Stability of oil-in-water emulsions stabilised by silica particles

B. P. Binks* and S. O. Lumsdon

Surfactant Science Group, Department of Chemistry, University of Hull, Hull, UK HU6 7RX

Received 19th March 1999, Accepted 23rd April 1999

We describe the preparation and properties of oil-in-water emulsions stabilised by colloidal silica particlesalone. The charge on the particles and their extent of Ñocculation, assessed via turbidity measurements, can bemodiÐed by pH control and addition of simple electrolytes. The stability of emulsions to both creaming andcoalescence is low in the absence of electrolyte, and the e†ects of adding salt are dependent on the type of salt.In systems containing NaCl, emulsions are less stable once the particles are Ñocculated. In the presence ofeither or tetraethylammonium bromide (TEAB), emulsion stability increases dramatically for conditionsLaCl3where the silica particles are weakly Ñocculated ; extensive Ñocculation of the particles however leads todestabilisation of the emulsions. For TEAB, relatively large emulsions of diameter around 40 lm remain verystable for up to 3 months at salt concentrations corresponding to the onset of coagulation of the colloid. Suchemulsions are themselves strongly Ñocculated.

Introduction

Solid colloidal particles constitute an important class of emul-sifying agents, and are commonly the cause of the high stabil-ity of water-in-crude oil emulsions. The e†ectiveness of theÐnely divided solid in stabilising oilÈwater emulsions dependson factors like particle size and shape, inter-particle inter-actions and the wettability of the particles by both liquidphases. Pickering noted that particles which were wetted moreby water than by oil acted as emulsiÐers for oil-in-water (o/w)emulsions by residing at the interface.1 Finkle et al.2 con-cluded that in an emulsion containing solid particles, the morepoorly wetting liquid becomes the dispersed phase of theemulsion, and the importance of the contact angle h which theoil/water interface makes with the soild was realised. Theseideas were given stong support by the experiments of Schul-man and Leja3 using barium sulfate crystals and powders,measuring the relevant contact angles and determining theemulsion type and stability. For conditions such that h (asmeasured through the aqueous phase) was slightly \90¡, theparticles stabilised o/w emulsions, but for h [ 90¡ particleswere still held at the interface but now stabilised water-in-oil(w/o) emulsions. Particles which were completely wetted bywater or oil became dispersed in either phase and were inca-pable of stabilising emulsions. The strength with which par-ticles are attached to an oil/water interface is related not onlyto h but also to the tension of the interface, For a smallcow .spherical particle of radius r (say \5 lm), assuming that theoil/water interface remains planar up to the contact line withthe particle, the energy, U, required to remove the particlefrom the interface into one of the bulk phases is given by4

U \ pr2cow(1^ cos h)2 (1)

where the sign in the bracket is positive for removal into oiland negative for removal into water. It follows that the parti-cle is most strongly held in the interface for h \ 90¡, is moreeasily removed into oil for h [ 90¡ and is more easily removedinto water for h \ 90¡.

Interestingly, good emulsion stabilisation was achievedwhen the particles were weakly Ñocculated (by salt in the caseof o/w5 or by surfactant in the case of w/o6) ; less stable emul-sions resulted when the particles were completely Ñocculated.

More recent work by Tambe and Sharma7 has shown thatsolid particles of various types stabilise emulsions by provid-ing both a steric barrier to dropÈdrop coalescence and bymodifying the rheological properties of the interface. The useof silica particles of diameter 12 nm as emulsion stabilisers hasalso been studied. Hassander et al.8 report that o/w emulsionswith drops between 5 and 20 lm were stable to coalescence inthe presence of a co-stabliser. The co-stabiliser (polymer orsurfactant) was responsible for causing Ñocculation of thesilica particles in solution, followed by adsorption of the Ñocsto the oil/water interface. Midmore9 describes the preparationof very stable o/w emulsions of high volume fraction ([0.8)with silica particles Ñocculated by the polymer hydroxypropylcellulose. The necessary stabilisation is achieved when theinterface is coated with the equivalent of only 29% of close-packed spheres. Thus, complete coverage of the oil drops witha rigid crust of silica particles is unnecessary. The unam-biguous role of the particles in these emulsion studies is diffi-cult to ascertain in the presence of other species capable ofadsorption. We describe here the preparation of o/w emul-sions with silica of the same size as used in the previous work,and examine the e†ect Ñocculating the particles has, usingelectrolyte alone, on the subsequent emulsion stability. Thesalts chosen are NaCl (1 : 1), (3 : 1) and the organic elec-LaCl3trolyte tetraethylammonium bromide. In this way, a clearerpicture as to the origins of stabilisation by silica may begleaned.

ExperimentalMaterials

Water was passed through a reverse osmosis unit and then aMilli-Q reagent water system. Toluene (Fisher, [99.9%) wascolumned twice through chromatographic alumina in order toremove polar impurities. The electrolytes used were NaCl(Prolabo, 99.5%), (Vickers, 98%) and tetra-LaCl3 É 7H2Oethylammonium bromide, TEAB (Acros, [99%) and wereused as received. Sodium hydroxide (Prolabo, 98%) andhydrochloric acid (May & Baker) were used to e†ect the solu-tion pH. Aerosil 200 silica powder was a gift from Degussa,with a quoted surface area of 200 m2 g~1 and a primary parti-cle diameter of 12 nm.

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Methods

(a) Preparation and characterisation of aqueous silica col-loids. Aqueous silica colloids were prepared by dispersing aknown mass of Aerosil 200 powder into 10 cm3 of pure water(of pH around 6) using a high intensity ultrasonic vibracellprocessor (Sonics & Materials, tip diameter 3 mm), operatingat 20 kHz and up to 10 W for 2 min. During sonication, it wasnecessary to cool the vessel in an ice-bath and the resultingsolutions were clear, bluish in appearance. Aqueous electrolytesolutions of various concentrations were then added beforeadjusting the solution pH using acid (9 M HCl) or base (1 MNaOH). The pH was measured using a Jenway PHM 6 meterwith a saturated KCl glass electrode. Transmission electronmicroscopy (TEM) of the colloids was performed using a Jeol100C 80 kV electron microscope. Ten microlitres of 0.125wt.% aqueous colloid were placed on a copper grid containinga very thin layer of carbon which acts as a support for thesolution. The solutions were freeze dried by immersing thegrid in liquid nitrogen for 2 min. Excess nitrogen was removedand the cell was loaded into an Edwards freeze drier undervacuum for 3 h before being observed in the microscope.

As a guide to Ñocculation, the turbidities of aqueous col-loids were measured using a Hach Analytical NephelometerModel 2424 at room temperature. 25 cm3 of solution wasplaced in glass vessels (25] 100 mm) and the intensity of lightscattered at 90¡ to the sample and at a height of 10 mm fromthe solution base is detected with a photomultiplier tube. Theinstrument records the turbidity in empirical NTU(nephelometric turbidity units) based on the amount of lightscattered by particles of a polymer reference standard For-mazin. At each salt concentration and pH, the turbidity wasmeasured immediately after preparation and 24 h later.

(b) Preparation, stability and characterisation of oil-in-wateremulsions. For 20 vol% toluene-in-water emulsions, 2 cm3 ofoil and 8 cm3 of aqueous colloid (in the presence of electrolyteand at the required pH) were emulsiÐed using a Janke andKunkel T25 homogeniser (rotor-stator) with an 18 mm headoperating at 8000 rpm for 2 min. The emulsions were trans-ferred into stoppered, graduated glass vessels of id 13 mm andlength 10 cm and thermostatted at 25 ¡C. Immediately afterhomogenisation, the conductivity of the emulsions was deter-mined using a PTI-58 digital conductivity meter with Pt/Ptblack electrodes. Their stability to creaming was assessed bymonitoring the increase with time of the position of the clearwater (serum)/emulsion interface. Stability to coalescence wasmeasured by following the change in height of the clear oil/emulsion interface. Emulsion drop size distributions weredetermined using a Malvern 2600c laser di†ractometer.Approximately 0.5 cm3 of emulsion was diluted to 50 cm3with its own continuous phase i.e. using the aqueous colloid atthe respective pH and salt concentration. This was to ensurethat, as far as is possible, dilution occurred without a†ectingthe size distribution. Optical microscopy of the emulsionsinvolved adding a small sample to a haemocytometer cell(Weber ScientiÐc) and viewing it with a Nikon Labophotmicroscope Ðtted with a DIC-U camera (World PrecisionInstrument). Images were processed using Aldus Photostyler2.0 software.

Results and discussion(i) Systems containing NaCl electrolyte

There are two main types of silica particles, Precipi-SiO2 .10tated silica particles like Ludox HS (BDH) are prepared byalkaline hydrolysis of sodium silicate solutions via nucleationand growth and are sold as an aqueous dispersion at high pH([9). Pyrogenic or fumed silica like Aerosil 200 (Degussa) isproduced by hydrolysis of silicon tetrachloride in a hydrogen/

oxygen Ñame and is sold as a Ñu†y, white powder. Both typesform as spherical, non-porous particles of amorphous silica ofdensity around 2.2 g cm~3. When initially dispersed in water,there appears to be a signiÐcant di†erence between the twoparticle types. In the case of precipitated silica, silanol(SiÈOH) groups present on the surface can ionise at theappropriate conditions of pH and render the surface charged.For the fumed silica, surface silanol groups also exist but someare not free to ionise as they are linked to adjacent groups viasiloxane (SiÈOÈSi) bonds. This di†erence manifests itself in thesensitivity of the aqueous colloidal dispersions to Ñocculationby electrolyte.

The isoelectric point (i.e.p.) and point of zero charge of silicaare both at pH 2 i.e. above this pH silica is negatively charged.The coagulation behaviour of colloidal dispersions of silica inwater has been described as ““anomalous ÏÏ in that their stabil-ity in terms of electrolyteÈpH control does not follow thepattern seen for almost all other oxide and amphoteric latexcolloids.11 For the latter systems, the critical coagulation con-centration (c.c.c.) of salt just required to Ñocculate the sol isvery low around the i.e.p. and increases progressively above itas the repulsion between charged particles increases. The wayin which the c.c.c. increases with pH can be accounted for interms of the classical DLVO theory which takes into accountthe electrostatic energy of repulsion and the attractive van derWaals energy between charged particles as they approacheach other. However, Iler10 noted that silica sols were remark-ably stable to electrolyte at their i.e.p. and in the presence ofhigh concentrations of salt at around neutral pH, contrary tothe predictions above. The Ðrst deÐnitive study of the anom-alous behaviour of silica sols was by Allen and Matijevic12who looked at Ludox HS sols of diameter equal to 15 nm.With NaCl as electrolyte, the sols remained dispersed (hencestable) up to the solubility limit of NaCl (around 5 M) at pHvalues between 2 and at least 5. Above pH 6, the c.c.c.decreased with increasing pH. Depasse and Watillon13observed the same trends with 120 nm diameter silica particlesprepared by polymerisation of silicic acid in alkaline solution.Very similar e†ects were reported by Rubio and Goldfarb14for particles of 12 nm diameter of the Aerosil 200 type. Thusthis behaviour is not restricted to precipitated silicas, and asHarding15 showed, is rather conÐned to silicas havingprimary particles below a certain size.

Silica sols, like all oxide sols, show an increasing negativezeta potential with increasing pH as the pH is raised abovethe i.e.p.16 The magnitude of the zeta potential decreases uni-formly at each pH as the salt concentration is increased. Theusual result of such variation in zeta potential with pH and1 : 1 electrolyte concentration is that the c.c.c. increases withpH as calculated by DLVO theory. Thus, the opposite trendmeasured experimentally for silica sols has another origin notaccounted for in DLVO theory. Possible reasons for theenhanced stability of silica at low pH have been postulated.Allen and Matijevic12 proposed that the stability of silica inacidic conditions is caused by particle hydration, with thesilanol groups being capable of hydrogen bonding with water,such that silica behaves more like a lyophilic than a lyophobiccolloid. Substitution of silanol protons by cations of the elec-trolyte as the pH increases leads to destabilisation of the solby eliminating sites for hydrogen bonding (dehydration). Dueto the low Hamaker constant for silica, the dispersion forcesare very low and it appears that one monolayer of water isquite enough to screen dispersion forces between small par-ticles preventing coagulation. By contrast, direct inter-particlebridging was suggested by Depasse and Watillon13 as amechanism for the coagulation of silica. At pH [ 6 and at therespective c.c.c., coagulation occurs because the particlesbecome linked by acidÈbase bonds formed by silanol groupson one particle and dissociated SiÈO~ groups on another.Very quickly these bonds are transformed into siloxane ones

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(SiÈOÈSi). Recently, Depasse17 has argued in favour of thelatter hypothesis in the light of new experimental data. A thirdexplanation of the high stability of silica in terms of stericstabilisation e†ects has been given by Healy11 and requiresthat oligomeric or polymeric silicate species are present at thesilica/water interface and that steric repulsion results duringoverlap of such layers.

The Ðndings mentioned above point to the likelihood thatsilica is subjected to a repulsive force not considered in theclassical DLVO theory. This non-DLVO force is generallyreferred to as a structural force, the origin of which is anongoing subject of debate and depends on the particularsystem.18 A well-known interpretation of this force is that apolar surface induces an ordering of the solvent which expo-nentially decays away from the surface. An overlap of theordered solvent layers near two mutually approaching sur-faces creates a force. Surface force measurements with glasshave conÐrmed the existence of a repulsive hydration force.19As described earlier,20 an extended form of the classicalDLVO theory may be written

Vt \ Vd] Ve ] Vh (2)

where is the net potential energy for the interaction betweenVttwo colloidal particles. The van der Waals dispersion energy,between two spherical particles of radius a is given byVd ,

Vd \ [aA13112H

(3)

where is the Hamaker constant for the system particleÈA131waterÈparticle and H is the surface-to-surface separationbetween the particles. The repulsive electrostatic energy, Ve ,for the case of constant potential is given by

Ve \ 32peakTCtanh

A et04kT

BD2e~2 exp([iH) (4)

where e is the permittivity of water (equal to its relative per-mittivity multiplied by the permittivity of a vacuum ise0), t0the surface potential (often substituted by the zeta potential f)and i is the inverse Debye length (or the reciprocal of thedouble layer thickness) given by

i \A2e2NA cel z2

ekTB1@2

(5)

where e is the elementary charge, is AvogadroÏs number,NAis the bulk concentration of electrolyte of valency z, k iscelBoltzmannÏs constant and T is temperature. For a 1 : 1 elec-trolyte, the double layer thickness is about 1 nm for a 0.1 Msolution and about 10 nm for a 0.001 M solution. Eqn. (4) isvalid when iH [ 1.

Since the abnormal stabilisation mechanism of silicadepends on salt concentration, the structural force shoulddepend on it in the same way. Assuming that the hydrationenergy, is directly proportional to salt concentration,Vh , cel ,Molina-Bolivar et al.21 have proposed the following expres-sion for Vh ,

Vh \ paNA Ch cel j2 exp([Hj) (6)

where is the hydration constant (in J) and j is the decayChlength. Fig. 1(a) shows the results of calculations of theseparate terms and along with vs. the particleVd Ve Vtseparation H using the classical DLVO theory (where Vt\ Vdfor the interaction of two spherical particles of radius] Ve),0.1 lm. The conditions chosen are 0.2 M of 1 : 1 electrolyte, anexperimentally determined zeta potential for silica16 at thisconcentration of [5 mV (corresponding to a pH of 2) and theHamaker constant is the average of theoretical and experi-mental values for silica.20 As seen, the overall energy is attrac-tive and the electrostatic contribution is virtually zero. Thus

Fig. 1 Interaction energies V vs. distance H between two sphericalparticles of radius 0.1 lm calculated using (a) classical DLVO theory,and (b) extended DLVO theory with J andCh \ 3 ] 10~19j \ 1 ] 10~9 m. The values of mV, M,t0\[5 cel\ 0.2 A131 \1.2] 10~20 J are taken in both cases.

DLVO theory would predict Ñocculation of the colloidwhereas it is found to be extremely stable. Fig. 1(b) shows thecalculations using the extended DLVO theory (Vt \ Vd ] Vefor the same system. The hydrationÈrepulsion term] Vh)dominates over the dispersionÈattraction one leading to atotal energy which is positive and hence repulsive. The valueof j taken is 1 nm (as determined directly for glass surfaces22)and the hydration constant has been varied until the sec-Chondary minimum in the total energy just disappears, as itshould since no Ñocculation occurs.

Using Aerosil 200 particles, we have investigated the e†ectsof pH and NaCl addition on the stability to Ñocculation of thecolloids and on the stability of toluene-in-water emulsionsprepared from them. The TEM image in Fig. 2 shows thatultrasonic dispersion breaks up the powder into primary par-ticles of diameter around 15 nm, some of which are joinedtogether in small chains. Fig. 3 shows the change in turbidityof 0.5 wt.% dispersions on addition of NaCl at pH 2, imme-diately after adding salt and 24 h later. Visibly, no change isobserved on adding salt or as a function of time i.e. no Ñoccu-lation occurs. The turbidity, q, of a dispersion of non-interacting particles of dimension much less than thewavelength of light is given by23

q\24p2P2NV p2

j4(7)

where j is the wavelength, N is the number of particles perunit volume, is the particle volume andVp

P\np2 [ nw2np2] 2nw2

(8)

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Fig. 2 TEM image of a 0.125 wt.% Aerosil 200 dispersion in water(pH 2) after freeze drying. The bar represents 0.1 lm.

with and being the refractive indices of the particle andnp nwwater solvent respectively. Noting that the product isNVpequal to the volume fraction of the particles, it is clear that theturbidity of a sample is proportional to the average particlevolume. At pH 2, since Ñocculation does not occur on addingNaCl up to 5 M and the colloids remain homogeneousthroughout, the decrease in the turbidity with salt concentra-tion is a result of the decrease in the magnitude of P due tothe increase in from 1.3330 in pure water to 1.3758 in 4.85nwM NaCl. Constraining the turbidity for no added salt to equalthe measured value of 310 NTU, the full line in Fig. 3 is thecalculated variation of q with [NaCl] taking equal to thatnpfor fused quartz (1.458 at 18 ¡C and 589 nm) and values of nwfrom ref. 24. As mentioned earlier,25 the decreased turbiditycan be explained entirely by the increase in the refractiveindex of the aqueous salt solution. An increase in q shouldthen be due to an increase in i.e. due to Ñocculation, overVpand above the decrease in N.

At the other extreme of pH (pH 10), the initial turbiditydecreases with salt concentration as before [Fig. 4(a)].However, above around 0.4 M NaCl the turbidity becomestime dependent and after 24 h the aqueous colloid separates

Fig. 3 Turbidity of 0.5 wt.% Aerosil 200 dispersions in water at pH2 as a function of NaCl concentration. Filled points, immediately afteradding NaCl ; open points, 24 h later. The line is a Ðt as described inthe text.

Fig. 4 (a) Turbidity of 0.5 wt.% Aerosil 200 dispersions in water atpH 10 as a function of NaCl concentration. Filled and open pointsrefer to immediately after adding NaCl and 24 h later respectively. (b)Change in turbidity of 0.5 wt.% Aerosil 200 dispersions in water atpH 10 in the presence of NaCl of given molarity.

into a clear upper phase (of low volume) and a turbid lowerphase in which particle Ñocs are visible. The open pointsshown refer to systems which have not been shaken. We showin Fig. 4b the development of the increased turbidity withtime for two high salt concentrations in unshaken colloids.The onset of the turbidity increase (around 2 h) coincides withthe Ðrst appearance of the clear, upper layer. The turbidityÈsalt concentration curves for unshaken colloids measured 24 hafter preparation are given in Fig. 5 for various pH values. AtpH 2, 4 and 5 no Ñocculation occurs and the turbidity onlydecreases. At pH 6 and above, Ñocculation is accompanied bya sharp increase in turbidity. The concentration of salt at theonset decreases with an increase in pH, whereas the maximumturbidity increases with pH possibly indicating a large Ñoc size(hence which forms when fully charged particles areVp)screened. Our Ðndings are summarised in Fig. 6 on a [salt]ÈpH plot, where data from the literature for Aerosil 20014 andLudox HS-4012 are included for comparison. Below the pointsthe colloid is stable and dispersed whereas above them it isunstable and Ñocculated. The anomalous behaviour is evidentin all cases.

The e†ect of particle concentration on the stability of 20vol% toluene emulsions in the absence of added salt is shownin Fig. 7 at two pH values. The left hand ordinate for cream-ing represents the fraction of emulsion which forms a clearaqueous phase (0 \ stable, 0.8 \ unstable) whilst the righthand ordinate for coalescence is the fraction of emulsionappearing as a separated oil phase. At pH 4 [the natural pHof the silica colloids, (a)], the two types of instability are initi-ated simultaneously and the induction times for each increasewith particle concentration. By 20 min, most of the emulsion

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Fig. 5 Turbidity of 0.5 wt.% Aerosil 200 dispersions in water 24 hafter adding NaCl. The pH values are 2 (open circles), 4 (Ðlleddiamonds), 5 (open squares), 6 (inverted Ðlled triangles), 7 (crosses)and 10 (Ðlled triangles).

has creamed and around 50% of oil has coalesced. Theremaining cream is a high volume fraction o/w emulsionwhich is very stable. At pH 10 (b), creaming and coalescenceoccur immediately and are similar at all particle concentra-tions. Coalescence was complete within 2 h. For an oil : watervolume ratio of 1 : 4, an average emulsion drop radius \ 20lm (later) and an average particle radius of 7.5 nm, theminimum mass of silica required to cover all drops with amonolayer is 1] 10~4 g. We have chosen an Aerosil 200mass in water of 0.5 wt.% for all subsequent emulsion studiessince this amounts to nearly 40 times as many particles as isnecessary to saturate the emulsion drop interfaces.

The stability of emulsions as a function of [NaCl] at di†er-ent pH values is given in Fig. 8. In all cases, the conductivitiesof equal volume oil and water emulsions are high indicatingemulsions are of the o/w type with no inversion to w/o at highconcentrations of salt. At all pH values between 2 and 10 and[NaCl] between 0 and 2 M, emulsions were unstable tocreaming such that after 30 min more than 90% of theaqueous phase was resolved. The major e†ect of adding NaCl

Fig. 6 Critical coagulation concentrations of NaCl for silica disper-sions in water as a function of pH and 24 h after mixing. Triangles arefrom ref. 12 with 0.2 wt.% Ludox HS silica, circles are present datawith 0.5 wt.% Aerosil 200 silica and squares are from ref. 14 with 0.2wt.% Aerosil 200 silica.

Fig. 7 Stability to creaming (open points, left hand ordinate) andcoalescence (Ðlled points, right hand ordinate) of 20 vol% toluene-in-water (no salt) emulsions stabilised by Aerosil 200. Wt.% values ofsilica in water are 0.2 (diamonds), 0.5 (squares) and 1.0 (triangles). (a)pH 4, (b) pH 10.

is on coalescence and e†ects are compared after 30 min in Fig.8(a). At low pH, emulsions become more stable on adding salt.At high pH, addition of salt causes signiÐcant destabilisation,whereas at intermediate pH emulsions are completely stable atall salt concentrations. Recasting the data as a function of pH,Fig. 8(b) reveals a maximum in coalescence stability aroundpH 4 for both pure water and 1 M NaCl systems. At pH \ 4where the particles possess a low surface potential, salt haslittle e†ect on emulsion stability as expected, whereas it inÑu-ences stability to an increasingly larger extent as the surfacepotential of the particles increases (pH [ 4). Maximum emul-sion stability occurs not then when the particles areuncharged (pH 2) but at some degree of low charge. Thedestabilising e†ect of NaCl on emulsions prepared at high pHmay be linked to the Ñocculation of the aqueous colloid. AtpH 10, the c.c.c. is 0.6 M NaCl so that in 1 M salt solution,emulsions are likely to contain some of the particles in a Ñoc-culated state. The present data indicate that Ñocs eitherprevent the formation of emulsions in the Ðrst place or act tobreak them once formed. Below pH 5, the colloids cannot beÑocculated even at 5 M NaCl, which is most likely the reasonfor the minor e†ect of salt on emulsion stability.

(ii) Systems containing electrolyteLaCl3

It is of interest to vary the nature of the Ñocculating cation insilica systems. The c.c.c. of electrolyte required to coagulate alyophobic colloid is much less for a trivalent ion than for amonovalent one.26 In addition, Wu et al.27 provide evidencethat adding, say, lanthanum chloride to aqueous dispersions

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Fig. 8 (a) Stability to coalescence after 30 min of 20 vol% toluene-in-water emulsions stabilised by 0.5 wt.% Aerosil 200 as a function ofNaCl concentration at three pH values. (b) Stability to coalescenceafter 30 min of 20 vol% toluene-in-water emulsions stabilised by 0.5wt.% Aerosil 200 as a function of pH at two NaCl concentrations.

of glass powder (similar to silica used here) results not only inÑocculation at low concentrations but also to an increase inthe hydrophobicity of the solid surface. For example, the zetapotential of glass particles falls from [52.7 mV in pure waterof pH 7.5 to [16.4 mV in the presence of 0.47 mM LaCl3 ,whilst the contact angle at the solid/liquid/vapour interfaceincreases from 49.4 to 65.9¡. It is argued that the main e†ect ofthe La3` ion is not the screening of the negative charge on thesolid but the neutralisation of the Lewis base sites on thesurface. This causes a change from an acidÈbase repulsionbetween particles in the absence of electrolyte to an acidÈbaseattraction (hydrophobic) with electrolyte, due to a lowering ofthe electron donor component of the solid surface free(cs~)energy.

The turbidities of 0.5 wt.% Aerosil 200 colloids at pH 2 and10 are given in Fig. 9(a) as a function of both initially[LaCl3]and after 24 h, shaking the dispersion just prior to the mea-surement. At pH 2 no change in turbidity occurs up to 20 mMand no phase separation was observed. This independence isdue to the fact that the refractive index of the salt solution ischanged by \0.075% in this concentration range. However,at pH 10, the turbidity increases by nearly a factor of three onaddition of 20 mM due to Ñocculation which occursLaCl3immediately. Concomitant with this the colloids separatedinto a clear, upper layer and a turbid lower layer for

mM. Fig. 9(b) shows the turbidity changes for[LaCl3]P 1various pH values. For pH[ 4 the turbidity begins to dependon and phase separation sets in at lower salt concen-[LaCl3]trations. A marked e†ect occurs between pH 6 and 8. The

Fig. 9 (a) Turbidity of 0.5 wt.% Aerosil 200 dispersions in water attwo pH values as a function of concentration. Filled points,LaCl3immediately after adding open points, 24 h later. (b) TurbidityLaCl3 ;of 0.5 wt.% Aerosil 200 dispersions in water 24 h after adding LaCl3 .The pH values are 2 (open circles), 4 (triangles), 4.5 (diamonds), 5(inverted triangles), 6 (asterisks), 8 (Ðlled circles) and 10 (squares).

e†ect of low concentrations of (O20 mM) on the stabil-LaCl3ity of o/w emulsions at pH 2 and 10 has been investigated.Fig. 10(a) shows the fraction of emulsion after 30 min whichremains as the aqueous phase (creaming) or the oil phase(coalescence) at low pH. At all creaming is more or[LaCl3],less complete and coalescence is over 50%. The resolvedaqueous phase is clear blue as it was before addition of oil. Atthis pH (the i.e.p.) little e†ect of salt is anticipated in line withthe Ðndings.

In contrast to the above, the emulsion stability is verydependent on once the particles are charged. It can[LaCl3]be seen from Fig. 10(b) at pH 10 that the stability to bothcreaming and coalescence shows a marked maximum at inter-mediate concentrations around 2È5 mM decreasing sharplyeither side of this range. Impressively, these emulsionsremained at this stability for over 3 months. Below 2 mM,rapid creaming occurs in which the resolved aqueous phase isclear blue. Between 2 and 5 mM, emulsions are stable and thesmall volume of aqueous phase resolved is colourless, implyingthat the particles originally present in water have transferredelsewhere either into the dispersed oil phase or to the oil/water interface around drops. Above 6 mM, the resolvedaqueous phase is viscous and turbid and most likely containsÑocculated particles. The e†ect of increasing pH on emulsionstability is shown in Fig. 11. In the presence of 5 mM LaCl3(a), emulsions formed at pHO 5 are unstable to creaming withmore than 90% of blue aqueous phase resolving. For pH P 6the low volume of aqueous phase appearing is colourless. Thestability to coalescence increases progressively with pH. For10 mM (b), maximum stability to both creaming andLaCl3

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Fig. 10 Stability to creaming (open points, left hand ordinate) andcoalescence (Ðlled points, right hand ordinate) after 30 min of 20 vol%toluene-in-water emulsions stabilised by 0.5 wt.% Aerosil 200 as afunction of concentration. (a) pH 2, (b) pH 10.LaCl3

coalescence is now observed at intermediate pH around 6 andthe sequence of regimes of behaviour is similar to thatdescribed earlier upon increasing at pH 10 [Fig.[LaCl3]10(b)].

As with NaCl as electrolyte, the state of the initial aqueouscolloid in the presence of is important in dictating theLaCl3Ðnal stability of the emulsions. Emulsions are very unstable atconditions where no Ñocculation takes place (low pH). At pH10, the marked increase in turbidity of the colloids occursbetween 1 and 2 mM marking the onset of Ñocculation.LaCl3This is exactly where the stability of emulsions begins toincrease markedly. Unlike NaCl however, in the case of LaCl3the appearance of Ñocculated particles initially enhances theemulsion stabilty. Further increase in the concentration of

results in destabilisation. It may be that small Ñocs ifLaCl3formed at low adsorb to interfaces resulting in stabili-[LaCl3]sation, whereas at higher the Ñoc size and extent may[LaCl3]be larger and adsorption is reduced. Our Ðnding that partialÑocculation, i.e. moderately prevalent attraction between thesolid particles, is necessary for emulsion stabilisation agreeswith the conclusions of Lucassen-Reynders and van denTempel6 on the e†ect of added surfactant on the stability ofwater-in-oil emulsions containing glycerol tristearate crystals.Since the c.c.c. decreases with pH, inducing Ñocculation byeither an increase in at Ðxed pH or an increase in pH[LaCl3]at Ðxed has the same e†ect.[LaCl3]The initial emulsion drop size distributions have been mea-sured as a function of at extremes of pH (Fig. 12).[LaCl3]The average volume diameter, d(v, 0.5), of 78 lm is reasonablylarge and is independent of salt concentration at pH 2. Theselarge drops thus cream rapidly at a rate also independent ofconcentration [Fig. 10(a)]. The polydispersity is fairly high,

Fig. 11 Stability to creaming (open points, left hand ordinate) andcoalescence (Ðlled points, right hand ordinate) after 30 min of 20 vol%toluene-in-water emulsions stabilised by 0.5 wt.% Aerosil 200 as afunction of pH. (a) 5 mM (b) 10 mMLaCl3 , LaCl3 .

with the span deÐned as [d(v, 0.9)[ d(v, 0.1)]/d(v, 0.5) equal to0.75. Optical microscopy revealed that the emulsions consistedof individual, spherical dispersed drops. At pH 10 however,the average drop diameters are considerably smaller and passthrough a minimum value of 42 lm between 2 and 3 mM

increasing by a factor two at higher salt concentra-LaCl3 ,tions. The minimum position corresponds closely to that forwhich emulsions are most stable [Fig. 10(b)]. Microscopyindicated evidence of some degree of Ñocculation of emulsion

Fig. 12 E†ect of concentration on the initial volume averageLaCl3diameter of 20 vol% toluene-in-water emulsions stabilised by 0.5 wt.%Aerosil 200 at pH 2 (open points) and 10 (Ðlled points).

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drops also only around 2È4 mM Since the velocity ofLaCl3 .creaming depends on the square of the drop radius,28 the rateof creaming is reduced for these smaller sized emulsions, andpartial Ñocculation does not enhance the rate.

(iii) Systems containing TEAB electrolyte

In contrast to the cases above for non-adsorbing, indi†erentelectrolyte we have chosen to investigate another type of elec-trolyte where one of the ions may adsorb preferentially at theparticle surface. Such ions are potential-determining ionswhen they alter the i.e.p. of a colloid.29 Short chain symmetri-cal quaternary ammonium salts like tetraethylammoniumbromide (TEAB) cause charged silica to Ñocculate whenadded at low concentrations but restabilise the colloid athigher concentrations.14 An argument based on the formationof a bilayer of TEAB in order to explain the restabilisation isunlikely.14 Instead, Rutland and Pashley30 describe a site-binding model to account for the observations. Due to theirhydrophobic character, TEA` ions can adsorb onto neutral,undissociated silanol groups without displacing hydratedprotons in addition to adsorbing directly onto negative, disso-ciated sites in competition with protons. At low concentra-tions of TEAB, “hydrophobic Ï adsorption is greater thandirect adsorption and the surface charge density (p) fallsslightly. As the concentration increases, p remains constantsince adsorbed TEA` ions then begin to displace protons.Eventually the surface becomes covered with TEA` ions andthereafter the e†ect of increasing [TEAB] is equivalent toincreasing the concentration of indi†erent electrolyte i.e. pincreases with concentration leading to restabilisation of thedispersion.

The turbidities of silica dispersions at pH 2, 6 and 10 aregiven in Fig. 13 24 h after addition of TEAB. At low and highpH, they do not change with [TEAB], apart from a decreaseabove 0.3 M due to refractive index reasons. Thus up to1 M TEAB, no Ñocculation is possible. At intermediate pHhowever, the turbidity increases by a factor of 1.5 at B0.003M remaining constant to 0.07 M and then decreasing tovalues at high salt recorded at other pHÏs. From this andvisual observations of phase separation on standing, silicaremains dispersed at low [TEAB], is Ñocculated between

Fig. 13 Turbidity of 0.5 wt.% Aerosil 200 dispersions in water 24 hafter adding TEAB. The pH values are 2 (circles), 6 (squares) and 10(triangles). The tubes are for the colloids at pH 6.

0.003 and 0.6 M and is re-dispersed above 0.7 M. The criticalconcentrations are in good agreement with those of Rubioand Goldfarb.14

The stabilities of o/w emulsions stabilised by silica in thepresence of TEAB are represented in Fig. 14 at these three pHvalues. At pH 2, emulsions were unstable to creaming at all[TEAB] with the resolved aqueous phase resembling the bluecolloid. The cream is stable to coalescence and large (mm) oildrops were visible. The lack of adsorption of TEA` ions ontosilica at this pH is consistent with the increase in the i.e.p. ofsilica to around pH 5 in 0.01 M TEAB rendering the surfacepositively charged at pH 2. At pH 10, the emulsions areextremely unstable with creaming and coalescence completewithin 5 min at all concentrations. Here, adsorption of TEA`occurs on silica as a monolayer and the reason for the insta-bility is possibly due to the van der Waals attraction betweenthe alkyl chains coating the outer surface of the drops, leadingultimately to coalescence. The stability to creaming of emul-sions prepared at pH 6 however is highly dependent on[TEAB], although all were stable to coalescence. At 0.001 M,over 90% of blue aqueous phase is resolved in 30 min. Thevolume of aqueous phase resolved decreases as the concentra-tion increases to 0.005 M, and remains very low (i.e. stableemulsion) up to 0.05 M. The aqueous phase is colourless inthis range indicating that Ñocculated particles are incorpor-ated into the creamy white emulsion. Emulsions were com-pletely stable for extended periods of up to 3 months aroundthis [TEAB]. The stability decreases at higher [TEAB] andabove 0.3 M the resolved aqueous phase is viscous and turbid.By 1.0 M, the aqueous phase is clear blue again. We thusobserve again that the most stable emulsions are formed whenthe particles are slightly Ñocculated, whereas more extensiveÑocculation leads to a decrease in emulsion stability. Thevariation in the average emulsion drop diameter with [TEAB]at pH 6 is shown in Fig. 15. A marked minimum is size is seenat intermediate salt concentrations, which parallels theincrease in stability to creaming, although the curve is shiftedto higher salt concentrations. The optical microscopy imagesin Fig. 16 are more revealing in terms of changes in the struc-ture of these emulsions. At low (0.002 M) and high (0.3 M) saltconcentrations, emulsion drops are dispersed as individualentities. Drop Ñocculation takes place from 0.005 M, with itsdegree increasing to 0.05 M and then decreasing again. Thehighly Ñocculated emulsion is very stable and Ñocs can be seen

Fig. 14 Stability to creaming after 30 min of 20 vol% toluene-in-water emulsions stabilised by 0.5 wt.% Aerosil 200 as a function ofTEAB concentration. The pH values are 2 (circles), 6 (squares) and 10(triangles). The dotted lines mark the onset and limit of the Ñocculatedregion for the aqueous colloids.

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Fig. 15 Initial volume average diameter of 20 vol% toluene-in-wateremulsion drops stabilised by 0.5 wt.% Aerosil 200 as a function ofTEAB concentration at pH 6.

separated by areas of drop-free continuous phase, resemblingthe situation seen for surfactant-stabilised emulsions depletionÑocculated by non-adsorbing micelles.31

ConclusionsThe following conclusions can be drawn concerning theproperties of toluene-in-water emulsions stabilised by silicaparticles alone as studied here :

(1) Aerosil 200 colloids exhibit anomalous DLVO behav-iour. In the presence of NaCl, emulsions are unstable tocreaming at all salt concentrations (0È5 M) and at pH valuesbetween 2 and 10. The stability to coalescence passes througha maximum around pH 4 where the particles have a low,negative zeta protential. Flocculation of the aqueous colloidat higher pH results in destabilisation of the emulsions.

(2) In the presence of emulsions are unstable at con-LaCl3 ,ditions where the silica is not Ñocculated (low pH). At highpH, emulsion stability passes through a pronouncedmaximum between 2 and 5 mM salt. The onset of the increasein stability coincides with the onset of Ñocculation of thecolloid. Such emulsions remain stable to both creaming andcoalescence for over 3 months, even though relatively large (40

Fig. 16 Optical micrographs of 20 vol% toluene-in-water emulsions stabilised by 0.5 wt.% Aerosil 200 in the presence of TEAB (of molaritygiven) at pH 6. The bar of 200 lm is the same for all the images.

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lm diameter) drops are formed. Extensive Ñocculation athigher results in destabilisation of the emulsions.[LaCl3](3) In the presence of TEAB, emulsions are relativelyunstable at both high and low pH corresponding to condi-tions where the silica particles are dispersed. At intermediatepH, emulsions are stable to coalescence but exhibit maximumstability to creaming around salt concentrations correspond-ing to the onset of colloid Ñocculation. Emulsion drops them-selves are highly Ñocculated too. Further Ñocculation of thecolloid de-Ñocculates the emulsion leading to destabilisation.

Acknowledgementswould like to thank the EPSRC and ICI Paints (Slough)We

for a CASE award to fund SOL.

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