Colloids and Surfaces A: Physicochem. Eng. Aspects 400 (2012) 44–51
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Review
Oil-in-water emulsions stabilized by Laponite particles modified with short-chain aliphatic amines Wei Li a , Lijie Yu a , Guopeng Liu a , Junjun Tan a,b , Shangying Liu a , Dejun Sun a,∗ a b
Key Laboratory for Colloid & Interface Chemistry of Education Ministry, Shandong University, Jinan, Shandong, 250100, People’s Republic of China School of Chemical and Environmental Engineering, Hubei University of Technology, Wuhan, Hubei, 430068, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 November 2011 Received in revised form 15 January 2012 Accepted 24 February 2012 Available online 5 March 2012 Keywords: Pickering emulsion Laponite particles Emulsion stability Amine Adsorption
a b s t r a c t Liquid paraffin-in-water emulsions stabilized by Laponite RD clay particles modified with short-chain aliphatic amines (diethylamine, DEA or triethylamine, TEA) were prepared in the absence of electrolytes. The formation and stability of the emulsions were investigated by macroscopic and microscopic observations, contact angle, zeta potential and rheology measurements. The hydrophilic Laponite particles are rendered partially hydrophobic by the in situ surface modification with DEA and TEA. Contact angles, zeta potentials, and emulsion stability reach a plateau above a certain amine concentration, implying the full coverage of amine molecules on Laponite surface. At low clay concentration (0.5 wt%), stable emulsions can only be obtained when the clay–amine suspension is flocculated. And the attachment of the amine-modified particles on the droplet surface results in emulsion stabilization, which was observed by laser-induced fluorescent confocal micrographs. However, at high clay concentration (4.0 wt%), extremely stable emulsions form even the clay–amine suspension remains unflocculated. In this case, the emulsion stability can be attributed to the particle shell of the droplet, and more importantly, the gel structure of the suspension. © 2012 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. In situ modification of Laponite with short-chain aliphatic amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Three-phase contact angle measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Zeta potential measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Stability of the Laponite/short-chain amine aqueous suspensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5. Preparation of emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6. Stability and characterization of emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.7. Observation of particles at the emulsion droplet surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.8. Rheology measurements of concentrated Laponite–amine suspensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. The variation of surface wettability of Laponite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Properties of Laponite/short-chain aliphatic amine aqueous suspensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Emulsions stabilized by Laponite particles modified by short-chain aliphatic amine at low particle concentration . . . . . . . . . . . . . . . . . . . . . . 3.4. Adsorption of particles on the droplet surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Emulsions stabilized by the 4.0 wt% amine-modified Laponite particles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding author. Tel.: +86 531 88364749; fax: +86 531 88365437. E-mail address:
[email protected] (D. Sun). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.02.044
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1. Introduction Emulsions stabilized by colloid particles, commonly named Pickering emulsions, have been studied extensively for their widespread use in practical applications, such as food, pharmaceutics, cosmetics, oil recovery and wastewater treatment since the beginning of 20th century [1]. In many of these applications, the stability of emulsions is necessary to keep their properties over long periods of time. Up to now, various theories about emulsions stabilized by solid particles have been developed and recently a thorough understanding of them has been achieved. According to these theories, particle wettability is a crucial parameter which determines the type and stability of the emulsions [2–4]. Stable emulsions are prepared with particles of intermediate wettability. When the contact angles of the particles are slightly smaller than 90◦ or slightly greater than 90◦ , stable oil-in-water (o/w) emulsions or water-in-oil (w/o) emulsions will be produced respectively. A wide variety of solid materials have been used as particulate emulsifiers such as silica, clay, alumina, barium sulfate, and calcium carbonate [5]. To meet the need of appropriate wettability for stability of emulsions, the particles are usually modified by chemical surface treatment (e.g., silanization of silica particles) [2,5–9] or via adsorption of long-chain amphiphilic molecules such as surfactants and cosurfactants [10–12]. Cationic, anionic, and nonionic surfactants have all been applied successfully to tailor the wettability of particles in liquids and thus prepare stable emulsions [10–14]. Emulsion stability usually enhances with the particles concentration [2,5,15,16]. However, in the above cases, the concentrations of the particles modified by the longchain amphiphilic molecules are usually limited as a result of the formation of amphiphile micelles and particle agglomeration when used in stabilizing emulsions. Excellent stability of emulsions was achieved when amphiphile concentrations are high enough to accomplish the surface hydrophobization and sufficiently low to avoid particle agglomeration in the initial suspension [17]. The problem discussed above becomes more complex in clay particles stabilized emulsions, for aqueous clay suspensions gel above a certain concentration, depending on the clay type and ionic strength [18,19]. Meanwhile, it is a little surprise that very few studies exist in which clay particles have been used as stabilizers in preparing emulsions, though these materials have been widely used in various fields for a long history, and there have been multitude of academic works in literature [18,20–27]. Among them, Laponite received much more attention recently due to its high purity and having a small and uniform particle size [28–30]. Laponite is a smectite clay of the hectorite type. It forms clear suspensions in water containing discrete particles similar to circular discs of diameter ca. 30 nm and thickness ca. 1 nm [31]. Laponite particles have been used as stabilizers in preparing Pickering emulsions. Ashby et al. [32] have found that stable emulsions are only formed under conditions where the Laponite are flocculated via salt. Whitby et al. [33] have prepared emulsions by homogenizing Laponite aqueous suspensions with oil solutions containing lipophilic surfactants. Recently, Ding et al. have shown the short amines may improve the affinity of Laponite particles to partly hydrolyzed alkenyl succinic anhydride and favor emulsification in paper making process [34]. Poly(ethylene imine) [35] and poly(oxypropylene) diamines [36] have been utilized to modify Laponite nanoparticles in order to produce an effective Pickering emulsifier. Although it is wellknown that the emulsion stability usually enhances with the particle concentration, a restriction was found for Laponite stabilized emulsions. The aqueous Laponite suspension gels above 2.0 wt% and emulsification can be hindered where strong attractive interactions between particles gel the suspension. In the
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preparation of emulsions stabilized by concentrated Laponite particles flocculated with salt, the authors found that emulsions could not be produced with Laponite concentrations above 3.5 wt% [32,33]. Herein, we introduce short-chain aliphatic amines, diethylamine (DEA) and triethylamine (TEA) as surface modifying agents for they can adsorb onto the particles surface but do not selfassemble into aggregated structures in bulk solution [37–39]. Furthermore, it has been found that addition of amine molecules results in decrease of viscosity of aqueous Laponite suspensions, which may facilitate emulsification. Given that circumstances, we suppose it would be possible to prepare emulsions with excellent stability in highly concentrated Laponite suspensions with modification of short-chain aliphatic amines. In this work, we first investigated the increase of the contact angle and zeta potential of the particles induced by the adsorption of two kinds of short-chain aliphatic amines on the solid surface. At low particle concentration, the in situ modification by our short-chain aliphatic amines results in the particle agglomeration but also brings about the good stability of the emulsions. It was further demonstrated that by adding small amount of amines, extremely stable emulsions could be prepared in concentrated suspensions even without particle flocculation. 2. Experimental 2.1. Materials Laponite RD, a synthetic hectorite, was supplied by Rockwood Additives Ltd. (UK) as a white powder. It is composed of rigid disk-shaped crystals with a well defined thickness of 1 nm and an average diameter about 30 nm. The empirical formula of Laponite is Na0.7 [(Si8 Mg5.5 Li0.3 )O20 (OH)4 ]. Within a single crystal of the Laponite particles, each sheet of octahedrally coordinated magnesium (as Mg oxide) is sandwiched between two silicate layers in a tetrahedral coordination environment with oxygen atoms. Some of the Mg2+ sites of the central layer are substituted with Li+ cations. This creates a charge imbalance, which is compensated by Na+ counterions located at the surface of the outer layers. The particles are negatively charged due to the release of Na+ ions on the particle surface. The short-chain aliphatic amines used in this work were diethylamine and triethylamine, supplied by Sinopharm Chemical Reagent Co., China. The general chemical formula of DEA is NH(CH2 CH3 )2 and the formula for TEA is N(CH2 CH3 )3 . The oil material used in this work was paraffin oil (Yongda Chemical Reagent Co., China) with purity greater than 99% (d420 = 0.835–0.855). The viscosity of liquid paraffin is 20.8 mpa s at 25 ◦ C [40]. The compositions are mainly isoalkane and the main carbon number distribution measured with Agilent 6820 GC (Agilent Co., USA) is between 16 and 26. The water used in experiments was double-distilled deionized water. 2.2. Methods 2.2.1. In situ modification of Laponite with short-chain aliphatic amines The Laponite stock suspension was prepared by dispersing a known mass of Laponite into deionized water using a Multimixer (Baroid Co., USA). The Laponite suspensions were sealed and laid aside at room temperature (25 ◦ C) for 2 weeks before use. The Laponite/amine suspensions were prepared by diluting the stock suspensions with clear DEA or TEA solutions and the pH was adjusted to 10.5 with NaOH or HCl. The prepared suspensions were stirred for at least 24 h to attain adsorption equilibrium.
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2.2.2. Three-phase contact angle measurements The contact angles of the modified particles were evaluated by using mica to mimic the Laponite particles’ surface because of the difficulties encountered in measuring the contact angles of particulate materials. Mica is analogous in both chemical composition and crystal structure to many layered clay minerals [41]. The mica flakes were immersed in amine aqueous solutions covering a certain range of concentrations (from 3 to 500 mM) at pH 10.5 for 24 h to allow amine molecules to adhere. Then the mica flakes were rinsed with water and dried in a vacuum. The treated mica flake was placed at the bottom of an open, transparent glass vessel filled with paraffin oil. The contact angles of sessile water drops (20 mL) under liquid paraffin were determined (Tracker, I.T.Concept, France). This procedure is similar to the one reported by Binks et al. [11,13]. The water drop was photographed and the contact angles were obtained with an appropriate software. 2.2.3. Zeta potential measurements The zeta potentials of the particles were measured with a JS94H microelectrophoresis instrument (Shanghai Zhongchen Digital Technic Apparatus Co., China). The particles in the sediment phase were diluted with the upper clear liquid before measurement. 2.2.4. Stability of the Laponite/short-chain amine aqueous suspensions The prepared suspensions were first transferred into a stoppered, graduated glass tube with internal diameter 1.6 cm and length 15 cm. Then the phase behavior of the suspensions was observed after 24 h.
Fig. 1. Variation of contact angle on mica as a function of amine concentration at 25 ◦ C.
2.2.8. Rheology measurements of concentrated Laponite–amine suspensions Viscosity measurements were performed using RS75 rheometer (Haake Co., Germany) with a cone-plate sensor system (model C60/0.5◦ ). We measured the apparent viscosity of the suspensions at the constant shear rate of 1000 s−1 because it is approximately equal to the shear rate during the process of emulsifying. The changing viscosity as a function of time was observed and the finally constant value was recorded. 3. Results and discussion
2.2.5. Preparation of emulsions All the emulsions were prepared by slowly adding oil to Laponite/amine suspensions, and then emulsifying the mixed liquids with a lab homogenizer (Shanghai Forerunner M&E Co., China) operated at 6000 rpm for 5 min at 25 ◦ C. The oil volume fraction was fixed at 0.5. All the suspension and emulsion tests were performed at 25 ◦ C. 2.2.6. Stability and characterization of emulsions The emulsions were transferred into the glass vessels mentioned in step 2.2.4 for observation of emulsion stability at 25 ◦ C. The stability of the emulsions to creaming and coalescence was assessed 24 h after preparation by monitoring the positions of the water-emulsion and emulsion-oil interfaces respectively. The volume fraction of emulsion phase was taken after 24 h of preparation. The morphology of emulsion droplets was observed with an Axioskop 40 optical microscope (ZEISS, Germany). The average size of emulsion droplets was obtained by processing the image using the microscopic image analysis software. 2.2.7. Observation of particles at the emulsion droplet surfaces The adsorption of Laponite particles at the droplet surfaces was observed under a laser-induced confocal microscope (Olympus Fluoview 500, Japan). The Laponite particles were labeled with Auramine O through electrostatic attractive interactions. Auramine O is positively charged in basic solution and has a maximum excitation wavelength at 505 nm. The concentration of Auramine O in the mixed aqueous suspension was fixed at 1.0 × 10−5 M. Then the Laponite particles modified with short-chain aliphatic amines were washed with deionized water to remove the unadsorbed Auramine O molecules in the bulk. Emulsions stabilized by the suspensions of labeled particles were prepared and the fluorescent images of the emulsion droplets were observed under the microscope.
3.1. The variation of surface wettability of Laponite Laponite was hydrophobized due to the adsorption of shortchain aliphatic amine molecules. We have sought to assess the change in the surface wettability of hydrophilic Laponite following amine adsorption by measuring the contact angles of mica surface with increased adsorbed amines. In Fig. 1, the contact angles are plotted as a function of the initial concentration of amine in the water phase. With the increase of amine concentration, the contact angle increases rapidly in the beginning and then reaches a plateau. The increased contact angle suggests the increase in oil wettability. The plateau appears at 100 mM for both DEA and TEA, from which point the contact angles do not change anymore. This means that the contact angle reaches maximum at the adsorption saturation point of the adsorbed amine layer. It should be pointed out that the real contact angle for Laponite particle surface with the same adsorbed amines is different from the value measured on mica surface, because the two materials have distinct surface composition and surface charge density. But the trends that the contact angle varies with amine adsorption would be similar. It can be inferred that the adsorption of amines on the Laponite particle surface increases oil wettability and the contact angle reaches maximum at full coverage. Besides, the contact angle is larger for TEA than DEA at the same amine concentration. This is own to the additional C2 H5 of each TEA molecule, which makes TEA modified surface more hydrophobic than those modified with DEA. The increase in wetting by oil is in line with our prediction that the adsorption of amine molecules results in flocs of increased hydrophobicity that can strongly attach to oil–water interfaces, allowing the stabilization of emulsions. There may be two kinds of driving forces for adsorption here, electrostatic attraction and H-bonds interaction. Part of the short-chain aliphatic amine molecules are protonated in aqueous
W. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 400 (2012) 44–51
Fig. 2. Zeta potential of Laponite/short-chain aliphatic amine aqueous suspensions at pH 10.5. The particle concentration in the suspensions is 0.5 wt%.
suspensions. From the pKa value of the amines (10.8 for DEA and 10.7 for TEA) and pH of the aqueous suspensions, it can be calculated that about 60% of the short-chain aliphatic amine molecules are protonated in aqueous suspensions [42]. Therefore, the amine molecules can adsorb on the surface of the Laponite by the electrostatic force between negatively charged particles and protonated amine molecules, and the H-bonds interaction between the silanol groups of Laponite surface and the headgroups of amine molecules [43,44]. This is similar to the case that hexylamine molecules adsorbed on the surface of Laponite or silica [45,46]. 3.2. Properties of Laponite/short-chain aliphatic amine aqueous suspensions The adsorption of amine molecules on Laponite will change the properties of Laponite/amine suspensions. The variations of the zeta potential of the Laponite particles are shown in Fig. 2. The potential of the pure particles is approximately −59 mV at pH 10.5. Upon adding small amount of DEA or TEA, the potential decreases in magnitude sharply. The reasons of the reduction of potential may be as follows. First, the headgroups of amine molecules are able to be protonated in aqueous suspensions, making it positively charged. The protonated amine molecules adsorb on the negatively charged particle surface through electrostatic attractive interactions and electrostatic neutralization can lead to the decrease of potential [47]. Secondly, the surface charge of modified Laponite is screened by DEA or TEA molecules adsorbed on the particle surface. As a result of this screening effect, the
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thickness of the electrical double layer around Laponite is reduced [48]. Moreover, clusters of bridging flocculation induced by the adsorption of amines have less mobility compared with single particles [49]. A plateau appears after 40 mM. The phenomenon is similar to the contact angle measurements on mica surface, but with different threshold value. It can also be inferred that the Laponite surface is monolayer-saturated at 40 mM. The plateau value is about −25 mV for DEA and −15 mV for TEA respectively. Larger particle aggregates may form in Laponite-TEA suspensions because of the more hydrophobic nature of TEA, which will lead to the less mobility in the same electric field. The adsorption of amine molecules on Laponite also affects the stability of the aqueous suspensions. The photographs of the mixed suspensions of Laponite and short-chain aliphatic amines are shown in Fig. 3. At a fixed particle concentration of 0.5 wt%, addition of DEA or TEA leads to a dramatic change in the stability of the suspensions. With the addition of a low concentration of amines, flocculations can be seen. With the further increasing of the amine concentration, the flocs sedimentate quickly and become more compact in both systems. The tightest sediment appears around 40 mM, which is consistent with potential measurements. The variation of the suspension stability can be related to the adsorption of amine molecules on the particles. The adsorbed amine molecules lead to the decreased potential and the increased hydrophobicity for Laponite particles, which bring about flocculations. 3.3. Emulsions stabilized by Laponite particles modified by short-chain aliphatic amine at low particle concentration To study the influence of amine-modified particles on the type and stability of emulsions, equal volumes of paraffin oil are mixed with the aqueous suspensions described above. Either Laponite or short-chain aliphatic amines (DEA and TEA) alone are poor emulsifiers. Attempts to prepare stable emulsions with Laponite or short-chain aliphatic amines alone were all failed and completely phase separation of oil and water can be observed soon after emulsification. In contrast, when it comes to the Laponite modified with amine, oil-in-water emulsions with good stability form. All the emulsions are o/w and the addition of DEA or TEA leads to critical effect to their stability to creaming and coalescence. The variation in emulsion volume fraction with the amine concentration is remarkable, as shown in Fig. 4. Both the volume fraction of emulsion increase rapidly in the beginning with the increase of amine concentration, which means the enhanced stability of emulsions stabilized by amine-modified particles. Laponite is too hydrophilic to be held at the oil/water interface [32,36]. The adsorption of amine molecules brings about the enhanced hydrophobicity of modified Laponite. The enhanced hydrophobicity of modified Laponite is
Fig. 3. Photographs of vessels after 24 h containing aqueous suspensions of Laponite particles (0.5 wt%) at pH 10.5 as a function of DEA (a) or TEA (b) concentration (given in mM) at 25 ◦ C.
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W. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 400 (2012) 44–51
Fig. 4. Emulsion volume fraction as a function of short-chain aliphatic amine concentration obtained from photographs of o/w emulsion prepared by 0.5 wt% amine-modified Laponite particles at different amine concentrations 24 h after preparation.
reflected by the increased contact angle, which rapidly increases with the increasing amine concentration (Fig. 1). The increase of volume fraction of emulsions does not cease until the appearance of a plateau. A marked maximum in stability occurs at 40 mM for amine-modified Laponite, after which the volume fraction of emulsions does not change any longer. The effect of amine concentration on mean droplet diameters of emulsions stabilized by the suspensions of 0.5 wt% Laponite modified with TEA or DEA is shown in Fig. 5. The mean droplet size could reach 100 m in the absence of DEA and TEA. Emulsions with so large drop size are unstable. At the beginning, even very small amount of DEA and TEA can cause the sharp fall of the curve. Then the further increasing amine concentration leads to the gradually decrease of mean droplet size. Finally the mean diameter of droplet reaches a plateau. The appearances of droplets shown in the optical microscopy images can reflect this change more intuitionisticly (Figs. S1 and S2). The variation of short-chain aliphatic amines concentration on droplet diameters can be linked to the stability of emulsions stabilized by the amine-modified Laponite. The enhancement of emulsion stability is consistent with the reduction of mean droplet size. The plateau of the mean droplet size can be linked to the final unchanged volume fraction of emulsions (Fig. 4).
Comparing the emulsions stabilized by TEA-modified Laponite with those by DEA-modified Laponite, the former always owns smaller mean droplet size and higher emulsion volume fraction at the same concentration of amine in the research range (Figs. 4 and 5). It means that emulsions stabilized by TEA-modified Laponite possess higher stability. This is associated with the difference of hydrophobicity of the modified particles [34]. It has been referred above that the adsorption of DEA or TEA on the surface of particles contributes to moderate hydrophobicity of Laponite, which can be reflected by the variation of the contact angles. Owing to the additional C2 H5 of each TEA molecule, the contact angle is always closer to 90◦ for TEA than DEA at the same concentration (Fig. 1). Therefore, the Laponite particles modified by TEA take on more suitable surface hydrophobicity to attach to the oil/water interface and the better stabilization of emulsions is achieved. Excellent emulsion stabilization was achieved when the particles are weakly flocculated. This is a common finding in particle-stabilized emulsions. Many systems have been investigated to confirm this phenomenon. Midmore [12] described the preparation of stable o/w emulsions with colloidal silica particles in the presence of the water-soluble hydroxypropylcellulose. Binks et al. [10,11,32,50] examined the effect of the silica particles flocculated by electrolytes, CTAB or SDS on the emulsion stability, and deduced that stable o/w emulsions of toluene, water and Laponite RD clay particles are only formed when the particles are flocculated (via salt). Our previous work [51] also referred to the enhanced stability of emulsions induced by the layered double hydroxides particle flocculations. In this work, Laponite flocculations induced by adsorption of short-chain aliphatic amines have a notable influence on the stability of emulsions. As discussed in Section 3.2, the flocculation appeared and became tighter upon adding short-chain aliphatic amines (Fig. 3). Finally it reached a plateau at 40 mM, where the maximum of emulsion volume fraction and the smallest droplet size also appeared. It seems that emulsions of best stability are formed from the most unstable particle suspensions. 3.4. Adsorption of particles on the droplet surfaces The adsorption of particles on droplet surfaces plays a significant role in the high emulsion stability studied here. Laser-induced confocal scanning microscopy experiments were performed to confirm the adsorption of particles on the emulsion droplet surfaces, as shown in Fig. 6. Obviously, labeled Laponite particles (green) adsorb on the emulsion droplet surfaces. These adsorbed particles and particle aggregates create a more or less steric sheath around the droplets. When the emulsion droplets approach and collide with each other, the steric protective sheaths are able to impede coalescence. In addition, there are also free particles in the surrounding continuous phase. 3.5. Emulsions stabilized by the 4.0 wt% amine-modified Laponite particles.
Fig. 5. Mean diameter of o/w emulsion droplets prepared by 0.5 wt% Laponite particles modified with short-chain aliphatic amine at different concentrations at pH 10.5.
It has been well accepted that the gel structure of the particles around the droplets in Pickering emulsions enhances emulsion stability [2,5,52]. However, some authors reported that emulsification could be hindered where strong attractive interactions between particles gel the particle suspension, and emulsions can only be produced with particle concentrations below 3.5 wt% in the Laponite suspensions flocculated by electrolytes [32]. In this work, we attempted to prepare emulsions in highly concentrated Laponite suspensions with the addition of amines. The Laponite concentration was fixed at 4.0 wt%. We first discuss the effect of adding amines on Laponite suspensions and then the emulsification of these amine/Laponite suspensions with the oil phase is described. The stability of the aqueous suspensions of Laponite
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Fig. 6. Confocal fluorescence microscope images of liquid paraffin–water (1:1 by volume) emulsions stabilized by Laponite modified with (a) 20 mM DEA and (b) 20 mM TEA immediately after preparation. The Laponite concentration in all aqueous suspensions is 0.5 wt%. (For interpretation of the reference to color in the text for Fig. 6, the reader is referred to the web version of the article.)
particles was monitored after mixing with DEA and TEA. The photographs are shown in Fig. 7. Laponite particles are well dispersed in the absence of amines. With the addition of DEA or TEA, the transparent gel become slightly turbid, which means the occurrence of flocculations. It can be contributed to the increased hydrophobicity of Laponite and the decrease of electrostatic repulsion among the particles caused by the adsorption of amine molecules. But as a result of the high viscosity of the suspensions, the flocs do not sedimentate even at very high amine concentrations after 24 h. The addition of amine molecules also affects the viscosity of suspensions. As seen in Fig. 8, for the concentrated Laponite suspensions, the addition of amine molecules results in decrease of viscosity. The lower viscosity during the process of emulsification is helpful for oil to enter the internal structure of the bulk phase and form emulsions. Therefore, with the enhanced particle hydrophobicity and reduced suspension viscosity, emulsification in the concentrated Laponite suspensions may be realized. We have prepared emulsions using the corresponding aqueous suspensions of Laponite modified with amines studied above. The appearance of the emulsions 24 h after preparation is shown in Fig. 9. Without amines, the oil phase separates immediately after emulsification with part of it trapped in the lower suspension. When amines are added at low concentrations as 10 mM in
both systems, the similar phase separation of oil and water can be observed immediately. That may be due to the shortage of amine molecules to modify the particles and meet the need of appropriate wettability for stability of emulsions. The incomplete emulsification is also observed at 20 and 40 mM for TEA. The curious phenomenon may be related to the internal structure difficult to break formed under these conditions. By adding amines to a certain amount, we prepared emulsions with excellent stability to creaming and coalescence. There is no oil released and aqueous phase separated within a time period of 6 months after emulsification. This kind of stability is of utmost importance to keep the properties of emulsions in many applications such as food, cosmetics, pharmaceuticals and paints [17]. And the enhanced stability achieved at high particle concentrations is particularly important for the further processing of emulsions into macroporous materials [53]. The mean droplet diameters have been obtained only for the stable emulsions, as shown in Fig. 10. Increasing the amine concentration leads to slightly smaller average droplet sizes. Compared to the emulsions prepared in 0.5 wt% Laponite suspensions, the mean droplet diameter here is obviously much smaller. It can be attributed to the larger number of particles available for oil/water interface.
Fig. 7. Photographs of vessels after 24 h containing aqueous suspensions of Laponite particles (4.0 wt%) as a function of DEA (a) or TEA (b) concentration (given in mM) at 25 ◦ C.
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be obtained when the clay–amine suspensions are flocculated. The emulsion stability improves with amine addition and reaches maximum above a certain amine concentration. The adsorption of the modified clay particles on the droplet surface results in emulsion stabilization, which was confirmed by laser-induced fluorescent confocal micrographs. Furthermore, extremely stable emulsions, where no oil and aqueous phase release within the observation time (6 months after emulsification), were prepared in high concentrated suspensions (4.0 wt%). The emulsion stability can be attributed to the particle adsorption onto the droplet surface, and more importantly, the gel structure of the suspension. This kind of emulsions with high particle loading and outstanding stability can be potential candidates for macroporous materials. Moreover, by replacing the amines with non-toxic short-chain molecules, they may find significant applications in the areas of food, pharmaceuticals and cosmetics. Acknowledgment Fig. 8. Viscosity of aqueous suspensions of Laponite particles (4.0 wt%) as a function of amine concentration (measured at 25 ◦ C).
We acknowledge financial support from the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program, No. 2009CB930100). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2012.02.044. References
Fig. 9. Appearance of emulsions stabilized by 4.0 wt% Laponite and DEA (a) or TEA (b) with different initial concentrations 24 h after preparation. The amine concentrations are shown in the figure (mM).
Fig. 10. Mean diameter of o/w emulsion droplets prepared by 4.0 wt% Laponite particles modified with short-chain aliphatic amine at different concentrations.
4. Conclusions In this work, liquid paraffin-in-water emulsions were successfully prepared by mixing oil with Laponite suspensions after in situ modification of the particles with short-chain aliphatic amines, DEA and TEA. The adsorption of DEA or TEA brings about enhancement of particle hydrophobicity and decrease of zeta potential, which make it easier for them to adsorb at oil–water interfaces. It was found that, at low clay concentration (0.5 wt%), stable emulsions could
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