Article pubs.acs.org/crystal
Ion-Exchange Processes between Surface Layer and Core of Reverse Micelles of NaAOT+Tergitol NP‑4 during Evaporation Crystallization of KNO3 Alexander I. Bulavchenko, Darya I. Beketova,* Marina G. Demidova, and Tatyana Yu. Podlipskaya Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3, Acad. Lavrentiev Avenue, Novosibirsk, 630090, Russia S Supporting Information *
ABSTRACT: Liquid extraction and microdroplet probing of the composition of reverse microemulsions (NaAOT+Tergitol NP4, NaAOT, and Tergitol NP-4 in n-decane) with aqueous salt solutions (KNO3, NaNO3, and KNO3+NaNO3) were used to determine the distribution of Na+ and K+ between the surface layer and the micelle core in the processes of evaporation crystallization of KNO3 and NaNO3 salts. It is shown that, at high content of NaAOT, K+ cations in micelle core are virtually completely replaced by Na+ and no crystallization of KNO3 in NaAOT micelles is observed. At low content of NaAOT in mixed micelles and small solubilization capacity (less than 1 vol %) partial exchange takes place and crystallization of KNO3 begins from a mixture of KNO3+NaNO3 salts. During crystallization, micelle cores become depleted in potassium nitrate, which causes reverse exchange of K+ cations, associated with AOT−, for Na+ contained in the micelle core. As a result, mainly KNO3 crystallizes in mixed micelles.
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INTRODUCTION Study of crystallization mechanism for producing ultradisperse powders in the restricted volumes of various nanoreactors and templates is a topical problem.1−8 In our previous work9−11 we carried out the detailed study of the evaporation microemulsion crystallization of KNO3 in simple (NaAOT, Tergitol NP-4) and mixed (NaAOT+Tergitol NP-4) micelles. We managed to reduce the dispersity of powders only in mixed micelles. It is surprising that in mixed micelles only KNO3 crystallized despite the fact that the molecules of NaAOT contain sodium cations which are able to exchange with the potassium ions and pass into the polar core of micelles with formation of nitrate salt solution NaNO3: NaAOTsurface + (KNO3 × nH2O)core → KAOTsurface + (NaNO3 × nH2O)core. At complete exchange in the studied systems, the concentration of Na+ ions in micelle cores may change from 2 to 50 M, which equals or considerably exceeds the content of KNO3 in the initial injected solution (2 M). The possibility of cation exchange in the micelles of NaAOT has been discussed in many publications. For example, numerical simulation by the method of molecular dynamics © 2017 American Chemical Society
in refs 12 and 13, shows that, if the diameter of the solubilized cation is larger than that of Na+, sodium ions bound with AOT− in the surface are replaced by the ions of potassium or cesium and pass to the micelle core. Results of experiments conducted using liquid extraction14 also support the possibility of exchange. The authors think that the exchange results from the gain in hydration energy. Large ions (K+ or Cs+) are hydrated weakly compared to Na+; therefore, its conversion to the surface layer with a low water content is thermodynamically more favorable. Experiments in ref 15 show a complete replacement of some monovalent cations by Na+ in the production of chlorides by the procedure of evaporation crystallization from microemulsions of NaAOT. The authors explain the replacement by a more than 50-fold excess of NaAOT concentration compared to that of salt introduced into the microemulsion. Received: May 23, 2017 Revised: August 30, 2017 Published: August 31, 2017 5216
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additionally dried in a desiccator over P2O5 for a day and the content of ions was determined. To determine sodium and potassium by the ICP AES analysis, two samples about 30 mg in weight were gradually dissolved (5−6 portions of 5 mL) in deionized water and heated to 60−70 °C. The solution was transferred to a flask and diluted with water to 50 mL. When required, the solution was additionally diluted. For the flame photometry determination two sets of reference solutions with sodium and potassium concentration 2−20 μg/mL were used. The main reference solutions were prepared using KAOT (content 90%) and NaAOT (content 96%, Sigma-Aldrich firm) that were synthesized following the procedure in ref 18. The content of the component was proven by the CHN and ICP AES analysis. To calculate the content of NO3− and AOT− in the samples separated from extraction, nitrogen and carbon were determined by CHN analysis (sample weight is 1 mg). It is worth noting that in the liquid extraction by NaAOT solutions, the surface layer exchanges ions as well as salt solutions form the micelle core. Therefore, for the correct estimation of exchange processes, it is also necessary to assess the content of cations in the core of micelles. On the assumption of electric neutrality, the total content of cations in the core was considered to be equal to the content of NO3− anions and the concentrations of cations were supposed to be determined by the composition of feed and equal to each other. This approach allowed estimation of the degree of substitution of Na+ by K+ in the surface layer. Liquid extraction studies can only partly model the exchange processes between the core and surface layer of micelles, which take place during evaporation crystallization, because the volume of the feed significantly exceeds the volume of solubilized aqueous pseudophase. Therefore, we have additionally developed a unique technique for testing the composition of polar cores of microemulsions by water microdrops. Microdrop Probing of Ionic Composition of Micelle Cores. Through a certain volume of microemulsion (25−100 mL) with preliminarily solubilized solution of salts, with the help of a micropipette, 200 μL of pure water was injected as ∼2 μL drops. After sedimentation, the drops combined into a separate water phase at the bottom of a conical test tube. The water phase was separated, dried, and analyzed. The content of sodium and potassium was determined in two solutions using the flame photometry: after dissolving 2−5 mg samples in 6−8 mL deionized water and after diluting it 5−10 times. The concentration of ions in the micelle core obtained from the results of microdrop probing was calculated by the following procedure. Microdrop probing provides a correct ratio of concentration of cations (A) in the micelle core: C(K+)/C(Na+) = A. The total amount of cations is calculated on the assumption of electric neutrality: C(K+) + C(Na+) = 2 (as initially 2 M solution of KNO3 is introduced into the microemulsion). As a result, we have two independent equations with two unknowns. It is worth noting that, at chosen optimal conditions, the microdrops only probe the composition of polar cores of micelles owing to the micellar diffusive transfer of salts, resulting from osmosis. As only pure water is used for probing, the osmotic ionic flow will be directed to the drops and the micelles will preferably transport the cations that are not bound with AOT. The total volume of drops does not exceed 20% of the initial solubilized volume of KNO3 solution and, owing to this specific sampling, no significant shift takes place in the equilibrium of the system. Testing of the procedure on microemulsions of known composition showed (details are provided in Supporting Information) that microdrop probing allows adequate determination of cation concentrations in the polar cores of microemulsion micelles. Crystallization. Further to verify the influence of exchange processes on crystallization, we crystallized KNO3 and NaNO3 and their mixtures from simple and mixed micelles with and without exchange. The crystallization procedure is described in ref 10. The obtained powders were washed 5−6 times in n-heptane, dried for several hours in the air and for 2 days in a desiccator over P2O5.
The aim of this work is to study the ion-exchange processes between the surface layer and the polar core of micelles in the process of water evaporation from microemulsion prior to and during crystallization. To achieve the goal, we had to determine experimentally the concentration of sodium and potassium cations bound with AOT− and those located in the core of micelles in the form of nitrate salts. To solve these tasks we used the following procedures: (1) liquid extraction of cations by microemulsions from salt solutions; (2) water microdrop probing of the composition of polar cores of microemulsions; (3) comparative evaporative crystallization of KNO3 and NaNO3 salts and their mixtures from microemulsions with and without cation exchange.
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METHODS OF RESEARCH
Experimental Section. Materials. We used n-decane, n-heptane, potassium nitrate (99%), and sodium nitrate (99%). N-decane was dried using aluminum oxide and twice distilled in the dust-free equipment. Oxyethylated nonylphenol with the oxyethylation degree of four Tergitol NP-4 (Dow Chemical Co) served as the micelleforming surfactant, and NaAOT (sodium bis(2-ethylhexyl) sulfosuccinate) produced by Sigma-Aldrich (96%) was used as the anionic surfactant. The concentration of aqueous solutions of KNO3 was 2.0 M. The total content of surfactants was 0.25 M. Solubilization of water and water solutions of KNO3 and NaNO3 was performed using the procedure of successive injections. Determination of Micelle Sizes. The hydrodynamic diameter of micelles (Dh) was determined by photon correlation spectroscopy (PCS). This technique is also known as dynamic light scattering (DLS). Prior to the measurements, the microemulsions and the initial “dry” micellar solutions were de-dusted by 5-fold cyclic filtering through a membrane filter (Pall) with an average diameter of pores 0.2 μm. The measurements were performed in 1 cm quartz cells on the 90Plus spectrometer produced by Brookhaven Inst., USA. The power of solid-state laser (Lasermax) was 35 mW. Scattered photons were accumulated using a highly sensitive APD detector (PerkinElmer). The z-average hydrodynamic diameter16 was calculated by the Stokes−Einstein equation for spherical particles as the average of 10 measurements.17 Photons for one measurement were accumulated for 10 s; soaking time for each temperature was 4 min. Temperature was kept with an accuracy of 0.1 °C; inaccuracy of the measurement of hydrodynamic radius was no more than 5%. Determination of Water Content. 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 watercontaining 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 3600−3000 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, microemulsions of Tergitol NP-4, Tergitol NP-4 (0.24 M) + NaAOT (0.01 M), and NaAOT in n-decane were used; water content was 0.5−4.0 vol %. The error of procedure was 3−5% and the limit of detection was 0.05 vol %. Determination of Ions. Sodium and potassium were determined using the flame photometry and ICP AES on the spectrometers Z8000 Hitachi and iCAP-6500 Termo Scientific. CHN analysis was performed on Euro EA 3000. Liquid Extraction. The 10 mL water solution of salts KNO3+NaNO3 in equimolar ratio (from 0.5 to 3.5 M) was slowly added by 0.25 M solution of NaAOT in n-decane in a volume ratio 1:1 and stirred at a rate of 160 rpm by a magnetic stirrer in a closed vessel for 3 h. The micellar phase (extract) was separated and centrifuged, and the hydrodynamic diameter of micelles and water content of pseudophase were determined. The aliquot of the extract was dried in the airflow (fan) for 7 h to a dry residue. Further, the sample was 5217
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nH2O)core ↔ KAOTsurface + (NaNO3 × nH2O)core in mixed micelles. It is worth noting that, initially, an increase in the ionic strength promotes the exchange of sodium for potassium but under the conditions of more severe competition for water (field of high ionic strength) the equilibrium of ion exchange reaction shifts in opposite direction. Taking account of the salt content in micelle cores (calculated data are given in Table S1) provides a minor correction owing to the low solubilization capacity of NaAOT microemulsions. The research conducted on model extraction systems only qualitatively confirms the possibility of reversible exchange, as they demonstrate a huge discrepancy between the volumes of the organic phase of microemulsion, feed, and the volume of solubilized aqueous pseudophase. Also, as calculations of distribution of ions between the core and surface layer of micelles are based on a number of assumptions, more detailed studies of micelle composition in mixed microemulsions were performed using the microdrop probing procedure. Composition of Microemulsions from Data of Microdrop Probe. Starting Microemulsions. At first, we studied the distribution of ions between the micelle core and surface layer, depending on the content of NaAOT in microemulsion immediately after the solubilization of 1 vol % 2 M of KNO3 solution (prior to evaporation of water).
Powder samples 1−2 mg in weight were dissolved in 10 mL of deionized water and further analyzed using the flame photometry.
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RESULTS AND DISCUSSION Exchange Processes in Liquid Extraction. In the beginning, ion-exchange processes in microemulsion were studied using extraction solubilization. In contrast to injection solubilization, microemulsion itself “chooses” how much water and salts are to be solubilized from the feed. Figure 1 shows the spectra of micellar solutions after extraction from salt solutions of various compositions.
Figure 1. IR spectra of NaAOT micellar solutions during extraction from feed composition: 1, 0.25 M KNO3 + 0.25 M NaNO3; 2, 0.5 M KNO3 + 0.5 M NaNO3; 3, 1 M KNO3 + 1 M NaNO3; 4, 2 M KNO3 + 2 M NaNO3; 5, 3.5 M KNO3 + 3.5 M NaNO3 (1−5 upward). Time of extraction is 3 h. Systems 1−5 correspond to 3−7 in Table 1, respectively.
Compositions of extracts are given in Table 1. An increase in the ionic strength of the feed leads to a decrease in the total content of aqueous pseudophase in microemulsion and hydrodynamic diameter of micelles. An increase in the concentration of KNO3 to 2.5 M results in growth of the fraction of K+ in the microemulsion to 87%. Introduction of NaNO3 into the feed results in the considerable suppression of the exchange. At equal contents of NaNO3 and KNO3, with growing ionic strength in the feed, the fraction of K+ in the extract gradually grows. On the whole, a conclusion can be drawn on model extraction systems supporting the possibility of reversible exchange of NaAOTsurface + (KNO3 ×
Figure 2. Dependence of concentrations of K+ and Na+ in the core of micelles on the content of NaAOT in microemulsion (dashed lines). Gray lines denote calculated concentrations in case of full exchange.
Table 1. Total Content of Ions (in Moles) in 30 mg of Dried Extract and Mole Fraction of K+ in the Extract and Surface Layer, Depending on the Composition of Feeda composition of dried extract, mole × 10‑5 cations feed, M
K+/(Na++K+), %
anions
N
KNO3
NaNO3
Vs/Vob, vol %
Dh, nm
AOT‑
NO3‑
Na+
K+
extract
surface layerc
1 2 3 4 5 6 7
0.25 2.5 0.25 0.5 1.0 2.0 3.5
0 0 0.25 0.5 1.0 2.0 3.5
5.3 2.7 5.3 4.5 3.4 2.8 1.9
6.8 4.7 7.1 6.4 5.5 5.2 4.5
6.5 6.2 6.5 6.4 6.2 6.4 6.1
0.2 1.7 0.4 1.1 1.7 1.8 2.2
3.4 0.9 4.0 3.8 4.1 4.5 5.4
3.3 5.8 2.6 3.0 3.4 3.8 3.5
49 87 40 44 45 46 39
47 82 39 43 44 45 36
a
Extraction was carried out during three hours with the initial microemulsion of 0.25 M NaAOT in n-decane at room temperature. bEquilibrium content of aqueous pseudophase after extraction. cCalculated values of cation content in core and surface layer are given in Table S3. 5218
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Table 2. Dependence of Ion Composition of Microemulsion (Calculation and Experiment) on the Content of NaAOT at the Initial Moment of Time (Without Evaporation of Water)a K+/(Na++K+), %
calculation concentration of NaAOT, M 0
localization Core Surface layer
0.01
Core Surface layer
0.06
Core Surface layer
0.125
Core Surface layer
0.188
Core Surface layer
0.24
Core Surface layer
0.25
Core Surface layer
a
full exchange
without exchange
experiment
core
2 M KNO3 0 M NaNO3 0 M KAOT 0 M NaAOT 1 M KNO3 1 M NaNO3 1 M KAOT 0 M NaAOT 0 M KNO3 2 M NaNO3 2 M KAOT 4 M NaAOT 0 M KNO3 2 M NaNO3 2 M KAOT 10.5 M NaAOT 0 M KNO3 2 M NaNO3 2 M KAOT 16.8 M NaAOT 0 M KNO3 2 M NaNO3 2 M KAOT 22 M NaAOT 0 M KNO3 2 M NaNO3 2 M KAOT 23 M NaAOT
2 M KNO3 0 M NaNO3 0 M KAOT 0 M NaAOT 2 M KNO3 0 M NaNO3 0 M KAOT 1 M NaAOT 2 M KNO3 0 M NaNO3 0 M KAOT 6 M NaAOT 2 M KNO3 0 M NaNO3 0 M KAOT 12.5 M NaAOT 2 M KNO3 0 M NaNO3 0 M KAOT 18.8 M NaAOT 2 M KNO3 0 M NaNO3 0 M KAOT 24 M NaAOT 2 M KNO3 0 M NaNO3 0 M KAOT 25 M NaAOT
2 M KNO3 0 M NaNO3 0 M KNO3 0 M NaAOT 1.49 M KNO3 0.51 M NaNO3 0.51 M KAOT 0.49 M NaAOT 0.68 M KNO3 1.32 M NaNO3 1.32 M KAOT 4.68 M NaAOT 0.24 M KNO3 1.76 M NaNO3 1.76 M KAOT 10.7 M NaAOT 0.17 M KNO3 1.83 M NaNO3 1.83 M KAOT 17.0 M NaAOT 0.29 M KNO3 1.71 M NaNO3 0.77 M KAOT 23.2 M NaAOT 0.33 M KNO3 1.67 M NaNO3 1.67 M KAOT 23.3 M NaAOT
100
surface layer 0
75
51
34
22
12
14
9
9.7
15
3.3
17
6.7
Total surfactant NaAOT+Tergitol NP-4 concentration is 0.25 M. Initial solubilization capacity is 1 vol% of 2 M KNO3.
The obtained data (Figure 2; Table 2) show that, with the increase in NaAOT content, the fraction of K+ in micelle core initially drastically drops (at NaAOT concentration equal to 0.125 M about 10% KNO3 remains in the core) and then gradually increases. Thus, immediately after the injection of KNO3 into microemulsions with NaAOT, K+ cations in the core of micelles are replaced by Na+. At high content of NaAOT, a reverse process of transition of K+ from the surface layer into the core takes place, which seems to be due to the toughening of hydration conditions in the micelles of anionic NaAOT compared to oxyethylated Tergitol NP-4. Thus, even at low (0.01 M) content of NaAOT in mixed micelles as early as at the beginning of crystallization a significant proportion of KNO3 is replaced by NaNO3. Microemulsions during Evaporation Prior to Crystallization. For more detailed comparative study of the change in salt concentration during water evaporation, three systems were used: simple NaAOT, Tergitol NP-4, and mixed micelles of composition 0.01 M NaAOT + 0.24 M Tergitol NP-4. The systems were analyzed prior to the beginning of crystallization (prior to turbidity point). Data reported in Table 3 show that water evaporation naturally decreases the hydrodynamic diameter of micelles and solubilization capacity of microemulsions. Crystallization in microemulsions begins when less than half the aqueous pseudophase evaporates; crystallization in NaAOT micelles was not observed at all. The ratio of molar concentrations KNO3/NaNO3 during evaporation did not change and was 1.74 and 0.19 for 0.01 M NaAOT + 0.24 M
Table 3. Dependence of Hydrodynamic Diameter, Content of Aqueous Pseudophase (Vs/Vo) and Concentrations of NaNO3 and KNO3 in Micelle Cores on the Time of Evaporation of Water Prior to Crystallization concentration in micelle core, M t, min
Dh, nm
Vs/Vo, vol %
crystal
NaNO3
0.25 M Tergitol NP-4 0 10.0 1.0 5 9.1 0.9 10 8.5 0.7 + Calculated from phase diagram: 0.01 M AOT + 0.24 M Tergitol NP-4 0 10.0 1.0 0.84 7 9.0 0.8 1.09 14 8.2 0.6 + 1.43 Calculated from phase diagram: 2.1 0.25 M NaAOT 0 9.9 1.0 1.67 10 5.7 0.7 2.27 20 4.9 0.6 3.03 30 4.4 0.4 3.93 Calculated from phase diagram: 7.25
KNO3
C(KNO3) C(NaNO3)
2.00 2.32 2.70 4.21
-
1.49 1.91 2.38 3.73
1.77 1.75 1.66 1.77
0.33 0.44 0.58 0.76 1.42
0.19 0.19 0.19 0.19 0.19
Tergitol NP-4 and 0.25 M NaAOT, respectively. This implies that no additional exchange of cations takes place during 5219
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Figure 3. Dependence of the calculated concentration of K+ (a, c) and Na+ (b, d) cations in the surface layer (1) and in micelle core (2) on the crystallization degree of KNO3. Content of NaAOT in microemulsions: 0.01 M (a, b) and 0.06 M (c, d). Total calculated (3) and experimental concentration of KNO3 (markers) is shown in part a.
concentration increases. During evaporation, prior to crystallization, the ratio of cation concentrations does not change. Crystallization Process. Microemulsions with Cation Exchange. According to the “water” phase diagram, the presence of salt mixture must result in simultaneous crystallization of KNO3 and NaNO3. The content of NaNO3 in the extracted powder must be 22 wt % (for 1.5 M KNO3 + 0.5 M NaNO3). However, in mixed micelles of Tergitol NP-4 + NaAOT no simultaneous crystallization of KNO3 and NaNO3 was observed. From data reported in ref 10, the content of sodium nitrate in the crystals isolated from the microemulsions is, from the most overestimated values, 1 and 8 wt %. The presence of carbon in the powder evidences the presence of sodium, most likely, in the form of NaAOT rather than NaNO3. The evidence in favor of the absence of crystallization of NaNO3 is also the constant concentration of sodium in the microemulsion throughout the crystallization process.11 Let us explain this fact from the viewpoint of ionic exchange. Unfortunately, the application of microdrop probing of micelle content during crystallization from mixed microemulsions is a rather controversial procedure. The water content is very low and the testing microdrops of pure water simply begin to be solubilized by the microemulsion. Therefore, for the analysis we will also use the results described in our previous work.11 These clearly established facts allowed us to calculate the compositions of cores and surface layers of micelles in the crystallization process of KNO3 (Figure 3). The calculations are based on the initial contents of ions and the principle of electric
evaporation, and the composition of micelles is only predetermined by the initial composition prior to evaporation and the content of aqueous pseudophase. The presence of a mixture of salts in the cores is very important, as it is known19 that the presence of foreign salts shifts the region of saturation. Experimental values corresponding to the beginning of concentration are shown in bold. To compare crystallization in ordinary aqueous solutions, we used data from the phase diagrams KNO3−NaNO3−H2O20 extrapolated to 35 °C (adjusted phase diagram is given in Figure S1). The compositions corresponding to the beginning of crystallization and calculated from the triangular phase diagram for water solutions of salt mixtures are also given in Table 3. The experimentally found concentrations of salts, corresponding to the beginning of crystallization, are 1.5 times lower than calculated values. The discrepancy of experimental and calculated values can be explained as follows. At injection solubilization of aqueous solutions, in addition to the exchange of cations (in mixed micelles), hydration of polar groups of surfactant molecules by water molecules takes place. As a result, part of the water passes from micelle core to surface layer and the concentration of salts in the core increases at the stage of solubilization prior to evaporation. Calculated concentrations will be close to experimental values when 35% of water is bound. Thus, at the stage of solubilization of KNO3 the cores of mixed micelles contain a mixture of salts; due to the binding of part of water by the polar groups of surfactant molecules, salt 5220
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and decrease in water content, there takes place a reverse exchange of K+ cations bound with AOT− for Na+ that are in the micelle core. The driving force of reverse transfer is toughening of hydration conditions of cations in micelles. As a result, only KNO3 crystallizes from the initial mixture of salts. It is worth noting that the situation is similar to the loss of potassium from the intracellular environment at hypotonic dehydration of living organisms. The role of surface-active AOT− anion in exchange processes is binding and retention of K+ cation in the surface layer and its release into the micelle core at a proper moment of time. It is most likely that in the studied systems the possibility of selective crystallization of KNO3 from the mixture of salts KNO3 + NaNO3 is governed by AOT−. It is also worth noting that Na+ cations do not virtually influence the dispersity of obtained salts. This is clearly demonstrated by the SEM-images of crystals obtained from simple and mixed microemulsions (Figure 4, photo in the left). In the systems without exchange (Tergitol NP-4) particles of micron size crystallize both in the case of simple and KNO3 + NaNO3 mixtures. The influence of AOT− on the morphology and dispersity of formed powder was not studied in detail in this work. We can only state that exchange processes with participation of AOT− somewhat
neutrality. The results obtained show that during the transition of KNO3 from micelle cores to solid phase the cores become completely depleted in KNO3. This situation is marked in the figures by vertical dashed lines at 0.75% and 0.4% degrees of crystallization for microemulsions of 0.24 M Tergitol NP-4 + 0.01 M NaAOT and 0.19 M Tergitol NP-4 + 0.06 M NaAOT, respectively. However, the crystallization process continues and the only way to enrich potassium is its reverse transition from surface layer to micelle core. To preserve the electric neutrality, an equivalent amount of sodium must return to the surface layer of micelles. The possibility of the reversible occurrence of successive ion-exchange processes was demonstrated using numerical simulation of electrostatic interactions of all ions in our work.11 The calculations performed with the help of direct optimization evidence that, at a high content of aqueous pseudophase, the difference in hydration energies of exchanging ions exceeds the energy difference of Coulomb interaction. 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 decreasing content of water molecules in micelles during evaporation, the difference in Coulomb interaction becomes most determining (Na+ ions can approach AOT− ions closer than K+).13 Gibbs energy becomes positive and a reverse transition takes place. Additional confirmation of the above arguments are the experimental data (solid symbols in Figure 3a) on the total content of KNO3 in microemulsion.11 They fall well on the solid line calculated from the content of K+ in the core and in the surface layer. It is worth noting that the possibility of reverse exchange is also supported by the data obtained in extraction studies. Microemulsions without Cation Exchange. To confirm the role of exchange processes, we conducted “background” crystallization from a mixture of salts in simple micelles of Tergitol NP-4 in the absence of cation exchange. For this purpose, we injected a mixture of salts: 1 M KNO3 + 1 M NaNO3 (molar ratio Na+/K+ in initial solution = 1) and 2 M KNO3 + 1 M NaNO3 (molar ratio Na+/K+ in initial solution = 1.6) into microemulsion of Tergitol NP-4. Data of flame photometry and energy dispersion analysis showed that the content of salts in obtained powders (Figure 3) was 57 wt % KNO3 and 43 wt % NaNO3 (molar ratio Na+/K+ = 0.90) for the former system and 65 wt % KNO3 and 35 wt % NaNO3 (molar ratio Na+/K+ = 1.56) for the latter. As expected, the composition of initial salts in crystallization from simple microemulsion Tergitol NP-4 did not change. Thus, mixed microemulsions of Tergitol NP-4 + NaAOT are unique nanoreactors allowing selective crystallization of KNO3 from a mixture of salts KNO3 + NaNO3.
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CONCLUSIONS We have studied the exchange of Na+ and K+ cations between the surface layer and core of mixed micelles of Tergitol NP-4 + NaAOT at all stages of evaporation microemulsion crystallization. Cation K+ in micelle cores is partly replaced by Na+ and passes into the surface layer at the stage of injection solubilization of aqueous solution of KNO3. The degree of substitution increases with the growth of NaAOT content. Further, during evaporation of water from microemulsion prior to crystallization, the concentration ratio KNO3/NaNO3 in micelle core does not change. With the beginning of crystallization, growth of particles is due to the “reserves” of KNO3 in micelle cores. Upon the depletion of the “reserves”
Figure 4. Morphology of KNO3 + NaNO3 powders obtained by evaporation microemulsion crystallization from 0.25 M Tergitol NP-4 (on the left) and mixed 0.24 M Tergitol NP-4 + 0.01 M NaAOT (on the right). Vs/Vo = 1 vol %. 5221
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(16) Cottet, H.; Biron, J.-Ph.; Martin, M. Anal. Chem. 2007, 79, 9066−9073. (17) Berne, B.; Pecora, R. Dynamic Light Scattering with Application to Chemistry, Biology and Physics; Wiley-Interscience: Hoboken, NJ, 1974. (18) Eastoe, J.; Towey, T. F.; Robinson, B. H.; Williams, J.; Heenan, R. K. J. Phys. Chem. 1993, 97, 1459−1463. (19) Mullin, J. W. Crystallization, 4th ed.; Butterworth-Heinemann, 2001; p 160. (20) Phase diagram of KNO3-NaNO3-H2O system at different temperatures; http://crct.polymtl.ca/ (accessed 15 Nov, 2016).
extend the time of the beginning of crystallization compared to simple micelles. This leads to an increase in the time of evaporation of water and formation of smaller size cores and, hence, to a decrease in the size of nuclei. However, the main factor responsible for the dispersity and morphology of crystallized powders is, most likely, the adsorption of AOT− on the faces of growing nucleus. This results in the formation of strongly elongated crystallites of whisker-type (Figure 4, photo in the right).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00718. Description of procedures: liquid extraction, microdrop probing, and tables of data based on the experiments (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Darya I. Beketova: 0000-0002-6590-9245 Notes
The authors declare no competing financial interest.
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ABBREVIATIONS Vs/Vo, solubilization capacity is a ratio of the volume of solubilized aqueous phase (Vs) to the total volume of organic phase (Vo) IR, infrared AOT, aerosol OT, sodium bis (2-ethylhexyl) sulfosuccinate PCS, photon correlation spectroscopy AES, atomic emission spectroscopy ICP, inductively coupled plasma
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REFERENCES
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DOI: 10.1021/acs.cgd.7b00718 Cryst. Growth Des. 2017, 17, 5216−5222
