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Bernard P. Binks Surfactant & Colloid Group Department of Chemistry University of Hull Hull

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Bernard P. Binks Surfactant & Colloid Group Department of Chemistry University of Hull Hull COLLOIDAL PARTICLES AS EMULSION AND FOAM STABILISERS Bernard P. Binks Surfactant & Colloid Group Department of Chemistry University of Hull Hull. HU6 7RX. U.K. bubbles films drops 1 mm COLLOIDAL PARTICLES: nm - mm Particles may be surface-active Exception: but not amphiphilic Janus particles oil/air water HYDROPHILIC HYDROPHOBIC silica clay (disk) polymer latex carbon ADSORPTION OF PARTICLES AT FLUID INTERFACES Free energy gain by losing an area of fluid-fluid interface hydrophilic hydrophobic 3000 oil/air oil/air DG r2 1 cosq 2 2000 q q DG/kT water water r = 10 nm, 1000 = 36 mN m-1 0 0 30 60 90 120 150 180 q/o Particles strongly held at interfaces: irreversibly adsorbed Contact angle is particle equivalent of surfactant HLB number Planar air & oil-water interfaces Simple emulsions air/oil oil water water cleaning of apples SOLID PARTICLES mayonnaise AT LIQUID flotation of ores INTERFACES crude oil oil air water water water or oil Foams Multiple emulsions Surface modification of silica particles hydrophilic hydrophobic H3C H CH3 H O Si H Cl O Si Cl Si O Si DCDMS H SiO2 SiO2 O Si Si O O HCl H H Formationq < 10o of planar monolayers q = 60o - 160o (oil-water) monodisperse precipitatedCCD amorphouscamera silica particles VCR particles in isopropyl alcohol 3 particle density: 2 g/cm microscope computer particle diameter: 1 or 3 mm + image analysis oil (air) software Petri dish water particle monolayer Planar Monolayers 1 mm monodisperse silica particles octane-water interface q disordered monolayers ordered monolayers q 50 mm 50 mm 50 mm 50 mm q = 70o q = 115o q = 129o q = 150o Planar Monolayers 3 mm silica particles at octane-water hydrophilic pH = 5.7 hydrophobic no salt q = 65o no salt q =152o due to charges at particle-oil surface? oil loose aggregates L water 28 mm 50 mm repulsion through water long-range repulsion through oil Optical tweezers: close-packed aggregates Coulombic L-4 repulsive force 50 mm 10 mM NaCl 100 mM NaCl Spencer U. Pickering, M.A., FRS (1858-1920) Professor of Chemistry, Bedford College Director of Woburn Experimental Fruit Farm CuSO4 (insecticide) + CaO (lime) ppt. basic CuSO4 superior emulsifier to soap; “pellicle around globules” J. Chem. Soc., 91, 2001 (1907) CXCVI - Emulsions “…the subject had already been investigated by Walter Ramsden, but his work, unfortunately, did not come under the notice of the writer until that here described had been completed.” Proc. Roy. Soc., 72, 156 (1903) Particle-Stabilised Emulsions Limited coalescence dilute particle monolayers monolayer density dense particle monolayers increases interfacial area decreases a steric barrier against coalescence w/o 1 cm What is the limiting monolayer density needed to prevent coalescence? Does it depend on particle hydrophobicity? Simple Emulsions 3 mm silica particles; 1 wt.% in emulsion; octane:water = 1:1 hydrophilic particles q = 65o hydrophobic particles q = 152o 20 mm 25 mm octane silica particles25 mm in octane silica particles water in water shaking shaking O / W W / O 100 mm Particle-Stabilised Emulsions Stabilising Mechanisms o hydrophilic particles q = 65o hydrophobic particles q = 152 water oil oil oil water water particle bilayer bridging particle monolayer uses fewer particles EMULSION INVERSION • Fumed silica nanoparticles • 20-30 nm primary particle diameter Me Me Si OH Me Me O HO OH HO O Si Si Si Si Si Si Si Cl Cl HO Si Si OH HO Si Si OH Si Si Si Si Si Si HO OH HO OH OH OH 100% SiOH, hydrophilic < 100% SiOH, more hydrophobic INFLUENCE OF PARTICLE WETTABILITY • Limonene oil 8 7 γow = 40.2 mN/m 6 1 5 0.1 wt.% particles - fw = 0.5 4 w/o o/w / µS cm µS / κ 3 2 1 0 0 20 40 60 80 100 % SiOH Phase inversion occurs at 47% SiOH TRANSITIONAL INVERSION OF OIL-WATER SYSTEM w/o o/w 14% 23% 32% 42% 51% 58% 67% 78% 87% 100% • Stability is greatest at phase Phase inversion for inversion this system occurs • Drop size is smallest at phase at 47% SiOH inversion Scalescale barbar == 200500 µµmm THEORY: CALCULATING CONTACT ANGLE o vs. % SiOH s w θ (a) ow: by experiment limonene 40.2, benzyl acetate 18.4 mN m-1 (b) sw: Owens & Wendt, J. Appl. Polym. Sci., 13, 1741 (1969). dispersion & polar SOLID-WATER INTERFACIAL ENERGY 100% SiOH 0% SiOH (c) so: SOLID-OIL INTERFACIAL ENERGY limonene benzyl acetate CALCULATED qow FOR LIMONENE 180 4000 150 Calculated qow and DG as a function of % SiOH: 3000 120 D o G/ / •Predict phase inversion 90 2000 ow kT at 46% SiOH (q = 90o) q ow 60 •Predict stable emulsions 1000 for 14 to 100% SiOH 30 0 0 0 20 40 60 80 100 % SiOH SUMMARY FOR OILS OF DIFFERENT POLARITY 100 80 60 phase inversion at 40 SiOH 20 exptl. % exptl.% 0 0 20 40 60 80 100 calculated % SiOH at θ = 90° MULTIPLE EMULSIONS inner oil water drop water oil globule oil water external water phase Drugs, enzymes… particles stay at interfaces Major advantage no mixing to break the emulsion W/O/W O/W/O ‟phobic water ’philic ‟philic oil ’phobic silica silica silica silica in oil in oil in water in water water-in-oil w-in-o-in-w oil-in-water o-in-w-in-o 50 mm triglyceride 50 mm toluene 20 mm Completely stable for 8 years! PARTICLE-STABILISED AQUEOUS FOAMS (a) J.C. Wilson, Ph.D. thesis, 1980 fumed nano-silica treated with DCDMS polystyrene latex particles > 2 mm d ~ 30 nm (b) Alargova et al., 2004 „hairy‟ bubbles with 25 x 0.5 mm rods Effect on foaming of: Particle hydrophobicity-vary % SiOH 50 mm Electrolyte concentration-vary [NaCl] Silica-stabilised foams: no salt Powder: on water surface or dispersed in water (using ethanol if necessary) Foams of 1 wt.% made by hand-shaking or using rotor-stator mixer (7 cm3) 10 air 8 homogenised, t = 0 q t = 0 water ) 3 6 4 2 t = 2 days foam volume foam volume (cm 0 100 90 80 70 60 50 40 30 20 10 more hydrophobic → surface SiOH content (%) hand-shaken, t = 0, 2 hr homogenised, t = 2 days Influence of salt addition on foams 0 M 8 mM NaCl 50 mm Reducing surface charge increases hydrophobicity 70 o qaw/ 40 10 0.01 0.05 0.1 % SiOH [NaCl]/M Planar monolayers at air-water surface Fumed silica, 32 % SiOH spread from chloroform = 70 mN/m 70 ripples from 60 folding / mN/m/ 50 40 100 mm 30 20 10 surface pressure, pressure, surface 0 = 13 mN/m 10 20 30 40 50 60 70 disordered trough area, A/ cm2 particle aggregates 100 mm EXAMPLES OF PARTICLE-STABILISED DISPERSED SYSTEMS oil + water system oil (air) o/w emulsion w/o emulsion θ particle water oil water hydrophilic hydrophobic particle water oil particle q = 0 transitional phase inversion q = 180 air water water air a/w foam w/a material? air + water system What is the w/a material and how can we phase invert the system? TRANSITIONAL INVERSION OF AIR-WATER SYSTEM fw = 0.056 (95 mL water/1700 mL total vol.) 2 wt.% silica particles (relative to water) 2 weeks after mixing and aeration hydrophobic hydrophilic % SiOH 14 20 32 36 42 51 57 62 66 78 100 powder foam dispersion Inversion of the curvature of the air-water surface is induced by changing the particle hydrophobicity Air-in-water foam Water-in-air powder 1 cm (5 g particles + 95 g water + air) 1 cm phase partially hydrophobic inversion silica 20% SiOH silica particles very hydrophobic silica PARTICLE-STABILISED OIL FOAMS la increasing sunflower oil, eugenol, benzyl acetate .. air (a) i ii iii iv v q liquid (b) (c) air hexane glycerol oil low energy fluoro- particles Summary Hydrophilic particles Hydrophobic particles behave very differently at planar oil-water interface negligible charge significant charge weak repulsion strong repulsion through the water through the oil O/W emulsions W/O emulsions particle bilayer bridging particle monolayer W oil oil oil W W Aqueous & Oil foams with particles 100 µm ACKNOWLEDGEMENTS S.O. Lumsdon C.P. Whitby A.K.F. Dyab T.S. Horozov J.A. Rodrigues A. Desforges R. Murakami J. Philip T.J. Lees D.A. Braz Z-G. Cui B.L. Holt NATURALLY OCCURRING PARTICLES Spores from Club moss - Lycopodium clavatum Pyrotechnics, herbal remedies: diarrhea, dysentery, constipation, eczema! 2 water drops in oil o/w emulsion 30 mm 1 cm 200 mm INTERFACIAL ARRANGEMENT OF PARTICLES w/o emulsion at pH 6.5 o/w emulsion at pH 10.1 disordered, partially covered monolayer hexagonally, close packed monolayer - particles flocculated in bulk - particles discrete in bulk Emulsion appearance after 1 year hexadecane + water (1:1): 1 wt.% of d = 200 nm particles No coalescence! w/o o/w sedimentation creaming creaming sedimentation pH: 3.2 6.0 9.2 10.1 10.6 11.0 - -COOH HCl -COOH NaOH - COO - PS - COOH PS -COO - PS - COO - -COOH - COOH -COO hydrophobic partially hydrophobic hydrophilic (pH 5.3) EMULSION PHASE DIAGRAM o/w w/o w/o + o/w/o 1.2 w/o 1.0 0.8 0.6 o/w [particle]/ wt.% [particle]/ 0.4 w/o + 0.2 o/w/o 0.0 0.0 0.2 0.4 0.6 0.8 1.0 φw IONISATION OF CARBOXYL GROUPS AT SURFACES Apparent pKa,s ≈ 10, cf. 5 in bulk [COO] s pH pK log[(1)/] s a,s [COO ]s [COOH]s pHs pHb [e / 2.303kT] Since pHs < pHb, ionisation at surfaces weakened cf.
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