Journal of Colloid and Interface Science 314 (2007) 230–235www.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, Chinab 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 ◦Cis described. First, all the components were mixed at a certain composition to prepare a microemulsion concentrate, which was rapidly sub-jected into a large dilution into water to generate an emulsion. Bluish transparent oil-in-water (O/W) nanoemulsions were formed only whenthe concentrate was located in the bicontinuous microemulsion (BC) or oil-in-water microemulsion (Wm) region. The existence of an optimumoil-to-surfactant ratio (Ros) in the BC or Wm region indicates that both the phase behavior and the composition of the concentrate are importantfactors in nanoemulsion formation. To demonstrate potential applications of these systems, they were employed to formulate a water-insolublepesticide, β-cypermethrin (β-CP). The nanoemulsion was compared with a commercial β-CP microemulsion in terms of the stability of sprayedformulations.© 2007 Elsevier Inc. All rights reserved.

Keywords: Nanoemulsion; Equilibrium phase behavior; Pesticide formulation

1. Introduction

Nanoemulsions [1] have uniform and extremely small drop-let sizes, typically in the range of 20–200 nm. In addition, highkinetic stability, low viscosity and optical transparency makethem very attractive systems for many industrial applications;for example, in the pharmaceutical field as drug delivery sys-tems [2,3], in cosmetics as personal care formulations [4], inagrochemicals for pesticide delivery [5], and in the chemicalindustry as polymerization reaction media [6]. The use of na-noemulsions as colloidal drug carriers is well-documented [3,7–9]. The bioavailability of drugs was reported to be stronglyenhanced by solubilization in small droplets (below 0.2 µm);for example, submicronic emulsions were found to increase thebioavailability of cefpodoxime proxetil from 50 to 98%, com-pared to other oral formulations [9].

Unlike microemulsions, nanoemulsions are metastable sys-tems, and stability depends on the method of preparation. Themost common approach is high-energy emulsification [10], us-ing high-shear stirring, high-pressure homogenizers and ultra-

* Corresponding author.E-mail address: colloid@whu.edu.cn (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 emulsifi-cation method [11] has been developed, by taking advantageof phase behavior and properties, to promote the formation ofultra-small droplets. These low energy techniques include self-emulsification [12–14], phase transition [1,15,16] and phaseinversion temperature methods (PIT) [17–22]. To make useof these approaches, it is necessary to study the relationshipbetween the equilibrium phase behavior of the initial systemand the resulting nanoemulsions. For example, with the PITmethod, emulsions are obtained by increasing or lowering thetemperature quickly to pass through the HLB (hydrophile–lipophile balance) temperature in a system containing nonionicsurfactant. In particular, if the initial system is located in abicontinuous microemulsion region (D phase) or a two phase(W + D) system at the HLB temperature, nanoemulsions canbe readily generated [20]. In the phase transition method, wateris added dropwise to a mixture of surfactants and oil at constanttemperature. The formation of nanoemulsions is generally at-tributed to phase instabilities during emulsification, where thepresence of lamellar liquid crystallites and/or bicontinuous mi-croemulsions are thought to play critical roles [1,23,24]. How-ever, no direct evidence has yet been put forward to clarify theseissues. Further effort is required in order to fully understand the

L. Wang et al. / Journal of Colloid and Interface Science 314 (2007) 230–235 231

mechanism of nanoemulsion formation, and therefore, optimizenanoemulsification processes.

In this work, a new self-nanoemulsifying alcohol-free sys-tem was developed, where fatty acid methyl ester (methyl de-canoate) was chosen as the oil phase. In order to gain a betterunderstanding of the isothermal formation of these nanoemul-sions, a two-step process (method A), was compared with phasetransition methods (B and C); these experiments were designedto clarify which equilibrium phase is responsible for the for-mation of nanoemulsions during the phase transition. The rela-tionship between final droplet sizes and the equilibrium phasebehavior of the initial microemulsion concentrate was studied.One reason for employing fatty acid methyl esters as oily com-ponent is due to their mild irritation to eyes and skin [25,26].Indeed, fatty acid methyl esters derived from vegetable oilshave gained attention over recent years as solvents in a vari-ety of applications [27–29]; significantly they are also widelyused as economically viable solvents in pesticide delivery sys-tems [30,31].

To investigate potential applications of the system developedhere, a water insoluble pesticide, β-cypermethrin (β-CP), wasincorporated into the precursor microemulsion concentrate. Theeffect 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 in-corporating β-CP in a bicontinuous microemulsion, which canbe converted into a nanoemulsion spontaneously upon waterdilution. This technique provides a new method to formulatewater insoluble pesticides for spraying applications.

2. Experimental

2.1. Materials

A technical grade poly(oxyethylene) lauryl ether, with anaverage of 7 mol of ethylene oxide (EO) per surfactant mole-cule, was supplied by Xingtai Lantian Jingxi Chemical Co.Lt. Methyl decanoate (purity 98.7%) was supplied by WujiangTianhong Food Corporation. β-Cypermethrin (β-CP) (purity97%) and a commercial β-CP microemulsion were purchasedfrom Yetian Agrochemical Corporation. All products were usedwithout further purification. Water was twice distilled.

2.2. Methods

2.2.1. Phase diagramsAll components were weighed, sealed in ampoules, and ho-

mogenized with a vibromixer. The samples were kept at 25 ◦Cto equilibrate. Optically anisotropic liquid crystalline phaseswere identified by using polarizing light microscopy (PLM,BX51, Olympus, Japan) through identification of characteristictextures. The boundary lines were found by consecutive addi-tion of one component to mixtures of the other components.

2.2.2. Nanoemulsion preparationEmulsions were prepared using the following low-energy

emulsification methods (Fig. 1):

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

(A) A two-step process: first, mixing all appropriate compo-nents to generate a concentrate; then, a certain amount ofconcentrate was injected into a very much larger volumeof water under gentle stirring to achieve the final emulsion.The water concentration in the initial concentrate was fixedat 50 wt%. This could be termed a “crash dilution” method.

(B) Water was added dropwise to the surfactant and oil mix-ture. The addition rate was adjusted carefully, to ensure itwas slow enough that so the bicontinuous D phase or oil-in-water Wm phase was present, but not too slow to result inan increase in droplet size due to emulsion destabilization.

(C) An appropriate amount of water was poured into a startingsolution of the surfactant and oil mixture. No obvious dis-tinct “stable” phases were noted during the emulsificationprocedure, except for the starting isotropic mixtures of oiland surfactant and the final emulsion.

The final concentration of water was kept constant at97.5 wt% and the temperature was held at 25±1 ◦C (thermostatbath K20, ThermoHaake, Germany).

2.2.3. Droplet size measurementThe mean droplet size and distribution of the nanoemulsions

were determined by dynamic light scattering (DLS) at a scatter-ing angle of 173◦ (Zetasizer Nano-ZS, Malvern, UK) at 25 ◦C,employing an argon laser (λ = 633 nm).

2.2.4. Pesticide-loaded formulationsThe solubility of β-CP is much higher in methyl decanoate

(468 mg/ml) compared to in water (1.13 × 10−4 mg/ml) at25 ◦C. β-CP was incorporated to oil/surfactant mixtures priorto the addition of water to form the active-loaded concentrateat 25 ◦C. The concentrate was then injected into water undergentle stirring to generate nanoemulsions. Dispersions weresprayed onto glass slides which were examined visually 24 hafter spraying. The appearance of crystals observed by polariz-ing light microscopy (PLM) indicated that the sprayed solutionwas 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/methyldecanoate at 25 ◦C is shown in Fig. 2. Four distinct one-phaseregions are observed, consistent with a micellar solution of

232 L. Wang et al. / Journal of Colloid and Interface Science 314 (2007) 230–235

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 microemul-sion); M, multiphase region (phases not determined).

an O/W microemulsion (Wm), an inverse micellar solution orW/O microemulsion (Om), a bicontinuous microemulsion orO/W microemulsion (D or Wm) and lamellar liquid crystallinephases (Lα). The phases Wm, D and Om are isotropic, color-less fluids. The rest of the diagram consists of several two- andmultiple-phase regions. Along the oil–surfactant axis, a two-phase region (II) extends between oil–surfactant ratios of 70/30to 88/12. At higher water concentrations, the two-phase regiondenoted as (Lα + Om) consists of a lower liquid crystallinephase with an upper colorless liquid phase. The region M de-notes a multiphase region, where phase equilibria were not de-termined.

3.2. Effect of initial concentrate phase behavior on the finaldiluted nanoemulsion

The final emulsions were prepared according to method A(the two-step process) which was described in Section 2. Thedroplet sizes of the resulting nanoemulsions as a function of ini-tial water concentration in the concentrate are shown in Fig. 3for different oil–surfactant weight ratios (Ros). Broadly, thisfigure can be divided into three regions. In regions I and III,emulsions with large droplet sizes and high polydispersity in-dices were obtained, which appeared milky white or translu-cent. Whereas, in region II, for which the concentrate is the Dor Wm phase, the resulting emulsion is of very small dropletsize (∼30 nm) and narrow distribution (polydispersity index<0.2). These results illustrate a close relationship between theequilibrium phase behavior of the initial concentrate and thedroplet sizes and polydispersities of the resulting emulsions.Nanoemulsions, with small droplet sizes and narrow distrib-utions, are formed only when the concentrate starts off as abicontinuous D phase or Wm microemulsion. Similar resultshave 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 Dor W + D phases (no excess oil had separated at the HLB tem-perature). It is well known that hydration of the ethylene oxidegroups is dramatically increased by reducing temperature, andhence promoting a preferred curvature change of the surfactantmonolayer and consequently the tendency for oil droplet for-mation.

The results in Fig. 3 can also be explained by inversion ofthe interfacial monolayer curvature. In region I, the concentratebegins as an W/O microemulsion, the formation of O/W emul-sions by dilution with water requires the inversion of surfactantfilm curvature, which will demand more curvature free energyto generate small droplets. This effect would contribute towardsan increase of droplet size and polydispersity [33]. When theinitial concentrate was located in region II, the oil phase iscompletely solubilized in a bicontinuous (D phase) or oil-in-water (Wm) microemulsion. The dilution of this concentratewith excess water decreases the surfactant concentration in thesystem, which inevitably leads to a decrease in oil solubiliza-tion. Subsequently, the system becomes supersaturated in oil,which may lead to homogeneous nucleation of oil in the formof small monodisperse droplets [13]. When the concentrate waslocated in region III, the simple dilution by water producedhighly polydisperse emulsions, likely because of heterogeneousnucleation [33].

3.3. Effect of the emulsification method on the nanoemulsiondroplet sizes

In order to investigate the effect of the emulsificationprocess, related systems were prepared by three different meth-ods as described in Section 2. The droplet sizes of the emulsionsas a function of oil–surfactant weight ratio (Ros) obtained bydifferent approaches are shown in Fig. 4. A U-shape curve isevident, showing nanoemulsions obtained by both methods Aand B, with low polydispersity (<0.2) for Ros values between0.8 and 1.2. Highly polydisperse emulsions, characterized byindices of about 0.4 were obtained for Ros outside this range.Considering the equilibrium phase behavior of the concentrateused in method A, it can be observed that smaller droplet sizesare obtained when the initial concentrate was a D phase bi-continuous microemulsion or an oil-in-water microemulsion(Wm). Increasing Ros causes the separation of an excess oilphase, resulting in higher droplet sizes. On the other hand,a decrease in Ros produced the separation of a lamellar liq-uid crystalline phase, which also resulted in higher dropletsizes. The same results were obtained by method B, becausethe same phase evolution was experienced during the emulsi-fication process. However, emulsions obtained by method Cwere highly polydisperse and with large droplet sizes irre-spective of Ros. The reason is that no D phase or Wm waspresent during the emulsification process. This agrees withthe aforementioned results and shows clearly that the exis-tence of D or Wm phases is crucial to nanoemulsion forma-tion.

The slight increase of droplet size, as Ros increases from 0.8to 1.2, can be explained by following three points:

L. Wang et al. / Journal of Colloid and Interface Science 314 (2007) 230–235 233

Fig. 3. Droplet size of the nanoemulsions at 25 ◦C as a function of water con-tent 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 forthe corresponding system.

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 phasefor the concentrate used in method A.

First, the structure of concentrate changes with increasingRos, which increases the size of the oil domains of the bicontin-uous phase, leading to an increase in the droplet size.

Second, owing to the dynamic nature of surfactant adsorp-tion at the oil–water interface, the newly formed interfaceis more rapidly stabilized at higher surfactant concentrations;therefore, smaller, more stable droplets are formed at lower Ros.

Furthermore, at low surfactant levels the interfacial tensionwould decrease with increasing surfactant concentration, whichwould also favor the formation of smaller droplets.

Lamaallam et al. [14] have reported similar results by vary-ing surfactant concentration from 6 to 11 wt%.

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

It was also of interest to study the nanoemulsion droplet sizesprepared by method A as a function of water concentration inthe final emulsion. Note when Ros = 1 and the water content ofthe concentrate is fixed at 50 wt%, the systems are in the D orWm phase range. Bluish transparent O/W nanoemulsions withdroplet sizes on the order of 28 nm were obtained, independentof the water content (data shown in Supplementary material); inaddition, polydispersity indices were lower than 0.2. These re-sults show that nanoemulsions form when the D or Wm phaserange microemulsion contacts with extra water; most likely theexcess water acts as a dilution medium resulting in oil nucle-ation. The droplet sizes are mainly controlled by the structureof the D or Wm phase, independent of the volume of water inthe final emulsion.

3.5. Stability of nanoemulsions

The nanoemulsions prepared displayed good stability; al-though there was no phase separation after several weeks, anincrease in droplet size was noted with time. The two mostprobable breakdown processes in dispersed systems are coales-cence and Ostwald ripening. The Lifshitz–Slezov and Wagner(LSW) theory [34] gives the following expression for the rateof Ostwald ripening:

(1)ω = dr3/dt = (8/9)[(C∞γ VmD)/ρRT

],

where C∞ is the bulk phase solubility (the solubility of the oilin an infinitely large droplet), γ is the interfacial tension, Vm isthe molar volume of the oil, D is the diffusion coefficient of theoil in the continuous phase, ρ is the density of the oil, R is thegas constant, and T is the absolute temperature.

To determine if the main breakdown process was Ostwaldripening, the cube of the radius, r3, is plotted as a functionof time at different Ros in Fig. 5. The linear variation of r3

as a function of time indicates that the mechanism of instabil-ity can be attributed to Ostwald ripening. It is shown that theslope declines with the decreasing Ros, which means the morestable nanoemulsions are obtained with higher surfactant con-centration. This phenomenon is consistent with results reportedearlier [35–37]. It was suggested that excess micelles formedin the aqueous phase, which act a solubilization sites for addedoil. The oil solubilized in the micelles was not dispersed at themolecular level in the continuous phase. As a result, the in-crease of the amount of micelles actually lowers the solubilityof oil in the bulk phase, viz. the term C∞ in Eq. (1) and hencethe ripening rate. Besides, the interfacial tension reduced as theincrease of surfactant concentration, which is a factor to reducethe Ostwald ripening according to in Eq. (1).

4. β-Cypermethrin (β-CP) incorporation

Nanoemulsions have shown to be advantageous for opti-mizing delivery of water-insoluble compounds in agrochemi-cals [5]. The small size of the droplets allows them to deposit

234 L. Wang et al. / Journal of Colloid and Interface Science 314 (2007) 230–235

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

uniformly on plant leaves. Wetting, spreading and penetrationmay be also enhanced as a result of the low surface tension ofthe whole system and the low interfacial tension of the O/Wdroplets [38]. The utility of any given system for drug or pes-ticide delivery depends on whether it is likely to be diluted onuse, and whether the solubilization capacity is lost on dilution[39]. In this work, the agrochemical active β-CP was employedas a model water insoluble compound to investigate the po-tential application of nanoemulsions prepared by this two-stepprocess for pesticide delivery.

The concentrate with oil–surfactant weight ratio (Ros) = 1and 50 wt% water was chosen as a self-nanoemulsified sys-tem to prepare nanoemulsions for active-loading studies. β-CPwas dissolved in oil/surfactant mixtures prior formulation ofthe concentrate. The droplet size and stability of the β-CP-loaded nanoemulsions were evaluated in the same way as forthe active-free system. Fig. 6 shows the droplet size of na-noemulsions as a function of concentration of β-CP in methyldecanoate. The droplet size of nanoemulsions prepared withless than 12 wt% of β-CP was around 30 nm (polydispersitylower than 0.2). The droplet size and polydispersity with greaterthan 12 wt% β-CP increased dramatically, because of the ap-pearance of multi-phase M systems in the initial formulations.This observation confirms that nanoemulsions form only whenthe phase of the concentrate is the bicontinuous D, or water-continuous Wm phase. With a large amount of β-CP, the phasebehavior of the concentrate changes from a D phase or Wm toan M phase, thus leading to a sharp increase in droplet size. Thedroplet sizes of nanoemulsions after 24 h of preparation showno obvious difference compared to systems in the presence orabsence of the pesticide, which means pesticide has no notice-able effect on the stability/size of the final nanoemulsions.

The stability of sprayed solutions resulting from dilution ofthe prepared formulation, and the corresponding commercialβ-CP microemulsion, was compared in terms of droplet sizesand their solubilization capacity on dilution (Figs. 7 and 8). Al-though the droplet size does not show significant differencesin 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 phaseat 0 h (1) and 24 h (!), respectively. The corresponding phase behavior for theinitial 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 microemulsionwithin 24 h of dilution, whereas no precipitation was observedfrom the concentrate (Fig. 8). It has been shown that precipi-tated pesticides have lower bioavailability in spraying applica-tions [40]. The results of Figs. 7 and 8 indicate the solubilizationcapacity of the concentrate was maintained on dilution, whichshows this system is very suitable for agrochemical active de-livery.

5. Conclusions

O/W nanoemulsions have been obtained at constant temper-ature by a two-step process (method A), which involves crashdilution of a bicontinuous or oil-in-water microemulsion intoa large volume of water. A key factor for the formation of na-noemulsions at constant temperature has been identified as theuse of a bicontinuous D phase or oil-in-water microemulsion(Wm) as the initial concentrate. The two-step process is easy to

L. Wang et al. / Journal of Colloid and Interface Science 314 (2007) 230–235 235

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 greatinterest for practical applications.

The incorporation of β-CP in the concentrate showed no ef-fect on the phase behavior when present at less than 12 wt%.Compared with the commercial β-CP microemulsion, the ex-cellent stability of sprayed solution diluted from the concentratemakes this system an ideal candidate as a water-insoluble pesti-cide delivery system. Thus, the application of the new method-ology in designing spray formulations of β-CP may enable areduction in the applied amounts, relative to those formulatedas O/W microemulsions. These characteristics make the newmethodology promising from both environmental and econom-ical points of view.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (NSFC 20573079) and Ministry of Sci-ence and Technology (2006 BAE01A07-5).

Supplementary material

The online version of this article contains additional supple-mentary material.

Please visit DOI: 10.1016/j.jcis.2007.04.079.

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