J. Phys. Chem. B 2006, 110, 19479-19486
19479
Controllable Organization of a Carboxylic Acid Type Gemini Surfactant at Different pH Values by Adding Copper(II) Ions Xu Huang, Meiwen Cao, Jinben 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: May 17, 2006; In Final Form: July 26, 2006
The gemini surfactant sodium 4,8-dioctyl-3,9-dioxo-6-hydroxy-4,8-diaza-1,11-undecanedicarboxylate (SDUC), bearing two carboxylic headgroups and two hydrocarbon chains, has been synthesized, and its self-organization characteristics at strong basic and neutral environments have been studied. The results reveal that a transition from micelles to vesicles in SDUC aqueous solution occurs when the pH drops from 12.0 to 7.0. When a small amount of Cu2+ ion is added to the SDUC solutions at both pH 12.0 and 7.0, remarkable transformations were observed within the SDUC aggregate structures. At pH 12.0, micelles transform into vesicles as Cu2+ is added, while the vesicles of SDUC are formed at pH 7.0 and these small vesicles tend to fuse into larger ones slowly. Moreover, these SDUC vesicles at pH 7.0 undergo a relatively fast fission process when Cu2+ is added in. An attempt has been made to explain the mechanism of these transformations.
Introduction There is much current interest in the investigation of vesicles in aqueous solutions because of their applications in drug and gene delivery and the modeling of biomembranes. Many attempts have been made to control vesiculation by modifying experimental conditions, such as surfactant composition,1 ionic strength,2 temperature,3 UV light,4 pH values,5 etc. Among these modifiable factors, changing pH is of particular interest for drug or gene delivery systems since the pH value around damaged tissue is always different from that around normal tissue. For example, Huang and co-workers6 used a series of mixed pHsensitive liposomes to deliver diphtheria toxin A fragments into the cytoplasm of cultured cells. Since these liposomes are stable at neutral and basic pH, they are effective drug carriers in human plasma. The environment around the damaged cultured cells is weakly acid, and in this more acid environment the liposomes become leaky and exhibit extensive lipid mixing, so the drug molecules are released. Vesicles undergoing fusion and fission processes are thought to be significant for modeling biomembranes.7 Even for vesicles formed by only one kind of amphiphilic molecule, the influence of changing conditions on fusion and fission processes can be followed. Menger and co-workers8 have investigated the chemical-induced aggregation, budding, fusion, and fission process in giant vesicles of cationic surfactant DDAB. They found that additive salts have a strong influence on the compactness of the molecules in the vesicles, so a “packing” or “stiffness” factor must be taken into account along with van der Waals attractions, electrostatic repulsion, and hydration repulsion in the membranemembrane interactions. Sommerdijk at al.9 have reported the stereodependent fusion and fission behavior of the vesicles formed by three different stereoisomers of a phosphatidic acid gemini surfactant. When calcium ions are added to the vesicle suspensions of the isomers, the vesicles formed by the (S,S) and (R,R) isomers undergo fusion, whereas the ones formed by * To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn.
the (R,S) isomer show vesicle fission. From these observations, it can be concluded that the molecular packing and the orientation of the hydrophilic headgroups may markedly influence the morphology of the vesicular system. More recently, vesicles formed by a range of different synthetic amphiphiles have attracted attention. These systems have nearly the same functionality and applicability as phospholipids. However, phospholipids do not dissolve well in water and ultrasonication or extrusion procedures are therefore required for the preparation of liposomes. In contrast, synthetic surfactants that mimic phospholipid molecules, e.g. gemini surfactants with extended spacers, can form vesicles directly in water.10 Many articles on catanionic surfactant mixtures report that various aggregates, including spherical micelles, rodlike micelles, nanodisk micelles, vesicles, and lamellar structures, can be generated simply by blending cationic and anionic surfactant solutions together.1b,11 It is also well-known that alkali-metal alkanoates (e. g. decanoate, oleate) can form vesicles from micelles when the pH is decreased to the pKa of the carboxylate.12 Recently, the surface properties of anionic gemini surfactants with carboxylic headgroups, hydrophobic linking spacers, and different alkyl chain lengths in very strong alkaline environment have been reported.13 These gemini surfactants have excellent micellization abilites at low concentrations and much greater efficiency at lowering surface tension than the corresponding single-chain surfactants. However, the aggregate behavior of these carboxylic acid based gemini surfactants at around neutral pHs has been little reported. Systematic research on the organization of the molecular assemblies of this kind of gemini surfactant as a function of change of environments is therefore of some importance. In this paper, the synthesis of the gemini surfactant sodium 4,8-dioctyl-3,9-dioxo-6-hydroxy-4,8-diaza-1,11-undecanedicarboxylate (SDUC) is reported. This molecule possesses amide groups between the hydrocarbon chains and the carboxylic headgroups, and the two chains are linked by a hydrophilic spacer with a hydroxyl group. The hydroxyl group increases the solubility of the surfactant, and the presence of amide groups
10.1021/jp0630121 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006
19480 J. Phys. Chem. B, Vol. 110, No. 39, 2006 SCHEME 1
makes the surfactant more easily hydrolyzable, so this surfactant should be easily biodegradable and environmentally acceptable. The surface and aggregation behavior of the gemini surfactant at different pH values have been studied in buffer solutions. In that the surfactant contains two carboxylic headgroups, its selfassembly can be influenced either by pH or by adding transition metal ions, because both the hydrogen bonds between the headgroups and the coordination between the carboxyl groups and the metal ions can alter the molecular packing in the aggregate.14 We observe the micelle to vesicle transition in aqueous SDUC solutions on changing the pH from 12.0 to 7.0 and on adding a small amount of copper(II) ions to the micellar solution of SDUC at pH ) 12.0. Furthermore, it is noteworthy that vesicles are formed at pH ) 7.0 before going through a slow fusion process and after mixing with a small amount of copper(II) ions undergo a relatively fast vesicle fission. Experimental Section Materials. Octylamine, epichlorohydrin, succinic anhydride, and all of the inorganic salts were purchased from Beijing Chemical Co. and were of analytical grade. Pyrene used in the steady-state fluorescence measurement was from Aldrich and recrystallized from ethanol before use. All the organic solvents were dried and distilled. Triply distilled water was used in all experiments. Synthesis. Compound SDUC was synthesized according to Scheme 1 and characterized by 1H NMR, 13C NMR, the mass spectrum, and elemental analysis. 1,3-Bis(octylamino)propan-2-ol (1). Epichlorohydrin (1.85 g, 20 mmol) was added to a solution of octylamine (5.68 g, 44 mmol) in 25 mL of 2-propanol. This solution was stirred and refluxed for 16 h, and then excess 10% ethanol solution of sodium hydroxide was added. The mixture was refluxed for an additional 2 h and then filtrated and concentrated in vacuo. Crystallization from ethyl acetate gave pure 1 as a white solid in 42% yield (mp ) 57 °C). 1H NMR (CDCl3, ppm): δ 0.88 (t, 6H, CH3, J ) 6.6 Hz), 1.28 (m, 20H, (CH2)5CH2CH3), 1.48 (m, 4H, CH2CH3), 2.52-2.72 (m, 8H, NHCH2CHOH and NHCH2CH2), 3.75 (m, 1H, CHOH). 13C NMR (CDCl3, ppm): 14.1, 22.6, 27.3, 29.2, 29.5, 30.1, 31.8, 49.9, 53.7, 68.2. MSESI (m/z): calcd, 314; found, 315 (M + 1). Anal. Calcd for C19H42N2O: C, 72.55; H, 13.46; N, 8.91. Found: C, 72.67; H, 13.59; N, 8.75. 4,8-Dioctyl-3,9-dioxo-6-hydroxy-4,8,-diaza-1,11-undecanedicarboxylic Acid (2). Compound 1 (1.89 g, 6 mmol) and succinic anhydride (1.98 g, 18 mmol) were dissolved in 30 mL of anhydrous ether, and the mixture was refluxed for 6 h. After completion of the reaction, the mixture was filtered to remove insoluble material and the filtrate was washed with water for 3 times. Then the solvent was removed in vacuo to yield 2 as a
Huang et al. glassy solid. Flash chromatography on silica with 80:20 chloroform/methanol gave a colorless oil that solidified on prolonged standing. A solid paste was obtained in 30% yield. 1H NMR (CDCl , ppm): δ 0.87 (t, 6H, CH , J ) 3.6 Hz), 1.28 3 3 (m, 20H, (CH2)5CH3), 1.55 (m, 4H, NCH2CH2), 2.62-2.66 (m, 8H, NCOCH2CH2COOH), 3.19-3.55 (m, 8H, NHCH2CHOH and NHCH2CH2), 4.14 (m, 1H, CHOH), 5.36 (br, 1H, CHOH), 11.21 (br, 2H, COOH). 13C NMR (CDCl3, ppm): 14.0, 22.5, 26.7, 28.6, 29.2, 31.6, 46.8, 48.6, 69.9, 172.3, 177.3. MS-ESI (m/z): calcd, 498; found, 497 (M - 1). Anal. Calcd for C27H50N2O7: C, 63.01; H, 9.79; N, 5.44. Found: C, 63.40; H, 10.05; N, 5.16. Sodium 4,8-Dioctyl-3,9-dioxo-6-hydroxy-4,8,-diaza-1,11undecanedicarboxylate (SDUC). Compound 2 was dispersed in water and the pH of the solution adjusted to 10 with 1 M NaOH solution to make the solution clear. Then the aqueous solution was lyophilized and repeatedly recrystallized from acetone/ethanol (95:5 in volume). The purity of SDUC was checked by surface tension until the surface tension curve did not have a minimum below the critical micelle concentration (cmc). Anal. Calcd for C27H48N2Na2O7‚1/2H2O: C, 57.13; H, 8.70; N, 4.93. Found: C, 57.26; H, 8.80; N, 4.89. Sample Preparation. A 10 µmol amount of solid SDUC was dissolved in 5 mL of 10 mM NaOH solution or 5 mM phosphate buffer (I ) 10 mM), and then the solution was vortex mixed and equilibrated at room temperature before investigation. The pH values of the resultant aqueous samples were 12.0 and 7.0, respectively, and could be maintained stable for more than 2 months. All the samples used in the following experiments except for the potentiometric pH titration were prepared as described above. Potentiometric pH Titration. SDUC was first dissolved in water at a concentration of 2.0 mM, and then the solution pH was adjusted to 12.5 with a small volume of 1 M NaOH. Then 1 M HCl was gradually added into this solution in small portions. The titration process was monitored with a pHS-2C acidity meter, and the temperature was kept at 25.0 ( 0.1 °C throughout the measurement. Three titrations were performed, and the mean pKa values were calculated. Surface Tension Measurements. The surface tension of the surfactant solutions at different pH was measured by the drop volume method at 25.0 ( 0.1 °C.15 The adsorption amount of surfactant was calculated according to the Gibbs adsorption equation,
Γ)-
(
)
dγ 1 2.303nRT d(lg C)
T
Here γ is the surface tension in mN m-1, C is the concentration of the surfactant in the buffer solutions, Γ is the saturated adsorption amount in mol m-2, T is the absolute temperature, R ) 8.314 J mol-1 K-1, and dγ/d(lg C) is the maximal slope in each case. For an ionic surfactant, we use buffer solutions to keep the ionic strength of the counterion constant, so n ) 1. Then the minimum average surface area/surfactant molecule (Amin) is obtained from the saturated adsorption by
Amin )
1 NAΓ
where NA is the Avogadro constant. Steady-State Fluorescence Measurements. The micropolarity was investigated by the measurements of the pyrene polarity ratio (I1/I3) in the concentration range from below to above the cmc.16 The concentration of pyrene probe in solution
Controllable Organization of a Gemini Surfactant was kept around 2 µM. The pyrene in solution was excited at 335 nm, and the emission spectrum was scanned from 350 to 500 nm. I1/I3 is the ratio of the fluorescence intensities of peaks I (373 nm) and III (384 nm) of the pyrene emission spectrum. The measurements were taken with a Hitachi model F-4500 spectrophotometer using a 10 mm path length quartz cuvette at 25.0 ( 0.5 °C. Each measurement was repeated at least twice, and the mean value was recorded. Transmission Electron Microscopy (TEM). The TEM images were obtained using a negative-staining method.17 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. Then 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 drying, the samples were imaged under a JEM-200CX electron microscope at a working voltage of 100 kV. Because uranyl acetate solution is quite acidic, we carefully adjusted the pH value of the staining solution above 4.5 before staining to prevent effects of acid on the sample. Turbidity Measurements. Turbidity measurements were carried out with a Shimadau 1601 PC UV/vis spectrometer. The turbidity of the solutions of SDUC was monitored by UV absorbance at 350 nm. A cuvette with 1 cm pathway was used. All the measurements were conducted at 25.0 ( 0.5 °C. 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 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 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 the measurements were performed at 25.0 ( 0.1 °C. X-ray Diffraction (XRD) Studies. Self-supported cast films for the XRD study were prepared by dispersing the solutions of SDUC onto precleaned silicon wafers and then air-dried at room temperature. Finally the wafers 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 10°, using a step width of 0.01°. Results and Discussion pH Titration and Surface Tension Measurements. Since surfactant SDUC is a pH-sensitive compound, the aggregate behavior of SDUC in aqueous solutions will be highly dependent on pH. Therefore a potentiometric pH-titration was carried out at 25.0 °C to determine the protonation constants of SDUC. Figure 1 shows the titration curve at a SDUC concentration of 2.0 mM. Upon addition of HCl to the surfactant solution, to which we had added 10% excess of NaOH to make the solution basic (for the observation of the first flat region), the carboxylic groups are gradually protonated and the pH value drops. The solution remains transparent above pH 6.5. In the range of 4.1 < pH < 6.5, the system is homogeneously turbid. When pH
