Journal of Colloid and Interface Science Volume 306 Issue 2 2007 [Doi 10.1016%2Fj.jcis.2006.10.062] Fei Yang; Quan Niu; Qiang Lan; Dejun Sun -- Effect of Dispersion pH on the Formation

8/14/2019 Journal of Colloid and Interface Science Volume 306 Issue 2 2007 [Doi 10.1016%2Fj.jcis.2006.10.062] Fei Yang; Qu…

http://slidepdf.com/reader/full/journal-of-colloid-and-interface-science-volume-306-issue-2-2007-doi-1010162fjjcis200610062 1/11

Journal of Colloid and Interface Science 306 (2007) 285–295

www.elsevier.com/locate/jcis

Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles

Fei Yang, Quan Niu, Qiang Lan, Dejun Sun ∗

Key Laboratory for Colloid & Interface Chemistry of Education Ministry, Shandong University, Jinan, Shandong 250100, People’s Republic of China

Received 15 August 2006; accepted 15 October 2006

Available online 28 October 2006

Abstract

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilizedsolely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of theemulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particleaggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potentialdecreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions isenhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variationis very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at theplanar oil/water interface is controlled by dispersion pH. We consider that the particle–particle (at the interface) and particle–interface electrostaticinteractions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particlelayer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particleconcentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and

the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. Theemulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEMimages of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized bythe adsorbed particle layers. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with theoil phase and a relatively unstable outer particle layer surrounding the inner layer.© 2006 Elsevier Inc. All rights reserved.

Keywords: Layered double hydroxides (LDHs) particles; Dispersion; pH; Adsorption; Structural strength; Gel-like structure; Pickering emulsions; Formation andstability

1. Introduction

Emulsions stabilized by colloidal particles were first dis-covered by Pickering in 1907 [1], subsequently this type of emulsions is also called Pickering emulsions. Much attentionnow focuses on Pickering emulsions because: (a) the use of colloidal particles as emulsifiers provides a new method of emulsion preparation with many practical applications in fieldssuch as cosmetics and pharmaceutics; (b) Pickering emulsionsprovide templates for the preparation of new materials [2–5];

* Corresponding author. Fax: +86 531 8564750. E-mail address: [email protected] (D. Sun).

(c) Pickering emulsions provide model systems for the inves-tigation of particle mobility, aggregation and assembly at the

curved oil/water interface [6–8].The adsorption of colloidal particles at the oil/water inter-

face is a crucial factor for preparing stable Pickering emul-sions [9,10]. In most cases, the adsorption or assembly of col-loidal particles at oil/water interfaces is controlled by adjustingthe surface hydrophobicity (also expressed by the three-phasecontact angle θ ) of colloidal particles. Yan and Masliyah [11]have investigated the adsorption of asphaltene-treated kaoliniteclay particles at the drop interfaces of oil-in-water emulsions.They found that the adsorption of clay particles was in directrelation to the particle hydrophobicity: the adsorption density

0021-9797/$ – see front matter ©

2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.10.062

http://www.elsevier.com/locate/jcis

mailto:[email protected]

http://dx.doi.org/10.1016/j.jcis.2006.10.062


mailto:[email protected]

http://www.elsevier.com/locate/jcis



286 F. Yang et al. / Journal of Colloid and Interface Science 306 (2007) 285–295

was lower when θ < 65◦ but higher when θ > 65◦. Binksand Lumsdon [12] have investigated the influence of particlewettability on the type and stability of nanometer-sized silicaparticle stabilized emulsions. The wettability of particles wascontrolled by chemisorbing silane on the particle surface. It wasfound that stable emulsions could only be prepared with parti-

cles of intermediate hydrophobicity because of the high affinityof particles to the oil/water interface when θ is close to 90◦.Reincke et al. [13] reported that the introduction of ethanolcould pull hydrophilic citrate-stabilized Au-nanoparticles intothe water/heptane interface, leading to a closely packed mono-layer. They observed that the addition of ethanol rendered thecontact angle of Au-nanoparticles at the water/hexane interfaceclose to 90◦, which was a crucial factor for the interfacial en-trapment of nanoparticles in their work. Meanwhile, Wang andco-workers [5,14] studied the direct self-assembly of nanopar-ticles at oil/water interfaces. They demonstrated that a contactangle of 90◦ was prerequisite for nanoparticles to localize at

the interface, which is determined by the terminal groups of thecapping ligands. In the recent years, a new kind of particulateemulsifier, also called pH-responsive particulate emulsifier, hasbeen prepared and widely reported [15–18]. Special ionizableorganic groups were first grafted at the surface of pH-responsiveparticles. When the dispersion pH changed, the charge densityof the particles changed due to the ionization or deionizationof the surface organic groups, which in turn changed the par-ticle hydrophobicity. Then the type and stability of emulsionsstabilized by such particles were well controlled.

As far as charged colloidal particles are concerned, the ad-sorption of the particles at air/water or oil/water interface may

be controlled by adjusting the particle–interface electrostatic in-teractions without changing particle hydrophobicity. Many re-searchers have investigated the effects of particle surface chargeand ionic strength on the adsorption of charged colloidal par-ticles at a stagnant air/water interface [19–21]. It was foundthat the presence of salt in colloidal dispersions promoted theadsorption of negatively charged particles at the interface bydecreasing the electrical barrier between the particles and theinterface. Positively charged particles showed greater affinity tothe air/water interface due to the attractive electrostatic interac-tion between the oppositely charged particles and the interface.Through the adsorption of octadecylamine at a toluene/water

interface, Mayya and Sastry [22] made the interface positivelycharged. Then the negatively charged nanoparticles dispersedin water assembled into nanostructured films at the oil/waterinterface due to the electrostatic attraction between the parti-cles and the interface. Using the same idea, a hybrid film of alkylammonium cations and smectite clay particles was fabri-cated by Umemura et al. [23] at an air/water interface. Velevet al. [24] found that the adsorption of lysine onto sulfate latexparticles (at concentrations insufficient to cause flocculation)enabled particle adsorption at the oil/water interface. This wasattributed to the reduction of the electrostatic repulsion betweenthe particles and the oil drops and possibly the reduction of hy-dration interactions between charged sites on the latex particlesand the oil.

Layered double hydroxides (LDHs), also called hydrotal-cite anionic clays, can be represented by the general formula[M2+

1−xM3+

x (OH)2]x+An−x/ n · mH2O, where M2+ is a divalent

metal ion such as Mg2+; M3+ is a trivalent metal ion suchas Al3+, and An− is an anion such as Cl−. The net positivecharge, due to substitution of divalent by trivalent metal ions, iscompensated by an equal negative charge of the interlayer sol-vated anions. When LDHs are dispersed in aqueous solution,the dispersed particles are plate-like and have positive chargesdue to the diffusion of interlayer anions into the aqueous phase.In a former paper, we have investigated the effect of NaClconcentration in LDHs aqueous dispersions on the formationand stability of liquid paraffin-in-water emulsions stabilized byLDHs particles alone [25]. Salt addition into dispersions pro-moted particle concentration at the oil/water interface due tothe increase of attachment energy E and the decrease of theparticle–particle (at the interface) and particle–interface elec-trostatic repulsions, which was prerequisite for preparing stable

emulsions. To further understand the effect of electrostatic in-teractions between particles and the interface on the stability of emulsions stabilized by LDHs particles, in this paper, we firstinvestigated the electrostatic interaction-tailored adsorption of LDHs particles at a planar liquid paraffin–water interface. Then,liquid paraffin-in-water emulsions stabilized by LDHs particleswere prepared at different dispersion pHs. The properties of theemulsions are described and discussed in relation to the adsorp-tion behavior of LDHs particles and the structural strength of the dispersions. The stabilization mechanisms of the emulsionsare also discussed here.

2. Experimental

2.1. Materials

The water was deionized water purified by reverse osmosis.The oil phase was liquid paraffin (Yongda Chemical ReagentCo., China) with purity greater than 99% (d 20

4 = 0.835–0.855).The composition of liquid paraffin is mainly isoalkanes and themain carbon number distribution measured with Agilent 6820GC (Agilent Co., USA) is between 16 and 26. The paraffinoil/water interfacial tension measured with a JYW-200 inter-

facial tensiometer (Chengde test instrument Co., China) was32.5 mN/m. The NaOH and hydrochloric acid used here wereall analytical grade.

The layered double hydroxides were magnesium aluminumhydroxides prepared according to a literature method [26]. Thefinal product was an aqueous dispersion of LDHs with a con-centration around 11 wt%. The pH value of the dispersion isaround 9.2–9.4. The morphology and thickness of the LDHsparticles measured with a multimode AFM (Stanta Barbara,CA) working under tapping mode are shown in Fig. 1a. Asseen in the figure, the dispersion was composed of hexagonalplate-like particles around 140 nm in size. The thickness of theparticles was smaller than 10 nm (Fig. 1b) in agreement withthe literature [27].



F. Yang et al. / Journal of Colloid and Interface Science 306 (2007) 285–295 287

(a)

(b)

Fig. 1. The morphology (a) and thickness (b) of LDHs particles measuredby AFM.

2.2. Methods

2.2.1. Preparation of LDHs dispersions

All experiments were performed at room temperature(25 ◦C). Aqueous dispersions of LDHs particles (0.5–2 wt%)

were prepared by diluting the original 11 wt% aqueous dis-persion under a magnetic stirring. The ionic strength of LDHsdispersions was kept constant by fixing the NaCl concentra-tion at 1 × 10−4 M. The dispersions were laid aside for 24 hbefore pH adjustment. The pH was adjusted by adding concen-trated aqueous NaOH or HCl solution under a N 2 atmosphereand measured with a PHS-25 meter (Leici Co., China).

2.2.2. Zeta potential measurements and aggregation of LDHs

dispersions

The zeta potential was measured using a DXD-II microelec-trophoresis instrument (Jiangsu Optical Instrument Co., China)with a flow-through sample cell. The electrophoretic mobilityof the particles obtained from the unit was converted to the zeta

potential using Smoluchowski equation. The Smoluchowski’sequation is shown as follows:

(1)ζ = ηu/εε0,

where η is viscosity of the continuous phase, u is elec-trophoretic mobility at actual temperature, ε is relative dielec-

tric constant, ε0 is dielectric constant in vacuum. The average of 10 measurements was taken to represent the measured potential.The applied voltage during the measurements was generallyvaried in the range of 50–100 mV.

The aggregation behavior of LDHs particles in aqueous so-lution was investigated through measurement of the averageparticle size of LDHs at different pHs with a laser particle-sizer (Zetasizer3000, England). The particle concentration of the dispersions was fixed at 0.5 wt% and the dispersion pH wasadjusted in the range of 9–12.5. All the samples were laid asideat room temperature for 3 h before experimentation.

2.2.3. Contact angle measurementsThe three-phase contact angle, θ , of the LDHs particles was

measured across water phase using the classic captive dropmethod [28,29]. Aqueous dispersions of LDHs were first driedand crushed into powders. Then the powders were compressedinto a circular disk with a Shimadzu Press at a pressure of 400 kgf /cm2. The thickness of the compressed disk was about2 mm. The LDHs disk was then placed at the bottom of anopen, transparent glass vessel of internal diameter 3.6 cm. Thecontact angle was measured as follows: the liquid paraffin wasfirst poured into the vessel and then a drop of water was placedon the particle layer. The water drop on the particle layer wasimmediately photographed and the contact angle was directlymeasured using a protractor.

2.2.4. Flow behavior of LDHs dispersions

Using RS75 rheometer (Haake Co., Germany) with a rotor-cylinder geometry, the flow curves (shear rate/shear stresscurves) of LDHs dispersions at different pHs were recorded be-tween 0 to 1000 1/s in 2 min. The dispersions were first shearedat a shear rate (D) of 50 1/s for 30 s, then remained at rest for20 min before shearing. The particle concentration was con-trolled between 0.5–2 wt% and the pH value was adjusted inthe range of 9–12.5. The yield stress of the dispersions was ob-tained by extrapolating the curves to D = 0 using a rheological

model.

2.2.5. Adsorption behavior of LDHs particles at planar

oil–water interface

Visual observations for the adsorption of LDHs particles atthe planar oil–water interface were conducted in a columniformglass cell of internal diameter 5 cm. LDHs dispersions with dif-ferent pHs were first prepared at a certain particle concentration(0.075 wt%). Then a portion (50 ml) of each dispersion wastransferred to the cell and 50 ml of liquid paraffin was gen-tly poured over the dispersion. The glass cell containing oiland dispersion was gently agitated by hand to promote particleadsorption at the oil/water interface. After that, visual observa-tions were performed at different time intervals.




The morphology of the interfacial particle layers was inves-tigated with a JSM-6700F SEM (Jeol, Japan). Most of the liquidparaffin was first removed from the top of the container with asyringe. Then, a substrate (mica) was touched to the oil–waterinterface horizontally and then the interfacial particle layer wastransferred to the surface of the substrate. In order to increase

the conductivity of the samples, the surface of the samples wascoated with Pt by sputtering before observation.

2.2.6. Preparation and properties of emulsions

Using a lab homogenizer (Shanghai Forerunner M&E Co.,China) operated at 8000 rpm for 2 min, we prepared emulsionsfrom LDHs dispersions (0.5–2 wt%) and liquid paraffin (50 cm3

total) at different dispersion pHs. The head of the homogenizeris composed of an inner rotor and an outer stator. The diameterof the inner rotor is 3 cm while the diameter of the outer sta-tor is 3.5 cm. The oil phase volume fraction of the preparedemulsions was always 0.5. The pH value of the dispersions

was changed over the range of 9–12.5. Immediately after ho-mogenization, emulsion conductivities were determined usinga DDS-370 digital conductivity meter (Leici Co., China) to de-termine the emulsion type. The emulsions were transferred intoa stoppered, graduated glass vessel of internal diameter 1.6 cmand length 15 cm for the observation of emulsion stability. Thestability of emulsions to dilution, shaking and pH adjustmentwas also studied here. The morphology of emulsion drops wasobserved with an Axioskop 40 microscope (ZEISS, Germany)and a TEM-100VII (Jeol Co., Japan). The emulsion sampleswere first diluted 10 times with an aqueous solution of the samepH to make the emulsion drops appear clearly under the micro-scope. Emulsion drop size distribution was obtained by imageprocessing using microscopic image analysis software.

3. Results and discussion

3.1. Properties of LDHs dispersions

3.1.1. Zeta potential and aggregation behavior

It is well known that the LDHs particles dispersed in aque-ous phase have permanent positive charges [30]. The concen-tration of H+ (or OH−) in LDHs dispersions determines thesurface charge density of LDHs particles because H+ and OH−

are both potential-determining ions [31]. The reactions between

H+ (or OH−) and surface hydroxyls may be expressed as fol-lows:

Sur-OH + H+ → Sur-OH+2 , (2)

Sur-OH + OH− → Sur-O− + H2O. (3)

The zeta potential of colloidal dispersions has great influ-ence on the stability, aggregation, flow, sedimentation, and fil-tration of the dispersions [32,33]. The zeta potential of LDHsdispersions was measured to investigate the effect of disper-sion pH on the electrokinetic properties of LDHs dispersions. InFig. 2a there is no significant effect of particle concentration onzeta potential. Therefore, in the subsequent potential measure-ments the particle concentration was kept constant at 0.5 wt%.

(a)

(b)

Fig. 2. Variation of zeta potential with (a) particle concentration and (b) pH of LDHs dispersions.

Fig. 3. Variation of the average size of LDHs particles with dispersion pH.D0 is the initial average diameter of LDHs particles; D is the average particlediameter at different dispersion pHs.

The variation of zeta potential with dispersion pH is shown inFig. 2b. The zeta potentials gradually decrease with the increaseof pH because that the OH− ions are potential-determining ionswhich can react with the surface hydroxyls of LDHs particlesand then decrease the particle surface charge density [34].

The aggregation behavior of LDHs particles in aqueous so-lution is displayed in Fig. 3. The LDHs particles become rel-atively unstable and aggregate into large flocs at high pHs,corresponding to the decrease of particle zeta potential with in-creasing dispersion pH. It should be noticed that when pH 11,the increase of particle size is relatively small (from the origi-nal diameter 140.8 to 194.4 nm); but above pH 11, the particlesize increases quickly.




Fig. 4. Variations of the three-phase contact angle of LDHs particles with dis-persion pH.

3.1.2. Contact angle measurement

The three-phase contact angle θ of a colloidal particle at anoil–water interface is an important parameter in determining theadsorption of particles at the oil/water interface and the stabil-

ity of Pickering emulsions [11–18]. The variation of θ of LDHsparticles with dispersion pH is shown in Fig. 4. The value of θ increases slowly as dispersion pH increases, possibly becausethe thickness of the hydration layer of LDHs particles decreaseswith the decrease of particle zeta potential, leading to increasedhydrophobicity of the particles. When the pH 12.5, the varia-tion of θ with increasing pH is very small (from 32.5◦ to 40.5◦).Even at pH 13, the value of θ (49.5◦) is still far smaller than 90◦.

3.1.3. Flow behavior

As seen in Figs. 5a and 5b, for our dispersion systems, theviscosity first decreases with increasing shear rate due to thedestruction of the structure of the dispersions; after that, the vis-

cosity slightly increases because that the recovery of dispersionstructure is faster than the destruction of dispersion structure.Therefore, the dispersions are initially shear-thinning fluids andthen shear-thickening fluids after the further increase of shearrate. Exceptions exist when pH 11.92 for 2 wt% LDHs dis-persions. Fig. 5c shows the variation of yield stress of LDHsdispersions with the dispersion pH and particle concentration.Increasing particle concentration and dispersion pH enhancesthe structural strength of the dispersions.

3.1.4. Adsorption behavior of LDHs particles at flat oil/water

interface

LDHs particles adsorbed at a planar liquid paraffin/waterinterface after different times are shown in Fig. 6. When theamount of adsorbed particles at the interface is small, the in-terface is mirror-like; whereas, when the amount of adsorbedparticles is large, the interface is milky white due to the re-flection of the interfacial particles. In Fig. 6a, the interface ismirror-like at low pHs; as the pH increases, the mirror-like areadecreases and finally an integrated milky white particle layer isformed at the oil/water interface. But when the dispersion pH istoo high (12.47), the oil–water interface becomes mirror-likeagain. Thus, pH adjustment controls the adsorption behavior of LDHs particles at the planar oil/water interface.

The SEM images of the interfacial particle layers formed atdifferent dispersion pHs are shown in Fig. 7, which also shows

(a)

(b)

(c)

Fig. 5. Flowcurvesof LDHsdispersions atdifferent pH: (a) 0.5 wt%,(b) 2 wt%;(c) variation of the yield stress of LDHs dispersions with dispersion pH andparticle concentration.

that the dispersion pH plays an important role in the adsorp-tion of LDHs particles at the planar oil/water interface and theformation of an interfacial particle monolayer or multilayer.When dispersion pH 9.3, there are few particles available tobe adsorbed at the interface and then the interface is mirror-like. A particle layer forms at the oil/water interface at pH 10.1,which is composed of many regular hexagonal plate-like LDHsparticles, meaning that the aggregation of particles at the inter-face is not obvious. But we can not find a clear particle layer inFig. 6a at pH 10.1. The possible reason is that the particle layermay be a monolayer and the thickness of the layer is too thinto be clearly observed. When pH 11.03, the adsorbed particlelayer is not a monolayer, but more like a multilayer. Many ag-gregates composed of plate-like LDHs particles were observed




(a) (b)

(c)

Fig. 6. Photographic images of the interfacial adsorbed layers of LDHs particles (0.075 wt%) at different times: (a) 1 h after preparation; (b) different view of thesample at pH 12.47, 1 h after preparation, showing sediment clearly against a black background; (c) one week after preparation.

at the interface. This is due to the decrease of the electrosta-tic repulsions between particles dispersed in water or at theinterface. Compared with the interfacial particle layer formedat pH 10.1, the particle layer formed at pH 11.03 is thicker andthe amount of absorbed particles is larger. Therefore, a corru-

gated particle layer can be visually observed in Fig. 6a. WhenpH 11.98, the particle layer formed at the oil/water interface be-comes thicker and particle aggregation becomes more serious.The individual plate-like particles can hardly be found in thelayer. This phenomenon is in accord with that in Fig. 6a, fromwhich we find a clear corrugated particle multilayer at the inter-face. When pH 12.47, the particle layer becomes loose and wecan not find a particle layer in Fig. 6a because the particles dis-persed in water aggregate into large flocs at high pH (see Fig. 3)and then gravity becomes dominant and the particles sedimentat the bottom of the vessel (see Fig. 6b).

The formation of the interfacial particle layer is also affectedby the resting time according to Fig. 6. The particle layersformed at low pH become more visible and uniform with in-creasing resting time. Even at pH 9.26, an integrated particlelayer is visible at the oil/water interface one week after prepa-ration (see Fig. 6c).

It is well known that the adsorption of colloidal particles atoil/water interface is driven by the reduction of interfacial en-ergy E (also called attachment energy) [35], which is calculatedas follows:

(4)E = −π R2γ ow(1 − cos θ)2,

where R, γ ow, θ represent particle radius, oil–water interfa-cial tension and three-phase contact angle respectively. In thispaper, the average radius of LDHs particles is 70 nm, and

the liquid paraffin–water interfacial tension is 32.5 mN/m.At a fixed value of θ = 32◦, the value of E is −2803kT ,much greater than the thermal motion energy of a colloidalparticle (about several kT ). It means that the adsorption of LDHs particles at oil–water interface is thermodynamically fa-

vored even at very low pH (9.26) due to the high value of E. Given enough time, a particle layer can be formed even atvery low pHs (see Fig. 6c). Moreover, when particles adsorbat the oil–water interface, desorption of the particles is verydifficult due to the high attachment energy E. According tothe former results, increasing dispersion pH greatly increasesthe particle size, causing an increase of E. Although the in-crease of E favors particle adsorption, we consider here thatit has little influence on the adsorption behavior of particles atoil–water interface, which is a kinetic process. Instead, the ad-sorption behavior in Fig. 6 is mainly controlled by the particle–interface and particle–particle interactions. There should be an

energy barrier between the particles dispersed in water and theoil/water interface. Colloidal particles must overcome the en-ergy barrier in order to be adsorbed at the interface. Accordingto Fig. 4, the variation of θ with pH is very small when pH12.5. Therefore, the variation of hydrophobic interactions be-tween the particles and the interface could be neglected here.It is well known that the bare oil–water interface is negativelycharged (in neutral aqueous solution) because of the adsorp-tion of OH− ions and that the electrical potential of the in-terface increases greatly with increasing solution pH [36–38].In our experiments, as the dispersion pH increases, the neg-ative potential of the oil/water interface increases. Then theparticle–interface electrostatic attractions increase, promotingparticle adsorption at the interface. Moreover, when the inter-




Fig. 7. SEM images of the interfacial adsorbed layer of LDHs particles (0.075 wt%) 1 h after preparation. Samples correspond to images in Fig. 6a.

face is positively charged due to the adsorption of LDHs par-ticles, the particle–interface repulsions are relatively small dueto the decrease of particle surface charge density with increas-ing pH. In addition, increasing dispersion pH also decreases theelectrostatic repulsions between the particles adsorbed at the

oil/water interface, which also promotes concentration of theparticles at the interface [13]. At high pH, however, the particleflocs are so large that gravity plays a crucial role. Therefore,the particle adsorption becomes difficult again due to parti-cle sedimentation. Compared with the formation of the inter-facial particle layer induced by NaCl [25], the formation of the interfacial particle layer induced by pH is faster (within0.5 h), possibly due to the high electrostatic attractions be-tween the particles and the interface. According to the abovediscussion, we conclude that the pH adjustment controls theparticle–particle (at the interface) and particle–interface elec-trostatic interactions, leading to pH-tailored particle adsorp-tion.

3.2. Preparation and characterization of emulsions stabilized

by LDHs particles

3.2.1. Appearance and stability of the emulsions stabilized by

LDHs particles

Emulsions stabilized by LDHs particles were prepared atdifferent particle concentrations (the concentration of particlesdiscussed here is its concentration in the aqueous phase) anddispersion pHs. Conductivity measurements of the emulsionsindicate that all the emulsions prepared are o/w type, meaningthat the particles dispersed in aqueous solution are hydrophilic.The appearance of the emulsions prepared at different parti-cle concentrations and pHs 24 h after preparation is shown inFig. 8. Dispersion pH plays a crucial role in emulsion stability.With the increase of dispersion pH, the coalescence stabilityand creaming stability of the emulsions improve. Increasingparticle concentration also improves emulsion stability. Thevariation of emulsion drop size distribution with dispersion pHand particle concentration is displayed in Fig. 9. The emulsion




Fig. 8. The appearance of liquid paraffin-in-water emulsions 24 h after preparation. The pHs of the dispersions are shown in the figure: (a) emulsions stabilized by0.5 wt% LDHs; (b) emulsions stabilized by 2 wt% LDHs; (c) photograph of emulsions stabilized by 2 wt% LDHs; (d) appearance of emulsions after inversion of the test tube.

(a)

(b)

Fig. 9. Drop size distributions of liquid paraffin-in-water emulsions immedi-ately after preparation: (a) emulsions stabilized by 0.5 wt% LDHs; (b) emul-sions stabilized by 0.5 wt% LDHs.

drop size decreases with increasing dispersion pH, correspond-ing to the adsorption behavior of LDHs particles at a planaroil/water interface. Increasing particle concentration also de-creases the emulsion drop size. According to the above results,we conclude that the improvement of the emulsion stabilitywith increasing particle concentration and dispersion pH is be-

cause that more particles are available to be adsorbed at theoil–water interface. In addition, the enhancement of the struc-tural strength of the dispersions (see Fig. 5) with the increaseof dispersion pH and particle concentration also improves theemulsion stability, but it is not necessary for emulsion forma-tion [25].

According to Fig. 6, the adsorption of LDHs particles at pla-nar oil/water interface is difficult at high pH (12.46) due to thesediment of large particle flocs (induced by gravity), which is

adverse for emulsion formation and stability. However, we seein Figs. 8 and 9 that emulsions prepared at high pH have smallerdrop size and are very stable. The possible reason is that (a) theemulsification process imparts the particles high kinetic energypromoting particle adsorption at the interface; (b) the particleflocs can also be fragmented into small pieces under emulsifi-cation. It is also seen in Fig. 8 that when the pH is above 11.5,the separated emulsion subphase changes from an aqueous dis-persion to pure water. According to Yan and Masliyah’s [11] as-sumption, in our high pH experiments, the change of separatedsubphase from aqueous dispersion to pure water shows that allthe particles have been adsorbed at the oil–water interface. We

do not think so and deduce that: the particle flocs formed athigh pH are very unstable in water, but they do not sediment.Instead, they are liable to go into the creamed emulsion phaseand flocculate with the emulsion drops so as to decrease theparticle–water interface. Therefore, the separated subphase of the emulsions is clear water above a certain pH. This deduc-tion is supported by the latter TEM images of the emulsions(Figs. 12b and 12c).

All the prepared emulsions cream slowly with time and thefinal state of the creamed emulsions is gel-like, as seen inFig. 8d. It is obvious that the flocculation stability of emulsionsstabilized by LDHs particles is poor. The gel-like structure isformed due to the flocculation of emulsion drops and the in-dividual particle flocs into a continuous network. When the




(a)

(b)

Fig. 10. Variations of the drop size distributions of emulsions after dilution andshaking: (a) emulsions stabilized by 0.5 wt% LDHs at pH 11.95; (b) emulsionsstabilized by 2 wt% LDHs at pH 12.01.

gel-like structure of emulsions is formed, the emulsions do not

cream water any more and no oil phase is separated from theemulsions after 3 moths. We consider that the gel-like structurealso improves the emulsion stability by immobilizing emulsiondrops in the network. Here, we studied the stability of emul-sions after the gel-like structure is destroyed to evaluate theeffect of gel-like structure on emulsion formation and stabil-ity. The emulsion samples (stabilized by 0.5 and 2 wt% LDHsdispersions at pH 12) were first diluted 10 times with an aque-ous solution of the same pH and then shaken for 10 min at150 rpm. The gel-like structure of the emulsions was com-pletely destroyed under dilution and shaking. After that, thedroplet size distributions of the samples were measured. As

seen in Fig. 10, dilution and shaking have little effect on thedroplet size distribution. Therefore, the formation of emulsionsis mainly determined by the adsorbed particle layer around theoil drops. The gel-like structure of the emulsions is responsiblefor the emulsion stability, but has little relation to the emulsionformation.

An attempt was made to demulsify the emulsions by adjust-ing the pH of emulsions to a relatively low value ( Fig. 11). Thechange of the pH has little effect on the droplet size distribu-tions of the emulsions, meaning that the adsorption of LDHsparticles at the oil–water interface is irreversible. This result isin accord with the high attachment energy E of the LDHs par-ticles.

(a)

(b)

Fig. 11. Variation of the drop size distributions of emulsions with emulsion pH:(a) emulsions stabilized by 0.5 wt% LDHs at pH 11.95; (b) emulsions stabilizedby 2 wt% LDHs at pH 12.01.

3.2.2. Microscopic images of the emulsions stabilized by

LDHs particlesThe optical microscope image and TEM images of the

emulsions stabilized by LDHs particles are shown in Fig. 12.Fig. 12a is an optical microscope image of the emulsions stabi-lized by 2 wt% LDHs at 12.01. Spherical drops with diametersabout 10–20 µm are visible and show a tendency to flocculate.Fig. 12b shows the TEM image of an emulsion drop with di-ameter about 15 µm. The morphology of the emulsion drop isa dark sphere under TEM, meaning that a thick particle layerforms around the oil drop. Fig. 12c shows the TEM image of three emulsion droplets which have a great tendency to floccu-late. The thickness of the interfacial particle layer is not uniform

and many individual particle flocs coexist with the emulsiondrops. We consider that the thick particle layers formed at thesurface of the emulsion drops should be divided into two parts:the first part is a dense thin particle layer (inner particle layer)in direct contact with the oil phase; the second part is a rela-tively loose but thick particle layer (outer particle layer), whichis composed of many individual particle flocs adsorbed at thesurface of the inner particle layer. The outer particle layer iseasily destroyed while the inner particle layer is very stable dueto the high attachment energy E of the adsorbed particles at theoil/water interface. When the emulsion samples are diluted be-fore TEM observation, the outer particle layers will be partiallydestroyed and then the individual particle flocs will diffuse fromthe outer particle layer into the bulk phase. Therefore, both the




(a)

(b)

(c)

Fig. 12. Optical microscope image (a) and TEM images (b, c) of liquid paraf-fin-in-water emulsions stabilized by 2 wt% LDHs at pH 12.01 immediatelyafter preparation.

emulsion drops and the individual particle flocs can be observedunder TEM.

4. Conclusions

We have systematically studied the effect of dispersion pHon the stability of emulsions stabilized by positively chargedplate-like LDHs particles. We show that changing the pH of

LDHs dispersions controls the adsorption behavior of LDHsparticles at oil–water interface, which in turn determines thestability of emulsions stabilized by such particles. The follow-ing conclusions are drawn:

The zeta potential of LDHs dispersions decreases with in-creasing dispersion pH, causing the flocculation of LDHs par-

ticles into large flocs. The contact angle of LDHs particles in-creases with the increase of pH, but the variation is very small.The structural strength of LDHs dispersions is enhanced by in-creasing pH or the particle concentration.

The adsorption behavior of positively charged plate-likeLDHs particles at a flat oil/water interface is thermodynami-cally favorable at all pH range measured. The adsorption be-havior is controlled by dispersion pH. As the pH increases, theparticle–interface and particle–particle (at the interface) elec-trostatic interactions are well controlled, leading to electrostaticinteraction tailored particle adsorption. At high pH, however,the size of particles is so large that they sediment, then particle

adsorption at the oil/water interface becomes difficult.Liquid paraffin-in-water emulsions stabilized by LDHs par-ticles were prepared. The formation of an adsorbed particlelayer at the oil drops is crucial for the formation and stabilityof emulsions. With the increase of dispersion pH and particleconcentration, the stability of emulsions improves and the dropsize of emulsions decreases. In addition, the enhancement of dispersion structural strength with increasing pH and particleconcentration also improves the stability of emulsions. The fi-nal state of emulsions is gel-like. The gel-like structure is notnecessary for emulsion formation, but improves the stability of emulsions. Emulsions can not be demulsified by adjusting theemulsion pH.

TEM images of the emulsion drops confirm that a thick par-ticle layer forms at the surface of the emulsion droplets. Thethick adsorbed particle layer may be composed of a stable innerparticle layer which is in direct contact with the oil phase anda relatively unstable outer particle layer surrounding the innerlayer.

Acknowledgments

This work was financially supported by a grant from the Na-tional Natural Science Foundation of China (No. 20373036).The authors thank Prof. X.S. Feng (Shandong University) and

Dr. Pamela Holt for their comments on this manuscript.

Supporting information

The online version of this article contains additional support-ing information.

Please visit doi: 10.1016/j.jcis.2006.10.062.

References

[1] S.U. Pickering, J. Chem. Soc. 91 (1907) 2001.[2] A.D. Dinsmore, M.F. Hsu, M.G. Nikolaides, M. Marquez, A.R. Bausch,

D.A. Weitz, Science 298 (2002) 1006.

[3] M.F. Hsu, M.G. Nikolaides, A.D. Dinsmore, A.R. Bausch, V.D. Gordon,X. Chen, J.W. Hutchinson, D.A. Weitz, Langmuir 21 (2005) 2963.






[4] P. Noble, O. Cayre, R. Alargova, O. Velev, V. Paunov, J. Am. Chem. Soc.126 (2004) 8092.

[5] H. Duan, D. Wang, N.S. Sobal, M. Giersig, D.G. Kurth, H. Möhwald,Nano Lett. 5 (2005) 949.

[6] S. Tarimala, S.R. Ranabothu, J.P. Vernetti, L.L. Dai, Langmuir 20 (2004)5171.

[7] L.L. Dai, R. Sharma, C. Wu, Langmuir 21 (2005) 2641.

[8] C. Wu, S. Tarimala, L.L. Dai, Langmuir 22 (2006) 2112.[9] B.P. Binks, Curr. Opin. Colloid Interface Sci. 7 (2002) 21.

[10] R. Aveyard, B.P. Binks, J.H. Clint, Adv. Colloid Interface Sci. 100–102(2003) 503.

[11] N. Yan, J.H. Masliyah, Colloids Surf. A 96 (1995) 229.[12] B.P. Binks, S.O. Lumsdon, Langmuir 16 (2000) 8622.[13] F. Reincke, S.G. Hickey, W.K. Kegel, D. Vanmaekelbergh, Angew. Chem.

Int. Ed. 43 (2004) 458.[14] D. Wang, H. Duan, H. Möhwald, Soft Matter 1 (2005) 412.[15] J.I. Amalvy, S.P. Armes, B.P. Binks, J.A. Rodrigues, G.-F. Unali, Chem.

Commun. (2003) 1826.[16] S. Fujii, E.S. Read, B.P. Binks, S.P. Armes, Adv. Mater. 17 (2005) 1014.[17] T. Ngai, S.H. Behrens, H. Auweter, Chem. Commun. (2005) 331.[18] B.P. Binks, J.A. Rodrigues, Angew. Chem. Int. Ed. 44 (2005) 441.[19] F.D. Williams, J.C. Berg, J. Colloid Interface Sci. 152 (1992) 218.

[20] J. Wan, J.L. Wilson, J. Water Resour. Res. 30 (1994) 11.[21] A.I. Abdel-Fattah, M.S. El-Genk, J. Colloid Interface Sci. 202 (1998) 417.[22] K.S. Mayya, M. Sastry, Langmuir 15 (1999) 1902.[23] Y. Umemura, A. Yamagishi, R. Shoonheydt, A. Persoons, F.D. Schryver,

Langmuir 17 (2001) 449.

[24] O.D. Velev, K. Furusawa, K. Nagayama, Langmuir 12 (1996) 2374.

[25] F. Yang, S. Liu, J. Xu, Q. Lan, F. Wei, D. Sun, J. Colloid Interface Sci. 302(2006) 159.

[26] X.J. Wang, D.J. Sun, S.Y. Liu, R. Wang, J. Colloid Interface Sci. 289(2005) 410.

[27] N. Wang, S.Y. Liu, J. Zhang, Z.H. Wu, J. Chen, D.J. Sun, Soft Matter 1(2005) 428.

[28] N. Yan, J.H. Masliyah, J. Colloid Interface Sci. 181 (1996) 20.[29] T. Kostakis, R. Ettelaie, B.S. Murray, Langmuir 22 (2006) 1273.

[30] Q.Z. Yang, D.J. Sun, C.G. Zhang, X.J. Wang, W.A. Zhao, Langmuir 19(2003) 5570.

[31] W.G. Hou, C.Y. Xia, S.H. Han, G.T. Wang, C.G. Zhang, Acta Chim.Sinica 56 (1998) 514.

[32] K.E. Bremmel, G.J. Jameson, S. Biggs, Colloids Surf. A 146 (1999) 75.

[33] M. Alkan, Ö. Demirbas, M. Dogan, Microporous Mesoporous Mater. 83(2005) 51.

[34] (a) G. Lagaly, O. Mecking, D. Penner, Colloid Polym. Sci. 279 (2001)1090;(b) Z.L. Jin, W.G. Hou, G.G. Dai, D.J. Sun, C.G. Zhang, Z.W. Sun, ActaChim. Sinica 62 (2004) 429.

[35] Y. Lin, H. Skaff, T. Emrick, A.D. Dinsmore, T.P. Russell, Science 299

(2003) 226.[36] K.G. Marinova, R.G. Alargova, N.D. Denkov, O.D. Velev, D.N. Petsev,

I.B. Ivanov, R.P. Borwankar, Langmuir 12 (1996) 2045.

[37] J.K. Beattie, A.M. Djerdjev, Angew. Chem. Int. Ed. 43 (2004) 3568.

[38] G.V. Franks, A.M. Djerdjev, J.K. Beattie, Langmuir 21 (2005) 8670.

Documents

Journal of Colloid and Interface Science Volume 306 Issue 2 2007 [Doi 10.1016%2Fj.jcis.2006.10.062] Fei Yang; Quan Niu; Qiang Lan; Dejun Sun -- Effect of Dispersion pH on the Formation