Particle-stabilized foams

Transcription

1 Particle-stabilized foams Brent S. Murray, Bernie P. Binks*, Eric Dickinson, Zhiping Du, Rammile Ettelaie & Thomas Kostakis Food Colloids Group Procter Department of Food Science, University of Leeds, UK *Surfactant & Colloid Group, Department of Chemistry, University of Hull, UK 1 Acknowledgements Maria P. Bilbao-Montoya, Krystel Maisonneuve, Phillip V. Nelson & Ingrid Söderberg Food Colloids Group, Leeds, UK Ton van Vliet, Martin Bos Wageningen Centre for Food Sciences (WCFS) A. Cox, I. Campbell Unilever Research, Colworth House, UK Richard Buscall Strategic Technology Group, ICI plc Wacker-Chemie GmbH 2

2 Overview Disproportionation: ultimate instability mechanism in foams Impossibility of preventing disproportionation with molecular stabilizers, e.g., surfactants, proteins Possibilities with particles as stabilizers 3 Particles may be involved in stabilization of many emulsions and foams e.g., in foods: droplets and fat crystals 4

3 Apparatus designed to study bubble stability mica sheet microscope piston bubbles light source 5 Originally designed to look at bubble coalescence But with time bubbles disappear anyway 0 min

4 40 min min

5 100 min min

6 Bubbles lost not through coalescence, but through disproportionation. 140 min 6 Air in the bubbles dissolves in the aq. phase 2 diffuses to the planar interface (and/or to larger bubbles). 11 Disproportionation aq. Driving force = the Laplace pressure, P = 2γ/r Ultimately destroys foams and emulsions (Ostwald ripening) even in the absence of coalescence, flocculation, etc. (?) 12

7 (?) Theory shows that if the interfacial film (or surrounding bulk phase) is strong enough, this can resist compression of the bubble and so stop disproportionation. Mechanical stability. Kloek, W., van Vliet, T., and Meinders, M., J. Colloid Interface Sci., 237, 158 (2001). Meinders, M. B. J., Kloek, W. and van Vliet, T., Langmuir, 17, 3923 (2001). 13 Theory dilatational elasticity (ε) stops shrinkage 100 r/ µm 80 ε = 0 ε= 0.5γ 0, larger bubble ε= 0.5γ 0, smaller bubble e+5 1e+6 2e+6 2e+6 3e+6 3e+6 Time τ 14

8 Experiment: bubble radius v. time for bubbles stabilized by different proteins (0.05 wt%, ph 7) r / µm 100 gelatin r / µm 100 WPI or caseinate t / min r /µ m t / min β-lactoglobulin t / m in 15 Conclusions from experiments: rates not very dependent on type of protein no protein (or surfactant) appears able to stop disproportionation, or even slow it down appreciably Despite the fact that the surface elasticities & viscosities of proteins film are different & sometimes very high Can any adsorbed film stop disproportionation?! 16

9 Particle-stabilized bubbles? Ashby, N.P. & Binks B.P.(2000) Pickering emulsions stabilised by Laponite clay particles, Phy. Chem..Chem. Phys., 2, Binks, B.P. & Lumsdon, S.O. (2000) Influence of Particle wettability on the type and stability of surfactant-free emulsions, Langmuir, 16, 8622 (2) hydrophobic phase Particle water θ particles on surface of bubble 17 Silica particles ( 20 nm ) treated with silylating reagent so 60% of surface Si OH groups Si O R Bubbles injected beneath interface 300 r / µm 200 All using 0.08wt% of 60% SiOR particles min min t / min 1 18

10 Bubbles must obtain sufficient particle coverage to remain stable to coalescence or disproportionation 19 Bubbles must obtain sufficient particle coverage to remain stable to coalescence or disproportionation 20

11 21 Bubbles nucleated by suddenly lowering the pressure 300 r / µm r / µm t / HOURS 0 min 100 min cf. protein stabilized t / min Typical stability with best proteins 22

12 Desirable to have better control over particle aggregation Desirable to use silica particles with less hydrophobic (%SiOR) treatment 23 Typical picture using 0.08wt% of 33% SiOR particles high density of particles at planar interface lower density of particles at planar interface 24

13 Effect of particle aggregation on stability (33% SiOR particles + 3M NaCl) 1.0 fraction remaining time/ min 25 Typical bubble shrinkage with 0.08wt% of 33% SiOR particles 100 radius /µm 50 0 proteinstabilized time/ min 26

14 Effect of NaCl on bubble stability with 33% SiOR particles 1.0 fraction remaining M M 2 M 1.8 M time/ min 27 Effect of NaCl conc. on 20% SiOR particles 1.0 fraction remaining M 4 M M time/ min 28

15 Effect of particle hydrophobicity (%SiOR) on stability (in 3M NaCl) 1.0 fraction remaining % SiOR time/ min 20% SiOR 29 Video 1 particle rafts with embedded bubbles 30

16 Effect of particle concentration (33% SiOR, 3M NaCl ) 1.0 fraction remaining wt% particles (+ 10 min sonication) 0.08 wt% particles time/ min 31 Three-dimensional array of bubbles formed with 1 wt% silica bubble-particle gel? 32

17 Video 2 3D bubble-particle gel 33 Greater stability with higher concentration of particles cf. protein 100 bubble radius /µm 50 1% particles 1% particles (33% SiOR, 3M salt) % particles best proteinstabilized time/ min 34

18 Effect of sonication time for (33% SiOR) 1.0 fraction remaining 0.5 1%, 60 min sonicate 0.08%, 60 min sonicate 1%, 10 min sonicate %, 10 min sonicate time/ min 35 Conclusions Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 36

19 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 37 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 38

20 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 39 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 40

21 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 41 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 42

22 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough 43 Bubbles can be stabilized almost indefinitely, provided enough particles adsorb fast enough This depends on the relative rates of particle adsorption versus particle aggregation 44

23 Theoretically-stabilized radii, assuming diffusion-controlled adsorption of 200 nm particle agglomerates to 50% surface coverage on bubbles 100 final bubble radius/ µm where close-packing particle shell formed initial bubble radius/ µm 45 In more concentrated particle systems, bubbles may be formed in, or reinforce a gel structure 46

24 In more concentrated particle systems, bubbles may be formed in, or reinforce a gel structure nucleate bubbles by pressure drop 47 In more concentrated particle systems, bubbles may be formed in, or reinforce a gel structure bubble-particle gel 48

25 Future work? Higher wt% particles + greater sonication Higher P to nucleate more bubbles NaCl effects on particle contact angles Microscopy of adsorbed particle layer Bulk rheology effects of 3-D particle network Other kinds of particles (fat, protein particles?) Mixed particle, protein & surfactant systems 49 Publications Brent S. Murray, Iain Campbell, Eric Dickinson, Krystel Maisonneuve, Phillip V. Nelson and Ingrid Söderberg, Langmuir 2002, 18, A Novel Technique for Studying the Effects of Rapid Surface Expansion on Bubble Stability. Dickinson Eric Dickinson, Rammile Ettelaie, Brent S. Murray and Zhiping Du. Kinetics of disproportionation of bubbles beneath a planar -water interface stabilised by food proteins. J Colloid Interface Sci 2002, 252, Brent S. Murray, Benoit Cattin, Elke Schüler and Zahlia O. Sonmez. The Response of Adsorbed Protein Films to Rapid Expansion. Langmuir 2002, 18, Ingrid Söderberg, Eric Dickinson, Brent S. Murray. Coalescence stability of gas bubbles subjected to rapid pressure change at a planar /water interface. Colloids Surf. B 2003, 30, Coalescence stability of gas bubbles subjected to rapid pressure change at a planar /water interface. Rammile Ettelaie Eric Dickinson, Zhiping Du and Brent S. Murray. The influence of disproportionation between bubbles on the stability of protein-stabilised bubbles at a planar -water interface. J. Colloid Interface Sci. 2003, 263, Zhiping Du, Maria P. Bilbao-Montoya, Bernie P. Binks, Eric Dickinson, Rammile Ettelaie and Brent S. Murray. Extreme stability of particle-stabilized bubbles. Langmuir 2003, 19, Thank you 50

26 END 51 Theoretical description of diffusion problem aq. Using the mathematical technique of Images, the flux (Q) of gas from a bubble is given by Q = 4π Dr cα ( r / L) = 4πDr c q i i= 0 ( β ) where q i values satisfy the following recurrence relations, valid for i > 0 r L L = 2R g 10 nm q i = βq λ i 1 i 1 D = diffusion constant of gas c = excess gas concentration in bubble 2 β λ i = λ0 λ i 1 52

27 Henry s equation for the gas solubility, S, c o = SP 0 So c o + c = S(P 0 + ( 2γ / r ) ) P o = the pressure in the gas above the liquid gas interface S the solubility constant of the gas in the liquid. Thus c = 2Sγ r Assuming P o >> 2γ/r and equating Q with -dr/dt,? = constant = γ 0 2 r α( r / L) dr dt 2DγSRT = P o T = temperature, R the ideal gas constant. 53 Approximate values of surface rheological parameters, measured at a shear rate of 10-3 s -1 / 10-3 Hz protein η app /mn s m -1 G app /mn m -1 ε' mn m -1 ε'' mn m -1 κ = ε''/ω /mn s m - 1 sodium caseinate β-lactoglobulin gelatin WPI η app = apparent surface shear viscosity G app = apparent surface shear elasticity ε' = apparent dilatational storage modulus ε'' = apparent dilatational storage modulus κ = ε''/ω apparent dilatational viscosity 54

28 Theoretical dilatational elasticities (ε) required to give 'perfect' fit to radius v. time protein ε / mn m -1 caseinate 0 WPI 0 gelatin 2.3 β-lactoglobulin 7.0 soy glycinin Bubbles stabilised by globular proteins shrink to non-spherical particles e.g., pure β-lact, ph µm 104 min e.g., soy glycinin, ph µm 215 min

29 Better model of film collapse required for high modulus films adsorbed protein film time 57