J. Phys. Chem. B 2005, 109, 5627-5630
5627
Salt-Induced Vesicle Formation from Single Anionic Surfactant SDBS and Its Mixture with LSB in Aqueous Solution Limin Zhai,*,† Mei Zhao,† Dejun Sun,‡ Jingcheng Hao,‡ and Lungjun Zhang† College of Chemistry and Chemical Engineering, Jinan UniVersity, Jinan, 250022, P.R.China, and Key Lab of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan, 250100, P.R. China ReceiVed: NoVember 27, 2004; In Final Form: January 28, 2005
Vesicles can be formed spontaneously in aqueous solution of a single anionic surfactant sodium dodecyl benzenesulfonate (SDBS) just under the inducement of salt, which makes the formation of vesicle much easier and simpler. The existence of vesicles was demonstrated by TEM image using the negative-staining method. The mechanism of the formation may be attributed to the compression of salt on the electric bilayer of the surfactant headgroups, which alters the packing parameter of the surfactant. The addition of the zwitterionic surfactant lauryl sulfonate betaine (LSB) makes the vesicles more stable, expands the range of formation and vesicle size, and reduces the polydispersity of the vesicles. The vesicle region was presented in a pseudoternary diagram of SDBS/LSB/brine. The variations of vesicle size with the salinity and mixing ratios, as well as the surfactant concentration, were determined using the dynamic light scattering method. It is found that the vesicle size is independent of the surfactant concentration but subject to the salinity and the mixing ratio of the two surfactants.
Introduction Spontaneously formed vesicles have become more and more acquainted to researchers in the field of surfactant associates. They are formed from various surfactant mixtures in aqueous solutions, such as catanionic,1-9 cationic/cationic,10 zwitterionic/ anionic,11 and nonionic/ionic mixtures12,13 or upon addition of cosurfactants in ionic mixtures.14,15 Vesicles can also be formed directly from single surfactant with double tails, for example, DDAOH, through changing polar headgroups,16 and Aerosol OT with salt.17 However, it seems impossible to form vesicles from simple single-tailed surfactant alone, except for special bola surfactant18 and oligomers.19 In fact, according to the theory of Israelachvili,20 surfactants spontaneously form aggregates when their concentration is above a certain “critical” value. The aggregate morphology depends on the well-known packing parameter P:
P ) V/lca0 where V and lc are the volume and chain length of the hydrophobic group, respectively, and a0 is the cross sectional area of the headgroup dictated by the electrostatic repulsion between adjacent headgroups in the associates. Alternatively, reducing the electrostatic repulsion between adjacent headgroups can increase the packing factor P. The pH,21,22 temperature,23,24 and salinity17 can all alter the electrostatic repulsion and thus the aggregation morphology. Therefore, if increasing the salinity of the aqueous solution of single-tailed ionic surfactant, the compressed headgroups may increase the value of P, which may result in the formation of the required structures. * To whom correspondence should be addressed. E-mail: lmzhai@ hotmail.com. † Jinan University. ‡ Shandong University.
In this paper, we have found evidence of the inducement of salt on the vesicle formation in aqueous solution of the single anionic surfactant sodium dodecyl benzenesulfonate (SDBS), which was demonstrated by TEM using the negative-staining method. Usually, SDBS is mixed with cationic surfactants such as CTAB to form vesicles.1 No large structure was expected to form from this single-tailed surfactant except micelles. But after the salt was introduced, there was an unexpected phenomenon. The addition of zwitterionic surfactant LSB expands the range of vesicle formation and changes the formation salinity, which was described in a pseudoternary diagram. The polydispersity and the stability are also markedly improved after the participation of LSB as measured using a light scattering method and visual observation, where no cloudy wisp was formed even in a year. Experimental Section Materials and Preparation of Vesicles. Sodium dodecyl benzenesulfonate (SDBS) (C18H29SO3Na) (purity > 90%, Na2SO4 < 7%, H2O < 3%) was from Beijing Chemical C.O. and was purified according to the following procedures: (1) SDBS was dissolved in hot methanol and filtered to remove Na2SO4 and (2) the solution was mixed with water and evaporated at 70 °C until dry sample was obtained. Lauryl sulfonate betaine (LSB) (C17H37SO3N) was purchased from Sigma and used as received, NaCl from Shanghai Chemical Co. was A.R. grade, and water was doubly distilled. Stock solutions were equilibrated at room temperature and filtered through a 0.2 µm filter prior to preparing samples. NaCl was added after being accurately weighed. Mixed vesicles were prepared by mixing stock solutions of both anionic and zwitterionic surfactant at the desired molar ratio. After brief vortex mixing, the solutions were not subject to any type of mechanical agitation. All samples were equilibrated at 25 °C in a thermostated bath for hours. Pseudoternary Diagram. Since the formation of vesicle is accompanied by the increase of scattering light, it can be judged
10.1021/jp044596e CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005
5628 J. Phys. Chem. B, Vol. 109, No. 12, 2005 roughly by the appearance of turbidity. It can be proved by dynamic light scattering that the blue scattering light from the 30-nm-sized spherical aggregates can be perceived by visual observation. During the preparation of the diagram, the total concentration of the two surfactants was fixed and NaCl was added into the mixture at different mixing ratios as a solid to avoid a change of concentration. Samples were placed in a beam of strong light with a dark background and observed from the vertical direction. The amount of NaCl in the mixture when turbidity appeared was recorded. The pseudodiagram was differently coordinated. All measurements were carried out at 25 °C. Negative-Staining TEM. The vesicles were observed with a transmission electron microscope using the negative-staining method. A drop of vesicle solution was spread on a 200-mesh copper grid with a Formvar film and the extra droplet was instantly removed by filter paper. After being naturally desiccated, a drop of 2% uranyl acetate in ethanol solution was dripped on the copper grid for about 60s and the extra droplet was also removed. Then the grid was dried naturally for about 3 h before TEM observation. Dynamic Light Scattering. The diameter of vesicles was determined using the dynamic light scattering (DLS) method which was done with a spectrometer of standard design (Brookhaven Model BI-300SM goniometer and Model BI9000AT correlator) and a 300 mW Ar laser. Prior to measurements, samples were allowed to equilibrate in a water bath at the desired temperature for at least 16 h and filtered through a 0.2 µm Millipore. All measurement was made at the scattering angle of 90° at 25 °C, and the intensity of the function was analyzed by the method of cumulants. The polydispersity values for the particle 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. Results and Discussions Vesicle Formation from Single-Tailed SDBS. Most singletailed surfactants, including SDBS and LSB in dilute solutions, have a P of less than one-third. This means that only spherical micelles are formed. For double-tailed surfactants, such as DDAB or AOT, the volume of the hydrophobic group (V) is increased, resulting in a P value in the one-half-1 range, and surfactants will form a bilayer or vesicle through mechanic forces. If the hydrophilic headgroup is further compressed by a salt, vesicles may be formed spontaneously.17 But for singletailed surfactant, it seems difficult to show such behavior. But when salinity reaches a rather high value, things may be different. In the experiment, the salinity was increased by adding solid NaCl into 0.01 M SDBS solution, and the sample was observed from a vertical side to the incident light. There is no apparent change at the first increase of salinity. However, while the salinity reaches 0.36 M, a bluish turbidity is detected in the solution, which is usually regarded as the precipitation at so high a salinity. Samples are then observed with TEM using a negative-staining method as shown in Figure 1a,b. Figure 1a shows some spheres with a diameter of about 50 nm. The staining may be a little heavy, so careful observation is needed to distinguish the typical character of the vesicles, bilayers, from the spheres. But in Figure 1b, the bilayers are exhibited clearly. The image shows many spherical rings crowded at the edge of the copper grid, which can be regarded definitely as vesicles. The diameter of vesicles is about 60 nm. The hollow or semi-hollow shape should be attributed to the
Zhai et al.
Figure 1. TEM images of pure SDBS vesicles. CSDBS ) 0.01 M, CNaCl ) 0.4 M.
Figure 2. Pseudoternary diagram of SDBS/LSB/brine. Csurfactant ) 0.01 M.
flow of the solution while staining, which makes the stained surfactants of vesicles shift out as a black cloud in the image. The vesicle solution from single SDBS is turbid and may produce cloudy wisps after being deposited for a long time. To make the vesicles more stable and their properties diversified, a zwitterionic surfactant lauryl sulfonate betaine (LSB) was introduced into the system. Vesicles from Mixtures of SDBS/LSB/Brine. The synergy of the zwitterionic surfactant LSB with AOT has been described in a previous paper,11 which stabilizes the AOT vesicles. A monodisperse vesicle system was obtained with the participation of salt, although the addition of salt has no influence on the phase behavior of the zwitterionic surfactant LSB itself, even at high salinity. In this paper, a pseudoternary diagram of SDBS/ LSB/brine was made to exhibit the vesicle region, as shown in Figure 2. In the diagram, the total surfactant concentration was fixed, salinity was varied through adding solid NaCl. It is shown that vesicles only appear after salt is added for all the mixing ratios. The mixture without salt is very clear, and no large structures are found in the TEM image. After the addition of salt, vesicles begin to be formed at different salinity. From the ternary diagram it can be seen that the salinity of vesicle formation decreases markedly after LSB is added, which is from 0.36 M in pure SDBS solution sharply to 0.06 M at xSDBS ) 0.9 and to the minimum of 0.01 M at a mixing ratio of xSDBS ) 0.7. But when the mixing ratio surpasses xSDBS ) 0.3, vesicles cannot be formed any more at any salinity. Vesicles become very stable with the participation of LSB, with no cloudy wisp being found during a year deposition. The TEM images of the mixed vesicles are also presented; one can clearly observe the bilayers of a large vesicle in Figure 3a and many crowed vesicles near the copper grid in Figure 3b. The size of the mixed vesicle is at about 200 nm, which is much larger than the single composed vesicle due to the decrease of the curvature with the participation of LSB.
Salt-Induced Vesicle Formation from SDBS and LSB
J. Phys. Chem. B, Vol. 109, No. 12, 2005 5629
Figure 3. TEM images of mixed vesicles. CNaCl ) 0.2 M, xSDBS ) 0.6, Csurfactant ) 0.01 M.
Figure 5. Variation of vesicle size and polydispersity with the mixing ratios at the same salinity: Csurfactant ) 0.01 M; CNaCl ) 0.05 M; 9, diameter; 0, polydispersity.
Figure 4. The diameter of the aggregates as a function of salinity at different mixing ratios: b, xSDBS ) 0.7; 9, xSDBS ) 0.4; Csurfactant ) 0.01 M.
Influence of Mixing Ratios and Salinities on the Vesicle Size. The variations of the vesicle size with different mixing ratios and salinities were determined by dynamic light scattering. Figure 4 shows the variation of the size of the aggregates with the salinities at different mixing ratios. The diameter increases evidently with salinity, due to the compression of salt on the polar headgroups, which reduces their volume and results in the decrease of the curvature of the spherical aggregates. But the increase is different for different mixing ratios, because of the cooperation of the zwitterionic surfactant LSB. In a SDBSrich mixture (xSDBS ) 0.7), the diameter increases abruptly at the beginning of the vesicle formation from micelles, which is from 4 to 103 nm at salinity of 0.01 M, and then turns to a gradual increase with salinity. But in the LSB-rich mixture (xSDBS ) 0.4), the vesicle size always keeps a slow increase with the addition of salt all along. No vesicle larger than 50 nm was observed, even at high salinity. The variation of vesicle size and polydispersity with mixing ratios is shown in Figure 5 at fixed salinity. The diameter increases at first with the increase of the LSB content, since its incorporation in the membrane reduces the curvature. But after a maximum, the diameter begins to decrease, which means the conic-structured LSB increases the curvature instead at high content. On the contrary, the polydispersity of the vesicles passes through a minimum at the mixing ratio of xLSB ) 0.3, which almost reaches a monodisperse state. It suggests that we can obtain different sized vesicles with low polydispersity through varying the salinity and mixing ratios, although the mechanism needs further investigation. Influence of Surfactant Concentration on Vesicle Size. The influence of surfactant concentration on vesicle size is shown
Figure 6. Variation of vesicle size with surfactant concentration at the same salinity: xSDBS ) 0.6; CNaCl ) 0.02 M.
in Figure 6. The salinity was fixed for different surfactant concentration from 0.001 to 0.01 M in the experiment. It is shown that the vesicle size is at about 95 nm for all the concentrations, which indicates that the size is independent of the surfactant concentration. The increase of surfactant concentration just increases the quantity of the vesicles, not the size. A similar conclusion that the formation concentration of vesicles from zwitterionic/anionic surfactants mixture is also independent of surfactant concentration but subject to the salinity was drawn in a previous paper,25 which implies that the vesicle size and formation concentration are all dependent on the area of the surfactant headgroups. Conclusions Spontaneous vesicles can be formed in single anionic surfactant SDBS solution under the inducement of salt, which is still a novel and interesting field in vesicle preparation. It makes the vesicle formation much easier and may contribute to the exact calculation of the surfactant packing parameter P. The participation of LSB makes the properties of the vesicle diversified, which may favor the practical application in various situations. Acknowledgment. We wish to thank the financial support from Key Laboratory of Colloid and Interface Chemistry of Shandong University, Ministry of Education of China.
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