COLLOIDAL PARTICLES AS EMULSION AND FOAM STABILISERS Bernard P. Binks

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1 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

2 COLLOIDAL PARTICLES: nm - mm Particles may be surface-active but not amphiphilic Exception: Janus particles oil/air HYDROPHILIC HYDROPHOBIC silica clay (disk) polymer latex carbon

3 ADSORPTION OF PARTICLES AT FLUID INTERFACES Free energy gain by losing an area of fluid-fluid interface hydrophilic hydrophobic DG/kT oil/air q oil/air q DG r 2 1 cosq 2 r = 10 nm, = 36 mn m q/ o Particles strongly held at interfaces: irreversibly adsorbed Contact angle is particle equivalent of surfactant HLB number

4 Planar air & oil- interfaces Simple emulsions air/oil oil cleaning of apples flotation of ores SOLID PARTICLES AT LIQUID INTERFACES mayonnaise crude oil oil air or oil Foams Multiple emulsions

5 Surface modification of silica particles hydrophilic H H O O Si Si H O Si H SiO 2 O Si Si O O H H H 3 C CH 3 Cl Si Cl DCDMS HCl SiO 2 hydrophobic Formation q < 10 o of planar monolayers monodisperse precipitated CCD amorphous camera silica particles Petri dish particles in isopropyl alcohol particle density: 2 g/cm 3 particle diameter: oil (air) 1 or 3 mm microscope q = 60 o o (oil-) VCR com puter + image analysis software particle monolayer

6 Planar Monolayers 1 mm monodisperse silica particles octane- interface q disordered monolayers ordered monolayers q 50 mm 50 mm 50 mm 50 mm q = 70 o q = 115 o q = 129 o q = 150 o

7 Planar Monolayers 3 mm silica particles at octane- hydrophilic ph = 5.7 hydrophobic no salt q = 65 o no salt q =152 o due to charges at particle-oil surface? oil loose aggregates L 50 mm 28 mm repulsion through long-range repulsion through oil Optical tweezers: close-packed aggregates Coulombic repulsive force L-4 10 mm NaCl 50 mm 100 mm NaCl

8 Spencer U. Pickering, M.A., FRS ( ) Professor of Chemistry, Bedford College Director of Woburn Experimental Fruit Farm CuSO 4 (insecticide) + CaO (lime) ppt. basic CuSO 4 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)

9 Particle-Stabilised Emulsions Limited coalescence dilute particle monolayers interfacial area decreases monolayer density increases dense particle monolayers 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?

10 Simple Emulsions 3 mm silica particles; 1 wt.% in emulsion; octane: = 1:1 hydrophilic particles q = 65 o hydrophobic particles q = 152 o octane silica particles in shaking 20 mm 25 mm silica particles 25 mm in octane shaking O / W W / O 100 mm

11 Particle-Stabilised Emulsions Stabilising Mechanisms hydrophilic particles q = 65 o hydrophobic particles q = 152 o oil oil oil particle bilayer bridging particle monolayer uses fewer particles

12 EMULSION INVERSION Fumed silica nanoparticles nm primary particle diameter Me Me HO HO Si Si OH Si Si Si OH OH Me Me Si Cl Cl HO HO Si Si O Si Si Si Si O OH HO Si Si OH Si OH HO Si Si OH Si OH 100% SiOH, hydrophilic < 100% SiOH, more hydrophobic

13 INFLUENCE OF PARTICLE WETTABILITY Limonene oil 8 γ ow = 40.2 mn/m wt.% particles f w = 0.5 κ/ µs cm w/o o/w % SiOH Phase inversion occurs at 47% SiOH

14 TRANSITIONAL INVERSION OF OIL-WATER SYSTEM w/o o/w 14% 23% 32% 42% 51% 58% 67% 78% 87% 100% Phase inversion for this system occurs at 47% SiOH Stability is greatest at phase inversion Drop size is smallest at phase inversion Scale scale bar = µm

15 THEORY: CALCULATING CONTACT ANGLE vs. % SiOH o w s 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

16 SOLID-WATER INTERFACIAL ENERGY 100% SiOH 0% SiOH (c) so : SOLID-OIL INTERFACIAL ENERGY limonene benzyl acetate

17 CALCULATED q ow FOR LIMONENE Calculated q ow and DG as a function of % SiOH: Predict phase inversion at 46% SiOH (q ow = 90 o ) q ow / o DG/ kt Predict stable emulsions for 14 to 100% SiOH % SiOH

18 exptl. % SiOH at phase inversion SUMMARY FOR OILS OF DIFFERENT POLARITY calculated % SiOH at θ = 90

19 MULTIPLE EMULSIONS inner drop oil oil globule oil external phase Drugs, enzymes Major advantage particles stay at interfaces no mixing to break the emulsion

20 W/O/W O/W/O phobic silica in oil philic silica in -in-oil w-in-o-in-w philic oil silica in oil-in- phobic silica in oil o-in-w-in-o 50 mm triglyceride 50 mm toluene Completely stable for 8 years! 20 mm

21 PARTICLE-STABILISED AQUEOUS FOAMS (a) J.C. Wilson, Ph.D. thesis, 1980 polystyrene latex particles > 2 mm fumed nano-silica treated with DCDMS d ~ 30 nm (b) Alargova et al., 2004 hairy bubbles with 25 x 0.5 mm rods Effect on foaming of: 50 mm Particle hydrophobicity-vary % SiOH Electrolyte concentration-vary [NaCl]

22 foam volume (cm 3 ) Silica-stabilised foams: no salt Powder: on surface or dispersed in (using ethanol if necessary) Foams of 1 wt.% made by hand-shaking or using rotor-stator mixer (7 cm 3 ) 10 8 q air homogenised, t = 0 t = t = 2 days surface SiOH content (%) more hydrophobic hand-shaken, t = 0, 2 hr homogenised, t = 2 days

23 Influence of salt addition on foams 0 M 8 mm NaCl 50 mm Reducing surface charge increases hydrophobicity q aw / o [NaCl]/M % SiOH

24 surface pressure, / mn/m Planar monolayers at air- surface Fumed silica, 32 % SiOH spread from chloroform trough area, A / cm mm 100 mm = 70 mn/m ripples from folding = 13 mn/m disordered particle aggregates

25 EXAMPLES OF PARTICLE-STABILISED DISPERSED SYSTEMS θ oil (air) particle oil + system o/w emulsion w/o emulsion hydrophilic particle oil oil hydrophobic particle q = 0 transitional phase inversion q = 180 air a/w foam air w/a material? air + system What is the w/a material and how can we phase invert the system?

26 TRANSITIONAL INVERSION OF AIR-WATER SYSTEM f w = (95 ml /1700 ml total vol.) 2 wt.% silica particles (relative to ) 2 weeks after mixing and aeration hydrophobic hydrophilic % SiOH powder foam dispersion Inversion of the curvature of the air- surface is induced by changing the particle hydrophobicity

27 Air-in- foam 1 cm Water-in-air powder (5 g particles + 95 g + air) 1 cm phase partially hydrophobic silica inversion 20% SiOH silica particles very hydrophobic silica

28 PARTICLE-STABILISED OIL FOAMS la increasing sunflower oil, eugenol, benzyl acetate.. air liquid q (a) i ii iii iv v (b) (c) air hexane glycerol oil low energy fluoro- particles

29 Summary Hydrophilic particles Hydrophobic particles behave very differently at planar oil- interface negligible charge significant charge weak repulsion through the strong repulsion through the oil particle bilayer O/W emulsions W/O emulsions bridging particle monolayer W oil oil W W oil Aqueous & Oil foams with particles 100 µm

30 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

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32 NATURALLY OCCURRING PARTICLES Spores from Club moss - Lycopodium clavatum Pyrotechnics, herbal remedies: diarrhea, dysentery, constipation, eczema! 2 drops in oil o/w emulsion 30 mm 1 cm 200 mm

33 INTERFACIAL ARRANGEMENT OF PARTICLES w/o emulsion at ph 6.5 o/w emulsion at ph 10.1 disordered, partially covered monolayer - particles flocculated in bulk hexagonally, close packed monolayer - particles discrete in bulk

34 Emulsion appearance after 1 year hexadecane + (1:1): 1 wt.% of d = 200 nm particles No coalescence! w/o o/w sedimentation creaming sedimentation creaming ph: COOH PS - COOH - COOH HCl - COOH PS - COO - - COOH NaOH - COO - PS - COO - - COO - hydrophobic partially hydrophobic (ph 5.3) hydrophilic

35 [particle]/ wt.% EMULSION PHASE DIAGRAM w/o o/w w/o w/o + o/w/o o/w w/o + o/w/o φ w

36 IONISATION OF CARBOXYL GROUPS AT SURFACES Apparent pk a,s 10, cf. 5 in bulk [ COO ] s [ COO ] s [ COOH] s ph s pk a, s log[(1 )/ ] ph s ph b [ e / 2.303kT] Since ph s < ph b, ionisation at surfaces weakened cf. in bulk Ionisation hindered two-fold: oil COOH proximity of groups/particle proximity of particles/drop

37 EMULSION STABILITY- W/O/W All w/o/w emulsions stable to coalescence: w drops & o globules Stability to creaming depends on particle concentration philic silica cream 0.5 % 0.75 % 1 % 2 % 4 % Smaller oil globule size & increased viscosity of continuous - creams (f o =0.7) still stable to coalescence for 4 years

38 Foam stability by light scattering sample hol der stopper sample tube dispersion mirror scattered light silica, 30 s - 72 hr Plexiglas window flatbed scanner SDS, 30 s - 6 hr Particles: drainage of slow over in few hours Surfactant: fast drainage coalescence & ripening

39 Charges at particle/nonpolar fluid interfaces Some Directions for Future Research charging mechanism particle hydrophobicity? Adsorption kinetics of nanoparticles at fluid interfaces large R and ow intermediate q DE des kt irreversible Amphiphilic particles DE des R 2 ow 1 cosq 2 particle adsorption reversible small R and ow small or large q DE des kt hydrophilic Particle-stabilised foams biphilic (Janus) hydrophobic Particles + Surfactants surfactants decrease ow change q competitive adsorption

40 U / / kt m Particle pair interactions through the oil P dipoles due to polar groups interactions are screened by electrolyte disorder Coulombic s po = 20 mc/m 2 l octane-; R = 1.5 mm P order R Dipolar repulsion (Earnshaw 1993) P smr 2 sin 2 q at l/2r > 4 U d P 2 q 4 l Coulombic repulsion surface charges dipole moment of one polar group polar groups per unit area s = 4.6 OH groups per nm 2 0 m = C m 3 po A po s po s po ~ 1-80 mc m dipolar contact angle, q/ o U C 2 q po l R 3 cosq l

41 Walter Ramsden, M.A., M.D. ( ) Professor of Biochemistry, Univ. of Liverpool Fellow of Pembroke College, Univ. of Oxford Proc. Roy. Soc., 72 (1903), 156 Separation of Solids in the Surface -layers of Solutions and Suspensions.. bubble retains particles obstinately,.. grotesque shapes of globules. John Betjeman, Poet Laureate-to-be; A Few Late Chrysanthemums (1954): Dr. Ramsden cannot read The Times obituary to-day, he s dead. Let monographs on silk worms by other people be thrown away unread, for he who best could understand and criticize them, he lies clay in bed. Master, Bursar, Senior Tutor, these, his three survivors, all feel old.

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43 oil NOVEL POROUS SOLIDS evaporation air air solid 75 % air! emulsion drop size solid pore size Uses: sorption media, filters, catalysts 50 mm

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