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Adsorption and Aggregation Behavior of Tetrasiloxane-Tailed Surfactants Containing Oligo(ethylene oxide) Methyl Ether and a Sugar Moiety Guoyong Wang ,† Wenshan Qu,‡ Zhiping Du,*,† Qianyong Cao,§ and Qiuxiao Li† †
China Research Institute of Daily Chemical Industry, Taiyuan, Shanxi 030001, P. R. China Department of Chemistry, Shanxi Datong University, Datong, Shanxi 037009, P. R. China § Department of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, P. R. China ‡
bS Supporting Information ABSTRACT: Three novel amphiphilic dicephalic (double-headed) surfactants containing oligo(ethylene-oxide)methyl-ether and a sugar moiety TGA-m (m = 1, 2, and 3) that incorporate a tetrasiloxane at the terminus of a hydrocarbon chain were designed and synthesized. Their surface activity and aggregation behavior in aqueous solution were systematically investigated by surface tension, dynamic light scattering (DLS), and transmission electron microscopy (TEM) techniques at 298 K. The surface tension measurements provided the critical aggregation concentration (CAC) and the surface tension at the CAC (γcac). In addition, with application of the Gibbs adsorption isotherm, the maximum surface excess concentration (Γmax) and the minimum surface area/molecule (Amin) at the airwater interface were estimated. The effect of EO chain length on the surface activity and aggregation behavior was also investigated. It was found that both the γcac and the CAC were lower than those for reported traditional hydrocarbon surfactants. Aggregates of three surfactants, TGA-m (m = 1, 2, and 3), formed in aqueous solutions could be assigned as spherical vesicles as suggested by analysis using DLS and TEM. Moreover the formation of vesicles can be confirmed by the encapsulation of bromophenol blue. These results indicate that these three surfactants have excellent efficiencies of vesicle formation and surface tension reduction in the aqueous phase.
’ INTRODUCTION Siloxane surfactants comprising a methylated siloxane hydrophobic moiety coupled to one or more polar groups are used extensively in industry, finding applications in textile manufacture, cosmetic formulations, polyurethane foam manufacture, and as paint additives. Most commonly used silicone surfactants are medium weight copolymers with either rake-type (also known as comb-type) or ABA-type structures. These molecules have superior properties when compared to normal hydrocarbon surfactants. Siloxane surfactants are surface active in both aqueous and nonaqueous media. It is believed that the flexibility and low cohesive energy of the dimethylsilicone chain are responsible for the unusual properties of silicone surfactants.1,2 The aggregation behavior of silicone surfactants in aqueous solutions has attracted considerable attention during the past two decades. Kunieda and colleagues have reported that in concentrated solutions of silicone surfactants that contain ethylene oxide (EO) groups, the types of liquid crystals or self-organized structures formed are highly dependent on the total surfactant concentration and the ratio of EO and dimethylsilicone.3 In dilute aqueous solutions, they can form globular and wormlike micelles and vesicles.4 Although no definitive relationships between the molecular structure and aggregate’s morphology were found, it is believed that the flexible dimethylsilicone chain r 2011 American Chemical Society
will fold over in the aggregated state. Gradzielski and Hoffmann and colleagues concluded from determination through SANS of the micellar radii that the silicone chain must be coiled in the aggregates.5 Schmaucks and colleagues have presented evidence that the silicone hydrophobe is highly flexible.6 Hill and colleagues have also found that the bilayer thickness of vesicles formed by comb-type silicone surfactants is significantly smaller than their extended molecular length.1,7 Yan and colleagues investigated the phase behavior and aggregation behavior of an ABAtype nonionic polymeric silicone surfactant IM-22 [(EO)15(PDMS)15-(EO)15] in water/glycerol mixtures and a mixed system of IM-22 and sodium dodecylsulfate (SDS) in aqueous solution. They found that globular and threadlike micelles and vesicles coexist in the single IM-22 aqueous solution, while both the threadlike micelles and vesicles were destroyed by the addition of SDS; over a large concentration range, of 060% (w/v), IM-22 in water/glycerol mixtures forms turbid solutions and well mixed solutions. The turbid solutions contain small unilamellar (SUV) and large multilamellar vesicles (MLV) with a thickness of ∼3.4 nm.8,9 Kickelbick and colleagues synthesized a series of Received: November 5, 2010 Revised: March 1, 2011 Published: March 17, 2011 3811
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Scheme 1. Synthetic Routes and Acronyms for TGA-m (m = 1, 2, and 3)
short-chain amphiphilic diblock copolymers consisting of a PDMS hydrophobic block and a poly(ethylene-oxide) hydrophilic block and explored their aggregation in aqueous solution. They observed the spontaneous formation of vesicles and the thickness of the vesicle walls increased in parallel with the length of the block constituting the hydrophobic moiety.10,11 The studies that have been carried out on this category of detergents have not been particularly extensive, however. Most works have focused on siloxane surfactants with polyether as a headgroup. Furthermore they have considered mixtures of surfactants with a variable distribution of molecular weight and/or isomers. Recently, siloxane surfactants produced from carbohydrates have been the focus of substantial research because incorporating carbohydrates into siloxane offers interesting properties for diverse applications, such as solubility enhancers for hydrophobic drugs, transdermal penetration enhancers,12 cosmetic formulations, self-assembling polymers, and stabilizers for nanoparticles.13,14 Although characterization of some of these families of surfactants has been reported, research on the aggregation behavior of carbohydrate modified siloxane surfactants remains rare, and much more remains to be done. In previous work, we have investigated the adsorption and aggregation behavior of some carbohydrate-modified tri- and
tetra-siloxane surfactants in aqueous solution. It was found that these surfactant solutions displayed low critical aggregation concentrations (CACs) and low surface tension; moreover they formed vesicles spontaneously above their CAC.15 As a continuation of our research aiming to characterize this new class of surfactants, we present a novel family of dicephalic (two-headed) siloxane surfactants containing oligo(ethylene-oxide)methylether and a sugar moiety (Scheme 1), TGA-m (m = 1, 2, and 3). We have explored the effects of surfactant structure, specifically the ethylene-oxide chain length of molecules, on (1) surfactant adsorption at the air/water surface, (2) critical aggregation concentration (CAC) of the surfactant, and (3) aggregation properties in aqueous solution. The adsorption properties were studied using surface tensiometry, whereas the aggregation behavior was monitored by dynamic light scattering (DLS) and transmission electron microscopy (TEM).
2. EXPERIMENTAL SECTION 2.1. Materials. N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (A.R. grade), D-(þ)-glucose δ-lactone (A.R. grade), and triethylene glycol monomethyl (A.R. grade) were purchased from Aldrich. Bromophenol blue (A.R. grade) was purchased 3812
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The Journal of Physical Chemistry B from the Beijing Chemical Reagent Company. All other reagents were of analytical grade and were used as received. Double distilled water was used for analytical experiments and for the measurement of primary properties. 2.2. Characterization of Surfactant Structure. Both nuclear magnetic resonance (NMR) (1H and 13C NMR) spectroscopy (Varian INOVA-400 MHz spectrometer) and Fourier transform infrared spectroscopy (FT-IR) (Hitachi 270-30) were used to characterize the structures of the prepared surfactants. Because the signals produced by some methyl groups in the surfactants produced overlapping peaks, TMS was not suitable as an internal standard for NMR, and so CDCl3 (residual protons 7.26 ppm) was selected. Elemental analysis was also carried out with a PerkinElmer 2400 CHN analyzer. 2.3. Measurement of the Aqueous Solution Behavior of Surfactants. Surface Tension. Surface tension was determined using a KRUSS K12 Processor Tensiometer by the Wilhelmy plate method at 298.15 ( 0.1 K. The length and thickness of the platinum plate was 19.9 mm and 0.2 mm, respectively. The dipping distance was 2 mm. Concentrated stock solutions of surfactants (pH ≈ 7.5) were freshly prepared in doubly distilled water and then diluted in the same medium to the appropriate concentration. The different solutions were incubated for at least 24 h before each experiment was carried out. The tensiometer was calibrated with doubly distilled water, while the platinum plate and glassware were also cleaned with a strong base and rinsed with doubly distilled water, prior to each experiment. For every concentration, the surface tension was measured three times with an interval of 60 s after stirring and with an average deviation of less than 0.2 mN 3 m1. Dynamic Light Scattering (DLS). The aggregation behavior of surfactant solutions was studied by dynamic light scattering (DLS) with a Zeta Plus Particle Size Analysis instrument (Brookhaven, USA) at a scattering angle of 90°. All solutions were filtered with a 0.45 μm (mixed cellulose acetate membrane) filter and equilibrated at 298 K for 8 h prior to each experiment. The resulting autocorrelation functions were fitted using a nonnegatively constrained least-squares algorithm to estimate the diffusion coefficient (D), which is then related to hydrodynamic diameter (dh, equivalent to two Stokes’ radii) of the surfactant aggregates (e.g., micelles or vesicles) through the Stokes Einstein relationship, dh = kT/(3πηD), where k is Boltzmann’s constant, T is the absolute temperature, and η is the viscosity of the disperse medium. Transmission Electron Microscopy (TEM). The microstructure of surfactant aggregates in solution was also observed by transmission electron microscopy of aggregates in negative stain, using a JEM-1011 electron microscope at 100 kV. A droplet of the surfactant solution was placed on a carbon-coated grid and allowed to equilibrate for 2 min. Excess liquid was removed by carefully touching one end of the grid with filter paper. When the grid was partially dried, a drop of staining solution (2% (w/v) phosphotungstic acid) was added onto the grid. Again, excess liquid was removed by filter paper after 2 min, and the grid dried at room temperature. X-ray Diffraction (XRD). Self-supported cast films were prepared by dispersing the surfactant solutions over precleaned glass plates and then air-drying these samples at room temperature. Finally, the plates were placed under low vacuum for 15 min. X-ray diffraction (XRD) studies were carried out using an X-ray diffractometer (Rigaku model D/MAX2500). The X-ray beam was generated with a Cu anode at 40 kV and 200 mA, and the
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Figure 1. Surface tensions of aqueous TGA-m (m = 1, 2, and 3) solutions as a function of the concentration at 298 K: 2= TGA-1, 9 = TGA-2, and b =TGA-3.
wavelength of the KR1 beam was 1.5406 Å. The X-ray beam was directed to the edge of film, and the scanning 2θ was recorded from 1° to 15°, using a step width of 0.01°. Encapsulation of Bromophenol Blue and Size Exclusion Chromatography. Dye encapsulation experiments for the surfactant solutions were also performed to identify the structure of the aggregated phase. The solutions were prepared with 5 104 mol 3 L1 bromophenol blue and incubated at room temperature for 6 h. An aliquot (0.4 mL) of the surfactant solution was passed at a velocity of 0.5 mL 3 min1 through a 1 cm 35 cm chromatography column packed with Sephadex G-25 gel (medium grade). When the water-soluble dye is encapsulated inside the vesicles, it elutes earlier than when in solution, as the volume of the column into which the vesicles can pass is lower due to the pore size of the resin. The concentration of the dye in the eluent was determined from the absorption of UV/visible light at 595 nm.
3. RESULTS AND DISCUSSION Three dicephalic surfactants TGA-m (m = 1, 2, 3) were synthesized by a multistep reaction as shown in Scheme 1. The purity of TGA-m (m = 1, 2, 3) was assayed by element analysis, IR, 1H, and 13C NMR (see Supporting Information for details). 3.1. Surface Properties of Surfactant TGA-m. The surface tension was measured to evaluate the surface activity of three dicephalic surfactants in aqueous solutions. Figure 1 depicts the surface tension (γ) of the solutions versus concentration (C) plots for TGA-m (m = 1, 2, 3) in aqueous solution at 298 K. The surface tension of TGA-m (m = 1, 2, 3) aqueous solutions progressively decreased with increasing concentration and then reached a plateau, indicating that the micelles (aggregates) had formed, this concentration transition point corresponding to the critical aggregation concentration (CAC).15 The CAC and the surface tension at the CAC (γcac) for the three TGA-m dicephalic surfactants were determined by extrapolation of the sharp turning points and are listed in Table 1. It can be seen from Figure 1, that TGA-m in solution reduces the surface tension at low concentrations, indicating that these molecules pack densely at the airwater interface. The values of γcac for TGA-m were 20.18, 20.58, and 20.64 mN 3 m1 for TGA-1, TGA-2, and TGA3, respectively. These values of γcac are significantly lower than those commonly achieved with hydrocarbon-based surfactants16 and are comparable to those reported for other low molecular 3813
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Table 1. Parameters of Aggregation and Adsorption of TGA-m (m = 1, 2, and 3) at 298 K surfactant
γcac (mN 3 m1)
CAC (mol 3 L1)
Amin (Å2/molecule)
ΔG0mic (KJ 3 mol1)
ΔG0ads (KJ 3 mol1)
Γmax (mol 3 cm2)
TGA-1
20.18
3.1 105
52.0
35.7
51.9
3.2 1010
54.2
34.1
50.9
3.1 1010
56.0
33.3
50.6
3.0 1010
TGA-2 TGA-3
20.58 20.64
6.0 10
5
8.2 10
5
Figure 2. Log(CAC) as a function of EO chain length in TGA-m solutions.
weight silicone surfactants.2,17,18 This result could be attributed to the branched tetrasiloxane moiety of these molecules lying flat at the airwater interface, exposing the highly surface-active methyl groups to air. The closely spaced values of γcac are reasonable, since these surfactants differ only slightly in their EO chain length. The values for the CAC are 3.1 105, 6.0 105, and 8.2 5 10 mol 3 L1 for TGA-1, TGA-2, and TGA-3, respectively. The CAC increases in the order TGA-1 < TGA-2 < TGA-3, in accordance with increased hydrophilicity owing to the extension of the EO chain. These CACs follow an inverse linear function relative to the length of the EO chain as demonstrated in Figure 2. This is identical to what has been observed in micelle formation by organic ethoxylates16 and trisiloxane ethoxylates.19 Clearly, the effect of the EO chain length on the CAC is greater than its effect on γcac. The saturation adsorption values, Γmax, at the airwater interface and the minimum area occupied by a single surfactant molecule at the airwater interface, Amin, were obtained for the three dicephalic siloxane surfactants by applying the approximate form of the Gibbs adsorption isotherm equations (eq 1 and eq 2) to the slopes of the surface tension versus log concentration plots (Figure 1). The standard free energy of aggregation and adsorption can be calculated from the eq 3 and eq 4 ! 1 Dγ Γmax ¼ ð1Þ 2:303RT Dlog c
Figure 3. The intensity-weighted size distributions of as-prepared siloxane surfactants: (a) TGA-1, 0.60 wt %; (b) TGA-1, 1.0 wt %; (c) TGA-2, 0.64 wt %; (d) TGA-2, 1.1 wt %; (e) TGA-3, 0.66 wt %; (f) TGA-3, 1.1 wt %. The data points are fitted by Gaussian curves.
T
ΔG0ads Asm ¼
1016 NA Γmax
ΔG0mic
cac ¼ RTln 55:5
ð2Þ
ð3Þ
CΠ ¼ RTln 6:022ΠAsm 55:5
ð4Þ
where R is the gas constant, T is the absolute temperature, NA is Avogadro’s number, Π (= γ0-γ) is surface pressure in the region of surface saturation, and CΠ is the molar concentration of the surfactant in the aqueous phase at a surface pressure Π (in mN 3 m1).16 The adsorption data and Amin are summarized in Table 1. 3814
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Figure 5. Small angle XRD measurements of the cast film of TGA-2 (1.1 wt %) vesicles on a glass plate. Inset: the proposed arrangement of the molecules in the vesicle wall.
Figure 4. Negative-stained transmission electron micrographs of vesicles formed in siloxane surfactant solutions: (a) TGA-1, 0.60 wt %; (b) TGA-1, 1.0 wt %; (c) TGA-2, 0.64 wt %; (d) TGA-2, 1.1 wt %; (e) TGA3, 0.66 wt %; (f) TGA-3, 1.1 wt %.
The surface area per molecule, Amin, appears to be determined by the area occupied by the hydrated hydrophilic groups, because the chains in surfactants with hydrophilic groups at one end of the molecule do not lie flat at an interface but rather are somewhat tilted with respect to it. When a second hydrophilic head is present in the molecule, Amin increases. The excess of surface concentration Γmax decreases as Amin increases. The standard free energy of micelle formation and adsorption are always negative. These negative values imply the ability to form micelles in solution and to adsorb at the airwater interface. ΔG0ads is larger than the corresponding ΔG0mic, indicating that adsorption occurs predominantly over the aggregation that is seen in aqueous solutions. Such a predominantance of absorption has similarly been reported in previous literature focusing on the adsorption and aggregation behavior of carbohydrate surfactants2022 and siloxane surfactants.23 Furthermore the ΔG0mic and ΔG0ads of these surfactants increase with decreasing numbers of oxyethylene units in the hydrophilic head, suggesting that an increase in the number of oxyethylene units results in a decrease in adsorption at the airwater interface and favors micelle formation in solution. 3.2. Aggregation Properties of Surfactant TGA-m in Aqueous Solution. Unexpectedly, the aqueous solutions of the TGA-m samples (above CAC) were a transparent blue, which is indicative of the formation of larger aggregates. Therefore, dynamic light scattering (DLS) and negative-staining TEM measurements were performed to further investigate the selfassembly of TGA-m aqueous solutions. Using DLS, which analyzes the hydrodynamic size distribution of larger aggregates,24 the intensity-weighted distribution graphs of TGA-m are shown in Figure 3. These aggregates were shown
to have monomodal or bimodal distribution functions. In bimodal distributions the maximum corresponding to larger aggregates indicates that there is only a small population of these aggregates. These aggregates have an average diameter of around 158.5 nm (0.60 wt %) and 111.4 nm (1.0 wt %) for TGA-1, 190.0 nm (0.64 wt %) and 177.6 nm (1.1 wt %) for TGA-2, and 105.7 nm (0.66 wt %) and 186.3 nm (1.1 wt %) for TGA-3. The aggregates were found to be much larger in size than the small spherical micelles, which have diameters typically in the range of 35 nm,5 indicating that large aggregates such as vesicles might exist in these solutions as well as smaller micelles. The morphology of the aggregates was visualized by TEM in negative stain. Figure 4 shows TEM images of TGA-m aqueous solutions by negative-staining methods. Images from TEM confirmed the presence of spherical assemblies with dark outer rings consistent with the expected 2D projection of vesicular structures as typically observed by TEM.25,26 In these electron micrographs (Figure 4a-f) the presence of vesicles with diameters between 100 and 500 nm was observed. The vesicles were typically spherical in structure, with a closed, stained periphery. As the particles were prepared from aqueous solutions, the hollow, water-filled, lumen of the vesicles is “loaded” with solution. Sometimes a swelling due to staining agent entering the vesicles was visible. The size of the structures seen in the images does not exactly reflect the size distribution of the vesicles observed by DLS. Taking into account this swelling, the sizes of the aggregates observed by TEM were in agreement with those derived by DLS. The influence of ethylene oxide chain length in TGA-m on aggregate size was unremarkable. The thickness of the vesicle walls can be estimated by XRD at small angles (a powerful method for elucidating long-range structure in ordered molecular assemblies25). A typical diffraction curve for TGA-2 is shown in Figure 5. TGA-2 systems exhibited periodic peaks in X-ray scattering intensity. The highest intensity peak is at high spacing (corresponding to the lowest 2θ value). The wall thickness can be calculated according to the Bragg equation (d = nλ/2sinθ, where d is distance between scattering planes, n is an integer, λ is wavelength, and θ is the angle of incidence to the scattering planes). The calculated wall thickness (3.1 nm) is smaller than two times the corresponding evaluated molecular length as estimated from a spacefilling model (CPK model) but much larger than the length of one molecule (1.9 nm), indicating that TGA-2 assembles into vesicles in a tilted tail-to-tail or interdigitated packing arrangement. 3815
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Figure 6. Absorption spectrum of bromophenol blue (BPB) in water (inset: the structure of BPB) and size exclusion profiles of the separation of the dye encapsulated (small peaks) in vesicles of TGA-1, TGA-2 (inset: inclusion of BPB inside vesicles of TGA-2) and TGA-3 at 1.1 wt % from the corresponding free dye (bromophenol blue).
Figure 7. Schematic illustration of vesicle formation.
The existence of vesicles was confirmed by the encapsulation of an aqueous dye. Encapsulation is an important characteristic of vesicles27 and may make vesicles excellent drug delivery vehicles. We choose bromophenol blue (BPB) as our vesicle cargo. BPB is a widely used dye for vesicle encapsulation assays.28,29 Separation of the free dye from BPB bearing vesicles was achieved by size exclusion chromatography (SEC), as described in our experimental protocols. The column resolved two independent peaks. The leading band contained vesicles and the second band contained free dye. An elution profile was obtained by plotting the absorbance at 595 nm of successive fractions. These profiles are shown in Figure 6. In all cases it was observed that there was a small initial portion containing vesicles encapsulating 13% of total dye, followed by a large peak that consisted of free dye. Clearly, the BPB molecule associated with vesicles could be distinctly separated from the free dye. The encapsulation efficiency (EF) was calculated by following equation EF ¼ Ce, dye =Ct, dye where Ce, dye and Ct, dye are the encapsulated and total dye concentrations, respectively. EF for TGA-m was 2.58% for TGA-1, 2.82% for TGA-2, and 1.84% for TGA-3, respectively.
Changes in vesicle size with time were also measured to examine the stability of the vesicles. As an example, the TEM images of TGA-2 on different days are shown in Figure 7. It can be observed that vesicles in TGA-2 solution still existed even after 30 days although the size of vesicles became heterogeneous. The siloxane tail appeared to play an important role in the formation of vesicles. Dicephalic surfactants consist of two polar head groups at one end of a hydrophobic group.30 Usually, they have comparably large headgroup sections when compared with the hydrocarbon moiety. According to the structure-shape concept, formation of vesicles is not expected when they are dispersed in water.31,32 Owing to the bulky tetrasiloxane structure of TGA-m, the surfactant has a cylindrical shape, and thus vesicles are the preferred morphology of aggregates formed in water (Figure 6). As the tetrasiloxane chains are bulkier and stiffer than hydrocarbon chains, the rigid structure generally leads to the formation of aggregates with lower curvature. Vesicle formation can also be considered from the viewpoint of the surfactant packing parameter of Israelachvili.33 In this model, the aggregate morphology is related to the surfactant structure through a molecular packing parameter P = υ/Roιc, where υ, Ro, and ιc are the volume, area, and length of the hydrophobic moiety. Cone or truncated-cone shaped surfactants usually possess a P value less than 0.5 and preferentially form micelles. For surfactant molecules with P values in the range of 0.51.0, the molecule is more cylindrically shaped and the formation of bilayers or closed bilayers (vesicles) is favored. Micelle-forming surfactants have a relatively large headgroup compared to the tail; thus, they have P values considerably lower than 0.5. If the polar headgroup becomes small compared to the tail, micelles will not form; instead there may be a direct transition from a very dilute solution of surfactant into a vesicular 3816
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Figure 8. TEM images of TGA-2 (1.1 wt %) at different days: (a) 7 days; (b) 20 days; (c) 30 days.
phase.34 One example of such behavior is trioxyethylene cholesterol, abbreviated ChEO3.35 The branched tetrasiloxane tails of the surfactants used in this work are bulky, rendering micelle formation unfavorable and favoring formation of a lamellar phase already at low concentration, just as with ChEO3. This is an alternative way to form spontaneous aggregates in water with respect to catanionic surfactants, where single chain surfactants are electrostaticaly coupled in an appropriate surfactant ratio.36,37
4. CONCLUSIONS Three tetrasiloxane dicephalic surfactants containing oligo(ethylene oxide)methyl-ether and a sugar moiety were successfully synthesized and characterized by elemental analysis, 1H and 13 C NMR, and FT-IR spectral techniques. Aggregates of three surfactants, TGA-m (m = 1, 2, and 3), formed in aqueous solutions could be assigned as spherical vesicles as suggested by DLS and TEM. Moreover the formation of the vesicles can be confirmed by encapsulation of bromophenol blue, indicating potential applications for these vesicles as microsphere drug delivery systems and as models of biomembranes. The γcac of the three surfactants in water evaluated by surface tension measurements was found as 2021 mN/m. These results indicate that the three surfactants have excellent efficiencies of vesicle formation and surface tension reduction in the aqueous phase. ’ ASSOCIATED CONTENT
bS
Supporting Information. IR, NMR, and elemental analyses data of each surfactant are reported here. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 008613453437591. Fax: 00863514040802. E-mail:
[email protected],
[email protected]. Corresponding author address: China Research Institute of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan, Shanxi Province, 030001 P. R. China.
’ ACKNOWLEDGMENT This project is funded by the Shanxi Province Youth Fund (Grant No. 2009021012) and the National Natural Science Fund of China (Grant No. 21073234). We would like to express our gratitude to Reinhard Miller (Max Planck Institute for Colloids and Interfaces) for many fruitful discussions. ’ REFERENCES (1) Hill, R. M. Siloxane surfactant. Surfactant Sci. Ser. 1999, 86, 1–47.
(2) Wang, G. Y.; Du, Z. P.; Zhang, W.; Cao, Q. Y. Synthesis and surface properties of trisiloxane-modified oligo(ethylene oxide). Tenside, Surfactant, Det. 2009, 46, 214–217. (3) Kunieda, H.; Uddin, M. H.; Horii, M.; Furukawa, H.; Harashima, A. Effect of hydrophilic-and hydrophobic-chain lengths on the phase behavior of AB-type silicone surfactants in water. J. Phys. Chem. B 2001, 105, 5419–5426. (4) Talmon, Y. Imaging surfactant dispersions by electron microscopy of vitrified specimens. Colloids and surfaces 1986, 19, 237–248. (5) Gradzielski, M.; Hoffmann, H.; Robisch, P.; Ulbricht, W.; Gruning, B. The aggregation behaviour of silicone surfactants in aqueous solutions. Tenside, Surfactants, Deterg. 1990, 27, 366–379. (6) Schmaucks, G.; Sonnek, G.; W€ustneck, R.; Herbst, M.; Ramm, M. Effect of siloxanyl groups on the interfacial behavior of quaternary ammonium compounds. Langmuir 1992, 8, 1724–1730. (7) Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Lyotropic liquid crystal phase behavior of polymeric siloxane surfactants. Langmuir 1993, 9, 2789–2798. (8) Yan, Y.; Hoffmann, H.; Drechsler, M.; Talmon, Y.; Makarskys, E. Influence of hydrocarbon surfactant on the aggregation behavior of silicone surfactant: observation of intermediate structures in the vesicle-micelle transition. J. Phys. Chem. B 2006, 110, 5621–5626. (9) Yan, Y.; Hoffmann, H.; Makarsky, A.; Richter, W.; Talmon, Y. Swelling of L [alpha]-phases by Matching the refractive index of the water- glycerol mixed solvent and that of the bilayers in the block copolymer system of(EO)15-(PDMS)15-(EO)15. J. Phys. Chem. B 2007, 111, 6374–6382. (10) Kickelbick, G.; Bauer, J.; Huesing, N.; Andersson, M.; Holmberg, K. Aggregation behavior of short-chain PDMS-b-PEO diblock copolymers in aqueous solutions. Langmuir 2003, 19, 10073–10076. (11) Kickelbick, G.; Bauer, J.; Husing, N.; Andersson, M.; Palmqvist, A. Spontaneous vesicle formation of short-chain amphiphilic polysiloxane-b-poly(ethylene oxide) block copolymers. Langmuir 2003, 19, 3198–3201. (12) Akimoto, T.; Kawahara, K.; Nagase, Y.; Aoyagi, T. Preparation of oligodimethylsiloxanes with sugar moiety at a terminal group as a transdermal penetration enhancer. Macromol. Chem. Phys. 2000, 201, 2729–2734. (13) Racles, C.; Hamaide, T.; Ioanid, A. Siloxane surfactants in polymer nanoparticles formulation. Appl. Organomet. Chem. 2006, 20, 235–245. (14) Racles, C.; Hamaide, T. Synthesis and characterization of water soluble saccharide functionalized polysiloxanes and their use as polymer surfactants for the stabilization of polycaprolactone nanoparticles. Macromol. Chem. Phys. 2005, 206, 1757–1768. (15) Wang, G. Y.; Du, Z. P.; Li, Q. X.; Zhang, W. Carbohydratemodified siloxane surfactants and their adsorption and aggregation behavior in aqueous solution. J. Phys. Chem. B 2010, 114, 6872–6876. (16) Rosen, M. J. Surfactants and interfacial phenomena; John Wiley and Sons: 2004. (17) Svitova, T.; Hoffmann, H.; Hill, R. Trisiloxane surfactants: surface/ Interfacial tension dynamics and spreading on hydrophobic surfaces. Langmuir 1996, 12, 1712–1721. (18) He, M.; Lin, Z.; Scriven, L. E.; Davis, H. T.; Snow, S. A. Aggregation behavior and microstructure of cationic trisiloxane surfactants in aqueous solutions. J. Phys. Chem. 1994, 98, 6148–6157. 3817
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