Letter pubs.acs.org/Langmuir
Direct Measurement of Contact Angles of Silica Particles in Relation to Double Inversion of Pickering Emulsions Bernard Paul Binks,*,† Lucio Isa,‡ and Andrew Terhemen Tyowua† †
Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, U.K. Laboratory for Surface Science and Technology, ETH, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland
‡
ABSTRACT: In an alkane−water system containing submicrometer silica particles at high pH, double emulsion inversion from oil-in-water (o/w) to water-in-oil (w/o) to oil-in-water can be effected by increasing the concentration of a dichain cationic surfactant in water. The contact angle θ of the particles at the planar oil−water interface has been measured directly using freeze−fracture shadow-casting cryo-scanning electron microscopy, enabling single-particle measurements of high accuracy. θ passes through a maximum with respect to surfactant concentration. It is shown that particles undergo a hydrophilic−hydrophobic−hydrophilic transition corresponding closely to the o/w−w/o−o/w transformation observed in emulsions. These results unequivocally link the single-particle contact angles to the type of particle-stabilized emulsion, confirming macroscopic emulsion inversion on the microscopic level.
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and anionic sodium dodecylsulfate (SDS),12 silica and dialkyldimethylammonium bromides (diCnDMAB) with different chain lengths,13 silica and di-C12DMAB,14 and cationic layered double hydroxide particles and SDS.15 However, because the determination of the contact angle submicrometer particles exhibit at the oil−water interface is challenging,16−23 the unequivocal link between contact angles and emulsion inversion has not been established. Instead, one study referred to above14 showed that for glass substrates the oil−water contact angle passed through a maximum value with respect to surfactant concentration and this value was >90°, in line with simple predictions. Recently, a new method for measuring the contact angle of single particles with a planar oil−water interface was described.24 It was named freeze−fracture shadow-casting cryo-scanning electron microscopy (FreSCa cryo-SEM) and was shown to be applicable to particles as small as 10 nm in diameter at a number of alkane−water interfaces. In summary, a particle-laden oil−water interface is created, frozen rapidly, and fractured, exposing immobilized particles that are then unidirectionally metal coated at an oblique angle. This creates a shadow behind each of the features protruding from the interface, from which their height can be deduced, leading simply to θ in the case of spherical particles. Here we make use of the technique to determine the contact angle of silica nanoparticles at the decane−water interface as a function of diC10DMAB concentration in a double inversion emulsion system in an attempt to verify the above hypothesis.
INTRODUCTION It is now understood that the type of emulsion (i.e., oil-inwater, o/w, or water-in-oil, w/o) obtained in an oil−particle− water mixture depends on the magnitude of the three-phase contact angle θ of the particle with the oil−water interface (measured through water).1−10 For relatively hydrophilic particles, θ is normally less than 90° and o/w emulsions are preferred, whereas for more hydrophobic particles θ is higher and w/o emulsions are preferred. If the wettability (θ) of the particles can be changed in situ at a fixed oil/water ratio then transitional phase inversion of the emulsions is obtained.1 The phenomenon of double phase inversion of emulsions stabilized partially by particles was described by one of us in dodecane− water systems containing anionic Ludox HS-30 nanoparticles and cationic surfactant.11 At a fixed particle concentration, increasing the concentration of dichain surfactant resulted in emulsions inverting from o/w to w/o initially and then back to o/w at high enough concentration. Using complementary experiments, these findings were discussed in terms of the adsorption of surfactant to particle surfaces and the influence this had on the wettability of the particles at the oil−water interface. Upon monolayer formation on particle surfaces driven by electrostatic attraction between cationic surfactant headgroups and negatively charged sites on the particles, particles became hydrophobic and the first emulsion inversion occurred (o/w → w/o). Surfactant adsorption on particles via van der Waals chain−chain interactions continued with the formation of a bilayer on particle surfaces, rendering them charged and hydrophilic again; the second inversion was seen (w/o → o/w). Since then double emulsion inversion has been demonstrated in a number of other particle−surfactant systems including those containing cationic calcium carbonate particles © 2013 American Chemical Society
Received: February 26, 2013 Revised: April 5, 2013 Published: April 9, 2013 4923
dx.doi.org/10.1021/la4006899 | Langmuir 2013, 29, 4923−4927
Langmuir
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Letter
EXPERIMENTAL SECTION
Materials. Silica particles of diameter 0.404 μm and polydispersity 0.02 were obtained as a high-pH (ca. 9.5) aqueous dispersion (50 mg mL−1) from Microparticles (Germany). The cationic surfactant was diC10DMAB (purity >98%) from TCI Europe (Belgium). n-Decane (purity 99% from Avocado Research Chemicals) was passed through basic alumina twice to remove polar impurities and used as the oil. Water was passed through an Elga Prima reverse osmosis unit and then a Milli-Q reagent water system. The treated water had a surface tension of 71.8 mN m−1 and a resistivity of 18 MΩ cm at 25 °C. Methods. Aqueous Dispersions and Oil−Water Emulsions. Various concentrations (0−10 mM) of the surfactant were allowed to adsorb on a fixed mass (1.5 wt %) of charged silica particles in water at room temperature without adjusting the pH. pH values were measured 30 min after mixing using a Jenway 3510 pH meter. Batch emulsions containing equal volumes (1 mL) of decane and aqueous dispersions (particles plus surfactant) were prepared at room temperature using an IKA Ultra Turrax T25 homogenizer with a 0.8 cm head operating at 11 000 rpm for 1 min. Immediately after preparation, the emulsion type was inferred from the drop test and by conductivity measurements using a Jenway 4510 conductivity meter. The stability of emulsions to creaming, sedimentation, and coalescence was assessed by monitoring the heights of resolved water and oil from the emulsion phase over time. Micrographs of drops of diluted emulsions were obtained on a dimple glass slide (Fisher Scientific) using an Olympus BX-51 microscope fitted with a DP50 digital camera using Image-Pro Plus 5.1 software (Media Cybernetics). Single-Particle Contact Angle Measurements by FreSCa CryoSEM. The sample preparation for FreSCa cryo-SEM proceeded as follows.24 The 1.5 wt % aqueous nanoparticle dispersion (0.5 μL) at the various di-C10DMAB concentrations was loaded into custom-made copper holders with a 200 μm depression and carefully covered with 3 μL of n-decane to create a macroscopically flat particle-laden oil−water interface at room temperature. The sample was closed with a flat copper plate, clamped, and frozen in a liquid propane jet freezer (BalTec/Leica JFD 030, Balzers, Vienna) at a cooling rate of 30 000 K s−1 to avoid water crystallization and arrest the nanoparticles in their original positions at the liquid interface. After being frozen, the samples were mounted under liquid nitrogen on a double-fracture cryo-stage and transferred under inert gas in a cryo-high-vacuum airlock (
