Journal Of Colloid And Interface Science Volume 314 Issue 1 2007 [doi 10.1016_j.jcis.2007.04.079] Lijuan Wang; Xuefeng Li; Gaoyong Zhang; Jinfeng Dong; Julian Eas -- Oil-in-water Nanoemulsions For P.pdf

Journal of Colloid and Interface Science 314 (2007) 230–235 www.elsevier.com/locate/jcis

Oil-in-water nanoemulsions for pesticide formulations Lijuan Wang a , Xuefeng Li a , Gaoyong Zhang a , Jinfeng Dong a,∗ , Julian Eastoe b a College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China b School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK

Received 3 March 2007; accepted 15 April 2007 Available online 13 May 2007

Abstract A two-step process for formation of nanoemulsions in the system water/poly(oxyethylene) nonionic surfactant/methyl decanoate at 25 ◦ C is described. First, all the components were mixed at a certain composition to prepare a microemulsion concentrate, which was rapidly subjected into a large dilution into water to generate an emulsion. Bluish transparent oil-in-water (O/W) nanoemulsions were formed only when the concentrate was located in the bicontinuous microemulsion (BC) or oil-in-water microemulsion (Wm) region. The existence of an optimum oil-to-surfactant ratio (Ros ) in the BC or Wm region indicates that both the phase behavior and the composition of the concentrate are important factors in nanoemulsion formation. To demonstrate potential applications of these systems, they were employed to formulate a water-insoluble pesticide, β-cypermethrin (β-CP). The nanoemulsion was compared with a commercial β-CP microemulsion in terms of the stability of sprayed formulations. © 2007 Elsevier Inc. All rights reserved. Keywords: Nanoemulsion; Equilibrium phase behavior; Pesticide formulation

1. Introduction Nanoemulsions [1] have uniform and extremely small droplet sizes, typically in the range of 20–200 nm. In addition, high kinetic stability, low viscosity and optical transparency make them very attractive systems for many industrial applications; for example, in the pharmaceutical field as drug delivery systems [2,3], in cosmetics as personal care formulations [4], in agrochemicals for pesticide delivery [5], and in the chemical industry as polymerization reaction media [6]. The use of nanoemulsions as colloidal drug carriers is well-documented [3, 7–9]. The bioavailability of drugs was reported to be strongly enhanced by solubilization in small droplets (below 0.2 µm); for example, submicronic emulsions were found to increase the bioavailability of cefpodoxime proxetil from 50 to 98%, compared to other oral formulations [9]. Unlike microemulsions, nanoemulsions are metastable systems, and stability depends on the method of preparation. The most common approach is high-energy emulsification [10], using high-shear stirring, high-pressure homogenizers and ultra* Corresponding author.

E-mail address: [email protected] (J. Dong). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.04.079

sound generators. More recently a neat low-energy emulsification method [11] has been developed, by taking advantage of phase behavior and properties, to promote the formation of ultra-small droplets. These low energy techniques include selfemulsification [12–14], phase transition [1,15,16] and phase inversion temperature methods (PIT) [17–22]. To make use of these approaches, it is necessary to study the relationship between the equilibrium phase behavior of the initial system and the resulting nanoemulsions. For example, with the PIT method, emulsions are obtained by increasing or lowering the temperature quickly to pass through the HLB (hydrophile– lipophile balance) temperature in a system containing nonionic surfactant. In particular, if the initial system is located in a bicontinuous microemulsion region (D phase) or a two phase (W + D) system at the HLB temperature, nanoemulsions can be readily generated [20]. In the phase transition method, water is added dropwise to a mixture of surfactants and oil at constant temperature. The formation of nanoemulsions is generally attributed to phase instabilities during emulsification, where the presence of lamellar liquid crystallites and/or bicontinuous microemulsions are thought to play critical roles [1,23,24]. However, no direct evidence has yet been put forward to clarify these issues. Further effort is required in order to fully understand the

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mechanism of nanoemulsion formation, and therefore, optimize nanoemulsification processes. In this work, a new self-nanoemulsifying alcohol-free system was developed, where fatty acid methyl ester (methyl decanoate) was chosen as the oil phase. In order to gain a better understanding of the isothermal formation of these nanoemulsions, a two-step process (method A), was compared with phase transition methods (B and C); these experiments were designed to clarify which equilibrium phase is responsible for the formation of nanoemulsions during the phase transition. The relationship between final droplet sizes and the equilibrium phase behavior of the initial microemulsion concentrate was studied. One reason for employing fatty acid methyl esters as oily component is due to their mild irritation to eyes and skin [25,26]. Indeed, fatty acid methyl esters derived from vegetable oils have gained attention over recent years as solvents in a variety of applications [27–29]; significantly they are also widely used as economically viable solvents in pesticide delivery systems [30,31]. To investigate potential applications of the system developed here, a water insoluble pesticide, β-cypermethrin (β-CP), was incorporated into the precursor microemulsion concentrate. The effect of this active pesticide on stabilities of the concentrate, and the corresponding nanoemulsion, were also investigated. The formulation process presented in this work consists of incorporating β-CP in a bicontinuous microemulsion, which can be converted into a nanoemulsion spontaneously upon water dilution. This technique provides a new method to formulate water insoluble pesticides for spraying applications.

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Fig. 1. Schematic representation of the experimental path in method A.

(A) A two-step process: first, mixing all appropriate components to generate a concentrate; then, a certain amount of concentrate was injected into a very much larger volume of water under gentle stirring to achieve the final emulsion. The water concentration in the initial concentrate was fixed at 50 wt%. This could be termed a “crash dilution” method. (B) Water was added dropwise to the surfactant and oil mixture. The addition rate was adjusted carefully, to ensure it was slow enough that so the bicontinuous D phase or oil-inwater Wm phase was present, but not too slow to result in an increase in droplet size due to emulsion destabilization. (C) An appropriate amount of water was poured into a starting solution of the surfactant and oil mixture. No obvious distinct “stable” phases were noted during the emulsification procedure, except for the starting isotropic mixtures of oil and surfactant and the final emulsion. The final concentration of water was kept constant at 97.5 wt% and the temperature was held at 25 ± 1 ◦ C (thermostat bath K20, ThermoHaake, Germany).

2. Experimental 2.1. Materials A technical grade poly(oxyethylene) lauryl ether, with an average of 7 mol of ethylene oxide (EO) per surfactant molecule, was supplied by Xingtai Lantian Jingxi Chemical Co. Lt. Methyl decanoate (purity 98.7%) was supplied by Wujiang Tianhong Food Corporation. β-Cypermethrin (β-CP) (purity 97%) and a commercial β-CP microemulsion were purchased from Yetian Agrochemical Corporation. All products were used without further purification. Water was twice distilled. 2.2. Methods 2.2.1. Phase diagrams All components were weighed, sealed in ampoules, and homogenized with a vibromixer. The samples were kept at 25 ◦ C to equilibrate. Optically anisotropic liquid crystalline phases were identified by using polarizing light microscopy (PLM, BX51, Olympus, Japan) through identification of characteristic textures. The boundary lines were found by consecutive addition of one component to mixtures of the other components. 2.2.2. Nanoemulsion preparation Emulsions were prepared using the following low-energy emulsification methods (Fig. 1):

2.2.3. Droplet size measurement The mean droplet size and distribution of the nanoemulsions were determined by dynamic light scattering (DLS) at a scattering angle of 173◦ (Zetasizer Nano-ZS, Malvern, UK) at 25 ◦ C, employing an argon laser (λ = 633 nm). 2.2.4. Pesticide-loaded formulations The solubility of β-CP is much higher in methyl decanoate (468 mg/ml) compared to in water (1.13 × 10−4 mg/ml) at 25 ◦ C. β-CP was incorporated to oil/surfactant mixtures prior to the addition of water to form the active-loaded concentrate at 25 ◦ C. The concentrate was then injected into water under gentle stirring to generate nanoemulsions. Dispersions were sprayed onto glass slides which were examined visually 24 h after spraying. The appearance of crystals observed by polarizing light microscopy (PLM) indicated that the sprayed solution was unstable. 3. Results and discussion 3.1. Equilibrium phase behavior of three-component systems: water/nonionic surfactant/methyl decanoate The phase diagram of the water/nonionic surfactant/methyl decanoate at 25 ◦ C is shown in Fig. 2. Four distinct one-phase regions are observed, consistent with a micellar solution of

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Fig. 2. Phase behavior of the water/poly(oxyethylene) nonionic surfactant/ methyl decanoate system at 25 ◦ C. II, two-liquid isotropic phases; Om, isotropic liquid phase (inverse micellar solution or W/O microemulsion); Wm, isotropic liquid colorless phase (micellar solution of O/W microemulsion); Lα, optically anisotropic phase; D, isotropic liquid phase (bicontinuous microemulsion); M, multiphase region (phases not determined).

an O/W microemulsion (Wm), an inverse micellar solution or W/O microemulsion (Om), a bicontinuous microemulsion or O/W microemulsion (D or Wm) and lamellar liquid crystalline phases (Lα). The phases Wm, D and Om are isotropic, colorless fluids. The rest of the diagram consists of several two- and multiple-phase regions. Along the oil–surfactant axis, a twophase region (II) extends between oil–surfactant ratios of 70/30 to 88/12. At higher water concentrations, the two-phase region denoted as (Lα + Om) consists of a lower liquid crystalline phase with an upper colorless liquid phase. The region M denotes a multiphase region, where phase equilibria were not determined. 3.2. Effect of initial concentrate phase behavior on the final diluted nanoemulsion The final emulsions were prepared according to method A (the two-step process) which was described in Section 2. The droplet sizes of the resulting nanoemulsions as a function of initial water concentration in the concentrate are shown in Fig. 3 for different oil–surfactant weight ratios (Ros ). Broadly, this figure can be divided into three regions. In regions I and III, emulsions with large droplet sizes and high polydispersity indices were obtained, which appeared milky white or translucent. Whereas, in region II, for which the concentrate is the D or Wm phase, the resulting emulsion is of very small droplet size (∼30 nm) and narrow distribution (polydispersity index <0.2). These results illustrate a close relationship between the equilibrium phase behavior of the initial concentrate and the droplet sizes and polydispersities of the resulting emulsions. Nanoemulsions, with small droplet sizes and narrow distributions, are formed only when the concentrate starts off as a bicontinuous D phase or Wm microemulsion. Similar results have been observed by employing the PIT method [1,16,21, 22,32]. Bluish transparent O/W nanoemulsions were obtained

when the equilibrium phases at the HLB temperature were D or W + D phases (no excess oil had separated at the HLB temperature). It is well known that hydration of the ethylene oxide groups is dramatically increased by reducing temperature, and hence promoting a preferred curvature change of the surfactant monolayer and consequently the tendency for oil droplet formation. The results in Fig. 3 can also be explained by inversion of the interfacial monolayer curvature. In region I, the concentrate begins as an W/O microemulsion, the formation of O/W emulsions by dilution with water requires the inversion of surfactant film curvature, which will demand more curvature free energy to generate small droplets. This effect would contribute towards an increase of droplet size and polydispersity [33]. When the initial concentrate was located in region II, the oil phase is completely solubilized in a bicontinuous (D phase) or oil-inwater (Wm) microemulsion. The dilution of this concentrate with excess water decreases the surfactant concentration in the system, which inevitably leads to a decrease in oil solubilization. Subsequently, the system becomes supersaturated in oil, which may lead to homogeneous nucleation of oil in the form of small monodisperse droplets [13]. When the concentrate was located in region III, the simple dilution by water produced highly polydisperse emulsions, likely because of heterogeneous nucleation [33]. 3.3. Effect of the emulsification method on the nanoemulsion droplet sizes In order to investigate the effect of the emulsification process, related systems were prepared by three different methods as described in Section 2. The droplet sizes of the emulsions as a function of oil–surfactant weight ratio (Ros ) obtained by different approaches are shown in Fig. 4. A U-shape curve is evident, showing nanoemulsions obtained by both methods A and B, with low polydispersity (<0.2) for Ros values between 0.8 and 1.2. Highly polydisperse emulsions, characterized by indices of about 0.4 were obtained for Ros outside this range. Considering the equilibrium phase behavior of the concentrate used in method A, it can be observed that smaller droplet sizes are obtained when the initial concentrate was a D phase bicontinuous microemulsion or an oil-in-water microemulsion (Wm). Increasing Ros causes the separation of an excess oil phase, resulting in higher droplet sizes. On the other hand, a decrease in Ros produced the separation of a lamellar liquid crystalline phase, which also resulted in higher droplet sizes. The same results were obtained by method B, because the same phase evolution was experienced during the emulsification process. However, emulsions obtained by method C were highly polydisperse and with large droplet sizes irrespective of Ros . The reason is that no D phase or Wm was present during the emulsification process. This agrees with the aforementioned results and shows clearly that the existence of D or Wm phases is crucial to nanoemulsion formation. The slight increase of droplet size, as Ros increases from 0.8 to 1.2, can be explained by following three points:

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3.4. Nanoemulsion formation at constant oil–surfactant weight ratio (Ros )

Fig. 3. Droplet size of the nanoemulsions at 25 ◦ C as a function of water content in the concentrate with various oil-to-surfactant weight ratios Ros : 0.9 (1), 1 (!), and 1.1 (P). The dotted lines indicate the range of D or Wm phase for the corresponding system.

It was also of interest to study the nanoemulsion droplet sizes prepared by method A as a function of water concentration in the final emulsion. Note when Ros = 1 and the water content of the concentrate is fixed at 50 wt%, the systems are in the D or Wm phase range. Bluish transparent O/W nanoemulsions with droplet sizes on the order of 28 nm were obtained, independent of the water content (data shown in Supplementary material); in addition, polydispersity indices were lower than 0.2. These results show that nanoemulsions form when the D or Wm phase range microemulsion contacts with extra water; most likely the excess water acts as a dilution medium resulting in oil nucleation. The droplet sizes are mainly controlled by the structure of the D or Wm phase, independent of the volume of water in the final emulsion. 3.5. Stability of nanoemulsions The nanoemulsions prepared displayed good stability; although there was no phase separation after several weeks, an increase in droplet size was noted with time. The two most probable breakdown processes in dispersed systems are coalescence and Ostwald ripening. The Lifshitz–Slezov and Wagner (LSW) theory [34] gives the following expression for the rate of Ostwald ripening:   ω = dr 3 /dt = (8/9) (C∞γ VmD)/ρRT , (1)

Fig. 4. Droplet sizes at 25 ◦ C as a function of Ros by method A (1), method B (!), and method C (P). The dotted lines indicate the range of D or Wm phase for the concentrate used in method A.

First, the structure of concentrate changes with increasing Ros , which increases the size of the oil domains of the bicontinuous phase, leading to an increase in the droplet size. Second, owing to the dynamic nature of surfactant adsorption at the oil–water interface, the newly formed interface is more rapidly stabilized at higher surfactant concentrations; therefore, smaller, more stable droplets are formed at lower Ros . Furthermore, at low surfactant levels the interfacial tension would decrease with increasing surfactant concentration, which would also favor the formation of smaller droplets. Lamaallam et al. [14] have reported similar results by varying surfactant concentration from 6 to 11 wt%.

where C∞ is the bulk phase solubility (the solubility of the oil in an infinitely large droplet), γ is the interfacial tension, Vm is the molar volume of the oil, D is the diffusion coefficient of the oil in the continuous phase, ρ is the density of the oil, R is the gas constant, and T is the absolute temperature. To determine if the main breakdown process was Ostwald ripening, the cube of the radius, r 3 , is plotted as a function of time at different Ros in Fig. 5. The linear variation of r 3 as a function of time indicates that the mechanism of instability can be attributed to Ostwald ripening. It is shown that the slope declines with the decreasing Ros , which means the more stable nanoemulsions are obtained with higher surfactant concentration. This phenomenon is consistent with results reported earlier [35–37]. It was suggested that excess micelles formed in the aqueous phase, which act a solubilization sites for added oil. The oil solubilized in the micelles was not dispersed at the molecular level in the continuous phase. As a result, the increase of the amount of micelles actually lowers the solubility of oil in the bulk phase, viz. the term C∞ in Eq. (1) and hence the ripening rate. Besides, the interfacial tension reduced as the increase of surfactant concentration, which is a factor to reduce the Ostwald ripening according to in Eq. (1). 4. β-Cypermethrin (β-CP) incorporation Nanoemulsions have shown to be advantageous for optimizing delivery of water-insoluble compounds in agrochemicals [5]. The small size of the droplets allows them to deposit

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Fig. 5. Droplet sizes at 25 ◦ C as a function of time at various Ros : 0.8 (1), 0.9 (!), 1 (P), 1.1 (夽) and 1.2 (E).

uniformly on plant leaves. Wetting, spreading and penetration may be also enhanced as a result of the low surface tension of the whole system and the low interfacial tension of the O/W droplets [38]. The utility of any given system for drug or pesticide delivery depends on whether it is likely to be diluted on use, and whether the solubilization capacity is lost on dilution [39]. In this work, the agrochemical active β-CP was employed as a model water insoluble compound to investigate the potential application of nanoemulsions prepared by this two-step process for pesticide delivery. The concentrate with oil–surfactant weight ratio (Ros ) = 1 and 50 wt% water was chosen as a self-nanoemulsified system to prepare nanoemulsions for active-loading studies. β-CP was dissolved in oil/surfactant mixtures prior formulation of the concentrate. The droplet size and stability of the β-CPloaded nanoemulsions were evaluated in the same way as for the active-free system. Fig. 6 shows the droplet size of nanoemulsions as a function of concentration of β-CP in methyl decanoate. The droplet size of nanoemulsions prepared with less than 12 wt% of β-CP was around 30 nm (polydispersity lower than 0.2). The droplet size and polydispersity with greater than 12 wt% β-CP increased dramatically, because of the appearance of multi-phase M systems in the initial formulations. This observation confirms that nanoemulsions form only when the phase of the concentrate is the bicontinuous D, or watercontinuous Wm phase. With a large amount of β-CP, the phase behavior of the concentrate changes from a D phase or Wm to an M phase, thus leading to a sharp increase in droplet size. The droplet sizes of nanoemulsions after 24 h of preparation show no obvious difference compared to systems in the presence or absence of the pesticide, which means pesticide has no noticeable effect on the stability/size of the final nanoemulsions. The stability of sprayed solutions resulting from dilution of the prepared formulation, and the corresponding commercial β-CP microemulsion, was compared in terms of droplet sizes and their solubilization capacity on dilution (Figs. 7 and 8). Although the droplet size does not show significant differences in the two systems, the precipitation of pesticide appeared in

Fig. 6. Droplet sizes at 25 ◦ C as a function of the β-CP present in the oil phase at 0 h (1) and 24 h (!), respectively. The corresponding phase behavior for the initial system is shown above.

Fig. 7. Droplet sizes obtained from dilution of nanoemulsion formulation (-2-) and commercial microemulsion (-"-), at 25 ◦ C as a function of time.

the sprayed solution of the commercial β-CP microemulsion within 24 h of dilution, whereas no precipitation was observed from the concentrate (Fig. 8). It has been shown that precipitated pesticides have lower bioavailability in spraying applications [40]. The results of Figs. 7 and 8 indicate the solubilization capacity of the concentrate was maintained on dilution, which shows this system is very suitable for agrochemical active delivery. 5. Conclusions O/W nanoemulsions have been obtained at constant temperature by a two-step process (method A), which involves crash dilution of a bicontinuous or oil-in-water microemulsion into a large volume of water. A key factor for the formation of nanoemulsions at constant temperature has been identified as the use of a bicontinuous D phase or oil-in-water microemulsion (Wm) as the initial concentrate. The two-step process is easy to

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Fig. 8. Polarizing light microscopy images taken of spray solution diluted from (a) the commercial β-CP microemulsion; (b) the nanoemulsion formulation after 24 h.

scale up and with less energy consumption, which is of great interest for practical applications. The incorporation of β-CP in the concentrate showed no effect on the phase behavior when present at less than 12 wt%. Compared with the commercial β-CP microemulsion, the excellent stability of sprayed solution diluted from the concentrate makes this system an ideal candidate as a water-insoluble pesticide delivery system. Thus, the application of the new methodology in designing spray formulations of β-CP may enable a reduction in the applied amounts, relative to those formulated as O/W microemulsions. These characteristics make the new methodology promising from both environmental and economical points of view. Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC 20573079) and Ministry of Science and Technology (2006 BAE01A07-5). Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2007.04.079. References [1] A. Forgiarini, J. Esquena, C. Gonza’lez, C. Solans, Langmuir 17 (2001) 2076. [2] H.L. Wu, C. Ramachandran, N.D. Weiner, B.J. Roessler, Int. J. Pharm. 220 (2001) 63. [3] E.I. Taha, S. Al-Saidan, A.M. Samy, M.A. Khan, Int. J. Pharm. 285 (2004) 109. [4] O. Sonneville-Aubrun, J.-T. Simonnet, F. L’Alloret, Adv. Colloid Interface Sci. 108–109 (2004) 145. [5] G. Lee, T. Tadros, Colloids Surf. 5 (1982) 105. [6] X. Liu, Y. Guan, Z. Ma, H. Liu, Langmuir 20 (2004) 10278. [7] N. Sadurni, C. Solans, N. Azemar, M.J. Garcia-Celma, Eur. J. Pharm. Sci. 26 (2005) 438. [8] J.S. Schwarz, M.R. Weisspapir, D.I. Friedman, Pharm. Res. 12 (1995) 687. [9] G. Nicolaos, S. Crauste-Manciet, R. Farinotti, D. Brossard, Int. J. Pharm. 263 (2003) 165.

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