Nanoparticle Induced ME Phase Behavior

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Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 8–15

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o l s u r f a

Influence of nanoparticle addition to Winsor surfactant microemulsion systems

B.P. Binks ∗, P.D.I. Fletcher, L. Tian

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

a r t i c l e i n f o

Article history:

Received 18 February 2010

Received in revised form 25 March 2010

Accepted 26 March 2010

Available online 3 April 2010

Keywords:

Nanoparticle

Surfactant

Microemulsion

Winsor

Emulsion

a b s t r a c t

The influence of addingnegatively charged silicananoparticles to multiphase Winsormicroemulsion sys-

tems of cationic surfactant/alcohol cosurfactant is reported. It is found that the particles do not change

the salt-induced progression of Winsor systems to any great extent, even when added at the same con-

centration as the surfactant. We find that all of the particles transfer from water where they originate tooil at all salt concentrations, although the distribution of surfactant between phases is unaffected. It is

ascertained that alcohol addition renders particles more hydrophobic promoting this transfer. Emulsions

prepared from the equilibrium microemulsionand excess phase(s) invert from oil-in-water to water-in-

oilwith increasing salt concentration, such thatthe continuousphaseis theone containingthe surfactant

aggregates. Their stability to coalescence is extremely low, due to mainly the ultralow tensions at the

oil–water interface. Particle addition does not alter the emulsion stability, implying that they are not

adsorbed to drop interfaces.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Surfactant molecules self-assemble in water into a large vari-

ety of morphologies, including micelles (or microemulsions if oil

is solubilised within) and liquid crystalline phases such as lamel-

lar or hexagonal phases. In a lamellar phase, surfactant molecules

form bilayers that are regularly stacked, while in a hexagonal

phase cylinders are organised on a triangular lattice. In the lit-

erature, solid particles in the nm–m range have been added at

low concentrations to these phases in order to obtain, for exam-

ple, micro-rheological information. For a hexagonal liquid crystal

made of anionic surfactant/alcohol cosurfactant, dos Santos et al.

[1] showed that gelling the continuous aqueous phase by incorpo-

ration of inorganic particles did not disrupt the hexagonal order of

the system. Using an amphiphilic nonionic copolymer to prepare

a hexagonal phase, the influence of added disk clay nanoparticles

on its structure and rheology has been studied [2]. Although the

shear modulus was found to be independent of the amount of par-

ticles, particles caused a lowering of the transition temperaturefrom a hexagonal phase at low temperature to a lamellar phase

at high temperature. The disc particles are intercalated between

the lamellae due to their entropically favoured packing.

Studies detailing the influence of particle addition to lamellar

phases are more numerous. They range from those with planar

lamellae [3–10] to curved multilamellar vesicles (onions) [11,12]

and from anionic [3–5,7,12,13] to zwitterionic [9] to nonionic sur-

∗ Corresponding author.

E-mail address: [email protected] (B.P. Binks).

factant [6,8,10]. Various effects are observed. These include an

increasein the bendingconstant of thesheets whenmagnetic parti-

clesare incorporated [4] a transition to prolate micelles [8] a change

in the temperature at which the lamellar phase phase separates [6]

and different locations of the particles from the centre to the edges

of onion phases depending on shear history [12]. Interestingly, sil-

ver particles may be directed to the water layers if hydrophilic

or the oil-swollen layers if hydrophobic in oil-containing lamellar

phases [5].

Regarding microemulsions which are thermodynamically sta-

ble mixtures of oil and water, very few studies exist describing

the effect of added particles. However, droplet microemulsions,

particularly those of water-in-oil (w/o), have been used for some

time as templates for the preparation of nanoparticles of metals

and single or mixed metal oxides with a broad range of appli-

cation in catalysis; see Ref. [13] f or a recent review. Of the two

reports on the effect of adding nanoparticles to microemulsions,

Kline and Kaler [14] investigated oil-in-water, o/w, microemul-

sions stabilised by the anionic surfactant AOT in the absence andpresence of negatively charged silica particles of diameter 22 nm.

No observable change in the stability or extent of the one-phase

microemulsion region was evidenced. It is likely that no adsorp-

tion of surfactant to particle surfaces occurs since they are of the

same sign and particles remain dispersed in the continuous aque-

ous phase. By contrast, Puech et al. [15] have recently shown that

addition of negatively charged silica particles of diameter 20nm to

o/w microemulsion networks of nonionicsurfactant/alcohol cosur-

factant drops bridged by polyethylene oxide increases the shear

modulus of the mixture markedly, even though it is verified that

the particles remain dispersed in water. It is suggested that there

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B.P. Binks et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 8–15 9

Fig. 1. Photograph of vessels containing Winsor microemulsion systems in the

absence of particles for (upper) S0.5P0 (centre) S1P0 and (lower) S2P0.

Fig. 2. Volume fraction of thirdphase versus salt concentration for systems in Fig.1.

is a re-organisation of the network around the particles, leading

to an increase in the number of rheologically active bridges. The

possibility of surfactant/polymer adsorption on particles cannot be

ruled out however. In this study, we have investigated what hap-

pens when anionic silica nanoparticles are added to multiphase

microemulsion systems of cationic surfactant/alcohol cosurfac-

tant, for which surfactant adsorption on particles may occur. The

changes in the partitioning of surfactant and particles between oil

and water are determined. Since both surfactant and particles can

potentially act as efficient emulsifiers of oil and water, we have

also measured the type and stability of emulsions prepared from

equilibrium systems.

2. Experimental

2.1. Materials

Water was first passed through an Elgastat Prima reverse osmo-

sis unit and then a Millipore Milli-Q reagent water system. It had a

resistivity of >18 M cm and a pH 5.6. The surfactant used to pre-

pare microemulsions was dodecyltrimethylammonium bromide,

Fig. 3. (a) Equilibriumdistribution of surfactantbetween the phases in Winsor sys-

temsof S2P0(no particles). (b)Variationof theequilibrium surfactantconcentration

in the aqueous phase with salt concentration for Winsor III and II systems of S2P0.



10 B.P. Binks et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 8–15

Table 1

Composition of microemulsion systems investigated.

System Toluene/wt.% Aq. NaBr/wt.% Butan-1-ol/wt.% DTAB/wt.% Ludox HS-30/wt.%

S0.5P0 47 48.5 4 0.5 0

S1P0 47 48.0 4 1.0 0

S2P0 47 47.0 4 2.0 0

S0.5P1.5 47 47.0 4 0.5 1.5

S1P1 47 47.0 4 1.0 1.0

S1P0.5 47 46.5 4 1.0 0.5

S1P2 47 46.0 4 1.0 2.0

DTAB, purchased from Sigma of purity 99%. The titrant used in

the quantitative analysis of DTAB was sodium dodecyl sulphate,

SDS (Sigma, >99%). The oil used in microemulsions was toluene

(Fisher, >99%) which wascolumnedthrough neutral alumina before

use. Butan-1-ol cosurfactant was from Lancaster of purity 99% and

the electrolyte was sodium bromide (Fluka, >99.5%). The reagents

required for the analysis of surfactant were AnalaR grade from

various sources, including dimidium bromide, disulphine blue,

hydrochloricacid andchloroform. The colloidal silica particleswere

thoseof LudoxHS-30from Aldrich,bought as an aqueous dispersion

at 30wt.% and pH 9.8. Theparticles arespherical and monodisperse

of diameter = 16nm.

Fig. 4. (a) Photograph of vessels containing Winsor microemulsion systems in the

presence of silica nanoparticles for (upper) S1P1 and (lower) S0.5P1.5. (b) Volume

fractionof thirdphase versus saltconcentrationfor S2P0,S1P1 andS0.5P1.5systems.

2.2. Methods

2.2.1. Preparation of equilibrium microemulsion systems

The system chosen (without particles) was investigated over 20

years ago andconsists of a cationicsurfactant alongwith an alcohol

cosurfactantin toluene–watermixtures [16]. Forequal masses ofoil

and aqueous electrolyte, addition of salt causes a progression from

Fig. 5. Equilibrium distribution of (a) surfactant and (b) particles between the

phases in Winsor systems of S1P1 containing silica nanoparticles.




Fig. 6. Photograph of vessels containing Winsor microemulsion systems in the presence of silica nanoparticles for (upper) S1P0.5 and (lower) S1P2.

a two-phase system (Winsor I) consisting of an o/wmicroemulsion

plus excess oil to a three-phase system (Winsor III) consisting of a

middle phase microemulsion plus excess water and oil phases to

a two-phase system (Winsor II) consisting of a w/o microemulsion

plus excess water. The curvature of the oil–water interface passesfrom positive through net planar to negative respectively, and the

effectof salt is to change the ratio of the headgroup to chain area in

the mixed monolayer. Experimentally, aqueous DTAB solutions of

differentNaBrconcentrations at pH 5.6werefirst mixed with aque-

ous silica dispersions of different salt concentrations also at pH 5.6

(using HCl), after which butanol and toluene were subsequently

added. The mixtures, of total volume between 19 and20 cm3, were

hand shaken in screw cap glass vessels. They were left to separate

into two or three phases in a thermostat bath at 20 ◦C for up to 1

week.The relative volumes of thecoexisting phases weremeasured

from the graduations. Photographs of the vessels were taken with

a Samsung NV3 digital camera. Various series of Winsor systems

were investigated, in which the surfactant and particle concen-

trations were varied. The composition of the different systems isgiven in Table 1, where the abbreviation SxPy refers to a mixture

containing x wt.% of DTAB and y wt.% of Ludox HS-30.

2.2.2. Analysis for surfactant and particles in microemulsion

systems

For equilibrated phases, the surfactant concentration was deter-

mined using the two-phase Epton titration with SDS as titrant

[17]. Typically, 5×10−2 M SDS was used for concentrated phases,

whereas 5×10−4 M was used for dilute phases. The volume of

phase sampled varied from 50L to 1c m3 depending on the

expected surfactant concentration. In order to determine the con-

centration of particles in the different phases, 2 g of a particular

phase was dried in an oven at 60 ◦C for 24 h. The sample was then

washed twice with anhydrous methanol to remove bothsurfactant

and salt leaving only theparticles. Thesample was re-dried at 60◦C

for a further 24 h and its mass recorded.

2.2.3. Emulsification of microemulsion systems

It is normally found that the macroemulsion type (o/w or w/o)of emulsions prepared from equilibrium microemulsion systems is

the same as the precursor microemulsion [18]. Thus, emulsifying

the two phases of a Winsor I system would produce an o/w emul-

sion. This is a manifestation of Bancroft’s rule in that thecontinuous

phase of the emulsion is the one containing aggregated surfactant

(microemulsion in this case). Both surfactant and suitably wettable

particles can act as stabilisers of emulsions, although the ability

of the latter emulsifier is substantially reduced if the oil–water

interfacial tension is ultralow (<1 mN m−1) as here [19]. Emulsions

were prepared from equilibrium microemulsion systems using a

Janke and Kunkel T25 ultra turrax rotor-stator homogeniser with a

0.8cm head operatingat11,000 rpmfor2 minat room temperature.

The conductivity of emulsions was measured during emulsifica-

tion using a Jenway4310 digital conductivity meter equipped withPt/Pt black electrodes. Their stability was assessed by monitoring

the positions of the water–emulsion and emulsion–oil interfaces

with time.

3. Results and discussion

3.1. Winsor microemulsion systems in the absence of particles

Since the surfactant is ionic, its behaviour at an oil–water inter-

face is sensitive to salt addition since added electrolyte will screen

the repulsion between adjacent headgroups thus reducing their

effective size which in turn affects the curvature of the interface

they areadsorbedto. At lowsalt concentration, the headgrouparea

exceeds that of the chain and molecules pack such that they are on




the exterior of the aggregate, i.e . an o/w microemulsion forms in

equilibrium with excess oil not solubilised in their interior. At high

salt concentration, the chain area (possibly swollen by penetrated

oil)exceeds thatof the headgroup andthe molecules assemble with

headgroups on the interior, i.e. a w/o microemulsion exists with

excess water. At intermediate salt concentration, the surfactant

has no preference for water or oil but instead forms an aggregated

bicontinuous or sponge phase between excess water and oil. Here

the net curvature of the oil–water interface is zero since surfac-

tant headgroup and chain areas are approximately equal [16]. In

the absence of particles, the sequence Winsor I–Winsor III–Winsor

II is affected by the concentration of surfactant, at fixed concen-

tration of cosurfactant. In Fig. 1 we show the appearance of the

vessels at differentsalt concentrations 1 weekafter mixing for three

concentrations of surfactant, S0.5P0, S1P0 and S2P0. It can be seen

that for Winsor I and Winsor II systems, the volume fraction of the

microemulsion phase increases on approaching the Winsor III sys-

tems, at low salt for the former and high salt for the latter as oil or

water is solubilised in spherical aggregates respectively. In Winsor

III systems, the middle microemulsion phase, containing approx-

imately equal volumes of water and oil, contains the majority of

surfactant andits volume fractionthus increasesprogressivelywith

initial surfactant concentration. From the plot in Fig. 2, it is also

seen that both the extent of the three-phase regions and the saltconcentrations at which they occur increase with surfactant con-

centration.

The partitioning of surfactant between the different phases has

been determined by quantitative analysis of DTAB at equilibrium in

the S2P0 system. Fig. 3(a) displays the results. In Winsor I systems,

the surfactant is located exclusively in the aqueous phase both as

monomer and stabilising o/w microemulsion droplets. In Winsor

III systems, no surfactant transfers to oil. Instead, the majority is

located in the middle microemulsion phase in equilibrium with a

low concentration, equivalent to the critical micelle concentration

(cmc), inthe aqueous phase. InWinsor II systems, nearlyall the sur-

factant has transferred to the oil phase, leaving the aqueous phase

around its cmc also. The reduction of the aqueous phase surfactant

concentration with salt concentration in Winsor III and II systemsis shown in Fig. 3(b), which is close to the variation of the cmc in

the absence of oil dueto thescreening of the charge on headgroups

promoting micellisation at lower concentrations. The cmc of DTAB

in pure water at 20 ◦C is 1.59×10−2 M [20].

3.2. Winsor microemulsion systems in the presence of

nanoparticles

Since the behaviour of the microemulsion systems with sur-

factant alone has been established, it is of interest to investigate

the effect of adding silica nanoparticles of opposite charge to

that of the surfactant. Potentially, inherently hydrophilic particles

could become more hydrophobic via surfactant adsorption as a

monolayer and transfer from water to oil as a result. Do suchparti-cles interfere with microemulsion formation? We prepared 4 new

systems containing surfactant plus particles, keeping the concen-

tration of cosurfactant constant at 4 wt.%. Two of these, S1P1 and

S0.5P1.5, can be compared to S2P0 in which the particle concen-

tration is increased as the surfactant concentration is decreased at

fixed overall amount. The other two systems, S1P0.5 and S1P2, can

be combined with S1P1 to investigate the influence of increasing

the particle concentration at fixed surfactant concentration.

3.2.1. Effect of varying the ratio of surfactant to particles (fixed

total)

The appearance of themultiphase systems for S1P1and S0.5P1.5

at various salt concentrations can be seen in Fig. 4(a) and com-

paredto thatwithout particles, S2P0, in Fig.1 (lower). As before, salt

Fig. 7. Equilibrium distribution of (a) surfactant and (b) particles between the

phases in Winsor systems of S1P2.

induces the progression from two phases to three phases back to

two phases. Increasing the particle concentration from0 to 1.5 wt.%

hardly affects the [NaBr] at the Winsor I/III border but decreases

that at theWinsor III/II border, i.e. the widthof the WinsorIII region

decreases.Thisisakintothedecreaseinwidthobservedondecreas-

ing the surfactant concentration in particle-free systems (Fig. 2). It

can also be seen that although the oil phase in Winsor III systemsis colourless in the absence of particles, it is bluish in their pres-

ence. This will be discussed later but is indicative of the presence

of particles. The volume fraction of the middle phase decreases by

approximately a factor of two for each decrease in the surfactant

content, Fig. 4(b). It thus appears that silica particles do not modify

the Winsor progression to any great extent.

We have verified that the presence of nanoparticles does

not change the partitioning of surfactant between the coexisting

phases. An example of the results is given in Fig. 5(a) for the S1P1

system. As before, aggregated surfactant transfers from water to

oil via a middle phase upon increasing the salt concentration. The

aqueous phase concentration in Winsor III and II systems falls pro-

gressively also, as the cmc is lowered by added salt. Significantly,

at two [NaBr] where S2P0 and S1P1 exhibit three phases (2 and




Fig. 8. Photograph of vessels containing emulsions after homogenisation of Winsor system S2P0 after (a) 15 s and (b) complete breakdown.

3 wt.%), the concentration of DTAB in the excess aqueous phase is

ca. 5 times lower in the particle-containing system compared with

the particle-free system. In addition, for the same salt concentra-

tions in Winsor II systems, the difference is ca. a factor 2 in the

same direction. The reason is probably due to the loss of surfactant

to particle surfaces via adsorption.Using an extraction method to analyse for particles, we have

determined their partitioning between phases also. The results

are presented in Fig. 5(b) for the S1P1 system. Despite the parti-

cles originally being dispersed in the aqueous phase, within the

error, virtually all of them added transfer to the oil phase for all

three Winsor systems. Thus, a Winsor I system comprises an o/w

microemulsion coexisting with an oil dispersion of particles. A

Winsor II system consists of an aqueous phase of monomeric sur-

factant coexisting with an oil phase containing particles and w/o

microemulsion droplets. The presence of (presumably) hydropho-

bic nanoparticles in oil causes it to scatter light and take on a blue

hue. A Winsor III system comprises an aqueous phase containing

monomeric surfactant, a third phase containing the majority of the

surfactant and an oil phase containing dispersed particles.The transfer of silica particles to oil even at low salt concen-

tration is intriguing since it has been shown that no transfer takes

place for single chain cationic surfactants in the absence of both

salt and cosurfactant [21,22]. We thus prepared three more mix-

tures inorderto elucidatethe reasonfor the transfer.Thesesamples

all contained 1wt.% DTAB and 1 wt.% Ludox HS-30 (like S1P1); one

containedno NaBrbut 4 wt.%butan-1-ol,anotherno butan-1-ol but

0.25 wt.% NaBr and the third had no salt or alcohol. In the absence

of salt only (i.e. in pure water), all the silica particles were found

to have transferred to toluene. In the absence of butan-1-ol alone

or without both butan-1-ol and NaBr, the particles remained in the

aqueous phase. We thus conclude that butan-1-ol is responsible

for promoting the transfer of particles from water to oil. In related

work, it was found that addition of the short chain alcohol ethanol

to water also caused the transfer of gold nanoparticles from water

to the heptane–water interface [23]. The authors showed that par-

ticles become less charged (decreased charge density) and more

hydrophobic encouraging their loss from water, due to the compet-

itive adsorption of ethanol molecules displacing citrate anions from

their surfaces. It is likely that a similar mechanism operates in ourcase, augmented by the simultaneous adsorption of a monolayer of

surfactant.

3.2.2. Effect of increasing particle concentration (fixed surfactant)

At a fixed concentration of surfactant (1 wt.%) and cosurfactant

(4 wt.%), we also investigated the effect of increasing the particle

concentration. The Winsor progression can be seen in Fig. 6 f or the

S1P0.5and S1P2 systems, whichforma serieswith S1P1 in Fig.4(a).

Winsor IIIsystems appear at the same salt concentrations through-

out and the volume fraction of the middle phase is unaffected by

particle concentration. The distribution of surfactant between the

phases in S1P2 shown in Fig. 7(a) displays no difference with that

for S1P1 discussed earlier. In addition, all the particles partition to

the oilphaseat allsalt concentrations, Fig.7(b). It thus appears that,since particles transfer to the oil phase, they do not influence the

Winsor series to any great extent.

3.3. Emulsions of Winsor microemulsion systems

If the coexisting phases of a Winsor surfactant system are

homogenised, unstable macroemulsions form which ultimately

revert with time to the equilibrium multiphase systems. The

emulsion type, o/w or w/o, is nearly always the same as the

microemulsion type [18] although exceptions do exist at low sur-

factant concentration [24]. The continuous phase of the emulsion

is the one containing the surfactant aggregates (microemulsion

droplets). For particle emulsifiers, it is common that hydrophilic

particles originating in water prefer to stabilise o/w emulsions,




Fig. 9. Conductivity of emulsions as a function of salt concentration in Winsor sys-

tem S2P0.

whereas hydrophobic particles dispersed in oil stabilise w/o emul-

sions. The stability of these emulsions to coalescence has been

shown to be related to the energy of attachment of particles

to the oil–water interface [25] itself a function of the particle

contact angle, particle size and interfacial tension, with more sta-

ble emulsions being those for which particles are most strongly

adsorbed. For systems exhibiting ultralow interfacial tensions

(<10−2 mN m−1) like those here [26] it is expected that emul-

sionscontainingadsorbed nanoparticleswill be veryunstable, since

adsorption energies are only several kT . The interfacial tension

passes through a minimum value withinthe Winsor III region [26].We havedetermined theemulsiontype andensuingstability in two

Winsor series—S2P0 (no particles) and S1P1 (same overall amount

of surfactant plus particles).

Emulsification of the S2P0 system produced the emulsions

shown in Fig. 8, immediately after homogenisation (a) and when

all the emulsion phases have coalesced (b). It can be seen that in

addition to theformation of turbidemulsions,some foam is formed

during aeration. The systems at low salt concentration in the Win-

sor III range are particularly unstable, separating oil and water

immediately. With the exception of the emulsion at the lowest

[NaBr], all emulsions are very unstable to coalescence, leading to

complete phase separation in under 3 min. That at 0.25wt.% NaBr

took 130 min to separate. The type of emulsion can be assessed

by measuring its conductivity, since o/w emulsions will conductwhereas w/o will not. Fig. 9 shows how the emulsion conduc-

tivity varies with salt concentration in S2P0 systems. Winsor I

systems giveo/w emulsionsof high conductivity, whose magnitude

increases with salt concentration. On entering the Winsor IIIrange,

the conductivity remains high and then falls by over two orders of

magnitude. It is possible that multiple emulsions form temporarily

from three-phase systems, probably inverting from w/o/wto o/w/o

in this region. The conductivity continues to fall to very low values

comparable to that of pure oil in the Winsor II region, indicative

of w/o emulsion formation. Thus the prediction of emulsion type

from a knowledge of the microemulsion type is borne out.

In the presence of particles (S1P1), the appearance of the emul-

sions immediately after formation and after complete breakdown

is shown in Fig. 10. As before, emulsions are most unstable in the

Fig.10. Photograph ofvesselscontainingemulsions afterhomogenisationof Winsor

system S1P1 after (a) 15s and (b) complete breakdown.

Fig. 11. Conductivity of emulsions as a function of salt concentration in Winsor

system S1P1.

Winsor III region, coalescing completely within 2 min. All except

that at 0.25 wt.% NaBr were also phase separated in around 3 min.

The presence of the particles neither enhances nor diminishes the

emulsion stability, whichprobablyimplies that they do not remain

at droplet interfaces. The corresponding conductivity variation is

given in Fig. 11 which again passes through a maximum in the

Winsor III region. For this series, it is particularly interesting which

emulsifier,surfactantor particle,dictates the emulsiontype. For the

three emulsions prepared from Winsor I systems, all the surfactant

is dissolved in water and all the particles are dispersed in oil prior

to emulsification (Fig. 5). The preferred emulsion is o/w, implying

that surfactant molecules dominate the coverage of droplet inter-

faces. For Winsor II systems, the majority of surfactant and all of




the particles originate in the oil phase and emulsions are w/o as

expected from the arguments earlier.

4. Conclusions

Charged silica nanoparticles originating in water have little

influence on the salt-induced progression of Winsor systems in

cationic surfactant–alcohol cosurfactant mixtures. It is found that

all the particles transfer to oil at all salt concentrations, an effect

due entirely to the hydrophobising effect of adsorbed butan-1-ol.

Emulsions prepared from the equilibrium microemulsion phases

phase invertfrom o/wto w/oon progressing from Winsor I to Win-

sorII systems. Theparticles,residingin oilbefore emulsification,do

not enhance or diminish the emulsion coalescence stability, which

is very lowdue to the ultralow interfacialtensions. Particles arenot

wellheld at dropinterfaces in these conditions andact as spectators

to the underlying surfactant/cosurfactant behaviour.

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