sig talk –surfactants for fluid dynamics · 2020. 3. 22. · nature of the surfactant layer...
TRANSCRIPT
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SIG talk – surfactants for fluid dynamics
Stuart ClarkeUniversity of Cambridge
Department of Chemistry and BP Institute
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Outline• Surfactant types – a reminder:
• Roles in fluid flow problems (some of our work)
1) Surfactant bulk solution behaviour:Ø‘Low’ conc: CMC è ‘micelles’ à mass transport issues/viscosity
Ø‘High’ conc: Mesophases è viscosity, non-Newtonian flows, anisotropic materials -
alignment, Shear induced phase transitions…
2) Interfaces – air/liquid, solid/liquid...How much is there –reduced surface tension
(Drop shape.. static systems)
Nature of the surfactant layer solid/liquid (Slip/non-slip boundary conditions)
Gradients in surface composition: Marangoni flows of the bulk
Dynamic systems: Foams, Dynamic surface tension etc..
Adsorbed layers under shear/flow è adsorbed species under extreme conditions
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Molecular adsorption:What’s the problem?
Tiny quantities of material at the surface; often one molecule thick
Lots of bulk material – ‘buried’ interfaces Bulk dominates most techniques
How can we ‘see’ the monolayer at the surface – without disturbing it?
Rather specialised methodsRather a lot of them
Fluid‘bulk’ solution
Solid Substrate
Monolayer
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Techniques
‘Wet’ surface methods (solid/liquid and liquid/liquid)Thermodynamic methods- Adsorption isotherms (S/L): - IFT/Contact angle/’spinning drop’ (S/L and L/L) How much adsorbs/competitive ads, temperature dependence è DH, DS.Spectroscopy: -ATR/AFM-IR (S/L) (including T, P and shear)-RAIRS/SFG (L/L) : Composition/binding/molecular orientation‘UHV’ methods:-XPS/SIMS/EDX/BSED (S/L) (depth profiling) inorganic composition.. (What is the surface you have? Fe oxide, Cr oxide …?)Neutron/synchrotron methods:Reflection (S/L): competitive ads., layer structure
Temperature, pressure, shearSANS/SAXS/Coherent/incoherent.. etc..Others:QCM – Ultra thin/SFGAFM/STM
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Techniques
‘Wet’ surface methods (solid/liquid and liquid/liquid)Thermodynamic methods- Adsorption isotherms (S/L): - IFT/Contact angle/’spinning drop’ (S/L and L/L) How much adsorbs/competitive ads, temperature dependence è DH, DS.Spectroscopy: -ATR/AFM-IR (S/L) (including T, P and shear)-RAIRS/SFG (L/L) : Composition/binding/molecular orientation‘UHV’ methods:-XPS/SIMS/EDX/BSED (S/L) (depth profiling) inorganic composition.. (What is the surface you have? Fe oxide, Cr oxide …?)Neutron/synchrotron methods:Reflection (S/L): competitive ads., layer structure
Temperature, pressure, shearSANS/SAXS/Coherent/incoherent.. etc..Others:QCM – Ultra thin/SFGAFM/STM
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Surfactant types – a reminder:
• Basic structure: ‘head’ and ‘tail’èAmphiphilic è preference for interfaces:
èRange of architectures:
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Molecule Name StructureSDS Sodium dodecyl sulfate CH3(CH2)11OSO3 Na
CTAB Cetyl trimethylammonium bromide
CH3(CH2)15NMe3 Br
CnEOm Alkyl ethylene oxides CnH2n+1O(CH2CH2O)mOHClass Head Group Applications
Anionic ¾ CO2- Na+ Soaps
¾ SO3- Na+ Synthetic detergents
¾ OSO3- Na+ Detergents, personal care
products¾ OPO3
- Na+ Corrosion inhibitors, emulsifiers
¾ (OCH2CH2)n OSO3-
Na+Detergents, toiletries, emulsifiers
Cationic ¾ NMe3+ Cl- Bitumen emulsions,
disinfectants> NMe2
+ Cl- Fabric and hair conditionersNon-ionic ¾(OCH2CH2)nOH Detergents, emulsifiersZwitterionic ¾ NMe2
+¾CH2¾SO3- Shampoos, cosmetics
Most important surfactants:Chemical nature of heads and tails:
THREE most important types:Anionic/cationic and non-ionic
Class Tail GroupAlkyl chains
¾ CH2 (CH2)nCH3
Alkyl benzene
Alkyl aryl
a-olefinCH3 (CH2)n CH =CH-
Polypropylene oxide* ¾(OCH2CHMe)nOH
¾ CH2(CH2)nCH(CH2)mCH3
CH3(CH2)n ¾ O ¾
Surfactant types – a reminder: ionic/non ionic/zwitterion/ mixtures
Our interest and commerically:Corrosion inhibitorsAnti wear agentsFriction modifyers.... Etc.
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Molecule Name StructureSDS Sodium dodecyl sulfate CH3(CH2)11OSO3 Na
CTAB Cetyl trimethylammonium bromide
CH3(CH2)15NMe3 Br
CnEOm Alkyl ethylene oxides CnH2n+1O(CH2CH2O)mOHTHREE most important types
Ionics: respond strongly to added salt (electrostatics)Non-ionics: respond strongly to temperature
Surfactant types – a reminder: ionic/non ionic/zwitterion/ mixtures
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Bulk solution Phase behaviour –’self assembly’• Micelles• Mesophases
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Micelles
• Surfactants form ‘association colloids’ with increasing concentration (in water).
(c.f non-aqueous systems)Evident in many physical properties -particularly surface tensionè‘Critical micelle concentration’ (CMC)Driven by ‘entropy’ of the water
Hydrophilic ‘heads’ outside in waterHydrophobic ‘tails’ inside
NOTE: generally still very dilute solutions: CMC for SDS 8 mMVERY non-ideal even when dilute
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Micelles
Surfactant mobility (complex!):• Low conc: C< CMC have ‘monomer diffusion• High conc: C> CMC Micelle diffusion.Some studies show a pronounced fall in mobility as CMC is crossed:Others that ‘Although micelles have a lower mobility than monomers have, the average mobility of surfactants is shown to increase rather than decrease upon micellization’*.
* A. I. Rusanov Colloid Journal, 2016, 78, 102–108) On the theory of surfactant mobility in micellar systems
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Micelles: Shape
• Micelles – NOT only spherical• Depends on ‘head’ and ‘tail’ sizes
Rules of thumb:
‘Big’ head + ‘small’ tail è spheres
‘head’ = ‘tail’ è layers
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Micelles: Shape• Micelles – NOT only spherical• Depends on ‘head’ and ‘tail’ sizes
‘Big’ head + ‘small’ tail è spheres
‘packing parameter’: !"#$%
Shape ConditionsSpherical Micelles v/a0lc < 1/3
Non-spherical/cylindrical micelles
1/3 < v/a0lc <1/2
Vesicles or bilayers 1/2 < v/a0lc <1Inverted micelles 1 < v/a0lc
Used to guestimate micelle shape
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Micelles: Shape
• Micelles – NOT only spherical• Depends on ‘head’ and ‘tail’ sizes
VERY big tails è ‘inverted’ micelles(‘heads’ on inside)
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Surfactant phase behaviour: concentrated solutions
• Mesophases -Typically:Hexagonal (H) and Lamella (La)
Different viscosities (higher than micellar fluids)Shear induced phase changes (La è ‘onions’- Multilamella vesicles)
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Surfactant phase behaviour: concentrated solutions
• Mesophases: Hexagonal (H) and Lamella (La)Liquid crystals: Intermediate degrees of order: Not fully ordered like a crystal.. But not completely disordered like a normal liquid(‘Lyotropic’ vs ‘thermotropic’)
Nematic SmecticIsotropic
Liquid crystalline phases(Doug Cleaver)
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Surfactant phase behaviour: concentrated solutions
• Mesophases -Sequence: TypicallyMicelles (L1) è Hexagonal (H) è Lamella (La) è Inverted phasesWhy: complex (amount of water and ‘head group’ size – same rule of thumb as before)
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Surfactant Behaviour at Surfaces:Adsorption
! "d ln & = −)* Γ
Surfactants preferentially adsorb at interfaces:
Reflected in falling surface tension (g):Gibbs equation:Surface ‘concentration’: Gbulk concentration: c
More at surface è lower surface tension(cf salt solutions. E.g NaCl surface tension rises)Can estimate area per molecule…
Adsorption stops when micelles form(chemical potential of monomer stops changing)
g
At air / liquid interface:Delocalised adsorption
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Surface tension used as a measure of impurity• If there is a MORE surface active impurity in sample.• Get characteristic ‘dip’ at the CMC
‘extra’ material at surface è lower surface free energy.Prefers to move to micelles when formed Cleanliness/purity: BIG problem
of experiment surface science
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Surfactant adsorp.on – solid/liquidBelow CMC – rather little adsorption: Molecules like being ‘free’> CMC - surfactant adsorbs cooperatively.
Hydrophobic (water hating) surface in water: monolayerHydrophilic (water loving) surface in water: bilayer
(Can be ‘patchy’ or micellar)
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Surfactant adsorption – specific adsorptionBelow CMC – ‘Normally’ rather little adsorption: Molecules like being ‘free’Below CMC but with ‘specific’ interactions (e.g. electrostatics)-surfactant can adsorb.
At solid / liquid interface:Can be localised adsorptionSurface ‘sites’/reaction…
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Chemistry at the surface
• Wide range of interactions of ‘surfactants’ with surface groups.
Van der waals forcesHydrogen bondsElectrostatics – cation bridgingLigand exchange......
Electrostatic
Ligand exchange
Cation Bridging
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Anion Binding Mechanisms• Equilibrium constants
(surface (S), adsorbate (A), divalent ions (D) )
SOH2+ ↔ SOH + H+ K1 (dissociation)
SOH ↔ SO- + H+ K2 (dissociation) SOH + D2+ ↔ SOD+ + H+ Kd (divalent adsorption)SO- + D2+ ↔ SOD+ Kd2 (divalent adsorption)SOH + A- ↔ SA + OH- KL (ligand exchange)SA1 + A2- ↔ SA2 + A1- KL2 (ligand exchange)HA ↔ A- + H+ KA (acid dissociation) SOD+ + A- ↔ SODA Kb (binding)
Electrostatic
Ligand exchange
Cation Bridging
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Ligand exchange pH dependence• Hexanoic acid on alumina from water
• The anion of the acid binds to the Al on the surface• Displaces Al-OH
• Low pH protonated acid è no adsorp<on• Around pKa of acid get anion è adsorp<on• High pH lots of OH- so acid is displaced.
Adsorbate ‘reacts’ with surface sites: localised adsorp<on
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Drop Shape – measuring surface tension
• Pendent drop• Gravity è elongates drop• Surface tension èspherical
èBalance used to measureSurface tension
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Nature of Surface layers (‘solid’ or ‘liquid’)• Amount at the surface
Tends to max out at the CMC
‘Inter-surfactant’ van der Waals interactions:
Longer chains, more ‘solid’ like films
(cohesive strength)
(whole range on non-covalent inter-surfactant interactions: Hydrogen bonding/halogen bonding..)
èSlip vs no-slip boundary conditions
Example: Bubble rise:
è Important for bubble stability
No surfactant: slip conditionWith surfactant: No-slip condition(Jie Li)
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Behaviour at Surfaces: Foams
Kine%cs: Foam forma%on
To make a stable foam.. (Guinness!!)à Need to trap gas rapidly (before escape)Pure water will not support a foam.
à Surfactants need to migrate to surface rapidlyà Need to make a stable surfactant film at surface
(Micelles/monomer mass transport)
èSmall surfactantèBig (long tail) surfactant
è Optimum often around C12 tail
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Film drainage (bubble stability): • As water film thins: repulsive surfactant interactions prevent rupture:(and viscosity)
Details of the interaction of two charged surfaces has been well studied.(salt, pH etc. dependent)
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Marangoni effect• Different amounts of surfactant è surfactant will move• This surfactant flow can ‘drag’ the bulk phase with it(flow along surfaces can be fast)
Eg prevents bubble coalescenceDepleted surfactant in gap.Other surfactant flows in from surroundsDrag water with itStops thinning of water filmStabilises the bubbles and prevents coalescence
High surface conc: ! low
Low surface conc: ! high
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Detergency
• Surfactant adsorbs at ‘soil/water’ and water/solid interfacesè roll up• Adsorbs at dispersed soil/water interface to stabilise
emulsions/prevent re-deposi:on
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IS IT TRUE?
• Lots of interesting behaviour..How do we know..?
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How do we study surfactant layers: Static/Shear• LOTS of ways.Adsorbed amounts: Depletion isotherms (amount as a function of concentration)
Chemistry: surface specific spectroscopy (SFG/AFM-IR..)
Structure: Scattering e.g. neutron reflection (today)
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Molecular adsorption:What’s the problem?
Tiny quantities of material at the surface; often one molecule thick
Lots of bulk material – ‘buried’ interfaces Bulk dominates most techniques
Often multicomponent mixtures (commercerially)
How can we ‘see’ the monolayer at the surface – without disturbing it?
Rather specialised methods
Fluid‘bulk’ soluFon
Solid Substrate
Monolayer
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! = 4$%&'()
Neutron Reflection: how does it work?• Reflection of neutrons from the
solid/liquid interface at grazing angles• Collect reflected intensity as a function of ‘angle’ (q)..
(
• Recently new solid/liquid Interfaces: iron oxides, ss, alumina, Ti oxides, Ni, Cu …
(previously: silica, Al2O3)• New conditions:
Temperature/shear/pressure
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Neutron Sca,ering Facili1es WorldwideInstitut Laue-Langevin, Grenoble, France(Reactor Source)
ISIS, Didcot, U.K.(Spallation Source)
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Neutron Reflection Theory
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.00 0.05 0.10 0.15 0.20 0.25
Refle
ctiv
ity
q, Å-1
No Layer
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Neutron Reflec,on Theory
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.00 0.05 0.10 0.15 0.20 0.25
Refle
ctiv
ity
q, Å-1
No Layer 20 Å Layer
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Neutron Reflection Theory
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.00 0.05 0.10 0.15 0.20 0.25
Refle
ctiv
ity
q, Å-1
No Layer 20 Å Layer 100 Å Layer
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Neutron Reflec,on Theory
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.00 0.05 0.10 0.15 0.20 0.25
Refle
ctiv
ity
q, Å-1
No Layer 20 Å Layer 100 Å Layer
Film composition
Film thickness
Layer thickness
Substrate
Liquid
‘Fit’ experimental data to model.è How much material at the surfaceè Layer ‘structure’è How much solventè Nature of solid surfaceè etc…è Non-invasive/in-situ
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Neutron: ‘Magic’
• Contrast matching..• Making things disappear!
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Mixtures: Contrast matching: ‘magic’
• Scattering from silica in water: Mixtures of H2O and D2O
Change scattering of water(refractive index = ‘colour’): silica –disappears
‘See’ each component of a mixture separatelySimplify complex systems
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Contrast variation and matching• Sample
Can you see what the sample is? Can you now?
FLUIDS SIG
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Neutron reflection: StaticSurfactant (AOT) on Calcite (CaCO3)/ oil
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.006 0.06
Refle
ctiv
ity
q, Å-1
Calcite - d-heptane (bare)
Calcite - AOT - d-heptane
AOT bilayer
Adsorption of a monolayer on a surface:(essentially extended chain length of AOT thick and very little solvent)(The bare surface, monolayer and bilayer calculations)
Stocker et al JCIS, 418 (2014), 140
oil
Calcite
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Example:Additives on calcite..
Monolayers, bilayers, multilayers and adsorbed polymers.
Effects of added electrolyte on binding and structure
Surface corrosion
Molecular precision!
Multi-layered assemblies è
Stocker et al. Prog. Coll. Polym. Sci. (2012) 139, 91Stocker, PhD Thesis (2013)
Note: surfactant on hydrophilic solid in water adsorbs as a bilayer
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Calcite/Surfactants/water:In-situ
• In-situ neutron shows bilayer adsorp1on from water.
(hydrophilic surface)
• Contact angle suggests hydrophobic surface?
• QCM suggests a monolayer?
è Leaflet ‘peeling’è Poorly coupled second leaflet
Important to study in-situ
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Adsorbed layers under shear
• Neutron reflection under shear…
Inherited wisdom: ‘organic inhibitor layers can be removed by flow of fluids in a pipe leading to worse corrosion’
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Shear set-up• Can apply both steady and oscillatory shear
• Modest shear rates –pipe flow, or flow over rock-beds:
steady up to 500s-1 and oscillatory up to 500%, 100Hz.
Mount
Alumina
Solution added here
Ti cone (1.0o, 0.097mm) In-situ NR on Figaro at ILL, Grenoble, France
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Surfactant - AOT
CMC (2.5 mM)– bilayer adsorp6on 3̴3Å
2wt% – bilayer plus lamellae
Langmuir 2010, 26(18), 14567–14573
Langmuir 2011, 27, 4669–4678
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Results: Specular Reflection
Thin molecular layers under varying shear – no change
Inherited wisdom: ‘organic inhibitor layers can be removed by flow of fluids in a pipe leading to worse corrosion’è Will need VERY high fluid shear flow to move molecular adsorbed layers!
Lamella data showing Bragg peaks from very thick layers which are lost under shear.
Some evidence of ‘peeling’!
10-4
2
4
68
10-3
2
4
68
10-2
2
4
68
Inte
nsity
2 3 4 5 6 7 8 90.1
qz (at qx=0.0) / Å-1
Specular Reflection
446365 446399 446400 446401
Exploring higher shear regimesElevated pressure and temperature
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Not JUST neutrons
• Very complicated systems• Combine many experimental methods
• Surfactants adsorbed on metals…
Examples: Iron/fatty acids (c=c)/oil
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Fatty acids: effect of double bonds
C18 fatty acids with zero, one and two double bonds:What is the structure on the surface of iron (from oil/dodecane)?Depletion isotherms:Polarised neutron reflectionSum frequency generation spectroscopy
Stearic: dense well-packed upright layer..Linoleic: rather disordered layer
NR:Layer thickness: oleic acid: reasonably upright, Oleic acid is NOT washed off.(similar but slightly higher adsorbed amount than depletion: roughness)
SFG:in D-dodecaneMore disordered chain packingStearic<oleic<linoleic
C=C-H peaks (3000cm-1):(Isomerisation on heating: cisètrans)
DOI: 10.1021/acs.langmuir.5b04435 Langmuir 2016, 32, 534−540
CombinaYon: Very detailed studies
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Conclusions• Surfactant types – a reminder:
ionic/non ionic• Roles in fluid flow problems
1) Surfactant bulk behaviour:ØCMC è ‘micelles’ à mass transport issues
ØMesophases è viscosity, non-Newtonian flows, anisotropic materials - alignment, Shear induced phase transitions.
2) Interfaces – air/liquid, solid/liquid...How much is there –reduced surface tension
(Drop shape.. static systems)Nature of the surfactant layer solid/liquid (Slip/non-slip boundary conditions)Gradients in surface composition: Marangoni flows of the bulkDynamic systems: Foams, Dynamic surface tension etc..
Adsorbed layers under shear è extreme conditions
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Thank You