Tunable Parameters for the Structural Control of Reverse Micelles in

Feb 25, 2009 - Formation of reverse micelles in surfactant/oil binary systems without ... is made available by participants in Crossref's Cited-by Lin...
2 downloads 0 Views 4MB Size
Download PDF
pubs.acs.org/Langmuir © 2009 American Chemical Society

Tunable Parameters for the Structural Control of Reverse Micelles in Glycerol Monoisostearate/Oil Systems: A SAXS Study Lok Kumar Shrestha,† Rekha Goswami Shrestha,† Dharmesh Varade,‡ and Kenji Aramaki*,† †

Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan and ‡School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland Received November 27, 2008. Revised Manuscript Received January 29, 2009

Formation of reverse micelles in surfactant/oil binary systems without water addition and the tunable parameters for the structure control of such micelles are presented. The small-angle X-ray scattering (SAXS) technique has been used for the structural characterization of micelles. It has been found that the nonionic surfactant glycerol monoisostearate (abbreviated as iso-C18G1) forms reverse micelles in different organic solvents such as cyclohexane, n-decane, and n-hexadecane without the addition of water. The structure (shape and size) of the reverse micelles has been found to depend on the solvent nature (alkyl chain length of oil), composition, temperature, and added water. Phase behavior study has shown that iso-C18G1 forms isotropic single-phase solutions in the aforementioned oils at 25 C. At lower temperatures (97% was purchased from Wako Chemical Industry, Tokyo, Japan. The surfactant was used without further purification. The nonpolar organic solvents cyclohexane, n-decane, and n-hexadecane were purchased from Tokyo Chemical Industry, Tokyo, Japan. All the oils were 99.5% pure, and Millipore-filtered water was used. The schematic molecular structure of iso-C18G1 is given in Scheme 1. 2.2. Methods. 2.2.1. Phase Behavior. First of all, binary phase diagrams of iso-C18G1 in n-decane and n-hexadecane were constructed. For this purpose, different samples (∼0.50 g each) composition range from 2 to 100 wt % were prepared in clean and dry glass ampules (size 12 mm  100 mm) and immediately flamesealed. The chemicals were properly weighed so that the total uncertainty for the values of concentration of each component is (0.01%. The samples were mixed with a dry thermobath, vortex mixture, and repeated centrifugation in order to get homogeneity. The phase behavior was then studied from 0 C until the liquid crystal phase melts to an isotropic solution phase, keeping the samples in a temperature-controlled water bath (with accuracy (0.05). The samples were left for at least 50-60 min at each temperature to equilibrate before identifying the equilibrium phases. The optical properties of the samples (birefringence) were checked through a crossed polarizer. 2.2.2. Small-Angle X-ray Scattering (SAXS). To investigate the reverse micellar structure, samples of iso-C18G1/oil mixtures (∼2 g) with concentrations of iso-C18G1 from 5 to 15 wt % were prepared in the aforementioned oils in clean and dry glass ampules (5 mL) with screw cap. The samples were mixed using dry thermobath, vortex mixer, and repeated centrifugation to achieve homogeneity. After mixing, the samples were placed in a temperature-controlled water bath at 25 C for 1 h before SAXS measurements. In the SAXS measurements, we used a SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealed-tube anode X-ray generator (PANalytical, Netherlands), which was operated at 40 kV and 50 mA. An :: equipped Gobel mirror and a block collimator enabled us to obtain a focused monochromatic X-ray beam of Cu KR (39) Shrestha, L. K.; Sato, T.; Aramaki, K. Langmuir 2007, 23, 6606. (40) Shrestha, L. K.; Sato, T.; Aramaki, K. J. Phys. Chem. B 2007, 111, 1664. (41) Shrestha, L. K.; Aramaki, K. J. Dispersion Sci. Technol. 2007, 28, 1236.

Langmuir 2009, 25(8), 4435–4442

Shrestha et al.

Article

Scheme 1. Schematic Molecular Structure of Glycerol Monoisostearate (iso-C18G1).

radiation (λ = 0.1542 nm) with a well-defined line shape. A thermostated sample holder unit (TCS 120, Anton Paar) was used to control the sample temperature. The 2D scattering pattern was recorded by an imaging-plate (IP) detector (a Cyclone, Perkin-Elmer) and integrated into to onedimensional scattered 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 measured intensities were semiautomatically calibrated for transmission by normalizing a zero-q attenuated primary intensity to unity, by taking advantage of a semitransparent beam stop. All I(q) data were corrected for the background scattering from the capillary and the solvents, and the absolute scale calibration was made using water as a secondary standard. To obtain the real-space structure function, the so-called pair-distance distribution function, p(r), of the reverse micelles via a virtually model-free routine, we used the generalized indirect Fourier transformation (GIFT) method.42-45 This procedure 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 static structure factor, and n is the number of particles in unit volume. As p(r) is mathematically connected to P(q) as Z

¥

PðqÞ ¼ 4π

pðrÞ 0

sin qr dr qr

ð1Þ

an experimental p(r) can be calculated as inverse Fourier transformation of P(q). To suppress the influence of interparticle interference scattering on the evaluation of p(r) that generally leads to highly underestimated maximum size of the scattering object, an interaction potential model for S(q) was to be involved, where we chose the averaged structure factor model46,47 of hard-sphere and PercusYevick closure relation to solve the Ornstein-Zernike equation. The detailed theoretical description on the method has been given elsewhere.48-50

2.2.3. Rheometry. The rheological measurements were performed in a stress-controlled rheometer (AR-G2, TA Instruments) using cone-plate geometries (diameters 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.

3. Results and Discussion 3.1. Phase Behavior of iso-C18G1/Oil Systems. The binary phase diagrams of iso-C18G1 in n-decane and (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. (50) Glatter, O. J. Appl. Crystallogr. 1980, 13, 577.

Langmuir 2009, 25(8), 4435–4442

n-hexadecane as a function of composition and temperature are presented in Figure 1. As can be seen in the phase diagram, there appears a small solid phase domain at lower temperatures in the surfactant-rich region of the iso-C18G1/ decane system and a wider domain of two-phase (II phase) dispersion. The II phase transforms into isotropic singlephase solution upon heating. The solid phase exists at all mixing fractions of surfactant in the iso-C18G1/hexdecane system below 18 C, and after that the II phase channel extends from lower to higher surfactant concentrations. Finally, upon heating the II phase, solution transforms into isotropic single-phase solution consisting of reverse micelles at normal room temperature. In the case of the iso-C18G1/ cyclohexane system, the isotropic single phase is present at and above 2 C. The shape and size of the reverse micellar aggregates formed in the isotropic solution phase have been determined by the SAXS measurements and are discussed in the following sections. 3.2. Reverse Micellar Structures. Formation of reverse micelles in surfactant/oil binary systems and their tunable structural variation have implications from both fundamental and practical viewpoints. Owing to this fact, we intended to study the structure of reverse micelles in iso-C18G1/oil binary systems in water-free systems and the structural modulation caused by the added water. In the following sections, we will discuss about the tunable parameters that can control the structure of the reverse micelles based on isoC18G1 nonionic surfactant. 3.2.1. Effect of Solvent Properties. First we discuss about the solvent dependent reverse micellar structure and then correlate these structures with the rheological properties. We have performed the SAXS measurements on the 5 wt % isoC18G1 in cyclohexane, n-decane, and n-hexadecane at 25 C. At this temperature, all the systems investigated exist in an isotropic single liquid phase (see phase diagrams). Figure 2 shows the scattering functions, I(q), and the resulting pairdistance distribution functions (PDDFs), p(r), of 5 wt % isoC18G1/oil systems, to compare the effects of cyclic and straight chain hydrocarbon oils and those of the different chain lengths in the straight ones on the aggregation behavior of the iso-C18G1. In addition to the p(r) functions, we obtain for each data set the structure factors and their parameters such as volume fraction, interaction radius, and polydispersity. These parameters are supplied as Supporting Information. The dependence of I(q) to q in the I(q)-q curves indicates the presence of micellar structure in iso-C18G1/oil systems at 25 C. The increase of the forward scattering intensity (I (q=0)) upon changing oil from cyclohexane to decane and decane to hexadecane as shown in Figure 2a can be taken as the evidence of the micellar growth. We used GIFT method as mentioned in the Experimental Section to evaluate our SAXS data. Using GIFT one can extract the p(r) functions (form factor in real space) and the structure factor. Note that in nonaqueous systems the contrast between the oil and the lipophilic part of the surfactant is low. As a result, SAXS selectively detects the hydrophilic core of the reverse micelles. Therefore, p(r) must be recognized as a measure of the micellar core structure. The micellar growth is well reflected in the p(r) functions in a more intuitive way. A slightly asymmetric, bell-shaped p(r) function of the iso-C18G1/cyclohexane system confirms that the micelle has a slightly elongated but nearly spherical shape, whose maximum core diameter, Dmax, is estimated to ∼2.5 nm at 25 C. The DOI: 10.1021/la8039225

4437

Article

Shrestha et al.

Figure 1. Phase diagrams of iso-C18G1 in decane (left) and hexadecane (right). S and I represent a solid and isotropic solution phase, respectively.

Figure 2. Effect of solvent nature on the reverse micellar structures in the 5 wt % iso-C18G1/oil systems as obtained by SAXS: (a) the scattered

intensities I(q) for different oils at 25 C in absolute unit, (b) the pair-distance distribution functions, p(r), obtained as an output of GIFT analysis, (c) the normalized p(r) functions, and (d) the zero-shear viscosity (η0) as a function of carbon number of oils. The solid and broken lines in (a) represent GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The arrows in panels b and c indicate the maximum length of micellar core, Dmax.

iso-C18G1/decane system exhibits a more asymmetric shape and a considerably greater Dmax (∼4 nm), demonstrating the formation of a prolate-type elongated aggregate. Further micellar growth is achieved upon gradual increase of the alkyl chain length of oil from decance to hexadecane. As shown in Figure 2b, p(r) function of the iso-C18G1/hexadecane system shows the typical behavior of an elongated particle with a greatly enhanced Dmax of ∼6.2 nm. The inflection point even seen on the higher-r side of the maximum of p(r), which is indicated by a dotted line at r ∼ 1.95 nm in Figure 2b, approximately points to the cross-section diameter of the reverse micellar core. In order to see the transition in the internal structure of the micelles depending on the solvent nature, we plot the normalized p(r) functions (p(r)/p(rmax)) as a function of r for different oil systems (see Figure 2c), which shows that the cross-sectional diameter of the micellar structure slightly increases upon changing oil from cyclohexane to decane or hexadecane. The reason for micellar growth (both dimension and cross section) can be attributed to the fact that cyclohexane due to 4438

DOI: 10.1021/la8039225

some degree of polarity favors a close contact with the hydrophilic/lipophilic interface of the surfactant, which increases the critical packing parameter. In contrast, unfavorable direct contact between the glycerol moiety and the nonpolar hydrocarbon oil like decane is likely to decrease the critical packing parameter, leading to the elongated structure of the micelles. Figure 2d shows the zero-shear viscosity (η0) of iso-C18G1 reverse micellar solutions as a function of total carbon number of oils. With increasing carbon number (or molecular weight) the η0 increases, which are in good agreement with the SAXS data. Namely, as the micellar length increases, the η0 increases. 3.2.2. Effect of Temperature. Figure 3 shows the results of SAXS measurements (I(q) and p(r) functions) at different temperatures (25, 50, and 75 C) for the 5 wt % iso-C18G1/ hexadecane system. Increasing temperature decreases the scattering intensity in the forward direction without affecting the scattering behavior in the high-q regions. The decreasing trend of the forward intensity with increasing temperature Langmuir 2009, 25(8), 4435–4442

Shrestha et al.

Article

Figure 3. Effects of temperature on the reverse micellar structures of iso-C18G1 in hexadecane as obtained by SAXS and rheometry: (a)

normalized X-ray scattering intensities, I(q), in an absolute unit at different temperatures of 25, 50, and 75 C; (b) corresponding real-space p(r) functions, obtained by GIFT procedure; (c) viscosity (at constant shear rate of 0.1 s-1) vs temperature plot. The solid and broken lines in (a) represent the GIFT fit and calculated form factor for n particles in unit volume nP(q), respectively. Arrows in panel b highlight the maximum length, Dmax, of the micellar core.

can usually be taken as a measure of the decreasing micellar size, which may also be affected by the modified contrast as a function of temperature. As shown in the p(r) functions (Figure 3b), increasing temperature from 25 to 75 C; the reverse micellar size seems to gradually decrease from ∼6.2 to 3.2 nm in the 5 wt % iso-C18G1/hexadecane, i.e., the micelles shrink by ∼50%. Thus, the SAXS data show that the length of the aggregates decreases with the rise of temperature, which is essentially a short rod-to-sphere type transition in the reverse micellar structure. Note that as temperature increases, the data quality of the experimental scattering intensity becomes noisy. One possible reason is the poor contrast of the system at higher temperatures. Next, the aggregates are about to break into monomer due to high thermal energy. If we further increase temperature, there may be a possibility of surfactant’s monomer dissolution. Contrary to aqueous poly(oxyethylene)-type nonionic surfactant systems, where the miscibility of surfactant decreases with the rise of temperature and the systems show clouding behavior at higher temperatures, the miscibility of oil and surfactant in nonaqueous systems increases with temperature. Rising temperature increases the thermal agitation and, hence, increases the space available for the solubilization in the micelle. Furthermore, with increasing temperature, the lipophilicity of the surfactant generally increases, so that the van der Waals interaction between the hydrocarbon chain of the surfactant and the oils increases, which leads to an increase in the oil solubilization capacity of the surfactant. Increasing temperature enhances the penetration of oil in the surfactant chain increases, thereby increasing packing parameter decreases the length of the reverse micelles. To complement the SAXS data, we have measured viscosity of the iso-C18G1/hexadecane system as a function of temperature (Figure 3c). It was found that the viscosity decreases continuously with rising temperature. Thus, the SAXS and rheology data are well correlated. Langmuir 2009, 25(8), 4435–4442

3.2.3. Effect of Surfactant Concentration. SAXS studies on the effect of surfactant concentration on the micellar structure have shown that the present surfactant iso-C18G1 favors micellar growth with increasing concentration. In Figure 4, we present the I(q) and the resulting p(r) functions for the iso-C18G1/decane system at different surfactant concentrations (5-15 wt %) at 25 C. With increasing surfactant concentration, the scattering intensities throughout the q range increases. Minute observation of I(q) vs q curves reveals that with increasing concentration the forward scattering intensity in the low-q region tends to suppress with a growing interaction peak at intermediate q range (q ∼ 1.8 nm-1). Both these features can be attributed to the fact that the osmotic compressibility of the micellar aggregates decreases and intermicellar interaction comes into play at higher surfactant concentrations. Coming to the p(r) functions, the position of the inflection point seen on the higher-r side of the maximum of p(r), as highlighted by a dotted line at r ∼ 1.9 nm in Figure 4b, is virtually unchanged despite change in composition, indicating that the cross-section structure of the reverse micelles is unaffected by the surfactant concentration. Increasing surfactant concentration increases the maximum length of the particle, Dmax, as highlighted by the arrows in Figure 4b. Figure 4c shows the normalized p(r) function (p(r)/Φs, where Φs is the volume fraction of surfactant). The maximum length of the micellar core changes from ca. 4.1 to 6.2 nm when the surfactant concentration is increased from 5 to 15 wt %. These observations highlight the onset of one-dimensional micellar growth with surfactant concentration. Judging from the downward convex shape of the high-r tail in p(r) function for the 5 and 10 wt % systems, one can say the presence of prolate spheroid-type aggregates as the major constituent. On the other hand, a pronounced peak in the low-r side with an extended long tail in the higher-r side of the DOI: 10.1021/la8039225

4439

Article

Shrestha et al.

Figure 4. (a) Scattering curves I(q) of the iso-C18G1/decane system at different surfactant concentrations of 5, 10, and 15 wt % on absolute scale at 25 C. (b) Corresponding real-space p(r) function. (c) Normalized p(r) functions (p(r)/φs). The solid and broken lines in (a) represent the GIFT fit and the calculated form factor for n particles existing in unit volume, nP(q), respectively. The arrows in panel b highlight the maximum length of the micellar core, and the broken line semiquantitatively indicates the cross-section diameter of the core.

p(r) function for the 15 wt % iso-C18G1 system can be taken as an evidence of rodlike or cylindrical particles as a major constituent. Thus, it can be concluded that the composition favors not only micellar growth but also the modulation in the shape of the particles. 3.2.4. Effect of Water. In the studies of water effect on the reverse micellar structure, first, we have investigated the modulation in the structure of micelles as a function of water concentration and then the temperature effect on the water added system at fixed water concentration. We will discuss stepwise in the following section. 3.2.4.1. Effect of Concentration of Added Water. It has been found that water induces micellization in nonaqueous systems. The water or other polar additives tend to be soluble in a reverse micellar core and cause an increase in the aggregation number.51 In the present study, we have investigated the effect of added water on the geometry of isoC18G1 reverse micelles in the 10 wt % iso-C18G1/decane system at 25 C. The structural transition induced by the added water can be seen in the scattering functions presented in Figure 5a. The addition of traces water enhances the forward scattering intensity, and the scattering curve of the high-q region (∼2 nm-1 indicated by a broken arrow) shifts toward the forward direction. The corresponding real-space information based on p(r) functions shown in Figure 5b reveals that addition of water induces dramatic changes in the structure of the micelles in terms of both its maximum dimensions and the internal structure. Dmax shows a monotonic increase with water until phase separation. (In the present system, phase separation occurred above 0.2% water.) To have a quantitative insight into the water induced transition in the internal structure of reverse micelles, we plot the normalized pairdistance distribution functions p(r)/p(rmax) in a limited r-range, as shown in Figure 5c. With increasing water (51) Mathews, M. B.; Hirschhorn, E. J. Colloid Sci. 1953, 8, 86.

4440

DOI: 10.1021/la8039225

concentration, the position of the rmax shifts toward a higher-r side. This indicates the formation of swollen micelles with a water pool in the micellar core.37 Thus, the added water causes simultaneous changes in the maximum dimension and the cross-section structure of the reverse micellar aggregates; i.e., water induces two-dimensional micellar growth. Finally, we have checked if the added water caused any significant change in the shape of the particles. For this reason, we plot the normalized p(r) functions versus r* = r/ rmax for the 10 wt % iso-C18G1/decane systems at different concentration of added water at 25 C (Figure 6). One can see that there is not significant change in the form of these functions. This indicates that there is no significant change in the shape of the reverse micelles in water added systems. Only the size and the scattering power vary. 3.2.4.2. Effect of Temperature on Water Added System. Here we present the temperature-induced microstructural transition on the water added system. With rising temperature, the forward scattering intensity decreases keeping highq scattering intensity virtually the same (Figure 7a), indicating the micelle shrinkage. The Dmax decreases continuously with increasing temperature (see Figure 7b). In other words, one can say temperature induces rod-sphere type transition in the reverse micellar structure. Since the system contains a small amount of water in the micellar core, the present situation is similar to the W/O type microemulsion. Increasing temperature decreases the hydration of the hydrophilic head group so that the critical packing parameter tends to increase. The increase of critical packing in the reverse system means the aggregates with more negative curvature. 3.2.5. Direct Cross-Section Analysis and Model Calculations. When the axial length of an elongated rodlike or cylindrical scattering particle is at least 3 times longer than the cross-sectional diameter, direct cross-section analysis of the micellar core can be carried out. Theoretically, the Langmuir 2009, 25(8), 4435–4442

Shrestha et al.

Article

Figure 5. (a) X-ray scattered intensities I(q) of 10 wt % iso-C18G1/decane at different concentrations of added water in absolute unit at 25 C. (b) Corresponding pair-distance distribution functions. (c) Normalized pair-distance distribution p(r)/p(rmax) in a limited r range. Solid and broken lines in (a) represent the GIFT fit and the calculated form factor for n particles in unit volume, nP(q), respectively.

Figure 6. Normalized pair-distance distribution p(r)/p(rmax) vs r* = r/rmax for the 10 wt % iso-C18G1/decane systems at different concentration of added water at 25 C. radial electron density profile, ΔFc(r), is connected to the cross-sectional pair-distance distribution function, pc(r), as52 pc ðrÞ ¼ rΔ~ F c 2 ðrÞ

ð2Þ

Using an extended technique of indirect Fourier transformation (IFT), pc(r) can directly be calculated from the experimental scattered intensity I(q) via Z IðqÞq ¼ πLIc ðqÞ ¼ 2π2 L

¥

pc ðrÞJ0 ðqrÞ dr

ð3Þ

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.53,54 The core diameter for all the iso-C18G1/decane or isoC18G1/hexadecane systems is ∼1.8-2.0 nm, read out from the inflection point of p(r) located on the higher-r side of its sharp maximum. To quantify this parameter more accurately, we have used the model-free cross-section structure (52) Glatter, O. J. Appl. Crystallogr. 1980, 13, 7. (53) Glatter, O. J. Appl. Crystallogr. 1981, 14, 101. (54) Glatter, O.; Hainisch, B. J. Appl. Crystallogr. 1984, 17, 435.

Langmuir 2009, 25(8), 4435–4442

analysis outlined above, whose typical results are given in Figure 8. The core cross-section diameter of ∼1.95 nm judged from Dc,max in pc(r) and rc,max ∼ 0.9 nm in ΔFc(r) for the 10 or 15 wt % iso-C18G1/decane systems and the 5 wt % iso-C18G1/hexadecane system is almost identical with those obtained in the total p(r) functions discussed before. Addition of water significantly increases the cross-sectional radius of the micelles, giving rc,max ∼ 1.0 and 1.1 nm for 0.15 and 0.20 wt % water added systems, respectively. Note that the rc,max of the ternary mixtures far exceeds those of waterfree systems. This clearly indicates the formation of water pool at the micellar core and emphasizes an efficiency of the polar additive for producing a thicker core of the reverse micelles. To confirm the consistency between the results of our GIFT/IFT technique reported in the previous sections and those obtained from a more conventional analytical approach, we have performed model calculations for selected systems by testing different plausible models for the investigated reverse micelles. The results are presented in Figure 9. The calculation was carried out based on the method described elsewhere.37,55 Judging from the shape of the p(r) functions derived from the GIFT method, we have chosen a prolate ellipsoid model for 5 wt % iso-C18G1/cyclohexane and 5 wt % iso-C18G1/decane system, respectively. The short and long axes of a spheroid a and b obtained from the theoretical model calculations are largely consistent with the results of the GIFT analysis (see Figure 9).

4.

Conclusion

In this paper, we have first described the formation of reverse micelles in iso-C18G1/oil systems at normal room temperature without water addition. Then, we have discussed about the tunable parameters for the structural control of nonionic surfactant micelles in organic solvents. The small-angle X-ray scattering (SAXS) technique has been used (55) Glatter, O. Acta Phys. Austriaca 1980, 52, 243.

DOI: 10.1021/la8039225

4441

Article

Shrestha et al.

Figure 7. (a) Scattering intensities I(q) of the 10 wt % iso-C18G1/decane + 0.20 wt % H2O system in an absolute unit at different temperatures (25, 50, and 75 C). (b) Corresponding pair-distance distribution functions.

Figure 8. (a) Cross-sectional pair-distance distribution function, pc(r), for the iso-C18G1/oil and iso-C18G1/oil/water systems. (b) Corresponding cross-section radial electron density profile, ΔFc(r), calculated from the deconvolution of the pc(r).

Figure 9. Model calculation and fit to the experimental SAXS intensities of 5 wt % iso-C18G1/cyclohexane (square) and 5 wt % iso-C18G2/decane (circle) as typical examples. The data were fitted using a geometrical model of a homogeneous prolate ellipsoid with hard-sphere interaction. for the structural characterization of micellar aggregates. Using the GIFT method, SAXS data have been evaluated which indicates that structure of iso-C18G1-based reverse micelles can be controlled by tuning solvent nature, temperature, composition, and water addition. Contrary to

4442

DOI: 10.1021/la8039225

the general reverse micellar systems that usually form small spherical type aggregates, we are able to achieve a variety of aggregates such as sphere, ellipoid prolate, and short-rod to a cylindrical type depending on the solvent nature, i.e., the chain length of the hydrocarbon oils, composition, or temperature. We saw clear evidence of one-dimensional (1-D) elongation with increasing surfactant concentration preserving the micellar core cross-section diameter, despite its certainly small hydrophilic head group. Addition of a small amount of water induces simultaneous elongation and core swelling. The results derived from the GIFT method are well supported by theoretical model calculations and rheometry. Acknowledgment. L.K.S. thanks JSPS for Postdoctoral Fellowship for Foreign Researchers. Supporting Information Available: Structure factors and their parameters (volume fraction, interaction radius, and polydispersity) obtained by the GIFT evaluation of the SAXS data for each data set. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(8), 4435–4442