Investigation of Phase Behavior and Thermodynamic Stability in

Article pubs.acs.org/jced

Investigation of Phase Behavior and Thermodynamic Stability in Kolliphor HS 15/Caprylic/Capric Triglycerides (GTCC)/Water Microemulsions System Wen-ting Wang,†,‡ Jiao-jiao Wu,†,‡ Dan Li,†,‡ Hong Liu,*,†,‡ and Hong-chun Pan*,†,‡ †

College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, China Chongqing Engineering Research Center for Pharmaceutical Process and Quality Control, Chongqing 400716, China

‡

S Supporting Information *

ABSTRACT: Phase behavior of microemulsions composed of Kolliphor HS 15, caprylic/capric triglycerides (GTCC), and water was investigated by phase diagrams and conductivity measurements. In addition, ΔG0CP, ΔH0CP, and ΔS0CP were also calculated because the clouding phenomenon is controlled by energy. On the basis of the preliminary contribution to the variation of both specific conductance and peak current, the monophasic phase could be divided into self-microemulsion and microemulsion regions. Besides, a shorter bicontinuous phase was formed as the composition closing to the boundary between two subregions. Furthermore, thermodynamic parameter showed that self-microemulson was more stable than that of microemulsion, which was further supported by cyclic voltammetry characterizing the two subregions. Dehydration of solvation layer as αGTCC increasing is responsible for the mechanism of the liquid−liquid phase separation process and therefore of the thermodynamic stability.

1. INTRODUCTION In the early 1940s, Hoar and Schulman1,2 had put forward the concept of microemulsion first. However, it was not until 1981 that the definition was given by Danielsson and Lindman.1,2 Microemulsions could be seen as a system in which water, oil, and surfactant form a translucent, low viscosity, isotropic, and thermodynamically stable mixture. Three different microemulsion types can be observed according to the composition: water-in-oil (w/o), bicontinuous phase, and oil-in-water (o/ w).3−6 The water-in-oil type is a dispersion of hydrophilic solution in oil, the continuous phase. On the contrary, the oilin-water type is a dispersion of oil in water acting as the continuous phase.7 A so-called bicontinuous phase forming in the process of the phase inversion of microemulsions is reported as a staggered network combined with oil and water as they both are continuous phases.8−10 On the basis of the advantages of nanometer size, excellent biocompatibility, and high solubilization capacity, microemulsions have playing an increasingly important part in pharmaceuticals industry such as kinds of drug-loaded supporters, polymers, and nanoparticles. Particularly, the application in drug delivery is promising.11−16 For instance, Ryoo et al.17 studied the influence of the surfactant−cosurfactant ratio on the size and stability of propofol-loaded miceoemulsion. Kolliphor HS 15 (HS 15) used in our research was a kind of new nonionic surfactant, which had been reported safer than current solubilizers such as Cremophor EL and polysorbate 80 because the latter could cause more severe impairment.18,19 In © XXXX American Chemical Society

our previous study, we have successfully established the coenzyme Q10-loaded micellar based on HS 15. Meanwhile, the properties and stability of micelles were also studied and the results showed the new coenzyme Q10-loaded micellar was very stable as a potential clinical drug below 25 °C.20 Caprylic/ capric triglycerides (GTCC) is a kind of excellent high-purity moisturizing grease with good spreadability and hence often used as the base material of moisture and stabilizers of cosmetics. The cloud point (CP) was defined as such a certain temperature in which nonionic ethoxylated surfactants of microemulsion and micelles occurred in a phase-separation phenomenon.21−23 The phenomenology of clouding and its technological applications should be considered as a doubleedged sword, because the phase separation process should sometimes be avoided for negatively affecting the performance and stability whereas in other times it should be necessary in the solvent-extracting part.21,23−25 We have succeeded studying the cloud point thermodynamics of paclitaxel-loaded microemulsion in the presence of glucose and NaCl.26 The results obtained revealed the glucose enhanced the thermodynamic stability of HS 15/GTCC/water/paclitaxel microemulsion system whereas on the contrary NaCl reduced the stability. Received: July 18, 2016 Accepted: November 28, 2016

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DOI: 10.1021/acs.jced.6b00636 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Cyclic voltammetry is one of the commonly used electrochemical research methods, which can be used to research the nature and mechanism of the electrode reaction and to quantitatively determine the concentration of the reactants. Gowda and Nandibewoor27 successfully studied the complete quality analysis of insoluble anticancer drug paclitaxel formulations and elucidated the electron transfer mechanism by cyclic voltammetry using self-made pencil electrode. In addition, cyclic voltammetry is also widely used to study the surfactants including monitoring the micellar growth behavior,28 measurement of the diffusion coefficient,29 calculation of the critical micelle temperature,28,30,31 as well as the observation on the structure transformation of the microemulsion7,32 and determination of droplet size.33 To the best of our knowledge, the related properties of HS 15/GTCC/water microemulsions have not been reported up to now. The purpose of this study is to research the phase behavior and microstructure in two different subregions of monophasic phase domain respectively and further study the effect of temperature and composition on the related properties of thermodynamic stability. As one of the most common electrochemical methods, cyclic voltammetry (CV) can be used in various studies in surfactants for its significant advantages of convenience and accuracy and thus it is considered to characterize this mixed system. Results obtained in the present study might provide some favorable significance for applications in preparation of nanoparticles, optimization of prescription, and stability study of delivery microemulsion systems.

Table 1. Information on Source, Purity and Lot Number of the Materials Used in This Experiment chemical name Kolliphor HS 15 GTCC potassium dihydrogen phosphate dipotassium phosphate

source

lot no.

BASF (Shanghai, China) Tieling North Medicinal Oil Co. Ltd. (Liaoning, China). Chengdu area of the industrial development zone xindu mulan (Chengdu, China) Chengdu area of the industrial development zone xindu mulan (Chengdu, China)

48328768E0 y120401-3-01 20120501 20120522

2.3. Determination of Electrical Conductivity. Different mass ratios of GTCC and HS 15 were mixed as oil phase, and then the water content φ was recorded with six drops of water added continuously. The electrical conductivity of mixture was detected subsequently at T = 298.15 ± 0.05 K through a conductivity meter used ac with 60 Hz working frequency, (DDS-11A, Frontour Instrument Co. Ltd., Shanghai, China). The measurement was repeated until the conductivity value was constant. All data were performed as mean ± standard deviation (SD), n = 3. 2.4. Determination of Cloud Point. Color-comparison tubes containing appropriate microemulsion solutions were put into water bath. While temperature increased gradually, the initial value of cloud point temperature was obtained when the solutions became visually cloudy. The sample was heated at the rate of about 0.2 K/min. The process was repeated four times and the average temperature was received as the final value. 2.5. CV Experiments. A mixed solvent containing 20 vol % microemulsion solution and 80 vol % supporting electrolyte (pH = 7, ionic strength = 0.2 M phosphate buffer solutions) placed in the electrolytic cell was taken as a sample solution to be tested. Every measurement was repeated three times and the average value was the final electrochemical signal. The threeelectrode system was consisted of a glassy carbon working electrode (GCE), a saturated calomel reference electrode (SCE), and a platinum wire counter electrode. The GCE was polished with alumina powder and then kept in ethanol and water for 5 min via sonication before each measurement. It should be noted that owing to the strong adsorption, the GCE was scanned in ethanol five times every time to remove the adsorbed substances. The parameters for cyclic voltammetry were scan range, 0 to −1.2 V; scan rate, 0.1 V/s; sensitivity, 10 μA/V; quiet time, 2 s. CHI600E electrochemical workstation was purchased from Chenhua Instrument Co. Ltd. (Shanghai, China).

2. EXPERIMENTAL SECTION 2.1. Materials. Along with polyoxyethylene chain, this nonionic surfactant named Kolliphor HS 15 (molecular mass 963.24, HLB = 14−16, ≥80%) in which the chemical structure was presented in Figure 1 was purchased from BASF (Shanghai,

Figure 1. Chemical structure of Kolliphor HS 15.

China). Caprylic/capric triglyceride (GTCC, molecular mass 408.56, ≥98%) was purchased from Tieling North Medicinal Oil Co. Ltd. (Liaoning, China). HS 15 and GTCC mentioned above were used without further purification. Other reagents and materials were of analytical reagent (AR) grade and supplied by Chengdu Kelong Chemical Reagent Company (Sichuan, China). Phosphate buffer solutions (pH = 7, ionic strength = 0.2 M) were prepared according to the literature method.34 All sample solutions were prepared in double distilled water. The details of the materials used in this work were shown in Table 1. 2.2. Phase Diagrams of the Microemulsions. Different mass ratios of GTCC and HS 15 were mixed (αGTCC = mGTCC/ mHS15, varying from 0.042 to 0.30). Subsequently, water added into the mixed oil phase by stirring until turbidity-totransparency occurred at T = 298.15 ± 0.05 K. The weight of HS 15, GTCC, and water were recorded using a MettlerToledo EL104 balance with an accuracy of ±0.0001 g (Shanghai, China). Weight percentage (%) of the three components were calculated and phase diagrams of the microemulsions were constructed based on the fitted curve with critical values obtained at the phase boundary.

3. RESULTS AND DISCUSSION 3.1. Phase Diagrams of the HS 15/GTCC/Water Microemulsion. In order to provide valuable information on the roles of polar phase, nonpolar phase, and surfactant play while determining the properties of the system, phase diagram or pseudoternary phase diagram is often constructed to investigate the phase behavior of microemulsion.7,16,35,36 Herein, the phase diagram of HS 15/GTCC/water microemulsion at T = 298.15 ± 0.05 K was shown in Figure 2. It obviously can be observed that the monophasic phase was relatively small for the absence of cosurfactant based on the fitted curve, which was simultaneously divided into selfmicroemulsion and microemulsion according to the tangent pointed along the fitted line. It is reported that the area toward B



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Figure 3. Electric conductivity (κ) of the HS 15/GTCC/water microemulsion in self-microemulsion region at T = 298.15 K. αGTCC = 0.053 (■); 0.034 (●); 0.026 (▲); 0.020 (★); 0.017 (*). Inset: the transition of microstructure with water content in the system of αGTCC = 0.053.

Figure 2. Phase diagrams of the HS 15/GTCC/water microemulsions with different subregions at T = 298 ± 0.05 K. (A) self-microemulsion (αGTCC ≤ 0.053), (B) microemulsion (0.053 < αGTCC ≤ 0.25), and (C) two-phase region (αGTCC > 0.25).

to a maximum φm, which is often used to reveal the formation of bicontinuous microemulsion. This could be interpreted from the network staggered by conductive chains. The microemulsion as a whole may be considered as a transitional intermediate structure with oil and water because the two are continuous phases at the same time. While the final decrease can be a consequence of decrease of the concentration by dilution of added water, the same result was also found in microemulsion region and as exhibited in Figure 4.

oil apex can be infinitely diluted by water and thus is called selfmicroemulsion. By contrast, the microemulsion was just clear within a certain range of the mass of water as presented in Figure 2.37 The self-microemulsion was shown in panel A when αGTCC was less than 0.053; a microemulsion region appeared when αGTCC was in the range of 0.053 and 0.25. With the increasing of oil phase proportion, when αGTCC was higher than 0.25, a turbid state named two-phase region was obtained regardless of how much water added. Because of the meaninglessness of twophase region in theoretical discussion and practical application, our following study focused on self-microemulsion and five points (αGTCC = 0.017, 0.020, 0.026, 0.034, 0.053) were taken in self-microemulsion and three points (αGTCC = 0.071, 0.11, 0.25) were taken in microemulsion for examples. 3.2. Microstructure in Microemulsion. According to the structure of microemulsions, monophasic phase can be divided into three subregions depending on the variation of water content φ, including w/o, bicontinuous phase (BC), and o/w. Conductivity measurement, based on the percolative conduction model, has been frequently used to study and evaluate the microstructure of microemulsion system.38 Electric conductivity data for the HS 15/GTCC/water system were presented in Tables S2 and S3. As an example, Figure 3 shows the change of conductivity κ versus water content φ in self-microemulsion, which is consistent with the classical theory.8,39,40 A w/o type microemulsion was formed when φ reached to φb, which however contained two mechanism within this process. When φ was less than the penetration threshold φc, the conductivity does not increase significantly because the behavior of most small flow formed by conductive components is prohibited. However, with the further increase of φ it suddenly increases almost linearly and reaches φb. One of the accepted mechanisms is the model of “sticky droplet collisions” suggested by Fletcher and Robinson.41 Because of these frequently sticky collisions, the small flow may turn into a large one and rapidly spread out to form narrow channels called conductive chains, which results in an abrupt and steep increase of conductivity until φ value reaches to φb. The conductivity then continues to increases, deviating from the former line up

Figure 4. Electric conductivity (κ) of the HS 15/GTCC/water microemulsion in microemulsion region at T = 298.15 K. The region of bicontinuous phase were (a), αGTCC = 0.071; (b), αGTCC = 0.11; (c), αGTCC = 0.25.

It can be seen from Figures 3 and 4 that during phase transition the process of bicontinuous phase experienced different in different αGTCC and therefore the values of threshold obtained were compared in Table 2. In selfmicroemulsion domain, Δφ decreased from 0.13 to 0.07 with the increasing GTCC, which suggested a narrower bicontinuous phase region. However, the result observed was interesting because of the exactly opposite phenomenon in microemulsion domain. As is shown in Table 2, the closer to the boundary of self-microemulsion and microemulsion, the shorter a bicontinuous phase experiences and the easier the phase transition is. C



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Table 2. Region of Bicontinuous Phase in HS 15/GTCC/Water Microemulsion at the Temperature T = 298.15 K and Pressure p = 0.1 MPaa,b self-microemulsion αGTCC φb φm Δφ

0.017 0.50 0.63 0.13

0.020 0.52 0.64 0.12

0.026 0.53 0.64 0.11

microemulsion 0.034 0.56 0.64 0.080

0.053 0.58 0.65 0.070

0.071 0.62 0.68 0.060

0.11 0.56 0.68 0.12

0.25 0.52 0.66 0.14

a αGTCC is the mass ratios of GTCC in each system; φb is the penetration threshold, φm is the water content when the conductivity is maximum, and Δφ is the water content in the range of BC. bStandard uncertainties u are u(φb) = 0.002, u(φm) = 0.001, u(Δφ) = 0.001 and u(p) = 10 kPa (0.68 level of confidence).

3.3. Cloud Point Thermodynamics. 3.3.1. Cloud Point of Microemulsion. The clouding phenomenon is studied from both theoretical and practical standpoints since it is closely related to the way in which surfactant−water interactions occur. Herein, the cloud point temperatures of different mixtures were illustrated in Figure 5. It was clear that the cloud points

0 ΔHCP

0 ⎡ ΔGCP ⎢d T = − T 2⎢ ⎢ dT ⎣

( ) ⎤⎥⎥ ⎥ ⎦

0 0 0 T ΔSCP = ΔHCP − ΔGCP

(2) (3)

where XO is the mole fraction of GTCC in mixtures, R is the gas constant, and T is the cloud point temperature. The enthalpy of the cloud point was calculated from the slope of a plot of ΔG0CP/T versus T for all studied solution where Figure 6 represents. It can be seen from this figure that

Figure 5. Cloud point temperature variation with the increasing of oil phase proportion (αGTCC) in self-microemulsion region (solid line) and microemulsion region (dashed line) of the HS 15/GTCC/water microemulsion.

Figure 6. Plots of ΔG0CP/T versus T in self-microemulsion region (dashed line) and microemulsion region (solid line) of HS 15/ GTCC/water microemulsion.

decrease as the ratio of GTCC in the two subregions increases, and the value in microemulsion region is lower compared with that in self-microemulsion. It is widely assumed that phase separation is due to the reduction of inter-repulsions and the increasing importance of interactions as a result of dehydration of the solvation layer of nonionic surfactant as the temperature increases.42 The increase of GTCC therefore reduces hydrogen bonds knitted between water and HS 15, thus reinforces the dehydration of the solvation layer and CP decreases. 3.3.2. Calculation of Thermodynamic Parameters. On account that the clouding phenomenon process is controlled by energy, an estimation of the energetic of this process could provide a theoretical explanation of the stability based on the different ratio of composition in different systems. Thus, the standard Gibbs free energy ΔG0CP, the enthalpy ΔH0CP, and the entropy ΔS0CP of the clouding phenomenon were calculated from the following eqs 1 to 3.44,45 The following equations were based on the phase-separation model42,43 0 ΔGCP = −RT ln XO

the data fit a straight lines (R2 > 0.98). The entropy of the microemulsion was estimated according to the enthalpy and Gibbs free energy obtained. The derived thermodynamic parameters were given in Table 3. The positive Gibbs free energy, negative enthalpy, and entropy showed that the clouding process is nonspontaneous, thermodynamically unfavorable, exothermic, and is governed by both enthalpic and entropic contributions. The negative value of ΔH0CP and ΔS0CP implied that the repulsive force resulting from the hydration decreases whereas the attractive forces (van der Waals and hydrophobic interactions) become increasingly important. This increase in interactions induces an increase in droplet aggregates, which shows a turbid state at the macro level. Both enthalpy and entropy contribute to the net interaction.42 It is noteworthy that the ΔG0CP values are slightly smaller in microemulsion rather than in self-microemulsion, further suggesting that it favors the clouding process in microemulsion region. This result that self-microemulsion is more thermodynamically stable than microemulsion may provide a potential application for drug-loaded system.

(1) D



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basis. Hence, the compensation temperature, TC, can be considered as the judgment criterion of the strength of the surfactant−water interactions. Larger values of TC, as a specific example, indicate that more energy is required to induce the release of a certain number of water molecules.44,52,53 The values of TC calculated from the slope of the plots in Figure 7 are 367.69 and 345.48 K in self-microemulsion region and microemulsion region, respectively. Our TC value for clouding in microemulsion region is lower than those for surfactant mixtures in self-microemulsion region, thus suggesting that less energy is required to induce clouding in microemulsion region. This variation in TC is due to a difference in the water− surfactant interaction mode. 3.4. Characterization of Microemulsion by CV. Electrochemical analysis method, viz. CV has been applied in the evaluation of the properties of microemulsion in this study and the result was shown in Figure 8. This picture indicates the

Table 3. Thermodynamic Parameters in HS 15/GTCC/ Water Microemulsion for the Mass Ratios of GTCC αGTCC, the Cloud Point CP, Gibbs Free Energy ΔG0CP, Enthalpy ΔH0CP, and Entropy ΔS0CP at the Pressure p = 0.1 MPaa,b

selfmicroemulsion

microemulsion

αGTCC

CP/K

ΔG0CP (kJ/mol)

ΔH0CP (kJ/mol)

TΔS0CP (kJ/mol)

0.017 0.020 0.026 0.034 0.053 0.071 0.11 0.25

364.0 361.5 359.0 357.5 354.0 353.2 351.7 348.5

24.03 22.92 22.31 21.54 20.30 21.12 19.97 17.38

−119.25 −117.61 −116.00 −115.03 −112.78 −264.47 −262.23 −257.48

−143.28 −140.53 −138.31 −136.57 −133.08 −285.59 −282.20 −274.85

αGTCC is the mass ratios of GTCC in each system; CP is the cloud point of each system, ΔG0CP is Gibbs free energy, ΔH0CP is the enthalpy, and ΔS0CP is the entropy. bStandard uncertainties u are u(T) = 0.01 K, u(ΔG0CP) = 0.01, u(ΔH0CP) = 0.1, u(ΔS0CP) = 0.0013, and u(p) = 10 kPa (0.68 level of confidence). a

The net heat change of the clouding phenomenon involved two important contributions, which were dehydration of the polyoxyethylene accompanied by heat absorption and subsequent association of the dehydrated system involves heat release.42,46,47 It should be noted that the calculated ΔH0CP are negative, becoming significantly more negative in microemulsion region, indicates that the net balance is exothermic, further suggesting heat release is the dominant factor. In addition, the enthalpy−entropy compensation experiment results were also consistent with the clouding thermodynamic parameters. Figure 7 displays plots of ΔH0CP as a function of

Figure 8. Relation between peak current and the oil phase proportion (αGTCC) in self-microemulsion region (solid line) and microemulsion region (dashed line) of HS 15/GTCC/water microemulsion. Inset: The cyclic voltammograms for self-microemulsion when αGTCC varies from (a) 0.017 to (e) 0.053.

electrochemical signals decrease with increasing GTCC in both subregions. It is known that GTCC as oil phase and HS 15 as nonionic surfactant can obstruct the transfer of electrons of supporting electrolyte, thus weaken the electrochemical signals resulting from the decrease of rate of oxidation−reduction.28,54 In other words, our experimental data shows the insoluble GTCC weakens more current signals comparing with HS 15 containing hydrophilic head groups. It might be interpreted that electrons wrapped by GTCC in solution are more difficult to adsorb in electrode surface, considering the fact that the electroactive species exist apart away from the surface. Similar result has also been found by Atta and Galal; they reported how the peak current decreases with incremental additions of the nonionic surfantants Triton X-405.54 In addition, the trend of decrease in microemulsion region is more significant than selfmicroemulsion, which may be attributed to the better thermodynamic stability in latter subregion, suggesting droplets are easier to hinder the electron transfer in the electrolyte.

Figure 7. Enthalpy−entropy compensation plots for HS 15/GTCC/ water microemulsion in self-microemulsion region (■) and microemulsion region (●).

ΔS0CP in self-microemulsion region and microemulsion region. A linear dependence between enthalpy change and entropy change,42,48−51 which is usually described in the form of eq 4: 0 0 * + TC ΔSCP ΔHCP = ΔHCP

(4)

where TC is the compensation temperature; ΔH*CP, which is the intersection of the compensation plot, suggests the enthalpy effect under the condition of zero entropy change. It is known that the clouding process can be considered as the balance of a “solvation” part and a “chemical” part based on the theoretical

4. CONCLUSION This article established HS 15/GTCC/water microemulsion. Ternary phase diagram showed the monophasic phase can be E



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(11) Ganguli, A. K.; Ganguly, A.; Vaidya, S. Microemulsion-based synthesis of nanocrystalline materials. Chem. Soc. Rev. 2010, 39, 474− 485. (12) Sjoblom, J.; Lindberg, R.; Friberg, S. E. Microemulsions-phase equilibria characterization, structures, applications and chemical reactions. Adv. Colloid Interface Sci. 1996, 65, 125−287. (13) Lawrence, M. J.; Rees, G. D. Microemulsion-based media as novel drug delivery systems. Adv. Drug Delivery Rev. 2000, 45, 89−121. (14) Magdassi, S. Delivery systems in cosmetics. Colloids Surf., A 1997, 123-124, 671−679. (15) Lawrence, M. J.; Rees, G. D. Microemulsion-based media as novel drug delivery systems. Adv. Drug Delivery Rev. 2012, 64, 175− 193. (16) Acharya, D. P.; Hartley, P. G. Progress in microemulsion characterization. Curr. Opin. Colloid Interface Sci. 2012, 17, 274−280. (17) Ryoo, H. K.; Park, C. W.; Chi, S. C.; Park, E. S. Development of propofol-loaded microemulsion systems for parenteral delivery. Arch. Arch. Pharmacal Res. 2005, 28, 1400. (18) Coon, J. S; Knudson, W.; Clodfelter, K.; Lu, B.; Weinstein, R. S. Solutol HS15, nontoxic polyoxyethylene esters of 12-hydroxystearic acid, reverses multi-drug resistance. Cancer Res. 1991, 51, 897−902. (19) Ku, S.; Velagaleti, R. Solutol HS15 as a novel excipient. Pharm. Technol. 2010. (20) Liu, L.; Mao, K.; Wang, W. T.; Pan, H. C.; Wang, F.; Yang, M.; Liu, H. Kolliphor® HS 15 Micelles for the Delivery of Coenzyme Q10: Preparation, Characterization, and Stability. AAPS PharmSciTech 2016, 17, 757−766. (21) Garenne, D.; Navailles, L.; Nallet, F.; Grélard, A.; Dufourc, E.; Douliez, J. P. Clouding in fatty acid dispersions for charge-dependent dye extraction. J. Colloid Interface Sci. 2016, 468, 95−102. (22) Lindman, B.; Medronhoe, B.; Karlstrom, G. Clouding of nonionic surfactants. Curr. Opin. Colloid Interface Sci. 2016, 22, 23−29. (23) Gürkan, R.; Korkmaz, S.; Altunay, N. Preconcentration and determination of vanadium and molybdenum in milk, vegetables and foodstuffs by ultrasonic-thermostatic-assisted cloud point extraction coupled to flame atomic absorption spectrometry. Talanta 2016, 155, 38−46. (24) Heidarizadi, E.; Tabaraki, R. Simultaneous spectrophotometric determination of synthetic dyes in food samples after cloud point extraction using multiple response optimizations. Talanta 2016, 148, 237−236. (25) Jalbani, N.; Soylak, M. Preconcentration/separation of lead at trace level from water samples by mixed micelle cloud point extraction. J. Ind. Eng. Chem. 2015, 29, 48−51. (26) Wang, W. T.; Wang, M.; Zhang, J.; Liu, H.; Pan, H. C. Cloud point thermodynamics of paclitaxel-loaded microemulsion in the presence of glucose and NaCl[J]. Colloids Surf., A 2016, 507, 76−82. (27) Gowda, J. I.; Nandibewoor, S. T. Electrochemical characterization and determination of paclitaxel drugusing graphite pencil electrode. Electrochim. Acta 2014, 116, 326−333. (28) Mahajan, R. K.; Chawla, J.; Bakshi, M. S. Effects of monomeric and polymeric glycol additives on micellar properties of Tween nonionic surfactants as studied by cyclic voltammetry. Colloids Surf., A 2004, 237, 119−124. (29) Asakawa, T.; Sunagawa, H.; Miyagishi, S. Diffusion coefficients of micelles composed of fluorocarbon surfactants with cyclic voltammetry. Langmuir 1998, 14, 7091. (30) Hassan, P. A.; Yakhmi, J. V. Growth of Cationic Micelles in the Presence of Organic Additives. Langmuir 2000, 16, 7187−7191. (31) Li, G. B.; Hao, J. C.; Li, H. G.; Fan, D. W.; Sui, W. P. Determination of the critical micellar temperature of F127 aqueous solutions at the presence of sodium bromide by cyclic voltammetry. Colloid Polym. Sci. 2015, 293, 787−796. (32) Shrikrishnan, S.; Lakshminarayanan, V. Electron transfer studies of redox probes in bovine milk. J. Colloid Interface Sci. 2012, 370, 124− 131. (33) Xing, Z. Z.; Zhu, H. Compound reverse microemulsion drop sizes determination by cyclic voltammetry. Modern Chemical Industry 2010, 30, 88−90.

respectively divided into two subregions, self-microemulsion, and microemulsion. The microstructure observed by conductivity measurement indicated that the closer to the boundary of self-microemulsion and microemulsion, the shorter a bicontinuous phase experiences and the easier phase transition is. In addition, thermodynamic parameters showed clouding process for both subregions was nonspontaneous, negative and exothermic. Meanwhile, the self-microemulsion is more thermodynamic stable than microemulsion region, because the Gibbs free energy is slightly larger in selfmicroemulsion. Cyclic voltammetry was applied to evaluate the properties of microemulsion according to electrochemical signals based on different thermodynamic stabilities. As a summary, our present work for HS 15/GTCC/water microemulsion may have potential value in applications such as optimization of pharmaceutical formulations, stability research of HS 15/GTCC/water microemulsion systems, and may embody certain significance in studying delivery systems.

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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.jced.6b00636. Additional tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.L.). *E-mail: [email protected] (H.P.). ORCID

Wen-ting Wang: 0000-0002-5536-2645 Funding

The present study was funded by Southwest University Dr. Fund projects (SWU110056, SWU110057), Chongqing Engineering Research Center for Pharmaceutical Process. Notes

The authors declare no competing financial interest.

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REFERENCES

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