6-10N have been synthesized, and their aggregation properties in aqueous solution have been studied by conductivity, ITC, NMR, and DLS. The CMC of N10-6-10N is much lower than that of N10TAB, as investigated by different techniques. Calorimetric investigations indicate that the nitrophenoxy group promotes the aggregation process, because the replacement of water molecules nearby the aliphatic chain by aromatic groups is beneficial to the exothermic enthalpy change for micellization. The gradual shifts of 1H NMR signals with the increase of surfactant concentrations indicate that the environmental change can be probed by different protons upon the micellization process. It can be deduced from the NMR data that the middle few methylenes sense the strong electron anisotropy of the aromatic nitrophenoxy group. This conclusion implies that the aliphatic chains are partially overlapped with the aromatic groups at the end of the hydrophobic tails. Further analysis of 1H NMR data gives the aggregation numbers of about 20 and 30 for N10TAB and N10-6-10N above their CMC and indicates an obvious premicelle formation process for the gemini surfactant N10-6-10N at the concentration well below its CMC. The DLS results demonstrate that both N10TAB and N10-6-10N form rather small micelles, while N10-6-10N molecules also form vesicles at the concentration much higher than its CMC. Acknowledgment. We greatly appreciate the valuable discussion with Dr. Robert K. Thomas. We are grateful for financial support from the National Natural Science Foundation of China and National Basic Research Program of China (grants 20633010, 20473101, 2005cb221300).

Figure 10. (a) DLS measurement of the size distributions of 50.0 mM of N10TAB (lower) and 10.0 mM of N10-6-10N (upper) at 25.0 °C. Rh ) 0.78 nm for N10TAB, and Rh ) 0.63 and 62 nm for N10-6-10N, respectively. (b) TEM images of N10-6-10N vesicles in 10 mM solution.

Although the micelle size determined from DLS measurements should be smaller than the actual one, because the measured diffusion coefficients are influenced by the intermicelle repulsion among the highly charged micelles,23 we can still make a conclusion that the micelle size of N10TAB is a little larger than that of N10-6-10N. Considering there are more hydrophobic chains in the micelles of N10-6-10N, the packing of N10-6-10N micelles should be tighter than that of N10TAB, which is the common feature of gemini surfactants and can be attributed to their higher tendency to aggregate. The larger size distribution around 62 nm for N10-6-10N in DLS has been observed by TEM, which indicates the vesicle existence at high concentration as shown in Figure 10b. The same phenomenon has been observed in other gemini or oligomeric surfactant systems.24 It is believed that the vesicles can be derived from the stronger attractive interaction between the multiple hydrocarbon chains in gemini molecules. The formation of vesicles does not conflict with the model based on the assumption that these two surfactants form micelles, because the NMR experi-

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