Rough Glass Surface-Mediated Formation of Vesicles from Lauryl

Article pubs.acs.org/Langmuir

Rough Glass Surface-Mediated Formation of Vesicles from Lauryl Sulfobetaine Micellar Solutions Xiaoyu Zhu, Na Du, Ruiying Song, Wanguo Hou,* Shue Song, and Renjie Zhang Key Laboratory of Colloid and Interface Chemistry (Ministry of Education), Shandong University, Jinan 250199, People’s Republic of China S Supporting Information *

ABSTRACT: We report novel vesicles composed of the zwitterionic surfactant lauryl sulfobetaine (LSB), which is a simple single-tailed surfactant (STS). The novel vesicles spontaneously formed from LSB micellar solutions with the mediation of a rough glass surface (RGS) in the absence of any cosurfactants or additives. Importantly, the obtained STS vesicles displayed good stability upon long-term storage, exposure to high temperature, and freeze−thawing after the RGS was removed. The pH of the LSB solution (4.0−9.0) and the presence of NaCl (1.0 × 10−5 and 1.0 × 10−4 mol/L) in the LSB solution had no obvious influence on the formation and stability of the vesicles. The adsorption configuration of LSB on the RGS was investigated via water contact angle measurements and atomic force microscope observations. The results showed that LSB adsorption bilayers could form on the RGS, and the bilayer adsorption of LSB on the RGS and the roughness of the solid surface played a key role in the vesicle formation. A possible mechanism for the RGS-mediated formation of LSB vesicles is proposed: LSB micelles and molecules adsorb on the RGS to form curved bilayers, and the curved bilayers are then detached from the RGS and close to form vesicles. To the best of our knowledge, this is the first report of LSB alone forming vesicles. This finding extends our understanding of the nature of vesicle systems.

1. INTRODUCTION

investigated, and the stability of the LSB vesicles was determined. It is well-known that many environmental factors can induce the spontaneous formation of vesicles, such as the variation of pH,17 temperature,18 and salinity,19 as well as the addition of organic additives20 or heavy metal ions.21 In addition, it has been revealed that charged particles can induce the spontaneous formation of vesicles from fatty acids22 or in surfactant mixtures,23−25 giving composites of particles encapsulated in vesicles. In contrast to these methods, the RGS-mediated strategy does not require the introduction of new components into the surfactant system and can obtain individual vesicles rather than composites. This study aims to improve the understanding of the formation and features of vesicle systems.

Vesicles, one of the amphiphilic molecule organized assemblies constructed by unilamellar or multilamelar closed bilayers, have attracted much interest over the past several decades because of their fundamental and practical importance.1−5 A large number of vesicles in aqueous solutions of surfactants6−10 and amphiphilic polymers11−13 have been reported. For the surfactant vesicles, the surfactants used are either surfactants with double tails6 or mixtures of two or more surfactants.7−10 A simple single-tailed surfactant (STS), such as dodecyltrimethylammonium bromide (DTAB) and lauryl sulfobetaine (LSB), is usually considered to be unable to form vesicles in water in the absence of cosurfactants or additives.14,15 However, we recently found that the cationic surfactant DTAB could form vesicles from its micellar solution with the mediation of a rough glass surface (RGS) in the absence of cosurfactants and additives.16 Importantly, the obtained DTAB vesicles displayed good stability upon long-term storage, exposure to high temperatures, and freeze−thawing after removal of the RGS.16 It is interesting to examine the generality of this RGS-mediated strategy for STS vesicle formation and to further understand the features of STS vesicles. In this work, we report another STS vesicle system formed from zwitterionic betaine surfactant LSB micellar solutions with the mediation of RGS. The effects of pH and electrolyte (NaCl) on the vesicle formation were © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Chemicals. All of the chemicals used were of analytical reagent grade. LSB was purchased from Tokyo Chemical Industry Co., Ltd., Shanghai, China and was recrystallized three times from a mixed solvent of ethanol−acetone. The molecular structure of LSB is shown in Figure 1. Received: July 25, 2014 Revised: September 8, 2014 Published: September 14, 2014 11543

dx.doi.org/10.1021/la502965q | Langmuir 2014, 30, 11543−11551

Langmuir

Article

solutions containing 1.0 × 10−5 (pH 6.7) and 1.0 × 10−4 (pH 6.6) mol/L NaCl were used instead of the original LSB solution in the tests of RGS-mediated vesicle formation through the same procedure described earlier. Each experiment was repeated three times with different etched glass plates. 2.3. Characterization. The morphology of the etched glass surfaces was observed using a Nanoscope IIIa multimode atomic force microscope (AFM, Digital Instruments Corp., Santa Barbara, California, USA) and a JSM-6700F field emission scanning electron microscope (SEM, JEOL, Tokyo, Japan). Observations of the vesicle morphology were performed using a JEM-1011 transmission electron microscope (TEM, JEOL). Samples for TEM were prepared by freezefracture (FF) and negative-staining (NS, with uranyl acetate−ethanol solution). Fracturing and replication were carried out in a Balzers BAF400D high-vacuum freeze-etching system (Leica, Wetzlar, Germany). Dynamic light scattering (DLS) measurements were performed using a BI-200SM DLS instrument (Brookhaven, Worcestershire, UK). The incident laser light had a wavelength of 532 nm, and the incident angle was 90°. The DLS data were analyzed using the Contin method, and size distribution information was obtained by measuring the light intensity f(Dh) = ΓG(Γ) as a function of the hydrodynamic diameter (Dh). Steady-state fluorescence measurements were performed using a F-7000 fluorescence spectrophotometer (Hitachi, Ibaraki, Japan) at 25.0 ± 0.2 °C, using 1.0 × 10−7 mol/L pyrene as the fluorescence probe. The fluorescence emission spectra of pyrene were obtained by excitation at 335 nm. The water contact angles (θw) on the glass surfaces were measured using a DSA10 contact angle goniometer (Krüss, Hamburg, Germany). A droplet of deionized water (∼30 μL) was carefully placed on the surface of glass substrate using a syringe. Photographs of the droplet were then recorded after equilibrium had

Figure 1. Molecular structure of LSB. Pyrene was purchased from Aladdin Industrial Co., Shanghai, China and was recrystallized three times from absolute ethanol. Sodium hydroxide (NaOH), hydrochloric acid (HCl), and hydrofluoric acid (HF) were purchased from Tianjin Dengke Chemical Reagent Co., Ltd., Beijing Chemical Works, and Sinopharm Chemical Reagent Co., Ltd., respectively. Ultrapure water with a resistivity of 18.2 MΩ·cm was obtained using a Milli-Q plus purification system (Millipore, Beijing, China). 2.2. RGS-Mediated Formation of Vesicles. The RGS was obtained by etching a plate glass surface using HF. Glass microscope slides with length, width, and thickness of 5.5 cm, 2.5 cm, and 0.1 cm, respectively, were immersed in HF for 3 min. The resulting etched glass plates were washed thoroughly with ultrapure water and then ethanol, and then dried at 80 °C. LSB (20 mM) micellar solution was prepared by dissolving LSB in water. An etched glass plate was immersed in 50 mL of the micelle solution in a well-sealed plastic tube. The samples were shaken in a thermostatic water bath shaker (Jiangsu Medical Instrument Factory, China) for a given time (tsm) at 30 °C. The vesicle formation in the aqueous solution was characterized before and after the removal of the etched glass plates. The pH of the LSB solution was 6.8, and no obvious change in its pH was observed during the test. To study the effects of pH and electrolyte on vesicle formation, LSB solutions with pH 4.0 and 9.0 (adjusted using HCl and NaOH) and

Figure 2. SEM images of (a) GS and (b) RGS. AFM height images and section analyses of (c) GS and (d) RGS. AFM height images and section analyses of LSB-adsorbed RGS from (e) 1, (f) 3, and (g) 20 mM LSB solutions. 11544


Langmuir

Article

Figure 3. (a) Negative-stain and (b) freeze-fracture TEM images of vesicles in LSB solution with pH = 6.8 at a RGS-mediation time of 9 days.

Figure 4. (a) Hydrodynamic diameter distributions and (b) vesicle peak area of the LSB solution (pH 6.8) at different times with RGS and after RGS removal. been reached (∼2 min) to obtain the θw values. Each water contact angle was the average of three measurements on different locations of a glass surface.

images, possibly arising from the momentary adhesion of vesicles. The kinetic information about the vesicle formation under RGS mediation can be obtained from the change in vesicle peak area (Av) with time (t) of the LSB solution (pH 6.8), as shown in Figure 4b. The initial 20 days were with RGS mediation, and the subsequent 20 days were with the RGS removal. Under the mediation of the RGS, the Av gradually increased, indicating that the LSB vesicles gradually formed in the micellar solution. After the RGS was removed, no significant change in Av was observed, demonstrating that no significant change occurred in the vesicle phase. In contrast to the time dependence of Av, no significant changes in the average size and the most probable diameter (Dmp) of the vesicles with t were observed in the TEM images and DLS plots before and after the RGS removal, respectively (Figure S1 in the Supporting Information). This reveals that the number of vesicles increased while their size did not change with increasing tsm. The preceding results are similar to those of the DTAB solution.16 In addition, the influences of pH and NaCl on the vesicle formation in the 20 mM LSB solution were investigated. Figure 5 shows the NS- and FF-TEM images of vesicles in the LSB solutions with pH 4.0 and 9.0, and containing 1.0 × 10−5 and 1.0 × 10−4 mol/L NaCl, corresponding to the RGSmediation time of 9 days. A large number of vesicles with an average size of approximately 76 nm were observed in all of the systems, similar to the LSB solution with pH 6.8. The DLS results for these LSB solutions (Figure S2 in the Supporting Information) were also similar to those of the LSB solution with pH 6.8. These results demonstrate that the pH (4.0−9.0) of the micellar solution and the presence of NaCl (1.0 × 10−5 and 1.0 × 10−4 mol/L) have no obvious influence on vesicle formation in the LSB solution under RGS mediation.

3. RESULTS AND DISCUSSION 3.1. Feature of RGS. The surface of the glass microscope slide (GS) and the RGS obtained by etching the glass microscope slide using HF were characterized by SEM and AFM, as shown in Figure 2a−d. The GS was very smooth, with only a small number of convex points with a height of approximately 4−7 nm existing on the GS (Figure 2a,c). In contrast, the surface of the HF-etched glass microscope slide was very rough, and a large number of “islands” with height and width of approximately 18−39 and 210 nm existed on the RGS (Figure 2b,d). Thus, the RGS most likely consisted of curved surfaces.16 3.2. RGS-Mediated Formation of Vesicles. After the etched glass plate was immersed in a 20 mM LSB micellar solution with pH 6.8, significant formation of vesicles was observed. Figure 3 shows the NS- and FF-TEM images of the LSB solution taken at a RGS-mediation time (tsm) of 9 days. A large number of vesicles with an average size of approximately 78 nm existed in the system, indicating that the RGS induced the formation of vesicles in the STS micellar solution. DLS measurements confirmed vesicle formation, as shown in Figure 4a. Before the addition of the etched glass plate, the hydrodynamic diameter distribution of the LSB solution exhibited a single peak at about Dh = 3 nm, which is a typical value for a micellar system. After the etched glass plate was added to the LSB solution, the intensity of the micelle peak significantly decreased, and a new peak at around Dh = 240 nm was observed, corresponding to the formation of vesicles. The most probable vesicle diameter (approximately 240 nm) was greater than the average size (∼78 nm) obtained from the TEM 11545


Langmuir

Article

Figure 5. Negative-stain (left) and freeze-fracture (right) TEM images of vesicles in LSB solutions at a RGS-mediation time of 9 days. (a, b) pH = 4, (c, d) pH = 9, (e, f) 1.0 × 10−4 mol/L NaCl, and (g, h) 1.0 × 10−5 mol/L NaCl.

were obviously determined, and the S/N atom ratio in the system was just 1, consistent with that of LSB (Figure S3 in the Supporting Information). These verification tests proved that the formation of the vesicles in the STS solutions was solely induced by the RGS. 3.3. Microenvironment of the LSB Vesicles. The microenvironment of the LSB vesicles was examined using the fluorescence of pyrene as an extrinsic probe. Figure 6 shows the fluorescence emission spectrum of pyrene in the LSB vesicle solution with pH 6.8. It is known that the ratio of the intensity of the first vibrational peak (at 374 nm) to that of the third peak (at 385 nm), I1/I3, is very sensitive to the polarity of the microenvironment in which the pyrene is located.26 The I1/ I3 value for the LSB vesicle solution with pH 6.8 was 1.42, which is close to that of the LSB micelle solution before the etched glass plate was added (1.45). Similar results were

In contrast to the preceding results, no vesicles were observed by NS-TEM in all of the LSB solutions without RGS mediation. Furthermore, similar tests using smooth (unetched) glass plates were performed for the LSB solutions under the same conditions. As expected, no vesicles were observed using either TEM or DLS, which demonstrated that the HF etch or roughness of the solid surface played an important role in the formation of the vesicles. Moreover, to rule out the possibility that substances dissolving from the etched glass plates induce the formation of vesicles, we prepared LSB solutions using water soaking the RGS for 6 days at the same pH and NaCl concentrations. No vesicle aggregates were observed in these systems using either TEM or DLS. Actually, we analyzed the chemical elements of the vesicle solution obtained with RGS mediation by X-ray photoelectron spectroscopy (XPS); no substances dissolving from the RGS 11546


Langmuir

Article

because there was no significant change in the Av value over a storage period of 20 days at room temperature (Figure 4, and Figure S2 in the Supporting Information). This high long-term stability was confirmed by NS-TEM, as shown in Figure 7, where the vesicles remained stable over the storage period in all of the LSB solutions studied. In fact, the obtained STS vesicle solutions were still stable at room temperature approximately 3 months after the initial experiments. The thermal and freeze−thaw stabilities of the LSB vesicle systems were also investigated. To investigate their thermal stability, the LSB vesicle systems were placed in a thermostatic bath at 80 °C for 2 h. To investigate their freeze−thaw stability, the STS vesicle systems were frozen at −20 and −196 °C (liquid nitrogen) for 2 h, and the frozen samples were then thawed at an ambient temperature of approximately 25 °C. Figure 8 shows the NS-TEM images and DLS plots of the LSB vesicle solution with pH 6.8 after the different treatments. The vesicles were still present in the systems, and their size did not significantly change compared with those before the treatments. Similar results were obtained for the LSB vesicle solutions with pH 4.0 and 9.0 and containing 1.0 × 10−4 and 1.0 × 10−5 mol/ L NaCl (Figures S5−S8 in the Supporting Information). According to the preceding results, we can conclude that, after the removal of RGS, the LSB vesicles had good stability for long-term storage, exposure to high temperatures, and freeze−thawing. Furthermore, the pH (4.0−9.0) and the

Figure 6. Fluorescence spectrum of pyrene in LSB vesicle solution with pH 6.8.

obtained for the LSB vesicle solutions with pH 4.0 and 9.0, and containing 1.0 × 10−4 and 1.0 × 10−5 mol/L NaCl (Figure S4 in the Supporting Information). These results demonstrate that the micropolarity in the vesicle bilayers is the same as that in the micelles, because the I1/I3 values for the vesicle and micelle solutions are similar. These results are consistent with those of previously reported conventional27 and DTAB vesicles.16 3.4. Stability of LSB Vesicles. The long-term stability of the LSB vesicles after removing the etched glass plates was investigated using DLS and NS-TEM. The DLS results indicated that the LSB vesicles had high storage stability,

Figure 7. Negative-stain TEM images of various LSB vesicle systems at 20 days after RGS removal. (a) pH 6.8, (b) pH 4, (c) pH 9, (d) 1.0 × 10−4 mol/L NaCl, and (e) 1.0 × 10−5 mol/L NaCl. 11547


Langmuir

Article

Figure 8. Negative-stain TEM images of LSB vesicle solution (pH 6.8) after (a) thermal treatment at 80 °C for 2 h, (b) freezing at −20 °C for 2 h, and (c) freezing at −196 °C for 2 h and then thawing. (d) Hydrodynamic diameter distributions for the LSB vesicle solution after the different treatments.

presence of NaCl (1.0 × 10−5 and 1.0 × 10−4 mol/L) have no obvious influence on the stability of the LSB vesicles. 3.5. Possible Formation Mechanism of LSB Vesicles. The molecular packing parameter (P) proposed by Israelachvili et al.28 has been widely applied to explain how the molecular structure of a surfactant controls the shape and size of resulting aggregates: P = v0/a0l0, where v0 is the surfactant tail volume, l0 is the tail length, and a0 is the area per amphiphile at the aggregate surface. As a general rule,15 spherical micelles are favored when P ≤ 1/3, cylindrical micelles are favored when 1/ 3 ≤ P ≤ 1/2, and bilayers or vesicles are favored when 1/2 ≤ P ≤ 1. Based on the concept of the molecular packing parameter, simple STSs, such as DTAB and LSB, are generally considered to be unable to form vesicles in water in the absence of cosurfactants or additives,29 because their P values obtained from experimental a0 values (usually at gas−liquid interfaces) are not in the range of 1/2 to 1. This is confirmed by the fact that no simple STS vesicles have been reported except for the DTAB vesicles reported in our recent work.16 However, the intrinsic areas of the head groups (or the cross-sectional areas of the nonhydrated head groups) for simple STSs are generally much lower than their experimental a0 values at gas−liquid interfaces. The higher experimental a0 values are due to the hydration of the head groups and the electrostatic repulsion between head groups.29 In fact, adsorbed bilayers of STSs at solid−liquid interfaces generally exhibit lower a0 values than their adsorbed layers at gas−liquid interfaces,30 because the enrichment effect of the solid surface toward the surfactant molecules can obstruct the hydration of the surfactant head groups and decrease the electrostatic repulsion between head groups. This effect of the solid surface on STS molecules may alter their P values in adsorbed bilayers to be in the range of 1/ 2 to 1, and these bilayers may act as precursors to form vesicles when they are detached from solid surfaces. Thus, solid surface mediation is a possible strategy for formation of STS vesicles, which was preliminarily confirmed by the formation of DTAB vesicles.16

A lot of studies have been published on the adsorption of sulfobetaine surfactants at gas−liquid 31,32 and solid− liquid30,33−36 interfaces. The l0 and v0 values of LSB are 1.67 nm and 0.35 nm3, respectively.37 The literature a0 values of LSB at gas−liquid interfaces are approximately 0.44−1.05 nm2,30−32 and their P values obtained from the reported a0 values are therefore approximately 0.20−0.48. These P values suggest that spherical micelles are favored for the surfactant. However, the intrinsic a0 value of LSB is only about 0.21 nm2, and its intrinsic P value calculated from the intrinsic a0 value is about 1.0, indicating the possibility of LSB vesicle formation. Previous studies30,34 have reported that the a0 values for LSB in adsorption bilayers at solid−liquid interfaces, such as silica− water30 and borosilicate glass−water,34 are ∼0.24−0.29 nm2, giving P values of 0.72−0.88 for the STS. We measured the adsorption amount of LSB on HF-etched glass particles in 20 mM LSB aqueous solution at pH ∼7, 25 °C, and a solid dosage of 20 g/L. The glass particles were obtained by milling a glass microscope slide and were then etched with HF prior to the adsorption measurements (see section S1 in the Supporting Information). The a0 value for LSB in the adsorption bilayers on the HF-etched glass particles was calculated to be approximately 0.33 nm2, which corresponds to a P value of 0.63. Thus, the LSB bilayers detached from solid surfaces most likely form vesicles based on the concept of the molecular packing parameter. In addition, the configurations of LSB on the GS and RGS were investigated via θw measurements and AFM observations. The GS and RGS were immersed in LSB solutions with different concentrations (c) for 24 h to reach adsorption equilibrium. The resulting GS and RGS adsorbed LSB (LSBadsorbed GS and RGS) were air dried and used for the θw and AFM measurements. Figure 9 shows the θw values of the GS and RGS before and after adsorbing LSB from LSB solutions. The GS and RGS without adsorbed LSB had θw values of approximately 42° and 21°, respectively. For RGS, with increasing c, the θw initially increased and then decreased, 11548


Langmuir

Article

contrast to the RGS, no significant differences were observed in the AFM images for the GS before and after LSB adsorption (Figure S10 in the Supporting Information), which is consistent with the θw result. Considering the θw and AFM results, we can conclude that the adsorption ability of LSB on the RGS is significantly stronger than on the GS, suggesting that etch of the glass microscope slide with HF can produce active sites for LSB adsorption. The adsorption of LSB on the RGS can form a bilayer, and it is the bilayer adsorption that plays an important role in the formation of vesicles. Based on the preceding discussion, we propose a possible mechanism for the RGS-mediated formation of vesicles,16 which is shown in Figure 10. As an etched glass plate is

Figure 9. Variation of water contact angles of LSB-adsorbed GS and RGS versus LSB concentration.

with a highest value of approximately 42° at c ≈ 3 mM, which is the critical micelle concentration (cmc) of the LSB solution. A similar result has been reported in the literature.33 It has been reported that the adsorptions of surfactants at solid−liquid interfaces commonly form monolayers and bilayers when the c values of the surfactant solutions are below and above their cmc, respectively.39 Therefore, the initial increase in θw can be attributed to the formation of a monolayer of LSB on the RGS, and the subsequent decrease can be attributed to the formation of a bilayer or hemimicelle.33,39 That is, in the case of c = 20 mM, the LSB molecules form a bilayer on the RGS. In contrast to the RGS, the GS showed no significant change in its θw before and after LSB adsorption, with only a slight increase from about 42° to 48°. This suggests that the adsorption of LSB on the GS is weak and only forms a monolayer. AFM images of LSB-adsorbed RGSs obtained from LSB solutions with different c are shown in Figure 2e−g. For LSBadsorbed RGS from 1 mM LSB solution, islands with a width of approximately 200 nm were observed (Figure 2e), which is consistent with the structure of the bare RGS. However, the height of the islands of LSB-adsorbed RGS was ∼74 nm, which is higher than that of the bare RGS (∼39 nm). Only a small number of islands were observed for LSB-adsorbed RGSs obtained with 3 and 20 mM LSB solutions (Figure 2f,g), and the islands had a low height of approximately 11−18 nm; that is, relatively smooth surfaces were obtained. These results suggest that the initial adsorption of LSB primarily occurs on the islands (or peaks) of the RGS, resulting in an increase in the island height. Subsequent adsorption then occurs in troughs on the RGS, resulting in a decrease in the island height. The formation of a monolayer of LSB, possibly with some aggregates,40,41 smoothens the RGS. In addition, careful analysis of Figure 2f,g indicated that the configurations of the LSB layers on the RGSs in 3 and 20 mM LSB solutions were different: the former was relatively smooth while the latter was relatively rough. It appears that there were some flocculent aggregates of LSB on the RGS in 20 mM LSB solution. This is consistent with the θw result that a monolayer and bilayer of LSB formed on the RGS in 3 and 20 mM LSB solutions, respectively. Furthermore, many fishlike (or cylindrical) aggregates of LSB were observed on the RGS in 20 mM LSB solution (Figure S9 in the Supporting Information), and these aggregates, possibly hemimicelles, had length, width, and height of approximately 300, 100, and 15−60 nm, respectively. The AFM images in Figures 2g and Figure S9 of the Supporting Information were obtained from different regions of one slide. Because the original islands of the RGS almost disappeared, the fishlike aggregates possibly formed on the adsorption bilayer. In

Figure 10. Schematic illustration of the RGS-mediated vesicle formation.

immersed in the LSB solution, the surfactant micelles and molecules are adsorbed on the curved surfaces of the glass under the influence of electrostatic interaction, hydrogen bonding, and van der Waals forces, forming curved bilayers.38 In the curved bilayers, the P value of LSB is in the range of 1/2 to 1. Under the forces imposed by shaking, the curved bilayers (perhaps together with fishlike aggregates) are detached from the glass surface. These bilayers then close to form vesicles to decrease the interfacial energy of the aggregates. With repetition of this process, the micelle phase is gradually transformed into a vesicle phase, until equilibrium is achieved between the two phases. The formation of STS vesicles under RGS mediation is consistent with the concept of the molecular packing parameter, but it is surprising that the obtained STS vesicles are stable. Thus, many essential questions concerning the nature of STS vesicles need to be answered.

4. CONCLUSION We report for the first time that vesicles form from LSB (a simple zwitterionic STS) micellar solutions under the mediation of a RGS in the absence of cosurfactants or any other additives. The obtained LSB vesicles show good stability upon long-term storage (at least 3 months at room temperature), exposure to high temperature (80 °C for 2 h), and freeze−thawing (−20 or −196 °C for 2 h to approximately 25 °C). The pH (4.0−9.0) of the LSB solution and the presence of NaCl (1.0 × 10−5 and 1.0 × 10−4 mol/L) in the LSB solution have no obvious influence on the formation and stability of the vesicles. LSB can adsorb on the RGS to form bilayers, and the 11549


Langmuir

Article

(12) Blanazs, A.; Armes, S. P.; Ryan, A. J. Self-Assembled Block Copolymer Aggregates: From Micelles to Vesicles and their Biological Applications. Macromol. Rapid Commun. 2009, 30, 267−277. (13) Chen, H.; Xiao, L.; Anraku, Y.; Mi, P.; Liu, X.; Cabral, H.; Inoue, A.; Nomoto, T.; Kishimura, A.; Nishiyama, N.; Kataoka, K. Polyion Complex Vesicles for Photoinduced Intracellular Delivery of Amphiphilic Photosensitizer. J. Am. Chem. Soc. 2014, 136, 157−163. (14) Alberty, R. A. Physical Chemistry; Wiley: New York, 1987. (15) Šegota, S.; Težak, Đ. Spontaneous formation of vesicles. Adv. Colloid Interface Sci. 2006, 121, 51−75. (16) Du, N.; Song, R.; Zhu, X.; Hou, W.; Li, H.; Zhang, R. Vesicles Composed of One Simple Single-Tailed Surfactant. Chem. Commun. (Cambridge, U. K.) 2014, 50, 10573−10576. (17) Scarzello, M.; Klijn, J. E.; Wagenaar, A.; Stuart, M. C.; Hulst, R.; Engberts, J. B. pH-dependent aggregation properties of mixtures of sugar-based gemini surfactants with phospholipids and single-tailed surfactants. Langmuir 2006, 22, 2558−2568. (18) Majhi, P. R.; Blume, A. Temperature-induced micelle-vesicle transitions in DMPC−SDS and DMPC−DTAB mixtures studied by calorimetry and dynamic light scattering. J. Phys. Chem. B 2002, 106, 10753−10763. (19) Zhai, L.; Zhao, M.; Sun, D.; Hao, J.; Zhang, L. Salt-induced vesicle formation from single anionic surfactant SDBS and its mixture with LSB in aqueous solution. J. Phys. Chem. B 2005, 109, 5627−5630. (20) Yin, H.; Lei, S.; Zhu, S.; Huang, J.; Ye, J. Micelle-to-vesicle transition induced by organic additives in catanionic surfactant systems. Chem.Eur. J. 2006, 12, 2825−2835. (21) Wang, J.; Song, A.; Jia, X.; Hao, J.; Liu, W.; Hoffmann, H. Two routes to vesicle formation: Metal−ligand complexation and ionic interactions. J. Phys. Chem. B 2005, 109, 11126−11134. (22) Hanczyc, M. M.; Fujikawa, S. M.; Szostak, J. W. Experimental models of primitive cellular compartments: Encapsulation, growth, and division. Science 2003, 302, 618−622. (23) Du, N.; Hou, W.; Song, S. A novel composite: Layered double hydroxides encapsulated in vesicles. J. Phys. Chem. B 2007, 111, 13909−13913. (24) Nie, H.; Hou, W. Vesicle formation induced by layered double hydroxides in the catanionic surfactant solution composed of sodium dodecyl sulfate and dodecyltrimethylammonium bromide. Colloid Polym. Sci. 2011, 289, 775−782. (25) Nie, H.; Song, S.; Hou, W. Vesicles Formation Induced by Layered Double Hydroxides in Mixture of Lauryl Sulfonate Betaine and Sodium Dodecyl Benzenesulfonate. Chin. J. Chem. 2011, 29, 1373−1379. (26) Kalyanasundaram, K.; Thomas, J. Environmental effects on vibronic band intensities in pyrene monomer fluorescence and their application in studies of micellar systems. J. Am. Chem. Soc. 1977, 99, 2039−2044. (27) Huang, J.; Zhao, G. Fluorescence probes study on the mixed cationic-anionic surfactant solutions. Colloid Polym. Sci. 1996, 274, 747−753. (28) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Theory of selfassembly of hydrocarbon amphiphiles into micelles and bilayers. J. Chem. Soc. 1976, 72, 1525−1568. (29) Miller, D.; Evans, D. Fluorescence quenching in double-chained surfactants. 1. Theory of quenching in micelles and vesicles. J. Phys. Chem. 1989, 93, 323−333. (30) Zajac, J.; Chorro, C.; Lindheimer, M.; Partyka, S. Thermodynamics of micellization and adsorption of zwitterionic surfactants in aqueous media. Langmuir 1997, 13, 1486−1495. (31) Ribera, R.; Velázquez, M. M. Effect of water-soluble polymers on the surface properties of 3-(dimethyldodecylammonio)propanesulfonate in aqueous solutions. Langmuir 1999, 15, 6686− 6691. (32) Qu, G.; Cheng, J.; Wei, J.; Yu, T.; Ding, W.; Luan, H. Synthesis, characterization and surface properties of series sulfobetaine surfactants. J. Surfactants Deterg. 2011, 14, 31−35.

bilayer adsorption and the roughness of the solid surface played an important role in vesicle formation. A possible mechanism for the RGS-mediated formation of the LSB vesicles was proposed. The LSB micelles and molecules are adsorbed on the RGS to form curved bilayers; the curved bilayers subsequently detach from the RGS and then close to form vesicles. Further research is needed to fully understand this phenomenon, and many essential questions concerning the nature of STS vesicles remain unanswered.

■

ASSOCIATED CONTENT

S Supporting Information *

Text describing adsorption experiments of LSB on HF-etched glass particles and figures showing changes in average size and most probable diameter of vesicles, hydrodynamic diameter distributions of LSB solutions, XPS survey spectra, fluorescence spectra of pyrene in LSB vesicle solutions, and TEM and AFM images. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 531 88365460. Fax: +86 531 88564750. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (Grant Nos. 21173135 and 21273135) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110131130008).

■

REFERENCES

(1) Singer, S. J.; Nicolson, G. L. The fluid mosaic model of the structure of cell membranes. Science 1972, 175, 720−731. (2) Sharma, A.; Sharma, U. S. Liposomes in drug delivery: Progress and limitations. Int. J. Pharm. (Amsterdam, Neth.) 1997, 154, 123−140. (3) Cevc, G. Lipid vesicles and other colloids as drug carriers on the skin. Adv. Drug Delivery Rev. 2004, 56, 675−711. (4) Gröschel, A. H.; Walther, A.; Löbling, T. I.; Schmelz, J.; Hanisch, A.; Schmalz, H.; Müller, A. H. Facile, solution-based synthesis of soft, nanoscale janus particles with tunable janus balance. J. Am. Chem. Soc. 2012, 134, 13850−13860. (5) Dong, R.; Liu, W.; Hao, J. Soft vesicles in the synthesis of hard materials. Acc. Chem. Res. 2012, 45, 504−513. (6) Liang, Z.; Wang, C.; Huang, J. The research on the vesicle formation and transformation in novel Gemini surfactant systems, Colloids and Surfaces A: Physicochemical and Engineering Aspects. Colloids Surf., A 2003, 224, 213−220. (7) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. Spontaneous vesicle formation in aqueous mixtures of single-tailed surfactants. Science 1989, 245, 1371−1374. (8) Viseu, M. I.; Edwards, K.; Campos, C. S.; Costa, S. M. Spontaneous vesicles formed in aqueous mixtures of two cationic amphiphiles. Langmuir 2000, 16, 2105−2114. (9) Kadi, M.; Hansson, P.; Almgren, M. Determination of isotherms for binding of surfactants to vesicles using a surfactant-selective electrode. J. Phys. Chem. B 2004, 108, 7344−7351. (10) Du, N.; Song, S.; Hou, W. Vesicle stability in aqueous mixtures of zwitterionic/anionic surfactants. Colloids Surf., A 2008, 312, 104− 112. (11) Hamley, I. Nanoshells and nanotubes from block copolymers. Soft Matter 2005, 1, 36−43. 11550


Langmuir

Article

(33) Chavez, P.; Ducker, W.; Israelachvili, J.; Maxwell, K. Adsorption of dipolar (zwitterionic) surfactants to dipolar surfaces. Langmuir 1996, 12, 4111−4115. (34) Lokar, W. J.; Ducker, W. A. Forces between glass surfaces in mixed cationic-zwitterionic surfactant systems. Langmuir 2004, 20, 4553−4558. (35) Hu, X.; Li, Y.; Sun, H.; Song, X.; Li, Q.; Cao, X.; Li, Z. Effect of divalent cationic ions on the adsorption behavior of zwitterionic surfactant at silica/solution interface. J. Phys. Chem. B 2010, 114, 8910−8916. (36) Li, N.; Zhang, G.; Ge, J.; Luchao, J.; Jianqiang, Z.; Baodong, D.; Pei, H. Adsorption behavior of betaine-type surfactant on quartz sand. Energy Fuels 2011, 25, 4430−4437. (37) Macakova, L.; Blomberg, E.; Claesson, P. M. Effect of adsorbed layer surface roughness on the QCM-D response: Focus on trapped water. Langmuir 2007, 23, 12436−12444. (38) Johnson, R.; Nagarajan, R. Modeling self-assembly of surfactants at solid−liquid interfaces. II. Hydrophilic surfaces. Colloids Surf., A 2000, 167, 21−30. (39) Zhang, R.; Somasundaran, P. Advances in adsorption of surfactants and their mixtures at solid/solution interfaces. Adv. Colloid Interface Sci. 2006, 123−126, 213−229. (40) Ducker, W. A.; Wanless, E. J. Surface-Aggregate Shape Transformation. Langmuir 1996, 12, 5915−5920. (41) Xu, S.; Chen, M.; Cholewa, E.; Szymanski, G.; Lipkowski, J. Electric-Field-Driven Surface Aggregation of a Model Zwitterionic Surfactant. Langmuir 2007, 23, 6937−6946.

11551


Rough Glass Surface-Mediated Formation of Vesicles from Lauryl

Recommend Documents