Composition and Size of Reverse Micelles of Tergitol NP-4 and

Publication Date (Web): February 10, 2014 ... In simple microemulsions of Tergitol NP-4 crystallization begins well before that in mixed microemulsion...
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Composition and Size of Reverse Micelles of Tergitol NP‑4 and Tergitol NP‑4 + AOT in N‑Decane during Evaporation Crystallization of KNO3 Alexander I. Bulavchenko, Darya I. Beketova,* Tatyana Yu. Podlipskaya, and Marina G. Demidova Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences 3, Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia S Supporting Information *

ABSTRACT: We have determined the solubilization and crystallization fields of KNO3 for simple (Tergitol NP-4 and AOT) and mixed (Tergitol NP-4 + AOT) microemulsions at 35 °C, depending on water content and concentration of KNO3. An increase in the content of AOT in mixed microemulsions leads to widening of the solubilization field, but the crystallization field narrows up to complete disappearance (in simple microemulsions of AOT). Fourier transform-infrared spectroscopy, photon−correlation spectroscopy, and flame photometry were used to determine the rate of water evaporation (2.6−7.4 μL/min), hydrodynamic diameter of micelles (4.3−10 nm), and the content of water and K+ and Na+ ions in the studied microemulsions at different stages of evaporation crystallization of KNO3. In simple microemulsions of Tergitol NP-4 crystallization begins well before that in mixed microemulsions; in mixed microemulsions the beginning of crystallization requires higher supersaturation (up to 13 M) compared to simple microemulsions (∼4 M at initial solubilization capacity of 0.5 vol %). In accordance with the Gibbs−Thomson (Kelvin) theory, high supersaturations suggest that in mixed micelles cores of smaller sizes are formed. During crystallization, micelles with KNO3 solution coexist with KNO3 crystals. The function of micelles is to supply “building material” for their growth. Numerical simulation with the help of direct optimization showed that in mixed micelles at low water contents, Na+ ions are stronger bound with AOT−, owing to which Na+ ions occur only as impurities in powders.

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INTRODUCTION Evaporation crystallization is a simple and available procedure for producing various powder materials with particles of a specified size.1 On crystallization of water-soluble salts, supersaturation is mostly achieved by evaporation of the solvent (i.e., water). A promising alternative for developing mass crystallization is the use of various types of emulsions and microemulsions, direct, reverse,2 and multiple,3 instead of water. The use of more complex structurally organized systems is reasonable because first, lower water content results in a shorter time of crystallization and second, emulsion drops create a limiting volume and allow the size and morphology of formed crystals to be controlled. In our work,4 powdered potassium and ammonium nitrates were produced by evaporating water from reverse microemulsions formed by nonionic surfactant Tergitol NP-4 in decane. It was shown that crystallization requires distillation of a small volume of solubilized water pseudophase (only 1−2 vol % of the total volume). Microemulsion after evaporation of water and precipitation of powder can be used repeatedly without additional treatment. Introduction of anionic AOT into the microemulsion allowed production of ultradispersed KNO3 powders with whisker-type particles, which have been described using a complex of physicochemical procedures.5 © 2014 American Chemical Society

This paper is a continuation of research published in ref 5, and the aim of the work is to establish the mechanism of evaporation crystallization in reverse microemulsions. As the problem of crystallization of salts in restricted volumes is complicated and multiform, in this study we made attempts to solve the following specific tasks: (1) to determine the regions of solubilization and crystallization for simple (Tergitol NP-4 and AOT) and mixed (Tergitol NP-4 + AOT) microemulsions in decane, (2) to evaluate the evaporation rate of water from microemulsions of varying composition and volume, (3) to study the dynamics of changes in size and composition of simple and mixed micelles at the beginning of and during crystallization, and (4) to estimate the possibility of exchange processes (K+ ↔ Na+) between the surface layer and core of mixed micelles.

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EXPERIMENTAL SECTION

Materials. We used n-decane, n-heptane, and potassium nitrate (99%). Decane was dried using aluminum oxide and twice distilled in dust-free equipment. Oxyethylized nonylphenol with the oxyethylation Received: October 29, 2013 Revised: January 28, 2014 Published: February 10, 2014 1142

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was kept with an accuracy of 0.1 °C; inaccuracy of the measurement of hydrodynamic radius was no more than 5%. Potassium and sodium concentration were determined by a procedure specially developed for microemulsion systems. Two aliquots of 1.0 mL were placed into quartz cups with a capacity of about 20 mL. Then the samples were heated in a muffle furnace to evaporate the solvent (T = 100−150 °C) and to decompose the surfactant (T = 200−550 °C). When analyzing the AOT solutions, NH4NO3 was used as an oxidizing reagent. The decomposition products were dissolved with hot 0.3 M HNO3 and transferred into glasses and flasks with a capacity of 10−50 mL. A blank experiment was subjected to all steps of analysis. Correctness of the analysis was confirmed by the “added-found” method for different salt concentrations. The error of procedure was 2−4% (n = 5; P = 0.95). Potassium and sodium content were determined on the atomic absorption spectrometer Z-8000 in an air-acetylene flame (λ = 766.5 nm). Diffraction study was carried out at the station of the 4th channel of VEPP-3 accumulator (Siberian Synchrotron and Terahertz Radiation Centre, Budker Institute of Nuclear Physics of the Siberian Branch of Russian Academy of Sciences, Novosibirsk).8 Radiation with the wavelength of 0.3685 Å was used. Diffracted radiation was registered using the detecting system mar345. A liquid sample was placed into a thin capillary produced by Hildenberg.

degree of four Tergitol NP-4 (Dow Chemical Company) served as the micelle-forming surfactant, and AOT [sodium bis(2-ethylhexyl) sulfosuccinate] produced by Sigma Aldrich (96%) was used as the anionic surfactant. The concentration of water solutions of KNO3 varied from 0 to 2.0 M, whereas surfactant concentration in decane was stable and equal to 0.25 M. Solubilization of water and water solutions of KNO3 was performed using the procedure of successive injections. The value of solubilization capacity varied from 0.5 to 4.0 vol %. Experimental Technique. Study of the evaporation rate of water (T = 20−45 °C) and crystallization of potassium nitrate (T = 35 °C) from microemulsions was carried out in an open thermostatted cell at constant temperature by stirring with a magnetic agitator at a rate of 150 rpm. Microemulsion volume was 10 and 20 mL; during evaporation of water, after a definite time, the stirring was stopped and aliquots were picked up to determine micelle sizes by photoncorrelation spectroscopy (PCS), content of water (IR), and potassium and sodium (flame photometry). The solutions in which crystallization started were preliminarily centrifuged for 5 min at a rate of 1500 rpm to remove the precipitated salt. For each determination, 2−5 parallel samples were prepared. Water content during evaporation and crystallization of salt was determined by IR-Fourier spectroscopy. IR spectra were registered on the IR-Fourier spectrometer Scimitar FTS 2000. Spectra of anhydrous and water-containing microemulsions of Tergitol NP-4 were registered in the CaF2 cells (l = 0.11 mm) in the region of 4000−940 cm−1. To determine the concentration of water, we used the stretching band ν(OH) in the region of 3600−3200 cm−1. The stretching band of the OH-group of surfactant was compensated by subtracting the areas of ν(OH) of initial “dry” reverse micellar solution. For calibration dependences (Figure 1, panels a and b), microemulsions of Tergitol

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RESULTS AND DISCUSSION Solubilization and Crystallization Regions. When determining the maximum solubilization capacity, we have established the solubilization fields of KNO3 at 35 °C for three solutions of surfactant in decane: 0.25 M Tergitol NP-4, 0.24 M Tergitol NP-4 + 0.01 M AOT, and 0.25 M AOT (Figure 2, regions 1 + 3). Microemulsions on the basis of AOT have a high capacity relative to water. An increase in salt concentration results in a decrease in solubilization capacity for all systems; microemulsions based on oxyethylized Tergitol NP-4 and a mixture of Tergitol NP-4 + AOT are more tolerant to salt

Figure 1. (a) IR spectra of microemulsions Tergitol NP-4 + 2 M KNO3 during evaporation of water (T = 35 °C), (b) dependencies of integral absorption area of the OH group on solubilization capacity of microemulsions AOT, Tergitol NP-4 + AOT, and Tergitol NP-4 for H2O.

NP-4, Tergitol NP-4 (0.24 M) + AOT (0.01 M), and AOT were used, which contained 2 M KNO3 in decane at different solubilization capacities (0.5−2.0 vol % of 2 M KNO3 or H2O). The total content of surfactants was 0.25 M. The error of procedure was 3−5%, and the limit of detection was 0.05 vol %. The hydrodynamic diameter of micelles (Dh) was determined by PCS. Prior to the measurements, the microemulsions and the initial “dry” micellar solutions were dedusted by 5-fold cyclic filtering through a membrane filter (Pall) with an average pore diameter of 0.2 μm. The measurements were performed in 1 cm quartz cells on the 90Plus spectrometer produced by Brookhaven Instruments. The power of solid-state laser (Lasermax) was 35 mW. Scattered photons were accumulated using a highly sensitive APD detector (PerkinElmer). The z-averaged hydrodynamic diameter6 was calculated by the Stokes−Einstein equation for spherical particles as the average of 10 measurements.7 Photons for one measurement were accumulated for 10 s; soaking time for each temperature was 4 min. The temperature

Figure 2. (1) Regions of KNO3 crystallization and (1 + 3) solubilization of aqueous solutions of KNO3 and (2) microemulsion with reverse emulsion for microemulsions of the following composition: (a) 0.25 M Tergitol NP-4, (b) 0.24 M Tergitol NP-4 + 0.01 M AOT, and (c) 0.25 M AOT (T = 35 °C). 1143

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solutions. In the regions of compositions 2, there coexist microemulsion and reverse emulsion (when minor contents of excess water phases are added). Thereafter, microemulsion crystallization zones were determined. We think that the beginning of crystallization is the moment when the solution grows drastically turbid during evaporation of water, which is observed either visually or with the help of a device. For the latter case, a spectrophotometer was used to register the decrease in the intensity of transmitted light and the static and dynamic scattering spectrometer4 was used to measure the increase in the intensity of light scattering at an angle of 90°. The upper crystallization boundary is restricted to a solubilization line. To establish the lower boundary, crystallization was conducted at a constant salt concentration, starting from the maximum initial solubilization capacity to its gradual decrease. In region 3, turbidity of solution was not observed during 2 h of mixing and further during more than one week of storage. In Figure 2, the established crystallization regions are designated by 1 and shaded. It is worth noting that AOT additives somewhat narrowed the crystallization field. In the case of microemulsions on the basis of AOT, no crystallization was observed at any concentrations of KNO3 and solubilization capacities. The curve bounding the lower crystallization region is welldescribed by the relationship Vs/Vo × 10−2 × cs = co = const, where const = 0.3 and 0.5 M for all systems of Tergitol NP-4 and Tergitol NP-4 + AOT, respectively (co is the salt concentration in terms of the whole microemulsion volume). Most likely, crystallization begins only when salt concentration exceeds some critical concentration, depending on the content of a specific system. In the regions where crystallization is absent, transparent “dry” micellar solutions containing salt are formed as water evaporates. At this stage of research, we do not know in what forms it occurs (amorphous or crystalline nuclei, ionic pairs (molecules) uniformly distributed in micelles). Synchrotron radiation diffraction study showed the lack of any reflexes typical of KNO3 crystals. This fact, however, might also suggest a low sensitivity of the procedure, resulting from low concentration of salt (co = 2 × 10−3 M). Total lack of crystallization at evaporation of water from AOT microemulsions agrees with the observations of the author of the procedure for producing “free” nanoparticles of water-soluble salts.9−12 Nanoparticles (or rather a composite consisting of nanoparticles in the matrix of “dry” AOT) were produced under total evaporation of water and volatile solvent (hexane or toluene). However, this procedure cannot be attributed to a microemulsion procedure because, as it was fairly mentioned in ref 13, during the evaporation of solvent, a microemulsion system passes into a liquid-crystal form. Rate of Water Evaporation from Microemulsions. To choose optimal crystallization conditions, in the beginning we studied the rate of water evaporation from microemulsions. Dynamics of water evaporation from Tergitol NP-4 microemulsions was determined, depending on solubilization capacity (0.5−4 vol %), concentration of KNO3 (0−2 M), temperature (20−45 °C), and volume (20 and 10 mL) of microemulsion. Figure 3 shows experimental dependencies for some studied systems obtained from the data of IR spectrometry at evaporation of water from microemulsion with the volume of 20 mL. The rate of water evaporation from microemulsions was constant, increased with growing temperature, and was virtually independent of KNO3 concentration

Figure 3. Evaporation of water from 20 and 10 mL microemulsion containing 0.25 M Tergitol NP-4 and 2 vol % H2O at different temperatures.

and solubilization capacity. At equal rotation rates of magnetic stirrer, water from the 10 mL of emulsion evaporates faster than from 20 mL; when more than 70% water evaporates, the dependence stops being linear. This seems to be related to the fact that the intensity of mixing of the upper layer decreases with the increasing height of microemulsion column, and the “bound” water is removed from the emulsion with more difficulty than the “free” water. Figure 4 shows dependencies of the dynamics of water evaporation from 10 mL of microemulsions of different

Figure 4. Evaporation of water from microemulsions containing 0.25 M Tergitol NP-4, 0.24 M Tergitol NP-4 + 0.01 M AOT, and 0.25 M AOT at 35 °C. In all cases, the volume equals 10 mL; all microemulsions contained 1 vol % of 2 M KNO3.

compositions (Tergitol NP-4, AOT, and Tergitol NP-4 + AOT). Table 1 lists the values of interfacial tension, hydrodynamic diameter of micelles and evaporation rates for studied systems calculated from the inclination of lines from Figure 4. They evidence that the rate of water evaporation is virtually independent of the composition of microemulsions (for studied systems), which is probably due to the close values of interfacial intension and hydrodynamic diameter. It is interesting that the evaporation rate of water molecules from surfaces of water and aqueous solution of KNO3 is only 2−2.5 times higher than from the surface of microemulsion, though water content in the surface layer of microemulsion is 2 orders of magnitude lower, and the estimated area of evaporation surface occupied by water drops is 3 orders of magnitude smaller. When studying the rate of water evaporation from microemulsion of AOT under “controlled gas flow conditions”, it was nearly 2 orders of magnitude lower than that from the water surface.14 In further study of the crystallization process, 1144

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Table 1. Interfacial Tension, Hydrodynamic Diameter of Micelles and Rate of Water Evaporation for 10 mL of Water, Aqueous Solution of KNO3 and Microemulsions Containing 0.25 M Tergitol NP-4, 0.24 M Tergitol NP-4 + 0.01 M AOT, and 0.25 M AOTa system water aqueous solution of 2 M KNO3 Tergitol NP-4 0.24 M Tergitol NP-4 + 0.01 M AOT AOT a

Table 2. Time of the Beginning of Crystallization (the Point of Turbidity), Solubilization Capacity, Hydrodynamic Diameter, Diameter of Polar Micelle Core and Salt Concentration in Simple and Mixed Microemulsions Depending on Initial Solubilization Capacity Vs/Vo and Wo (ratio of initial water content to surfactant, [H2O]/ [surfactant]) (T = 35 °C)

interfacial tension (mN/ m)

hydrodynamic diameter (nm)

evaporation rate (μL/min)

− −

− −

7.4 6.1

Vs/Vo (vol %)

Wo

1.1 1.2

9.8 9.1

2.8 2.6

1.1

4.3

2.3

0.50 0.75 1.00 1.25 1.50 2.00

1.1 1.65 2.2 2.75 3.30 4.4

0.50 0.75 1.00 1.50 2.00

1.1 1.65 2.2 3.3 4.4

Solubilization capacity of microemulsions is 1 vol % (T = 35 °C).

we worked with 10 mL of microemulsion because with this small volume the start time of crystallization is considerably reduced. Beginning of Crystallization. In our previous work,5 we showed that at the same salt concentration and solubilization capacity, only powders with micrometer-sized particles form in the case of simple microemulsions and whisker-type particles 3−4 μm long and 200−300 nm thick in the case of mixed microemulsions. A decrease in solubilization capacity in the latter case to 0.5 vol % resulted in a considerable reduction of particles sizes. Figure 5 shows a variation in salt concentration

time (min)

Vs/Vo (vol %)

W

Dh (nm)

0.25 M Tergitol NP-4 9 0.23 0.51 6.9 10 0.37 0.81 7.4 10 0.50 1.10 7.9 9 0.67 1.47 8.6 9 0.86 1.89 9.3 13 1.15 2.53 10.4 0.24 M Tergitol NP-4 + 0.01 M AOT 35 0.07 0.15 6.2 18 0.13 0.29 6.3 14 0.57 1.25 8.0 12 0.92 2.02 9.2 10 1.34 2.95 10.6

dpol (nm)

C (KNO3) (M)

1.3 1.8 2.3 3.0 3.7 4.8

4.4 4.1 4.0 3.7 3.5 3.5

0.6 0.8 2.4 3.6 5.0

12.9 8.9 4.1 3.3 3.0

Figure 6. Concentration of KNO3 in polar cavities of mixed micelles in the point of turbidity, depending on the diameter of polar micelle core on semilogarithmic scales. Plotted using data from Table 2

pseudophase, the beginning of crystallization in mixed microemulsions requires considerably higher supersaturations. Crystallization Process. During crystallization, part of the salt precipitates as a solid phase and the content of salt in microemulsion decreases. Compositions of microemulsion phases during crystallization are reported for simple and mixed microemulsions in Table 3. In both cases, the initial solubilization capacity was 1 vol %. In simple and mixed micelles, the hydrodynamic diameter and the contents of water and KNO3 continuously decrease and the content of Na+ ions remains the same (within estimation error). The concentration of KNO3 in micelles during crystallization is no more than 4 M. Only at the final stages, when crystallization does not occur any longer does its concentration drastically increase. In the first case, about 10% KNO3 remains in the micelles, whereas in the second, nearly two times more. As by this moment virtually no water remains, residual salt in micelles is present either as molecules (ionic pairs) or as nanoparticles (amorphous or crystalline). Diffraction study using synchrotron radiation also showed lack of any reflexes typical of KNO3 crystals. In comparison with Tergitol NP-4 + AOT, crystallization of KNO3 in Tergitol NP-4 begins earlier. At one and the same

Figure 5. Change in solubilization capacity and concentration of KNO3, depending on time during evaporation of water for microemulsion containing 0.24 M Tergitol NP-4 + 0.01 M AOT and 0.5 vol % 2 M KNO3 (T = 35 °C).

during evaporation of water from microemulsion containing 0.5 vol % water pseudophase up to the turbidity point. It is worth noting that in simple microemulsions, crystallization begins at 9 min at the attainment of the salt concentration close to the saturated concentration at 35 °C (∼4.2 M). In mixed microemulsions, turbidity appears only after 35 min at a KNO3 concentration of about 13 M. Further, by the same procedure, we determined the composition for simple and mixed microemulsions in the turbidity point at constant initial 2 M salt concentration and solubilization capacities of 2, 1.5, 1, and 0.75 vol % (see Table 2 for data). Figure 6 shows salt concentration in micelles, depending on the diameter of polar micelle cavity (on semilogarithmic scale as well) at the turbidity point. Thus, at high solubilization capacities, crystallization in simple and mixed microemulsions begins at concentrations close to saturated; at low similar contents of water 1145

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a number of univalent cations by Na+ is shown experimentally in ref 12, when producing chloride by the procedure of complete evaporation crystallization from AOT microemulsions. The authors think that the replaceability is explained by the more than fifty-fold excess of NaAOT concentration compared to salt concentration introduced into the microemulsion. Therefore, initial data evidence that K+ ions are stronger bound with AOT−, and the NaNO3 salt must crystallize, which is in conflict with our experimental data. Due to the abovementioned facts, we carried out a more detailed study of this problem, while considering the following two factors: solubility of NaNO3 in water, which at 35 °C equals 11.8 M, and the Gibbs energy of exchange reaction:

Table 3. Changes of H2O, K+, and Na+ Concentration and Hydrodynamic Diameter of Micelles during Evaporation of Water in Microemulsions (T = 35 °C) Ct/Coa (%)

time (min)

Vs/Vo (vol %)

0 5 10 15 25 40 70

1.04 0.81 0.66 0.37 0.18 0.05 ≤0.05

0 7 14 21 28 35 50 70 a

1.06 0.87 0.64 0.40 0.22 0.08 0.08 0.05

W

Dh (nm)

H2O

K+

0.25 M Tergitol NP-4 2.29 10.0 100 100 1.78 9.1 78 97 1.45 8.5 63 90 0.81 7.4 36 61 0.40 6.7 17 33 0.11 6.2 5 12 0.11 6.2 5 11 0.24 M Tergitol NP-4 + 0.01 M 2.55 10.0 100 100 1.91 9.0 75 99 1.41 8.2 55 94 0.88 7.4 34 61 0.48 6.7 19 46 0.18 6.3 7 37 0.18 6.3 7 33 0.11 6.2 4 27

Na+ − − − − − − − AOT 100 107 113 106 101 96 98 99

CK in polar cavity (M)

CNa in polar cavity (M)

1.9 2.4 2.7 3.3 3.7 4.8 ≤4.8

− − − − − − −

1.9 2.3 3.0 3.1 4.2 9.3 8.3 10.8

NaAOT + K+ × nH 2O ↔ KAOT + Na + × nH 2O

Let us consider the first simpler factor. Assume that Na+ ions passed into the micelle core and totally replaced K+ ions. Calculated values of the concentrations of Na+ ions in terms of the volume of water pseudophase are listed in the last column of Table 3. The reported data suggest that an 11.8 molar concentration corresponding to saturated NaNO3 solutions at 35 °C can be achieved only at the end of the process, when, as a result of evaporation, solubilization capacity will amount to no more than 0.1 vol %. Results obtained in this work, evidence that at these low water and AOT contents, salt crystallization in mixed micelles is complicated. Now let us consider the possibility of ion exchange. Our calculations conducted using the direct optimization method (see the Supporting Information) agree with data from refs 15 and 16 and evidence that at a high content of water pseudophase, the difference in hydration energies of exchanging ions exceeds the difference in Coulomb energies. Gibbs energy of exchange reaction is negative and sodium ions pass into the micelle core, whereas potassium ions pass into the surface layer. However, with the decreasing content of water molecules in micelles during evaporation, the difference in Coulomb interaction becomes determinative. Gibbs energy becomes positive (Na+ ions might approach closer to AOT− ions than K+), and a reverse transition takes place. Dependencies of Na+ concentration on solubilization capacity calculated from ΔG are given in Figure 7. The concentration of sodium ions in the polar core increases no more than 1.7 times and then drops. In case of partial penetration of ions into the surface layer of AOT−, concentration begins to decrease instantly. Thus, at the

0.9 1.2 1.7 2.5 4.3 11 12 19

Co, initial ion concentration.

time of crystallization 15% more salt passes into a solid phase for case of Tergitol NP-4 solution. Crystal growth rates are virtually identical. We made an attempt to measure the size of solid phase particles, beginning from the moment when turbidity appeared, by the PCS procedure. However, we did not manage to perform qualitative measurement. During this estimation at the beginning of crystallization the sizes varied from micelle sizes reported in Table 3 to several micrometers. The estimated size during crystallization was several micrometers, and the intensity of light scattering increased from 20 kcps to 2 Mcps. Thus, during crystallization, crystals coexist with micelles in which KNO3 solution occurs. The role of the micelles is to supply building material for crystal growth. Exchange Processes in Mixed Micelles. From Table 3 one can see that the content of sodium ions in micelles is constant. As it was mentioned,5 with growing concentration of AOT, in mixed microemulsions the concentration of Na+ ions drastically grows as well. For example, at AOT contents 0.01; 0.02 and 0.05 M the concentration of Na+ in terms of the volume of water pseudophase at 1 vol % solubilization capacity is 1, 2, and 5 M, respectively. The concentration of K+ ions is 2 M. In mixed microemulsions of Tergitol NP-4 + AOT, only KNO3 crystallizes; NaNO3 is present in the precipitated powder as an admixture of no more than 3%. This result is inconsistent with the data published in literature. For example, from the results of numerical simulation15,16 carried out using the molecular dynamics method, sodium ions in the surface layer (i.e., bound with AOT−) must be replaced by potassium ions and migrate to the micelle core. Results of experiments conducted with the help of liquid−liquid extraction17 evidence the possibility of exchange. The exchange is favored by the gain in hydration energy. A large K+ ion is hydrated weaker than Na+, and so its migration into the surface layer with a low water content is thermodynamically more preferable. Complete replacement of

Figure 7. Change in the calculated concentration of Na+ in the micelle core during evaporation of water at various concentrations of AOT in microemulsions and distances of counterions from the surface of potential-determining ions (values are given in brackets in this figure). 1146

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The influence of nucleus size is clearly observed on the morphology of grown samples: in mixed microemulsions, the size of particles decreases with decreasing solubilization capacity (see Figure 4 in ref 5). Most likely, the AOT− anion is adsorbed on the nucleus surface and forms the surface layer from hydrocarbon “tails”. They hinder the growth of nuclei and prevent the lateral intergrowth of crystallites because overlapping of adsorption layers results in the occurrence of shortrange repulsive force of osmotic and steric nature. An increase in AOT concentration promotes adsorption, and the described effects become more apparent up to complete inhibition of the crystallization of salt in the simple microemulsions of AOT. A simplified scheme of crystallization in the systems studied can be presented as follows (Scheme 1). At the first stage of

initial stages of evaporation process, the salt concentration is no higher than 2 M, and at the final stage, virtually all sodium ions are totally bound with AOT−. Thus, consideration of both factors (solubility of NaNO3 and Gibbs energy of exchange reaction) leads us to an unambiguous conclusion about the dominating crystallization of KNO3 under studied conditions, which was observed experimentally. Scheme of Crystallization in Simple and Mixed Microemulsions. Admittedly, many models have been developed that describe crystallization processes in the presence of different templates, polymers, and surfactants.18,19 To qualitatively analyze the obtained results, we will use the general scheme of microemulsion crystallization described in ref 20. In conformity with this model, at the first stage nucleation, takes place and at the second stage, the nuclei grow as a result of aggregation. At the third stage, aggregated nuclei agglomerate to form crystallites. The fourth stage is characterized by a self-assembly of crystallites. At the fifth stage, one can observe the side growth of crystallites resulting from intergrowth with nuclei, and at the sixth stage, the crystallites laterally intergrow to form a monolith. At this stage of research, our task is to carry out a preliminary comparative analysis of crystallization processes in simple and mixed microemulsions. The information obtained from the study of compositions and sizes of micelles will, most likely, be useful in the investigation of the nucleation stage. Data of PCS evidence that the size of the nucleus is smaller than that of the polar cavity of micelles because micelles prior and during crystallization have the same hydrodynamic diameter as upon the introduction of the same volumes of water pseudophase into microemulsion. The hydrodynamic diameter of simple and mixed micelles during water evaporation and crystallization changes virtually identically (Table 3). Thus, it is impossible to directly determine the size of nuclei in the studied systems with the help of PCS. However, nucleus size can be judged indirectly from the concentration of supersaturated solution which is in equilibrium with the assembly of nuclei. The concentration of supersaturated solution Cs, equilibrated with the spherical nuclei, is estimated taking into consideration the surface energy of the nucleus by the Gibbs−Thomson (Kelvin) equation:21 ln

Scheme 1. General Scheme of KNO3 Crystallization in Different Microemulsions: 0.25 M Tergitol NP-4, 0.24 M Tergitol NP-4 + 0.01 M AOT, and 0.25 M AOT

Cs 4V σ = M C∞ dRT

where d is the nucleus diameter, Cs is the concentration of crystal-forming species, C∞ is the equilibrium concentration, σ is the interfacial tension (energy) of a particle, VM is the molar volume of liquid or solid body, and R is the universal gas constant. It follows from the equation that the smaller the nucleus radius, the higher supersaturation required (at constant interfacial energy) for the formation of a nucleus (Figure 6). All these facts lead us to a conclusion that in mixed microemulsions of Tergitol NP-4 + AOT at low initial solubilization capacities (and, hence, small volumes of polar cavities), the size of nuclei is smaller than in simple micelles of Tergitol NP-4, since the concentration at the point of turbidity in the former case is 4 and in the latter it is 13 M (Table 2). At high capacities (sizes of polar cavities), the concentrations of supersaturation weakly change and are close to each other (and to the concentration of supersaturated solution), which suggests close nuclei sizes.

evaporation, crystallization in simple and mixed microemulsion nuclei form. Their size decreases with growing content of AOT. Micelles with nuclei coexist with micelles containing a saturated or supersaturated solution of KNO3. At the second stage, nuclei aggregate into crystallites. At the third stage, the side growth of crystallites and their lateral intergrowth takes place. The binding energy of molecules of nonionic Tergitol NP-4 with the surface of crystallites is negligible, which provides a possibility for the side growth of crystallites with the formation of tubular crystals of micrometer size. Micelles containing KNO3 solution coexist with crystallites. They supply KNO3 for crystal growth. Adsorption of AOT− anion results in blocking of lateral growth of crystallites. In simple microemulsions of AOT, the crystallization process is 1147

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Article

restricted to nucleation and finally results in the formation of stable organosols. The size of nuclei is no bigger than the size of the polar cavity of micelles.

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CONCLUSION Thus, the following results were obtained: (1) zones of solubilization and crystallization of KNO3 for simple and mixed microemulsion on the basis of Tergitol NP-4 and AOT in decane were established; (2) rates of water evaporation from microemulsions of various compositions were estimated; (3) variation in the size of micelles and composition of microemulsions during crystallization was found; (4) exchange processes in the Tergitol NP-4 + AOT system were studied; (5) scheme of crystallization in simple and mixed microemulsions was proposed; and (6) it was shown that ultradispersed powders and organosols can be produced from mixed microemulsions consisting of nonionic oxyethylated Tergitol NP-4 and anionic AOT, whereas microemulsions of AOT can yield only organosols.

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ASSOCIATED CONTENT

S Supporting Information *

Initial model, optimization procedures, coulomb interactions, hydration energy and composition of surface layer of mixed micelles, scheme of exchange between surface layer and micelle core, and tables including ion parameters used for calculation of Coulomb energy, values of electrostatic energy of interaction between K+ ions and AOT− in micelle, changes in Coulomb energies, hydration and total energy during evaporation of water from microemulsion, changes in Gibbs energy, equilibrium constants, and composition of surface layer as a result of exchange reactions. 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: +73-833-165349, Fax: +73-833-308248. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was partly supported by Grant 09-03-00511 from the Russian Foundation for Basic Research. We are grateful to Prof. A.Yu. Manakov for the diffraction study of microemulsions with the help of synchrotron radiation.

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REFERENCES

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dx.doi.org/10.1021/cg401610m | Cryst. Growth Des. 2014, 14, 1142−1148