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Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 48–54

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

ultiple W/O/W emulsions—Using the required HLB for emulsifier evaluation

. Schmidts, D. Dobler, A.-C. Guldan, N. Paulus, F. Runkel ∗

nstitute of Biopharmaceutical Technology (IBPT), University of Applied Sciences Giessen-Friedberg, Wiesenstr. 14, 35390 Giessen, Germany

r t i c l e i n f o

rticle history:eceived 27 July 2010eceived in revised form7 September 2010ccepted 20 September 2010vailable online 25 September 2010

eywords:/O/W multiple emulsions

LBydrophilic surfactantequired HLBlectrolyte encapsulation

a b s t r a c t

Stable emulsions are best formulated with emulsifiers or combinations of emulsifiers, which possess HLBvalues close to the required HLB of the oil phase. In this work, we have investigated the application ofthis established method to the development of multiple emulsions. This is of particular interest, sincemultiple emulsions are highly sensitive in terms of variations of the individual components as a resultof the presence of two thermodynamically unstable interfaces. However, multiple W/O/W emulsionsare potential skin delivery systems for water-soluble active pharmaceutical ingredients as a result oftheir pronounced encapsulation properties. Firstly, a suitable primary emulsion was developed based onrequired HLB determinations of the investigated oils. Secondly and based on the required HLB, multipleW/O/W emulsions were developed using the most appropriate primary emulsion and 1% of hydrophilicemulsifier blends in order to stabilise the second interface. In order to find the appropriate mixtures ofhydrophilic emulsifiers, the required HLB for the primary W/O emulsion was determined using two dif-ferent chemical classes of emulsifier blends, i.e. polyethoxylated ethers and polyethoxylated esters. Thephysicochemical parameters of the formulations were characterised by means of rheological measure-ments, droplet size and creaming volume observations as well as by means of conductivity analysis. As

discovered, all methods are appropriate for determining the required HLB determination with the excep-tion of the rheological data. Referring to the primary emulsions tested, required HLB values of 4.3–4.7using paraffin as the oil phase resulted in stable emulsions. Irrespective of the emulsifiers used, thefinest droplets, lowest conductivity and minimal creaming volume were obtained for the multiple emul-sions at required HLB values between 15 and 15.5 using paraffin as the oil phase. What is more, usinga polyethoxylated ether instead of a polyethoxylated fatty acid ester resulted in more stable multiple emulsions.

. Introduction

Owing to their distinct structure and properties, multiple emul-ions are of particular interest for several drug delivery approaches,ncluding carrier systems for the dermal application of pharmaceu-ical drugs [1–5].

Carrier systems in particular are developed by empirical means.hese empirical procedures are often extremely time-consuming,ince particularly sensitive carrier systems require a well-definedatio of ingredients [6]. It is therefore of great importance to incor-orate the physicochemical properties of the selected constituentst an early stage of the formulation development process. Twof these major parameters are the hydrophilic–lipophilic balance

HLB) value of the emulsifiers used and, determined by empiricalests, the required HLB (rHLB) value of the oil phase investigated.sing these parameters, the number of experiments can be reducedarly during the formulation screening stage [7–10].

∗ Corresponding author. Tel.: +49 641 3092550.E-mail address: Frank.Runkel@tg.fh-giessen.de (F. Runkel).

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.09.025

© 2010 Elsevier B.V. All rights reserved.

Griffin first established the hydrophilic–lipophilic balance (HLB)system to classify non-ionic surfactants [7]. According to Grif-fin, low HLB values are ascribed to lipophilic surfactants, whereashydrophilic surfactants are considered to possess high HLB val-ues. With regard to this system, W/O emulsions are obtained usingsurfactants with HLB values ranging between 3 and 8 and O/Wemulsions are formed using surfactants with HLB values between9 and 12. Stable emulsions are best formulated with emulsifiers orcombinations of emulsifiers, which possess HLB values close to theso-called rHLB of the oil or oil phase used. In order to determinethe rHLB of an oil or oil phase, emulsions are produced with differ-ent ratios of emulsifier blends, representing various HLBs, and areinvestigated in respect of their separating properties [24]. However,not only the HLB value but also the chemical type of emulsifier caninfluence the stability of emulsions. For example, depending on theemulsifier systems used and the phase obtained, the required HLB

for paraffin oil is proposed by some authors to range between 10 [7]and 12 [9] for simple O/W emulsions. For simple W/O emulsions, itis thought to be 4 [7].

Water-in-oil-in-water (W/O/W) multiple emulsions are sys-tems where both W/O and O/W emulsions appear simultaneously.

T. Schmidts et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 48–54 49

Table 1Results of the production of primary W/O emulsions. Numbers in brackets represent the HLB value of the lipophilic surfactant.

Sorbitan trioleate (1.8) Sorbitan sesquioleate (3.7) Sorbitan monooleate (4.3) Sorbitan monostearate (4.7) Sorbitan laurate (8.7)

Medium-chain triglycerides No emulsion No emulsion No emulsion No emulsion No emulsionPhaPhaEmEm

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2.5. Droplet size measurement

Mean water droplet size (z-average) in the primary W/Oemulsion was determined by dynamic light scattering (High Per-formance Particle Sizer (HPPS), Malvern Instruments, UK). Samples

Table 2Composition of multiple W/O/W emulsions.

Isopropyl palmitate No emulsion Phase separationOctyldodecanol No emulsion Phase separationLight paraffin No emulsion Phase separationHeavy paraffin No emulsion Emulsion

he main problem regarding their stability, however, is the pres-nce of two thermodynamically unstable interfaces, i.e. the W/Onterface of the primary emulsion and the O/W interface of the mul-iple emulsion. Two different emulsifiers are therefore necessaryor their stabilisation: one with a low HLB for the W/O interfacend a second one with a high HLB for the O/W interface. Theffect of the nature and quantity of both emulsifiers on the proper-ies of multiple emulsions is discussed in several works [6,11–14].

hen choosing lipophilic surfactants and determining the rHLBor the primary W/O emulsion of multiple emulsions, surfactantsor interface stabilisation can be screened in the same way as isone for simple W/O emulsions. However, the determination of arequired HLB” for W/O/W multiple emulsions and thus the selec-ion of hydrophilic emulsifiers, which are necessary to stabilise theecond interface and form a multiple emulsion, is more complex.t has been shown that both lipophilic and hydrophilic emulsifiersan be absorbed on O/W interfaces [15]. The HLB value of the O/Wnterface of multiple emulsions is therefore, if both hydrophilic andipophilic emulsifiers are added, the sum of the HLB values of allbsorbed surfactants. The amount of absorbed lipophilic emulsifiero the O/W interface is however not defined and dependent on itsotal amount in the formulation and the phase volume ratio, forxample. Magdassi et al. [15] discovered that the ideal HLB valueor hydrophilic emulsifiers increases when the concentration of theipophilic emulsifier added is higher or when the amount of theydrophilic emulsifier is lowered.

The purpose of our research was first to screen for an appropri-te primary W/O emulsion using rHLB considerations. Secondly, inrder to determine the rHLB, which will permit the most appro-riate primary emulsion to be incorporated into the multiplemulsion, W/O/W emulsions were prepared with different blendsf hydrophilic emulsifiers representing a range of HLB values andhe properties and long-time stability of the multiple W/O/W emul-ions obtained were investigated. The following parameters weresed to determinate the stability of the formulation as well as the

deal rHLB allowing the primary W/O emulsion to be incorporatednto the multiple emulsions: conductivity, multiple droplet size,reaming volume and viscosity. All of these methods have beenvaluated and compared. Their suitability for determining the rHLBalue of multiple W/O/W emulsions was investigated.

. Materials and methods

.1. Materials

The following chemicals were used to prepare the emulsions.ll chemicals are of Ph. Eur. quality:

Heavy and light paraffin, the oils isopropyl palmitate, octyldo-ecanol and medium-chain triglycerides were supplied by FagronmbH Co. KG (Barsbüttel, Germany).

The lipophilic surfactants sorbitan monooleate, sorbitan

esquioleate, sorbitan monostearate, glyceryl stearate and sor-itan trioleate were supplied by Croda GmbH (Kaldenkirchen,ermany). The hydrophilic surfactant polysorbate 20 was suppliedy Caelo GmbH (Hilden, Germany), steareth-10 and ceteareth-30ere obtained from Cognis GmbH (Monheim, Germany), whereas

se separation Phase separation No emulsionse separation Phase separation No emulsionulsion Emulsion No emulsionulsion Emulsion Phase separation

PEG-8 stearate was supplied by Croda GmbH (Kaldenkirchen, Ger-many). A NaCl solution (0.1 M) was used as the inner water phase,NaCl was supplied in Ph. Eur. grade by Merck KG (Darmstadt, Ger-many).

2.2. Preparation of the emulsions

Multiple emulsions were prepared using a 2-step procedure, asreported by Matsumoto et al. [16]. In the first step, the primary W/Oemulsion is prepared and in the second step, 40% of the primaryemulsion is dispersed in an aqueous solution of the hydrophilicemulsifier in order to obtain a multiple emulsion.

The primary W/O emulsion was prepared by adding the aqueousphase (50%) containing 0.1 M NaCl solution to the oil phase (50%).The oil phase consisted of 20% lipophilic emulsifier and 80% oil. Thecompositions of the primary emulsions and the multiple emulsionsare shown in Tables 1 and 2, respectively.

The two phases were heated separately to approx. 70–75 ◦C.After adding the water phase to the oil phase, the emulsion washomogenised for 2 min using a rotor/stator homogeniser Diax 600(Heidolph, Germany) at 9500 rpm. In the second step, the primaryemulsion was cooled down to room temperature and then slowlyadded to the outer water phase while the system was stirred at1200 rpm using a EUROSTAR digital stirrer (IKA, Germany) until ahomogeneous emulsion was produced. The obtained phase ratioW1:O:W2 was 1:1:3.

2.3. Conductometric analysis

Conductivity measurements were carried out using a WTWMicroprocessor Conductivity Meter LF 96 (WTW, Germany) at roomtemperature. Measurements were performed directly in the undi-luted emulsion (mean ± S.D., n = 3). To estimate the released massfraction of NaCl, a calibration curve was plotted.

2.4. Microscopic observation

The W/O/W multiple emulsions were analysed using an opti-cal immersion microscope TR 300 connected to a DV 2B camera(VWR, Germany) at ×1000 magnifying power (oil immersion). Thismethod was used in order to facilitate a standardised quality con-trol procedure as well verification of the multiple emulsions.

0.1 M NaCl solution 20% Inner water phaseHeavy paraffin oil 16% Oil phaseSorbitan monoolete 4%Aqua dest. 59%Hydrophilic emulsifier 1% Outer water phase

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ere diluted 1:1000 using light paraffin oil (viscosity: 33 mPa s)rior to measurement.

Oil droplet size and distribution in multiple W/O/W emulsionsere determined using a laser diffraction particle size analyser

Mastersizer S, Malvern Instruments, England). The fundamentalize distribution obtained with this technique is based on volumeistribution. The particle size distribution was calculated accord-

ng to the Mie theory. Measurements were performed directlyfter dilution in iso-osmotic glucose solution (0.1 M), thereby tak-ng into account the osmotic pressure of the internal aqueoushase. (mean ± S.D., n = 3). Microscopic observations revealed thathe bimodal particle size distribution obtained by static light scat-ering measurements can be attributed to the occurrence of simpleil droplets without an encapsulated water phase and oil dropletsith multiplicity (containing inner water droplets). The peak size of

he larger multiple droplets was therefore chosen for characterisinghe W/O/W multiple emulsions.

.6. Rheological measurement

Rheological analysis was performed at 25 ◦C using a RheoStress00 Rheometer (Thermo Haake, France) with a 2 cm diameter conend plate geometry measuring system. The geometry had a 2◦

ngle. The apparent viscosity was measured over a shear rate of.1–100 s−1. The results are presented as mean values (mean ± S.D.,= 3).

.7. Degree of creaming

A 40 ml sample of emulsion was poured into a 50 ml gradu-ted cylinder immediately prior to preparation. The volume ratiof the separated aqueous phase to the total volume of the emulsionas determined at room temperature (22 ◦C) as a function of time

ver 12 weeks. The values obtained were averages of three deter-inations. The results are presented as mean values (mean ± S.D.,= 3).

. Results and discussion

.1. Development of W/O primary emulsion

A basic principle of stable multiple W/O/W emulsion formationnd efficient drug encapsulation is the creation of homogeneousnd small inner water droplets (W1 < 1 �m) [17]. A uniform dis-ribution of droplet size counteracts Ostwald ripening, a processn which large droplets grow at the expense of small ones [18].y way of contrast, decreasing droplet size results in an increase

n Laplace pressure, which can induce coalescence. Laplace pres-ure can be compensated for by a small quantity of salt in the W1roplets [19]. However, the salt concentration in W1 is critical dueo the osmotic gradient between the inner (W1) and outer (W2)ater phase. Therefore, the electrolyte concentration should be suf-ciently high so as to compensate for Laplace pressure but, at theame time, be low enough so as to inhibit osmotic swelling of the

1 droplets in the multiple emulsion [17].Another crucial parameter for stable W/O/W emulsions is the

election of the appropriate type and amount of surfactants as wells the prevention of negative interactions between the W/O andhe O/W emulsifier. It was found that increasing the oil-soluble sur-actant concentration can hamper the exchange of water molecules11] as well as the transport of NaCl ions [18] between the two water

hases. Moreover, it is preferable to use low molecular emulsifiersor W/O (e.g. polyglycerol polyricinoleate or sorbitan monooleate)n combination with a high molecular emulsifier (e.g. proteins) [20]r a lipophilic polymeric emulsifier for the outer water phase [21].urthermore, the instability of W/O/W emulsions can be increased

icochem. Eng. Aspects 372 (2010) 48–54

by the use of unsaturated oils in the oil phase, which prevents aclose packed, condensed interfacial film [22,23].

The present work pursued the approach of the rHLB forthe development of appropriate primary W/O emulsions. There-fore, several saturated oils (isopropyl palmitate, octyldodecanol,medium-chain triglycerides, light paraffin and heavy paraffin) andfive lipophilic surfactants with HLB values ranging from 1.8 to 8.7were chosen for the preparation of primary W/O emulsions. Thesurfactants are of the same surfactant class (sorbitan ester) withdifferent side chains resulting in various HLB values. All primaryW/O emulsions consisted of 50% of an aqueous phase (0.1 M NaClsolution) and 50% of an oil phase (80% oil and 20% lipophilic surfac-tant).

It was found that, using medium-chain triglycerides, isopropylpalmitate or octyldodecanol, neither homogeneous nor stable(phase separation occurs in a few minutes) formulations wereobtained. Both paraffin oils, particularly the heavy paraffin, formedmore stable formulations (Table 1). More precisely, light paraffinresulted only with sorbitan monooleate and sorbitan monostearatein formulations that were stable for approximately 3–5 h. Heavyparaffin exhibited the same behaviour for these two surfactants, butadditionally resulted in a metastable W/O emulsion (∼2 h) usingsorbitan sesquioleate.

Referring to the results presented in Table 1, one can concludethat, using a 0.1 M NaCl solution as the water phase, the rHLB val-ues for light and heavy paraffin range between 4.3 and 4.7 and 3.7and 4.7, respectively. The most stable emulsion was obtained usingheavy paraffin stabilised with sorbitan monooleate. The viscosityof this primary emulsion was 1.9 ± 0.1 Pa s and the mean waterdroplet size was approximately 650 nm.

In comparison to the other tested oils in this work, paraffin oil isstrong non-polar. Therefore, the emulsions containing this oil areprobably more stable. Furthermore, the rHLB of paraffin oil is about4 [24] and consequently emulsifiers with an HLB of approximately4 will create the most stable formulations.

The stability of W/O/W emulsions can be further improved bythe addition of a thickener to the inner water phase [25]. There-fore, we investigated the influence of the two thickeners cetylpalmitate (2%) and hydrogenated castor oil (1%) on the stabilityof the primary emulsion prepared with heavy paraffin and sorbi-tan monooleate. Both thickeners increased the viscosity of the W/Oemulsion (∼4 Pa s) and its stability. No phase separation occurredover at least seven days. Nevertheless, from our experience usingthese more highly viscous primary emulsions for the productionof W/O/W emulsions resulted in phase inversion into simple W/Oemulsions. By way of contrast, less stable and low-viscous primaryW/O emulsions (without thickener) produced stable W/O/W emul-sions.

Therefore, the formulation consisting of heavy paraffin sorbi-tan monooleate and 0.1 M NaCl solution was used as the primaryemulsion for the following study.

3.2. Multiple emulsion development using rHLB considerations

Multiple emulsions have potential for many applications. Theiruse as drug delivery systems in particular seems to be most promis-ing. They can be developed as systems for oral, intravenous [26]or topical [4] applications. The advantages of multiple emulsionsas opposed to simple W/O or O/W emulsions are based on thefollowing properties:

- protection of drugs incorporated in the inner phase;- controlled release of drugs;- ability to integrate incompatible substances into the different

phases.

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at the same time, increases the number of oil droplets. When thecritical limit is reached, the oil layer breaks down and the innerwater phase migrates into the outer water phase. As a result, theoil droplet size decreases [31].

T. Schmidts et al. / Colloids and Surfaces A

Despite their advantages, multiple emulsions are, in the main,xtremely unstable. Two processes in particular must be bornen mind when it comes to multiple emulsion destabilisation: theoagulation of multiple droplets and multiple drop break-down.revious studies have shown that the composition of multiplemulsions, especially the selection of appropriate emulsifiers, playsmajor role in terms of producing stable formulations [6]. Referring

o emulsifiers, the significance of an ideal chemical compositionnd HLB value for producing simple emulsions was studied in the0s by Griffin [7]. However, in the case of multiple W/O/W emul-ions, these results can only be applied directly to primary W/Omulsions. Multiple W/O/W emulsions always contain two dif-erent emulsifiers (lipophilic and hydrophilic), the distribution ofhich in the emulsion is, in the main, not precisely known andependent on the total amount of emulsifiers in the formulationnd the phase volume ratio, for example. Therefore, the ideal HLBor hydrophilic emulsifiers varies from system to system and haso be determined on an individual basis for each formulation.

In the present work, the rHLB of hydrophilic emulsifiers wasetermined by the constant composition of W/O/W formulations.o verify the results, two different emulsifier combinations weresed: steareth-10 (HLB = 12.4) in combination with cetosteareth-0 (HLB = 17) and PEG-8 stearate (HLB = 10.8) in combinationith polysorbate 20 (HLB = 16.7). The emulsifiers represent chem-

cal substances with different functional groups. Steareth-10 andetosteareth-30 are polyethoxylated fatty alcohol ethers, whereasEG-8 stearate is a polyethoxylated fatty acid ester. Polysorbate0 is polyethoxylated sorbitan and a fatty acid ester, where bothunctional groups, i.e. esters and ethers, are present. The chemicalomposition of emulsifiers has no influence on the general trendsn the HLB range. Therefore, both emulsifier mixes were able toe used for determining the rHLB in order to further optimise theormulations. Emulsions with different physical properties (dropletize, viscosity) are nevertheless to be expected. Therefore, the influ-nce of the chemical composition of the emulsifiers on emulsiontability was also able to be determined in the process.

The composition of the multiple emulsions is presented inable 2. Based on the results of the development of the primary/O emulsion, sorbitan monooleate was chosen as the lipophilic

mulsifier and heavy paraffin as the oil. In the second prepara-ion step, the emulsifier mixes mentioned above were used ashe hydrophilic emulsifier: steareth-10 (HLB = 12.4) in combinationith cetosteareth-30 (HLB = 17) and PEG-8 stearate (HLB = 10.8)

n combination with polysorbate 20 (HLB = 16.7). The HLBs weredjusted by mixing the emulsifiers in order to obtain HLB valuesanging between 13 and 17 (0.5 unit step). For the steareth-0/cetosteareth-30 emulsifier system, the emulsions were pre-ared 3 times in order to determine the reproducibility of thereparation process. Repetition of the experiments is particularly

mportant, as the properties of W/O/W emulsions are highly sensi-ive in relation to the parameters applied during their preparation.eferring to the samples studied, relevant differences in the proper-ies of the obtained formulations with the same composition werelso found. The differences are probably associated with the man-al production process, as the individual steps were not able to bearried out in identical manner. However, a general trend in formu-ation properties (droplet size, viscosity, conductivity) and stabilityubject to the adjusted HLB value was able to be demonstrated.ecause of their instability, the formulations containing PEG-8tearate (HLB = 10.8) and polysorbate 20 were only prepared once.

The ideal HLB value for surfactant in respect of the composition

f an emulsion can be determined using several methods. Dropletize measurements as well as the determination of the creamingolume are the methods most commonly used [27,28]. In addition,ther data such as viscosity [8], turbidity [29] or conductivity [8]an be helpful. Conductivity measurements are of interest, espe-

icochem. Eng. Aspects 372 (2010) 48–54 51

cially in the case of multiple W/O/W emulsions, as the transport ofelectrolytes between both water phases is able to be detected quiteeasily [11,14].

All of the formulations produced in the present work werestored for 12 weeks at RT and parameters such as droplet size,conductivity and viscosity were observed.

No multiple W/O/W emulsions were created at HLB values lowerthan 14 and phase inversion into W/O emulsions was observed.Multiple W/O/W emulsions were created at higher HLB values. Incontrast to Frenkel et al. [30], no phase inversion into O/W emul-sions during the preparation process was observed at high HLBvalues.

One parameter to greatly influence emulsion stability is dropletsize. In relation to simple emulsions, it is, in most cases, advanta-geous when small droplets with minimal droplet distribution areformed [8]. Reducing the size of the droplets results in better emul-sion stability to gravitational separation, as described by Stokes’law. The uniformity of the droplets prevents coalescence as a resultof Oswald ripening.

According to this information, the ideal droplet size for innerdroplets in multiple W/O/W emulsions should be below 1 �m.However, oil droplets must be considerably greater and in the orderof magnitude of several �m [17].

The inner water droplets in the primary W/O emulsions pre-pared in the present study were approximately 650 nm. It isassumed that, after formation of multiple droplets in the sec-ond preparation step, the inner water droplets remain unchanged.On the other hand, the oil droplet size of the multiple W/O/Wemulsions is greatly dependent on the emulsifiers used as wellas on the HLB value of the emulsifier mix. Fig. 1 shows theeffect of the HLB value of a hydrophilic emulsifier on the sizeof multiple droplets. Using steareth-10/cetosteareth-30 emulsi-fiers, considerably smaller droplets are obtained than with PEG-8stearate/polysorbate 20. However, despite significant differences indroplet size, the minimum droplet size is found for both emulsifierpairs at the same HLB value, i.e. approximately 15–15.5.

The other stability factor associated with droplet size is thechange in droplet size during storage. Referring to simple emul-sions, the change in droplet size is mostly associated with acoalescence process. For multiple emulsions, many more reasonscould be responsible for such changes. One such reason in particu-lar might be the flow of water between the inner and outer aqueousphases resulting from the osmotic gradient between the two sidesof the oil layer. This causes the internal water drops to swell and,

Fig. 1. Droplet size as a function of the HLB of hydrophilic surfactant mixture formultiple emulsions containing 4% of lipophilic and 1% of hydrophilic emulsifier.

52 T. Schmidts et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 48–54

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ig. 2. Evolution of droplet sizes over time for formulations containing steareth-0 and cetosteareth-30 as hydrophilic emulsifiers. Droplet size is shown by way ofxample for one series of emulsion.

Droplet size development during storage for the formulationontaining steareth-10/cetosteareth-30 is presented in Fig. 2. Nohange in droplet size was able to be detected. It can be assumedhat, within the tested HLB range, no coalescence process occursnd that the water flow between both aqueous phases is negligible.

By way of contrast, a decrease in droplet size within thehole HLB range is observed for formulations containing PEG-8

tearate/polysorbate 20. This is probably linked to the instability ofster bounds in emulsifiers in the presence of electrolytes, whichesults in a loss of multiplicity.

Conductivity measurements are commonly used for multiplemulsions to detect the release of electrolytes from the internalater phase [32,33]. The emulsions are stable when no migration of

lectrolytes and other substances encapsulated in the inner waterhase occurs. An increase in conductivity indicates a release of elec-rolytes. It can occur as a result of diffusion or droplet breaking.oth processes alter the emulsion and can lead to destabilisationhenomena.

The conductivity of multiple emulsions subject to the HLB valueirectly after preparation is presented in Fig. 3. For the emulsi-er mix containing steareth-10/cetosteareth-30, a minimum HLBalue of approximately 15–15.5 is found. For the second emulsifierix consisting of PEG-8 stearate/polysorbate 20, the minimum HLB

alue is observed at approximately 14.5. However, the differences

n conductivity in the HLB range between 14.5 and 15.5 are verymall.

Irrespective of the emulsifiers used, all formulations show anncrease in conductivity during storage for formulations containingteareth-10/cetosteareth-30, as plotted in Fig. 4. For the emulsion

ig. 3. Conductivity as a function of the HLB of hydrophilic surfactant mixture forultiple emulsions containing 4% of lipophilic and 1% of hydrophilic emulsifier.

Fig. 4. Increase in conductivity over time for formulations containing steareth-10and cetosteareth-30 as hydrophilic emulsifiers. Conductivity shift is shown by wayof example for one series of emulsion.

with an HLB value of 15.5, conductivity remained lowest over timecompared with other formulations. This indicates that this emul-sion featured the highest level of efficiency in terms of electrolyteencapsulation.

The creaming volume measurements are consistent with theresults obtained from the measurements relating to droplet sizeand conductivity. The creaming of the emulsions containingcetosteareth-30 and steareth-10 as emulsifiers after 3 months ofstorage is shown in Fig. 5. The lowest creaming volume and thusmost stable formulation was found at an HLB value of approxi-mately 15.5.

The viscosity data for formulations containing steareth-10 andcetosteareth-30 are plotted as a function of the HLB (Fig. 6). Theviscosity shows a maximum at HLB = 15. Recent studies of simpleO/W emulsions show significantly different characteristics [8,27].Two areas are identified for simple O/W emulsions: one featuring astrong decrease in viscosity (low HLB values) and another featuringa linear profile, where only small changes in viscosity with increas-ing HLB values are observed. The HLB value, where the breakingpoint between both areas is estimated to be equates roughly to idealstability. The differences between O/W and W/O/W emulsions are

in multiple W/O/W emulsions. Emulsion viscosity is mostly depen-dent on several parameters. Some of these parameters, such as,for example, the viscosity of the continuous phase, remained con-

Fig. 5. Creaming volume after 3 months as a function of the HLB for formulationscontaining steareth-10 and cetosteareth-30 as hydrophilic emulsifiers.

T. Schmidts et al. / Colloids and Surfaces A: Phys

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ig. 6. Viscosity as a function of the HLB of hydrophilic surfactant mixture forultiple emulsions containing 4% of lipophilic and 1% of hydrophilic emulsifier.

teareth-10 and cetosteareth-30 were used as hydrophilic emulsifiers.

tant during the present experiment. The other parameters, suchs phase ratio associated with the flow of water and electrolytesetween both water phases and droplet size, were greatly influ-nced by the HLB of the emulsifiers used. Whereas for simple O/Wmulsions the disperse-to-continuous phase ratio always remainsonstant, this ratio can vary for multiple W/O/W emulsions depend-ng on the water flow between both phases. The flow from theuter water phase into the inner water phase results in an increasen viscosity as a result of a decrease in the amount of continuoushase. By way of contrast, the flow in the reverse direction leadso a decrease in viscosity. Phase inversion into an O/W emulsion isven possible above the critical weighted HLB value [30]. The elec-rolytes released into the outer water phase can also disturb theydrophilic emulsifier and result in a drop in viscosity.

The influence of droplet size on viscosity is described in severalublications [34,35]. It is found that the reduction in droplet sizeesults in a dramatic increase in the viscosity of concentrated W/Ond O/W emulsions [35]. As shown above, the HLB of hydrophilicmulsifiers can strongly influence droplet size. Therefore, differ-nces in viscosity because of this are expected in the HLB rangetudied.

The viscosity therefore reflects all previously described param-ters such as conductivity and droplet size. It could theoreticallye assumed that a maximum level of viscosity might correspondoughly to the emulsions featuring ideal parameters: droplet sizend the water flow into the outer water phase are minimal and,y way of contrast, the encapsulation of electrolytes occurs on aaximum level. However, not all parameters influence viscosity

n the same way. What is more, a modified emulsion compositionan also result in different viscosity characteristics. Therefore, theesults obtained cannot be discussed without knowledge of othereasurements.The results presented above show that methods such as droplet

ize measurements, conductivity and creaming behaviour could beuitable for determining the ideal HLB for hydrophilic emulsifiersy means of the preparation of multiple W/O/W emulsions. Under

deal conditions, viscosity could also be helpful for this purpose.owever, because viscosity projects several emulsion parameters,

t can, under certain circumstances, yield ambiguous informationbout the stability of the formulation in question.

As mentioned above, the appropriate rHLB for multiple emul-ions is strongly linked to several parameters such as the HLB of

he lipophilic emulsifier used as well as the amount of lipophilicnd hydrophilic emulsifier. Therefore, the results obtained in thisork are not able to be directly applied to formulations with dif-

erent compositions. Generally, it must be taken into account that

icochem. Eng. Aspects 372 (2010) 48–54 53

the HLB of the hydrophilic emulsifier in multiple W/O/W emulsionsmust be higher than its HLB in simple O/W emulsions. For the for-mulations containing paraffin oil, 4% of lipophilic emulsifier (HLB ofapproximately 4) and 1% hydrophilic emulsifier, it was found thatits HLB should be approximately 15–15.5.

In addition to determining the appropriate HLB, the clear influ-ence of emulsifier chemistry on the stability of multiple emulsionswas observed in the present study. The emulsions containing PEG-8stearate and polysorbate 20 were characterised by very low stabil-ity. Phase separation was observed for all formulations within 4weeks. The emulsions containing steareth-10 and cetosteareth-30were much more stable. After 4 weeks, phase separation was foundonly for the formulation with an HLB value of 14. However, initialcreaming was observed for all samples after 12 weeks. It can be con-cluded that, for the presented formulations, emulsifiers containingether groups have a positive effect on stability. By way of contrast,ester bonds have a negative effect. This might be associated withthe hydrolysis of esters in the presence of electrolytes and, as aresult, destabilisation of the emulsion [36]. The strong influence ofhydrophilic emulsifiers on the physicochemical parameters of theemulsions is described elsewhere [6].

4. Conclusions

In the present study, the determination of the ideal HLB value forthe hydrophilic emulsifier in multiple W/O/W emulsions was stud-ied. Several common techniques such as droplet size measurement,conductometric analysis, creaming volume and rheological prop-erties were analysed and compared. It was found that most of themethods used yield similar results and are suitable for the determi-nation of the HLB value in multiple emulsions. However, rheologicaldata are only useful for this purpose under certain conditions. Thisis associated with the complexity of parameters influencing viscos-ity, for example how water flow between both water phases resultsin changes in the disperse-to-continuous phase ratio, the amountof electrolytes released into the outer water phase or droplet size.

The optimum HLB value determined for hydrophilic emulsifiersfor W/O/W emulsions containing 4% lipophilic (HLB = 4.3) and 1%of hydrophilic surfactant was found to range between 15 and 15.5.

In addition, the strong influence of emulsifier chemistry on thestability of multiple emulsions was able to be demonstrated. In thepresented formulation, hydrophilic emulsifiers containing ethergroups had a positive effect on stability, whereas emulsifiers con-taining ester groups resulted in unstable emulsions.

Acknowledgement

We would like to thank the Hessen State Ministry of Higher Edu-cation, Research and the Arts for the financial support within theHessen initiative for scientific and economic excellence (LOEWE-Program) and also grant number 137/07-01.

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