Intrinsic Parameters for the Structure Control of Nonionic Reverse

ARTICLE pubs.acs.org/Langmuir

Intrinsic Parameters for the Structure Control of Nonionic Reverse Micelles in Styrene: SAXS and Rheometry Studies Lok Kumar Shrestha,*,† Rekha Goswami Shrestha,‡ and Kenji Aramaki§ †

International Center for Young Scientists (ICYS), WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba Ibaraki, 305-0044, Japan ‡ Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda Chiba 278-8501, Japan § Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan

bS Supporting Information ABSTRACT: Shape, size, and internal structure of nonionic reverse micelle in styrene depending on surfactant chain length, concentration, temperature, and water addition have been investigated using a small-angle X-ray scattering (SAXS) technique. The generalized indirect Fourier transformation (GIFT) method has been employed to deduce real-space structural information. The consistency of the GIFT method has been tested by the geometrical model fittings, and the micellar aggregation number (Nagg) has been determined. It was found that diglycerol monocaprate (C10G2), diglycerol monolaurate (C12G2), and diglycerol monomyristate (C14G2), spontaneously self-assemble into reverse micelles in organic solvent styrene under ambient conditions. The micellar size and the Nagg decrease with an increase in surfactant chain length, a scenario that could be understood from the modification of the critical packing parameter (cpp). A clear picture of one-dimensional (1-D) micellar growth was observed with an increase in surfactant weight fraction (Ws) in the C10G2 system, which eventually formed rodlike micelles at Ws g 15%. On the other hand, micelles shrunk favoring a rod-to-sphere type transition upon heating. Reverse micelles swelled with water, forming a water pool at the micellar core; the size of water-incorporated reverse micelles was much bigger than that of the empty micelles. Model fittings showed that water addition not only increase the micellar size but also increase the Nagg. Zero-shear viscosity was found to decrease with surfactant chain but increase with Ws, supporting the results derived from SAXS.

1. INTRODUCTION Studies on the formulation, structure, dynamics, and rheological properties of reverse micelles have attracted significant interest in recent years due to their wide ranges of practical applications. Reverse micelles are composed of hydrophilic polar core and lipophilic nonpolar shell; i.e., the structure of reverse micelles is opposite to the conventional micelles in aqueous systems. Reverse micelles have been utilized as a micro/nanoreactor for several aqueous chemical reactions and also templated in the tailored synthesis of nanoparticles utilizing their various geometries as the structure of nanomaterials would be a replica of template micelles.1
9 These observations demonstrate the need and importance of in depth knowledge of free structure control of reverse micelles. Though enough evidence on the formation of reverse micelles is available in the literature, most of the studies deal with ternary mixtures of ionic surfactant/water/oil usually in r 2011 American Chemical Society

oil-rich regions, in which water is considered as an essential component.10
15 On the other hand, reverse micelles have also been formulated with aqueous systems of lipophilic surfactants at a high concentration close to the surfactant axis in the temperature
composition diagram.16
25 However, the formation of reverse micelles in surfactant/oil binary systems, in particular without water addition, has sparsely been studied and is still a matter of discussion. There exist only a few reports describing the structure and properties of reverse micelles in surfactant/oil binary systems.26
30 Although ethylene oxide (EO)-based nonionic surfactants are the well-known and most-studied systems in the family of nonionic amphiphiles, they do not undergo spontaneous self-assembly, unless a trace amount of water is incorporated into Received: February 20, 2011 Revised: March 27, 2011 Published: April 13, 2011 5862

dx.doi.org/10.1021/la200663v | Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

Scheme 1. Schematic Molecular Structures of Diglycerol Monocaprate (C10G2), Diglycerol Monolaurate (C12G2), Diglycerol Monomyristate (C14G2), and Styrene

the system, as the hydrophilic part of these surfactants is assumed to dissolve in organic solvents. In such systems, it is the mole ratio of surfactant and water that determines the geometries of reverse micelles and also the dynamic and rheological properties.31,32 Contrary to this, glycerol- or sucrose-based nonionic surfactants have recently been shown to be more solvophobic, having potential to self-assemble into various structures in many nonpolar organic solvents without water addition from outside.33,34 Nevertheless, these systems are not absolutely water free, as the hydrophilic moiety of the surfactants are hygroscopic and contain some water as impurity. Similarly to the aqueous systems, the self-assembly behavior of these surfactants depends on composition, temperature, surfactant, and solvent molecular structures.29,35
37 In our previous studies, we have shown that a subtle difference in solvent polarity greatly affects the reverse micellar geometries in surfactant/oil binary systems. For instance, diglycerol monomyristate (C14G2) selfassembles into spheroid-type micelles in cyclohexane and ellipsoid prolate type in n-octane, which undergoes a 1-D or 2-D microstructure transition with an increase in alkyl chain length of alkane oils; however, reverse micelles are possible to structure only at higher temperatures after melting of the lamellar phase, limiting the use of these structures in practical applications.37,38 Surfactants having potential for forming reverse micelles at room temperature have long been desired for real practical applications. In this context, we have recently successfully formulated reverse micelles in branched chain diglycerol poly isostearate nonionic surfactant/oil systems and observed different types of reverse micellar structures depending on temperature, surfactant concentration, and surfactant or solvent molecular structures.39 The present paper is an extension of our ongoing project on the free structure control of nonionic surfactant reverse micelles in nonaqeuous media. In this contribution, we have studied the self-assembly of the short chain diglycerol fatty acid esters diglycerol monocaprate (C10G2) diglycerol monolaurate (C12G2), and diglycerol monomyristate (C14G2), all nonionic surfactants, in aromatic organic solvent styrene concentrating on the effect of surfactant chain length, concentration, temperature, and water on the structure of reverse micelles. We used small-angle X-ray scattering (SAXS) for structural characterization of reverse micelles, and SAXS data were evaluated by the generalized indirect Fourier transformation (GIFT) method and complemented by geometrical model fittings considering different plausible models. Rheological properties depending on micellar structure have also been studied.

2. EXPERIMENTAL SECTION 2.1. Materials. Diglycerol fatty acid esters diglycerol monocaprate (designated as C10G2 with purity >92%) diglycerol monolaurate (designated as C12G2 with purity >91.1%), and diglycerol monomyristate (designated as C14G2 with purity >92.9%), which are nonionic surfactants, were a generous gift from the Taiyo Kagaku Co., Ltd., Yokkaichi, Japan. The main impurities are unreacted diglycerol and diglycerol difatty acid esters. The surfactants were used as received. The nonpolar organic solvent styrene with purity >99% was purchased from Tokyo Chemical Industry, Tokyo, Japan. It is well-known that the presence of a small amount of impurities changes the interfacial properties of surfactants significantly. However, in the studies of interfacial properties of the present surfactants in water, we did not see any significant effects in the surface tension curves, especially close to critical micelle concentration (cmc) values.40 Thus, we speculate that the impurities present in the present surfactants may not have a significant effect on the micellar structure, at least in the studied concentration regime. The schematic molecular structures of surfactants and styrene are given in Scheme 1. 2.2. Methods. 2.2.1. Isothermal Phase Behavior in Dilute Region. Equilibrium phases in the dilute regions (5% e Ws e 25%) of binary mixtures of C10G2, C12G2, and C14G2 with styrene were identified by visual inspection through a crossed-polarizer at 25
C. Surfactant/oil binary mixtures with surfactant weight fractions (Ws) between 5% and 25% were prepared in styrene in a 5 mL clean and dry glass ampule with a screw cap. The samples were mixed using a dry thermobath and a vortex mixer with repeated centrifugation to achieve homogeneity. The samples were kept in a temperature-controlled water bath at 25
C for 2 h to observe the equilibrium phases. The accuracy of the temperature control was better than (0.5
C. Isotropic solutions were observed for all the systems. 2.2.2. Small-Angle X-ray Scattering (SAXS). SAXS experiments were carried out on a series of surfactant/oil binary mixtures to study the influence of surfactant chain length, surfactant concentration, temperature, and water addition on the reverse micellar structure. The samples were placed in the water bath at 25
C for 2 h prior to SAXS measurements. For SAXS measurements, a SAXSess camera (Anton Paar) attached to a PW3830 sealed-tube anode X-ray generator (PANalytical, Netherlands) was operated at 40 kV and 50 mA. A G€obel mirror and a block collimator provided a focused monochromatic X-ray beam of Cu KR radiation (λ = 0.1542 nm) with a well-defined lineshape. A thermostated sample holder unit (TCS 120, Anton Paar) controlled the sample temperature with an accuracy of 0.1
C. The 2-D scattering pattern was recorded by an image plate (IP) detector (Cyclone, Perkin-Elmer) and integrated into one-dimensional scattering intensities [I(q)] as a function of the magnitude of the scattering vector q = 4π/λ sin(θ/2) using SAXSQuant software (Anton Paar), where θ is the total scattering angle. All the measured 5863

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

intensities were semiautomatically calibrated for transmission by normalizing a zero-q attenuated primary intensity to unity. All I(q) data were corrected for background scattering, capillary effects, and the solvent, and an absolute scale calibration was made using water as a secondary standard.41 Using the GIFT method,42
45 SAXS data obtained in reciprocal space were transformed into a real-space, so-called pair-distance distribution function, p(r). The shape of p(r) curves resembles the shape of micelles and the value of r at which p(r) reaches to zero in the higher-r regime estimates the maximum dimension of micelles. The structures of micelles were further confirmed by testing geometrical models. The GIFT method relies on a basic equation of one-component globular particle systems, I(q) = nP(q) S(q), and its extension to polydisperse systems, where P(q) is the averaged form factor, S(q) is the averaged static structure factor, and n is the number of particles in unit volume. Theoretically, P(q) is the Fourier transformation of p(r) and can be described as Z ¥ sin qr dr PðqÞ ¼ 4π pðrÞ ð1Þ qr 0 Thus, one needs to calculate the inverse Fourier transformation of an experimental P(q) to deduce p(r) from the scattering experiments. To suppress the influence of interparticle interference scattering on the evaluation of p(r) that generally leads to oscillations and underestimates the maximum size of scattering object micelles at present, an interaction potential model for S(q) has to be involved in the analysis, where we chose the averaged structure factor model46,47 of hard-sphere and the Percus
Yevick closure relation to solve the Ornstein
Zernike equation. The detailed theoretical description on the method can be found elsewhere.48
50 In the case of elongated scattering particles with a high axial ratio— say the axial length is at least 3 times longer than the cross sectional diameter—one can apply a model-free cross section analysis for quantitative estimation of cross section structure. Theoretically, the radial electron density profile, ΔFc(r), is related to the cross section pairdistance distribution function, pc(r), as51 pc ðrÞ ¼ rΔ~F c 2 ðrÞ

ð2Þ

And using the indirect Fourier transformation (IFT) method, pc(r) can directly be calculated from the experimental scattering intensity I(q) based on following equation Z ¥ ð3Þ IðqÞq ¼ πLIc ðqÞ ¼ 2π2 L pc ðrÞ J0 ðqrÞ dr 0

where J0(qr) is the zeroth-order Bessel function. The yielded pc(r) can then be used to calculate ΔFc(r) by the deconvolution procedure.52,53 2.2.3. Rheometry. Steady-shear rheological measurements were performed in a stress-controlled rheometer (ARG2, TA Instruments), using a cone
plate geometry (diameter 60 mm with a cone angle of 1
) with the plate temperature controlled by a Peltier unit, which uses the Peltier effect to rapidly and accurately control heating and cooling. All the samples showed Newtonian-fluid-like behavior; viscosity is independent of shear-rate. Zero-shear viscosity (η0) was determined by extrapolating the plateau value to zero shear rates. 2.2.4. Densimetry. Densities of oil and reverse micellar solutions were measured at temperatures corresponding to SAXS measurement temperatures using a high-precision DSA5000 densimeter (Anton Paar). The DSA5000 instrument is based on the conventional mechanical oscillator method, which measures the natural resonant frequency of a U-shaped glass tube, filled with 1 mL sample. The highly tuned temperature control of the apparatus enables an accuracy of 10 mK in an absolute value. Densities of styrene and surfactant/styrene mixtures for different systems at SAXS measurements temperatures are supplied in Table S1 in Supporting Information.

Table 1. Water Content in Surfactants and Oil surfactants and oil

water level (%)

C10G2

0.96

C12G2

0.89

C14G2

0.66

styrene

0.12

2.2.5. Measurement of Water Content. Water content in surfactants and oil was determined by the Karl Fisher method using a CA-06 moisture meter (Mitsubishi Chemicals Co., Tokyo, Japan). Table 1 shows the water level in surfactants and oil.

3. RESULTS AND DISCUSSION 3.1. Isothermal Phase Behavior in the Dilute Regions. Equilibrium phases of the C10G2, C12G2, and C14G2 in organic solvent styrene were identified by visual inspection at 25
C in the dilute regions, 5% e Ws e 25%. It has found that these surfactants form isotropic solutions of reverse micelles in styrene under ambient conditions at 25
C. A series of SAXS and steady shear rheological measurements was carried out in the isotropic solution phase. In the following sections, we will discuss the effects of various intrinsic parameters such as surfactant chain length, surfactant concentration, temperature, and water addition on the geometry of reverse micelles and the rheological behavior. 3.2. Effect of Surfactant Chain Length. Figure 1 shows the X-ray scattering intensities, I(q), in the low-q region and the calculated pair-distance distribution functions, p(r), using the GIFT method for 5% surfactants (C10G2, C12G2, and C14G2) in styrene at 25
C. Presence of micellar structure can be seen from the q dependence behavior of I(q); in the absence of self-assembled structure, I(q) is independent with q in the SAXS region. Decreasing forward scattering intensities [I(q=0)] without affecting the scattering behavior at the higher-q region (Figure 1a) with the increase in chain length of surfactant from C10 to C14 is an indication of micellar shrinkage. Prominent peaks in the lower-r side with the extended tails with a downward convex shape of the p(r) curves shown in Figure 1b resemble ellipsoidal prolate geometry of micelle.54,55 Maximum dimensions of the micellar core, Dmax, decreases from ca. 6.0 to 3.0 nm upon the increase in surfactant chain from C10 to C14, while the position of maximum in p(r) apparently remains unchanged. We point out that this trend is equivalent to an ellipsoidal prolate-to-sphere transition and would be understood in terms of the increase of critical packing parameter, cpp. In addition to the GIFT method, we have also performed geometrical model fittings, and additional information such as the micellar aggregation number (Nagg) has been determined. The calculation was done on the basis of the method reported elsewhere.56 To fix the scattering length density difference of the micelle, densities of the investigated surfactant oil mixtures and oil were measured (see Table S1 in the Supporting Information). From the density data, the electron density difference values of the hydrophilic part of C10G2, C12G2, and C14G2 surfactants in styrene at 25
C, ΔFe, are estimated to 141.33, 99.04, and 61.88 el/nm3, respectively, where the partial electron density of the hydrocarbon group of the surfactant was extracted from the literature values.57 The contrast of hydrophilic core, ΔF, was determined with ΔFe and the scattering length of an electron, 5864

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

Figure 1. (a) X-ray scattered intensities, I(q), of the 5% C10G2, C12G2, and C14G2 in styrene at 25
C in absolute scales, (b) the corresponding pairdistance distribution functions, p(r), (c) model fitting (solid lines) to the experimental scattering intensities (symbols) with GIFT fit (dashed lines), and (d) comparison of p(r) curves obtained from the GIFT method (symbols) and model fittings (solid lines). The solid and broken lines in panel a represent the GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The arrows in panels b and d indicate the maximum diameter of the micellar core, Dmax. The data were fitted by considering a homogeneous ellipsoid prolate model.

l = 0.282  10
12 cm, for the individual systems and was used as an input parameter for the fittings. On the basis of the shape of the total p(r) function (Figure 1b), we chose the ellipsoidal prolate model for the reverse micellar core, and in addition to the short and long axes of an ellipsoidal prolate, a and b, it was possible to determine the volume of the geometrical model by the fittings, which when divided by the volume of the hydrophilic core (obtained from density measurements) gives Nagg. Optimum fit curves to the experimental I(q) and the resulting p(r) curves are presented in Figure 1c,d. We note that an ellipsoidal prolate model is able to explain the shape and size of the p(r) obtained by the GIFT method with a notable divergence in the high-q regime of the experimental I(q) functions (see Figure 1c). The absence of the theoretically predicted minima in the high-q part of the experimental I(q) could be attributed to the polydispersity in size and/or small electron density fluctuations inside the micellar core. It is well-established that, if polydisperse spheres are considered, such high-q minima can easily be smeared out with increasing polydispersity. This is because slightly different radii R gives minima at slightly shifted q-positions, and their superposition no longer represents distinct minima.54 It should be noted that the use of the simple equation I(q) = nP(q) S(q) is correct only for monodisperse spherical colloidal dispersions. However, Weyerich et al.,45 after testing various effective structure factors for polydisperse hard spheres, the averaged structure factor, the local monodisperse approximation, and the decoupling approximation, have shown that the recovery of nearly the exact form

Table 2. Effect of Surfactant Chain Length on the Geometrical Parameters Short Axis (a) and Long Axis (b) of Ellipsoid Prolate, Radius of Gyration (Rg), and Aggregation Number (Nagg) short axis long axis radius of gyration

aggregation

system

(a)/nm

(b)/nm

(Rg)/nm

number (Nagg)

5% C10G2/styrene 5% C12G2/styrene

1.40 1.40

3.10 2.20

1.56 1.27

49 36

5% C14G2/styrene

1.39

1.50

0.96

24

factor with a freely determined parameter set for individual structure factors can be done. Furthermore, successful application of GIFT for particle characterization of elongated particles was demonstrated. Although the resulting parameters for the averaged structure factor must be understood as “apparent” parameters with limited physical relevance, as our p(r) functions for the reverse micelles deduced with GIFT/IFT as the inverse Fourier transformation of the averaged form factor are still on the whole correct and sufficient for the purpose of particle characterization is dense reverse micellar solutions. The geometrical parameters short and long axes of the ellipsoidal prolate (a and b), the radius of the gyration (Rg), and the Nagg obtained from the model fittings are given in Table 2. The structure factor curves and structure factor parameters effective volume fraction (φ), effective interaction radius (R), and the polydispersity (μ), obtained from the GIFT method, 5865

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

Figure 2. (a) Viscosity against shear rate curves and (b) the corresponding zero-shear viscosity, η0, for the 5% surfactant/styrene system at 25
C depending on the surfactant chain length (C
C14).

Figure 3. Effect of surfactant concentration on reverse micellar structure: (a) The I(q) curves of the C10G2/styrene system at different surfactant concentrations, 5, 10, 15, 20, and 25%, on absolute scales at 25
C, (b) the corresponding real-space p(r) functions, (c) the p(r) normalized by surfactant weight fraction [p(r)/Ws], and (d) the static structure factor curves [S(q)]. The solid and broken lines in panel a represent the GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The arrows and broken lines in panels b and c highlight the maximum dimensions of the reverse micellar core, Dmax, and cross section diameter, Dc, respectively.

are supplied in the Supporting Information (see Figure S1 and Table S2). Note that the value of a remains apparently the same, whereas b decreases upon changing surfactant from the C10G2 to C14G2 via C12G2, indicating again a trend of ellipsoidal prolate-tosphere transition in the reverse micellar structure. This scenario could be understood in terms of the increase of cpp. On the other hand, the micellar shrinkage may also be caused due to repulsive excluded volume interactions, which increases in magnitude with the increase in the volume of the hydrophobic part of surfactant.

As a result, the micelle interface tends to become more curved, and as a consequence micelles shrink. We have also performed steady-shear rheological measurements on the 5% C10G2, C12G2, and C14G2 in styrene at 25
C. Figure 2 shows viscosity versus shear rate curves for the 5% surfactant/styrene system at 25
C and the corresponding zeroshear viscosity, η0, depending on the surfactant chain length (C10
C14). The systems showed Newtonian-fluid-like behavior, which can be judged from the shear-independent behavior of the viscosity. 5866

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

Figure 4. Comparison of results obtained from GIFT and model fittings: (a) model fitting (solid lines) to the experimental I(q)s (symbols) with GIFT fit (dashed lines) for the C10G2/styrene system at different surfactant concentrations (5% e Ws e 25%) on absolute scales and (b) comparisons of p(r) curves obtained from the GIFT method (symbols) and model fittings (solid lines). The arrows and broken line in panel b indicate the maximum diameter of the reverse micellar core, Dmax and cross section diameter, Dc, respectively. The data were fitted by considering homogeneous ellipsoid prolate (5% e Ws e 10%) and homogeneous cylinder (15% e Ws e 25%) models. For better visibility, scattering intensities are multiplied by factors 101 (10%), 103 (15%), 2  104 (20%), and 105 (25%).

The value of η0 determined by extrapolation of η value to zero shear rate decreases with increasing surfactant chain length, sustaining the results obtained from SAXS. Namely, the viscosity decay with the increase in surfactant chain length is caused by micelle shortening. 3.3. Effect of Surfactant Concentration on Reverse Micellar Structure. SAXS measurements were carried out in the dilute regions of the C10G2/styrene system at 25
C over a wider concentration range (5% e Ws e 25%). Figure 3 shows the I(q) and the corresponding p(r) curves. The maximum dimension, Dmax, of the ellipsoidal prolate micelles increases with the surfactant concentration and eventually rodlike micelles are formed at Ws g 15%. As can be seen in the I(q) curves (Figure 3a) upon increasing surfactant concentration from 5 to 10%, the I(q) values increase throughout the q-range, which is more than expected due to an increase in the number density of micelles in the unit volume. Minute observations of the I(q) curve for the 15% system reveal that though the scattering intensity is higher throughout the q-range, low-q scattering intensity [I(q=0)] is smaller than expected by the increase of surfactant concentration. Moreover, the weak interaction peak that appeared at qmax ∼ 1.2 nm
1 indicates the repulsive intermicellar interactions,58,59 which grows with concentration and also shifts toward the higher-q side (qmax ∼ 1.52 nm
1 for Ws = 25% system). This shows that the micellar interaction increases as the intermicellar distance between the neighboring micelles decreases. Note that Dmax increases from ca. 6.0 to 9.30 nm upon an increase in concentration from Ws = 5
25%. Nevertheless, the inflection point after the maximum of p(r) curves, as highlighted by the broken line, which measures the cross section diameter of micellar core, remains essentially constant at ca. 1.75 nm, suggesting that the micellar growth is 1-D. The monotonous increase of curve height with the weight fraction of surfactant in the normalized p(r) curves by surfactant weight fraction (Ws) further supports the concentration-induced 1-D growth (see Figure 3c). Careful observations of p(r) curves reveal that at a concentration Ws g 15% the micellar geometry transformed from ellipsoidal prolate to rodlike structure; the p(r) decay linearly in the higher-r regime.

Table 3. Effect of Surfactant Concentration on the Geometrical Parameters Short Axis (a) and Long Axis (b) of Ellipsoid Prolate, Cross Section Radius of Micellar Core (Rc), Maximum Length of Cylinder (L), Radius of Gyration (Rg), and Aggregation Number (Nagg) Obtained from the Results of Model Fittings for the C10G2/Styrene Systems at 25
C cross

surfactant

maximum

weight

short

long

section

axial

radius of

aggregation

fraction

axis

axis

core radius

length

gyration

number

(Rc)/nm

(L)/nm

(Rg)/nm

(Nagg) 49

(Ws)

(a)/nm (b)/nm

0.05

1.40

3.10

1.56

0.10

1.44

3.35

1.78

53

0.15

0.82

6.95

2.07

162

0.20

0.87

8.05

2.38

264

0.25

0.87

8.41

2.50

276

The S(q) curves (Figure 3d) obtained from the GIFT method reveal the presence of considerable intermicellar interactions, mainly at the higher concentrations Ws g 15%. S(q) peak position, which measures the mean distance between the micelles, shifts slightly toward higher-q regions with increasing concentration, inferring that the nearest neighboring micellar distance decreases. Note that the actual structure factor peaks may differ from what is predicted for monodispersed hard spheres, for which the extrapolated S(q) to zero q [S(q=0)] value reflects the osmotic compressibility of the system and is determined by the packing fraction of hard spheres.60 We found that S(q=0) decreases with the increase in surfactant concentration, illustrating that the osmotic compressibility as well as packing fraction of reverse micellar solutions may be explained in a quantitative manner, if the excluded volume of surfactant is taken into account. The structure factor parameters, effective volume fraction of surfactant (Φs), interaction radius (R), and polydispersity (μ) obtained from GIFT for the C10G2/styrene system as a function of surfactant concentration are supplied in Table S3 in the Supporting Information. We found that the effective volume fraction of surfactant (Φs) is slightly higher than the weight fraction of the surfactant (Supporting Information 5867

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

Figure 5. Direct cross section analysis: (a) cross sectional pair-distance distribution function, pc(r), of the C10G2/styrene systems at different surfactant concentrations (5
25%) at 25
C and (b) the resulting cross section radial electron density profile, ΔFc(r), calculated with the deconvolution procedure of pc(r) using the program DECON. Solid lines in part a represent the DECON fit. Arrows in panels a and b represent the maximum core cross section diameter, Dc max, and the maximum core radius, Rc, respectively.

Figure 6. (a) Viscosity versus shear rate curve for the C10G2/styrene system at 25
C at different surfactant concentrations (5
25%) as obtained from steady-shear rheological measurements and (b) the corresponding zero-shear viscosity, η0.

Figure S2) and could be attributed to partial hydration by the impure water. Figure 4 compares the experimental SAXS I(q) (symbols), the GIFT fit (dashed lines), and model fitting (solid lines) and the p(r) curves obtained from the GIFT method (symbols) and model fittings (solid lines). We note that an ellipsoidal prolate model could well-fit the experimental I(q) for the 5 and 10% C10G2/ styrene systems. On the other hand, at Ws g 15%, only cylinder models could explain the shape and size of the p(r) functions. A visible difference of experimental and theoretically predicted I(q) in the higher-q regime again implies polydispersity in micellar size and/or moderate electron density fluctuation inside the micellar core. For better visibility, scattering intensities are multiplied by factors 101 (10%), 103 (15%), 2  104 (20%), and 105 (25%). Table 3 shows the geometrical parameters obtained from model fittings at the different surfactant concentrations for the C10G2/styrene systems: a and b of ellipsoid prolate, cross section radius of micellar core (Rc), maximum length of cylinder (L), Rg, and Nagg. The Nagg increases only slightly with an increase in concentration from 5 to 10%, i.e., micellar growth is relatively less significant as predicted by GIFT. However, above this concentration, Nagg increases significantly, representing the formation of rodlike micelles. The cross section diameter of micellar core was estimated to be ca. 1.75
1.95 nm from the p(r) curves. Using the indirect

Fourier transformation (IFT) method the cross sectional pairdistance distribution function, pc(r), could be obtained from the experimental I(q) based on eq 3 as a quantitative measure of cross section structure. The radial electron density distribution profile, 4Fc(r), could further be achieved with the deconvolution of the pc(r).52,53 Figure 5 shows pc(r) and 4Fc(r) for the C10G2/styrene system at different surfactant concentrations. The pc(r) functions represent typical homogeneous aggregates with the maximum cross section diameter, Dc max, of ca. 1.85 nm at all concentrations and close to those estimated from the total p(r) functions. The positive electron density profiles in Figure 5b show the electron-rich hydrophilic reverse micellar core. The maximum core radius (Rc) estimated from the 4Fc(r) profile, ca. 0.95 nm, is closely related to twice the extended chain of the glycerol moiety, as one glycerol molecule accounts for ca. 0.4
0.5 nm. Figure 6 shows the results of steady-shear rheological measurements carried out on the C10G2/styrene system at 25
C as a function of surfactant concentration. Newtonian-fluid-like behavior persists over a wider concentration range, though viscosity increases appreciably (Figure 6a). Zero-shear viscosity, η0, increases monotonously with concentration, supporting concentration-induced 1-D micellar growth as pointed out by SAXS. Careful observation of the η0 curve reveals that the value of the η0 increases in two fashions; first, η0 increases slowly until the concentration reached 5868

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

Figure 7. Effect of temperature on the reverse micellar structures of the 10% C10G2/styrene system as obtained by SAXS: (a) X-ray scattering intensities, I(q), in absolute scales at different temperatures of 25, 50, and 75
C; (b) the corresponding real-space p(r) functions; (c) the experimental scattering intensities (symbols), model fits (full lines), and GIFT fits (dashed lines); and (d) comparison of experimental p(r) derived with GIFT (symbols) and those obtained from model fittings (solid lines). The data were fitted using a model of a homogeneous prolate ellipsoid. The solid and broken lines in panel a represent the GIFT fit and the calculated form factor for n particles in unit volume nP(q), respectively. Arrows and broken lines in panels b and d highlight the maximum core diameters, Dmax, and maximum micellar cross section diameters, Dc max.

Ws = 15% and then more significantly above this concentration, as distinguished by two straight lines in Figure 6b. The slope of the first line in the lower concentration range 5% e Ws e 15% is ca. to 0.0032, which increased to ca. 0.0036 at higher concentration range 15% e Ws e 25%. From this rheological behavior, we anticipated that the viscosity increase in the first case is caused by 1-D micellar growth, whereas it is caused by micellar growth followed by shape transition from ellipsoidal prolate-to-rod in the second case. Note that though the elongated rodlike micelles with maximum length ca. 8.41 nm are observed at Ws = 25%, network structures are yet to form; otherwise, viscosity would have increased by several orders of magnitude and shear-thinning behavior would have been observed in viscosity
shear rate curves.61 3.4. Temperature-Induced Microstructure Transition. Nonionic micelles in aqueous systems have been shown to grow with the rise of temperature; a sphere-to-rodlike transition is seen due to dehydration of surfactant’s headgroups, but an opposite trend has commonly been observed with nonaqueous media.62
66 It was anticipated that temperature enhances the penetration tendency of solvent molecules into the lipophilic chain of surfactant and the micellar curvature tends to be more curved. In order to investigate the effect of temperature on the reverse micellar structure, SAXS measurements were carried out on the 10% C10G2/styrene system at different temperatures of 25, 50, and 75
C. Figure 7 shows results obtained from SAXS measurements and model fittings.

Table 4. Effect of Temperature on the Geometrical Parameters Short Axis (a) and Long Axis (b) of Ellipsoid Prolate, Radius of Gyration (Rg), and Aggregation Number (Nagg) Obtained from the Results of Model Fittings for 10% C10G2/ Styrene Reverse Micelles short axis

long axis

radius of gyration

aggregation

temp/
C

(a)/nm

(b)/nm

(Rg)/nm

number (Nagg)

25 50

1.44 1.40

3.35 2.50

1.76 1.36

53 43

75

1.00

2.10

1.20

30

The decreasing tendency of low-q scattered intensity and the calculated form factor P(q) in the low-q region with the rise of temperature without affecting the scattering behavior in the high-q regions is a clear signature of micellar shrinkage. The Dmax as indicated by downward arrows in Figure 7b decreases from ca. 6.7 to 4.2 nm with the increase in temperature from 25 to 75
C, as well-supported by geometrical model fittings (Figures 7c,d). We observed that the Dmax decreases by ∼37% with the increase in temperature form 25 to 75
C. The geometrical parameters obtained from the results of model fittings are given in Table 4, and the structure factor curves along with structure factor parameters as a function of temperature are supplied in the Supporting Information (see Figure S3 and Table S4). 5869

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

Figure 8. (a) The pc(r) of the 10% C10G2/styrene systems at different temperatures (25, 50, and 75
C) and (b) the resulting ΔFc(r) profiles calculated via deconvolution from the pc(r). Solid lines in part a represent the DECON fit. Arrows in panels a and b represent the maximum core cross section diameter, Dc max, and maximum core radius, Rc, respectively.

Figure 9. (a) The I(q) of the 10% C10G2/styrene at different concentrations of water obtained on absolute scales at 25
C, (b) the corresponding p(r) curves, (c) normalized p(r) function, p(r)/p(rmax), and (d) the experimental scattering intensities (symbols), model fits (solid lines), and GIFT fits (dashed lines). Solid and broken lines in panel a represent the GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The broken lines in panel b and arrows in panels b and c indicate the maximum core cross section diameters and maximum dimensions of micelles.

Table 4 shows that the structure parameters of ellipsoid prolate (a and b), Rg, and Nagg decrease with the increase in temperature, representing temperature-induced micelle shortening, which would be understood to be caused by enhanced penetration of oil in the surfactant chain. Besides, increasing temperature also increases the effective hydrophobic volume of the surfactant chain by increasing the kink states in the chain. The present result supports our previous findings, where the temperature effect was found to be more efficient.67,68

Micellar core cross section diameters, Dc max, are estimated in the range of ca. 1.3
1.8 nm from total p(r) functions (Figure 7b,d). Values of Dc max are estimated to be quantitatively ca. 1.85, 1.65, and 1.42 nm at 25, 50, and 75
C, respectively, from the pc(r) curves obtained from the IFT method (see Figure 8a). Similarly, values of micellar core cross section radius Rc are estimated to ca. 0.74, 0.90, and 0.95 at 75, 50, and 25
C, respectively derived from the 4Fc(r). 3.5. Water-Induced Microstructure Transition. Reverse micelles are good candidates for the encapsulation of water or 5870

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

polar molecules, as they tend to be soluble in the reverse micellar core and cause a microstructure transition.69 Investigations have shown that water modulates reverse micellar geometry, producing bigger micelles.70
76 Presently, the 10% C10G2/styrene system could solubilize 1.3% water at 25
C and caused 2-D micellar growth. Figure 9 shows SAXS results at different concentrations of water. The low-q scattering intensity I(q=0) increases significantly and the I(q) curve in the cross section region (1.8 e q e 3.0 nm
1) shifts toward the lower-q side upon addition of water (Figure 9a), inferring that the water induced simultaneous changes in maximum dimension and the cross section structure of the C10G2-based reverse micelles (2-D micellar growth). Both the inflection point after the maximum of the p(r) and the Dmax as indicated by broken lines and arrows in Figure 9b increase with water concentration. The 2-D micellar growth is visible in a comprehensive manner in the normalized p(r) curves (Figure 9c) that the position of maximum of p(r), i.e., rmax sits toward higher-r value. We note that ellipsoidal prolate model well fits the data Figure 9d. The geometrical parameters obtained from the results of model fittings for the 10% C10G2/styrene reverse micelles as a function of water concentration is presented in Table 5. The S(q) curve and structure factor parameters as a function of water concentration are supplied in Figure S4 and Table S5 in Supporting Information. Note that water increases the micellar aggregation number significantly, which well supports the concept of water-induced reverse micelle formation in nonaqueous media causing a micellar growth. Table 5. Effect of Water on the Geometrical Parameters Short Axis (a) and Long Axis (b) of Ellipsoid Prolate, Radius of Gyration (Rg), and Aggregation Number (Nagg) Obtained from the Results of Model Fittings for the 10% C10G2/Styrene Reverse Micelles short axis

long axis

radius of gyration

aggregation

(a)/nm

(b)/nm

(Rg)/nm

number (Nagg)

0

1.44

3.35

1.78

53

0.32

2.10

4.20

2.00

90

0.72

2.50

4.85

2.68

155

1.29

3.70

6.00

3.25

273

% water

Direct cross section analysis has shown a continuous growth of core cross sectional diameter, Dc max and cross sectional radius, Rc [see Figure 10, which shows the pc(r) of the 10% C10G2/styrene systems at different water concentration and the resulting ΔFc(r) obtained from the deconvolution of the pc(r)]. Micellar cross section diameters of water-incorporated systems of ca. 2.25
3.70 nm cannot be explained, unless the formation of a water pool at the micellar core is considered. Thus, water-induced microstructure transition would be understood in terms of formation of the water pool. Besides, some water molecules hydrate the hydrophilic moiety of surfactant so that the overall hydrophilic size of the surfactant increases and favor micellar growth by reducing the cpp.

4. CONCLUSION In this paper, we have investigated the self-assembled structures and rheological properties of the diglycerol fatty acid esters (having different surfactant chain lengths C10, C12, and C14) in the organic solvent styrene in the dilute regions (5% e Ws e 25%) under ambient conditions, in particular the effects of surfactant chain length, concentration, temperature, and water on the geometry of reverse micelles. Shape, size, and internal structures of reverse micelles were determined by the generalized indirect Fourier transformation (GIFT) evaluation of the smallangle X-ray scattering (SAXS) data and were very well supported by geometrical model fittings. We found that present nonionic surfactants spontaneously self-assemble into reverse micellar aggregates in styrene without water addition from outside under ambient conditions of temperature and pressure and micelles shrink with an increase in surfactant chain length, causing a reduction in the aggregation number, Nagg. Nevertheless, it should be noted that the systems are not absolutely water free, as these surfactants and styrene contain traces amount of water as an impurity. Similar effects were observed with the rise of temperature; micelles shrunk and Nagg decreased. Note that these microstructure transitions are caused by the modification of the amphiphilicity of the surfactant. With an increase in surfactant chain length or temperature, the lipophilicity of the surfactant increases, and as a result, the critical packing parameter increases and reverse micelles with more negative curvature are formed. Moreover, the penetration tendency of oil to the lipophilic chain of surfactant increases upon heating, and micellar curvature tends to be more curved. However, an opposite trend

Figure 10. Direct cross section structure analysis of water-incorporated systems: (a) cross sectional pair-distance distribution function, pc(r), for the 10% C10G2/styrene þ water systems at different concentration of water (0, 0.32, 0.72, and 1.29%) and (b) the corresponding electron density profile, ΔFc(r), calculated via deconvolution from the pc(r). Solid lines in part a represent the DECON fit. Arrows in panels a and b represent the maximum core cross section diameter, Dc max, and maximum core radius, Rc, respectively. 5871

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir was observed with an increase in surfactant concentration and water addition. The size of the ellipsoidal prolate type micelles observed in the 5% C10G2/styrene system increases with an increase in concentration, and then micelles undergo a shape transition to rodlike structure at the higher concentration Ws g 15%. Reverse micelles of the 10% C10G2/styrene system could incorporate a significant amount of water (∼1.3%), enhancing the potential application of the system in water solubilization and/or in separation and extraction. Water caused a drastic change in the structure of host micelles; the maximum dimension and cross section diameter of the micellar core increase with the amount of water, demonstrating the formation of the water pool at the micellar core. As expected from SAXS data, the viscosity of the reverse micellar solution decreases with an increase in surfactant chain length; on the other hand, it increases significantly with an increase in surfactant concentration. The systems showed Newtonian-fluid-like behavior, demonstrating that though the micelles are elongated, network structures are yet to form.

’ ASSOCIATED CONTENT

bS

Supporting Information. Densities of solvent and surfactant/solvent mixtures for different systems at SAXS measurements temperatures and compositions, S(q) curves depending on surfactant chain length, temperature, and water addition, and derived structure factor parameters obtained from the GIFT evaluation of SAXS data. This information is available free of charge via the Internet at http://pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ81-29851-3354, ext. 8903. Fax: þ81-29-860-4706.

’ ACKNOWLEDGMENT L.K.S. thanks the International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), and World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA) Japan, for financial support. Fruitful discussion with Prof. Dr. Otto Glatter, University of Graz, Austria, and Dr. Takaaki Sato, Shinshu University, Japan, is highly acknowledged. ’ REFERENCES (1) Luisi, P. L. Strab, B. E. (Eds.) Reverse Micelles: Biological and Technological relevance of Amphiphilc Structures in Apolar Media; Plenum Press: New York, 1987. (2) Pileni, M. P. Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elasevier: Amsterdam, 1989; Vol 65. (3) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (4) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (5) Pileni, M. P. Langmuir 1997, 13, 3266. (6) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137. (7) Lopez-Quintela, M. A.; Tojo, C.; Blanco, M. C.; García Rio, L.; Leis, J. R. Curr. Opin. Colloid Interface Sci. 2004, 9, 264. (8) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893. (9) Pileni, M. P. Adv. Colloid Interface Sci. 1993, 46, 139. (10) De, T.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95.

ARTICLE

(11) Riter, R. E.; Kimmel, J. R.; Undiks, E. P.; Levinger, N. E. J. Phys. Chem. B. 1997, 101, 8292. (12) Cason, J. P.; Roberts, C. B. J. Phys. Chem. B. 2000, 104, 1217. (13) Li, Q.; Li, T.; Wu, J. J. Phys. Chem. B. 2000, 104, 9011. (14) Kanamaru, M.; Einaga, Y. Polymer 2002, 43, 3925. (15) Tung, S.-H.; Huang, Y.-E.; Raghavan, S. R. J. Am. Chem. Soc. 2006, 128, 5751. (16) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (17) Kunieda, H.; Shinoda, K. J. Dispers. Sci. Technol. 1982, 3, 233. (18) Friberg, S. E.; Blute, I.; Kunieda, H.; Stenius, P. Langmuir 1986, 2, 659. (19) Kunieda, H.; Solans, C.; Parra, J. L. Colloids Surf. 1987, 24, 225. (20) Solans, C.; Pons, R.; Davis, H. T.; Evans, D. F.; Nakamura, K.; Kunieda, H. Langmuir 1993, 9, 1479. (21) Uddin, H. Md.; Rodriguez, C.; Watanabe, K.; Lopez-Quintela, M. A.; Kato, T.; Furukawa, H.; Harashima, A.; Kunieda, H. Langmuir 2001, 17, 5169. (22) Kunieda, H.; Tanimoto, M.; Shigeta, K.; Rodriguez, C. J. Oleo Sci. 2001, 50, 633. (23) Kaneko, M.; Matsuzawa, K.; Uddin, H. M.; Lopez-Quintela, M. A.; Kunieda, H. J. Phys. Chem. B 2004, 108, 12736. (24) Kunieda, H.; Uddin, H. Md.; Horii, M.; Furukawa, H.; Harashima, A. J. Phys. Chem B 2001, 105, 5419. (25) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (26) Forster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956. (27) Zhong, X. F.; Varsheny, S. K.; Eisenberg, A. Macromolucules 1992, 25, 7160. (28) Desjardins, A.; van de Ven, T. G. M.; Eisenberg, A. Macromolecules 1992, 25, 2412. (29) Shrestha, L. K.; Masaya, K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Kunieda, H. Langmuir 2006, 22, 1449. (30) Rodriguez, C.; Uddin, Md. H.; Watanabe, K.; Furukawa, H.; Harashima, A.; Kunieda, H. J. Phys. Chem. B. 2002, 106, 22. (31) Schurtenberger, P.; Scartazzini, R.; Luisi, P. L. Rheol. Acta 1989, 28, 372. (32) Scartazzini, R.; Luisi, P. L. J. Phys. Chem. 1988, 92, 829. (33) Herrington, T. M.; Shali, S. S. J. Am. Oil. Chem. Soc. 1988, 65, 1677. (34) Rodriguez-Aberu, C.; Acharya, D. P.; Hinata, S.; Ishitobi, M.; Kunieda, H. J. Colloid Interface Sci. 2003, 262, 500. (35) Shrestha, L. K.; Sato, T.; Acharya, D. P.; Iwanaga, T.; Aramaki, K; Kunieda, H. J. Phys. Chem. B 2006, 110, 12266. (36) Shrestha, L. K.; Sato, T.; Aramaki, K. Langmuir 2007, 23, 6606. (37) Shrestha, L. K.; Sato, T.; Aramaki, K. J. Phys. Chem. B. 2007, 111, 1664. (38) Shrestha, L. K.; Aramaki, K. J. Dispers. Sci. Technol. 2007, 28, 1236. (39) Shrestha, L. K.; Shrestha, R. G.; Oyama, K.; Matsuzawa, M.; Aramaki, K. J. Phys. Chem. B 2009, 113, 12669. (40) Shrestha, L. K.; Saito, E.; Shrestha, R. G.; Kato, H.; Takase, Y.; Aramaki, K. Colloids Surf., A 2007, 293, 262. (41) Orthaber, D.; Bergmann, A.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 218. (42) Fritz, G.; Bergmann, A.; Glatter, O. J. Chem. Phys. 2000, 113, 9733. (43) Glatter, O.; Fritz, G.; Lindner., H.; Brunner, P. J.; Mittelbach, R.; Strey, R.; Egelhaaf, S. U. Langmuir 2000, 16, 8692. (44) Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Shubert, K. V.; Kaler, E. W.; Glatter, O. J. Chem. Phys. 1999, 110, 10623. (45) Weyerich, B.; Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1999, 32, 197. (46) Pusey, P. N.; Fijnaut, H. M.; Vrijm, A. J. Chem. Phys. 1982, 77, 4270. (47) Salgi, P.; Rajagopolan, R. Adv. Colloid Interface Sci. 1993, 43, 169. (48) Fritz, G.; Bergmann, A. J. Appl. Crystallogr. 2004, 37, 815. (49) Glatter, O. Prog. Colloid Polym. Sci. 1991, 84, 46. 5872

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873

Langmuir

ARTICLE

(50) Glatter, O. J. Appl. Crystallogr. 1980, 13, 577. (51) Glatter, O. J. Appl. Crystallogr. 1980, 13, 7. (52) Glatter, O. J. Appl. Crystallogr. 1981, 14, 101. (53) Glatter, O.; Hainisch, B. J. Appl. Crystallogr. 1984, 17, 435. (54) Glatter, O. J. Appl. Crystallogr. 1979, 12, 166. (55) Shrestha, L. K.; Sato, T.; Dulle, M.; Glatter, O.; Aramaki, K. J. Phys. Chem. B 2010, 114, 12008. (56) Glatter, O. Acta Phys. Austriaca 1980, 52, 243. (57) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934. (58) Stradner, A.; Sedgwick, H.; Cardinaux, F.; Poon, W. C. K.; Egelhaaf, S. U.; Schurtenberger, P. Nature 2004, 432, 492. (59) Stradner, A.; Glatter, O.; Schurtenberger, P. Langmuir 2000, 16, 5354. (60) Svensson, B.; Olsson, U.; Alexandridis, P.; Mortensen, K. Macromolecules 1999, 32, 6725. (61) Shrestha, L. K.; Yamamoto, Y.; Arima, S.; Aramaki, K. Langmuir 2011, 27, 2340. (62) Sharma, S. C.; Shrestha, L. K.; Tsuchiya, K.; Sakai, K.; Sakai, H.; Abe, M. J. Phys. Chem. B 2009, 113, 3043. (63) Shrestha, R. G.; Shrestha, L. K.; Sharma, S. C.; Aramaki, K. J. Phys. Chem. B 2008, 112, 10520. (64) Sharma, S. C.; Rodríguez-Abreu, C.; Shrestha, L. K.; Aramaki, K. J. Colloid Interface Sci. 2007, 314, 223. (65) Shrestha, L. K.; Shrestha, R. G.; Varade, D.; Aramaki, K. Langmuir 2009, 25, 4435. (66) Shrestha, L. K.; Glatter, O.; Aramaki, K. J. Phys. Chem. B 2009, 113, 6290. (67) Shrestha, L. K.; Shrestha, R. G.; Oyama, K.; Matsuzawa, M.; Aramaki, K. J. Oleo Sci. 2010, 59, 339. (68) Shrestha, L. K.; Sato, T.; Aramaki, K. Phys. Chem. Chem. Phys. 2009, 11, 4251. (69) Mathews, M. B.; Hirschhorn, E. J. Colloid Sci. 1953, 8, 86. (70) Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356. (71) Schurtenberger, P.; Magid, L. J.; King, S. M.; Lindner, P. J. Phys. Chem. 1991, 95, 4173. (72) Wu, G.; Zhou, Z.; Chu, B. Macromolecules 1993, 26, 2117. (73) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (74) Alexandridis, P.; Andersson, K. J. Colloid and Interface Sci. 1997, 194, 166. (75) Angelico, R.; Palazzo, G.; Colafemmina, G.; Crikel, P. A.; Giustini, M.; Ceglie, A. J. Phys. Chem. B 1998, 102, 2883. (76) Li, J.; Zhang, J.; Han, B.; Gao, Y.; Shen, D.; Wu, Z. Colloids Surf., A 2006, 279, 208.

5873

dx.doi.org/10.1021/la200663v |Langmuir 2011, 27, 5862–5873