EMULSIONSJANUARY/FEBRUARY 200416

INTRODUCTION

According to Becher, “an emulsion is aheterogeneous system, consisting of atleast one immiscible liquid intimatelydispersed in another in the form ofdroplets, whose diameters, in general,exceed 0.1µm. Such systems possess aminimal stability, which may beaccentuated by such additives as surface-active agents, finely-divided solids, etc.”(emulsifying agents) (1). Simple emulsionsbelong to two types: oil in water (O/W)and water in oil (W/O). There are also twokinds of multiple (double) emulsions:W/O/W and O/W/O. The so-calledaqueous phase may contain inorganic ororganic solutes and the oil phase, often amixture of species, can be of mineral,vegetable or animal origin. In spite of theirthermodynamic instability, naturalemulsions are not rare (e.g. milk, rubber-tree latex). Synthetic ones are more oftenformulated products used in extremelyvarious fields, such as food, cosmetics,pharmacy and medicine, biotechnology,agrochemicals, fabric, leather or metalprocessing, pulp and paper industry, paintsand lacquers, detergence, lubrication,road construction, automotive fuels,explosives, etc.. It also happens thatemulsions, desirable or not, areformed temporarily during industrialprocesses (e.g. polymerization, oilextraction).

While formulating emulsions,composition variables (nature andproportions of ingredients),temperature and process parametershave to be considered: the former,including temperature, will mainlydetermine the type of emulsion,whereas the latter will partly govern itsstability. In fact, sooner or later, anemulsion will break and undergophase separation. The observedchanges involve:

– reversible phenomena – flocculation,creaming or sedimentation according toStoke’s law: V=∆ρ g d2 /18 η, ifparticles only migrate;

– and irreversible phenomena (Ostwaldripening, coalescence), if particle size isaltered (Figure 1).

Among the properties enhancingemulsion stability, let us list: a lowdispersed-phase volume fraction, a lowdensity difference between phases, a low(but not too low) interfacial tension, ahigh viscosity of the continuous phase,high mechanical resistance and elasticityof the interface, a high ζ potential, a highsolubility of the emulsifier in thecontinuous phase (Bancroft’s rule), anarrow droplet size distribution. Increasingtemperature often accelerates emulsionbreaking.

The formulation, properties and stabilityof emulsions have been described in anumber of books and reviews (1-7). As forthe theoretical analysis and comparison ofemulsification processes, they have givenrise to some recent papers (8-15). Thepresent work is intended to brieflysummarize the state-of-the-art of emulsion

EMULSIONS Emulsion preparation:Theoretical notionsand practical aspects

J.P. CANSELIERM. POUX

Laboratoire de Génie Chimique (UMR CNRS 5503 ENSIACET-INPT/UPS)5 rue Paulin Talabot, BP 1301 F-31106 Toulouse, FranceTel +33-534-615254Fax +33-534-615253 JeanPaul.Canselier@ensiacet.fr

Firstly, some techniques foremulsion characterization (phaseidentification, droplet sizedistribution, viscosity) are brieflyreviewed. The energyrequirements and the mainmechanisms involved inemulsification processes are thenconsidered: the conditions ofdroplet formation and break-up inlaminar or turbulent regime arerecalled. The roles played by theemulsifier are explained andkinetic aspects are dealt with.Finally, the most frequently usedemulsification equipments aredescribed and compared.

AB

STR

AC

T

Figure 1 – Destabilization of an emulsion: droplet migration and droplet size alteration –from Ref. 49a, with permission

published by srlVia Cesare da Sesto, 1020123 Milano (Italy)Tel. 0039 02 83241119Fax 0039 02 8376457www.b5srl.com

characterization, mechanisms of formationand emulsification equipment.

EMULSION CHARACTERIZATION

Phase Identification

The continuous and dispersed phases canbe identified by three types of tests:addition of a water-soluble dye, dilution bywater and conductivity measurement. Inthe latter, recommended for instance byAFNOR (16), the higher conductivity of theaqueous phase, especially in the presenceof added electrolyte, is turned to account.According to Maxwell (17), the conductivityof not too concentrated emulsions(spherical droplets) obeys to:

[1]

where κ, κc and κd are emulsion,continuous phase and dispersed phasespecific conductivities, respectively and φ isthe dispersed phase volume fraction.

Droplet Size and Droplet Size Distribution

Unless a special process is applied tomake monodisperse emulsions (18-20),these systems are best characterized bytheir droplet size distribution (DSD),measured with a particle sizer or, possiblywith a microscope or an image analysissystem (21). All other things being equal,two emulsions can behave differently onlybecause different DSD (22). However, for amonomodal, not too wide DSD, the surfaceaverage diameter (Sauter diameter, d32) isthe most representative single parameter:d32 = Σ nidi

3/Σ nidi2 = 6V/A [2]

where ni is the number of dropletsbelonging to the class of average diameterdi, and V and A are the volume and area ofthe dispersed phase, respectively. Otheraverage diameters, di,i-1, defined similarly,are also in use. Let us notice that, amongthe commercial particle sizers, only thatbased on ultrasound velocity is able to dealwith concentrated emulsions (up to φ = 0.5or even 0.8) directly (23), whereas theother apparatuses need previous dilutions(φ of the order of 10-5). On the other hand,optical reflectance (24) and focussed beamreflectance devices (25) allow to extend therange of DSD techniques towardsconcentrated media. Besides, TurbiscanMA2000 (26) and Turbiscan “on-line” (27),operating by measuring back-scattered lightintensity, also yield an average droplet sizediameter for concentrated emulsionswithout dilution.

The shape of the DSD curve of roughlydispersed liquid-liquid systems correspondsgenerally to a normal distribution functionaround d32, with the stirrer diameter and φas main parameters (28). On the other

κ − κ c

κ + 2κ c=

κd − κ c

κd + 2κ cφ

hand, for colloidal or finely-dispersedmedia, a log-normal distribution isconsidered preferably (29):

where P is the probability of finding adroplet of diameter di, σ the geometricalstandard deviation and d50 the averagegeometrical diameter. Criteria foremulsion stability and techniques ofemulsion stabilization have beendetailed elsewhere (4-6,30).

Viscosity

The viscosity of very dilute emulsions(φ ≤ 0.02) only depends on the volumefraction of the dispersed phase,according to Einstein’s relation:ηr = η/ηc = 1+(5/2) φ [4]where η and ηc are the shear viscosities ofthe emulsion and the continuous phase.This relation is obeyed provided that thereis no interaction (repulsive Coulombic orattractive Van der Waals forces) betweenrigid, spherical droplets. If this is not thecase, especially for more concentratedemulsions, an empirical, polynomial formmay be used:ηr =1+ a φ + b φ2 + c φ3 + … [5]with b theoretically equal to 14.1, accordingto Guth, Gold and Simha (1), but found tovary between 3 and 6 experimentally. Moregenerally, emulsion viscosity also dependson droplet size and DSD (finer, lesspolydisperse emulsions are moreviscous)(31), oil/water interfacial tension,viscosity of both individual phases (mainlythe continuous one) and interfacialrheology.

Concentrated emulsions often shownon-Newtonian behaviour (namelyshear-thinning at low shear rate andshear-thickening at higher shear rate),with, possibly, a yield value at aminimum shear stress. Thixotropy, thatis apparent viscosity decrease with timeat constant shear rate, is observed invery viscous emulsions (e.g. gel-emulsions)(4). Viscoelastic propertiesalso characterize highly concentratedemulsions (φ > 0,70-0,74). Recentreviews on emulsion viscosity havebeen published (32,33).

EMULSIFICATION PROCESSES

Energy Requirements

The enormous increase of interfacialarea (∆A ~ A) involved duringemulsification requires an energy inputproportional to the interfacial tension, γ.However, the increase of interfacial freeenergy, including a very small entropic

term, T∆Sconfig (∆Gform = γ∆A – T∆Sconfig

~ γA > 0, unless γ < T∆Sconfig/A, wherespontaneous emulsification canoccur) only accounts for about1‰ of the total energyprovided. For instance, themagnitude of γA in theformation of an oil-in-water

emulsion (φ = 0,1, Ø = 1µm, thereforeA/V = 3.105 m-1, γ = 10 mN/m) is 3kJ/m3 to be compared with an actualenergy expense of 3 MJ/m3 necessary tomake such an emulsion. In fact, as far asemulsification mechanisms are known,the energy supply is divided betweenviscous dissipation and energy actuallyused for droplet formation and dropletbreak-up, that is interfacial free energyincrease, and terms related to dropletdeformation and involving dispersedphase viscosity, interfacial viscosity andLaplace pressure, ∆P: for a sphericaldroplet of radius r, this pressuredifference across the curved interface(the pressure being higher on theconcave side) is approximated by:∆P =2 γ / r [6]

So, ∆P reaches 1 bar for a dropletradius of 0,2 µm and an interfacialtension of 10 mN/m. Energy is moreoften supplied by mechanical agitation, amore vigorous dispersive action yieldingfiner droplets. Finally, except γA, thewhole amount of energy is dissipated asheat (13).

In fact, elementary processesinclude at least two steps: dropletformation and droplet breaking intosmaller ones. The second phenomenonis the most important one: the smallerthe droplets, the more stable theemulsion. That is why coalescencemust be avoided.

Mechanisms

From the hydrodynamic point of view,emulsification can take place under laminaror turbulent flow regimes (8,9), the latterbeing more often turned to account. In thefollowing, a mechanical energy supply will befirst considered. We shall see later that someprocesses are less energy-consuming.

EMULSIONS JANUARY/FEBRUARY 2004 17

Figure 2 – Instabilities of an interface: 1, capillary waves; 2 and 3, left, development of Rayleigh-Taylor instability; 2 and 3, right,combination of Kelvin-Helmholtz and Rayleigh-Taylor instabilities - fromRefs. 63, with permission

[3]

Droplet FormationEmulsification processes begin with theformation of a film of the futurecontinuous phase around the futuredispersed phase. Gibbs elasticity (E = dγ/d(lnA)) best explains thephenomena, with the emulsifierconcentration as the controlling parameter:with enough surfactant, the thinnest partof the film, possessing the highest E value,will be more resistant to stretching andcontribute to stabilization (8).

A coarse emulsion can form throughthe breaking of either a planar interface or aliquid cylinder. The first phenomenon isproduced by turbulence, capillary waves,Rayleigh-Taylor or Kelvin-Helmholtzinstabilities (14), and it is likely that dropformation results from a combination ofthose four mechanisms (Figure 2). Forinstance a low interfacial tension isfavorable to overcome the Laplace pressurein turbulent regime and to increase waveamplitude, but, even for very low γ values,Rayleigh-Taylor instability will only producerather large drops, while interfacial gradientsseem to be detrimental to Kelvin-Helmholtzinstability.

Thin cylinders can come from spikes,produced by interfacial instabilities, or resultfrom a liquid jet or from the stretching of alarge drop. The axisymmetricaldeformations of a stationary cylinder lead tobreaking into droplets surrounded bysatellites, especially for an optimumwavelength (Figure 3).

A very wide DSD may be observed with a turbulent jet: the average diameterdepends on the viscosities and densities of both phases as well as on the jet speed.On the other hand, for a thread comingfrom a large drop, the more intense the stretching, the smaller the resultingdroplets. In a laminar flow, cylinder breakingis controlled by two opposite forces, whoseratio of the associated pressures (Laplacepressure acting against deformation and shear stress favouring deformation) is the Weber number:We = ηc G r/γ [7]where ηc is the viscosity of the continuousphase and G the elongation rate (velocitygradient). If We is high enough, the cylinderwill be broken by viscous forces (8,14).

Droplet Break-UpViscous forces and inertial forces play a

role in droplet deformation and break-up.The former, generating stresses tangentialor perpendicular to the droplet surface,predominate in laminar flow, whereas thelatter, producing pressure differences, areessential in turbulent flow, although viscousforces may not be negligible. The Laplacepressure is always associated to an internal“restitution” force opposing dropletdisruption. Break-up, due to shear, andcoalescence phenomena take placesimultaneously and give rise to a dynamicequilibrium.

In laminar flow, a droplet will break up ifits Weber number is higher than a criticalvalue, Wecr, that is if its radius, r, is largerthan the critical rcr. Under simple shear andfor viscosity ratios (q = ηd/ηc) between ca.0.02 and 2, Wecr is of the order of 1, goingthrough a minimum (slightly lower) forequal viscosities (9,14,34) (Figure 4). So,highly viscous drops can hardly bedeformed. Practically, emulsification inlaminar flow can be achieved in a capillarytube (19) or in acolloid mill. Theformer, low-shearprocess isefficient with bothrather viscousphases and highφ values. Energyconsumption islow, DSD narrow,but theemulsificationtime may reachseveral hours.

The turbulentregime is moreefficient and morecommonly used; it will be supposedisotropic. If viscous forces appearnegligible compared with others (ηd < 10 mPa.s), droplet stability isrelated to deformation forces (turbulentfluctuations) and surface forces. TheWeber number of a moving droplet isthen the ratio of the correspondingenergies, εv and εs. Another hypothesis isthat the macro-turbulence scale is of thesame order of magnitude as the dropletdiameter, d. Then:We = εv/ εs = ρc ∆u2d3/γd2 =

ρc∆u2d/γ [8]∆u = (ε.d)1/3 [9]We = ρcε2/3d5/3/γ [10]ε is the powerdissipated per massunit and ∆u thefluctuation of thelocal velocity. Hereagain, it is assumedthat droplets breakup when We ishigher than Wecr(35): their diameteris then themaximum stablediameter, dmax:

dmax = (Wecr)3/5 (ρc/γ)-3/5 ε-2/5 = C1 (ρc/γ)-3/5 ε-2/5 [11]

Experimentally, dmax has been foundproportional to d32 (36,37):d32 = K dmax [12]with K ranging from 0.38 and 0.70 and increasing with viscosity (38,39).

In continuous processes, thedependence of the droplet diameter on theresidence time (tres) may be combinedwith that on power density into adependence on specific energy. On theother hand, in batch processes, anexpression analogous to [11] must includethe power density, a function of tres as wellas a term in ηd

α. (40).In short, under prevailing inertial forces,

d varies as γ0.6 and often as ε-0.4 (ε-0.6 in ahigh-pressure homogenizer)(9), theexponent of ηd being 0. However, whenviscous forces are predominant, d isproportional to γ, to ε-0.25 and to a certainpower of ηd,

Beside elongational flow in laminarregime andturbulence, athirdmechanisminvolved indropletdisruption iscavitation.Thisphenomenonresides in theformation, inlow-pressurezones and notvery deeply inthe liquid, of“stable” or

transitional tiny bubbles, containingdissolved gas and/or vapour of thevolatile components of the continuousphase. During ultrasound emulsification,cavitation takes place as the mainmechanism of droplet breakup(Figure 5), but it also plays a role inhigh-pressure homogenization and, to alesser extent, with classical rotor-statordevices. Cavitation is inhibited by anincrease of hydrostatic pressure. Energydensity (dissipated “instantaneously”per mass or volume unit) appears to bethe key parameter. For instance, atconstant energy density, drop size is notaffected by hydrostatic pressure (8,14).

EMULSIONSJANUARY/FEBRUARY 200418

Figure 3 – Instability of a stationary liquidcylinder - from Ref. 64, with permission

Figure 4 – Droplet break-up under simple shearflow (Grace curve) - from Ref. 65, with permission

Figure 5 – Acoustic emulsification: droplet formation and break-up -from Ref. 50, with permission

Process functions, such as[11], designed foremulsification, can thusprovide a guide for processoptimization and scaling-up(40).

Effect of the EmulsifierAlthough it is considered aspractically impossible toobtain a stable emulsion by only mixingtwo pure liquids, Reddy and Foglerclaim they made stable, very diluteO/W emulsions through insonation,without using any emulsifying agent(41). They assume that their emulsionsare stable thanks to OH- ion adsorptiononto oil droplets. Usually, however, theaddition of at least an emulsifier isrequired for enhancing emulsion kineticstability. The part played by this additiveis multiple. More often, it determinesthe emulsion type (O/W or W/O), itlowers the interfacial tension, producesinterfacial tension gradients duringemulsification and slows down orprevents coalescence.

The spectacular interfacial tensionlowering (from 30-50 mN/m down to afew mN/m with a surfactant, or even afew µN/m in the presence of acosurfactant) reduces the surface freeenergy change and the Laplacepressure considerably, but theequilibrium value is not the main issue,since adsorption times can be long,compared with drop break-up andcollision times.

Interfacial tension gradients maygenerate lateral motion of theinterface and even interfacialinstability, favourable to dropletdisruption. In addition, if thesurfactant is dissolved in thecontinuous phase (Bancroft’s rule), itopposes droplet coalescence, thanksto the Marangoni effect. Beforeequilibrium is reached, whilesurfactant adsorption occurs, itsamount is less and the interfacialtension higher where the filmbetween droplets is thinner, so thatliquid flows towards the centre ofthe film. This self-healingmechanism can only take place ifthe Gibbs elasticity, E, of the film ishigh enough. On the contrary, if thesurfactant is dissolved in thedispersed phase, E is low,there is no interfacial tensiongradient and no liquid flowinside the film: without theGibbs-Marangoni effect,droplets would coalesce.Coulombic repulsions couldalso play a role in preventingcoalescence. Anyway, thehigher the surfactantconcentration, the slower thecoalescence rate (8,14).

Kinetic AspectsThree characteristic time scales areinvolved in emulsification processes.The deformation time (or relaxationtime following a small deformation of adroplet), τdef, is of the order of0.01-0.1 ms, except for large drops inturbulent regime (τdef = 10 ms for r = 1 mm). The adsorption time, τads,necessary to the transport of thesurfactant (by convection) towards thenew interface, increases from 10-4 msin laminar flow to 0.5 ms in turbulentflow for large drops, and from 0.1 ms(laminar) to 6 ms (turbulent) for adroplet of critical radius. As regards theaverage time between collisions, τcol, itranges from 5.10-4 ms in turbulentregime to a few tenths of a ms inlaminar flow (8).

The time needed for disruption is ofthe same order as τdef: so, althoughdisruption must be repeated manytimes, emulsification should becomplete within a second or so. Now,practically, it takes minutes. In fact,some kinds of machines (colloid mills,high-pressure homogenizers, ultrasoundtransmitters, high-speed rotor-statordevices) are able to break smalldroplets very quickly, but they are notso suitable for the first steps ofemulsification. Inversely, mixers involvemuch slower processes, especially inthe final stages. Therefore,preemulsifying a mixture by stirring maybe advantageous (smaller droplets,narrower distribution).

On the whole, emulsification timedepends on the process variables andon the nature and amount of surfactant.For instance, as regards the supposeddynamic equilibrium between ruptureand coalescence during emulsification,it has been shown recently that, only athigh surfactant concentration, the finalDSD reflects the disruption efficiency ofthe device (no coalescence)(42).

Techniques and Devices

Mechanical Processes: Dispersers andHomogenizersAs stated above,mechanical processesoften consist of twosuccessive operations: afirst step of mixing and

dispersion, leading to a coarse emulsion(drops of ca. 100 µm), is followed by a“homogenization” step reducing dropletsize and stabilizing the medium. Thoseoperations can be effected in vessels orin pipes (batch or continuous processes)by means of dispersers andhomogenizers, respectively.

For dispersers, the main issue is toproduce a high shear able to form andbreak up droplets. Stronger shear forcesare required if the dispersion is harder toprepare. Radial stirrers, such as Rushtonor pitched blade turbines, are welladapted but must ensure a goodrecirculation of the fluid: in fact,coalescence is more likely to occurfarther from the stirrer.

In a homogenizer, the two-phasemixture, possibly containing rather largedrops (10-100 µm) is forced to enter aconfined zone (e.g. a few-millimeterwide gap) where it is subjected to veryhigh shear rates. Rotor-stator devices arethe most used ones: the product issucked into the head of the machine,due to the high speed of the rotor, thenexpelled by the blades, passing throughthe holes or slits of the stator (Figure 6).

A particular type of rotor-statorsystem is the colloid mill, characterizedby its truncated-cone shape and its verynarrow gap (= 0.1 mm). The fluid entersthe mill through its upper part (top of thecone) and is strongly sheared betweenthe smooth or rough internal surfaces ofthe gap, at a rotation speed of 3,000 to15,000 rpm.

A high-pressure homogenizer is amachine in which the mixture is forcedthrough a narrow valve slit (~ 0.1 mm).Under the effect of pressure (up to 100MPa), the valve opens against a spring.Very high liquid velocities (~200 m.s-1)and energy densities (of the order of 106

W.cm-3) may be reached (8). All otherthings being equal, homogenizers usuallyproduce finer emulsions (Ø ~ 0.2-1µm)

than colloid mills, but notso monodisperse (3).Various types of emulsifyingdevices are represented inFigure 7.

A novel technique toprepare monodisperseemulsions (σ/d50 ~ 0.05)uses a microchannel (MC)device consisting of achannel and a terrace.Because, on a smaller scale,

EMULSIONS JANUARY/FEBRUARY 2004 19

Figure 6 – Multistage rotor-stator device (VMI-Rayneri model)

Figure 7 – Various emulsifying devices: a) membrane, b) colloid mill, c)and d) high-pressure homogenizers - from Ref. 66, with permission

interfacial tension ismore significant thanother forces, itbecomes the drivingforce for dropletformation. Droplet sizemay be varied at willaccording to MCdimensions (43).

MechanicalProcesses: StaticMixers Pipe flow in laminar orturbulent regime mayturn to accountconstrictions and/orbaffles. By dividing theflow, whose parts arethen joined together,these obstaclesenhance velocitygradients or turbulence. The fluidsare circulated by means of a pump,acording to the pressure drop andthe desired flowrate (44).Emulsions, with rather narrow DSD,form above a speed threshold (e.g.3 m/s to yield droplets smaller than1 µm diameter). Static mixers canoften replace stirred vesselsadvantageously, because of theabsence of moving parts and theirperfect adaptation to continuousprocesses (45). The plug-flowmodel is quite convenient. Theirmain drawback resides in a difficultcleaning.

InjectionIn the classical technique, a cylindricaljet of the dispersed phase, entering thecontinuous one, is broken up intodroplets. A new system involves thecollision of two pre-emulsified flows(46). Several equations have beenproposed to evaluate droplet size as afunction of operating conditions (47).

InsonationEmulsification by high-power, low-frequency (~ 20 kHz)ultrasound has beenreported for the firsttime a long time ago(48). Since then,various types ofultrasound devices(1,28,49), mainlyoperating throughcavitation, have beenused. An example ofthose is represented inFigure 8. Insonationrepresents a very fast,efficient means ofmaking fine, thereforestable O/W, and evenW/O emulsions,compared with rotor-

stator classicalapparatus. Theonly drawbacksare the rathershortpenetrationlength ofultrasonicwaves, whichrequires pre-emulsificationin the casewheremaximumpower is notapplied (28,50)and therestriction toproducts of lowviscosity.Ultrasoundemulsification

has been recently reviewed (51).

Membrane ProcessesMembrane emulsification, usingmechanisms completely differentfrom the usual ones (52), is well-adapted to shear-sensitivespecies. In the more commonof these low-shear, low-energyprocesses, the dispersed phaseis forced towards thecontinuous one through thepores of a microfiltration orultrafiltration membrane (cross-flow process) (53) (Figure 9). Itmakes sense that, withoutstirring, ultrafiltrationmembranes yield fineremulsions than microfiltrationmembranes (54), but, inaddition to membrane averagepore size and pore sizedistribution, the adsorption kineticsof the emulsifier onto the dropletsurface also plays a role (55). Anarrow pore size distribution doesnot always yield a monodisperseemulsion (56), whereas a mixingdevice on the permeate side helps

producingsmaller dropletswith a narrowerDSD (54).

In the morerecent “dead-end”process, a coarseemulsion is forcedthrough themembrane:droplets getsmaller and ahigh flowrateaffords moremonodisperseemulsions (57).

The “ShirasuPorous Glass”technique, using a

module with outer and innermembranes has also been applied tothe preparation of double emulsions(e.g. by dispersing an aqueous phaseinto a vegetable oil containing alipophilic surfactant, then dispersing theW/O emulsion into an aqueous phasedissolving a hydrophilic surfactant)(58).

Less Common ProcessesSeveral processes, mentioned in theliterature, are seldom used, at least ona large scale: this is the case, forexample, of spontaneousemulsification, phase inversiontemperature (PIT) emulsification,shaking, electrical techniques, aerosolto liquid and condensation processes(8). Let us only consider a few low-energy processes.

In spontaneous emulsification therequired energy either comes from achemical reaction (e.g. pouringvegetable oil containing a few percentof fatty acid into an alkaline aqueoussolution) or from redistribution ofspecies inside the medium. Severalmechanisms can contribute to this

phenomenon: interfacial turbulence,diffusion and trapping of dispersedphase droplets in the presence of aco-solvent with possible liquid crystalformation (e.g. addition of anisooctane/isooctanol mixture to anaqueous solution of C12E5)(59), thevery disputed transitional occurrence ofa zero or even negative interfacialtension and the recently proposedbursting of swollen bilayers, leading tothe pulverization of oil micro-dropletsinto the aqueous phase (60).

Other low-energy techniques donot need sophisticated apparatuseither. They consist, for instance inproducing a phase transition bychanging temperature or salinity. Veryfine, blue emulsions can be preparedby the phase inversion temperature(PIT) method (61) (Figure 10). Highly-concentrated W/O emulsions (gel-emulsions) form spontaneously fromoil-swollen micellar solutions with anabrupt increase in temperature (62).Other examples are cited in (59).

EMULSIONSJANUARY/FEBRUARY 200420

Figure 8 – Piezoceramics ultrasoundtransducer (sandwich type) - from Ref. 50 with permission

Figure 9 – Membrane emulsification through a cross-flowdevice - from Ref. 58, with permission

Figure 10 – Preparation of a fineemulsion by the phase inversiontemperature (PIT) process - from Ref. 67,with permission

CONCLUSION

Although the tools for characterizingemulsions are now well developed andthe mechanisms of emulsificationreasonably understood, it is still difficultto predict the exact result of anemulsification process, since this is acombination of a lot of parameters,including formulation and processvariables. As regards the equipment,mechanical agitation apparatuses,especially rotor-stator devices, andhomogenizers will certainly continue toundergo wide use. On the other handinsonation techniques and lowerenergy systems, such as membranes,appear to be quite advantageous andof great interest in continuousprocesses.

ACKNOWLEDGEMENTThe authors thank Dr. B. Abismaïl for hiscontribution to this paper during his thesis.

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