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O microemulsion dilution method
Colloids and Surfaces A: Physicochem. Eng. Aspects 497 (2016) 101–108
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Formation of nanoemulsion with long chain oil by W/O microemulsion dilution method Kun Tong a , Chunhua Zhao b , Dejun Sun a,∗ a b
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, Shandong 250100, China Oilfield Chemistry Research Institute, Division of Oilfield Chemistry, China Oilfield Services Limited, Yanjiao, Hebei 065201, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Long chain oil nanoemulsion was formed by W/O microemulsion dilution method. • Nanoemulsions with different charge, even positive charge, were formed by W/O microemulsion dilution method. • Thermodynamically stable W/O microemulsion, can be diluted to nanoemulsion on demand, is an alternative to nanoemulsion. • Easy-to-prepare systems in this work show important application, especially in water-based drilling fluids.
a r t i c l e
i n f o
Article history: Received 17 December 2015 Received in revised form 19 February 2016 Accepted 24 February 2016 Available online 3 March 2016 Keywords: Nanoemulsion Long chain oil Microemulsion dilution method Positive charge Water-based drilling fluids
a b s t r a c t The preparation of nanoemulsions using long chain oil (C20 to C33 ) with remarkably small droplet size by microemulsion dilution method is generally difficult. In this work, a simple, W/O microemulsion dilution method was used to prepare O/W nanoemulsions with long chain oil in water/Span 80 − Tween 80/paraffin system. With the increase of dilution temperature from 40 to 80 ◦ C, the emulsion droplet diameter decreased from 1.2 m to 61 nm. The increase in the amount of dilution water led to the increase of the droplet diameter of nanoemulsions. Meanwhile, nanoemulsions with different charge, even positive charge, were also formed by adding various concentration of Jeffamine (D230) or cetyltrimethylammonium bromide (CTAB) in the W/O microemulsions. More importantly, paraffin nanoemulsions, formed in situ when W/O microemulsion was added to water-based drilling fluids, have effective lubrication and permeability plugging ability. Hence, formation of nanoemulsion by microemulsion dilution method demonstrated here is of great importance for practical applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. E-mail address: [email protected] (D. Sun). http://dx.doi.org/10.1016/j.colsurfa.2016.02.039 0927-7757/© 2016 Elsevier B.V. All rights reserved.
Emulsions are dispersions of at least two immiscible liquids. Nanoemulsions are a class of emulsions with droplet diameters in the nanometer scale, generically in the range of 50–500 nm [1,2]. Microemulsions, another type of emulsion, also have a
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droplet diameter at most 100 nm [3]. The major difference between microemulsions and nanoemulsions is that nanoemulsions are not thermodynamically stable, while microemulsions are. Additionally, nanoemulsions can be diluted by water without changing the droplet size distribution [4]. Hence, the formation of nanoemulsions requires the input of energy. There are two main approaches: high energy emulsification methods [5,6] and low energy emulsification methods [2,7]. Nowadays, the main limitation for nanoemulsion applications is their relatively low stability [8]. Ostwald ripening is the major process leading to the instability of nanoemulsions [9]. Using high energy emulsification methods, the stability of nanoemulsions can be easily improved by increasing the carbon chain length of oil [10]. Ostwald ripening can be decreased by an increase in carbon chain length of the oil phase owing to the decrease of oil solubility in water [11]. The droplet size and distribution of these nanoemulsions do not change over months. High energy emulsification methods are generally achieved by high shear stirring, high pressure homogenizers or with ultrasound generators. The high energy input is needed to overcome the Laplace pressure to break the droplets into smaller ones. However, the disadvantages of these high energy methods are the energy cost [12]. In contrast, low energy emulsification methods can make full use of the internal chemical characteristics to prepare nanoemulsions with low energy input. Low energy emulsification methods can be classified as phase inversion composition method [13–16], phase inversion temperature method [17–19], or microemulsion dilution method. Nanoemulsions are prepared by phase inversion method due to a change in surfactant spontaneous curvature and phase transition during the emulsification process. This change of curvature is obtained by maintaining constant temperature (phase inversion composition method, PIC) or composition (phase inversion temperature method, PIT). For microemulsion dilution methods, nanoemulsions can be prepared by diluting O/W microemulsions, bicontinuous microemulsions, or W/O microemulsions with water [20]. Dilution of an O/W microemulsion with water induces part of the surfactants to dissolve into the aqueous phase. The surfactants still at the oil/water interface cannot maintain the low interfacial tension required for thermodynamic stability and the microemulsion droplets give rise to nanoemulsion droplets [21]. When diluting bicontinuous microemulsions, the homogeneous nucleation that occurs during the spontaneous emulsification leads to the formation of nanoemulsions [22,23]. Despite this mechanism, nanoemulsions may be formed by the migration of surfactants or cosurfactants through the oil-water interface due to the Ouzo effect when diluting bicontinuous microemulsions or W/O microemulsions [24,25]. When diluting W/O microemulsions, oil is the continuous phase before dilution. The existing oil may act as nuclei, leading to heterogeneous nucleation, and resulting in droplets with larger sizes and polydispersity [23]. Nanoemulsions formed by diluting O/W microemulsions or bicontinuous microemulsions are more stable and have smaller droplets [26]. In fact, the effect of carbon chain length of various n-alkanes (C10 to C18 ) on nanoemulsion formed by phase inversion methods has been investigated [27,28]. The droplet diameter of nanoemulsions increases with the increase of carbon chain length due to the increase in the interfacial tension and oil viscosity. By increasing emulsification temperature, long chain oil (C20 to C33 ) nanoemulsion with droplet size about 51 nm can be successfully formed by phase inversion composition [14]. However, only a few studies focused on the preparation of nanoemulsions with long chain oil (>C20 ) by microemulsion dilution method in decades. In this work, we studied the formation of nanoemulsions by W/O microemulsion dilution method using a long chain (C20 to C33 ) and viscous paraffin oil. This study explored the effect of the dilution
temperature, the structure of the initial concentrate, the surfactant concentration and the amount of dilution water on the properties of nanoemulsions. Meanwhile, nanoemulsions with different charge, even positive charge, were also formed by adding various concentration of cosurfactant D230 or cationic surfactant CTAB in the W/O microemulsions. Furthermore, practical application of paraffin W/O microemulsions in water-based drilling fluids was evaluated. 2. Experimental section 2.1. Materials Polyoxyethylene (20) sorbitan monooleate (Tween 80, chemically pure grade), sorbitan monooleate (Span 80, chemically pure grade), Cetyltrimethyl Ammonium Bromide (CTAB, chemically pure grade), liquid paraffin (d25 = 0.86, chemically pure grade) and kalium chloricum (KCl, analytically pure grade) were obtained from Sinopharm Chemical Reagent. O,O -Bis(2-aminopropyl) polypropyleneglycol (D230) was obtained from BASF. All reagents were used as received without further purification. Deionized water was used in all experiments. 2.2. Preparation of nanoemulsions To prepare a nanoemulsion, W/O microemulsion was first formed by homogenizing with a magnetic stirrer after all components of were weighed and sealed in ampules. After that, nanoemulsions were prepared by diluting W/O microemulsions with water at 70 ◦ C. During emulsification, W/O microemulsions were kept under continuous magnetic stirring. The influence of composition parameters, including the oil to surfactant mass ratio (O/S) and the dispersed phase mass percentage () of nanoemulsions were investigated systematically. 2.3. Droplet size determination Nanoemulsion droplet diameter and distribution were measured by dynamic light scattering (DLS), using a Brookhaven BI-200SM research gonimeter. A 200 mW green laser ( = 532 nm) with variable intensity was used, and measurements were carried out at room temperature with a scattering angle of 90◦ . The nanoemulsions were diluted about 1000 times with deionized water just before the measurements. The average diameter and polydispersity index were calculated from intensity autocorrelation data with the cumulants method. Intensity−intensity time correlation functions were analyzed by the CONTIN method [29]. 2.4. Interfacial tension measurements The interfacial tension between water and surfactant-in-oil solution was measured using a spinning drop interfacial tension meter Model TX500C, that may be used over a wide range (10−5 −102 mN m−1 ). Experiments were carried out with special care to avoid water evaporation. 2.5. Electrical conductivity Liquid paraffin, Span 80, Tween 80 and water (0.01 mol/L KCl aqueous solution), and were mixed at room temperature using a magnetic stirrer at 200 rpm. Span 80/Tween 80 wt ratios were set at 0.44/0.56, where the hydrophilic-lipophilic balance (HLB) (10.3 at this ratio) is proper for emulsifying liquid paraffin. The conductivity of the resulting emulsion was measured as a function of temperature using a Leici DDS-307 conductivity meter and a Pt/platinized
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electrode with a cell constant of 1.02 cm−1 (25 ◦ C), determined using standard KCl solutions. 2.6. Rheological measurements Rheological measurements were carried out on a HAAKE RS 6000 rheometer with a cone-plate geometry at 25 ◦ C. The diameter and the cone angle of the cone-plate were 35 mm and 1◦ , respectively. In steady shear experiments, the shear rate was typically increased from 1 to 1000 s−1 within 5 min. 2.7. Phase diagram determination All components were weighed, sealed in ampules, and homogenized with a magnetic stirrer. The samples were equilibrated at 70 ◦ C. The boundary lines were found by consecutive addition of water to mixtures of the Span80/Tween 80 and paraffin. The nature of the different phases and their boundaries were identified by conductivity measurements and transparency along water dilution paths. 2.8. Electrophoretic properties The zeta potential of nanoemulsion droplets was measured using a ZetaPALS zeta Potential Analyzer instrument (Brookhaven, USA). The values of the zeta potential were calculated by Smoluchowski’s formula: =
4 v ε U/L
(1)
where and ε are the viscosity and dielectric constant of water; v, the mobile velocity of the oil droplets in an electric field; U, the voltage; and L, the distance between the two electrodes. Experiments were performed at room temperature. 2.9. Lubrication properties Adhesion coefficient reduced rates (Kf ) were measured to evaluate the lubricity performance of the specific evaluation of the technical standard from China National Petroleum Corporation (CNPC) for liquid lubricants in water-based drilling fluids (Q/SY 1088-2007) was used. 2.10. Permeability plugging ability Filtration tests were carried out in a standard OFITE Automatic Pressure Control System for PPT (OFI Testing Equipment, Inc.) with ceramic filter disc (OFITE 3 um, 400 mD) according to the Automatic Pressure Control System for PPT guidelines. In each run, 300 mL of the water-based drilling fluids was placed in the filter press at 100 ◦ C under a differential pressure of 1000 psi provided by N2 gas chargers. The volumes of filtrate through the ceramic filter disc were determined at 1.0, 3.0, 5.0, 7.5, 15, and 30 min for each measurement. 3. Results and discussion 3.1. Formation of long chain oil nanoemulsions by W/O microemulsions dilution method at high dilution temperature The paraffin oil used here mainly consists of long-chain isoalkanes (C20 to C33 ), which makes it hard to emulsify the dispersed phase by microemulsion dilution methods at low temperature. The effect of dilution temperature on nanoemulsion droplet diameter is shown in Fig. 1. With the increase of dilution temperature, the droplet diameter of the nanoemulsions decreases from 1.2 m to
Fig. 1. Effect of dilution temperature on nanoemulsion droplet diameter and interfacial tension for samples with O/S = 1:1, = 0.5.
61 nm. When the dilution temperature is greater than 60 ◦ C, lowpolydispersity nanoemulsions with droplets of about 61 nm are prepared. The droplet diameter decreases dramatically with the increase of dilution temperature from 40 to 70 ◦ C and remains unchanged with a further increase in the temperature. This result is similar to the curve of the interfacial tension (Fig. 1). The lower interfacial tension at high temperature favors the intermingling of the polar and nonpolar components of the surfactant and therefore facilitates the spontaneous formation of a larger oil-water interface [30]. Furthermore, the increase in temperature leads to the decrease of the viscous resistance of the long chain oil phase, leading to faster adsorption of surfactant at higher temperatures. To prepare nanoemulsions by microemulsion dilution method, an initial concentrate is first formed. The phase diagram of the water/Span 80 − Tween 80/paraffin at 70 ◦ C is shown in Fig. 2a. Multiphase regions were observed in many regions of the diagram, identified as Em . A single microemulsion domain, W/O microemulsion, Om , is present at low water content. The nature of the different phases and their boundaries were identified by conductivity measurements and transparency (Fig. 2b) along water dilution paths with a fixed O/S ratio at 70 ◦ C. Evolution of the droplet diameter of nanoemulsions as a function of water mass percentage in initial concentrate was shown in Fig. 2b. When the water mass percentage of initial concentrate was 0 − 18%, nanoemulsions with droplet diameter of 61 nm were formed. The initial concentrate was W/O microemulsion or reverse micelle here as shown in Fig. 2a. When diluting W/O microemulsions, Tween 80 dissolved in the W/O droplets migrates through the oil/water interface to the water phase, resulting in the formation of nano-sized O/W droplets [8]. In addition, the surfactants at the oil/water interface may also reverse and migrate to the water phase, which also will form O/W droplets. When diluting reverse micelles, surfactant can migrate and inverse at the oil-water interface leading to the spontaneous increase in the surface area of the interface, resulting in the formation of O/W droplets. Here, the droplet diameter of the nanoemulsions formed by diluting reversed micelles was a little larger than 61 nm. When the water mass percentage of the concentrate was more than 18%, the droplets diameter of nanoemulsions were about 100 nm. Due to the increase of internal phase mass percentage, the initial concentrate becomes multiphase with high viscosity (see supporting information). Hence, it is difficult for surfactant molecules to migrate and inverse to form small O/W nano-sized droplets. Therefore, the water mass percentage of the initial concentrate was fixed at 5% in the following study. The influence of oil/surfactant mass ratio on the nanoemulsion droplet diameter and polydispersity index is shown in Fig. 3.
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Table 1 Effect of paraffin nanoemulsion/microemulsion on water-based drilling fluids.
a
WBD WBD + NEb WBD + MEc a b c d e f
Addition amount (%)
Adhesion coefficient
Kf d (%)
FLe (mL)
KFL f (%)
N/A 1.5 0.75
0.573 0.125 0.112
N/A 78 81
18 11 10
N/A 39 44
Water-based drilling fluids (WBD). Paraffin O/W nanoemulsion (NE) (O/S = 4:1 with 50% water). Paraffin W/O microemulsion (ME) (O/S = 4:1 with 1% D230, 5% water). The adhesion coefficient reduced rate. Fluid loss volume. Fluid loss volume reduced rate.
Fig. 3. Evolution of the droplet diameter and polydispersity index of nanoemulsions as a function of the oil/surfactant mass ratio of W/O microemulsions.
Fig. 2. (a) Partial phase diagram (T = 70 ◦ C) of water/Span 80 − Tween 80/paraffin. (Om , inverse micellar solution or W/O microemulsion; Em , multiphase region). (b) Evolution of the droplet diameter of nanoemulsions (O/S = 1:1, = 0.5) and conductivity as function of water mass percentage in initial concentrate at 70 ◦ C. Inset: the visual aspects of the initial concentrate with 9% and 30% water mass percentage are shown, respectively.
At low O/S ratio (1:1), nanoemulsions with a droplet diameter of 61 nm and low polydispersity index may be prepared by W/O microemulsion dilution method. The polydispersity index is less than 0.2 at low O/S, reflecting a mono distribution of droplet diameter in the nanoemulsions. The droplet diameter and polydispersity of nanoemulsions increase with the increase of O/S ratio, due to the decrease of surfactant which stabilizes the droplets. A series of O/W emulsions with different water mass percentages were prepared by mixing the surfactant, oil, and aqueous phase (10 mmol/L KCl in water) together using a shear mixer. Fig. 4a shows the variation of conductivity with temperature for emulsions at different water mass percentages (O/S = 1:1). All emulsions show
similar general trends in the conductivity-temperature profiles. With the increase of temperature, the conductivity of emulsions increases steadily and then decreases suddenly at a particular temperature. This observation is indicative of phase inversion from an O/W emulsion to a W/O emulsion. The PIT temperature, TPIT , was taken as the average value of the maximum and the minimum values of this temperature range. As shown in Fig. 4a, the TPIT increases with water mass percentage of emulsions in the system. The droplet diameter of nanoemulsions with O/S = 1:1 with different water mass percentages formed by diluting W/O microemulsions at 70 ◦ C is shown in Fig. 4b. For nanoemulsions formed by PIC or PIT methods, the droplet size is mainly governed by the structure of the phase during the phase inversion. Excess water acts only as a dilution procedure in the system [19]. Here, however, the droplet diameter of nanoemulsions increases with the increase of the amount of dilution water. This may be a result of two possible factors. First, as shown in Fig. 4a, TPIT increases with the water mass percentage of the emulsions. When the water mass percentage of the system was 50%, the TPIT was about 75 ◦ C. The TPIT gradually increased with the increase of water mass percentage. As the interfacial tension of system is lowest at the TPIT , the interfacial tension would increase with the increase of water used for dilution when diluted at 70 ◦ C. Second, oil is the continuous phase in W/O microemulsions. Existing oil droplets may act as nuclei to trigger heterogeneous nucleation when W/O microemulsions are diluted by water. With the increase of the amount of dilution water, the number of nuclei and the local surfactant concentration around the nuclei decrease at the same time. Therefore, the droplet diameter increases with the increase of the water mass percentage in the nanoemulsions. To further explore the mechanism of the effect of dilution amount of water on the nanoemulsions, the apparent viscosity and shear stress as a function of shear rate of nanoemulsions with different water mass percentages is shown in Fig. 5. Herschel-Bulkley
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Fig. 4. (a) Conductivity as a function of temperature for samples at different water mass percentages with O/S = 1:1. (b) Evolution of the nanoemulsion droplet diameter as a function of water mass percentage in nanoemulsions formed by the W/O microemulsion dilution method at 70 ◦ C with O/S = 1:1.
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Fig. 5. (a) The shear stress and (b) Viscosity as a function of shear rate for nanoemulsions with different water mass percentages formed by W/O microemulsions with O/S = 1:1.
3.2. The effect of cosurfactant/cationic surfactant in W/O microemulsion on properties of nanoemulsion model is a generalized model of a fluid, in which strain experienced by the fluid is related to the stress in a complicated, non-linear way. The constitution equation of the Herschel-Bulkley model is = 0 + k␥˙ n
(2)
where is the shear stress, ␥˙ is the shear rate, k is the consistency index, and n is the flow index [31,32]. After Herschel-Bulkley model adjust, the flow behavior index was 0.2572, 0.7263, 0.9219, 0.9860, 0.9889 for nanoemulsions with 50%, 60%, 70%, 80%, 90% water, respectively. Hence, the nanoemulsions show shear-thinning behavior. The structure of the nanoemulsion is broken gradually, and locally dense clusters are disrupted into individual droplets with the increase of shear rate [10]. Meanwhile, with the increase of the water mass percentage, shear-thinning behavior of the nanoemulsions was weaker, indicating that the cluster aggregation and interdroplet attraction became weaker. In addition, the apparent viscosity of the nanoemulsions decreased rapidly with the increase of water mass percentages. These provide indirect evidence that the local surfactant concentration around the droplets decreases with the increase of the water mass percentage in the nanoemulsion.
Generally, small molecular amines can be used as cosurfactant to decrease the interfacial tension of a system [33]. As shown in Fig. 6a, the droplet diameter of nanoemulsions decreases with the increase of cosurfactant D230. However, the Zeta potential of nanoemulsions decreases with the increase of D230. In our previous work, droplets of paraffin O/W nanoemulsions stabilized by Span80 and Tween80 mixtures were found to be negatively charged and the zeta potential strongly influenced by the pH of the system [34]. The surface charge of the droplets may be caused by the special adsorption of hydroxyl ions, due to the existence of hydrogen bonds between water molecules in the boundary layer and hydroxyl ions [35]. In addition, the presence of impurities (fatty acids) in the nonionic surfactant could also lead to the negative charge on the droplets formed by dilution of pure water [36]. D230 is a small molecule with positive charge, thus, with the increase of D230, the Zeta potential of droplets in nanoemulsions decreased to about 0 mV when D230 concentration was greater than 1.5%. However, the long-term stability of nanoemulsions did not depend on the surface charge and increased with the decrease of the droplet diameter (Fig. 6b), indicating that the electrostatic repulsion is not important in nonionic surfactant nanoemulsions. In fact, Ostwald ripening is the major instability mechanism of nanoemulsions [9]. Lower droplet size and polydispersity would lead to more stable nanoemulsions [11,37]. Therefore, nanoemulsions formed by
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Fig. 7. Evolution of the droplet diameter and Zeta potential of nanoemulsions as a function of CTAB mass percentage in W/O microemulsions with O/S = 1:1.
make the surfactants on the droplet surface align more closely. When the CTAB concentration was over 0.8%, the droplet diameter of nanoemulsions increased dramatically to 85 nm, larger than the nanoemulsions without CTAB. When CTAB concentration was over 0.8%, the adsorption of CTAB on the droplet surface reached the maximum, and excess CTAB would stack together, forming micelles. These micelles can carry oil from a small oil droplet and deposit the solute into a larger drop [38]. Therefore, the droplet diameter of nanoemulsions increases with the increase of CTAB concentration here. 3.3. Applications of W/O microemulsion dilution method in water-based drilling fluids Fig. 6. (a) Evolution of the droplet diameter and Zeta potential of nanoemulsions as a function of D230 mass percentage in W/O microemulsions with O/S = 2:1. (b) The photographs of nanoemulsions without and with 1.5% D230 after standing for 2 months.
diluting W/O microemulsions with 1.5% D230 were stable after standing for 2 months, whereas nanoemulsions without D230 broke. More importantly, nanoemulsions could be formed at room temperature in the system with D230 due to the low interfacial tension, whereas system without D230 cannot (see Supporting information). As shown in Fig. 6, nanoemulsions were generally negatively charged. In fact, positively charged nanoemulsions have important applications in pharmaceuticals, cosmetics and drilling fluids due to the electrostatic interaction with negative surface. As shown in Fig. 7, by adding CTAB in W/O microemulsion, the Zeta potential value of the negatively charged droplets gradually decreases from −56 mV to 0 mV, then increases up to +48 mV. The positive charge is due to that CTAB adsorbs on the droplet surface and neutralizes the negative charge. Therefore, a charge reversal occurs and droplets with a positive charge are formed. When the CTAB concentration was over 0.8%, the Zeta potential of droplets remained +48 mV despite of the increase of CTAB concentration, indicating that the adsorption of the cationic surfactant CTAB on the droplet surface reached the maximum. Meanwhile, the evolution of droplet diameter of nanoemulsions showed a trend of decrease and then increased in response to CTAB concentration. When CTAB concentration was 0–0.8%, the droplet diameter decreased from 71 nm to about 55 nm with the increase of CTAB concentration. This may be caused by the decrease of interfacial tension due to the adsorption of CTAB on the droplet surface, which can
Even though the droplet diameter of the nanoemulsions prepared by W/O microemulsion dilution method is similar to those obtained using the PIT or PIC method, the advantages of the W/O microemulsion dilution method are as follows: First, the efficiency of emulsification is improved by the W/O microemulsion dilution method. When 20 g nanoemulsion was prepared by the PIC method, the total emulsification process was carried out within 10 min. By the PIT method, the sample needs to be equilibrated at TPIT for more than 30 min before the temperature quench. When the nanoemulsion was prepared by the W/O microemulsion dilution method, the total emulsification process can be achieved in seconds. Second, the emulsification in the PIT method should be fixed to a temperature close to TPIT . Compared with PIT method, nanoemulsions could be prepared over a wide range of preparation temperature by W/O microemulsion dilution method. Third, even though nanoemulsions are kinetically stable systems and can be stable against flocculation, creaming, sedimentation, or coalescence, Ostwald ripening could still cause the growth of droplets in nanoemulsions and even result in nanoemulsions breaking. However, W/O microemulsions are thermodynamically stable systems. Hence, W/O microemulsions, can be diluted to O/W nanoemulsions on demand, are a good alternative to nanoemulsions. Paraffin nanoemulsions have broad application in water-based drilling fluids [25]. However, the instability of paraffin nanoemulsions for long-term storage is a limit for practical application [8]. Since paraffin W/O microemulsions could form nanoemulsions when diluted by large amount of water as shown above, the application of paraffin W/O microemulsions in water-based drilling fluids was investigated. When 1.5% paraffin nanoemulsion or 0.75% W/O microemulsion was added to the water-based drilling fluids, the adhesion coefficient of drilling fluid decreased
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application in pharmaceuticals, cosmetics and drilling fluids due to the electrostatic interaction. In practical application, paraffin nanoemulsions formed in situ when the paraffin W/O microemulsion was added into water-based drilling fluids, demonstrated effective lubrication and permeability plugging characteristics. More importantly, a microemulsion is thermodynamically stable, whereas a nanoemulsion is thermodynamically unstable. Hence, these easy-to-prepare systems show potential applications in other fields. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant 21333005). Appendix A. Supplementary data
Fig. 8. Evolution of fluid loss volume as function of t1/2 with different W/O microemulsion concentration in water-based drilling fluids.
from 0.573 to 0.112 and the fluid loss volume decreased from 18 mL to 11 or 10 mL (Table 1). When paraffin W/O microemulsion was added to the water-based drilling fluids, paraffin nanoemulsion was formed in situ. Hence, compared paraffin nanoemulsion with W/O microemulsion, the lubrication and permeability plugging ability of them were similar when the addition amount of W/O microemulsion was half that of nanoemulsion in the water-based drilling fluids. To further study the permeability plugging ability of paraffin W/O microemulsion, Fig. 8 shows the fluid loss volume versus t1/2 in water-based drilling fluids with different W/O microemulsion concentrations. In all curves, the fluid loss volume versus t1/2 shows a linear relation. We noted that the fluid loss rate of water-based drilling fluids with W/O microemulsions is about two times smaller than the water-based drilling fluids without W/O microemulsions. When the concentration of W/O microemulsion was over 1.5%, the fluid loss rate was lowest. When W/O microemulsions were added to the water-based drilling fluids, nanoemulsions were formed spontaneously. The nano-scale oil droplets could help block nanoscale pores of sandstone. In addition, the hydrophilic surface of pores of sandstone would become hydrophobic due to the adsorption of droplets on the surface, preventing the transport of water. Furthermore, thermodynamic W/O microemulsions can remain stable during storage and decrease the cost of transport. Hence, W/O microemulsion dilution method forming paraffin nanoemulsions may have important practical application in water-based drilling fluids. 4. Conclusion Nanoemulsions with long carbon chain oil (C20 to C33 ) were prepared by W/O microemulsion dilution method in system water/Span 80 − Tween 80/paraffin. Nanoemulsions were formed only when the dilution temperature was over 60 ◦ C. The structure of nanoemulsions is mainly controlled by both the structure of the initial concentrate and the amount of dilution water. Nanoemulsions were negatively charged in the nonionic surfactant system. Even though the addition of cosurfactant D230 in the system shielded the droplet charge, it decreased the droplet diameter, and enhanced the stability of nanoemulsions. More importantly, by adding cationic surfactant CTAB to the W/O microemulsions, positively charged nanoemulsions were formed. These positively charged nanoemulsions have important
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Formation of nanoemulsion with long chain oil by W/O microemulsion dilution method Kun Tong a , Chunhua Zhao b , Dejun Sun a,∗ a b
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, Shandong 250100, China Oilfield Chemistry Research Institute, Division of Oilfield Chemistry, China Oilfield Services Limited, Yanjiao, Hebei 065201, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Long chain oil nanoemulsion was formed by W/O microemulsion dilution method. • Nanoemulsions with different charge, even positive charge, were formed by W/O microemulsion dilution method. • Thermodynamically stable W/O microemulsion, can be diluted to nanoemulsion on demand, is an alternative to nanoemulsion. • Easy-to-prepare systems in this work show important application, especially in water-based drilling fluids.
a r t i c l e
i n f o
Article history: Received 17 December 2015 Received in revised form 19 February 2016 Accepted 24 February 2016 Available online 3 March 2016 Keywords: Nanoemulsion Long chain oil Microemulsion dilution method Positive charge Water-based drilling fluids
a b s t r a c t The preparation of nanoemulsions using long chain oil (C20 to C33 ) with remarkably small droplet size by microemulsion dilution method is generally difficult. In this work, a simple, W/O microemulsion dilution method was used to prepare O/W nanoemulsions with long chain oil in water/Span 80 − Tween 80/paraffin system. With the increase of dilution temperature from 40 to 80 ◦ C, the emulsion droplet diameter decreased from 1.2 m to 61 nm. The increase in the amount of dilution water led to the increase of the droplet diameter of nanoemulsions. Meanwhile, nanoemulsions with different charge, even positive charge, were also formed by adding various concentration of Jeffamine (D230) or cetyltrimethylammonium bromide (CTAB) in the W/O microemulsions. More importantly, paraffin nanoemulsions, formed in situ when W/O microemulsion was added to water-based drilling fluids, have effective lubrication and permeability plugging ability. Hence, formation of nanoemulsion by microemulsion dilution method demonstrated here is of great importance for practical applications. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. E-mail address: [email protected] (D. Sun). http://dx.doi.org/10.1016/j.colsurfa.2016.02.039 0927-7757/© 2016 Elsevier B.V. All rights reserved.
Emulsions are dispersions of at least two immiscible liquids. Nanoemulsions are a class of emulsions with droplet diameters in the nanometer scale, generically in the range of 50–500 nm [1,2]. Microemulsions, another type of emulsion, also have a
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droplet diameter at most 100 nm [3]. The major difference between microemulsions and nanoemulsions is that nanoemulsions are not thermodynamically stable, while microemulsions are. Additionally, nanoemulsions can be diluted by water without changing the droplet size distribution [4]. Hence, the formation of nanoemulsions requires the input of energy. There are two main approaches: high energy emulsification methods [5,6] and low energy emulsification methods [2,7]. Nowadays, the main limitation for nanoemulsion applications is their relatively low stability [8]. Ostwald ripening is the major process leading to the instability of nanoemulsions [9]. Using high energy emulsification methods, the stability of nanoemulsions can be easily improved by increasing the carbon chain length of oil [10]. Ostwald ripening can be decreased by an increase in carbon chain length of the oil phase owing to the decrease of oil solubility in water [11]. The droplet size and distribution of these nanoemulsions do not change over months. High energy emulsification methods are generally achieved by high shear stirring, high pressure homogenizers or with ultrasound generators. The high energy input is needed to overcome the Laplace pressure to break the droplets into smaller ones. However, the disadvantages of these high energy methods are the energy cost [12]. In contrast, low energy emulsification methods can make full use of the internal chemical characteristics to prepare nanoemulsions with low energy input. Low energy emulsification methods can be classified as phase inversion composition method [13–16], phase inversion temperature method [17–19], or microemulsion dilution method. Nanoemulsions are prepared by phase inversion method due to a change in surfactant spontaneous curvature and phase transition during the emulsification process. This change of curvature is obtained by maintaining constant temperature (phase inversion composition method, PIC) or composition (phase inversion temperature method, PIT). For microemulsion dilution methods, nanoemulsions can be prepared by diluting O/W microemulsions, bicontinuous microemulsions, or W/O microemulsions with water [20]. Dilution of an O/W microemulsion with water induces part of the surfactants to dissolve into the aqueous phase. The surfactants still at the oil/water interface cannot maintain the low interfacial tension required for thermodynamic stability and the microemulsion droplets give rise to nanoemulsion droplets [21]. When diluting bicontinuous microemulsions, the homogeneous nucleation that occurs during the spontaneous emulsification leads to the formation of nanoemulsions [22,23]. Despite this mechanism, nanoemulsions may be formed by the migration of surfactants or cosurfactants through the oil-water interface due to the Ouzo effect when diluting bicontinuous microemulsions or W/O microemulsions [24,25]. When diluting W/O microemulsions, oil is the continuous phase before dilution. The existing oil may act as nuclei, leading to heterogeneous nucleation, and resulting in droplets with larger sizes and polydispersity [23]. Nanoemulsions formed by diluting O/W microemulsions or bicontinuous microemulsions are more stable and have smaller droplets [26]. In fact, the effect of carbon chain length of various n-alkanes (C10 to C18 ) on nanoemulsion formed by phase inversion methods has been investigated [27,28]. The droplet diameter of nanoemulsions increases with the increase of carbon chain length due to the increase in the interfacial tension and oil viscosity. By increasing emulsification temperature, long chain oil (C20 to C33 ) nanoemulsion with droplet size about 51 nm can be successfully formed by phase inversion composition [14]. However, only a few studies focused on the preparation of nanoemulsions with long chain oil (>C20 ) by microemulsion dilution method in decades. In this work, we studied the formation of nanoemulsions by W/O microemulsion dilution method using a long chain (C20 to C33 ) and viscous paraffin oil. This study explored the effect of the dilution
temperature, the structure of the initial concentrate, the surfactant concentration and the amount of dilution water on the properties of nanoemulsions. Meanwhile, nanoemulsions with different charge, even positive charge, were also formed by adding various concentration of cosurfactant D230 or cationic surfactant CTAB in the W/O microemulsions. Furthermore, practical application of paraffin W/O microemulsions in water-based drilling fluids was evaluated. 2. Experimental section 2.1. Materials Polyoxyethylene (20) sorbitan monooleate (Tween 80, chemically pure grade), sorbitan monooleate (Span 80, chemically pure grade), Cetyltrimethyl Ammonium Bromide (CTAB, chemically pure grade), liquid paraffin (d25 = 0.86, chemically pure grade) and kalium chloricum (KCl, analytically pure grade) were obtained from Sinopharm Chemical Reagent. O,O -Bis(2-aminopropyl) polypropyleneglycol (D230) was obtained from BASF. All reagents were used as received without further purification. Deionized water was used in all experiments. 2.2. Preparation of nanoemulsions To prepare a nanoemulsion, W/O microemulsion was first formed by homogenizing with a magnetic stirrer after all components of were weighed and sealed in ampules. After that, nanoemulsions were prepared by diluting W/O microemulsions with water at 70 ◦ C. During emulsification, W/O microemulsions were kept under continuous magnetic stirring. The influence of composition parameters, including the oil to surfactant mass ratio (O/S) and the dispersed phase mass percentage () of nanoemulsions were investigated systematically. 2.3. Droplet size determination Nanoemulsion droplet diameter and distribution were measured by dynamic light scattering (DLS), using a Brookhaven BI-200SM research gonimeter. A 200 mW green laser ( = 532 nm) with variable intensity was used, and measurements were carried out at room temperature with a scattering angle of 90◦ . The nanoemulsions were diluted about 1000 times with deionized water just before the measurements. The average diameter and polydispersity index were calculated from intensity autocorrelation data with the cumulants method. Intensity−intensity time correlation functions were analyzed by the CONTIN method [29]. 2.4. Interfacial tension measurements The interfacial tension between water and surfactant-in-oil solution was measured using a spinning drop interfacial tension meter Model TX500C, that may be used over a wide range (10−5 −102 mN m−1 ). Experiments were carried out with special care to avoid water evaporation. 2.5. Electrical conductivity Liquid paraffin, Span 80, Tween 80 and water (0.01 mol/L KCl aqueous solution), and were mixed at room temperature using a magnetic stirrer at 200 rpm. Span 80/Tween 80 wt ratios were set at 0.44/0.56, where the hydrophilic-lipophilic balance (HLB) (10.3 at this ratio) is proper for emulsifying liquid paraffin. The conductivity of the resulting emulsion was measured as a function of temperature using a Leici DDS-307 conductivity meter and a Pt/platinized
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electrode with a cell constant of 1.02 cm−1 (25 ◦ C), determined using standard KCl solutions. 2.6. Rheological measurements Rheological measurements were carried out on a HAAKE RS 6000 rheometer with a cone-plate geometry at 25 ◦ C. The diameter and the cone angle of the cone-plate were 35 mm and 1◦ , respectively. In steady shear experiments, the shear rate was typically increased from 1 to 1000 s−1 within 5 min. 2.7. Phase diagram determination All components were weighed, sealed in ampules, and homogenized with a magnetic stirrer. The samples were equilibrated at 70 ◦ C. The boundary lines were found by consecutive addition of water to mixtures of the Span80/Tween 80 and paraffin. The nature of the different phases and their boundaries were identified by conductivity measurements and transparency along water dilution paths. 2.8. Electrophoretic properties The zeta potential of nanoemulsion droplets was measured using a ZetaPALS zeta Potential Analyzer instrument (Brookhaven, USA). The values of the zeta potential were calculated by Smoluchowski’s formula: =
4 v ε U/L
(1)
where and ε are the viscosity and dielectric constant of water; v, the mobile velocity of the oil droplets in an electric field; U, the voltage; and L, the distance between the two electrodes. Experiments were performed at room temperature. 2.9. Lubrication properties Adhesion coefficient reduced rates (Kf ) were measured to evaluate the lubricity performance of the specific evaluation of the technical standard from China National Petroleum Corporation (CNPC) for liquid lubricants in water-based drilling fluids (Q/SY 1088-2007) was used. 2.10. Permeability plugging ability Filtration tests were carried out in a standard OFITE Automatic Pressure Control System for PPT (OFI Testing Equipment, Inc.) with ceramic filter disc (OFITE 3 um, 400 mD) according to the Automatic Pressure Control System for PPT guidelines. In each run, 300 mL of the water-based drilling fluids was placed in the filter press at 100 ◦ C under a differential pressure of 1000 psi provided by N2 gas chargers. The volumes of filtrate through the ceramic filter disc were determined at 1.0, 3.0, 5.0, 7.5, 15, and 30 min for each measurement. 3. Results and discussion 3.1. Formation of long chain oil nanoemulsions by W/O microemulsions dilution method at high dilution temperature The paraffin oil used here mainly consists of long-chain isoalkanes (C20 to C33 ), which makes it hard to emulsify the dispersed phase by microemulsion dilution methods at low temperature. The effect of dilution temperature on nanoemulsion droplet diameter is shown in Fig. 1. With the increase of dilution temperature, the droplet diameter of the nanoemulsions decreases from 1.2 m to
Fig. 1. Effect of dilution temperature on nanoemulsion droplet diameter and interfacial tension for samples with O/S = 1:1, = 0.5.
61 nm. When the dilution temperature is greater than 60 ◦ C, lowpolydispersity nanoemulsions with droplets of about 61 nm are prepared. The droplet diameter decreases dramatically with the increase of dilution temperature from 40 to 70 ◦ C and remains unchanged with a further increase in the temperature. This result is similar to the curve of the interfacial tension (Fig. 1). The lower interfacial tension at high temperature favors the intermingling of the polar and nonpolar components of the surfactant and therefore facilitates the spontaneous formation of a larger oil-water interface [30]. Furthermore, the increase in temperature leads to the decrease of the viscous resistance of the long chain oil phase, leading to faster adsorption of surfactant at higher temperatures. To prepare nanoemulsions by microemulsion dilution method, an initial concentrate is first formed. The phase diagram of the water/Span 80 − Tween 80/paraffin at 70 ◦ C is shown in Fig. 2a. Multiphase regions were observed in many regions of the diagram, identified as Em . A single microemulsion domain, W/O microemulsion, Om , is present at low water content. The nature of the different phases and their boundaries were identified by conductivity measurements and transparency (Fig. 2b) along water dilution paths with a fixed O/S ratio at 70 ◦ C. Evolution of the droplet diameter of nanoemulsions as a function of water mass percentage in initial concentrate was shown in Fig. 2b. When the water mass percentage of initial concentrate was 0 − 18%, nanoemulsions with droplet diameter of 61 nm were formed. The initial concentrate was W/O microemulsion or reverse micelle here as shown in Fig. 2a. When diluting W/O microemulsions, Tween 80 dissolved in the W/O droplets migrates through the oil/water interface to the water phase, resulting in the formation of nano-sized O/W droplets [8]. In addition, the surfactants at the oil/water interface may also reverse and migrate to the water phase, which also will form O/W droplets. When diluting reverse micelles, surfactant can migrate and inverse at the oil-water interface leading to the spontaneous increase in the surface area of the interface, resulting in the formation of O/W droplets. Here, the droplet diameter of the nanoemulsions formed by diluting reversed micelles was a little larger than 61 nm. When the water mass percentage of the concentrate was more than 18%, the droplets diameter of nanoemulsions were about 100 nm. Due to the increase of internal phase mass percentage, the initial concentrate becomes multiphase with high viscosity (see supporting information). Hence, it is difficult for surfactant molecules to migrate and inverse to form small O/W nano-sized droplets. Therefore, the water mass percentage of the initial concentrate was fixed at 5% in the following study. The influence of oil/surfactant mass ratio on the nanoemulsion droplet diameter and polydispersity index is shown in Fig. 3.
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Table 1 Effect of paraffin nanoemulsion/microemulsion on water-based drilling fluids.
a
WBD WBD + NEb WBD + MEc a b c d e f
Addition amount (%)
Adhesion coefficient
Kf d (%)
FLe (mL)
KFL f (%)
N/A 1.5 0.75
0.573 0.125 0.112
N/A 78 81
18 11 10
N/A 39 44
Water-based drilling fluids (WBD). Paraffin O/W nanoemulsion (NE) (O/S = 4:1 with 50% water). Paraffin W/O microemulsion (ME) (O/S = 4:1 with 1% D230, 5% water). The adhesion coefficient reduced rate. Fluid loss volume. Fluid loss volume reduced rate.
Fig. 3. Evolution of the droplet diameter and polydispersity index of nanoemulsions as a function of the oil/surfactant mass ratio of W/O microemulsions.
Fig. 2. (a) Partial phase diagram (T = 70 ◦ C) of water/Span 80 − Tween 80/paraffin. (Om , inverse micellar solution or W/O microemulsion; Em , multiphase region). (b) Evolution of the droplet diameter of nanoemulsions (O/S = 1:1, = 0.5) and conductivity as function of water mass percentage in initial concentrate at 70 ◦ C. Inset: the visual aspects of the initial concentrate with 9% and 30% water mass percentage are shown, respectively.
At low O/S ratio (1:1), nanoemulsions with a droplet diameter of 61 nm and low polydispersity index may be prepared by W/O microemulsion dilution method. The polydispersity index is less than 0.2 at low O/S, reflecting a mono distribution of droplet diameter in the nanoemulsions. The droplet diameter and polydispersity of nanoemulsions increase with the increase of O/S ratio, due to the decrease of surfactant which stabilizes the droplets. A series of O/W emulsions with different water mass percentages were prepared by mixing the surfactant, oil, and aqueous phase (10 mmol/L KCl in water) together using a shear mixer. Fig. 4a shows the variation of conductivity with temperature for emulsions at different water mass percentages (O/S = 1:1). All emulsions show
similar general trends in the conductivity-temperature profiles. With the increase of temperature, the conductivity of emulsions increases steadily and then decreases suddenly at a particular temperature. This observation is indicative of phase inversion from an O/W emulsion to a W/O emulsion. The PIT temperature, TPIT , was taken as the average value of the maximum and the minimum values of this temperature range. As shown in Fig. 4a, the TPIT increases with water mass percentage of emulsions in the system. The droplet diameter of nanoemulsions with O/S = 1:1 with different water mass percentages formed by diluting W/O microemulsions at 70 ◦ C is shown in Fig. 4b. For nanoemulsions formed by PIC or PIT methods, the droplet size is mainly governed by the structure of the phase during the phase inversion. Excess water acts only as a dilution procedure in the system [19]. Here, however, the droplet diameter of nanoemulsions increases with the increase of the amount of dilution water. This may be a result of two possible factors. First, as shown in Fig. 4a, TPIT increases with the water mass percentage of the emulsions. When the water mass percentage of the system was 50%, the TPIT was about 75 ◦ C. The TPIT gradually increased with the increase of water mass percentage. As the interfacial tension of system is lowest at the TPIT , the interfacial tension would increase with the increase of water used for dilution when diluted at 70 ◦ C. Second, oil is the continuous phase in W/O microemulsions. Existing oil droplets may act as nuclei to trigger heterogeneous nucleation when W/O microemulsions are diluted by water. With the increase of the amount of dilution water, the number of nuclei and the local surfactant concentration around the nuclei decrease at the same time. Therefore, the droplet diameter increases with the increase of the water mass percentage in the nanoemulsions. To further explore the mechanism of the effect of dilution amount of water on the nanoemulsions, the apparent viscosity and shear stress as a function of shear rate of nanoemulsions with different water mass percentages is shown in Fig. 5. Herschel-Bulkley
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Fig. 4. (a) Conductivity as a function of temperature for samples at different water mass percentages with O/S = 1:1. (b) Evolution of the nanoemulsion droplet diameter as a function of water mass percentage in nanoemulsions formed by the W/O microemulsion dilution method at 70 ◦ C with O/S = 1:1.
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Fig. 5. (a) The shear stress and (b) Viscosity as a function of shear rate for nanoemulsions with different water mass percentages formed by W/O microemulsions with O/S = 1:1.
3.2. The effect of cosurfactant/cationic surfactant in W/O microemulsion on properties of nanoemulsion model is a generalized model of a fluid, in which strain experienced by the fluid is related to the stress in a complicated, non-linear way. The constitution equation of the Herschel-Bulkley model is = 0 + k␥˙ n
(2)
where is the shear stress, ␥˙ is the shear rate, k is the consistency index, and n is the flow index [31,32]. After Herschel-Bulkley model adjust, the flow behavior index was 0.2572, 0.7263, 0.9219, 0.9860, 0.9889 for nanoemulsions with 50%, 60%, 70%, 80%, 90% water, respectively. Hence, the nanoemulsions show shear-thinning behavior. The structure of the nanoemulsion is broken gradually, and locally dense clusters are disrupted into individual droplets with the increase of shear rate [10]. Meanwhile, with the increase of the water mass percentage, shear-thinning behavior of the nanoemulsions was weaker, indicating that the cluster aggregation and interdroplet attraction became weaker. In addition, the apparent viscosity of the nanoemulsions decreased rapidly with the increase of water mass percentages. These provide indirect evidence that the local surfactant concentration around the droplets decreases with the increase of the water mass percentage in the nanoemulsion.
Generally, small molecular amines can be used as cosurfactant to decrease the interfacial tension of a system [33]. As shown in Fig. 6a, the droplet diameter of nanoemulsions decreases with the increase of cosurfactant D230. However, the Zeta potential of nanoemulsions decreases with the increase of D230. In our previous work, droplets of paraffin O/W nanoemulsions stabilized by Span80 and Tween80 mixtures were found to be negatively charged and the zeta potential strongly influenced by the pH of the system [34]. The surface charge of the droplets may be caused by the special adsorption of hydroxyl ions, due to the existence of hydrogen bonds between water molecules in the boundary layer and hydroxyl ions [35]. In addition, the presence of impurities (fatty acids) in the nonionic surfactant could also lead to the negative charge on the droplets formed by dilution of pure water [36]. D230 is a small molecule with positive charge, thus, with the increase of D230, the Zeta potential of droplets in nanoemulsions decreased to about 0 mV when D230 concentration was greater than 1.5%. However, the long-term stability of nanoemulsions did not depend on the surface charge and increased with the decrease of the droplet diameter (Fig. 6b), indicating that the electrostatic repulsion is not important in nonionic surfactant nanoemulsions. In fact, Ostwald ripening is the major instability mechanism of nanoemulsions [9]. Lower droplet size and polydispersity would lead to more stable nanoemulsions [11,37]. Therefore, nanoemulsions formed by
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Fig. 7. Evolution of the droplet diameter and Zeta potential of nanoemulsions as a function of CTAB mass percentage in W/O microemulsions with O/S = 1:1.
make the surfactants on the droplet surface align more closely. When the CTAB concentration was over 0.8%, the droplet diameter of nanoemulsions increased dramatically to 85 nm, larger than the nanoemulsions without CTAB. When CTAB concentration was over 0.8%, the adsorption of CTAB on the droplet surface reached the maximum, and excess CTAB would stack together, forming micelles. These micelles can carry oil from a small oil droplet and deposit the solute into a larger drop [38]. Therefore, the droplet diameter of nanoemulsions increases with the increase of CTAB concentration here. 3.3. Applications of W/O microemulsion dilution method in water-based drilling fluids Fig. 6. (a) Evolution of the droplet diameter and Zeta potential of nanoemulsions as a function of D230 mass percentage in W/O microemulsions with O/S = 2:1. (b) The photographs of nanoemulsions without and with 1.5% D230 after standing for 2 months.
diluting W/O microemulsions with 1.5% D230 were stable after standing for 2 months, whereas nanoemulsions without D230 broke. More importantly, nanoemulsions could be formed at room temperature in the system with D230 due to the low interfacial tension, whereas system without D230 cannot (see Supporting information). As shown in Fig. 6, nanoemulsions were generally negatively charged. In fact, positively charged nanoemulsions have important applications in pharmaceuticals, cosmetics and drilling fluids due to the electrostatic interaction with negative surface. As shown in Fig. 7, by adding CTAB in W/O microemulsion, the Zeta potential value of the negatively charged droplets gradually decreases from −56 mV to 0 mV, then increases up to +48 mV. The positive charge is due to that CTAB adsorbs on the droplet surface and neutralizes the negative charge. Therefore, a charge reversal occurs and droplets with a positive charge are formed. When the CTAB concentration was over 0.8%, the Zeta potential of droplets remained +48 mV despite of the increase of CTAB concentration, indicating that the adsorption of the cationic surfactant CTAB on the droplet surface reached the maximum. Meanwhile, the evolution of droplet diameter of nanoemulsions showed a trend of decrease and then increased in response to CTAB concentration. When CTAB concentration was 0–0.8%, the droplet diameter decreased from 71 nm to about 55 nm with the increase of CTAB concentration. This may be caused by the decrease of interfacial tension due to the adsorption of CTAB on the droplet surface, which can
Even though the droplet diameter of the nanoemulsions prepared by W/O microemulsion dilution method is similar to those obtained using the PIT or PIC method, the advantages of the W/O microemulsion dilution method are as follows: First, the efficiency of emulsification is improved by the W/O microemulsion dilution method. When 20 g nanoemulsion was prepared by the PIC method, the total emulsification process was carried out within 10 min. By the PIT method, the sample needs to be equilibrated at TPIT for more than 30 min before the temperature quench. When the nanoemulsion was prepared by the W/O microemulsion dilution method, the total emulsification process can be achieved in seconds. Second, the emulsification in the PIT method should be fixed to a temperature close to TPIT . Compared with PIT method, nanoemulsions could be prepared over a wide range of preparation temperature by W/O microemulsion dilution method. Third, even though nanoemulsions are kinetically stable systems and can be stable against flocculation, creaming, sedimentation, or coalescence, Ostwald ripening could still cause the growth of droplets in nanoemulsions and even result in nanoemulsions breaking. However, W/O microemulsions are thermodynamically stable systems. Hence, W/O microemulsions, can be diluted to O/W nanoemulsions on demand, are a good alternative to nanoemulsions. Paraffin nanoemulsions have broad application in water-based drilling fluids [25]. However, the instability of paraffin nanoemulsions for long-term storage is a limit for practical application [8]. Since paraffin W/O microemulsions could form nanoemulsions when diluted by large amount of water as shown above, the application of paraffin W/O microemulsions in water-based drilling fluids was investigated. When 1.5% paraffin nanoemulsion or 0.75% W/O microemulsion was added to the water-based drilling fluids, the adhesion coefficient of drilling fluid decreased
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application in pharmaceuticals, cosmetics and drilling fluids due to the electrostatic interaction. In practical application, paraffin nanoemulsions formed in situ when the paraffin W/O microemulsion was added into water-based drilling fluids, demonstrated effective lubrication and permeability plugging characteristics. More importantly, a microemulsion is thermodynamically stable, whereas a nanoemulsion is thermodynamically unstable. Hence, these easy-to-prepare systems show potential applications in other fields. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant 21333005). Appendix A. Supplementary data
Fig. 8. Evolution of fluid loss volume as function of t1/2 with different W/O microemulsion concentration in water-based drilling fluids.
from 0.573 to 0.112 and the fluid loss volume decreased from 18 mL to 11 or 10 mL (Table 1). When paraffin W/O microemulsion was added to the water-based drilling fluids, paraffin nanoemulsion was formed in situ. Hence, compared paraffin nanoemulsion with W/O microemulsion, the lubrication and permeability plugging ability of them were similar when the addition amount of W/O microemulsion was half that of nanoemulsion in the water-based drilling fluids. To further study the permeability plugging ability of paraffin W/O microemulsion, Fig. 8 shows the fluid loss volume versus t1/2 in water-based drilling fluids with different W/O microemulsion concentrations. In all curves, the fluid loss volume versus t1/2 shows a linear relation. We noted that the fluid loss rate of water-based drilling fluids with W/O microemulsions is about two times smaller than the water-based drilling fluids without W/O microemulsions. When the concentration of W/O microemulsion was over 1.5%, the fluid loss rate was lowest. When W/O microemulsions were added to the water-based drilling fluids, nanoemulsions were formed spontaneously. The nano-scale oil droplets could help block nanoscale pores of sandstone. In addition, the hydrophilic surface of pores of sandstone would become hydrophobic due to the adsorption of droplets on the surface, preventing the transport of water. Furthermore, thermodynamic W/O microemulsions can remain stable during storage and decrease the cost of transport. Hence, W/O microemulsion dilution method forming paraffin nanoemulsions may have important practical application in water-based drilling fluids. 4. Conclusion Nanoemulsions with long carbon chain oil (C20 to C33 ) were prepared by W/O microemulsion dilution method in system water/Span 80 − Tween 80/paraffin. Nanoemulsions were formed only when the dilution temperature was over 60 ◦ C. The structure of nanoemulsions is mainly controlled by both the structure of the initial concentrate and the amount of dilution water. Nanoemulsions were negatively charged in the nonionic surfactant system. Even though the addition of cosurfactant D230 in the system shielded the droplet charge, it decreased the droplet diameter, and enhanced the stability of nanoemulsions. More importantly, by adding cationic surfactant CTAB to the W/O microemulsions, positively charged nanoemulsions were formed. These positively charged nanoemulsions have important
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