Fd

QK

a

ARRAA

KAFAL

1

cpaths

itptltsotaao

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 355 (2010) 151–157

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

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

oams stabilized by Laponite nanoparticles and alkylammonium bromides withifferent alkyl chain lengths

ian Liu, Shuiyan Zhang, Dejun Sun ∗, Jian Xuey Laboratory for Colloid & Interface Chemistry of Education Ministry, Shandong University, Shan Da South Road 27#, Jinan, Shandong 250100, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 5 August 2009eceived in revised form 8 November 2009ccepted 2 December 2009vailable online 5 December 2009

eywords:

a b s t r a c t

We investigated the behavior of foams stabilized by Laponite nanoparticles combined with alkylam-monium bromides with different alkyl chain lengths. A four-region model based on electrostatic andhydrophobic interactions adequately explains the adsorption of the cationic surfactants on the negativelycharged Laponite particles. The results indicate that chain length has a minimal influence on surfac-tant adsorption via cation exchange, but a longer alkyl chain length can induce a stronger hydrophobicinteraction among the adsorbed alkylammonium molecules and hence a higher surfactant adsorption.

dsorption isothermoam stabilitylkyltrimethylammonium bromidesaponite

Adsorption of surfactants on the Laponite particles is crucial to foam stability. Surfactant addition ini-tially transforms particles from anionic to uncharged and hydrophobic and subsequently to cationic asa result of adsorption. The foam experiments indicate that the most hydrophobic particles, possessingan adsorbed monolayer of surfactant, yield foams which are completely stable to disproportionation andcoalescence. As the surfactant chain lengths increase from C12 to C16, the characteristic features of theadsorption isotherm are lowered by approximately an order of magnitude. For surfactants with longeralkyl chain surfactants, stable foams are obtained at lower concentrations.

. Introduction

Recently, there have been a number of reports on the use ofolloidal particles as sole stabilizers of aqueous foams. Aqueous sus-ensions of certain solid particles with inherent hydrophobicity areble to make extremely stable foams in the absence of any surfac-ant [1–10]. Particle shape, size, concentration and hydrophobicityave been identified as the main factors responsible for the foamtabilization [11–14].

Since many particle types are inherently hydrophilic, attach-ng weakly to fluid interfaces, one of the ways to increaseheir hydrophobicity is to hydrophobise the particles in situ byhysisorption. Wilson [15] investigated foams prepared by addi-ion of cationic surfactants to aqueous dispersions of polystyreneatex. They noticed that stable bubbles can be formed only whenhe latex was close to coagulating in the bulk. Wasan [16] pre-ented experimental observations on polyhedral foams containingver 90% air produced by irregularly shaped fine crystalline par-

icles of modified sodium chloride. The NaCl crystals were mademphiphilic by physical adsorption of a cationic surfactant. Studartnd co-workers [17–19] accomplished in situ hydrophobizationf metal oxide particles through the adsorption of short-chain

∗ Corresponding author. Tel.: +86 531 88364749; fax: +86 531 88564750.E-mail address: djsun@sdu.edu.cn (D. Sun).

927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.12.003

© 2009 Elsevier B.V. All rights reserved.

amphiphiles. High-volume and stable foams were prepared bythose particles. Sun et al. [20–22] have studied aqueous foamsprepared by cetyltrimethylammonium bromide (CTAB), tetraethy-lene glycol monododecyl ether (C12E4), hexylamine and disk-likeLaponite particle dispersions. Binks et al. [23] have investigated thebehavior of air-in-water foams stabilized by a mixture of LudoxHS-30 silica nanoparticles and di-C10DMAB cationic surfactantat high pH. They concluded that the foam stabilization changesfrom being surfactant-dominated at low surfactant concentrationto being particle-dominated at intermediate concentrations andreverting to surfactant-dominated at higher concentrations.

Addition of surfactants to particle dispersions does not alwayspromote foam stability. Alargova et al. [24] found that the sta-bility of foams prepared by mixtures of hydrophobic rod-shapedparticles and sodium dodecyl sulfate (SDS) was decreased com-pared with that by the particles alone, meaning that SDS acted asa defoamer. It is possible that the adsorption of SDS onto the par-ticles rendered them more hydrophilic and then the particles losttheir affinity for the solution/air interface. Fujii et al. [4] found thatfoams could not be formed using poly-N-vinylpyrrolidone (PNVP)-stabilized polystyrene particles if they contained excess PNVP.

Weak, short-lived foams were formed but collapsed within 10 safter two centrifugation/redispersion cycles to remove the non-adsorbed PNVP. After the third centrifugation/redispersion cycle,highly stable foams that remained stable for more than 1 yearwere obtained, but this behavior was not explained. Subramaniam

1 ysicochem. Eng. Aspects 355 (2010) 151–157

ebpos

alitmas

2

2

LApw3sotaN0itnhhtm(f>

2

kM

Fp

observed after 24 h.To separate the Laponite particles from supernatant, the pre-

pared dispersion was centrifuged for 60 min at 20,000 rpm in order.Alkylammonium ions in the supernatant are analyzed for total

52 Q. Liu et al. / Colloids and Surfaces A: Ph

t al. [25] reported the destabilization of particle-stabilized bub-les exposed to various concentrations of surfactant solutions. Theyroposed a microstructural mechanism, which recognized the rolef interfacial jamming and stresses in particle stabilization andurfactant-mediated destabilization of armored bubbles.

In this paper, we use a platelet Laponite particle and threelkyltrimethylammonium bromides with different alkyl chainengths. The effects of adding alkyltrimethylammonium bromidesnto Laponite dispersions on the particle zeta potential, adsorp-ion behavior and foam stability were studied. The foams were

ost stable when the particles were most flocculated, as a result ofdsorption of a surfactant monolayer adsorption onto the particleurfaces.

. Materials and methods

.1. Materials

The water was deionized water purified by ion exchange.aponite RD, a synthetic hectorite, was supplied by Rockwooddditives Ltd. (UK) as a white powder and used without furtherurification. It is composed of rigid disk-shaped crystals with aell defined thickness of 1 nm and an average diameter about

0 nm. The idealised structure would have a neutral charge withix magnesium ions in the octahedral layer, giving a positive chargef 12. In practice, however, some magnesium ions are substi-uted by lithium ions and some spaces are empty to give typically

composition which has the empirical formula of Laponite isa0.7+[(Si8Mg5.5Li0.4)O4(OH)20]0.7−. This has a charge deficiency of.7 per unit cell. The cation exchange capacity (CEC) of Laponite RD

s about 0.75 mequiv./g. When the powders are dispersed in water,he Na+ ions on the particle surface are released and a stronglyegative charge appears on the faces of the disks. On the otherand, because of the protonation of the hydroxyl groups with theydrogen atoms of water, a weakly positive charge appears onhe rim of the disks. The surfactants used were cetyltrimethylam-

onium bromide (CTAB), tetradecyltrimethylammonium bromideTTAB), dodecyltrimethylammonium bromide (DTAB), purchasedrom Sinopharm Chemical Reagent Co., Ltd. (China) with a purity99%.

.2. Preparation and characterization of dispersed phases

The Laponite stock dispersion was prepared by dispersing anown mass of Laponite powders into the deionized water using aultimixer (Baroid Co., USA). The Laponite dispersions were sealed

ig. 1. The adsorption isotherms of the three alkylammonium bromides on Laponitearticles at 25 ◦C.

Fig. 2. The zeta potentials for the Laponite particles (1 wt.%) as a function ofalkyltrimethylammonium bromide concentrations.

and laid aside for 1 week before use. The Laponite/alkylammoniumbromide dispersions were prepared by diluting the stock Laponitedispersions with alkylammonium bromide solutions. The prepareddispersions were further stirred for at least 12 h to attain adsorptionequilibrium. After 12 h, the dispersions were first transferred intoa stoppered, graduated glass tube with internal diameter 1.6 cmand length 15 cm. Then the phase behavior of the dispersions was

Fig. 3. Photographs of the aqueous mixtures of Laponite particles (1 wt.%) mixedwith alkyltrimethylammonium bromides (mM) after 24 h. The concentrations ofthe surfactants are shown in the figure.

Q. Liu et al. / Colloids and Surfaces A: Physicoc

Ft

oScc

piCau

2

pFFstwmsh

Fac

with their headgroups down. In region II adsorbed alkylammoniumions associate into patches or hemimicelles on the surface throughchain–chain interactions and an adsorbed monolayer is formed. Inthis concentration region a sharp increase of adsorption is observed,showing that increasing surface coverage enhances the affinity of

ig. 4. Half-lives of foams stabilized with alkyltrimethylammonium bromide solu-ions as a function of their concentrations.

rganic carbon (TOC) with TOC analyzer (Toc-VCPN FA, CN200,himadzu Corporation). The amount of surfactant adsorbed wasalculated from the difference between initial and equilibrium con-entrations of surfactant and divided by the mass of the dried solid.

The zeta potentials of Laponite/alkylammonium bromide dis-ersions were measured with a JS94H microelectrophoresis

nstrument (Shanghai Zhongchen Digital Technic Apparatus Co.,hina). The equilibrated dispersions were first sonicated for 20 minnd then diluted with deionized water to make the particles visiblender the microscope. After that, the zeta potential was measured.

.3. Preparation, stability, and characterization of foams

Foaming of 50 ml Laponite/alkylammonium bromide dis-ersions was carried out using a lab homogenizer (Shanghaiorerunner M&E Co., China) operated at 8000 rpm for 5 min.oams prepared by Laponite/alkylammonium bromide dispersionshowed a four- to fivefold increase in volume at optimum concen-rations of alkylammonium bromide. After homogenization, foams

ere immediately transferred into the measuring cylinder. Theorphology of bubbles was observed by an Axioskop 40 micro-

cope (ZEISS, Germany). Half-life which means the time it takes foralf of the liquid to drain out is used to evaluate the foam stability.

ig. 5. Half-lives of foams stabilized with Laponite (1 wt.%)/alkyltrimethyl-mmonium bromide mixtures as a function of alkyltrimethylammonium bromideoncentration.

hem. Eng. Aspects 355 (2010) 151–157 153

3. Results and discussion

3.1. Adsorption isotherm measurement

The adsorption isotherms of the three alkylammonium bro-mides on Laponite particles are given in Fig. 1. All the isothermshave similar shapes. The adsorption isotherms can be approxi-mately divided into four regions [26]. In region I the adsorbedamount increases only slowly with increasing surfactant concen-tration. In this region, alkylammonium ions adsorb individually

Fig. 6. Appearance of dried foams prepared with Laponite/alkyltrimethyl-ammonium bromide mixtures. The Laponite particles concentration is 1.0 wt.%. Thealkyltrimethylammonium bromide concentration in the mixtures is indicated above(mM).

1 ysicoc

tthp(hptosctmcos

emaeht

Fi

54 Q. Liu et al. / Colloids and Surfaces A: Ph

he surface. In region III, a second layer begins to adsorb on the ini-ial adsorbed monolayer through hydrophobic interactions of theydrocarbon chains. The adsorption in these two regions is accom-anied by a sharp change in the zeta potential and charge reversalFig. 2) because the surfactants are oriented with their chargedeadgroups toward the solid surface, while the hydrocarbon chainsrotrude into the aqueous phase forming hydrophobic patches onhe surface. Further adsorption results in an increasing numberf surfactant aggregates, with molecules adsorbing in an oppo-ite orientation once the surface is neutralized by the oppositelyharged surfactant. Region IV is located in the vicinity of the cmc ofhe surfactant. Above the cmc, the concentration of the surfactant

onomer remains nearly constant and hence adsorption remainsonstant in this region, indicating the absence of micelle adsorptionn the Laponite particle surface. The adsorbed layer possesses thetructure of a bilayer.

The shape of a surfactant adsorption isotherm depends to a greatxtent on its critical hemimicelle concentration (hmc) and critical

icelle concentration (cmc) which mark the onsets of regions II

nd IV, respectively. An increase in alkyl chain length is consid-red to decrease the Gibbs free energies of the micellization andemimicellization processes, resulting in a shift of cmc and hmcoward lower concentrations. Addition of a CH2 group to the chain

ig. 7. Optical microscope images of foams stabilized by Laponite and DTAB mixtures. Then the mixtures are 1.0, 4.0, 8.0, 10.0, 15.0 and 18.0 mM, respectively.

hem. Eng. Aspects 355 (2010) 151–157

is known to decrease the cmc and hmc by a factor of 3 (Traube’srule) [27]. The shifting of the regions in the isotherm is a result ofthe increased hydrophobicity imparted by the longer alkyl chains[28–31]. At the solid-aqueous interface, hydrophobic interactionsmay exist between the surfactant and the surface, and also laterallybetween the adsorbed surfactants molecules.

3.2. Properties of Laponite/cationic surfactant dispersions

The stability of aqueous dispersions of Laponite particles wasmonitored 24 h after mixing with positively charged surfactants.The appearances of the mixed dispersions are shown in Fig. 3. Inthe absence of alkyltrimethylammonium bromides, the Laponiteparticles in water are stable as an aqueous dispersion. Upon addingalkyltrimethylammonium bromides, the initial stable Laponite dis-persion became unstable and flocculated. With further increasesin the alkyltrimethylammonium bromide concentrations, the sed-iment becomes more and more compact and is packed the tightest

at concentrations corresponding to region II in the adsorptionisotherm. Then the sediment redisperses progressively at highercationic surfactant concentrations. This change comes from the sur-face adsorption of surfactants on the Laponite particles, which isreflected by the zeta potential (Fig. 2). The Laponite particles can be

Laponite concentration is fixed at 1.0 wt.%. From (a) to (f), the DTAB concentrations

ysicoc

wbUtcaTpactciictohh

cim

Fi

Q. Liu et al. / Colloids and Surfaces A: Ph

ell dispersed in water in the absence of alkyltrimethylammoniumromides due to a relatively low negative zeta potential (−45 mV).pon adding alkyltrimethylammonium bromides, the zeta poten-

ials gradually increase to about 40 mV at around cmc. In region IIations electrostatically adsorb on the Laponite surfaces mainly asmonolayer with their alkyl chains exposed to the bulk solution.his configuration can screen the electrostatic repulsion betweenarticles, and thus phase separation occurs. In region III furtherdsorption of alkyltrimethylammonium bromides can occur viahain–chain interaction, resulting in a surfactant bilayer or surfac-ant aggregates, with head groups exposed to the solution. Thisase causes a relatively positive zeta potential which results in anncreased electrostatic repulsion between particles, hence the sed-ments at the bottom of the tube will redisperse. In summary, theharge reversal is due to the initial formation of an adsorbed surfac-ant monolayer on the particle surface followed by the formationf a surfactant bilayer or surfactant aggregates which expose theead groups to the continuous aqueous phase. The particles became

ydrophilic again.

At a given concentration, the zeta potential of Laponite parti-les in alkyltrimethylammonium bromide solutions increased withncreasing alkyl chain length of alkyltrimethylammonium bromide

olecules. The results are consistent with the observed increase

ig. 8. Optical microscope images of foams stabilized by Laponite and TTAB mixtures. Then the mixtures are 1.0, 2.0, 2.5, 3.0 and 5.0 mM, respectively.

hem. Eng. Aspects 355 (2010) 151–157 155

in the adsorption isotherms with increasing alkyl chain length ofthe alkyltrimethylammonium bromide molecules. Charge reversalof the Laponite occurs at a lower concentration as the alkyl chainlength of the surfactant increases.

3.3. Properties of foams stabilized by Laponite/cationic surfactantmixtures

Foams cannot be prepared solely by Laponite dispersions, evenin the presence of electrolytes such as NaCl. The stability of foamsprepared by surfactant solutions first increases with increasing sur-factant concentration and then remains unchanged at surfactantconcentrations above its cmc (Fig. 4). After 24 h, the surfactant-onlyfoams have disappeared.

The stability of the foams in Laponite particle–alkyltrimethyl-ammonium bromides mixtures also varies with surfactant con-centration (Fig. 5). The most stable foams were prepared from thedispersions in which the Laponite particles are most aggregated at

intermediate surfactant concentrations. For all three surfactants,four regions, which are closely related to the regions in the adsorp-tion isotherms, were defined according to the foam stability. Inregion I, the stability of the foams prepared by particle–surfactantmixtures is much lower than that of foams prepared with surfac-

Laponite concentration is fixed at 1.0 wt.%. From (a) to (e), the TTAB concentrations

156 Q. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 355 (2010) 151–157

F s. Thei

tIidtadtwecappIs

m(tt

appbpfttofbtptt

In region III, a bilayer surfactant forms on the particle surfaces,

ig. 9. Optical microscope images of foams stabilized by Laponite and CTAB mixturen the mixtures are 0.5, 0.9, 1.2 and 2.0 mM, respectively.

ant solution alone. Stable foams are hardly formed. In region II andII, the stability of foams prepared by particle–surfactant mixturesncreases gradually with surfactant concentration (region II), thenecreases (region III). Foams prepared by particle–surfactant mix-ures are more stable than those stabilized by surfactant solutionlone. Foams are most stable at concentrations where the aqueousispersions are most sedimented. As seen from the zeta poten-ials in Fig. 2, this concentration is where the particles are coatedith monolayer have low charge and are most hydrophobic. The

nhanced stability arises from two mechanisms: (a) adsorption ofolloidal particles at the bubble surface preventing coalescencend disproportionation and (b) stratification of non-adsorbingarticles in the intervening thin film separating the dispersedhases, which improves the stability of foams against drainage.

n region IV, foam stability is almost the same as that of pureurfactant.

With increasing alkyl chain length of the CnTAB molecules, theost stable foams are obtained at about C16 (CTAB), 1.85 mM for C14

TTAB) and 8.90 mM for C12 (DTAB). The results are consistent withhe observed increase in the adsorption isotherms and zeta poten-ials with increasing alkyl chain length of the CnTAB molecules.

The photographs of the dried foams in Fig. 6 were taken 12 hfter preparation. The particles are located within the dried foamhase in regions II and III, which indicates that the particles takeart in stabilizing the foams. The adsorption of particles on the bub-le surface has been shown by confocal images of foams in therevious paper [20]. In region IV, there is nothing left on the driedoam phase. It can be seen from Fig. 5 that the foam stability inhis region is the same as that prepared from pure surfactant solu-ion. The results indicate that the particles actually do not adsorbn the bubble surface and the foams are stabilized only by free sur-actant in the dispersion. In regions II and III, the drained liquid

ecame clearer and more transparent. This means the concen-ration of particles increases in the foam phases; there are morearticles in the Plateau border. Upon increasing the chain length ofhe monomer, the stable foams can be obtained at lower concen-rations.

Laponite concentration is fixed at 1.0 wt.%. From (a) to (d), the CTAB concentrations

The optical microscope images of bubbles stabilized byalkyltrimethylammonium bromide–Laponite dispersions areshown in Figs. 7–9. The bubble size falls first then increases withincreasing surfactant concentration. The bubble size distributionin the foams is wide at low particle concentration and becomes thenarrowest for the most stable foams. In regions II and III, the bubblesurfaces are rough as a result of ripples. Since the particles areirreversibly adsorbed, further compression causes the air–watersurface to undulate giving it the rough appearance. The bubble sizeis smallest when the particles are most aggregated. The bubblesize results agree well with the foam stability.

4. Summary

1. The adsorption isotherms of the three cationic surfactants onLaponite have similar shapes

2. The adsorption mechanism for alkyltrimethylammonium bro-mides on Laponite is in accord with the four-region model (asshown in Fig. 10). Foam stability is closely related to the adsorp-tion conformation of the surfactants on Laponite.

In region I, surfactant ions adsorb individually with their head-groups down. The particles are still hydrophilic so no stablefoams can be produced.

In region II, a sharp increase of adsorption is observed withincreasing surfactant concentration. Particles become coatedwith a surfactant monolayer causing them to aggregate astheir hydrophobicity increases. The foam stability increases withincreasing surfactant concentration. Foams are most stable whenthe aqueous dispersions are most sedimented, corresponding totheir lowest charge and maximum hydrophobicity.

rendering them hydrophilic again. Foam stability decreases inthis range.

In region IV, adsorption remains constant in this region. Theadsorbed layer possesses the structure of a bilayer. Foam stabilityis almost the same as that of pure surfactant.

Q. Liu et al. / Colloids and Surfaces A: Physicoc

Fa

3

A

(

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[30] S.H. Wu, P. Pendleton, Adsorption of anionic surfactant by activated carbon:

ig. 10. Schematic representation of the stabilization of aqueous foams by Laponitend alkyltrimethylammonium bromides mixtures.

. Increasing the chain length of the monomer by four methyleneunits, from C12 to C16, lowers the concentration at which charac-teristic features of the isotherm occur by approximately an orderof magnitude. Therefore, for longer chained surfactants stablefoams can be obtained at lower concentrations.

cknowledgments

The authors thank Prof. Xusheng Feng and Dr. Pamela HoltShandong University) for help in preparation of the manuscript.

eferences

[1] B.P. Binks, T.S. Horozov, Aqueous foams stabilized solely by silica nanoparticles,Angew. Chem. Int. Ed. 44 (2005) 2–5.

[2] E. Dickinson, R. Ettelaie, T. Kostakis, B.S. Murray, Factors controlling the for-mation and stability of air bubbles stabilized by partially hydrophobic silicananoparticles, Langmuir 20 (2004) 8517–8525.

[3] T. Kostakis, R. Ettelaie, B.S. Murray, Effect of high salt concentrations on thestabilization of bubbles by silica particles, Langmuir 22 (2006) 1273–1280.

[4] S. Fujii, A.J. Ryan, S.P. Armes, Long-range structural order, moire patterns, and

iridescence in Latex-stabilized foams, J. Am. Chem. Soc. 128 (2006) 7882–7886.

[5] S. Fujii, P.D. Iddon, A.J. Ryan, S.P. Armes, Aqueous particulate foams stabilizedsolely with polymer Latex particles, Langmuir 22 (2006) 7512–7520.

[6] Z. Du, M.P. Bilbao-Montoya, B.P. Binks, E. Dickinson, R. Ettelaie, B.S. Mur-ray, Outstanding stability of particle-stabilized bubbles, Langmuir 19 (2003)3106–3108.

[

hem. Eng. Aspects 355 (2010) 151–157 157

[7] B.P. Binks, B. Duncumb, R. Murakami, Effect of pH and salt concentration on thephase inversion of particle-stabilized foams, Langmuir 23 (2007) 9143–9146.

[8] B.P. Binks, R. Murakami, Phase inversion of particle-stabilized materials fromfoams to dry water, Nat. Mater. 5 (2006) 865–869.

[9] S.L. Kettlewell, A. Schmid, S. Fujii, D. Dupin, S.P. Armes, Is Latex surfacecharge an important parameter for foam stabilization? Langmuir 23 (2007)11381–11386.

10] B.P. Binks, R. Murakami, S.P. Armes, S. Fujii, A. Schmid, pH-responsive aqueousfoams stabilized by ionizable Latex particles, Langmuir 23 (2007) 8691–8694.

11] B.P. Binks, Particles as surfactants similarities and differences, Curr. Opin. Col-loid Interface Sci. 7 (2002) 21–41.

12] B.P. Binks, T.S. Horozov, Colloidal particles at liquid interfaces: an introduction,in: B.P. Binks, T.S. Horozov (Eds.), Colloidal Particles at Liquid Interfaces, 1stedn., Cambridge University Press, Cambridge, 2006, pp. 1–73.

13] T.N. Hunter, R.J. Pugh, G.V. Franks, G.J. Jameson, The role of particles in stabil-ising foams and emulsions, Adv. Colloid Interface Sci. 137 (2008) 57–81.

14] T.S. Horozov, Foams and foam films stabilised by solid particles, Curr. Opin.Colloid Interface Sci. 13 (2008) 134–140.

15] J.C. Wilson, A study of particulate foams, Ph.D. Thesis, University of Bristol,Bristol, U.K., 1980.

16] K. Vijayaraghavan, A. Nikolov, D. Wasan, Foam formation and mitigation in athree-phase gas–liquid–particulate system, Adv. Colloid Interface Sci. 123–126(2006) 49–61.

17] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Ultrastable particle-stabilized foams, Angew. Chem. Int. Ed. 45 (2006) 3526–3530.

18] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Stabilization of foamswith inorganic colloidal particles, Langmuir 22 (2006) 10983–10988.

19] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Tailoring themicrostructure of particle-stabilized wet foams, Langmuir 23 (2007)1025–1032.

20] S. Zhang, Q. Lan, Q. Liu, J. Xu, D. Sun, Aqueous foams stabilized by Laponite andCTAB, Colloids Surf. A. 317 (2008) 406–413.

21] S. Zhang, D. Sun, X. Dong, C. Li, J. Xu, Aqueous foams stabilized with particlesand nonionic surfactants, Colloids Surf. A. 324 (2008) 1–8.

22] Q. Liu, S. Zhang, D. Sun, J. Xu, Aqueous foams stabilized by hexylamine-modifiedLaponite particles, Colloids Surf. A 338 (2009) 40–46.

23] B.P. Binks, M. Kirkland, J.A. Rodrigues, Origin of stabilisation of aqueous foamsin nanoparticle–surfactant mixtures, Soft Matter 4 (2008) 2373–2382.

24] R.G. Alargova, D.S. Warhadpande, V.N. Paunov, O.D. Velev, Foam superstabi-lization by polymer microrods, Langmuir 20 (2004) 10371–10374.

25] A.B. Subramaniam, C. Mejean, M. Abkarian, H.A. Stone, Microstructure, mor-phology, and lifetime of armored bubbles exposed to surfactants, Langmuir 22(2006) 5986–5990.

26] D.W. Fuerstenau, R. Jia, The adsorption of alkylpyridinium chlorides and theireffect on the interfacial behavior of quartz, Colloids Surf. A 250 (2004) 223–231.

27] A. Fan, P. Somasundaran, N.J. Turro, Adsorption of alkyltrimethylammoniumbromides on negatively charged Alumina, Langmuir 13 (1997) 506–510.

28] J. Wang, B. Han, M. Dai, H. Yan, Z. Li, R.K. Thomas, Effects of chain length andstructure of cationic surfactants on the adsorption onto Na–Kaolinite, J. ColloidInterface Sci. 213 (1999) 596–601.

29] S. Chandar, D.W. Fuerstenau, D. Stigter, R.H. Ottewill, C.H. Rochester, A.L. Smith(Eds.), Adsorption from Solution, Academic Press, New York, 1983, p. 197.

effect of surface chemistry, ionic strength, and hydrophobicity, J. Colloid Inter-face Sci. 243 (2001) 306–315.

31] R. Atkin, V.S.J. Craig, E.J. Wanless, S. Biggs, The influence of chain length and elec-trolyte on the adsorption kinetics of cationic surfactants at the silica–aqueoussolution interface, J. Colloid Interface Sci. 266 (2003) 236–244.