COLLOIDAL PARTICLES AS EMULSION AND FOAM STABILISERS Bernard P. Binks Surfactant & 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) 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 • 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/ 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

phaseinversion

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 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. in bulk

Ionisation hindered two-fold:

COOH proximity of groups/particle oil proximity of particles/drop

water 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 0.5 % 0.75 % 1 % 2 % 4 %

cream

water

Smaller oil globule size & increased viscosity of continuous water

- creams (fo=0.7) still stable to coalescence for 4 years Foam stability by light scattering

silica, 30 s - 72 hr sample hol der stopper sample tube dispersion mirror scattered light Plexiglas window

flatbed scanner

SDS, 30 s - 6 hr

Particles: drainage of water slow over in few hours

Surfactant: fast water drainage coalescence & ripening  Charges at particle/nonpolar fluid interfaces

Some Directions for Future Research charging mechanism particle hydrophobicity ?

 Adsorption kinetics of nanoparticles at fluid interfaces 2 2 large R and ow DEdes   R  ow1 cosq  small R and ow intermediate q small or large q irreversible particle adsorption reversible D E Amphiphilicdes  kT particles DEdes  kT

hydrophilic hydrophobic

biphilic (Janus)  Particle-stabilised foams

 Particles + Surfactants

surfactants decrease ow change q competitive adsorption Particle pair interactions through the oil

dipoles due to polar groups Dipolar repulsion (Earnshaw 1993) P 2 2 m P  smR sin q at l/2R > 4 l P R dipole moment of one polar group polar groups per unit area U  P2 4 l 3  interactions are screened by electrolyte d 0 s = 4.6 OH groups per nm2 octane-water; R = 1.5 mm m = 6.2 10-30 C m 305

25 Coulombic repulsion 4 disorder order

20 surface charges

3 q po  Apos po

kT kT

/ / 15

/ / Coulombic U U U U 2 2 -2 10 spo= 20 mC/m spo ~ 1 - 80 mC m 1 5 dipolardipolar q2 1 1  0 po UC     0 30 60 90 120 150 180 4 l 2 2 2 0  R 3 cosq   l  contact angle, q/o Walter Ramsden, M.A., M.D. (1868-1947)

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.”

NOVEL POROUS SOLIDS oil

air evaporation air

water solid 75 % air!

emulsion drop size ≈ solid pore size Uses: sorption media, filters, catalysts

50 mm