Simeon D. Stoyanov. Unilever R&D Vlaardingen, The Netherlands

Transcription

1 Simeon D. Stoyanov Unilever R&D Vlaardingen, The Netherlands Laboratory of Physical Chemistry and Colloid Science, Wageningen University, NL Department of Mechanical Engineering, University College London, UK

2 Why Saponins!?! Natural surfactants found in more than 500 plant species (can reach % of the dry mass) -> natural /sustainable sourcing feasible Molecules consist of hydrophobic part (triterpenoid or steroid aglycone) and hydrophilic oligosaccharide chains (1 3). Aglycone Quillaja Saponin: Triterpenoid aglycone. 2 sugar chains (2 5 residues). One of oldest detergents (x00 years of usage in America/Asia) In1890 saponins were used by Lord Rayleigh solve dispute between Maragoni & Plateau on the existence of surface viscosity

3

4 Saponin stabilized emulsions QD Escin BSC TS

5 Introduction / Why Saponis are relevant Natural surfactants found in more than 500 plant species (can reach % of the dry mass) -> natural /sustainable sourcing feasible Molecules consist of hydrophobic part (triterpenoid or steroid aglycone) and hydrophilic oligosaccharide chains (1 3). Aglycone Quillaja Saponin: Triterpenoid aglycone. 2 sugar chains (2 5 residues). One of oldest detergents (x00 years of usage in America/Asia) In1890 saponins by Lord Rayleigh to clarify the origin of interfacial viscosity and address the dispute between Maragoni & Plateau Have a variety of positive bio-effects: anti-viral, anti-microbial, antiinflammatory, cholesterol-lowering, anti-cancer, adjuvant etc.

6

7 So let use saponins and 1. Characterize the surface shear & dilatational behavior of adsorption layers of saponins from various sources and structures 2. Establish structure functionality relation. Quillaja (2 sugar chains) Escin (1 sugar chains)

8 Studied saponins Horse Chestnut (Aesculus hippocastanum) Sapindus Mukurossi Camellia Oleifera Abel Panax Ginseng Quillaja Saponaria Molina Acacia concinna Sapindus Trifoliatus Tribulus Terrestris Yucca Schidigera Trigonella foenum graecum Licorice

9 Mono/Bidesmosidics Horse Chestnut (HC/ES) (Aesculus hippocastanum) Sapindus Mukurossi (BSC (Berry Saponin) Camellia Oleifera Abel (TS) (Tea seed saponin) Panax Ginseng (GS) Triterpenoid aglycone Quillaja Saponaria Molina (QD) Oligosaccharide chains Monodesmosidic saponins Bidesmosidic saponins

10 Purity of Studied saponins How we can make science out of the mess? Type of aglycone Trade Name Saponins in Abbreviation extract % Horse chestnut extract HC 20 Escin ES 95 Tea Saponin TS 96.2 Berry Saponin BSC 53 Concentrate Triterpenoid Sapindin SAP 50 Quillaja Dry 100 QD 26 Ginsenosides GS 80 Ayurvedic Saponin Concentrate ASC 30 Tribulus terrestris TT 45 extract Steroid Foamation Dry 50 FD 9 Fenusterols FEN 50

11 Surface Tension vs Bulk Pressure Surface tension Energy per unit area or Force per unit length Pressure - Energy per unit volume or Force per unit area Surface tension vs bulk pressure: P eq = F/ (L)~ / E~Es/~ 1 mn/m / 1 nm~10 atm Equivalent bulk modulus: E= F/ (L)~Es/ E~Es/~30 mn/m / 1 nm~300 atm =F/L Es=F/L E~Es/~ 1000 mn/m / 1 nm~1 GPa

12 Surface tension, mn/m Surface tension isotherms Gibbs Escin concentration, mm Escin ph = natural k B T CMC Volmer adsorption isotherm e 0 kt Van der Waals adsorption isotherm 2 KC exp kt Gibbs adsorption isotherm d d d ktd ln( C) s b d e kt d ln C 2 d kt d 2 d ln( c) c dc KC exp r U kbt πk BT 1 e rdr πu r0 r0 rrdr

13 Surface tension, mn/m Area per molecule at CMC = 0.54 nm 2 Tea Saponin Mw = 1000 g/mol Volmer isotherm K = 1088 m 3 /mol inf = 3.9 mol/m 2 CMC = 3.1 mol/m Saponin concentration, mm

14 Surface tension isotherms for ASC, BSC and Sapindin Surface tension, mn/m ASC Surface tension, mn/m Berry saponin Saponin concentration, wt % Saponin concentration, wt % Surface tension, mn/m 80 Saponidin Saponin concentration, wt % Positive Curvature!!!! For these saponins we cannot determine the characteristics of adsorption layer (slope of the curves contradicts Gibbs isotherm) Mixture of components and presence of aggregates?!

15 Molecular packing mono vs bi desmosides A nm 2 A nm 2 Is there a link between molecular packing & surface modulus Shear

16 Surface Rheology Interactions between the molecules Kinetics of adsorption / desorption ( ), ( )

17 Surface Shear Rheology: bicone tool Lateral view Top view M, Shear stress: 2 М / 2R 1 Shear deformation: ( R R ) / 2( R R )

18 The Boussinesq number Bo surface viscous drag S bulk viscous drag. L B At Bo < 200: Surface and sub-surface flows coupling. Numerical procedure needed to calculate the surface viscosity. Surfactant layer regarded as a 2D body at Bo > 200.

19 Oscillatory Amplitude sweep (t A = 30 min) Elastic modulus, mn/m 10 3 GS ESC TS BSC QD Viscous modulus, mn/m 10 3 GS ESC TS BSC QD Strain amplitude, % Strain amplitude, % The bidesmosidic saponins have lower elastic and viscous modulus.

20 Creep Relaxation Experiments (1) Deformation at constant torque (1 N.m). (2) Strain relaxation. Visco-elastic (some of triterp/mono) Viscous (main steroidal) Group EV BSC 160x x x10 3 Group LV FD FEN Strain, % TS QD Strain, % 100x x x x10 3 ASC TT 0.05 ESC GS Time, s 20x10 3 LIC Time, s

21 Rheological model (compound Voigt) G 1 G 2 G 0 = 1/J Quillaja saponin. Maxwell + Kelvin (1) + Kelvin (2). 6 parameters (3 viscous and 3 elastic). 2 relaxation times. Compliance, m/n J 0 J Time, sec 0.5 wt % QD 10 mm NaCl 0.1 g/l NaN 3 = 0.94 mn/m t CR / 0

22 Molecular interpretation Molecules aggregated in domains. Burger Element: [Maxwell + 1 st Kelvin element] deformation and re-arrangement of domains. 2 nd Kelvin Element re-arrangement of molecules within the domains.

23 Viscoelasticity of triterpenoid saponins Highly elastic surface layer, G`>>G``. G` increases for more than 12 hours of aging of the layer. G`` decreases or stays constant. Saponins with one sugar chain exhibit much higher elasticity. G` ~ 1000 mn/m G` ~ 100 mn/m

24 Oscillatory dilatational deformations Langmuir trough 302LL/D1 (Nima Technology Ltd, UK) Surface pressure, mn/m % deformation s s Surface area, mm Time, sec Deformation created by the moving barrier(s) in the LM is uniaxial, i.e., it is a superposition of well defined dilatation and shear 1 2 K s s K α=ln(a/a0) - relative dilatation Boundary effects are neglected! s s K dilatational elasticity shear elasticity S dilatational viscosity S shear viscosity (Petkov et al. 2000)

25 Adsorbing system: 0.5 wt % saponins + 10 mm NaCl Oscillatory experiments done after equilibration has been reached (>30 min) Expansion 0.5 wt % ES parallel perpendicular Parallel plate K From the best fit K+ = 204 mn/m S + S = 163 mn.s/m s s For expanding Escin layer K = 154 mn/m; = 50 mn/m; s = 127 mn.s/m; s = 36 mn.s/m Perpendicular plate K From the best fit K- = 103 mn/m S - S = 91 mn.s/m s s

26 Analysis of experimental data from dilatation 14 stress-relaxation experiments Burger model Surface stress, mn/m E 1 E Time, sec 1 E t t 1 1 R1 R1 1 E1 tr 1 E2 tr2 tr2 tr 1tR 2 tr2 t R1 = 1 /E 1 t R2 = 2 /E 2 From the best fit of deformation and relaxation stages we determine E 1, E 2, t R1 and t R2

27 Surface rheological properties, as determined by oscillating drop method Surface moduli, mn/m ASC 40 Storage modulus Loss modulus mn/m 11 1 mn/m T = 10 s Deformation, % From this experiment we determine the surface dilatational moduli, as functions of surface deformation.

28 Expansion and contraction of large pendant drop QS initial state contraction final state equilibrium contraction state after 100 s expansion final state very close to the initial one The formation and destruction of elastic membrane is reversible!

29 Isotropic vs. Anisotropic Interfaces Fluid Interfaces: - Zero surface shear elasticity; - Isotropic: Single surface tension,, which is the same along the meridians and parallels ; - Uniform: The surface tension is the same in all points of the interface; - Method: DSA based on fit of meniscus profile by Laplace equation; and p adjustable parameters. Solid Interfaces (Membranes): - Nonzero surface shear elasticity; - Anisotropic: Two different surface tensions, s and φ, along the meridians and parallels ; - Noniform: The surface tensions s and φ vary from point to point throughout the interface; - Method: CMD (capillary meniscus dynamometry) based on fit of data for meniscus profile and p.

30 Force Balances per Unit Area of a Curved Interface Balance of linear momentum per unit surface area: s σ psn p s pressure difference across the interface; n n n running unit normal to the surface; s σ 0 0 Surface stress (tension) tensor (axial symmetry) d d r ( s r) (tangential projection) s const. s (uniformity ( non - uniformity isotropy) anisotropy) s s s p s (normal projection) (, the two principal curvatures) For reduces to Laplace s equation

31 Balance of Integral Surface Tension and Pressure Forces p s r 0 d r d d r ( r sin ) s p s r p 0 hydrostati c pressure: p s p in p out p 0 gz The result of integration can be expressed in the form: r sin F r p g[ r z r ( ~ z )d ~ z ] s F p integral surface tension force p integral pressure force 0 z contribution of hydrostatic pressure Details in: Danov et al., J. Colloid Interface Sci. 440 (2015)

32 Variations of s and along the Bubble Profile apex apex Anisotropic non-uniform surface tension Wrinkles in the zones with negative φ. Wrinkles theory: Danov, Kralchevsky, Stoyanov, Langmuir 26 (2010) 143.

33 DSA vs. CMD onset of anisotropy For anisotropic surface DSA gives greater nonphysical values DSA apex The error of the DSA fit is not so sensitive to surface stress anisotropy The onset of deviation of DSA from CMD may serve as criterion for surface stress anisotropy Surface tension DSA and the error of the Laplace fit given by the DSA apparatus, vs. surface tension at the drop apex, CMD (0), measured by CMD.

34 Surface storage modulus GS T = 10 s Storage modulus, mn/m ES TS QD BSC FEN TT SAP FD ASC HC Surface deformation, %

35 Take home messages & (Soft Matter) challenges The old new challenge -> The origin of interfacial rheology (1890, Rayleigh) is linked with saponins There are still unresolved theoretical & experimental issues with respect of surface rheology How we separate/extract/qc naturals? How/did we measure surface stress [tensor] and strain correctly Can we do in plane measurements / if not how we estimate local/global deformations due to the measurement Did we use the right constitutive and measurement protocols in order to convert surface storage and loss moduli into surface elasticity and viscosity? Saponins are large class of natural surfactants with unique architecture and surface properties, with multiple functionalities, which despite of old history of use and long list of functionalities are yet poorly understood -> and could be used as a proxy for validation of next generation theoretical and experimental soft matter studies Some of these natural compounds challenge our standard concepts of surfactants and surface behavior and might be at the limit of what our current methods can measure Naturals are hot consumer trend and we need to develop appropriate soft matter tools that allow us to study and conceptualize them -> similar to what we have for synthetic polymers and surfactant systems

36 Thank You! Contributors Lucy Bialek, Sergey Melnikov Unilever Research Netherlands Grahame Mackenzie, Vesselin Paunov University of Hull, UK