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Spontaneous Vesicle Formation from Poly(1,2-butylene oxide) Sulfate Oligomers
Chart 1. Structures of the Poly(butylene oxide) Oligomer before (a) and after (b) Sulfonation
J. Keith Harris,*,†,‡ Gene D. Rose,‡ and Merlin L. Bruening† Chemistry Department, Michigan State University, East Lansing, Michigan 48824, and The Dow Chemical Company, Corporate Research and Development, Midland, Michigan 48674 Received February 24, 2003. In Final Form: April 16, 2003
Introduction We recently showed that poly(1,2-butylene oxide/ethylene oxide) (BO/EO) diblock copolymers spontaneously form multilamellar vesicles (MLVs) when mixed with water if the average EO block length is between 5 and 18 units.1 If the EO block is larger than 18 units, spherical micelles result; if smaller than 5 units, an emulsion forms. This behavior is consistent with a packing model that suggests that micellar structure is determined by surfactant geometric properties such as headgroup area and hydrophobe length and volume.2 Interestingly, while numerous ethoxylated surfactants exist, few demonstrate spontaneous vesicle formation. The propensity of certain BO/EO copolymers to form MLVs is thus most likely due to the large volume-to-length ratio of the BO hydrophobe. If this is the case, BO surfactants with other headgroups (anionic, cationic, or nonionic) should also form vesicles as long as the headgroup has the appropriate size. On the basis of the similarity in headgroup areas calculated for an EO11 block and measured for -OSO3-, we thought that sulfonated BO surfactants would also form vesicles.3,4 In this note, we show that sulfonated BO is indeed capable of forming vesicles, thus expanding the versatility of BO surfactants. An ionic surfactant that spontaneously produces vesicles offers several advantages over ethoxylated materials. Due to charge repulsion, vesicles prepared from ionic surfactants are less likely than ethoxylated structures to form gels at elevated concentrations.5 Anionic vesicles should also be stable at higher temperatures than structures formed from nonionic surfactants.6 Finally, strongly ionic headgroups, such as sulfate, are relatively unaffected by changes in pH, although hydrolysis of the -OSO3- group could be problematic.7 Experimental Section BO oligomers (Chart 1a) were prepared following the procedure we previously used to make poly(BO/EO) diblock copolymers, except that the reaction was terminated with acetic acid instead of continuing with the addition of the EO block.1 The hydroxyl* To whom correspondence should be addressed. E-mail: kharris@ dow.com. † Michigan State University. ‡ The Dow Chemical Co. (1) Harris, J. K.; Rose, G. D.; Bruening, M. L. Langmuir 2002, 18, 5337. (2) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1525. (3) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Phys. Chem. 1963, 67, 1987. (4) Rosen, M. J. Surfactants and Interfacial Phenomenon; WileyInterscience: New York, 1989; pp 70-80. (5) Schmolka, I. R. J. Am. Oil Chem. Soc. 1991, 68, 206. (6) Rosen, M. J. Surfactants and Interfacial Phenomenon; WileyInterscience: New York, 1989; pp 10 & 23. (7) Asinger, F.; Geiseler, G.; Muller, G. Chem. Ber. 1960, 93, 2491.
terminated BO was then sulfonated using a DMF‚SO3 complex.8 After the reaction, the methylene chloride solvent was evaporated, and the product was dispersed in water. NaOH (0.5 N) was added dropwise until the solution was neutralized. The product (Chart 1b) was dried under a vacuum to produce a clear oil. Sulfonation was confirmed by Fourier transform infrared (FT-IR) spectroscopy, and average BO lengths were determined by 1H NMR in CDCl3.1 Interestingly, after a few months, EtBO4SO4Na and EtBO11SO4Na underwent phase separation. The oligomers formed a clear, upper phase and a viscous, opaque, lower phase which contained a white precipitate. Dispersions prepared from the upper phase did not form vesicles but themselves phase separated from water after mixing. Phase separation suggests surfactant degradation, so all the vesicle data presented here were obtained from oligomers prior to any evidence of phase separation. Vesicles were formed by placing 1 drop (∼0.03 g) of the surfactant into a 5 dram vial and adding 3 g of deionized (>17 MΩ cm) water. The vial was then agitated by hand to produce structures. To compare the thermal stabilities of EtBO11EO11 and EtBO11SO4Na, 1 wt % dispersions of each material were prepared by hand agitation and subsequent ultrasonication (Ultrasonic Processor, model 130, Cole-Parmer Instruments, Vernon Hills, IL; 3 mm probe).9 The dispersions were exposed to ultrasonication for 2 min at 10 W using a “4 s on/1 s off” cycle while the vials were cooled in an ice-water bath. Prior to light-scattering measurements (Brookhaven 90Plus, Brookhaven Corp., Holtsville, NY), 1 wt % dispersions were allowed to equilibrate in a water bath at the desired temperature for at least 16 h. The dispersion was evaluated at the equilibrium temperature and then returned to the water bath to equilibrate at a new temperature. Light-scattering data were analyzed using the CONTIN algorithm and intensity-weighted measurements. The average diameter was calculated from the mean values of at least five separate measurements. The polydispersity values for the particles reflect the intensity-weighted relative variance of the diffusion coefficient. Values should be less than 0.3 for best particle size data and close to zero for monodisperse systems. Structures were also examined using cryogenic scanning electron microscopy (cryo-SEM) (JEOL JSM-6320FV) as described previously.1
Results and Discussion Vesicle formation requires that the hydrophobe length exceed some minimal value. Nakamura et al. showed that BO sulfonates with less than 3 BO units displayed no (8) Kelly, K. K.; Matthews, J. S. J. Org. Chem. 1971, 15, 2159. (9) The EtBO11EO11 copolymer forms large MLV structures using the hand-agitation method, so ultrasonication was used to reduce these structures to a diameter similar to that spontaneously formed by the EtBO11SO4 oligomer. To be experimentally consistent, both dispersions were ultrasonicated.
10.1021/la034319j CCC: $25.00 © 2003 American Chemical Society Published on Web 05/21/2003
Notes
Figure 1. Cryo-SEM image of vesicles spontaneously formed from 1 wt % EtBO11SO4Na in water. The sample was aged for 3 weeks prior to examination.
interfacial tension or film formation characteristics on water, so we felt that the combination of the ethoxide initiator and 4 BO units would be about the minimum BO block length necessary to initiate micellization.10 We chose a maximum BO length of less than 20 units because our experience with EtBO20EOn copolymers showed that spontaneous vesicle formation from materials containing such a large hydrophobe is difficult. This is likely due to the high energy needed to disperse a large hydrophobe prior to self-assembly into an ordered structure. Therefore, the three oligomers synthesized for this study (EtBO4SO4Na, EtBO11SO4Na, and EtBO17SO4Na) were intended to represent probable minimum and maximum BO lengths for spontaneous vesicle formation as well as a composition comparable to that of the most common BO/EO material we previously studied (EtBO11EO11). While having low polydispersity, these materials are not monodisperse, and the formulas listed represent the average oligomeric compositions of each surfactant. Gentle agitation was sufficient to produce structures from EtBO4SO4Na and EtBO11SO4Na; however, EtBO17SO4Na required ultrasonication for complete surfactant dispersion. Light scattering showed that the EtBO4SO4Na oligomer spontaneously produced vesicles with an intensity-weighted mean particle diameter of 180 nm and a mean polydispersity of 0.12 ( 0.04. The polydispersity reflects the fact that 95% of the particles have diameters between 90 and 260 nm. The EtBO11SO4Na oligomer spontaneously formed vesicles with diameters of 140 nm and a polydispersity of 0.17 ( 0.01. More than 95% of these particles have diameters between 60 and 190 nm. A second dispersion of EtBO11SO4Na was ultrasonicated for 2 min to yield vesicles with diameters of 130 nm. EtBO17SO4Na produced vesicles with diameters of 150 nm and a polydispersity of 0.18 ( 0.01 after 30 s of ultrasonication. Vesicle sizes, which varied by less than 30% from measurement to measurement, are on the order of those seen with BO/EO surfactants after sonication.1 All vesicles showed less than a 10% change in particle size after standing at room temperature for 30 days. Cryo-SEM images are consistent with light-scattering data taken from the same dispersion. Images, such as Figure 1, show that EtBO11SO4Na spontaneously forms spherical particles with 30-300 nm diameters, and the majority of structures are in the 50-150 nm range. (10) Nakamura, M.; Sakashita, T.; Mori, T.; Yano, W.; Kimura, W. Yukagaku 1972, 21, 95.
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Figure 2. Diameters of vesicles formed from EtBO11EO11 (b) and BO11SO4Na (9) as a function of temperature. Dispersions were 1 wt % surfactant and were ultrasonicated prior to evaluation. Error bars represent the standard deviations of the mean of five measurements.
As would be expected for an anionic surfactant, structures formed from sulfonated copolymers are more stable at high temperatures than vesicles formed from ethoxylated copolymers. Figure 2 shows the diameters of EtBO11SO4Na and EtBO11EO11 structures as a function of temperature. The graph indicates that both ethoxylated and sulfonated BO structures increase in diameter at about the same rate upon heating. For both surfactants, the increase in particle size is likely due to a decrease in the headgroup area resulting from thermally induced dehydration.11,12 Dehydration decreases the radius of curvature for the bilayer and increases the vesicle diameter. However, at temperatures greater than 65 °C, the ethoxylated surfactant loses its amphiphilic properties and phase separates from the aqueous medium, while EtBO11SO4Na does not.1 (The EtBO11SO4Na structures are stable enough that reproducible light-scattering data can still be collected at 75 °C, though the ability of the instrument to maintain that temperature accurately is questionable.) To further evaluate the thermal stability of the anionic surfactant, a 1 wt % dispersion of EtBO11SO4Na was sealed in a glass bottle and placed in an oven at 100 °C for 9 days. Examination of the hot sample showed that while the dispersion had become more opaque, there was no indication of phase separation or an increase in sample viscosity. The cooled sample returned to its original appearance. Since hydrolysis of the sulfate group in an aqueous environment is a possibility, the long-term stability of the vesicles was evaluated. A 1 wt % dispersion of EtBO17SO4Na was prepared with ultrasonication and found to have a mean particle diameter of 140 nm by light scattering. The sample then stood at room temperature for 5 months with less than a 10% decrease in average particle diameter. Thus, at pH 6.8, these vesicles are extremely stable. Conclusions Spontaneous formation of vesicles from both sulfonated BO oligomers and BO/EO copolymers indicates that the BO hydrophobe, not the EO headgroup, is primarily responsible for the formation of BO/EO vesicles. Spon(11) Rosen, M. J. Surfactants and Interfacial Phenomenon; WileyInterscience: New York, 1989; p 142. (12) Corkill, J. M.; Goodman, J. T.; Tate, J. R. Trans. Faraday Soc. 1966, 62, 979.
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taneous structure formation by sulfonated BO17 oligomers occurs for average hydrophobe lengths of 4 and 11 BO units, and vesicles also form from sulfonated BO17 oligomers upon ultrasonication. Structures of all three sulfonated oligomers were between 120 and 175 nm in diameter, which is consistent with the size expected from a surfactant geometry model.2 Furthermore, the BO sulfate structures are stable for at least 5 months at room temperature and give no sign of phase separation after standing at 100 °C for more than a week. These structures are considerably more temperature stable than vesicles
Notes
formed from EtBO11EO11, as the ethoxylated materials phase separate at 65 °C. Acknowledgment. Our thanks to The Dow Chemical Company for sponsoring this research and to Andrew Schirmer and Darrell Mondeau for assistance with the oligomer synthesis, conversion, and characterization. Thanks also to Dave Williams (The Dow Chemical Company) for cryo-SEM work. LA034319J
