Letter pubs.acs.org/JPCL
Demonstration of Solvent-Induced One-Dimensional Nonionic Reverse Micelle Growth Lok Kumar Shrestha,*,† Rekha Goswami Shrestha,‡ Kenji Aramaki,§ Genki Yoshikawa,† and Katsuhiko Ariga† †
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan ‡ Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda Chiba 278-8510, Japan § Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Yokohama 240-8501, Japan S Supporting Information *
ABSTRACT: We report formulation and free morphology switching of nonionic surfactant reverse micelles in nonpolar solvent alkylbenzene. Microstructure transformations of nonionic reverse micelles depending on the solvent molecular architecture under ambient conditions and their thermoresponsive behavior have been investigated using small-angle Xray scattering (SAXS) and rheometry. SECTION: Biomaterials, Surfactants, and Membranes
S
Ethylene oxide (EO)-based surfactants are well-known and well-studied nonionic surfactants, but they require inclusion of water or other polar molecules to trigger self-assembly or nanostructure formation in nonaqueous media as the EO chain of the surfactant tends to be soluble in many organic solvents. However, glycerol- or sugar-based nonionic surfactants being more solvophobic in organic solvents shows superior solution behavior compared to conventional EO-based nonionic surfactants and spontaneously self-assemble into reverse micelles without adding water.21,22 Diglycerol fatty acid ester nonionic surfactants have been shown to form a variety of selfassembled structures (micelles, liquid crystals, and vesicles) in nonpolar organic solvents.21−23 In linear chain alkane oils, the short-chain (C10 to C14) diglycerol surfactants have been shown to assemble into reverse micelles at high temperature after melting of the lamellar liquid-crystal phase.23 Room-temperature reverse micelle formulation was achieved with the longerchain diglycerol polystearates or polyglycerol oleates.24,25 Although the shape, size, and internal cross-sectional structure of those reverse micelles were found to depend on the molecular structure of surfactants, we could not observe long rod formation. Besides, a sharp microstructure transition such as zero-to-one-dimensional (0-to-1D) was not realized. Similarly to the conventional ionic reverse micelles containing water, nonionic surfactant reverse micelles swell with water and
elf-assembly is the independent organization of atoms, molecules, or functional units into the well-defined patterns or structures without external interference. This process is common throughout nature and has been highly utilized in technology. Self-assembly is one of the few practical strategies for creation of ensembles of nanostructures in different length scales and, hence, has become an essential part of nanotechnology.1,2 Amphiphilic molecules such as surfactants or block copolymers are able to self-assemble into a variety of threedimensional (3D) morphologies in aqueous systems, whose characteristics and sizes can be flexibly tuned by surfactant molecular structure, composition, temperature, and other outer parameters. Surfactants usually show remarkable phase behavior in aqueous systems, forming various phases depending on the composition and temperature.3−7 Just above the critical micelle concentration (cmc), surfactants generally tend to form spheroid-type micelles, whose morphology can be modulated into other geometries such as prolate, rods, or disks or orderedliquid crystalline phases with the change of control parameters such as concentration, temperature, salinity, and pH.8−12 These self-assembled structures have extensively been utilized in various realistic applications including food, cosmetic and pharmaceutical formulations, and technology.13−15 However, despite the enormous applications in industry and technology, self-assembly of surfactants in nonaqueous media and the fundamental physicochemical properties, structure, and dynamic behavior of reverse micelles have not been deeply investigated in comparison to the aqueous system.16−20 © 2013 American Chemical Society
Received: June 19, 2013 Accepted: July 22, 2013 Published: July 22, 2013 2585
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systems on the absolute scale after subtraction of solvent scattering in a logarithmic plot of the scattering intensity, I(q), versus scattering vector, q. The solid and broken lines in Figure 1a represent a fit to the experimental data and calculated form factor, P(q), as obtained by the GIFT method. The results of the GIFT procedure, the pair−distance distribution functions p(r), are shown in Figure 1b. Figure 1c and d shows the results obtained from geometrical model fittings for the selected systems. Detail will be discussed later. As can be seen in Figure 1a, all of the I(q) curves are strongly dependent to the q, demonstrating the presence of aggregate (micellar) structures. The absence of correlation peaks in the lower-q region indicates the negligible micellar interactions in the studied systems. Figure 1a shows a clear dependence of the scattering curve on the alkyl chain length of alkylbenzene. The forward scattering intensity increases with the alkyl chain length of the solvent, while scattering in the higher-q region (q > 1.5 nm−1) remains essentially unchanged. Such scattering behavior is clear evidence of micellar growth.31 The low-q scattering behavior of the shortest alkyl chain, that is, the EB system, follows approximately q0 behavior, which is typical of the spheroid geometry of the scattering element. The low-q slope increases in parallel to the alkyl chain and follows q−1 behavior in the DDB system. Growth to rodlike micelles should lead to q−1 behavior in the scattering curve, at least for systems with negligible interaction effects.32 With a further increase in the alkyl chain, the slope further increases and reaches q−1.5 in the TDB system, which does not look straightforward. Generally, a growth to disk-like micelles should lead to q−2 scattering behavior in the small-angle region. We anticipated that the q−1.5 behavior is the signature of the attractive interactions. Essential information on the particle geometry can be obtained from the pair−distance distribution function, p(r), which is obtainable from the GIFT analysis of the SAXS data. One can easily estimate the maximum dimension Dmax within one particle from the abscissa value where the p(r) reaches zero. Even if the system is interacting, the GIFT procedure separates out the disturbing interparticle contribution from the overall scattering intensity. Details can be found elsewhere.33 Our systems are free from such effects. However, it should be noted that all of the p(r) curves described herein should be recognized as a measure of the reverse micellar core structure. This is because of the poor (negligibly small) contrast (electron density difference) between organic solvent and the lipophilic part of the surfactant in nonaqueous media; as a result, SAXS selectively detects the only hydrophilic core. For the EB system, the p(r) corresponds to the form factor and has the typical shape of almost a globular micelle with a maximum core diameter, Dmax, of ∼5.7 nm. From the p(r) function, one can clearly see micellar growth with increasing alkyl chain length of the solvent. Globular-like micelles observed in EB transform into rodlike micelles with a cross-sectional diameter of about 3.0 nm (judged from the inflection point after the p(r) peak, which semiquantitatively indicates the cross-sectional diameter of a rodlike micelle) and a maximum length of about 9.5 nm in HB. With further increase in the alkyl chain length, micelles grow axially (one-dimensionally), keeping the cross-sectional diameter essentially unchanged (see Figure 1b, p(r) decays to zero at higher r values). Variation of the maximum dimension of the micelles as a function of the carbon number of alkylbenzene is shown in Figure S2 of the Supporting Information. The p(r) function of the DDB system corresponds to a perfect rod shape of the micelles; a pronounced peak on
cause micellar growth. Free shape- and size-controlled reverse micelle formulation is very important from the viewpoint of technological applications such as in nanobio architectonics, enabling strategic design of nanostructured materials.26 Herein, we report experimental evidence of solvent (particularly the alkyl chain of alkylbenzene) induced 0-to-1D microstructure transitions of diglycerol monocaprate (C10G2) reverse micelles. Instead of linear chain alkane oils, we chose aromatic solvents alkylbenzenes, which being less nonpolar compared to alkanes, reduce the Krafft temperature of diglycerol-based nonionic surfactants. The C10G2 formed reverse micelles in alkylbenzene at room temperature. Moreover, we also discuss thermally induced size variation of C10G2 reverse micelles. Investigations are based on small-angle X-ray scattering (SAXS) and rheometry. The shape and size of reverse micelles were obtained by the generalized indirect Fourier transformation (GIFT) method,27,28 and the results were also complemented by geometrical model fittings. For the quantitative estimation of the internal cross-sectional structure (cross-sectional diameter) of rodlike reverse micelles, direct cross-sectional analysis was performed on the experimental scattering function using the indirect Fourier transformation (IFT) method.29,30 Details can be found in the Supporting Information. SAXS measurements were performed on C10G2/alkylbenzene binary mixtures (the concentration and temperature were fixed at 5% and 25 °C, respectively) at different alkyl chain lengths of the alkylbenzenes (ethylbenzene: EB; hexylbenzene: HB; octylbenzene: OB; decylbenzene: DB; dodecylbenzene: DDB; and tetradecylbenzene: TDB). SAXS results are presented in Figure 1. Figure 1a shows the experimental scattering curves (symbols) of the C10G2/alkylbenzene binary
Figure 1. (a) X-ray scattered intensities, I(q), of the 5% C10G2 in alkylbenzene at 25 °C on absolute scales, (b) the corresponding pair− distance 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 a unit volume, nP(q), respectively. The arrows and broken lines in panels (b) and (d) indicate the maximum dimension, Dmax, and the semiquantitative measure of the cross-sectional diameter, Dc max, respectively. The data were fitted considering homogeneous ellipsoid prolate (for EB and HB) and homogeneous cylinder (DDB) models. 2586
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experimental scattered intensity I(q) according to eq 4 (see the Supporting Information). The IFT approach based on eq 4 yields pc(r), from which Δρc(r) can be deduced via the deconvolution procedure. Figure 2 shows pc(r) and Δρc(r) for
the lower-r side with a linear decay at higher r is an indication of a rod shape of the scattering element. This is the reason for the clear q−1 behavior in the reciprocal space in the lower-q region. Note that the additional broad peak after the typical cross-sectional peak in TDB is the signature of the attractive interaction.34 We note that such a clear picture of solventinduced 1D nonionic reverse micellar growth has never been realized under ambient conditions. The results derived from the GIFT method are well supported by the geometrical model fittings (see Figure 1c,d). The optimum model fits to the experimental I(q), and the resulting p(r) curves are presented in Figure 1c,d. We noted that homogeneous ellipsoidal prolate and cylinder models explain well the shape and size of the micelles derived from the GIFT method, although a notable divergence in the higher-q region of the experimental I(q) curves is seen. The absence of the theoretically predicted minima in the high-q part of the experimental I(q) could be attributed to the polydispersity in the size and/or small electron density fluctuations inside of the micellar core.35 It is well-established that if polydisperse spheres are considered, such high-q minima can easily be smeared out with increasing polydispersity.30 This is because slightly different radii R give minima at slightly shifted q positions, and their superposition no longer represents distinct minima. The solvent-induced 1D micellar growth can be explained in terms of solvent polarity, which decreases with the chain length, thereby causing low interpenetration of the hydrophobic chain of the surfactant and solvent. Poor interpenetration causes an increase in the effective cross-sectional area of the hydrophilic moiety of the surfactant and, consequently, critical packing at the micellar interface decreases. In the reverse micellar system, a decrease in critical packing causes micellar growth.36 Moreover, the transfer free energy of the hydrophilic part of the surfactant from the hydrophilic to oily hydrophobic environment with different hydrocarbon chain lengths may be crucial for the determination of the self-assembly of amphiphiles in nonaqueous media.37 It can be naturally expected that the oil-to-oil transfer free energy of the hydrophobic moiety of C10G2 will be negligible for different chain length hydrocarbon oils. However, considering the highly hydrophilic nature of the diglycerol moiety, its transfer free energy from the hydrophilic environment to oil will considerably differ depending on the chain length of the solvent. That is, the unfavorable free energy of the diglycerol moiety with the longer alkyl chain alkylbenzene will decrease the cross-sectional area occupied by the C10G2 molecule on the hydrophilic/hydrophobic interface to optimize the contact with the solvent and promote the formation of longer cylindrical micelles. For rodlike scattering objects such as that observed in the C10G2/HB and in the longer-chain alkylbenzene, a model-free analysis of the micellar cross-sectional structure is available, under the assumption that the scattering length density profile of the cross section is simply a function of the radial position. The core diameter for the HB−TDB systems is found approximately in the range of 2.5−3.5 nm considering the inflection point of p(r) located on the higher-r side of the sharp maximum as its measure. To confirm this estimation, we used the model-free approach to the cross-sectional structure. The interplay between the cross-sectional (core) radial density profile Δρc(r) and the cross-sectional pair−distance distribution function pc(r) is given by eq 3 (see the Supporting Information). The pc(r) can directly be calculated from the
Figure 2. Results of direct cross-sectional analysis. (a) Cross-sectional pair−distance distribution function, pc(r), of 5% C10G2/alkylbenzene systems at 25 °C and (b) the resulting cross-sectional radial electron density profile, Δρc(r), calculated with the deconvolution procedure of the pc(r) using the DECON program. Solid lines in (a) represent the DECON fit. Arrows in panels (a) and (b) represent the maximum core cross-sectional diameter, Dc max, and maximum core radius, Rc max, respectively.
the 5% C10G2 in HB, OB, DB, DDB, and TDB systems at 25 °C. The pc(r) functions presented in Figure 2a represent typical homogeneous aggregates with a maximum cross-sectional diameter, Dc max, of ∼2.99 nm for the HB−DDB systems and 3.09 nm for the TDB system. These values are close to those estimated from the total p(r) functions (Figure 1b,d). The positive electron density profiles in Figure 2b are the indication of an electron-rich hydrophilic reverse micellar core. From the Δρc(r) profiles, the maximum core radii (Rc max) are ∼1.454 nm for the HB−DDB systems and 1.503 nm for the TDB system. These values are slightly higher than that of the extended chain of the two-glycerol moiety as one glycerol molecule accounts for ∼0.5 nm. This is caused due to the small amount of impure water present in C10G2 and solvents. Figure 3a shows the measured viscosity, η, of 5% C10G2 in EB−TDB as a function of the shear rate at 25 °C, while Figure
Figure 3. (a) Viscosity versus shear rate curve for the 5% C10G2/ alkylbenzne systems at 25 °C as obtained from steady-shear rheological measurements and (b) the estimated zero-shear viscosity, η0, and relative viscosity (ηR = η0(micelle)/η0(solvent), where η0(micelle) and η0(solvent) represent the zero-shear viscosity of the 5% micellar solution and alkylbenzenes, respectively) versus the number of carbons in the alkyl chain length of the alkylbenzene.
3b shows the relative zero-shear viscosity, (ηr = η0(micelle)/ η0 (solvent), where η0(micelle) and η0 (solvent) represent the zero-shear viscosity of the 5% micellar solution and alkylbenzenes, respectively) versus the carbon number in the alkyl chain of solvents. The viscosity of the solvents is shown in Figure S3 of the Supporting Information. As can be seen in Figure 3a, the systems display Newtonian-fluid-like behavior, which can be 2587
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judged from the shear-independent behavior of the viscosity.9 The calculated relative viscosity, ηr, remains apparently the same upon increasing the carbon chain the of solvent from 2 to 4, that is, changing the solvent from EB to HB but with further increasing the carbon chain up to 12, ηr increases significantly, which is expected to be caused by the 1D micellar growth. A decrease in the ηr value with changing solvent from DDB to TDB is expected to be caused by attractive interaction of the long rods, which leads to zero curvature at the intersection point of two rods for example. It is well-known that macroscopic rheological properties of the surfactant solution depend on the volume fraction of the surfactant, the number density of the micelles, and also the shape and size of the micelles.9 Just above the cmc, the viscosity of the spherical micelles is comparable to that of pure solvent and increases with the volume fraction of the surfactant. On the other hand, the viscosity of rodlike micelles is much higher than that of the pure solvent and/or spherical micelles.34,38 Presently, we have found that at fixed C10G2 concentration, the relative viscosity increases with the alkyl chain length of alkylbenzene. Therefore, it can be concluded that the viscosity gain is caused by the 1D micellar growth. Next, we discuss the thermoresponsive behavior. It is wellknown that conventional EO-based nonionic micelles in aqueous systems grows with the rise of temperature, and generally, a sphere-to-rod transition is inevitable due to dehydration of EO, or in other words, the EO-based surfactant becomes lipophilic at higher temperature. However, an opposite microstructure transition has been commonly observed in glycerol or sugar-based nonionic surfactants in nonaqueous media.23,34 It has been anticipated that temperature enhances the penetration tendency of solvent molecules into the lipophilic chain of the surfactant and micellar curvature tends to be more curved at higher temperature. In this Letter, we have investigated the effect of temperature on the reverse micellar structure of the 5% C10G2/TDB system by performing SAXS measurements at different temperatures of 25, 40, 50, 60, 75, and 85 °C. Figure 4 shows SAXS results. One can clearly see in Figure 4a the decrease in the forward scattering for higher temperatures while scattering in the higher-q region (q > 1.5 nm−1) remains essentially unchanged. This scattering behavior is opposite to the increase in chain length of alkylbenzene (see Figure 1a), inferring that an increase in temperature decreases the micellar size. Note that the low-q slope of the scattering curve is decreasing with temperature, and a q−1 line is added to the scattering curve at 85 °C. As mentioned earlier, a rodlike particle leads to a q−1 behavior in the scattering curve. These observations confirmed that an increase in temperature decreases the micellar size and avoids the attractive interactions. In Figure 4b, we have presented the corresponding results in real space. Here, we noted two important points. First, a broad peak after the typical crosssectional peak (the signature of the attractive interaction) decreases, and second, the maximum average length of the micelles as indicated by arrows also decreases with the rise in temperature. At the highest SAXS measurement temperature (85 °C), the p(r) apparently corresponds to the form factor and has the typical shape for a rod with a maximum length of ∼11.5 nm and cross-sectional diameter of approximately 3.0 nm. It should be noted that micelles still exhibit rodlike structure, but with a further increase in temperature, globulartype micelles are expected to be formed. Variation of the maximum dimension of the micelles versus temperature is
Figure 4. (a) X-ray scattered intensities, I(q), of the 5% C10G2/TDB at different temperatures (25, 40, 50, 60, 75, and 85 °C) on absolute scales, (b) the corresponding pair−distance distribution functions, p(r), (c) the pc(r) curves, and (d) the corresponding Δρc(r) profiles, calculated with the deconvolution procedure of the pc(r) using the DECON program. The solid and broken lines in panel (a) represent the GIFT fit and the calculated form factor for n particles existing in a unit volume, nP(q), respectively. The arrows and broken lines in panel (b) indicate the maximum dimension, Dmax, and cross-sectional diameter, Dc max, respectively. Solid lines in panel (c) represent the DECON fit. Arrows in panels (c) and (d) represent the maximum core cross-sectional diameter, Dc max, and maximum core radius, Rc max, respectively.
supplied in Figure S4 of the Supporting Information. Figure 4c and d shows the results of direct cross-sectional analysis. From the pc(r) functions, we found that the maximum cross-sectional diameter, Dc max, remains unchanged at ∼3.09 nm until 60 °C and then decreases slightly at higher temperatures, for example, Dc max is ∼2.99 and 2.95 nm for 75 and 85 °C, respectively. A similar trend is observed in the core cross-sectional radius (see Δρc(r) profiles in Figure 4d). In summary, we have presented a clear picture of solventinduced 1D nonionic reverse micelle growth under ambient conditions and their thermal size switching behavior. We noted that the temperature and chain length of the solvent function oppositely in the microstructure transitions of nonionic reverse micelles. Our results have demonstrated the fundamental importance of the solvent molecular structure and temperature for the free shape and size control of the nonionic reverse micelles, which is greatly desired, for instance, in soft-templatebased material nanoarchitectonics for the strategic design of novel nanomaterials.
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EXPERIMENTAL SECTION Sample Preparation. Binary mixtures of C10G2 with alkylbenzenes (ethylbenzene: EB; hexylbenzene: HB; octylbenzene: OB; decylbenzene: DB; dodecylbenzene: DDB; and tetradecylbenzene: TDB) were prepared 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 temperaturecontrolled water bath at 25 °C for 2 h to observe the equilibrium phases. Small-Angle X-ray Scattering (SAXS). First, SAXS experiments were carried out on a series of the 5% C10G2/alkylbenzene 2588
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The Journal of Physical Chemistry Letters systems to study the influence of the solvent molecular architecture, particularly the chain length of alkylbenzene, on the nonionic reverse micellar structure at a constant temperature of 25 °C. In the next step, the thermoresponsive behavior of nonionic reverse micelles was studied measuring SAXS on the 5% C10G2/TDB system at different temperatures (25, 40, 50, 60, 75, and 85 °C). For SAXS measurements, a SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealedtube anode X-ray generator (PANalytical, Netherlands) was operated at 40 kV and 50 mA. A Göbel mirror and a block collimator provided a focused monochromatic X-ray beam of Cu−Kα radiation (λ = 0.1542 nm) with a well-defined line shape. A thermostatted sample holder unit (TCS 120, Anton Paar) controlled the sample temperature with an accuracy of 0.1 °C. The two-dimensional (2D) scattering pattern was recorded by an image plate (IP) detector (Cyclone, PerkinElmer, U.S.A.) and integrated into 1D scattering intensities, I(q), as a function of the magnitude of the scattering vector, q, using SAXSQuant software (Anton Paar). The scattering vector, q, is related to the total scattering angle, θ, by a sample equation, q = (4π/λ) sin(θ/2). All of the measured 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 and the solvent), and an absolute scale calibration was made using water as a secondary standard. SAXS data evaluations were performed using the generalized indirect Fourier transformation (GIFT) method and geometrical model fittings.27,28,30 Details can be found in the Supporting Information. Rheometry. The rheological behavior of the nonionic reverse micelles as a function of alkyl chain length of alkylbenzene has been studied by performing steady-shear rheological measurements in a stress-controlled rheometer, ARG2 (TA Instruments), using a cone−plate geometry (diameter of 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 of the samples showed Newtonian-fluid-like behavior; the viscosity was independent of shear-rate. The zero-shear viscosity (η0) was determined by extrapolating the plateau value to zero shear rates. To determine the relative zero-shear viscosity of the reverse micellar solution, the viscosity of each solvent was also measured.
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ACKNOWLEDGMENTS
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REFERENCES
L.K.S. thanks the International Center for Materials Nanoarchitectonics (WPI-MANA) for partial financial support.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, SAXS data evaluation procedure, variation of Dmax versus the carbon number in the alkyl chain of alkylbenzene, Dmax versus temperature of the 5% C10G2/TDB system, and the zero-shear viscosity of alkylbenzene. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +81 (0)29860-4809. Fax: +81 (0)29-860-4832. Notes
The authors declare no competing financial interest. 2589
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The Journal of Physical Chemistry Letters
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dx.doi.org/10.1021/jz401273z | J. Phys. Chem. Lett. 2013, 4, 2585−2590
