SIG talk – surfactants for fluid dynamics

Stuart ClarkeUniversity of Cambridge

Department of Chemistry and BP Institute

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

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

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

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

Surfactant types – a reminder:

• Basic structure: ‘head’ and ‘tail’èAmphiphilic è preference for interfaces:

èRange of architectures:

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.

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

Bulk solution Phase behaviour –’self assembly’• Micelles• Mesophases

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

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

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

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

Micelles: Shape

• Micelles – NOT only spherical• Depends on ‘head’ and ‘tail’ sizes

VERY big tails è ‘inverted’ micelles(‘heads’ on inside)

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)

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)

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)

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

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

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)

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…

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

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

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

Drop Shape – measuring surface tension

• Pendent drop• Gravity è elongates drop• Surface tension èspherical

èBalance used to measureSurface tension

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)

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

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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+

+

+

+

+

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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

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

IS IT TRUE?

• Lots of interesting behaviour..How do we know..?

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)

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

! = 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

Neutron Sca,ering Facili1es WorldwideInstitut Laue-Langevin, Grenoble, France(Reactor Source)

ISIS, Didcot, U.K.(Spallation Source)

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

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

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

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

Neutron: ‘Magic’

• Contrast matching..• Making things disappear!

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

Contrast variation and matching• Sample

Can you see what the sample is? Can you now?

FLUIDS SIG

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

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

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

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’

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

Surfactant - AOT

CMC (2.5 mM)– bilayer adsorp6on 3̴3Å

2wt% – bilayer plus lamellae

Langmuir 2010, 26(18), 14567–14573

Langmuir 2011, 27, 4669–4678

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

Not JUST neutrons

• Very complicated systems• Combine many experimental methods

• Surfactants adsorbed on metals…

Examples: Iron/fatty acids (c=c)/oil

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

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

Thank You