Multiple Phase Inversion of Emulsions Stabilized by in Situ Surface

ARTICLE pubs.acs.org/Langmuir

Multiple Phase Inversion of Emulsions Stabilized by in Situ Surface Activation of CaCO3 Nanoparticles via Adsorption of Fatty Acids Z.-G. Cui,*,† C.-F. Cui,† Y. Zhu,† and B. P. Binks*,‡ † ‡

School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122. P. R. China Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K.

bS Supporting Information ABSTRACT: The in situ surface activation of raw CaCO3 nanoparticles by interaction with a series of sodium carboxylates of chain length between 6 and 12 as well as sodium 2-ethylhexylsulfosuccinate (AOT) was studied, and the impact of this on the stabilization and phase inversion of toluene
water emulsions was assessed. By using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of amphiphile, and relevant contact angles, the mechanism of this activation was revealed. The results show that hydrophilic CaCO3 nanoparticles can be surface activated by interaction with sodium carboxylates and AOT even if they are not surface-active themselves. Both the electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic amphiphile headgroups and the chain
chain interactions of the amphiphile result in monolayer adsorption of the amphiphile at the particle
water interface. This transforms the particles from hydrophilic to partially hydrophobic such that they become surfaceactive and stabilize oil-in-water O/W(1) emulsions and induce O/W(1) f water-in-oil W/O phase inversion, depending on the chain length of the carboxylate molecules. At high amphiphile concentration, bilayer or hemimicelle adsorption may occur at the particle
water surface, rendering particles hydrophilic again and causing their desorption from the oil
water interface. A second phase inversion, W/O f O/W(2), may occur depending on the surface activity of the amphiphile. CaCO3 nanoparticles can therefore be made good stabilizers of both O/W and W/O emulsions once surface activated by mixing with traces of suitable anionic amphiphile.

’ INTRODUCTION It is well-known that colloid particles partially wetted by both fluid phases can adsorb at oil
water or air
water interfaces and hence stabilize emulsions and foams, respectively.1
3 These particles are thus surface-active like surfactant molecules. It has been suggested that the contact angle, θ, of a particle at a fluid interface, conventionally measured through the water phase, is equivalent to the hydrophile
lipophile balance number of a surfactant, which determines not only the surface activity of the particle but also the type of emulsion or foam they stabilize.1,4 The adsorption free energy of a particle at a fluid interface is thus a function of θ, and that of a nanoparticle with θ between 60° and 120° may reach thousands of kT, indicating that the adsorption is more or less irreversible.3 The emulsions and foams stabilized by the surface-active nanoparticles can be ultrastable, in contrast to those stabilized by surfactants which are frequently unstable to coalescence. In particular, the inverse foam or “dry water” product, which has not been achieved using surfactants to date, can be prepared using surface-active nanoparticles.4 Although the surface-active nanoparticles are superior to surfactants in stabilizing emulsions or foams, the majority of nanoparticles commercially available are not surface-active due to either their extreme hydrophilicity or hydrophobicity. To obtain surface-active nanoparticles, some researchers have put r 2011 American Chemical Society

effort into preparing “Janus” particles5
10 having an asymmetry like surfactant molecules. The methods developed, however, are quite involved, and most are not designed for preparing large quantities of particles.6
8,10 Fortunately, the homogeneous surface coating route was found to be easy and reproducible for preparing surface-active nanoparticles, a typical example being silica. By controlled surface silylation, silica nanoparticles can be modified progressively from extremely hydrophilic to surfaceactive with a wide range of wettability.3 For inorganic nanoparticles which are usually charged in aqueous solution, this surface modification can also be achieved in solution by adsorbing oppositely charged amphiphiles.11
25 Comparatively, the in situ surface activation process is much simpler than the ex situ homogeneous surface coating route, and the surface activity of the particles can be controlled flexibly by selection of the molecular structure and concentration of the amphiphile.13
15,22,24,25 Importantly, when the amphiphile is a surfactant, novel interfacial phenomena may be induced. For example, double phase inversion has been reported in emulsions stabilized by silica particles and cationic surfactant15,22 and calcium carbonate (CaCO3) and Received: October 13, 2011 Revised: November 20, 2011 Published: November 21, 2011 314

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anionic surfactant16,25 mixtures, which has never been observed in emulsions stabilized by surfactants or particles alone. The interaction between surface-active nanoparticles and surfactant at fluid interfaces has become a subject of growing interest.26
31 Nanoparticles of CaCO3 are probably the cheapest commercial nanoparticles with the largest output in the world. Moreover, they are edible and are therefore safe to human beings and the environment. Although the raw particles are not particularly surface-active, they can be endowed with surface activity by surface modification.21 In our previous studies, it has been found that by simple in situ surface activation with anionic surfactants such as sodium dodecyl sulfate (SDS) or sodium 2-ethylhexylsulfosuccinate (AOT), CaCO3 nanoparticles can be converted to good emulsion and foam stabilizers.16,25 In this paper we describe the activation of CaCO3 nanoparticles using food-grade carboxylic acids and demonstrate how their function as an emulsion stabilizer depends markedly on the chain length of the amphiphile.

concentrations are expressed as weight percentage (wt %) and moles per liter (M) relative to the water phase, respectively. The emulsion type was identified by the drop test.24 Microscope images of the emulsion droplets were obtained using a Motic BA400 microscope system (Xiamen Motic Co.) as described elsewhere.24 Adsorption Isotherm of Amphiphiles on CaCO3 Nanoparticles in Water. 0.2 g of CaCO3 nanoparticles was weighed into a series of glass bottles of dimensions 7.5 cm (h) by 2.5 cm (d) followed by adding 20 cm3 of aqueous solution of anionic amphiphile at different concentrations. The particles were dispersed using ultrasound, and the dispersions were thermostated at 25 ( 0.5 °C for 24 h. After removing the suspended particles by centrifugation (5000 rpm for 30 min), the equilibrium concentration of the anionic amphiphile in the supernatant was measured, and the adsorbed amount of amphiphile on CaCO3 nanoparticles, Γ, was calculated using Γ¼

V ðC0
CÞ ðmmol=gÞ w

ð1Þ

where V is the volume of the solution (cm3), C0 and C are the initial and equilibrium concentrations (M) of anionic amphiphile in the dispersion, respectively, and w is the weight of the particles (g). In order to measure the equilibrium concentration of sodium carboxylate, an extraction
titration process was used as follows.32 A certain volume (V) of supernatant comprising aqueous sodium carboxylate was weighed into a 125 cm3 separation funnel, followed by adding water to a total of 10 cm3. Then 0.2 cm3 of l M hydrochloric acid was added (moles of HCl is at least double that of the carboxylate to ensure complete acidification), and the mixture was well shaken (>15 s). Then 7.5 cm3 of diethyl ether together with 0.5 cm3 of n-butyl alcohol was added into the funnel, and the mixture was fully shaken for about 60 s. After phase separation the lower water layer was abandoned, and the ether layer was washed twice with 7.5 cm3 of pure water. The ether layer was then transferred into a 50 cm3 conical flask, and the funnel was washed with a mixed solution containing 0.5 cm3 Span 80 (to improve miscibility of ether with water), 5 cm3 ethanol, and 94.5 cm3 water. Combining the washing liquor with the ether layer, the mixture was titrated with 0.01 M NaOH using phenolphthalein as indicator. For C12Na the washing solution was replaced by ethanol, and the mixture of washing liquor and ether was heated to 60 °C for 15 min to remove diethyl ether before titration. The equilibrium concentration of sodium carboxylate in the supernatant solution was calculated using

’ EXPERIMENTAL SECTION Materials. Calcium carbonate nanoparticles (NLS906) prepared by a precipitation method with a purity of g98% and a primary particle diameter of 80
100 nm were supplied by Henan Keli New Materials Co. Ltd., China. They were supplied as a powder and used as received. Sodium hexanoate (C6Na) of 99% purity, sodium octanoate (C8Na) of 99% purity, sodium decanoate (C10Na) of 98% purity, and sodium dodecanoate (C12Na) of 98% purity were purchased from Sigma and used as received. AOT of 96% purity was purchased from Alfa Aesar, SDS of 99% purity and Hyamine 1622 of 98% purity were purchased from Sigma, and sorbitan oleate (Span 80) of CP grade was purchased from Sinopharm Chemical Reagent Co.; all were used as received. Toluene of 99.5% purity was purchased from Sinopharm Chemical Reagent Co. and columned three times through neutral alumina before use. All other chemicals for analysis were analytically pure and purchased from Sinopharm Chemical Reagent Co. Ultrapure water with a resistance of 18.2 MΩ cm and a pH of 6.7 at 25 °C was purchased from Wuxi Huawei Co. Ltd. (China). Methods. Dispersion of CaCO3 Nanoparticles in Aqueous Media. CaCO3 powder was weighed into a glass bottle of dimensions 7.5 cm (h) by 2.5 cm (d) followed by adding pure water or aqueous solutions of amphiphile. The particles were then dispersed using an ultrasound probe (JYD-650, Shanghai) of tip diameter 0.6 cm operating at an output of 50 W for 2 min while cooling the vessel in an ice bath. Scanning electron microscopy (SEM) of the powder was obtained using a Hitachi S4800 SEM at 3 kV and 40 000 magnification by placing the powder directly on a conductive film followed by spraying a thin film of gold. Zeta Potential of Aqueous Dispersions of CaCO3 Nanoparticles. 1 wt % CaCO3 nanoparticles were ultrasonically dispersed in pure water of different pH, adjusted by adding HCl or NaOH, as described above and the dispersions were left overnight at 25 °C. The zeta potentials were measured using a Malvern Zetasizer 2000 instrument at 25 °C. The zeta potentials of the particle dispersions (1 wt %) were also measured as a function of the concentration of added amphiphile. Dilution of the dispersion using the same aqueous phase as that used for preparing the dispersion was usually required. Preparation and Characterization of Emulsions. CaCO3 nanoparticles were initially dispersed with sonication in 7 cm3 pure water or aqueous amphiphile solution contained in a glass bottle of dimensions 7.5 cm (h) by 2.5 cm (d) as described above. Then 7 cm3 of toluene was added and the mixture was homogenized at 5000 rpm for 2 min using a XHF-D homogenizer (Ningbo Xinzhi) fitted with a dispersing tool of outer diameter equal to 1.4 cm. The particle and amphiphile

C¼

VNaOH  NNaOH Vs

ð2Þ

where Vs is the volume of supernatant solution sampled (cm3) and VNaOH and NNaOH are the volume and concentration of NaOH solution consumed in the titration. The concentration of sodium carboxylate in blank solutions (without particles) was similarly measured and taken as the initial concentration C0, with the recovery found to be 98.6% on average compared with the concentration calculated based on the mass of the sodium carboxylate. For AOT, the equilibrium concentrations were measured using either a spectrophotometric method (C < 1 mM) or a two-phase titration method (C g 1 mM) as described elsewhere.16 Contact Angles of Aqueous Sodium Carboxylate Solution on a CaCO3 Substrate in Air. A piece of stone with a CaCO3 content of 99 wt % from Tianli Construction Material Co. Ltd. (China) was polished to give a plane of approximately 5  3 cm2. The contact angle θ of a drop of aqueous sodium carboxylate solution (volume = 5  10
3 cm3) on this substrate in air was measured by means of an optical drop shape analyzer (Drop Meter-A-100, Ningbo HaishuMaishi Scientific Test Co.).25 For each concentration, the θ value is an average from at least 5 drops at different positions on the plane. After such measurements, the stone was 315

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Emulsions Stabilized by CaCO3 Nanoparticles Alone. Using toluene as oil and CaCO3 nanoparticles as sole emulsifier, the emulsions formed are all of the O/W type but not stable as expected. The appearance of the vessels after 1 week is shown in Figure S1 for various particle concentrations. Although the emulsions appear to be stable when viewed from the side, coalesced oil can be observed from the top of the vessels when tilted and some particles sediment, indicating that the raw particles (without any surface modification) are not particularly surface-active as a result of their inherent hydrophilicity. The droplet diameters are in general between 100 and 200 μm, as shown in Figure S2. Particularly at low particle concentration (e0.5 wt %) the droplet are so big that they can be observed by eye. They are not stable, however, and coalesce immediately once placed onto a glass slide. Emulsions Stabilized by Sodium Carboxylate Alone. Toluene
water emulsions were prepared with the series of sodium carboxylates, from C6Na to C12Na, as a function of concentration. The pH values of the sodium carboxylate solutions were observed to increase with an increase in both the concentration and alkyl length. The results, shown in Figure S3, show that for both C6Na and C8Na no stable emulsion is formed up to a concentration as high as 60 mM. By contrast, C10Na and C12Na can stabilize O/W emulsions at concentrations higher than a minimum concentration, Cmin. These emulsions cream with time but are stable to coalescence above Cmin. The four homologous anionic amphiphiles, although with similar molecular structure, have different surface activity. The C12Na is a typical surfactant of the soap type, which can stabilize O/W emulsions as expected at concentrations beyond Cmin, close to the critical micelle concentration (cmc), and the droplet size decreases in general with increasing concentration. The C10Na is less surface-active than C12Na, and a much higher Cmin is needed. On the other hand, C6Na and C8Na are not surface-active due to their lower chain lengths, and no stable emulsion can be formed as expected. Values of Cmin for C10Na and C12Na, together with their surface activity parameters and other parameters found for all the sodium carboxylates are listed in Table 1 where the measured cmc values of C10Na and C12Na are in good agreement with those reported in the literature.33 Emulsions Stabilized by CaCO3 Nanoparticle/Sodium Carboxylate Mixtures. Once sodium carboxylate coexists with CaCO3 nanoparticles in aqueous solution, the stability of the emulsions is greatly improved and multiple phase inversion may be observed, as illustrated in Figure 3. It is seen that 1 wt % CaCO3 nanoparticles leads to stabilization of toluene-in-water emulsions in the presence of C6Na over a wide range of concentration although no phase inversion was observed up to 60 mM. Since C6Na is not surface-active at the oil
water interface, the oil droplets are stabilized by C6Na-coated particles and the O/W emulsions here are denoted as O/W(1). If these vessels are tilted, coalesced oil no longer appears at concentrations of C6Na g 0.6 mM, and the emulsions are more viscous than those stabilized by particles alone. Also an obvious decrease of droplet size, from 100 to 200 μm without amphiphile to 30
60 μm with amphiphile, was observed as illustrated in Figure S2. In the presence of C8Na, stable O/W(1) emulsions which cream are obtained and phase inversion to a W/O emulsion is observed at the highest amphiphile concentration studied (60 mM). For C10Na, phase inversion can also be effected, but it occurs at much lower surfactant concentration (between 3 and 6 mM). Finally, a double phase inversion is observed in the case of C12Na, in which

Figure 1. SEM image of powdered CaCO3 nanoparticles.

Figure 2. Zeta potential of 1 wt % CaCO3 nanoparticles dispersed in water of different pH. polished and washed to give a fresh plane for the measurement of the next concentration. Measurement of Air
Water Surface Tension. The surface tension of amphiphile solutions was measured by means of the du No€uy ring method using an apparatus described elsewhere.24 All experiments were carried out at room temperature (25 °C) unless specified otherwise.

’ RESULTS AND DISCUSSION Characterization of CaCO3 Nanoparticles. Figure 1 shows a SEM image of the CaCO3 powder sample, which indicates that the particles are quasi-spherical with primary diameters between 80 and 120 nm. The specific surface area of 16.21 m2 g
1, measured using the BET technique, is in good agreement with the sizes illustrated in the image. CaCO3 nanoparticles are almost insoluble in water, but once dispersed in water the Ca
OH group can be formed on the particle surface, which may become either positively (Ca
OH2+) or negatively (Ca
O
) charged by combining with or releasing a proton, depending on the pH of the medium.25 Figure 2 shows the zeta potential of the particles in water as a function of pH, from which an isoelectric point around pH = 9.35 is found which is common for most CaCO3 types. At pH > 9.35 the particles are negatively charged, and the magnitude of the zeta potential increases with increasing pH. When dispersed in pure water, the pH of the dispersion is 8.93, indicating that the CaCO3 nanoparticles are positively charged in neutral water. 316

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Table 1. Surface Activity Parameters of Sodium Carboxylates and AOT Together with Other Parameters Related to Emulsion Phase Inversion and Adsorption at the Particle
Water Interface amphiphile

C8Na a

C12Na

AOT

cmc/mM

250

30

5.9

2.2

Γ∞(a/w)/mol cm
2 A(a/w)/nm2 molecule
1

2.82  10
10 a 0.59a

2.54  10
10 0.65

2.50  10
10 0.66

1.35  10
10 1.23

>30

>3

not measured

36.30

1.98

0.57

0.13

4.40  10
5

3.80  10
5

2.40  10
5

8.0  10
6

0.61

0.71

1.12

3.36

Cmin/mM CPI(1)/mM Γ(s/w) at CPI(1)/mol g
1 2


1

A(s/w) at CPI(1)/nm molecule

a

C10Na

CPI(2)/mM

>100

5.0

Γ(s/w) at CPI(2)/mol g
1

>4.8  10
3

1.10  10
4

A(s/w) at CPI(2)/nm2 molecule
1

98% CaCO3) to give a plane of 5  3 cm2, and the contact angle θ measured through the aqueous phase is an average of at least 5 sessile drops placed at different positions on the substrate. The standard deviation in measuring θ is usually no more than 5%. Once the concentration of the solution is changed the surface is polished and washed again to give a fresh surface. It is found that the contact angle of pure water on the surface is nearly 10°, in good agreement with the inherent

Figure 3. Digital photographs of vessels containing toluene
water emulsions stabilized by CaCO3 nanoparticles (1 wt %) and sodium carboxylate: (A) C6Na, (B) C8Na, (C) C10Na, and (D) C12Na at different concentrations, taken 1 week after preparation. Concentration from left to right: (A, B, and C) 1, 3, 6, 10, 30, 60 mM; (D) 1, 3, 10, 30, 100, 300 mM. O/W emulsions cream, W/O emulsions sediment.

O/W(1) f W/O f O/W(2), with the first inversion occurring below 3 mM and the second taking place above 100 mM. The second O/W(2) emulsion is believed to be stabilized mainly by surfactant molecules16 and was observed also in CaCO3/SDS emulsions. We thus see that inversion to oil continuous emulsions is promoted as the chain length of the fatty acid is increased. In order to quantitatively evaluate the O/W(1) f W/O phase inversion, the equilibrium concentration at phase inversion, CPI(1), was determined by measuring the equilibrium amphiphile concentration in the resolved aqueous phase of the last O/W(1) emulsion before inversion separated 1 week after homogenization. These values are listed in Table 1. It is found that CPI(1) values decrease successively from C8Na to C12Na and are approximately 1/10 of their cmc’s. Here CPI(1) represents the concentration of CnNa required to ensure particle surfaces are hydrophobic enough to stabilize a W/O emulsion. Similarly, CPI(2) was estimated by measuring the equilibrium concentration in the aqueous phase of the first O/W(2) emulsions separated 1 week after homogenization, and values are also listed in Table 1. We note that in all O/W(1) emulsions no coalescence of oil droplets is observed over a few months. Figure 4 gives representative optical micrographs of the various emulsions, showing that droplets are spherical and polydisperse and that the droplet size in O/W(2) emulsions is in general smaller than that in O/W(1) and W/O emulsions. Mechanism of in Situ Surface Activation of CaCO3 Nanoparticles via Interaction with Sodium Carboxylates. Since the 317

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Figure 4. Micrographs of toluene
water emulsions stabilized by a mixture of 1 wt % CaCO3 nanoparticles and sodium carboxylate at different concentrations taken 1 week after preparation. (A) C6Na, 2 mM, O/W(1); (B) C6Na, 30 mM, O/W(1); (C) C8Na, 0.2 mM, O/W(1); (D) C8Na, 20 mM, O/W(1); (E) C10Na, 0.1 mM, O/W(1); (F) C10Na, 10 mM, W/O; (G) C12Na, 0.3 mM, O/W(1); (H) C12Na, 60 mM (W/O); (I) C12Na, 300 mM, O/W(2).

Figure 6. Adsorption isotherms of sodium carboxylates at the CaCO3 nanoparticle
water interface at 25 °C. Figure 5. Zeta potentials of 1 wt % CaCO3 nanoparticles dispersed in aqueous sodium carboxylate solutions of different concentration at 25 °C.

the O/W(1) f W/O phase inversion takes place once the particle surface is modified to a particular hydrophobicity, irrespective of the concentration and molecular structure of the amphiphile. The contact angle in the case of C6Na never reaches this value, in line with the fact that no phase inversion is observed. Theoretically, the O/W(1) f W/O phase inversion takes place when the contact angle of the particles at oil
water interface increases to beyond 90°.1
3 All the angles measured here will be higher at the oil
water compared with the air
water interface. On the other hand, the cross-sectional area, A(s/w), of the adsorbed carboxylate molecules at the solid
water interface just before the O/W(1) f W/O phase inversion can be estimated based on the measured CPI(1) and the adsorption isotherms together with the specific surface area of the particles (16.21 m2/g). As seen in Table 1, the values are found to be smaller than the

hydrophilicity of the particles. For solutions of carboxylates over a wide range of concentration, θ increases with increasing concentration, suggesting that the adsorbed amphiphile molecules form a monolayer with their headgroups toward particle surfaces and their hydrocarbon chains toward water. This configuration renders particles more hydrophobic such that the CaCO3 nanoparticles are surface activated in situ and become good emulsion stabilizers. The values of the surfactant concentration CPI(1) are displayed in Figure 7 as vertical broken lines. It is interesting to note that these lines cross the contact angle curve at almost the same value of θ, 25°, shown by the horizontal line. This indicates that 318

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effective emulsifiers. If a monolayer forms at the particle
water interface via electrostatic interaction of the headgroup, particles are rendered partially hydrophobic. This is responsible for their surface activation with the efficiency of the amphiphile depending on its chain length. Sodium hexanoate (C6Na) is effective in surface activation of CaCO3 nanoparticles but cannot induce emulsion phase inversion from O/W(1) f W/O due to the low hydrophobicity of the coated particles. Sodium octanoate (C8Na), sodium decanoate (C10Na), and sodium dodecanoate (C12Na) are more and more effective: they all greatly improve the stability of toluene-in-water emulsions and induce O/W(1) f W/O phase inversion; the concentration at which inversion takes place decreases with increasing alkyl chain length. At high amphiphile concentration, chain
chain interactions may result in bilayer or hemimicelle formation at the particle
water interface, which renders particles hydrophilic again enhancing their desorption from the oil
water interface. A second phase inversion, W/O f O/W(2), may be achieved when the amphiphile is sodium dodecanoate or AOT which are highly surface-active and stabilize the O/W(2) emulsion alone. Double phase inversion, O/W(1) f W/O f O/W(2), is thus achieved. AOT is the most efficient for the in situ surface activation of the CaCO3 nanoparticles and in inducing double phase inversion due to its twin-tailed structure.

Figure 7. Contact angles of aqueous sodium carboxylate drops on planar CaCO3 substrates in air at 25 °C. Vertical lines represent CPI(1) values. Horizontal line is at θ = 25°.

corresponding cross-sectional area at the air
water interface, A(a/w), determined from surface tension measurements (see Figure S4). We conclude therefore that only monolayer adsorption occurs at the solid
water interface when the O/W(1) f W/O phase inversion takes place. The second phase inversion, W/O f O/W(2), was only observed for the C12Na system at high amphiphile concentration (see Table 1). It was revealed16 that a bilayer or hemimicelle adsorption of the amphiphile at the particle
water interface is responsible for the second phase inversion. Because of this enhanced adsorption, the surface of the particles become hydrophilic again, inducing desorption of the particles from the oil
water or air
water25 interfaces and dispersing them in the aqueous phase. The bilayer or hemimicelle adsorption relies on chain
chain interactions of the amphiphile molecules and the ability to form micelles. It is known that C6Na and C8Na are not surface-active at liquid interfaces and do not form micelles at concentrations up to 60 mM. C10Na is slightly surface-active, but the chain
chain interaction is not strong enough for bilayer formation at the concentrations investigated. Only C12Na is able to form such a bilayer on particle surfaces. This is confirmed by the contact angle data in Figure 7 where the values for C6Na, C8Na, and C10Na amphiphiles increase monotonously with increasing concentration, whereas that of C12Na displays a maximum. The mechanism of the second phase inversion is further confirmed by using AOT as amphiphile, as shown by the data given in Table 1 and the adsorption isotherm and photographs given in Figures S5
S7. It is believed that the chain
chain interaction between AOT molecules in aqueous solution is much stronger than that between C12Na molecules due to the twintailed structure of the molecule. Indeed, the CPI(2) corresponding to W/O f O/W(2) phase inversion is found to be ∼5.0 mM, just beyond the cmc of 2.2 mM, whereas that for C12Na is estimated to be higher than 100 mM, much higher than its cmc of 6 mM. Moreover, CaCO3 nanoparticles are found to be well dispersed in AOT solutions at concentrations approaching the cmc, whereas they are not well dispersed in C12Na solutions (Figure S7). Nevertheless, in both cases the A(s/w) at CPI(2) is smaller than A(a/w), suggesting bilayer or hemimicelle adsorption at the particle
water interface.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1
S7. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.-G.C.); b.p.binks@hull. ac.uk (B.P.B.).

’ REFERENCES (1) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (2) Dickinson, E. Curr. Opin. Colloid Interface Sci. 2010, 15, 40. (3) Binks, B. P.; Horozov, T. S. In Colloidal Particles at Liquid Interfaces; Binks, B. P., Horozov, T. S., Eds.; Cambridge University Press: Cambridge, 2006; Chapter 1. (4) Binks, B. P.; Murakami, R. Nature Mater. 2006, 5, 865. (5) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708. (6) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lamic, E.; Duguet, E. J. Mater. Chem. 2005, 15, 3745. (7) Cayre, O.; Paunov, V. N.; Velev, O. D. J. Mater. Chem. 2003, 10, 2445. (8) Paunov, V. N.; Cayre, O. Adv. Mater. 2004, 16, 778. (9) Hong, L.; Jiang, S.; Granick, S. Langmuir 2006, 22, 9495. (10) Lattuada, M.; Hatton, T. A. Nano Today 2011, 6, 286. (11) Gelot, A.; Friesen, W.; Hamza, H. A. Colloids Surf. 1984, 12, 271. (12) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Langmuir 2006, 22, 10983. (13) Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 3626. (14) Lan, Q.; Yang, F.; Zhang, S.; Liu, S.; Xu, J.; Sun, D. Colloids Surf., A 2007, 302, 126. (15) Binks, B. P.; Rodrigues, J. A. Angew. Chem., Int. Ed. 2007, 46, 5389. (16) Cui, Z.-G.; Shi, K.-Z.; Cui, Y.-Z.; Binks, B. P. Colloids Surf., A 2008, 329, 67.

’ CONCLUSIONS Raw CaCO3 nanoparticles can be surface activated in situ by adsorbing anionic amphiphiles from water rendering them 319

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(17) Ravera, F.; Ferrari, M.; Liggieri, L.; Loglio, G.; Santini, E.; Zanobini, A. Colloids Surf., A 2008, 323, 99. (18) Zhang, S.-Y.; Lan, Q.; Liu, Q.; Xu, J.; Sun, D.-J. Colloids Surf., A 2008, 317, 406. (19) Zhang, S.-Y.; Sun, D.-J.; Dong, X.-Q.; Li, C.; Xu, J. Colloids Surf., A 2008, 324, 1. (20) Binks, B. P.; Kirkland, M.; Rodrigues, J. A. Soft Matter 2008, 4, 2373. (21) Zhou, W.; Cao, J.; Liu, W.; Stoyanov, S. Angew. Chem., Int. Ed. 2009, 48, 378. (22) Binks, B. P.; Rodrigues, J. A. Colloids Surf., A 2009, 345, 195. (23) Liu, Q.; Zhang, S.; Sun, D.; Xu, J. Colloids Surf., A 2010, 355, 151. (24) Cui, Z.-G.; Yang, L.-L.; Cui, Y.-Z.; Binks, B. P. Langmuir 2010, 26, 4717. (25) Cui, Z.-G.; Cui, Y.-Z.; Cui, C.-F.; Chen, Z.; Binks, B. P. Langmuir 2010, 26, 12567. (26) Pichot, R.; Spyropoulos, F.; Norton, I. T. J. Colloid Interface Sci. 2010, 352, 128. (27) Binks, B. P.; Fletcher, P. D. I.; Tian, L. Colloids Surf., A 2010, 363, 8. (28) Limage, S.; Kr€agel, J.; Schmitt, M.; Dominici, C.; Miller, R.; Antoni, M. Langmuir 2010, 26, 16754. (29) Vashisth, C.; Whitby, C. P.; Fornasiero, D.; Ralston, J. J. Colloid Interface Sci. 2010, 349, 537. (30) Santini, E.; Kr€agel, J.; Ravera, F.; Liggieri, L.; Miller, R. Colloids Surf., A 2011, 382, 186. (31) Bastrzyk, A.; Polowczyk, I.; Sadowski, Z.; Sikora, A. Sep. Purif. Technol. 2011, 77, 325. (32) Ni, Y.-B.; Gu, L. Chin. J. Blood Transfus. 1994, 7, 123. (33) Zhao, G.-X.; Huang, J.-B. China Surfactant Deterg. Cosmet. 1992, 22, 7. (34) Hou, Z.-S.; Li, Z.-P.; Wang, H.-Q. Chem. Res. Appl. 2000, 12, 39.

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dx.doi.org/10.1021/la204021v |Langmuir 2012, 28, 314–320