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emulsion, liquid droplets and/or liquid crystals are dispersed in

relatively high kinetic stability, even for several years [1 ]. Theother source of misunderstandings about nano-emulsions is

Evidently, the preparation method influences emulsion proper-

cellar phases, take place during emulsification by condensationor low-energy methods. However, this does not mean that atthe end of the emulsification process the system is in ther-modynamic equilibrium.

Current Opinion in Colloid & Interface Sciea liquid [2]. Obviously, microemulsions are excluded from thisdefinition if the word dispersed is interpreted as non-equi-librium and opposite to solubilized, a term that can be ap-plied to microemulsions and micellar systems. Therefore, thereis a fundamental difference between microemulsions and nano-emulsions: microemulsions are equilibrium systems (i.e. ther-modynamically stable), while nano-emulsions are non-equili-brium systems with a spontaneous tendency to separate into theconstituent phases. Nevertheless, nano-emulsions may possess a

ties (e.g. droplet size, stability, etc.), but the nature of the finaldispersion (the constituent phases) is the same whether themethod of preparation uses high shear (external energy, dis-persion methods) or the chemical energy stored in the system(condensation methods). The view expressed in Ref. [3] is basedin a misinterpretation of the mechanisms by which nano-emulsions form by low-energy methods, described, for instance,in Ref. [5]. Phase transitions involving equilibrium phases,such as lyotropic liquid crystalline, microemulsion and/or mi-1. Introduction

There are two major misunderstandings in the literatureregarding nano-emulsions. One arises from their similarities tomicroemulsions. Nano-emulsions are emulsions with an extre-mely small droplet size [1] which can overlap those of micro-emulsions. The definition of emulsions by the InternationalUnion of Pure and Applied Chemistry (IUPAC) states: In an

related to the method of preparation. For instance, in Ref. [3], areview on nano-emulsions, only emulsions with droplet size inthe nanometer range obtained by shear methods are consideredas nano-emulsions. According to these authors, emulsions withextremely small droplet size (i.e. nano-emulsions) obtained bythe so-called condensation methods (e.g. phase inversion tem-perature (PIT) or composition (PIC) methods, self-emulsifyingmethods, etc.) should not be considered as nano-emulsions.Keywords: Nano-emulsions; Preparation; Applications; Optimization; EmulsificationNano-emulsions: New applications

J.M. Gutirrez a,, C. Gonzlez a, A. Ma Chemical Engineering Department, University of Ba

b CSIC/IIQAB, CIBER-bbn, c/Jordi

Received 10 September 2007; received in reviseAvailable online

Abstract

Nano-emulsions, as non-equilibrium systems, present characteristicpreparation method. Although interest in nano-emulsions was developelast years that direct applications of nano-emulsions in consumer produapplications have made that studies on optimization methods for nano-erecent literature on developments of nano-emulsions as final applicatio 2008 Elsevier Ltd. All rights reserved. Corresponding author. Tel.: +34 934021292; fax: +34 934021291.E-mail address: josemaria.gutierrez@ub.edu (J.M. Gutirrez).

1359-0294/$ - see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.cocis.2008.01.005d optimization of their preparation

stro a, I. Sol a, C.M. Pey a, J. Nolla b

ona, c/ Mart i Franqus 1, 08020 Barcelona, Spainna 18-26, 08034-Barcelona, Spain

rm 29 January 2008; accepted 30 January 2008February 2008

nd properties which depend not only on composition but also on thence about 20 years ago, mainly for nanoparticle preparation, it is in theare being developed, mainly in pharmacy and cosmetics. These recentlsion preparation be a requirement. This review is focused on the mostroducts and on the optimization of their preparation.

nce 13 (2008) 245251www.elsevier.com/locate/cocisAs a summary of this point, nano-emulsions are emulsions(non-equilibrium systems, defined according to [1]) with aremarkable small droplet size (in the nanometer range, e.g. 20

200 nm), regardless of the preparation method. A photographyof an oil-in-water (O/W) nano-emulsion with a schematic ex-ample of the structure is presented in Fig. 1.

Evidently the size range may vary depending on the authors.Some authors consider 500 nm as the upper limit [1,6]. In anycase, the size limit is not a key issue because no qualitativedifferences are established by droplet size. The formation,properties and stability of nano-emulsions are well established innumerous papers which are reviewed [1,5]. Regarding ap-plications, nano-emulsions were firstly developed, and used fora long time, to obtain nanoparticles by polymerization [7], theso-called miniemulsion polymerization method, and morerecently to obtain solid lipid nanoparticles [810], and ceramicparticles [11]. At present, new applications are being developedto use nano-emulsions as consumer products.

In this review, recent literature on the new applications ofnano-emulsions as consumer products is reviewed and classifiedaccording to the field of application. This direct application ofnano-emulsions requires the optimization with respect to for-

246 J.M. Gutirrez et al. / Current Opinion in Collomulation and preparation variables in order to obtain the desiredcharacteristics. Recent literature on optimization of nano-emul-sion preparation is also reviewed and classified according to threeapproaches: considerations on phase behavior, selective variationof parameters and experimental designs.

2. Direct application of nano-emulsions in final products

Practically all original and review papers on nano-emulsionsstress their great potential for applications. However, after morethan 10 years since a growing interest in nano-emulsions wasdeveloped, reports on direct applications of nano-emulsionsare not as numerous as expected. The main limitation for de-veloping applications for nano-emulsions is their stability.Although practically all papers on nano-emulsions indicatethat nano-emulsions can be stable even by years, the smallFig. 1. Visual aspect of an O/W nano-emulsion and structural conformation ofthe droplets.droplet size makes nano-emulsions break by the Ostwald-ripening mechanism [1214] in time periods which pose a greatlimitation for developing applications different than nanoparti-cle preparation. In fact, only an extremely low solubility of thedispersed phase, as presented by silicone oils, would give thestability needed for most of the applications. In this context,Mason research group has published several papers studyingnano-emulsions with silicone oils as dispersed phase, [1517].These nano-emulsions show great stability even for such a highconcentration that droplets are deformed to a foam-like structure[3], but no applications have been yet developed for these nano-emulsions.

Due to the limitations in the stability of nano-emulsions, arevision of the most recent literature results in few papers pro-posing new applications, and in most of them, nano-emulsionsmust be prepared shortly before their use.

With respect to agrochemical applications, in a recentreference [18] nano-emulsions are proposed for solubilizingwater-insoluble pesticides, the classical potential application.Nano-emulsions are formed before the application by dilutionof a concentrate containing oil, surfactant, the active and 50% ofwater. It is claimed that the nano-emulsions obtained show goodstability, but the diameter increases about five times in 14 h,from 40 to 200 nm. Therefore, application should be carried outa few hours after preparation for obtaining the advantagesof small droplet size. In this paper, the preparation of nano-emulsions is presented as a novelty when comparing withcommercial microemulsion formulations which also result bydilution in nano-emulsions. This fact demonstrates that ap-plication of nano-emulsions for solubilizing pesticides is not apotential application but a commercial one.

Concerning the applications in food technology, two recentreviews about possibilities of nanotechnology [19,20] indicatethat there are potential applications of nano-emulsions, but nospecific applications are given. In Ref. [19], potential ap-plications for nanostructured materials in general are analyzed,but regarding nano-emulsions, only a mention about theirpotential application is made. In Ref. [20], a detailed review onemulsification techniques is presented but the possible applica-tions that are indicated are not likely to have an important impactin the future food technology. Finally, nano-emulsification ofcarotene containing hexane is described in Ref. [21] although thefinal application proposed after evaporation of hexane is adispersion of carotene, not a nano-emulsion.

In cosmetics, a generic paper [22] presents the possibilitiesfor improving stability through using different oils and oilmixtures. Adequate stability is only achieved by using morethan 50% of such heavy oil as isocetyl isostearate. Cosmeticproperties of nano-emulsions are analyzed and favorableconclusions are obtained. Other papers presenting studies fornano-emulsion application on skin were also published [2326].

Pharmacy is the field where more direct applications ofnano-emulsions are proposed. Many of them consist in self-emulsifying systems, so the stability problem is solved by

id & Interface Science 13 (2008) 245251using the nano-emulsions short after their preparation. Self-emulsifying nano-emulsions for parenteral application [27] ororal application [2830] have been described. Nano-emulsion

olloformulations are studied for their application as nanocarrierswhich allow the treatment of a variety of diseases. The followingare examples of the most recent proposals of drugs solubilizedin nano-emulsions for disease treatments: anticonvulsant[27], antihypertensive [28] antibiotic [30]; antinflammatoryapplied through skin [26]. There are reports on: drugs solubilizedin nano-emulsions for HIV/AIDS therapy [31]; mechanisms ofatherogenesis studied with cholesterol nano-emulsions [32];cancer therapy investigated by solubilizing the drug in acholesterol rich nano-emulsion [33]; intestinal absorption ofthree model drugs solubilized in nano-emulsions [34]; efficacy ofa schistosomicidal compound solubilized in nano-emulsions;[35]; and application of anthrax vaccine through W/O nano-emulsions [36]. Special magnetic nano-emulsions are alsobeing studied for medicine applications [3739]. A recentreview [40] shows extensively potential multifunctionalapplications of nanocarriers including nano-emulsions inpharmacy. As stated in the paper, multifunctional nanocarrierscould provide almost unlimited opportunities in producinghighly efficient and specialized systems for drugs, genes anddiagnostic agents. Antimicrobial generic activity of nano-emulsions is also being investigated [41] with W/O nano-emulsions that are diluted in water just before applicationreverse to O/W emulsions. More specifically, inactivation ofEbola virus by nano-emulsion was studied [42] with promisingresults. It was concluded that nano-emulsions could be used asdisinfectants.

3. Optimization of nano-emulsion preparation

The properties of nano-emulsions, as non-equilibrium sys-tems, depend not only on composition variables but preparationvariables such as emulsifying path, agitation or emulsificationtime. These variables can have a significant influence on thenano-emulsion final properties. Direct application of nano-emulsions requires optimization studies for achieving the bestproperties for specific applications. The most frequent aim foroptimization is to exploit the advantages of nano-emulsions withrespect to conventional emulsions (i.e. macroemulsion): smallsize and low polydispersity. Therefore, in general, optimizationis directed to obtain minimum droplet size and/or minimumpolydispersity. Another aim in nano-emulsion optimization isto improve the stability because, as stated above, stability isthe main problem to overcome to find practical applicationsfor nano-emulsions. Optimization is also directed to obtain anoptimum in the function for which the nano-emulsions are used(e.g. drug delivery).

The properties to be optimized, for example droplet size andpolydispersity, will depend, of course, on composition variables,and could depend on preparation variables, so optimization can becarried out with respect to these two types of variables. Con-cerning optimization methods, sometimes the characteristics ofemulsification path allow predicting optimum properties of nano-emulsions, so optimizations are carried out by studying the phase

J.M. Gutirrez et al. / Current Opinion in Cbehavior of the systems. In other occasions, optimization isexperimentally carried out by selective variation of one variable.Finally, given the high number of variables that can influence thefinal properties of nano-emulsions, optimization is carried outby experimental designs which allow reducing the numberof experiments needed. Review of papers about optimization ispresently classified according to these three types of methods.

3.1. Phase behavior studies for optimization

Studies on phase behavior for optimization of nano-emulsionproperties can be important when the so-called condensation orlow-energy emulsification methods are used, because the phasesinvolved during emulsification are determinant in order to obtainnano-emulsions of small droplet size and low polydispersity. Incontrast, if shear methods are used, there is not a compositionemulsification path and only phases at the final composition areimportant.

The importance of the phase behavior, namely crossingmicroemulsion (bicontinuous, D) or lamellar liquid crystallinephase regions during emulsification is described in detailin recent reviews [1,4,5]s. Some recent original works inwhich this conclusion is experimentally proved are [4346]for nano-emulsions obtained by the phase inversion tempera-ture method (PIT); [4749] for nano-emulsions obtained byphase inversion composition method (PIC), or [18,28,30]for nano-emulsions prepared by a self-emulsifying method.Only bicontinuous (D) or O/Wmicroemulsions are consideredappropriate for self-emulsifying while lamellar liquid crystalcompositions do not self-emulsify by dilution, probably dueto viscosity of the lamellar phase [18]. Comparing resultsfrom Refs. [47] and [48] with results from Ref. [18], it canbe concluded that by slow addition of water to a lamellarliquid crystalline phase nano-emulsions can be obtained,while emulsions with higher droplet size are obtained by rapiddilution (as in self-emulsifying methods).

In Ref. [50], nano-emulsions with a very small droplet size areobtained in an ionic surfactant system by adding aqueous phasethrough an emulsification path crossing a micellar cubic liquidcrystalline phase. Other recent not published experimental resultsdemonstrate that the initial droplet sizes of the nano-emulsionscoincide with the micelle size of the cubic liquid crystalline phasefrom which the nano-emulsion is obtained by dilution [Fig. 2].

Actually, conditions for obtaining O/W nano-emulsions witha minimum in droplet size and consequently low polydispersitycan be summarized as follows: in emulsification by phaseinversion temperature or composition methods an aqueouscontinuous phase, O/W or bicontinuous, with all the oilsolubilized must be crossed immediately before reaching thefinal two phase region where the nano-emulsions form. Theseare composition conditions necessary but not sufficient, becausethe kinetics of incorporation of oil to this water continuous phaseor the coalescence canmake that nano-emulsion droplet size alsodepends on preparation variables such as aqueous phase additionrate for PIC method or cooling rate for PIT method.

3.2. Optimization by selective variation of parameters

247id & Interface Science 13 (2008) 245251Parameters whose influence on nano-emulsion characteristicscan be studied may be classified as composition or preparation

ollovariables. For emulsification by low-energymethods compositionvariables will have a much higher influence than preparationvariables, however for shear emulsification, the influence ofpreparation variables will be determinant.

Examples of recent literature about optimization of nano-emulsions obtained by shear include the study of the influence ofdifferent variables and the correlation of droplet size with them[51]. In this paper two different industrial scale emulsifierequipments were studied, and the Sauter diameter was correlatedwith viscosities, stabilizers, volume fractions and pressure for a

Fig. 2. Nano-emulsion obtention by dilution of a cubic liquid crystal.

248 J.M. Gutirrez et al. / Current Opinion in Cjet mill, number of disc mixers for a static mixer, and passagenumber for the two equipments. For both, an equilibrium size isreach for high number of passages resulting in constant dropletsize.

For other systems, optimum pressure or passage number canexist if coalescence is facilitated by high pressures or passagenumber. In Ref. [52] a food system is studied with a high pressuremicrofluidizer to emulsify and using a surfactant and differentpolymers for stabilizing the emulsions. The competing phenom-ena of breaking and coalescence are discussed taking into accountthe effect of stabilizers.

In Ref. [53], optimization of nano-emulsion preparation bysubmitting a coarse emulsion to subcritical water conditions ispresented. The optimization was studied by selective variationof composition parameters (surfactant and oil concentration),and preparation parameter (temperature). For this system smallsizes, 40 nm, are obtained.

For other condensation methods, variables whose effect iscommonly studied are the surfactant oil ratio and the ratio be-tween surfactants when a surfactant mixture is used.

For nano-emulsions prepared by the phase inversion tempera-ture method, optimization by selective variation parameters ispresented in several cited references of recent bibliography. In[43,46] variation of droplet size is studied with respect to oilsurfactant ratio with the obvious result that the higher the oilsurfactant ratio the greater the droplet size, and in [45] variationof droplet size with surfactant mixing ratio is studied with theremarkable result that droplet size does not depend on surfactantmixing ratio if nano-emulsions are prepared by cooling from theHLB temperature.

For nano-emulsions prepared by the phase inversion com-position method, there are also several studies in recent bib-liography. In [54] optimization with respect to preparationmethod and variation of droplet size with oil surfactant ratio arepresented. In [49] different routes for emulsification are studiedand droplet size variation with HLB, water fraction and sur-factant concentration is also reported. In Ref. [55], effect ofvariables HLB and oil surfactant ratio are separately studied withthe expected result that there is an optimum HLB and that thehigher the oil surfactant ratio the greater the droplet size. In Ref.[56] optimization of W/O nano-emulsion preparation is pre-sented. For different combinations of Span-Tween surfactants,an optimum surfactant composition presenting a water solubilitymaximum is chosen, and droplet size variation is studied withrespect to water concentration. Also with W/O nano-emulsions,the result is, as expected and coinciding with Ref. [47], that thehigher the water concentration the greater the droplet size.

For nano-emulsions prepared by self-emulsification, there isa detailed work on optimization [57]. Droplet size variationwith oil, surfactant HLB, and solvents, was studied. The resultsindicated that there are optimum values for HLB and proportionsof solvents.

As an example of optimization of nano-emulsion function, inRef. [26] the influence of sucrose surfactants on percutaneouspenetration is studied, and in Ref. [35] the efficacy of aschistosomicidal agent is improved by incorporating the agentin nano-emulsions.

3.3. Experimental designs for optimization

Experimental designs allow to experimentally study theinfluence of several variables with a limited number ofexperiments. Statistical analysis of results will allow to knowwhich variables have a significant influence, and to correlatedesired response with variables by polynomial equations. InFig. 3 an example of experimental design is shown, and in Fig. 4there is an example of response surface.

Not many papers present optimization of nano-emulsionpreparation by experimental designs, andmost of them deal aboutpharmaceutical formulations for self-emulsification [27,5861].In Ref. [27] experimental design was used to determine theinfluence of two qualitative independent variables: type of oil andtype of lipophilic emulsifier. The other four references correspondto the same research group. In Refs. [58,59] the incorporation ofretinol to a self nanoemulsifying formulation is studied, beingoil, surfactant and cosurfactant amounts in the formulation thethree independent variables, and mean droplet size, turbidity, anddissolution rate at 10 and 30 min, the four response variablesstudied. Response equations are presented, and system is op-

id & Interface Science 13 (2008) 245251timized for dissolution rate at 30 min using the other threeresponses as restrictions. In Ref. [60] the surface responsetechnology explained in a more detailed way and six response

pre

J.M. Gutirrez et al. / Current Opinion in Collovariables are analyzed. In Ref. [61], authors apply the samemethodology to evaluate ultrasonic technique in characterizationof nano-emulsions.

In Ref. [48] a complete explanation of experimental designapplication to study the preparation of nano-emulsions ispresented. Methodology is applied to low-energy emulsificationby phase inversion composition method, and effects of com-position variables and preparation variables were all togetherevaluated. Droplet size as response surface was minimizedseparately, first with respect to composition variables, and after-wards with respect to preparation variables. The results confirmthat the higher the oil surfactant ratio the greater the droplet size,and that there is an optimum surfactant mixing ratio or, what isthe same, an optimum HLB. Concerning the preparation var-iables, addition and agitation rate have little but significantinfluence and an optimum agitation rate is found.

In Ref. [62], optimization methodology by experimentaldesign is applied to nano-emulsions in an ionic surfactant systemobtained by the phase inversion composition method. Again, thehigher the oil surfactant ratio the greater the droplet size, andthere is an optimum ratio of surfactants in the mixture used.Concerning the preparation variables, they present again no or

Fig. 3. Example of experimental design for thelow influence on droplet size. Other not published results of theauthors on nano-emulsions prepared by the phase inversiontemperature confirm that preparation variables such as coolingrate or agitation do not have a significant influence on dropletsize.

A general conclusion of papers using experimental designs isthat this methodology constitutes a very good tool for studyingpreparation of nano-emulsions.

Fig. 4. Example of surface response from the preparation variables agitation andaddition rate.4. Conclusions

Possible applications of nano-emulsions are stronglylimited by the stability of nano-emulsions, except for nanopar-ticle preparation when the process of physical or chemicalsolidification takes place within the period of stability of nano-emulsions.

Nano-emulsions are proposed for numerous applicationsin pharmacy as drug delivery systems because of their ca-pacity of solubilizing non polar active compounds. Due to thestability problems, most of proposed formulations are self-emulsifying systems and the nano-emulsions are producedjust before their application. Although there have not beenreported too many applications in other fields, there is a greatpotential for nano-emulsion applications if Oswald-ripeningdestabilization mechanism is limited by using very insolubleoils.

Concerning optimization in preparation of nano-emulsionsby shear, an optimum shear or time shearing can exist if breakingand coalescence are competing phenomena during the process.

Concerning optimization in the preparation of nano-emul-sions by low-energy methods, recent literature confirms that

paration variables agitation and addition rate.

249id & Interface Science 13 (2008) 245251crossing bicontinuous or aqueous continuous phases duringemulsification allows obtaining O/W nano-emulsions of smalldroplet size and low polydispersity.

Optimizations by selective variation of parameters or ex-perimental designs allow to conclude that, with respect tocomposition variables, generally there is an optimum surfac-tant mixture composition, or HLB, and that the higher the oilsurfactant ratio the greater the droplet size. The preparationvariables, as addition, agitation or cooling rate, generally donot have a significant influence if the system is optimized withrespect to composition.

This last conclusion has a very important derivation: ifpreparation variables do not have influence, the system can bescaled-up, from lab to industrial, and similar results can beexpected.

As a final comment, judging from the most recent literature,the interest in nano-emulsion preparation and application isgrowing, but few of the numerous ideas reported become com-mercial final applications.

250 J.M. Gutirrez et al. / Current Opinion in ColloAcknowledgment

Financial support from the Spanish Ministry of Scienceand Education, MEC (grants CTQ 2005-09063-C03-01/PPQand CTQ 2005-09063-C03-02/PPQ) is acknowledged.

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[43] Morales D, Gutierrez JM, Garcia-Celma MJ, Solans C. A study on therelation between bicontinuous microemulsions and O/W nano-emulsionformation. Langmuir 2003;19:7196200. Relation between properties ofnano-emulsions and phases crossed through emulsification path permits tooptimize the preparation of nano-emulsions.

[44] Izquierdo P, Esquena J, Tadros TF, Dederen JC, Feng J, Garcia-Celma MJ,Azemar N, Solans C. Phase behavior and nanoemulsion formation by thephase inversion temperature method. Langmuir 2004;20:65948.

[45] Izquierdo P, Feng J, Esquena J, Tadros TF, Dederen JC, Garcia MJ, AzemarN, Solans C. The influence of surfactant mixing ratio on nano-emulsionformation by the pit method. Journal of Colloid and Interface Science2005;285:38894. Optimization of preparation respect to surfactant mixingratio is presented.

[46] Morales D, Solans C, Gutierrez JM, Garcia-Celma MJ, Olsson U. Oil/water droplet formation by temperature change in the water/C16E6/mineral oil system. Langmuir 2006;22:301420. Mechanism of dropletformation depending on phases crossed through emulsification path arediscussed.

[47] Uson N, Garcia MJ, Solans C. Formation of water-in-oil (W/O) nano-emulsions in a water/mixed non ionic surfactant/oil systems prepared bylow energy methods. Colloids and Surfaces A, Physicochemical andEngineering Aspects 2004;250:41521.

[48] Pey CM, Maestro A, Sol I, Gonzlez C, Solans C, Gutierrez JM.Optimization of nano-emulsions prepared by low energy emulsificationmethods at constant temperature using experimental designs. Colloidsand Surfaces A, Physicochemical and Engineering Aspects 2006;288:14450. Complete description of experimental design application to

J.M. Gutirrez et al. / Current Opinion in Cstudy nano-emulsion preparation. Effect of variables and responsesurfaces are obtained.[50] Sol I, Maestro A, Pey CM, Gonzlez C, Solans C, Gutierrez JM. Nano-emulsions preparation by low energy methods in an ionic surfactant system.Colloids and Surfaces A, Physicochemical and Engineering Aspects2006;288:13843. Preparation of nano-emulsions by crossing a watercontinuous cubic liquid crystal phase through emulsification path is described.

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[55] Liu W, Sun D, Li C, Liu Q, Xu J. Formation and stability of paraffin oil-in-water nano-emulsions prepared by the emulsion inversion point method.Journal of Colloid and Interface Science 2006;303:55763. Results of thispaper clearly show the dependence of droplet size on HLB in surfactantmixture.

[56] Porras M, Solans C, Gonzlez C, Martnez A, Guinart A, Gutierrez JM.Studies on W/O nano-emulsions. Colloids and Surfaces A, Physicochem-ical and Engineering Aspects 2004;249:1158. Studies about influence ofrelevant variables on droplet size of W/O nano-emulsions.

[57] Bouchemal K, Brianon S, Perrier E, Fessi H. Nano-emulsion formulationusing spontaneous emulsification: solvent, oil and surfactant optimization.International Journal of Pharmaceutics 2004;280:24151.

[58] Taha E, Al-Saidam S, Samy AM, Khan MA. Preparation and in vitrocharacterization of self-nanoemulsified drug delivery system (SNEDDS)of all-trans-retinol acetate. International Journal of Pharmaceutics2004;285:10919.

[59] Taha E, Samy AM, Kassem AA, Khan MA. Response surface methodologyfor the development of self-nanoemulsified drug delivery system (SNEDDS)of all-trans-retinol acetate. Pharmaceutical Development and Technology2004;10:36370. Complete optimization is presented for a self-emulsifyingsystem, with experimental design, analysis of response surfaces andoptimization.

[60] Zidam AS, Sammour OA, Hammad MA, Megrab NA, Habib MJ, KhanMA. Quality by design: understanding the formulation variables ofcyclosporine a self-nanoemulsified drug delivery system by BoxBehnken design and desirability function. International Journal ofPharmaceutics 2007;332:5563. The same group that in reference [55]presents similar treatment of other system through surface response, butnow named the method Quality by design (QBD). A more detailedexplanation of methodology is presented.

[61] Shah RB, Zidam AS, Funck T, Tawakkul MA, Nguyenpho A, Khan MA.Quality by design: characterization of self-nanoemulsified drug deliverysystems (SNEDDSs) using ultrasonic resonator technology. InternationalJournal of Pharmaceutics 2007;341:18994. The same group again utilizeexperimental designs and response surface methodology, but this timeusing them to evaluate a new characterization ultrasonic technique.

[62] Sol I, Maestro A, Gonzlez C, Solans C, Gutierrez JM. Optimization ofnano-emulsion preparation by low energy methods in an ionic surfactantsystem. Langmuir 2006;22:832632. Complete study showing influenceof phases crossed through emulsification path and influence of relevantvariables as determined by experimental designs.

Nano-emulsions: New applications and optimization of their preparationIntroductionDirect application of nano-emulsions in final productsOptimization of nano-emulsion preparationPhase behavior studies for optimizationOptimization by selective variation of parametersExperimental designs for optimization

ConclusionsAcknowledgmentReferences and recommended readings,Of special interest.Of outstanding interest.