Gelation of Charged Catanionic Vesicles Prepared by a

pubs.acs.org/Langmuir © 2009 American Chemical Society

Gelation of Charged Catanionic Vesicles Prepared by a Semispontaneous Process Zheng-Lin Huang, Jhen-Yi Hong, Chien-Hsiang Chang, and Yu-Min Yang* Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received July 30, 2009. Revised Manuscript Received November 1, 2009 Various stable charged catanionic vesicles with mean ζ-potential values from þ59 mV to -96 mV were successfully prepared from an ion-pair amphiphile (dodecyltrimethylammonium-dodecylsulfate, DTMA-DS) and different amounts of the component ionic surfactants (dodecyltrimethylammonium bromide and sodium dodecyl sulfate) by using a simple semispontaneous process with the aid of cosolvent (1-propanol) addition in water. With the ensuring positively and negatively charged catanionic vesicles, gelation of them by four water-soluble polymers with various charge and hydrophobic characteristics was systematically studied by the tube inversion and rheological characteristic analyses. Four phase maps, which show regions of phase separation, viscous solution, and gel by varying the vesicle composition and polymer content, were thereby constructed. Furthermore, the experimental results of the relaxation time and the storage modulus at 1 Hz for the viscous solutions and gel samples revealed that the interactions at play between charged catanionic vesicles and the water-soluble polymers are of electrostatic and hydrophobic origin. The phase maps and the rheological properties obtained for mixtures of charged catanionic vesicles and polymers may provide useful information for the potential application of catanionic vesicles in mucosal or transdermal delivery of drugs.

Introduction Preparations of Charged Catanionic Vesicles. Since the first example of spontaneous vesicle formation from mixed cationic and anionic single-chained surfactants using cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzenesulfonate (SDBS) was given by Kaler et al.,1 the isothermal ternary phase diagrams of a few cationic surfactant-anionic surfactant-H2O (or D2O) systems have been constructed by using several techniques including imaging, NMR, and scattering methods.1-10 The term “catanionic vesicles” (or catansomes) is now commonly accepted to qualify the types of structures resulted from mixtures of anionic and cationic surfactants, whose association through the interaction of their polar heads can mimic the *Corresponding author. Telephone: 886-6-2757575 ext. 62633. Fax: 886-62344496. E-mail: [email protected]. (1) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371–1374. (2) Kaler, E. W.; Herrington, K. L.; Murthy, A. K. J. Phys. Chem. 1992, 96, 6698–6707. (3) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A. N.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792–13802. (4) Regev, O.; Khan, A. J. Colloid Interface Sci. 1996, 182, 95–109. (5) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W. J. Phys. Chem. 1996, 100, 5874–5879. (6) Soderman, O.; Herrington, K. L.; Kaler, E. W.; Miller, D. D. Langmuir 1997, 13, 5531–5538. (7) (a) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M.G..; Lindman, B. J. Phys. Chem. B 1998, 102, 6746–6758. (b) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M.G..; Lindman, B. J. Phys. Chem. B 1999, 103, 8353–8363. (8) Bergstrom, M.; Pedersen, J. S.; Schurtenberger, P.; Egelhaaf, S. U. J. Phys. Chem. 1999, 103, 9888–9897. (9) Sakai, H.; Matsumura, A.; Yokoyama, S.; Saji, T.; Abe, M. J. Phys. Chem. B 1999, 103, 10737–10740. (10) Lin, C.-C. Gelation of Spontaneously Formed Catanionic Vesicles by Polymers. Master Thesis, National Cheng Kung University: Taiwan, 2005. (11) Tondre, C.; Caillet, C. Adv. Colloid Interface Sci. 2001, 93, 115–134. (12) Marques, E. F.; Regev, O.; Khan, A.; Lindman, B. Adv. Colloid Interface Sci. 2003, 100-102, 83–104. (13) Bramer, T.; Dew, N.; Edsman, K. J. Pharm. Pharmacol. 2007, 59, 1319– 1334. (14) Wu, K.-C.; Huang, Z.-L.; Yang, Y.-M.; Chang, C.-H.; Chou, T.-H. Colloids Surf. A 2007, 302, 599–607.

2374 DOI: 10.1021/la902798n

type of structures encountered in double-chained amphiphiles like phospholipids.11-15 The true equilibrium composition of the catanionic vesicles, however, appears to be somewhat difficult to understand: on the one hand, the domains of phase diagrams where vesicles exist always exclude the equimolar line where precipitation is often observed; on the other hand, each time the microscopic composition of the individual particles has been determined, it was found close to equimolar condition. The excess surfactant obviously contributes to the stability of these systems, which is usually better than for pure ion pair amphiphile (IPA) systems.11 Near to the water corner upon a phase diagram, therefore, positively charged vesicle (Vþ) region and negatively charged vesicle (V-) region are usually found on either side of the equimolar line.1-10 Recently, vesicle-promoting method by means of cosolvent addition in water has been proposed for several 1:1 cationic-anionic mixed surfactant systems and found to endow the surfactant mixtures with outstanding vesicle-forming capability, especially for easily precipitated surfactant systems.16-20 This may provide an opportunity for enlarging the application scope of vesicular systems. For example, effects of four homologous cosolvents (methanol, ethanol, 1-propanol, and 1-butanol) on the spontaneous formation of catanionic vesicles from eight 1:1 cationic-anionic mixed surfactant systems, alkyltrimethylammonium bromides-sodium alkylsulfates (CmN(CH3)3Br-CnSO4Na; m = 8, 10, 12, 14; n = 12, 14), at a total surfactant concentration of 10 mM were systematically studied.20 This (15) Yang, Y.-M.; Wu, K.-C.; Huang, Z.-L.; Chang, C.-H. Langmuir 2008, 24, 1695–1700. (16) Huang, J. B.; Zhao, G. X. Colloid Polym. Sci. 1995, 273, 156–164. (17) Huang, J. B.; Zhu, B. Y.; Zhao, G. X.; Zhang, Z. Y. Langmuir 1997, 13, 5759–5761. (18) Zhang, X. R.; Huang, J. B.; Mao, M.; Tang, S. H.; Zhu, B. Y. Colloid Polym. Sci. 2001, 279, 1245–1249. (19) Wang, C. Z.; Tang, S. H.; Huang, J. B.; Zhang, X. R.; Fu, H. L. Colloid Polym. Sci. 2002, 280, 770–774. (20) Yu, W.-Y.; Yang, Y.-M.; Chang, C.-H. Langmuir 2005, 21, 6185–6193.

Published on Web 11/24/2009

Langmuir 2010, 26(4), 2374–2382

Huang et al.

practical vesicle-promoting method is exemplified by SDS/DeTMAB and STS/DeTMAB with methanol, ethanol, 1-propanol, and 1-butanol and by SDS/DTMAB with 1-propanol and 1-butanol. Under these conditions, values of ζ-potential around -10 mV or so were measured for the catanionic vesicles.21 A subtle distinction exists in the literature between simple cationic-anionic surfactant mixtures, where each surfactant is accompanied by its own counterion, and the so-called ion-pair amphiphiles (IPAs), in which the preceding counterions were removed leaving two amphiphilic ions oppositely charged.11,12 The most common IPAs (that is, catanionic surfactants or catanionics) are formed by pairing two single-chained oppositely charged amphiphiles with equimolar ratio. The resulting surfactants are thus uncharged and can be considered as pseudodoublechained surfactants, in the sense that the two chains are not covalently bonded to the same headgroup.12 Vesicle formation from IPAs, however, is usually made possible only by mechanical dispersion method through the preparation of thin film with the similar manner as for liposomes from most phospholipids. Unfortunately, vesicles formed from IPAs usually showed only a short-term stability.22-24 In our previous study,24 four IPAs, derived from the pairing of alkyltrimethylammonium chlorides and sodium alkylsulfates, were used to form vesicles in water upon mechanical dispersion method through the preparation of thin film. Short-chained alcohols including methanol, ethanol, 1-propanol, and 1-butanol were added as cosolvents at a variety of concentrations and their effects on the stability of the ensuing vesicles were systematically studied. The experimental results indicated that vesicles formed from one of the IPAs (that is, dodecyltrimethylammoniumdodecylsulfate, DTMA-DS) could be efficiently and successfully stabilized by the addition of appropriate amounts of 1-propanol and 1-butanol. Maximum lifetimes of more than 1 year and 132 days for stable vesicles in 5 vol % 1-butanol and 15 vol % 1-propanol solutions, respectively, were observed and this demonstrated that a method for the stabilization of vesicles formed from IPAs becomes available by means of cosolvent addition. On the other hand, a semispontaneous process was also proposed to prepare catanionic vesicles14,15 and liposomes15 in water with the aid of cosolvent addition by using IPAs as materials. This simple method, that is, dissolving the IPA in a liquid alcohol and then adding water with mixing by using of a homogenizer, of preparing catanionic vesicles becomes available for IPAs with short-chained alcohols. It has the advantage over the preceding spontaneous process and classic mechanical dispersion method through the preparation of thin film in purity and simplicity, in the sense that no counterions exist in the solution and no film preparation is needed, respectively. Furthermore, this method is potentially feasible for producing liposomes and catanionic vesicles in large scale under size control in a reproducible manner. It is noteworthy that the successful preparations of catanionic vesicles by spontaneous,20 semispontaneous,14,15 and mechanical dispersion24 processes with the aid of cosolvent addition in water is explained by the mechanism proposed on the basis of the viewpoint of a mixed solvent dielectric constant. The charges of the catanionic vesicles prepared by the above-mentioned (21) Yu, W.-Y. Master’s Thesis, National Cheng Kung University: Taiwan, 2004. (22) Fukuda, H.; Kawata, K.; Okuda, H. J. Am. Chem. Soc. 1990, 112, 1635– 1637. (23) Chien, C.-L.; Yeh, S.-J.; Yang, Y.-M.; Chang, C.-H.; Maa, J. R. J. Chin. Colloid Interface Soc. 2002, 24, 31–45. (24) Yeh, S.-J.; Yang, Y.-M.; Chang, C.-H. Langmuir 2005, 21, 6179–6184.

Langmuir 2010, 26(4), 2374–2382

Article

methods, however, have less been studied. Zero or near zero ζ-potential values for the catanionic vesicles are expected. Further evidence, however, should be produced in support of the intuition. In this work, intended charged catanionic vesicles were prepared by using an IPA (DTMA-DS) with excess component ionic surfactants (DTMAB or SDS) of the IPA. Negatively and positively charged catanionic vesicles are expected to be prepared by the addition of component anionic (SDS) and cationic (DTMAB) surfactants, respectively. The semispontaneous process with the aid of cosolvent (1-propanol) addition as mentioned above was used. Gelation of Charged Catanionic Vesicles. The modification of water-soluble homopolymers by grafting a low amount of hydrophobes, like alkyl chains, leads to amphiphilic hydrophobically modified polymers (HM-P) which have a tendency to selfassociate by hydrophobic interaction. This weak aggregation leads to an increase in viscosity and in other rheological characteristics, resulting in the use of these “associative thickners” as rheology modifiers in paints and other products.25 The interaction of hydrophobically modified polymers with surfactants has also been extensively studied.25-28 An added surfactant will interact strongly with hydrophobes of the polymer, leading to a strengthened association between polymer chains, and thus to an increased viscosity. At higher surfactant concentrations, however, the viscosity effect is lost. This stoichiometry dependent effect can be understood in terms of mixed micelle formation between the surfactant and the hydrophobically modified polymer. In order to have cross-linking and thus a viscosity effect, there must be a sufficiently high number of polymer hydrophobes per micelle. At higher surfactant concentrations, there will be only one polymer hydrophobe in a micelle and all of the cross-linking effect is lost.25 In contrast to those with surfactants, the interactions of hydrophobically modified polymers with larger surfactant aggregates, like vesicles, have much less been studied.29-42 Such systems are especially complicated when the polymer is a HMpolyelectrolyte and the vesicles also bear charges. Polymer-vesicle interactions in such cases will be mediated by a combination (25) Lindman, B. Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons: New York, 2001; Chap. 20. (26) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: London, 1993. (27) Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998. (28) Holmberg, K.; J€onsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; Wiley: New York, 2003; Chapter 13. (29) Loyen, K.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1995, 11, 1053–1056. (30) Kevelam, J.; vanBreemen, J. F. L.; Blokzijl, W.; Engberts, J. Langmuir 1996, 12, 4709–4717. (31) Murphy, A.; Hill, A.; Vincent, B. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1996, 100, 963–971. (32) Meier, W.; Hotz, J.; GuntherAusborn, S. Langmuir 1996, 12, 5028–5032. (33) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. G.; Lindman, B. Macromolecules 1999, 32, 6626–6637. (34) Regev, O.; Marques, E. F.; Khan, A. Langmuir 1999, 15, 642–645. (35) Ashbaugh, H. S.; Boon, K.; Prud’homme, R. K. Colloid Polym. Sci. 2002, 280, 783–788. (36) Auguste, D. T.; Prud’homme, R. K.; Ahl, P. L.; Meers, P.; Khon J. Biochim. Biophys. Acta 2003, 1616, 184–195. (37) Antunes, F. E.; Marques, E. F.; Gomes, R.; Thuresson, K.; Lindman, B.; Miguel, M. G. Langmuir 2004, 20, 4647–4656. (38) Zhai, L.; Lu, X.; Chen, W.; Hu, C.; Zheng, L. Colloids Surf. A 2004, 236, 1–5. (39) Lee, J.-H.; Gustin, J. P.; Chen, T.-H.; Payne, G..F.; Raghavan, S. R. Langmuir 2005, 21, 26–33. (40) Medronho, B.; Antunes, F. E.; Lindman, B.; Miguel, M. G. J. Dispers. Sci. Tech. 2006, 27, 83–90. (41) Antunes, F. E.; Brito, R. O.; Marques, E. F.; Lindman, B.; Muguel, M. G. J. Phys. Chem. B 2007, 111, 116–123. (42) Lin, C.-C.; Chang, C.-H.; Yang, Y.-M. Colloids Surf. A 2009, 346, 66–74.

DOI: 10.1021/la902798n

2375

Article

Huang et al.

of hydrophobic and electrostatic forces.39 Among these investigations, the addition of HM-P can stabilize vesicles by adsorbing polymer chains on them,31 give rise to faceted vesicles in contrast to spherical structures,33,39,40 and result in a vesicle gel by bridging the adjacent vesicles.33-35,37-42 It is noteworthy that vesicle gels may find their potential use in mucosal or transdermal delivery of drugs.13,43-45 Because the IPAs possess a similar double-chained structure as lipids, they tend to form catanionic vesicles and may have the potential use in drug delivery. With the ensuring charged catanionic vesicles mentioned previously, gelation of them by four polymers (P0, HMP0, Pþ, and HMPþ) were systematically studied by the tube inversion and rheological characteristic analyses in this work.

Experimental Section Surfactants and Polymers. The anionic and cationic surfactants used in this work were sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DMTAB), respectively, supplied by Sigma. All surfactants with >99% purity were used as received without further purification. The IPA as precipitate will come out when cationic and anionic surfactants in aqueous solutions of sufficiently high concentrations are allowed to react with each other. In this work, such precipitate, dodecyltrimethylammonium-dodecylsulfate (DTMA-DS), was prepared by mixing equal volumes (500 mL) of 20 mM solutions of DTMAB and SDS. A concentration of 20 mM is well beyond the critical micelle concentrations (CMCs) of these surfactants. After standing for 1 h, the precipitate was separated from the solution by repeated centrifuging and washing. It was then dried for 36 h under vacuum and ground into fine powders for further studies. The IPA, DTMA-DS, by pairing of two oppositely charged surfactants was thereby obtained. The resulting pure IPA can be subjected to elemental analysis and determination of mass and infrared spectra.23 There was enough evidence to prove that the pseudodoublechained amphiphilic compound contained amphiphilic cation and amphiphilic anion in an equimolar ratio. The two nonionic polymers used, 250 MR (hydroxyethyl cellulose, HEC, P0) and Plus 330 (hydrophobically modified hydroxyethyl cellulose, HM-HEC, HMP0) were manufactured by Hercules. The two cationic polyelectrolytes used, JR 400 (Pþ) and LM 200 (HMPþ) are HEC derivatives and were manufactured by Union Carbide Chemicals and Plastics Company, Inc. The JR 400, N,N,N-trimethylammonium derivative, has approximately one positive charge every 2 nm along the polymer chain. In a 1 wt % aqueous solution, this corresponds to a concentration of charges of about 10 mm.46 Since JR 400 has a molecular weight of about 500 kDa, each polymer chain has an approximate contour length of 1000 nm.47 LM 200 is a N,N-dimethyl-N-dodecylammonium derivative that on average has approximately one positive charge every 10 nm, or about 2 mm of charge in a 1 wt % aqueous solution. Since this polymer has a low molecular weight (approximately 100 kDa), the average contour length is much shorter (on average about 200 nm).46-48 Both polymers render their charges from quaternary ammonium groups. However, the ammonium group on LM 200 also contains a dodecyl chain. The degree of hydrophobic modification of LM 200 is about 5.4 hydrophobic chains per 100 sugar residues. All experiments were conducted with pure water that was passed through a Milli-Q plus purification system (Millipore, USA) with a resistivity of 18.2 MΩ-cm. (43) Rollan, A. Pharmaceutical Particulate Carriers. Therapeutic Application; Rollan, A., Ed.; Marcel Dekker: New York, 1993; 367-421. (44) Schreier, H.; Bouwstra, J. J. Controlled Release 1994, 30, 1–5. (45) Mourtas, S.; Fotopoulou, S.; Duraj, S.; Sfika, V.; Tsakiroglou, C.; Antimisiaris, S. G. Colloids Surf. B 2007, 55, 212–221. (46) Thuresson, K.; Nilsson, S.; Lindman, B. Langmuir 1996, 12, 530–537. (47) Dhoot, S.; Goddard, E. D.; Harris, E. C.; Murphy, D. S. Colloids Surf. 1992, 66, 91–96. (48) Guillemet, F. Thises de Doctorat, Universite de Paris VI: 1995.

2376 DOI: 10.1021/la902798n

Catanionic Vesicles Preparations by Semispontaneous Process. A very simple semispontaneous process for preparing catanionic vesicles in water with the aid of cosolvent addition has been developed.14,15 This process was adopted as a preparation method in this work. In a typical experiment of catanionic vesicle preparation, a suitable amount of DTMA-DS with or without excess DTMAB or SDS was dissolved in 1-propanol. Water was then added and the solution sample was passed through a homogenizer (Ultra-Turrax T-25, IKA) at 11000 rpm for 3 min at room temperature in a sealed container specially designed for this vesicle preparation process. The total surfactant concentration, however, was kept constant at 10 mM. The composition of mixed solvents was represented by the cosolvent volume percentage and was kept constant at 12.5%, which is the optimal cosolvent concentration revealed by the previous study of DTMA-DS at 1 mM concentration.14

Measurements of Diameter and ζ-Potential and Physical Stability Evaluation of Charged Catanionic Vesicles. The diameter of catanionic vesicles was determined by the dynamic light scattering method using a computerized analyzer (model Zetasizer 3000 HS, Malvern, U.K.). During a measurement, the count rate, that is, the sample scattering intensity, was also provided. It is noteworthy that the count rate determined for pure water is about 0.6 kcount/sec (kilo 3 count per second, kcps). In this work, a criterion of count rate for the existence of appreciable amounts of catanionic vesicles was set to be g150 kcps, which was used to justify the stability of the vesicles. Details of the measurements of diameter and count rate by the analyzer were reported elsewhere.14,15,20,24 The ζ potential of catanionic vesicles was determined by the laser-Doppler electrophoretic light scattering method using the same vesicle size analyzer employing a He-Ne laser (λ = 632.8 nm, 10 mW). It is obvious that the variation of catanionic vesicle size with time is a critical test for the physical stability of catanionic vesicles. Therefore, the catanionic vesicle stability was evaluated by measuring the catanionic vesicle size and size distribution as a function of time. A transmission electron microscope (model H-7500, Hitachi) was used to obtain the catanionic vesicle images with the negativestaining technique. For the sample preparation, a few drops of catanionic vesicle dispersions were applied to carbon-coated Cu grid and dried. A drop of uranyl acetate-ethanol solution was then added as the staining agent. Sample Preparation and Phase Characterization. The polymer-catanionic vesicle samples of desired compositions were prepared by adding solid polymer (1-9% by weight) into catanionic vesicle solution (10 mM total surfactant concentration) in test tubes and followed by rotary mixing for 2 h at room temperature. All samples were then let to stand at 25 °C to attain equilibrium for 7 days before any visual examination or rheological experiments were performed. It is worthy to note that polymer and catanionic vesicle solution were mixed in appropriate amounts to obtain samples along lines defined by constant polymer content (varying surfactant composition) or constant surfactant composition (varying polymer content). The phase boundary was evaluated visually by tube inversion (details under Results). Rheological Studies. A dynamic rheometer (RS 150, Haake, Germany) equipped with an automatic gap setting was used for the rheological measurements. The instrument can be run either in the controlled stress or in the controlled strain mode. A cone-andplate geometry of 35 mm diameter with a 2° cone angle was used. The sample temperature was maintained at 25 ( 0.1 °C by a circulating water bath and evaporation was minimized by a solvent trap. Prior to any oscillatory deformation test, the linear viscoelastic region for each sample was determined by stress sweep tests. Measurements within the linear regime ensure that the measured rheological properties are independent of the applied stress and that near equilibrium properties are probed. The Langmuir 2010, 26(4), 2374–2382

Huang et al.

Article

Figure 1. Solution appearance of DTMA-DS in 12.5% 1-propanol with different amounts of excess component ionic surfactants.

experiments were performed as frequency sweeps in the range of 0.1-100 rad/s at a constant stress to determine the sample storage, G0 , and loss, G00 , moduli. The storage modulus, G0 , gives a measure of the elastic properties of the material, associated with its ability to store energy. The loss modulus, G00 , gives a measure of the viscous properties associated with dissipation of work done on the material. Sample relaxation time, τ, was found as the inverse angular frequency where G0 and G00 intersect. At times shorter than this characteristic time, a solution has a response that is mainly elastic, while at longer times viscous behavior prevails. Moreover, the storage moduli at definite frequencies (1 Hz in this work), which gives information about the number of active links,49 were also determined.

Results and Discussion In order that negatively and positively charged catanionic vesicles can be prepared by using DTMA-DS, the addition of excess component anionic (SDS) and cationic (DTMAB) surfactants is desirable. Various compositions of DTMA-DS/ SDS or DTMAB mixed surfactants, therefore, were designed for the semispontaneous process. In this work, XDS is defined as the composition of dodecylsulfate (DS) in the mixed surfactants with DS and DTMA (dodecyltrimethylammonium), which are combined as an IPA, treated as two surfactants. For example, a solution with XDS = 0.5 means that only DTMA-DS is used and therefore DS and DTMA are of equal fractions. On the other hand, solution with XDS = 0.45 and XDS = 0.55 mean that DS is deficient and excess, respectively, in amounts as compared to DTMA. Because the total surfactant concentration was always kept constant at 10 mM in this work, XDS = 0.45 and XDS = 0.55 actually indicated that surfactant solutions were prepared with DTMA-DS/DTMAB = 4.5 mM/1 mM and DTMA-DS/ SDS = 4.5 mM/1 mM, respectively. The other compositions follow the same definition. Charged Catanionic Vesicles. As shown in Figure 1 for solutions with XDS = 0.1, 0.2, 0.3, 0.35, 0.4, 0.45, 0.55, 0.6, 0.7, and 0.8, the appearance of solutions of DTMA-DS in 12.5% 1-propanol with different amounts of excess component ionic surfactants exhibited an interesting trend. On the basis of XDS = 0.5, similar appearance changing with excess component ionic surfactants can be observed. To the right side with excess SDS, the solutions with XDS = 0.55 and 0.6 actually appeared bluish to the eye. Furthermore, a solution with XDS = 0.6 appeared less bluish than those with XDS = 0.55. For more excess SDS, however, the solutions with XDS = 0.7 and 0.8 appeared clear to the eye. To the left side with excess DTMAB, solutions with XDS = 0.45, 0.4, 0.35, and 0.3 appeared bluish to the eye. They appeared less and less bluish with more and more excess DTMAB. For more excess DTMAB, again, the solutions with XDS = 0.2 and 0.1 appeared clear to the eye. It is thought that bluish appearance is one manifestation of the existence of catanionic vesicles. A reasonable inference, therefore, is that there may exist a catanionic vesicle (49) Ferry, J. Viscoelastic Properties of Polymers, 3rd ed.; John Wiley & Sons: New York, 1980.

Langmuir 2010, 26(4), 2374–2382

Figure 2. Initial size and ζ potential distributions: (a) XDS = 0.5, (b) XDS = 0.45, and (c) XDS = 0.55.

regime between compositions XDS = 0.3 and 0.6. Stable catanionic vesicles can not exist beyond these compositions. Parts a-c of Figures 2 show, for example, the initial size and ζ potential distributions of the catanionic vesicles in solutions with XDS = 0.5, 0.45, and 0.55, correspondingly. Mean size values of 343, 141, and 172 nm, and mean ζ potential values of -20, þ30, DOI: 10.1021/la902798n

2377

Article

Huang et al.

Figure 3. TEM image of the catanionic vesicles formed from DTMA-DS/Ionic surfactants in 12.5% 1-propanol solution observed by the negative staining technique: (a) XDS = 0.45 and (b) XDS = 0.55.

and -76 mV were measured. For the process of semispontaneous formation, typical catanionic vesicles with mean sizes of tens to hundreds of nanometers are expected. The TEM micrographs shown in Figure 3, parts a and b, illustrate the DTMA-DS/ionic surfactant catanionic vesicles formed in 12.5% 1-propanol aqueous solution with XDS = 0.45 and 0.55, respectively. Electrical neutrality of catanionic vesicles formed from DTMA-DS without excess component ionic surfactants is expected by intuition. Mean ζ potential value of -20 mV, however, was observed. This implies that some cationic amphiphiles in bilayers of DTMA-DS might dissolve into the solvent and leave the catanionic vesicles negatively charged. Moreover, charge enhancement and reversal by the addition of excess component ionic surfactants are clearly observed for catanionic vesicles with XDS = 0.55 and 0.45, respectively, as compared to that with XDS = 0.5. The incorporation of ionic amphiphiles in vesicle membranes is proposed. Parts a-c of Figure 4 show the variations of initial count rate, size, and ζ potential measurements, correspondingly, as functions of XDS. As shown in Figure 4, parts a and b, the values of count rate and size indicated that the composition range for stable catanionic vesicle formation is around XDS = 0.25-0.65 according to the count rate criterion. The shaded area, which covers the composition range with count rate g150 kcps and exactly evidenced by the experimental data, therefore represents a somewhat cautious composition range (XDS = 0.3-0.6) for the 2378 DOI: 10.1021/la902798n

Figure 4. Composition range for stable catanionic vesicle formation as revealed by initial (a) count rate, (b) size, and (c) ζ potential.

formation of stable catanionic vesicles. In the stable catanionic vesicle regime, values of ζ potential range from 59 mV to -96 mV as shown in Figure 4c. The charge variations and reversal for catanionic vesicles can, therefore, be realized by the addition of excess component anionic or cationic surfactants in catanionic surfactants. The physical stability of charged catanionic vesicles was further tested by monitoring the catanionic vesicle size and size distribution with time. The data in Figure 5. parts a-c, suggest that lifetime of 30 days was observed for catanionic vesicles with XDS = 0.5 and lifetimes of more than 244 days might be observed for catanionic vesicles with XDS = 0.45 and 0.55. Significant Langmuir 2010, 26(4), 2374–2382

Huang et al.

Article Table 1. Characteristics of Stable Charged Catanionic Vesicles at Various Compositionsa IPA: DTMA-DS Cosolvent: 1-propanol

XDS

initial mean diameter (nm)

initial mean ζ potential (mV)

appearance time of smaller-sized aggregates after preparation(day)

lifetime (day)

0.3 289 59 2 128 0.35 222 49 10 128 0.4 177 41 20 143 0.45 141 30 45 >244 0.5 343 -20 30 0.55 172 -76 80 >244 0.6 160 -96 39 179 a Total surfactant concentration = 10 mM; cosolvent composition = 12.5 vol %.

Figure 5. Time-dependent size distributions and lifetimes of catanionic vesicles: (a) XDS = 0.5, (b) XDS = 0.45, and (c) XDS = 0.55.

lifetime enhancement by the addition of excess component ionic surfactants was demonstrated. Some larger particle signals could be observed in Figure 5, which might be due to the formation of precipitate or larger vesicles. However, the intensity of the larger particle signals was significantly lower than that of the vesicles, and no phase separation could be suggested from the visual observation when the signals first appeared. It should be noted that the upper and lower limits of an accurate size measurement by the particle size analyzer used in this work are 3000 and 3 nm, respectively. The arrow in Figure 5a indicates the time when the catanionic vesicle size exceeded the upper limit of an accurate size Langmuir 2010, 26(4), 2374–2382

Figure 6. (a) Phase map constructed by visual examination and tube inversion, (b) relaxation time, and (c) G0 of catanionic vesicle/ P0 system. DOI: 10.1021/la902798n

2379

Article

Figure 7. (a) Phase map constructed by visual examination and tube inversion, (b) relaxation time, and (c) G0 of catanionic vesicle/ HMP0 system.

measurement and, therefore, represents the lifetime of catanionic vesicles with XDS = 0.5. The arrows in Figures 5, parts b and c, on the other hand, indicate that catanionic vesicles were still alive at the time of 244 days after preparation. Furthermore, besides accompanied by larger-sized aggregates, catanionic vesicles with smaller-sized aggregates can also be observed at times after preparations for all the compositions studied except XDS = 0.5. This is attributed to the preferential dissolution of vesicle components and subsequent formation of mixed micelles in the solution. Table 1 summarizes the characteristics of the stable charged catanionic vesicles at various compositions. The data in Table 1 revealed that size of catanionic vesicles at various compositions departing from XDS = 0.5 is always smaller than that at XDS = 0.5. The anionic excess surfactant seems to demonstrate a stronger effect on the variation of ζ-potential than the cationic one. The larger the compositions departing from XDS = 0.5, the earlier the appearance of smaller-sized aggregates in the catanionic vesicle solutions. Except for the XDS = 0.5, catanionic vesicles with all compositions studied indicated lifetimes of more than 4 months. 2380 DOI: 10.1021/la902798n

Huang et al.

Figure 8. (a) Phase map constructed by visual examination and tube inversion, (b) relaxation time, and (c) G0 of catanionic vesicle/ Pþ system.

Phase and Rheological Characterization. Test tube inversion is frequently employed in studying gels and is basically a measure of sample yield stress. In this study, three typical “phases”, that is, phase separation, viscous solution, and gel, resulted from various mixtures of catanionic vesicle and polymer. “Phase separation” may occur with different viscosities in upper or lower phases and can be identified easily by visual examination. While a “viscous solution” with a nonexistent or low yield stress will drop down in 30 s from the beginning of test tube inversion, a “gel-like” sample with sufficient yield stress will be able to hold its own weight longer than 30 s in an inverted tube. Phase maps of catanionic vesicle/P0, catanionic vesicle/HMP0, catanionic vesicle/Pþ, and catanionic vesicle/HMPþ systems constructed by visual examination and tube inversion are shown in Figures 6a, 7a, 8a, and 9a, correspondingly. It is noteworthy that the phases exhibited by the pure polymers in water containing 12.5% 1-propanol (hydropropanolic solution) were also indicated Langmuir 2010, 26(4), 2374–2382

Huang et al.

Article

Figure 9. (a) Phase map constructed by visual examination and tube inversion, (b) relaxation time, and (c) G0 of catanionic vesicle/ HMPþ system.

in these phase maps for comparison. As shown in Figure 6a, phase separation was found for both Vþ and V- at lower polymer P0 (250MR) concentrations. With increasing polymer concentration, however, gel was formed directly from phase separation except for XDS = 0.35 and 0.3 where gel was formed from phase separation through a brief formation of viscous solution. On the other hand, gel was formed through viscous solution with increasing the concentration of pure polymer in water. As shown in Figure 7a for vesicle solutions and polymer HMP0 (Plus 330), the viscous solution region became larger at the expense of reduction in phase separation region. This is probably owing to hydrophobic effect of hydrophobically modified polymers. Obviously, the effect of vesicle charge on phase formation is insignificant due to charge neutrality of both nonionic polymers P0 and HMP0. Significant charge effect, in contrast, on phase formation was found for vesicles and cationic polyelectrolyte Pþ (JR400) as shown in Figure 8a. Almost all the positively charged vesicle solutions and the cationic polyelectrolyte resulted in phase separation on one hand, almost all negatively charged vesicle Langmuir 2010, 26(4), 2374–2382

Figure 10. Storage, G0 , and loss, G00 moduli as a function of frequency, ω, for 6 wt % LM200 and 10 mM total surfactant concentration with different compositions. (a) XDS = 0.45, (b) XDS = 0.5, and (c) XDS = 0.55.

solutions and the cationic polyelectrolyte resulted in viscous solutions on the other hand. However, only two phases, that is, phase separation and viscous solution, occurred. Gel accompanying viscous solution and phase separation were formed for vesicle solutions and HMPþ (LM200) as can be seen in Figure 9a. Phase separation, which prevails for positively charged vesicle solutions with Pþ at all polymer concentrations, was transformed to be viscous solution at intermediate polymer concentrations and gel at higher polymer concentrations, respectively, of HMPþ. Phase DOI: 10.1021/la902798n

2381

Article

separation region appeared at lower HMPþ concentrations for the negatively charged vesicle solutions, on the other hand, at the expense of disappearance of viscous solution region. This implies that both hydrophobic and electrostatic effects work for the system. Furthermore, the experimental results of the interactions between charged catanionic vesicles and polymers with various charge and hydrophobic characteristics also shed light on the roles of electrostatic and hydrophobic effects that may play in gelation. Viscous solution and gel samples of the catanionic vesicle/polymer systems were further subjected to rheological characterization. Plots of the rheological parameters G0 and G00 , the shear storage modulus and the shear loss modulus, respectively, versus the oscillation frequency for the mixtures of 6 wt % HMPþ (LM200) and vesicle solutions with XDS = 0.45, 0.5, and 0.55, respectively, are shown in Figure 10 for example. The angular frequencies where G0 and G00 intersect are also indicated in the figures. It should be noted again that the inverse of them represent the corresponding sample relaxation times. Figures 6b, 7b, 8b, and 9b show the variations of relaxation time (τ) and Figures 6c, 7c, 8c, and 9c show the storage modulus at 1 Hz, G0 (1 Hz), with composition for the catanionic vesicle/P0, catanionic vesicle/HMP0, catanionic vesicle/Pþ, and catanionic vesicle/HMPþ systems, correspondingly. Again, values of τ and G0 (1 Hz) for hydropropanolic solutions of pure polymers were also indicated in these figures for comparison. As shown in parts b and c of Figure 6, τ and G0 (1 Hz) values for mixtures of vesicle solutions and polymer P0 (250MR) at different polymer concentrations seem to vary insignificant with charge. This is also true for mixtures of vesicle solutions and polymer HMP0 (Plus330) as can be seen in Figure 7, parts b and c. Much higher values of τ and G0 (1 Hz) for mixtures as compared to those for hydropropanolic solutions of pure polymer with the same polymer contents, however, revealed that hydrophobic rather than electrostatic effect dominates the rheological properties of catanionic vesicle/ HMP0 system. Values of τ and G0 (1 Hz) for the viscous solutions of negatively charged vesicles and polymer Pþ (JR400) at different polymer concentrations, as shown in Figure 8, parts b and c, revealed that charge may consistently strengthen the rheological properties. However, the properties are not so much different from those of polymer solutions themselves. On the other hand, much higher values of τ and G0 (1 Hz) for mixtures as compared to those for hydropropanolic solutions of pure polymer with the same polymer contents and the consistent charge effect were found for

2382 DOI: 10.1021/la902798n

Huang et al.

catanionic vesicle/HMPþ (LM200) system as shown in Figure 9, parts b and c.

Conclusions A semispontaneous process was developed to prepare catanionic vesicles with controlled charge characteristics from mixed ion pair amphiphile (IPA)/single-chained ionic surfactant systems. The phase behavior and rheological properties of the catanionic vesicles with various types of water-soluble polymers were then elucidated. With the developed semispontaneous process, charge characteristics of catanionic vesicles prepared from an IPA, DTMA-DS, could be easily controlled through the presence of single-chained ionic surfactants, DTMAB or SDS. Moreover, stable catanionic vesicles were found with XDS between 0.3 and 0.6. For the charged catanionic vesicles with water-soluble polymers without charge and hydrophobic modification, one could notice that phase separation occurred at low polymer concentrations and gel structure was formed at high polymer concentrations. However, the phase separation behavior was inhibited when the uncharged water-soluble polymer was hydrophobically modified, most likely due to enhanced hydrophobic interactions between the catanionic vesicles and polymers. This was supported by that the rheological properties of the system was dominated by the hydrophobic effect rather than the electrostatic effect. For the mixture of charged catanionic vesicle/cationic polyelectrolyte, the phase behavior was strongly dependent on the charge characteristics of the vesicles. One generally found that viscous solutions were always formed when the vesicles were negatively charged and phase separation generally occurred when the vesicles were positively charged. With the presence of cationic polyelectrolytes with hydrophobic modification in the negatively charged catanionic vesicle dispersion, phase separation was detected at lower polymer concentrations with the disappearance of the viscous solution region, implying that both electrostatic and hydrophobic effects worked for the system. However, for the mixtures of the positively charged catanionic vesicle/polyelectrolyte with hydrophobic modification, the phase separation behavior was inhibited and viscous solution or gel was found at higher polymer concentrations, suggesting that hydrophobic effect was more pronounced than the electrostatic effect for the mixtures. Acknowledgment. This work was supported by the National Science Council of Taiwan through Grants NSC 93-2214-E-006006 and NSC 97-2221-E006-094. The authors are grateful to Professor Lynn L. H. Huang for her generous providing access to the rheometer.

Langmuir 2010, 26(4), 2374–2382