ARTICLE IN PRESS

Chemical Engineering Science 65 (2010) 2957–2964

Contents lists available at ScienceDirect

Chemical Engineering Science

0009-25

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/ces

High pressure emulsification with nano-particles as stabilizing agents

K. Kohler �, Aline S. Santana, Brigitte Braisch, Rebecca Preis, H.P. Schuchmann

Institute of Process Engineering in Life Sciences, Section Food Process Engineering, KIT Karlsruhe Institute of Technology, Karlsruhe University (TH), Germany (LVT)

a r t i c l e i n f o

Article history:

Received 12 November 2009

Received in revised form

26 December 2009

Accepted 20 January 2010Available online 25 January 2010

Keywords:

Pickering emulsion

Silica particles

High pressure homogenization

Droplet size distribution

Stability

09/$ - see front matter & 2010 Elsevier Ltd. A

016/j.ces.2010.01.020

esponding author. Tel.: +49 721 608 8586; fa

ail address: karsten.koehler@kit.edu (K. Kohle

a b s t r a c t

This work presents a technical possibility to produce PSE (particle stabilized emulsions, so called

Pickering emulsions) using inorganic nano-particles, which can be highly abrasive. The influence of

process parameters (such as inlet flow combination, volume throughput, pressure) and special

challenges arising from using nano-particles instead of emulsifier molecules are depicted. Furthermore,

the stability and droplet size distributions of oil-in-water (o/w) emulsions stabilized with x�200 and

12 nm Stober silica particles are discussed. Results are compared to those obtained by batch processing

using a rotor–stator-system. Process economics as well as efficiency will also be discussed.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Presently, high-pressure homogenization and emulsificationtechnologies are commonly used in the chemical, food andpharmaceutical industry. To ensure the stability of emulsions,amphiphilic molecules are usually applied to lower the interfacialtension and stabilize droplets against flocculation and coales-cence. Small-molecular weight emulsifiers or high-molecularweight (bio-) polymers are most commonly used (Horne, 1996).However, solid nano- or micro-particles, which still have not beenproperly explored, are capable of executing similar functions asemulsifiers do. Ramsden (1903) and Pickering (1907) reportedthat solid colloidal particles can be used to obtain stability ofthese systems. Particle stabilized emulsions (PSE), also calledPickering emulsions, have recently regained interest in scientificliterature (Aveyard et al., 2003) which is partially due to therecent advances in nanotechnology and the associated increasedavailability of suitable particles. As for all emulsions includingPSE, the extent of droplet fragmentation (break-up) seems to bedetermined by the amount of energy supplied during emulsifica-tion (Braisch et al., 2009). Therefore, high pressure homogeniza-tion could be a process of great interest, which can be runcontinuously and provides highest specific energies for droplets ina range well below 1mm. Despite of the good results obtainedwith rotor–stator systems, which are typically run batch-wise, theapplication of high pressure systems for Pickering emulsions is atechnical challenge. Nano-particles used for PSE stabilization are

ll rights reserved.

x: +49 721 608 9069.

r).

mostly inorganic and range in size from 10 to some hundrednanometers. These particles are highly abrasive and can damagethe plant, especially the pump and valve system, within minutesof processing (Sauter and Schuchmann, 2008). Most often, nano-particles also strongly tend to form big agglomerates. Thus addingparticles to emulsion premixes is not trivial, especially ifcontinuous processing is targeted as in high-pressure homogeni-zation.

This article investigates the parameters and special challengesconcerning the production of oil-in-water (o/w) emulsionsstabilized with Stober silica particles. The equipment used in thiswork was a high pressure pump followed by a special highpressure homogenizing valve. This new valve is based on the ideaof a Simultaneous Homogenization and Mixing (SHM-) valve(Kohler et al., 2007) allowing for a main stream and a mixedstream within the valve. As the main stream passes an orifice, itdelivers the local flow conditions required for emulsifying thedisperse phase, mixing, distributing, and deagglomerating thenano-particles. Abrasive ingredients, such as the nano-particles,are added as a side stream. They enter the energy dissipating mainflow stream without passing the orifice itself (Sauter andSchuchmann, 2008; Kohler et al., 2007). High mixing quality ina short time on a micro-scale is required, as the particles have tobe as close to the droplet surfaces as possible where they have toadsorb before the droplets start coalescing.

In this study the research was focused on the preparation ofemulsions via a continuous high pressure homogenizationprocess using this SHM-valve. As process parameters, thecomposition, volume throughput, and pressure of the flowstreams were varied. Advantages and disadvantages of inletcombinations of four different modes will be presented together

ARTICLE IN PRESS

K. Kohler et al. / Chemical Engineering Science 65 (2010) 2957–29642958

with the micro structural characteristics of the Pickering emul-sions.

2. Theoretical background

2.1. Pickering emulsions

In Pickering emulsions or PSE, the surface is stabilized byparticles instead of amphiphilic molecules as conventionallyapplied in emulsion technology. Today, Pickering emulsionsare produced with rotor–stator-systems such as tooth-rim-dispersing-machines (Xu et al., 2005). Rotor–stator-systems arecheap, flexible in use, and permit long residence times resulting inlong stabilization times (Schubert, 2005). However, the specificenergy input is limited resulting in big droplets usually above1mm (Kohler et al., 2007; Schuchmann and Danner, 2004).

The fundamental requirement for the stabilization of emulsiondroplets with particles is partial wettability of the particles at theinterface of both emulsion phases (Binks and Lumsdon, 1999).This can be achieved by a suitable modification of the particlesurface to obtain partly hydrophilic and partly hydrophobicsurface characteristics. Depending on their wettability theparticles tend to stabilize w/o- or o/w-emulsions. Otherapproaches include the addition of suitable surfactants (Binkset al., 2007) or suitable multivalent ions (Binks and Lumsdon,1999; Wu et al., 1994; Frith et al., 2008), or the adjustment of thepH (Wolf et al., 2007; Binks and Whitby, 2005). Overall, a weakflocculation of the particles is necessary to achieve goodemulsification results (Binks and Lumsdon, 1999; Briggs, 1921).The flocculation is commonly found at a zeta potential ofapproximately zero (iso electric point).

2.2. High pressure homogenization

High pressure homogenization is a common unit operation inthe chemical, pharmaceutical and food industry. High pressurehomogenizers are generally pumping the at pressures up to2000 bar through a disruption system such as flat or orifice valves(Schubert, 2005; Aguilar et al.; Kohler et al., 2008). Thus highlyspecific energy inputs can be realized within extremely shortperiods of time (order of milliseconds) and small volume of thefluid, stressing droplet interfaces resulting in their deformationand disruption (Schubert, 2005). The main disadvantage of highpressure processes, especially in particle loaded systems, are thehigh costs, which are driven by the abrasion of the plant,particularly of the piston packing and the valves. The abrasion isoften increased significantly by nano-particles such as silicaparticles, resulting in a service time of no longer than severalminutes, especially when high pressure is applied. For this reasonthis process has up to now been considered unsuitable as acommercial process. Furthermore, in high pressure applications,emulsions tend to form big droplet agglomerates, especially whenthe applied active surface material tends to form gel-typestructures as found in proteins (Kohler et al., 2008; Kessler,2002) or nano-particles (Abend et al., 1998). This problem anddamages (Sauter and Schuchmann, 2008) to the plant can besolved by the use of a mixing stream directly behind the valve(SHM-valve).

2.3. SHM valve

A simultaneous homogenizing and mixing valve (SHM-valve),as described in Kohler et al. (2007, 2008), is used in the presentwork. Here, a mono-piston pump is followed by a simple high

pressure orifice valve as in the conventional high pressurehomogenization processes. The fluid of the main stream iselongated with the first diameter reduction in front of the orificeborehole. In and after the borehole the fluid switches fromelongational to turbulent flow. The specific geometry of the SHM-valves ensures that the main stream passes the orifice itself anddelivers the local flow conditions required for emulsifying thedisperse phase, mixing and distributing the nano-particles anddeagglomerating them. However, contrary to the standardsystems the abrasive ingredients, such as the nano-particlesapplied, are mixed as a side stream, the second stream, into theenergy dissipating mainstream instead of following the main-stream and causing damages to the valve. In the SHM-valve, thesecond stream is mixed, respectively, sucked into the homo-genization stream directly after the orifice outlet, i.e. directly intothe droplet disruption zone (Fig. 1). Local mixing times(calculated by CFD) are below 100ms (Kohler et al., 2007). Thusa reduction of the coalescence and of the build-up rate ofaggregates of droplets directly after droplet disruption can beachieved. Both reductions can be explained by a significantreduction of the collision frequency between the droplets throughquasi-instant dilution. An additional hydrodynamic stabilizationeffect by natural surface-active compounds of the oil cannotbe excluded. At the same time, efficient micro-mixing ensuresthat additional emulsifier molecules/silica particles are locallyavailable to stabilize new interfaces.

3. Materials and methods

3.1. Materials

The nano-particles used in the experiments were Stober silicaparticles with a nominal diameter x of 200 and 12 nm. The 200 nmsilica particles were acquired from Blue Helix Ltd. (Angstrom-Sphere, Crawley, UK). The quality of these particles consideringthe sphericity was formerly established by Frith et al. (2008). The12 nm particles were obtained as Ludox HS-40 (Grace Davison,USA) with a density of 1.77 g/cm3. The results were comparedwith the emulsifier Tweens 20 (Carl Roth GmbH & Co, Germany).

A commercially available corn oil (Mazola ex Unilever, UK),acquired from a local supermarket without additional treatmentwas used as oil phase. The aqueous emulsion phase consisted ofde-ionized water (18.2 MO). Hydrochloric acid (32 M) and sodiumhydroxide (3 M) were added to adjust the pH. 3 mM of Lanthanum(III) chloride heptahydrate (Carl Roth GmbH & Co, Germany) wasdissolved in the aqueous phase, which is above the criticalflocculation concentration and has been previously applied tostabilize Pickering emulsions (Frith et al., 2008). In all experi-ments the disperse fraction (oil volume content) amounted to 20%(v/v) in reference to the complete emulsion.

To calculate the amount of silica particles that are necessary tostabilize Pickering emulsions by a monolayer, the theoreticalvalue of surface coverage regarding a hexagonal close packing of90.7% is applied. For the 200 nm silica particle the formation of amonolayer is confirmed (Braisch et al., 2009). When consideringfactors like particle size variation, electronic repulsive forcesbetween the particles, and different contact angles of the particlesa value of about 70% coverage is realistic (Balmer et al., 2009). InFig. 2 the calculated possible droplet size of emulsions stabilizedby 12 and 200 nm are depicted in comparison to experimentalresults. In this calculation we used the worst case scenario valueof 90.7% coverage in respect to a monolayer, assuming that thecomplete amount of silica particles was adsorbed on the dropletsurface and that droplets are coalescing until the surface iscompletely covered by particles (Arditty et al., 2003).

ARTICLE IN PRESS

a = 0.2 mmc, b = 2.0 mmd = 9.6 mme, g = 0.4 mmf = 0.6 mml = 1.0 mm

a

c

a

c

(I) High pressure stream(II) Mixing stream(III) Fine emulsion

f

mixing

c

b

e l

c

homogenization

ed

g(I)

(II)

(II)

f

c

(III)

Fig. 1. Principle design of the combined homogenization valve, the T-shaped micro-mixer (Kohler et al., 2007).

Fig. 2. Dependency of the droplet size on the silica concentration (Kohler).

K. Kohler et al. / Chemical Engineering Science 65 (2010) 2957–2964 2959

Considering this model, the droplet size is never limited by thetheoretical monolayer surface coverage. Using 12 nm silicaparticles the droplet size is nearly constant at around 7mm downto a concentration of 0.3% silica particles. Preliminary testswithout particles delivered unstable emulsions, where thecomplete phase separation occurred in minutes. The reasons forthe constant droplet size at higher Particle concentrations couldnot be fully clarified in this work.

If 200 nm silica particles are used to stabilize the emulsion thedroplet size decreases at higher silica concentration. Therefore apossible explanation could be the faster diffusion process of thesilica particles from the bulk phase to the surface through thehigher concentration gradient (Ward and Tordai, 1946) resultingin faster stabilization and smaller droplets.

In general the process is not limited by the particle concentra-tion, due to the fact that the particle sizes obtained in theexperiments are always higher than the theoretical sizes(sometimes a factor 20 above). All other process and substance

factors, which can influence the surface coverage, such as particlesize, electronic repulsive forces, or the contact angle cannotexplain this difference. Therefore we used a concentrationof 3% silica particles for the subsequent results, which was theminimal concentration for which both particle sizes were stillcomparable.

The silica particles (3% w/w referring to the continuous phase)were dispersed in the aqueous emulsion phase containinglanthanum chloride, if necessary. The zeta potential z wasmeasured with an AcoustoSizer II ex Colloidal Dynamics at pH-values of 2–10 (see Fig. 3).

The zeta potential was around zero at a pH of 2 for bothsilica particles independent of the presence of LaCl3. At higherpH values, the absolute value of the zeta potential increasedespecially for the 12 nm particles. Therefore the pH wasadjusted to a value of 2 for the experiments using sodiumhydroxide ensuring that the particles were close to their iso-electric point.

ARTICLE IN PRESS

Fig. 3. Zeta potential of the 12 and 200 nm silica particles at different pH-values

measured with and without LaCl3 (SDE5 mV).

1 vessel2 high pressure pump3 pressure indicator4 pulsation damper5 SHM-valve6 mixing vessel

pickeringemulsion

PI

1

23 4

6 5

Fig. 4. Layout of the pilot plant for the SHM-valve.

K. Kohler et al. / Chemical Engineering Science 65 (2010) 2957–29642960

3.2. Pilot plant

The pilot plant consists mainly of a mono-piston pump (seeFig. 4(2)), which is followed by the high pressure (SHM-) valve (5).The two inlets of the SHM valve are fed by two vessels (1 and 6).To reduce the pulsation of the mono-piston pump a pulsationdamper (4) was interconnected between the pump and the SHM-valve.

The main process parameter, which was investigated in thiswork, is the pressure of the main stream produced by the highpressure pump. It varied between 350 and 1000 bar. The mixingstream of vessel 6 was suck in caused by a negative pressureproduced with the specific geometry of the SHM-valve. Noadditional pump was required.

3.3. Operational modes of the SHM-valve

The used SHM-valve is described in detail in Kohler et al.(2007, 2008). This type of SHM-valve may be operated in fourdifferent operational modes, depending on the fluid which passesthe main (see Fig. 1, I) or the mixing inlet (see Fig. 1, II). In generalthe continuous as well as the disperse phase may be pumpedthrough the main inlet (orifice). However, for Pickering emulsionslimitations need to be considered with respect to thenano-particles causing abrasion to the pump and the orifice.Accordingly the nano-particles have to be added with the mixing

stream. The four operational modes described below were appliedfor particle stabilized o/w-emulsions.

The first two operational modes depicted in Fig. 5 use a premixemulsion, which needs to be prepared prior to processing. Incontrast, operational modes three and four start with the twoactual emulsion phases. Using the premix operational modes(I and II), already existing droplets are decreased in size, whereasin the operational mode III and IV, two separate phases enter theSHM valve and new droplets have to be formed and reduced totarget size within the homogenization step. This simplifies theprocess by omitting the pre-emulsification step, but results indisadvantages such as the lack of elongation in front of the orifice.

In operational mode I droplets of a non-stabilized premix arereduced in size after leaving the orifice and stabilized by nano-particles, which are added via the mix stream. Depending on thesubstances an elongation of the droplets in the orifice inlet couldimprove their disruption. However, producing a homogeneouspremix without a time-dependent fluctuation in respect to thedisperse phase fraction is a challenge, as no stabilizing agent isadded. An additional disadvantage is that, after droplet disruptionthe newly produced surfaces of the droplets need to be coveredcompletely.

In operational mode II only continuous phase is pumpedthrough the orifice increasing process stability significantly.The emulsion premix, already stabilized by the nano-particles(Pickering emulsion premix) is added to the main stream via themix stream. As an advantage the droplet surfaces are already pre-covered and stabilized with nano-particles and the nano-particlesare homogeneously distributed in the continuous phase and thusalready close to the droplet interfaces. The main disadvantage isthat highly concentrated emulsions have to be used to achieve areasonable disperse phase fraction in the finished product. At therequired concentrations, Pickering emulsions often show yieldstresses and high viscosities limiting the possibility to workwithout any mix stream pump.

In the operational mode III the disperse phase (oil) is pumpedthrough the orifice. In comparison to the operational modes I andII nano-particle dispersions with lower concentrations can bemixed into the main stream. Detrimental is that the turbulencenecessary for droplet formation and break up has to be producedby the disperse phase itself, which often has a much higherviscosity than the continuous phase. Higher viscosity results inreduced turbulence (Reynolds, 1883), if we consider fluids withonly a weak elastic behavior (Bertola et al., 2003) as used in thisstudy. This highly viscous fluid also needs to be highly pressurizedwhich is another disadvantage of this mode.

In the operational mode IV the continuous phase, which isusually of lower viscosity, is pumped through the orifice.However, the challenges here are the particles. At first theparticles need to be dispersible in the disperse phase and thenmixed with the continuous phase. To reach high particleconcentrations in the oily phase, the particles have to have ahigh wettability in the oil (disperse phase). High wettability ofparticles in oil contradicts the Bancroft rule, which states that thephase in which an emulsifier is more soluble constitutes thecontinuous phase (Bancroft, 1913). This rule can be explained bythe effect, that the interfacial active material has to cover a curvedsurface. Therefore the larger volume of it has to be wetted by thecontinuous phase (HLB 8-18 (Griffin, 1949) or contact angleYo901 (Binks, 2002; Binks and Horozov, 2006) for o/w emul-sions). A second challenge results from the fact that the particleshave to diffuse through a phase of higher viscosity to the dropletsurface. Especially for emulsions with small droplets (or a largesurface area) a high concentration of nano-particles in the oilphase is required again resulting in significantly increasedviscosities, which—in turn—make droplet forming, deforming,

ARTICLE IN PRESS

Premix

2 phases

I

IVIII

II

o/w

w + s

w + s

wo

o + s

w

o/w + s

Fig. 5. Operational modes split in two operational conditions using a premix and starting with two separate phases for the production of Pickering emulsions (o: oily

phase; w: watery phase; s: solid particles).

K. Kohler et al. / Chemical Engineering Science 65 (2010) 2957–2964 2961

and break-up more difficult. This operational mode is quitelimited. In this work we exclusively used mainly hydrophilicnano-particles; therefore no results are presented for thisoperational mode.

3.4. Preparation of Pickering emulsion premixes with a rotor–stator

system

Pickering emulsions of 200 ml were prepared by batchhomogenization using a rotor–stator overhead device (ULTRA-TURRAX& T25, Germany) in a 250 ml snap cap vial. The rim speedwas 28 m/s and the homogenization time accounted to 2 min. Thisemulsion was used as premix in the operational modes I and IIand as the rotor–stator-system reference emulsion.

3.5. Droplet size distribution measurement

The droplet size distributions of the emulsions were measuredusing a laser diffraction spectrometer combined with PIDStechnology (Beckman Coulter LS 230). Volume size distributionswere used to characterize the oil globule collective. Maximumdroplet diameters of the volume size distribution x90,3 weredepicted as a measure for the creaming stability: the higher themaximum droplet diameter, the lower the creaming stability orthe shorter the shelf life of the product. To calculate the existingsurface between the disperse and continuous phase in anemulsion the sauter diameter x1,2 was used.

The particle size measurement of the Pickering emulsions withlaser diffraction is a challenge, due to the interaction of the three-phase system of continuous and disperse phase and nano-particles. In this work size 12 nm nano-particles were mainlyused, which are below the measurement limit of the device.Therefore the results representing the drop size distribution areonly weakly influenced by the silica particles, which results in asmall measurement error of E5%, as found for repeated

measurements of the same sample. To validate the results,photographic pictures of some samples were taken additionally.

4. Results and discussion

4.1. Influence of specific energy input and nano-particle size

First experiments using both the SHM-valve in the operationalmode I and a rotor–stator system proved the feasibility ofcontinuous high pressure processing. However, droplet sizesresulting from the high pressure process with higher specificenergy inputs resulted in bigger droplets (mean size x50,3E60mm) compared to the rotor–stator-system (x50,3E20mm, seeFig. 6).

To prove whether the homogenization process with the SHM-valve is limited by the disruption (e.g. specific energy input) or thestabilization (e.g. stabilization kinetics of the 200 nm Stober nano-particles), a fast adsorbing emulsifier (Tween 20) was used inaddition. This resulted in small droplets as expected for theenergy input applied (x50,3E2mm). Consequently, the specificenergy input achieved in the SHM valve is high enough to producesmall droplets, but the droplets cannot be stabilized fast enoughby the large silica particles, even if the emulsion is produced bythe rotor–stator-system.

An explanation may be the residence time of the droplets inthe dispersing zone of the rotor–stator system, allowing for longerstabilization times. Due to this effect, rotor–stator systems arestate of the art for producing Pickering emulsions.

In order to use high pressure homogenization processes, thestabilization has to be improved. The stabilization process canbe divided into the transport of the emulsifying material (in ourcase the nano-particles) from the bulk phase to the dropletinterface, the diffusion through the sub layer, and the adsorptionof the particles and their embedment in the droplet interface(McClements and Dickinosn, 1996). Especially with respect to thelast two steps, the size of the molecules or particles responsible

ARTICLE IN PRESS

Fig. 6. Cumulative volume distribution of Pickering emulsions produced with a rotor–stator-system (RS), compared with two emulsions produced via high pressure

homogenization (HPH) using the SHM-valve in operational mode I.

Fig. 7. Influence of nano-particle size on the Pickering emulsion droplet size distribution at constant specific energy (pressure drop of 800 bar).

K. Kohler et al. / Chemical Engineering Science 65 (2010) 2957–29642962

for droplet stabilization has a huge impact on the stabilizationkinetics, which could be shown by using smaller nano-particles of12 nm size (see Fig. 7). The reduction of the nano-particle sizefrom 200 to 12 nm resulted in remarkably decreased droplet sizes.Compared to the results achieved with Tween 20, the 12 nm silicaparticles are able to stabilize droplets of the same size range.However, a bimodal size distribution was found. Whether theparticles measured below 1mm in size are actually agglomeratesof nano-particles or Pickering emulsion droplets could not beclearly resolved in this work. Therefore, instead of mean sizesx50,3, characteristic maximum droplet sizes x90,3 were usedsubsequently, as they are less sensitive to small particles in thecollective.

4.2. Influence of SHM operational mode

Homogenization results of the three operational modes I to IIIare depicted in Fig. 8.

In general, Pickering emulsions can be produced using all threeoperational modes. In all operational modes the standarddeviations were high at low pressures. In operational mode I thiscan be explained by the missing stabilization of the Pickeringemulsion premix droplets. Segregation and coalescence in thepremix on its way from the vessel to the orifice result in a hightime-dependent fluctuation of the disperse phase fraction andthus an unstable process. This consequently results in some largedroplets, which tend to cream significantly (see Fig. 9, left). In

ARTICLE IN PRESS

Fig. 8. Comparison of the homogenization results obtained with the different operational modes.

Fig. 9. Pickering emulsions produced with the operational mode III at homo-

genization pressures of 100, 500, and 1000 bar.

K. Kohler et al. / Chemical Engineering Science 65 (2010) 2957–2964 2963

order to decrease the fluctuation of the disperse phase fraction,the two phases have to be fed directly in front of the valve, e.g.with a static mixer, instead of preparing the Pickering emulsionpremix in a separate vessel. At higher pressures and thereforehigher volume throughputs this problem can also be reduced. Thesamples obtained at higher pressures were smaller in droplet size(x90,3E10mm at pressures of 500–800 bar) with low standarddeviation. The samples did not cream anymore and a yield stresswas found (see Fig. 9, right) (Braisch et al., 2009). Nonetheless,droplet stabilization with the silica particles limited the process,as increasing pressure did not result in decreasing droplet sizes,found when Tween 20 was used instead.

Operational mode II also shows a high standard deviation atlow pressures, most probably due to the high viscosity of thePickering emulsion premix added to the side stream. The mixing

stream tended to stick resulting in an instable mixing process. Anadditional pump or a geometric modification of the SHM-valvecould improve this operational mode. At higher homogenizationpressures this operation also ran at a constant flow and became astable process.

Operational mode III led to nearly the same results as mode II,but ran more stable due to the low viscosity of the mixing stream(no sticking of the mixing stream) and the pure disperse phase inthe high pressure stream (no phase separation inside the inlet).

In conclusion all three operational modes are practicableand do not significantly differ in the resulting particle sizes.Operational mode III ran most stably, especially at low homo-genizing pressures. As no premix has to be produced, it seemsquite an attractive possibility in a lot of applications.

With the SHM-valve no abrasive particles passes thehigh pressure pump and valve, wherefore common highpressure equipment can be used. This results in cheaper plants.Furthermore the efficiency of the process can be improved by areduction of volume which has to be pumped on high pressures.This could be demonstrated on cow milk (Kohler et al., 2008) andcan result in energy savings of up to 90%.

5. Conclusions

In this article, a new approach is presented for the continuousproduction of nano-particle stabilized emulsions (PSE or Pickeringemulsions), being also suitable for abrasive nano-particles. Thisapproach allows an operation without passing the nano-particlesthrough the high pressure area (pump and orifice). This permits acontinuous high pressure processing for the production ofPickering emulsions down to droplet sizes well below thosefound in rotor–stator systems used so far.

The basic idea depicted here is to let the highly abrasivenano-particles bypass the high pressure pump and orifice, addingthem only after the orifice outlet. A valve geometry allowing thisis presented (simultaneous homogenization and mixing (SHM)valve). It permits preparing micron-sized emulsion droplets and

ARTICLE IN PRESS

K. Kohler et al. / Chemical Engineering Science 65 (2010) 2957–29642964

stabilizing them by nano-particles within one continuous proces-sing step.

The stabilization of micron-sized droplets in fast processeshighly depends on the droplet stabilization. Nano-particle sizeclearly was a parameter of significant influence. Silica particles of200 nm could not stabilize the droplets, while 12 nm silicaparticles were able to stabilize the droplets at lower homogeniz-ing pressure (100–500 bar). The resulting droplet size distribu-tions were comparable to emulsions stabilized by a conventionalemulsifier. At higher pressures (800–1000 bar), however, stabili-zation could not follow droplet break-up.

The SHM valve can be used in operational modes I, II, and III,but mode III resulted in the most stable process at lowerhomogenizing pressures.

Acknowledgments

We would like to thank the Helm AG, Germany, for the Ludoxparticles and Prof. Helmar Schubert for the beneficial discussions.

References

Abend, S., Bonnke, N., Gutschner, U., Lagaly, G., 1998. Colloid and Polymer Science276, 730.

Aguilar, F., Kohler, K., Schubert, H., Schuchmann, H.P., Chemie Ingenieur Technik80 (5), 607.

Arditty, S., Whitby, C.P., Binks, B.P., Schmitt, V., Leal-Calderon, F., 2003. EuropeanPhysical Journal e 11, 273.

Aveyard, R., Binks, B.P., Clint, J.H., 2003. Advances in Colloid and Interface Science100, 503.

Balmer, J.A., Armes, S.P., Fowler, P.W., Tarnai, T., Ga‘uspa‘ur, Z., Murray, K.A.,Williams, N.S.J., 2009. Langmuir 25, 5339, doi:10.1021/la8041555.

Bancroft, W.D., 1913. The Journal of Physical Chemistry 17, 501.

Bertola, V., Meulenbroek, B., Wagner, C., Storm, C., Morozov, A., van Saarloos, W.,Bonn, D., 2003. Physical Review Letters, 90.

Binks, B.P., 2002. Current Opinion in Colloid & Interface Science 7, 21.Binks, B.P., Horozov, T.S., 2006. Colloidal Particles at Liquid Interfaces. Cambridge

University Press, Cambridge.Binks, B.P., Lumsdon, S.O., 1999. Physical Chemistry Chemical Physics 1, 3007.Binks, B.P., Rodrigues, J.A., Frith, W.J., 2007. Langmuir 23, 3626.Binks, B.P., Whitby, C.P., 2005. Colloids and Surfaces A—Physicochemical and

Engineering Aspects 253, 105.Braisch, B., Kohler, K., Schuchmann, H.P., Wolf, B., 2009. Chemical Engineering &

Technology 32.Briggs, T.R., 1921. Journal of Industrial and Engineering Chemistry 13, 1008.Frith, W.J., Pichot, R., Kirkland, M., Wolf, B., 2008. Industrial & Engineering

Chemistry Research 47, 6434, doi:10.1021/ie071629e.Griffin, W.C., 1949. Journal of the Society of Cosmetic Chemists 1, 311.Horne, D.S., 1996. Current Opinion in Colloid & Interface Science 1, 752.Kessler, H.G., 2002. Food and Bio Process Engineering—Dairy Technology. Verlag A.

Kessler, Munchen.Kohler, K., 2000, Thesis, dissertation, Universitat Karlsruhe (TH), in press.Kohler, K., Aguilar, F.A., Hensel, A., Schuchmann, H.P., 2007. In: Hessel, V., Renken,

A., Schouten, J.C., Yoshida, J. (Eds.), Micro Process Engineering, Vol. 3. WILEY-VCH, Weinheim.

Kohler, K., Aguilar, F.A., Hensel, A., Schubert, K., Schubert, H., Schuchmann, H.P.,2007. Chemical Engineering & Technology 30, 1590.

Kohler, K., Karasch, S., Schuchmann, H.P., Kulozik, U., 2008. Chemie IngenieurTechnik 80, 1107, doi:10.1002/cite.200800070.

McClements, D.J., Dickinosn, E., 1996. Advances in Food Colloids. Springer, Berlin.Pickering, S.U., 1907. Journal of the Chemical Society, Faraday Transactions 91,

2001, doi:10.1039/CT9079102001.Ramsden, W., 1903. Proceedings of the Royal Society London (1854–1905) 72, 156,

doi:10.1098/rspl.1903.0034.Reynolds, O., 1883. Philosophical Transactions of the Royal Society of London 174,

935.Sauter, C., Schuchmann, H.P., 2008. Chemie Ingenieur Technik 80, 365.Schubert (Hg.), H., 2005. Emulgiertechnik. Behr’s Verlag, Hamburg.Schuchmann, H.P., Danner, T., 2004. Chemie Ingenieur Technik 76, 364.Ward, A.F.H., Tordai, L., 1946. The Journal of Chemical Physics 14, 453.Wolf, B., Frith, W.J., Lam, S., Kirkland, M., 2007. Journal of Rheology 51, 465.Wu, W., Giese, R.F., Vanoss, C.J., 1994. Colloids and Surfaces A—Physicochemical

and Engineering Aspects 89, 241.Xu, Q.Y., Nakajima, M., Binks, B.P., 2005. Colloids and Surfaces A—Physicochemical

and Engineering Aspects 262, 94.