10. Surface interactions– Effect of polymersMonika ÖsterbergSpring 2017

CHEM-E2150 Interfacial Phenomena in

Biobased Systems

Content

1. Example of the effect of polymers on the surface properties2. Polymers in solution

1. What determines the solubility and radius of gyration?3. Theory of polymer (polyelectrolyte) adsorption4. More examples of effect of polymers

a) Flocculation and stabilization5. Summary of surface interaction forces

Learning objectives

• You are familiar with the main surface forces and their origin

• You understand how surface interactions can be modified by adsorption of polymers

• You understand the effect of surface charge, polymer charge and properties of the media (I, pH) on the conformation of adsorbed polymers

• And how polymer conformation affects interactions

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Bioinspired lubricating films of cellulose nanofibrils and hyaluronic acidRead Valle-Delgado, JJ, Johansson, L-S, Österberg, M, (2016) “Bioinspired lubricating films of cellulose nanofibrils and hyaluronic acid” Colloids and Surfaces B: Biointerfaces 138 86-93. dx.doi.org/10.1016/j.colsurfb.2015.11.047

Think about what type of forces are present in the systemsWhat is the effect of electrolyte concentration in the media?

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HA attached to CNF films by esterification reaction between hydroxyl and carboxyl groups

Objective: Durable lubricating layer

Surface force measurement Friction force measurement

AFM

Atomic force microscope (AFM) and colloid probe technique

Measurement of forces between a colloid probe and a

substrate using an AFM.

20 μm

Glass colloid probe

What forces are present?

Van der Waals Forces?Electrostatic Double-Layer Forces?Steric forces?

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Fig. 2PBS, pH ~7

PBS = phosphate buffered saline

Increase in steric repulsion

What is the reason for the steric repulsion?

Yes

Yes

Yes

dx.doi.org/10.1016/j.colsurfb.2015.11.047

Effect of ionic strength

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Fig 3. dx.doi.org/10.1016/j.colsurfb.2015.11.047

What is the effect of ionic strength on • van der Waals attraction?• Double-Layer repulsion?• Electrosteric repulsion?

Phosphate buffered saline (PBS):10 mM Na2HPO4, 1.8 mM KH2PO4, 137 mM NaCl, 2.7 mM KCl High IPhosphate buffer (PB): 10 mM Na2HPO4, 1.8 mMKH2PO4 Low I

Hydrated layer, high repulsion → Low friction

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Low IHydrated fibril and polymer layersVery low friction

High ICollapsed polymer layerHigher friction

Effect of ionic strength II

Reflections on the previous example

Lubrication was achieved by attachment of polymersHydrated polymer layerExtended polymer chains (good solvent)

Similar approach of surface modification can be used for:• Steric stabilization of nanoparticles• Antifouling surfaces (grafting of PEG chains)• In composites for better alignment of reinforcing fibers

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Summarize forces

Surface forces recap

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ε1, n1

ε2, n2

ε3, n3

vdW forces: Always present.

- +--

+ +++ +

-ø (Surface potential)

ø (Surface potential)

-- +++

++

Ionic strength

Electrostatic Double Layer forces: Present between charged surfaces.

-- -

Ca2+

Hydration force:Additional short range steric repulsion due to strongly bound water molecules.

Forces due to adsorbed polymers: Attractive bridging force or steric repulsion. Depends on coverage, conformation and interactions between polymer and solvent.

DLVO: Ftot= Fvdw + FDL

The forces were affected by the conformation of the polymerIn this case the polyelectrolyte was covalently boundSurfaces can also be modified via polymer adsorption

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We need to understand how polymers adsorb on surfaces

We need to understand polymer conformation in solution

Polyelectrolytes in solution

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14

Most important acid groups: ‐ Carboxylate. ‐COOH  ‐COO–+ H+

e.g. lignin, hemicellulose, starch, many other polysaccharides‐ Sulfonate,. ‐OSO3H  ‐OSO3

– + H+

e.g. polystyrenesulfonate, lignosulfonate

Polyacids

Polybases Most important basic groups:‐ Amines: primary: RNH2 + H+  R‐NH3

+

secondary: R2NH2 + H+  R2NH3+

quaternary: R(CH3)3N+Cl–

Ampholytes

The term polyelectrolyte is used for polymers consisting of a macromolecule carrying covalently bound anionic or cationic groups, and low-molecular ”counterions” securing for electroneutrality.

Polyelectrolytes

Polyelectrolytes containing both anionic and cationic groups covalently bound, e.g. proteins

15

PolyacrylamideCH CH2 CH CH2 CH

NH2 NH2NH2

O O O

Neutral in neutral and alkaline,cationic in acid soutions

Modified (cationised) PAM, C‐PAMcationic at all pH:s of practical interest

C‐PAM is extensively used as a flocculant in papermaking and water treatment

CH

NH2

O

CH

NH2

CH2

O

CH

NH2

O

CH2

CH

NH

O

CH2

CH2CH2

CH2

N+

CH3 CH3CH3

CH

NH2

O

CH2

NH

O

CH2

CH2

CH2

CH2

N+

CH3 CH3CH3

CH2

N+

CH3

CH2

CH3

CH2

CH CH

CH2CH2

N+

CH2

CH3CH3

N+

CH2

CH3CH3

CH2 CH2

Diacryloethylidimethyl-ammonium chloride(DADMAC)

Poly(N,N-dimethyl-3,4-diethylpyrrolidonium chloride(Poly-DADMAC)

Cationic flocculation polymers

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Characteristics of polyelectrolytes

Polyelectrolytes

1. The ionic groups are classified as weakly and strongly acid and basic groups.

• Different pH dependence

2. The average distance between the adjacent ionic charges along the chain is an important parameter determining the polyelectrolyte behaviour, especially in solution.

3. Location of the charges.

• In integral type polyelectrolytes the ionic sites are part of the polymer backbone.

• In pendant type polyelectrolytes the ionic site is attached to the backbone with a spacer.

4. The species of low molecular counterion influences strongly on the solution properties.

Parameters that describe the electric properties of polyelectrolytes

tionpolymerizaofdegreemonomersmodifiedofamountthe

DS

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• Degree of substitution, DS  = fraction of monomers in a homopolymer that are modified by some substitution (for example, by adding an ionic group). 

DS=0‐100 %,  The DS of cationic synthetic polymers vary for end use

• Charge density, CD: Often given as equivalents/mass (mmol/g or mekv/g)

α is only important  pH is near pKa(pKa of COOH is ~4.5 so carboxylic groups are charged only when pH is basic pH >7)(pKa of NH2 is ~9.5 so the amine groups are only charged when pH is acidic pH <7)

• Degree of dissociation, α groups ionizable ofnumber totalgroups ddissociate ofnumber

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Polyelectrolyte swelling: the Donnan equilibrium

Both the coil phase and the external phase are electrostatically neutral. Hence, the polyelectrolyte coils contain more freely mobilecounterions than co-ions

The solution consists of1) Polyelectrolyte coils that are swollen withan aqueous solution containing small, 

mobile cations and anions2) A surrounding solution that contains onlysmall, mobile cations and anions

All ions except those chemically bound to the polyelectrolyte  are freelymobile between the solution in the coils (f) and the external phase (e)

Ion distribution is determined by mass equilibrium and electroneutrality conditions (Donnan equilibrium)

+ ++

+

++

++ +

+–

––

– –

–

–

–

–

–

–

–

+

+

++

+

+

Swollen polyelectrolyte with free counterions

External solution

Distribution of ions

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[X] f z [X] e = distribution coefficient

The ionic strength is given by

I 1

2zi

2co,ii

Multivalent ions increase ionic strength more effectively than monovalent ions size of polymer coil is more effectively decreased by multivalent ions

The polymer coils swell with water until the chemical potentials of water inthe coil and in the surrounding solution are equal. At equilibrium, for any ionwith charge zi

The higher the ionic strength of the solution, the smaller will be the concentra‐tion differences of small ions between the coils and the external solution.Hence, swelling decreases with increasing ionic strength. i.e.  depends onionic strength.

Multivalent ions distribute more strongly into the coils than monovalent ions!

Based on the Donnan equilibrium predict how• Degree of substitution• Electrolyte concentration• Degree of dissociation Affects the polymer coil size

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• The higher the DS the larger the coil, due to repulsion between like charges and more counter ions in the coil

• The higher the electrolyte concentration of the media the smaller the coil size. Smaller difference between concentration of counter ions inside coil and outside, smaller osmotic pressure. At high enough I the polyelectrolyte behaves as a neutral polymer.

• The higher the degree of dissociation, the larger the coil size. α affects theeffective charge.

Polyelectrolytes in solution

• Polyelectrolytes generally dissolve well in water. A dissolved polyelectrolyte molecule acquires a charge by dissociation.

• The (small) counterions of polyelectrolytes are mobile, but are influenced by the strong electric field created by the polyelectrolyte.

• The counterion concentration inside the polymer coil is high.• The chain becomes more rigid (it is stretched) because of osmotic

repulsion between counterions of neighbouring segments.

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22

• The effective charge of a polyelectrolyte is lower than the stoichiometric charge because of counterion condensation.

• The swelling of a polyelectrolyte with water depends strongly on ionic strength (osmotic pressure).

• The mean dimensions of the coil increases as the charge density of the polyion increases.

• At high ionic strengths the polyelectrolyte behaves more or less like a neutral polymer.

Polyelectrolytes in solution

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Polymer Molecular weight(millions)

Radiusnm

Polyethyleneimine 0.0250.6

2890

Cationic polyacrylamide

DS 20% 0.751.42.4

280420560

DS 59% 0.791.93,4

300500700

DS 80% 0.881.853.54.2

320480720800

How does

a) the length of the polymer chain

b) the amount of charged groups (DS)

affect the size of the polymer coil in water?

Polyelectrolytes in solutionHydrodynamic radii of cationic polymers in aqueous solution

Polymers in solution

•Why polymers dissolve?

•Why polyelectrolytes dissolve?

•Size and molecular weights are determined from polymer solutions

Solubility Thermodynamics

For polyelectrolytes in water the enthalpy term is negative and polyelectrolytes dissolve easily

mixmixmix STHG

Polymers in solution

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•If there are no restrictions the polymer forms a random coil, whosesize depends on the solvent

•In a good solvent the chain will expand

•In a poor solvent the chain will contract, to reduce interactionswith the solvent

•Between the extremes is so called θ(theta) solvent (and the polymer behaves like an ideal polymer)

Good solvent: Theta solvent: Poor solvent:

Polymer solution theory

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Three concentration ranges can be distinguished:

I. Mean intermolecular distance >> Rg,‐ ”colloidal solution”

II Mean molecular distance ≈ Rg,.‐Strong interactions

III Mean molecular distance < Rg, ‐Polymer chains form solvent‐swollen polymer network. 

Flory-Huggins theory

Predicts the solubility of polymersIs based on a lattice theory, which makes certain assumptions:

• The mixture of solute and solvent is completely random.• One solvent or solute molecule in each lattice site.• The total volume of the system remains constant when solute and solvent is

mixed, Va +Vb = Vsolution.

• The number of neighbours is constant.• Only interactions between nearest neighbours are considered.

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The basis of Flory-Huggins theory

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This theory predicts the solubility of polymers, mixing of solutions andsolvent‐swelling of polymer.

NA solvent molecules

NB solutemolecules

SolutionN = NA+NB

A polymer solution is stable if for the process

Gibbs’ energy GM < 0.

The Flory‐Huggins theory is based on the theory of regular solutions, whichassumes:

‐ "Ideal" mixing entropy SM

‐That the changes in contact energies between polymer segments and solvent,the enthalpy of mixing, HM, describes these interactions

GM = HM ‐ TSM < 0

Entropy of mixing

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B A A + B• There are many more ways of arranging the molecules so that they form a solution.

• Hence, the solution is a more probable state than the state in which solvent andpolymer molecules are separately.

• Formation of solution is accompanied by an entropy increase. If nothing else affect the process of solution, polymer dissolves in solvent completely.

Enthalpy of mixing

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Solubility of polymers

Regular solutions

Before mixing After mixing

interaction energy between two molecules of type A

similarly If mixing on a molecular level, there’s a different molecule as the nearest neighbour

BBAAH 0

ABfH 2

2

BBAAABmixH

2

BBAAAB

BA

BAmix Z

NNNNH

1) 2) 3) 4)

1) Number of molecules of type A

2) Probability to achieve molecule of type B as the nearest neighbour to type A

3) Number of nearest neighbours (coordination number)

4) Contact energy

Mixing enthalpy

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The volume fractions of the polymer and the solvent are:

A NAVA

NAVA NBVB B NBVB

NAVA NBVB

VA = the volume occupied by a solvent moleculeVB = the volume occupied by the polymerNA = the number of solvent moleculesNB = the number of polymer molecules

• The basic assumption is: the volume of a solvent molecule and a polymer segment is equal so that VB = rVA

Mixing enthalpy

33

+

Pairwise segment‐segment interaction

BB

Pairwise solvent‐solvent interaction

AA

Pairwise segment‐solvent interaction AB

When A-A and B-B contacts are replaced by A-B contacts the change in contactenergy per A-B-bond formed is

AB AB 1

2AA 1

2BB

• Assume that each solvent molecule has z neighbours.

• In the solution, on the average, the fraction of polymer neighbours that are polymersegments is A and the fraction of segment neighbours that are solvents is BHence, the number of A‐B‐contacts in the solution is NBzrA = NAzB and theEnthalpy of mixing is

HM NAzBAB NBzrAAB

The interaction parameter of Flory and Huggins, chi parameter

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The dimensionless parameter is defined by  zAB

kTwhich is called Flory‐Huggins interaction parameter or ‐ (chi) parameter

= 1 when zAB = kT, i.e. the interaction energy is equal to the thermal energy.

• if < 1/2 the polymer is completely soluble

‐parameter can be calculated in solubility of polymer or activity of solventin solution (pressure of gas). Respectively the solubility of polymer can be estimated if ‐parameter in solution is known. 

• The temperature in which = 1/2 is called the ‐temperature (theta temperature).And the polymer behaves ideally. ‐> the polymers float in the solution without interferencefrom each other or from the chains the polymers are formed

The interaction parameter: chi

Good solvent 0< <0.5Theta solvent = 0.5Poor solvent >0.5

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Summary of polymers in solution

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GM = HM ‐ TSM < 0

The entropy of mixing positiveThe enthalpy term will vary depending on interactions between polymer and solvent

Polyelectrolytes in aqueous media: Conformation dependent on effective charge and electrolyte (salt) concentration

Adsorption of polymers and

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A typical adsorption isotherm for a monodispersepolymer

The plateau adsorbed amount is a function of molecular weight and solvency. The higher the molecular weight the higher the adsorbed amount is in general. Adsorbed amounts are as a rule higher in theta solvents than in good solvents.

Net adsorption depends both on the properties of the solvent and the surface

1

0.5

)mg/m( 2

C

39

Structural aspects

tailloop tail

train

tail

Polymers in solution have large number of internal degrees of freedom. Adsorption leads to changes in conformation and loss in conformational entropy for the polymer.At low coverage chains have a tendency to flatten (lowest energy).

Description of the interfacial region

• An increase in concentration of solute in the interfacial region is called adsorption.

• Chemisorption – adsorption involving formation of chemical bonds

• Physisorption – only physical interactions• Depletion – negative adsorption, reduce in solute

concentration near the interface• Adsorption depends on the net adsorption energy for polymer

segments, i.e. difference between free energy ∆G of segment/surface contact and solvent/surface contact. Should be sufficiently negative

Scheutjens-Fleer theory (lattice model)Application of Flory-Huggins theory to adsorption

Calculate G for the process:

polymer in solution adsorbed polymer

Segment/surface interaction parameter χs

Segment/solvent interaction parameter χ

STHG Free energy

Change in entropy

Change in enthalpy

∆G<0 for adsorption

surface

polymer

solvent

– Interactions between polymer segments and surface

– Interactions between solvent and surface

– Interactions between polymer and solvent in solution

– Interactions between polymer and solvent at the surface

Factors affecting the enthalpy, ∆H:

Based on these facts give some examples of surface, solvent or polymer properties that will promote adsorption.

Factors that affect the entropy, ∆S

Polymer concentration at interface increases entropy of mixing for polymer decreasesLarge amount of solvent molecules are released from polymer network entropy of mixing increasesTotal entropy of the system increases one driving force for adsorption

ExerciseWhat is the conformation of the adsorbed polyelectrolyte in the following cases and how does the situation change if the salt concentration in solution increases?

1. The surface is neutral and the polymer is charged.2. The surface and the polyelectrolyte have the same sign of charge.3. The polyelectrolyte and the surface have opposite charge. The surface

charge density is high and the charge of the polyelectrolyte is low. 4. The polyelectrolyte and the surface have opposite charge. The charge

density of both the surface and the polyelectrolyte is high.

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1. Neutral surface, charged polymer

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++ +

Low I: Repulsion between segments, low adsorption. Needs to be other driving force for adsorption than charge neutralization

++ +

++ +

+ + +

High I: Screening of repulsion between like charges and DL-repulsion. Adsorption easier. Still needs to be other driving force for adsorption than charge neutralization

--

- -

-+

+++

---

- --

--

Polyelectrolyte adsorption when polyelectrolyte and surface have the same sign

46

Low salt concentration –no adsorption

Intermediate salt concentration –adsorption in an extended conformation, provided other affinity drives adsorption

Fleer et al, Polymers at interfaces, Chapman& Hall, 1993, chapter 7

Polyelectrolyte adsorption on oppositely charged surface – low charge on polymer

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The surface has higher charge density than the polyelectrolyte – extended conformation, much loops and tail.

Fleer et al, Polymers at interfaces, Chapman& Hall, 1993, chapter 7

Low I

Extended conformation, much loops and tails

Less adsorption. Too high I might restrict adsorption (no affinity)

High I

Polyelectrolyte adsorption on oppositely charged surface – High charge on both surface and polymer

48

Extended conformation, much loops and tails.

Fleer et al, Polymers at interfaces, Chapman& Hall, 1993, chapter 7

The surface and polymer has the same charge density – flat conformation

Low I High I

Conformation of polyelectrolytes – effect of ionic strength

49

Low ionic strength

++++

+

++

+ +++++

High ionic strength

+++++

++ +

+

++

++

++

+

The effect of different parameters on polymeradsorption

Properties of surfaces- Surface charge, surface area, chemical consistency of surface

Structure of polymer- Solubility, molecular weight, degree of substitution, structure (linear/ branched)

Properties of solvent- pH, ionic strength

Practical applications of polymer adsorption• Retention aids in papermaking• Strength additives in papermaking and composites• Surface modification

• Steric stabilization• Charge reversal• Layer-by-layer deposition

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Layer-by-Layer deposition

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Theory

+- H2O H2O

Adsorption of charged polymers (polyelectrolytes) or charged particles on a surface with an opposite charge leading to charge reversal.

Driving force for build-up of multilayer:Electrostatic attraction, Donor/acceptor interactions, Hydrogen bonding, covalent bonds, or specific recognition.

For polyelectrolytes: Entropy gain by the release of the counterions

Examples

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~160º

Cationic PLL + anionic wax particles

Conformation of polymer affected adsorption

Breathable and water resistant textiles

LbL cationic polymer + CNF: Effect on paper strength and ductility

Wågberg’s group: dx.doi.org/10.1016/j.carbpol.2013.03.049

Adsorption of C-PAM on bleached kraft pulp. Mw ≈ 1 milj., pH 7 DS 7.8, 15.3, 27.8, NaCl

2/11/201954Lindström, T. and Wågberg, L. Tappi J.66:6 (1983) 83

•What is the effect of degree of substitution on adsorption?

•What is the effect of electrolyte concentration on adsorption in each case? Explain both the rise and decrease in adsorption upon adding salt.

DS

The effect of time on the adsorbed layer

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Adsorption on a porous surface; slow changes

Time

Conformational changes: very important in practical useof flocculants

Time

Summary of polymer adsorption at surfaces• The conformation in solution affects the conformation of

adsorbed layer: extended col – flat conformation

• Entropy of polymer decreases upon adsorption but entropy of solvent molecules increase – net positive effect

• Polyelectrolytes: Effect of ionic strength: High I coiled polymer in solution – more loops and tails upon adsorption

• + other factors affecting adsorption

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