Indirect Fourier Transformation Method Research Articles

Nanoparticle size distribution quantification: results of a small-angle X-ray scattering inter-laboratory comparison.

This paper presents the first worldwide inter-laboratory comparison of small-angle X-ray scattering (SAXS) for nanoparticle sizing. The measurands in this comparison are the mean particle radius, the width of the size distribution and the particle concentration. The investigated sample consists of dispersed silver nanoparticles, surrounded by a stabilizing polymeric shell of poly(acrylic acid). The silver cores dominate the X-ray scattering pattern, leading to the determination of their radius size distribution using (i) the generalized indirect Fourier transformation method, (ii) classical model fitting using SASfit and (iii) a Monte Carlo fitting approach using McSAS. The application of these three methods to the collected data sets from the various laboratories produces consistent mean number- and volume-weighted core radii of Rn = 2.76 (6) nm and Rv = 3.20 (4) nm, respectively. The corresponding widths of the lognormal radius distribution of the particles were σn = 0.65 (1) nm and σv = 0.71 (1) nm. The particle concentration determined using this method was 3.0 (4) g l-1 or 4.2 (7) × 10-6 mol l-1. These results are affected slightly by the choice of data evaluation procedure, but not by the instruments: the participating laboratories at synchrotron SAXS beamlines, commercial and in-house-designed instruments were all able to provide highly consistent data. This demonstrates that SAXS is a suitable method for revealing particle size distributions in the sub-20 nm region (at minimum), out of reach for most other analytical methods.

Structural features and surfactant properties of core–shell type micellar aggregates formed by gemini piperidinium surfactants

Abstract Aqueous micellar solution of piperidinium gemini surfactants was investigated by small angle X-ray scattering (SAXS) and the real space structural information of the micelles was deduced by indirect Fourier transformation (IFT) method. The maximum dimension of the micelles and their shape were determined by evaluating the pair distance distribution function (PDDF) and the electron density profile of the aqueous surfactant solution. These gemini surfactants formed core–shell type micelles in the aqueous solution above their cmc values having ellipsoidal structure. The dilute aqueous solutions of these surfactants were further investigated to determine their surfactant properties by surface tension and conductivity method. The self-aggregation and thermodynamic properties as well as the shape, size and internal structure of the micelles formed by these surfactants were found to be dependent on the methylene spacer units and hydrophobic alkyl chain length of the gemini surfactants.

Structure of Micelles Calcium Didodecyl Sulfate: A SAXS Study

This paper reports the structure of micelles of Calcium Didodecyl Sulfate (CDS), Ca(DS)2 , as studied using Small Angle X-ray Scattering (SAXS). CDS is a dianionic surfactant consisting of two DS- tails attached to Ca++ divalent ion. There is considerable interest in understanding the structure (especially the outer shell of counter-ions) of micelles of CDS as unlike conventional surfactants (e.g. CTAB, SDS) where counter-ion is monovalent, CDS has a divalent counter-ion. SAXS is an ideal technique for obtaining information about the outer shell of the Ca(DS)2 micelle, as the constituents (S, Ca, O etc.) of the shell are strong X-ray scatterers. The SAXS measurements have been made on salt-free aqueous solutions of calcium didodecyl sulfate for surfactant concentrations of 0.5, 1.0, 2.5, and 20 weight % (or 8.8, 17.7, 44.92 and 438.01 mM/dm3 ) respectively. Single step indirect Fourier transformation method has been utilized to generate particle distance distribution function. It is found that micelles are prolate ellipsoidal in shape. The size parameters of the ellipsoidal micelles have been determined. Other relevant parameters like mean aggregation number and effective fractional charge has been determined by fitting an ellipsoidal shaped core-shell model to the Fourier transformed scattering data. It is seen that increase in surfactant concentration results in lowering of aggregation number, increase of shell thickness and lowering of total charge and probably lowering of water association.

Two-dimensional indirect Fourier transformation for evaluation of small-angle scattering data of oriented samples

An extension of the indirect Fourier transformation method for two-dimensional small-angle scattering patterns is presented. This allows for a model-free investigation of real-space functions of oriented structures. The real-space function is built from two-dimensional basis functions. The Fourier transformed basis functions are approximated to the scattering pattern. The solution to this problem in reciprocal space can be used to compute the corresponding real-space functions. These real-space functions contain information on size, shape, internal structure and orientation of the structures studied. Information on structures that are oriented in different distinct directions can be partly separated. The applicability of the technique is demonstrated on simulated data of oriented cuboids and on two experimental data sets based on the nanostructure of spruce normal wood.

A SAXS Study of Aggregation in the Synergistic TBP–HDBP Solvent Extraction System

The macroscopic phase behaviors of a solvent system containing two extractants, tri-n-butyl phosphate (TBP) and di-n-butyl phosphoric acid (HDBP) in n-dodecane, were investigated through use of liquid-liquid extraction and small-angle X-ray scattering (SAXS) experiments. Five organic solutions, each containing a total extractant concentration (TBP + HDBP) of 1 M in varying molar ratios (0, 0.25, 0.5, 0.75, and 1.0 [TBP]:[TBP + HDBP]), were contacted with 0.2 M HNO3 aqueous solutions without and with dysprosium(III) at a concentration of 10(-4) M. An enhancement of the extraction of Dy(3+)--due to effects of synergism arising from the binary combination of extractants--was observed. SAXS data were collected for all solution compositions from 0 to 1 mol-fraction end ratios of TBP after contact with the acidic aqueous solutions both in the absence and presence of Dy as well as for the organic phases before aqueous contact. In the precontacted solutions, no notable changes in the SAXS data were observed upon combining the extractants so that the scattering intensity (I) measured at zero angle (Q = 0 Å(-1))--parameter I(0)--the experimental radius of gyration (R(g)), and the maximum linear extent (MLE) of the extractant aggregates were arithmetic averages of the two end members, 1 M HDBP, on the one hand, and 1 M TBP, on the other. In contrast, after contact with the aqueous phases with and without Dy(3+), a significant reorganization occurs with larger aggregates apparent in the extractant mixtures and smaller in the two end member solutions. In particular, the maximum values of the metrical parameters (I(0), R(g), and MLE) correlate with the apparent optimal synergistic extraction mole ratio of 0.25. The SAXS data were further analyzed using the recently developed generalized indirect Fourier transformation (GIFT) method to provide pair-distance distribution functions with real-space information on aggregate morphology. Before aqueous contact, the organic phases show a systematically varying response from globular-like reverse micelles in the case of 1 M TBP to rod-shaped architectures in the case of 1 M HDBP. After aqueous contact, the aggregate morphologies of the mixed extractant systems are not simple linear combinations of those for the two end members. Rather, they have larger and more elongated structures, showing sharp discontinuities in the metrics of the aggregate entities that are coincident with the synergistic extraction mixture for Dy(3+). The results in this initial study suggest a supramolecular, micellization aspect to synergism that remains underexplored and warrants further investigation, especially as it concerns the contemporary relevance to decades-old process chemistry and practices for high throughput separations systems.

Structure of Nonionic Reverse Micelles in Octylbenzene

We report structure and rheology (flow property) of diglycerol monomyristate (designated as C14G2) reverse micelles in an aromatic solvent octylbenzene. Structural characterization has been made using small-angle x-ray scattering (SAXS), while flow property is studied using rheometry. SAXS data have been evaluated by the generalized indirect Fourier transformation (GIFT) method and geometrical model fittings. We found that, under ambient conditions of temperature and pressure, C14G2 spontaneously self-organize into ellipsoidal prolate-type micelles with maximum core diameter of about 6.5 nm in octylbenzene. Shape and size of the micelles remain essentially unchanged over wide concentrations (3–20%). On the other hand, micellar size decreases with increase in temperature, which is anticipated to be due to an increase in the critical packing parameter (CPP). Since surfactant becomes lipophilic upon heating, micellar curvature becomes more negative at higher temperatures. Steady-shear rheological experiments showed that the present reverse micelles behave as a Newtonian fluids; measured viscosity is independent of the applied shear. Zero-shear viscosity tends to increase with weight fraction of the surfactant, which is assumed to be caused due to an increase in number density of micelles.

Bayesian reconstruction ofP(r) directly from two-dimensional detector imagesviaa Markov chain Monte Carlo method

The interatomic distance distribution, P(r), is a valuable tool for evaluating the structure of a molecule in solution and represents the maximum structural information that can be derived from solution scattering data without further assumptions. Most current instrumentation for scattering experiments (typically CCD detectors) generates a finely pixelated two-dimensional image. In contin-uation of the standard practice with earlier one-dimensional detectors, these images are typically reduced to a one-dimensional profile of scattering inten-sities, I(q), by circular averaging of the two-dimensional image. Indirect Fourier transformation methods are then used to reconstruct P(r) from I(q). Substantial advantages in data analysis, however, could be achieved by directly estimating the P(r) curve from the two-dimensional images. This article describes a Bayesian framework, using a Markov chain Monte Carlo method, for estimating the parameters of the indirect transform, and thus P(r), directly from the two-dimensional images. Using simulated detector images, it is demonstrated that this method yields P(r) curves nearly identical to the reference P(r). Furthermore, an approach for evaluating spatially correlated errors (such as those that arise from a detector point spread function) is evaluated. Accounting for these errors further improves the precision of the P(r) estimation. Experimental scattering data, where no ground truth reference P(r) is available, are used to demonstrate that this method yields a scattering and detector model that more closely reflects the two-dimensional data, as judged by smaller residuals in cross-validation, than P(r) obtained by indirect transformation of a one-dimensional profile. Finally, the method allows concurrent estimation of the beam center and Dmax, the longest interatomic distance in P(r), as part of the Bayesian Markov chain Monte Carlo method, reducing experimental effort and providing a well defined protocol for these parameters while also allowing estimation of the covariance among all parameters. This method provides parameter estimates of greater precision from the experimental data. The observed improvement in precision for the traditionally problematic Dmax is particularly noticeable.

Location of Cholesterol in Liposomes by Using Small-angle X-ray Scattering (SAXS) Data and the Generalized Indirect Fourier Transformation (GIFT) Method

We investigated the location of cholesterol (Chol) in liposomes and its interaction with phospholipids using small-angle x-ray scattering (SAXS) data and applying the generalized indirect Fourier transformation (GIFT) method. The GIFT method has been applied to lamellar liquid crystal systems and it gives quantitative data on bilayer thickness, electron density profile, and membrane flexibility (Caillé parameter). When the GIFT method is applied to the SAXS data of dipalmitoylphosphatidylcholine (DPPC) alone (Chol [-]) or a DPPC/Chol = 7/3 mixed system (Chol [+], molar ratio), change in the bilayer thickness was insignificant in both systems. However, the electron density for the Chol (+) system was higher than that for the Chol (-) system at the location of hydrophilic groups of phospholipids, and whereas Caillé parameter value increased with temperature for the Chol (-) system, no significant change with temperature was observed in the Caillé parameter for the Chol (+) system. These results indicated that Chol is located in the vicinity of the hydrophilic group of the phospholipids and constricts the packing of the acyl chain of phospholipids in the bilayer.

Nonionic Reverse Micelles near the Critical Point

We report shape, size, and internal cross-sectional structure of diglycerol monomyristate (C₁₄G₂) reverse micelles in n-hexadecane near the critical point using small-angle X-ray scattering (SAXS). Pair-distance distribution function, p(r), which gives structural information in real-space, was obtained by indirect Fourier transformation (IFT) method. The p(r) showed a clear picture of rodlike micelles at higher temperatures well above the critical point (micellar solution phase separates into two immiscible liquids at ~ 48°C). At a fixed surfactant concentration (5% C₁₄G₂), decrease in temperature increases the micellar size monotonously and surprisingly shape of the p(r) curve at 50°C; close to the critical point, mimics the shape of the two dimensional disk-like micelles indicating the onset of critical fluctuations (attractive interactions among rodlike micelles forming a weak network). A similar behavior has been observed with normal micelles in aqueous system near the critical point. When the system is heated to 60°C, shape of the p(r) curve regains rodlike structure. At fixed temperature of 60°C, increase in C₁₄G₂ concentration induced one dimensional micellar growth. Maximum length of micelles increases from ca. 23.5 to 46.0 nm upon increasing concentration from 1 to 12% keeping cross section diameter apparently unchanged at ca. 4.0 nm.

Morphology of Polystyrene Core–dPoly(ethylene oxide) Shell Arborescent Copolymer Micelles from Small-Angle Neutron Scattering Analysis

Arborescent (dendrigraft) copolymers with a branched polystyrene (PS) core grafted at the chain termini with deuterated poly(ethylene oxide) segments (PS-dPEO) were characterized in benzene and acetone by small-angle neutron scattering measurements using the contrast matching technique. While copolymers incorporating a G1 (twice-grafted) PS core aggregated to some extent, the portion of the scattering curve corresponding to non-aggregated copolymer molecules could still be analyzed to determine the shape and segment radial density profile for core and shell. These were derived from the pair distance distribution function P(r) and the scattering length density contrast profile Δρ(r) = ρ(r) − ρ(solvent), obtained by the indirect Fourier transformation and deconvolution methods. The profiles obtained for the G1 copolymer are consistent with a well-defined PS core–dPEO shell morphology, only observed previously for upper generation (G4) polymers with deuterated PS (dPS) chains grafted randomly on arborescent PS substrates. Detailed morphological analysis could not be carried out for an analogous G3 arborescent PS copolymer terminally grafted with dPEO segments due to extensive aggregation in both solvents.

Intrinsic Parameters for the Structure Control of Nonionic Reverse Micelles in Styrene: SAXS and Rheometry Studies

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 (N(agg)) has been determined. It was found that diglycerol monocaprate (C(10)G(2)), diglycerol monolaurate (C(12)G(2)), and diglycerol monomyristate (C(14)G(2)), spontaneously self-assemble into reverse micelles in organic solvent styrene under ambient conditions. The micellar size and the N(agg) 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 (W(s)) in the C(10)G(2) system, which eventually formed rodlike micelles at W(s) ≥ 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 N(agg). Zero-shear viscosity was found to decrease with surfactant chain but increase with W(s), supporting the results derived from SAXS.

SAXS Study on Azithromycin Loaded Nonionic Microemulsions

Abstract Structure of water/propylene glycol/ethoxylated mono-di-glyceride/peppermint oil/ethanol microemulsions solubilizing azithromycin has been investigated at room temperature by small-angle X-ray scattering (SAXS) technique. The scattering data were evaluated by Indirect Fourier Transformation (IFT) or Generalized Indirect Fourier Transformation (GIFT) methods depending on the aqueous phase content. The growth of the microemulsions droplets by increasing the water content and the effect of azithromycin incorporation in the microemulsions were investigated. The Indirect Fourier Transformation gives the real space pair-distance distribution function: a facile way for the quantitative estimation of structure parameters of the microemulsions. It was found that the size of the microemulsion aggregates or repeating distances in the microemulsion increases with the aqueous phase content. The maximum diameter of the drug free and drug loaded microemulsions aggregates depend on their microstructure. The values of the effective interaction radius of the microemulsions are higher for the drug free compared to the drug loaded microemulsions aggregates.

Reverse micelle microstructural transformations induced by oil and water

Effects of alkyl chain length of oil and water on the shape, size, and internal structure of glycerol monooleate (GMO) reverse micelle have been investigated using small-angle X-ray scattering (SAXS) and rheometry techniques. The generalized indirect Fourier transformation (GIFT) method has been employed to analyze the SAXS data, also supported by the direct modeling. It has been found that under ambient conditions the GMO spontaneously self-structure into an ellipsoidal prolate type micelle with a maximum size of ca. 5.7 nm in n-hexane and the size increases in parallel to the chain length of oil reaching a size of ca. 10.7 nm in n-hexadecane. However, the micellar core radius remains essentially the same, i.e., the growth is one dimensional. The increasing tendency of zero-shear viscosity, η0, with the chain length of oil further confirmed the micellar growth. The relative viscosity, ηr, of the reverse micelles was higher and exhibited a steeper solvent (chain length) dependence behavior than those predicted for a dispersion of spherical particles based on the Krieger–Dougherty relation sustaining the SAXS results. The micelles swell with water causing a significant micellar growth; the maximum size and core radius of 0.3% water incorporated reverse micelles were found to be respectively 53%, and 28% larger than the empty micelles.

SAXS and Rheometry Studies of Diglycerol Monolurate Reverse Micelles in Styrene

We have investigated the structure and rheology of diglycerol monolaurate (C(12)G(2)) reverse micelles in styrene using small-angle X-ray scattering (SAXS) and rheometry techniques. The SAXS data have been evaluated by generalized indirect Fourier transformation (GIFT) method and further supported by geometrical model fittings. We found that the C(12)G(2) when added into styrene spontaneously self-assemble into ellipsoidal prolate type reverse micelles under ambient conditions. Micelles grew and micellar aggregation number (N(agg)) increased with the increase in surfactant concentration demonstrating concentration induced one dimensional micellar growth, which was further supported by rheology data; the relative viscosity, η(r), for the C(12)G(2)/styrene mixtures exhibit far steeper concentration dependence behavior than those predicted for a dispersion of spherical particles based on the Krieger-Dougherty relation, due to the elongated micellar structures. On the other hand, a distinct modulation in the shape and size of micelles favoring ellipsoidal prolate-to-sphere type transition was observed upon heating, whose scheme could be attributed to the enhanced penetration tendency of oil into the lipophilic shell of the surfactant at the higher temperatures. As anticipated reverse micelles swelled with water causing two dimensional micellar growth; both the maximum dimension and maximum core radius increase with water.

Structural Characterization of Nonionic Surfactant Reverse Micelles in Diglycerol Monolaurate/Squalene System

Structure of nonionic surfactant diglycerol monolaurate (C12G2) reverse micelles in squalene at different surfactant concentrations and temperatures is presented. For the structural characterization of the micelles, small-angle X-ray scattering (SAXS) technique is used. The scattering data is evaluated by virtually model free generalized indirect Fourier transformation (GIFT) method. GIFT analysis of the SAXS data has shown the clear evidence on the formation of rod-like micelles in the C12G2/squalene system at higher temperatures. At fixed temperature, increasing surfactant concentration induces one-dimensional micellar growth; the maximum length of the micelles increases with concentration, but the micellar cross-section remains essentially the same. On the other hand, at fixed concentration, the maximum length of the micelles decreases with the rise of temperature. It is found that the length of micelles decreases by ~ 35% keeping cross-section diameter unchanged with the rise of temperature from 60 to 80 °C in 5 wt% wt% C12G2/squalene system.

Structure of Polyglycerol Oleic Acid Ester Nonionic Surfactant Reverse Micelles in Decane: Growth Control by Headgroup Size

The structure of polyglycerol oleic acid ester nonionic surfactant micelles in n-decane has been investigated at room temperature by small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and densiometry techniques. The scattering data were evaluated by indirect Fourier transformation (IFT) or generalized indirect Fourier transformation (GIFT) methods depending on the volume fractions of the surfactants and also by model fit. A simple route to the growth control of reverse micelles by headgroup size of the surfactant was investigated. Additionally, the dependence of reverse micellar structure (shape and size) on temperature, composition, and added water was also investigated. The indirect Fourier transformation gives the real space pair-distance distribution function, p(r): a facile way for the quantitative estimation of structure parameters of the aggregates. It was found that the size of the reverse micelles increases with increasing the headgroup size of the surfactant. Globular type of micelles with maximum diameter ca. 6 nm observed in the monoglycerol oleic acid ester/decane system at 25 degrees C transferred into elongated prolate type micelles with maximum diameter ca. 19.5 nm in the hexaglycerol oleic acid ester/decane system. In a particular surfactant and oil system, increasing temperature decreased the micellar size. The size of the micelle was decreased by approximately 25% upon increasing temperature from 25 to 75 degrees C in the 5 wt % diglycerol oleic acid ester/decane system. Concentration could not modulate the structure of micelles despite a wide variation in the surfactant concentration (5-25 wt %). Nevertheless, increasing surfactant concentration reduces the intermicellar distance, and a strong repulsive interaction peak was observed in the scattering curves at higher surfactant concentrations. Besides, the results obtained from the dynamic light scattering have shown the signature of diffusion hindrance relative to hard sphere with the surfactant concentration. Interestingly, the reverse micelles of the 10 wt % diglycerol oleic acid ester/decane system could incorporate approximately 1.2% water in the micellar core and cause a dramatic growth to the micelles size. The size of the water swollen micelles was approximately 40% bigger than the empty micelle.

Structure of Diglycerol Polyisostearate Nonionic Surfactant Micelles in Nonpolar Oil Hexadecane: A SAXS Study

Using a small-angle X-ray scattering technique, shape and size, and internal structure of diglycerol polyisostearate nonionic surfactant micelles in nonpolar oil n-hexadecane (HD) were investigated at 25 degrees C. Furthermore, the effect of added water on the structure of host reverse micelles was also investigated. The scattering data were evaluated by the generalized indirect Fourier transformation (GIFT) method and model fittings. It was found that diglycerol polyisostearate (abbreviated as (iso-C18)nG2, where n=2-4 represent the number of isostearate chain per surfactant molecule) spontaneously form reverse micelles in HD at 25 degrees C and their geometry (shape and size, and internal structure) could flexibly be controlled by a small change in the lipophilic tail architecture of the surfactant, temperature, and water addition. Increasing number of isostearate chain per surfactant molecule decreases the micelles size favoring prolate-to-sphere type transition. This phenomenon could be best understood due to voluminous lipophilic part of the surfactant. Increasing temperature decreases the size of the reverse micelles due to enhanced inter-penetration of the surfactant chain and the oil and also due to dominant hydrophobic character of the surfactant at higher temperatures. In the studies of effect of added water on the structure of micelles, it was found that the reverse micelles swell with water causing two dimensional micellar growths.

Structural Investigation of Diglycerol Polyisostearate Reverse Micelles in Organic Solvents

The structure of glycerol-based reverse micelles in the surfactant/oil binary system without external water addition has been investigated using a small-angle X-ray scattering technique, and different tunable parameters for the structure control of reverse micelles are determined. The scattering data were evaluated by the generalized indirect Fourier transformation (GIFT) method and complemented by model fitting. It was found that diglycerol polyisostearates (abbreviated as (iso-C18)nG2, n=2-4, where n represents the number of isosterate chains per surfactant molecule) form reverse micelles in a variety of organic solvents such as cyclohexane, n-decane, and n-hexadecane without the addition of water from outside, and their structure (shape and size) depends on solvent properties (alkyl chain length), tail architecture of the surfactant, temperature, and added water. Small globular types of micelles were observed in the (iso-C18)2G2/cyclohexane system at 25 degrees C. The micellar size and the aggregation number were increased with increasing the alkyl chain length of the oils resulting in elongated ellipsoidal prolate or rodlike type micelles in the (iso-C18)2G2/hexadecane system. This structural evolution is caused by the different penetration tendency depending on the chain length of oils to the lipophilic chain of the surfactant. At fixed oil, composition, and temperature, the tail architecture of the surfactant played a crucial role in the micellar structure. The micellar size and, hence, the aggregation number decreased monotonically with increasing number of isostearate chain per surfactant molecule due to the voluminous lipophilic part of the surfactant. Composition could not modulate the structure of micelles but led to strong repulsive interactions among the micelles due to reduced osmotic compressibility of the system at higher concentrations. Increasing temperature decreased the micellar size, while the cross-section structure remains essentially the same. The structure was modified significantly in terms of micellar size and cross-section diameter upon solubilization of traces water in the surfactant/oil/water system. Both the maximum size and the cross-section diameter of the micelles increase with water; i.e., reverse micelles swell with water forming a water pool in the micellar core. Furthermore, from the results of model fittings, it was found that the aggregation number increases with water concentration.

Structure of Nonionic Surfactant (Glycerol α-Monomyristate) Micelles in Organic Solvents: A SAXS Study

In this paper we present the study of reverse micellar aggregates formed by glycerol alpha-monomyristate (C(14)G(1)) in different organic solvents. We have investigated the structure (mainly shape and size) of the monomyristin reverse micelles depending on alkyl chain length of the oils, temperature, and the surfactant concentration. Moreover, we present how addition of trace polar additives such as water or glycerol and the hydrophilic size of the surfactant affect the structure of the micelles. Monomyristin could not form any self-organized structure in nonpolar media at room temperature because of its smaller headgroup size and tends to separate from the nonpolar media. In dilute regions this surfactant forms a solid dispersion, which upon heating transforms into an isotropic reverse micellar solution. For the structural characterization of the reverse micelles formed at elevated temperatures, we have performed small-angle X-ray scattering (SAXS) measurements, and the data were analyzed utilizing the generalized indirect Fourier transformation (GIFT) method. The SAXS data have shown that the size of the reverse micelles increases with the alkyl chain length of oils and surfactant concentration but decreases with temperature. A dramatic growth to the micelles could be achieved when the hydrophilic size of the surfactant is increased. Furthermore, addition of trace water or glycerol induces a significant change in the structure of micelles in terms of a cross-section and maximum length. The added water tends to form a water pool in the micellar core and results in swollen micelles. Model-free cross-section analysis of the SAXS data provided complementary information on the structural parameters. Thus, the present study has highlighted the possible ways of controlling the structure, mainly the shape and the size of the inverse micelles.

Tunable Parameters for the Structural Control of Reverse Micelles in Glycerol Monoisostearate/Oil Systems: A SAXS Study

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 degrees C. At lower temperatures (<20 degrees C) II phases (dispersion of solid or liquid crystal phase) has been observed. SAXS data were evaluated by the generalized indirect Fourier transformation (GIFT) method, which has drawn a clear picture on the structural variations of the reverse micellar aggregates. Small globular types of micelles are found in the iso-C18G1/cyclohexane system. On the other hand, elongated ellipsoidal prolatelike or rodlike micelles are found in iso-C18G1/decane or iso-C18G1/hexadecane systems. The underlying mechanism of this structural evolution may be explained in terms of the transfer free energy of hydrophilic glycerol moiety from hydrophilic to hydrophobic environment of oils with different chain lengths. Besides, the penetration of oils to the lipophilic chain of the surfactant in reverse micellar systems differs depending on the chain length of oils. Lowering temperature and increasing surfactant concentration similarly lead to micellar growth while the cross-section structure remains essentially unchanged. Addition of trace water induced micellar growth, which is accompanied by the rapid swelling of the micellar core. The results obtained by this study demonstrate that the solvent nature, temperature, composition, and water addition can be the tunable parameters for the size, shape, and internal structure control of the iso-C18G1-based reverse micelles.