Surface Charge Surface Potential Surface Energy. Course 3130, Dr. Lokanathan Arcot 2 Origin of surface charge 1. Dissociation or ionization of a surface.

Surface Charge

Surface Potential

Surface Energy

Course 3130, Dr. Lokanathan Arcot

2

Origin of surface charge1. Dissociation or ionization of a surface group

(NH 3)+

COOHCOO

-

Carboxylterminated

Dissociated carboxyl

Neutral AnionicpH > PI

COOHCOOH

COO-

COO-

NH 2 NH 2 NH 2

(NH 3)+

(NH 3)+

Amineterminated

Ammoniumterminated

Neutral CationicpH < PI

Eample 1a: Cationic surface Eample 1b: Anionic surface

2. Preferential adsorption of an ion from solution

Ag+

AgI

Silver ion adsorption on Silver Iodide

Eample 1a: Cationic surface Eample 1b: Anionic surface

Ag+

Ag+

Cl-

Au

Chloride ion adsorption on Gold

Cl-

Cl-

Principles of Colloid and Surface Chemistry, Chapter 11, 3rd Edn, Paul and Raj

Physical Chemsitry of Surfaces, Chapter 5, 6th Edn, Arthur and Alice


3

Why is surface charge important

Colloidal Stability

Most Relevant Stabilization StrategiesRepulsion between Like

charges


4

Relation between surface charge densityAnd surface potential

- Ve

Surface Charge Density:Number of Charges per unit area

Surface Potential: ( Electrical Potential)Charge produces Electric FieldThus a charged surface can influence the motion of other charged particles in surrounding

+Potential due to surface at point ’A’

Point A

Move the unit +ve chargeFrom infinity to point ’A’ Unit +ve

Electric Potential – Work done in moving a unit charge from infinity to a point- Ve potenital : -ve work done+ Ve potenital : +ve work done

-Ve chargedParticle


5

Effect of surface charge in Solution

Electrical double layerSimplified representation

Anionic ion-Ve

Cationic ion+Ve

Anionic colloidal particle-Ve


6

Charges in different situations

In water

Charges in aqueous solution - solvated

Charges giving rise to surface charge – not solvated

7

Electrical double layer model : Why named ‘Double layer’ ?

LAYER ICompact non-solvated

Non-columbicCharge contributing layer

LAYER IISolvated ions: ColumbicSubdivided based on if they can move aroundStern layer – No movement; Diffuse layer - mobile



8

Electrical double layer model : Slipping Plane or Shear Plane – Zeta potential

9Course 3130, Dr. Lokanathan Arcot

Electrical double layer model : Electrical Potential as a Func. Of Distance

ΨO - absolute value of surface potential (Very difficult to be experimentally measured)Ψd - Surface potential at distance ‘d’ζ – Potential at slipping plane (Easily experimentally measurable)

Conclusion: Surface charge is responsible for ΨO

For a given solvent condition ζ is drectly related to ΨO


Electrokinetic Phenomena

+

Electrophoresis – Induced motion of charged particles/ions in presence of applied electric field

-


Electrokinetic PhenomenaElectroosmosis – Induced motion of electrolyte solution located near a charged surface in presence of applied electric field

The movement of counterions in diffuse layer of double layer

under the influence of applied electric field drags the liquid and

hence the osmotic flow+ -


Electrokinetic PhenomenaStreaming Potential /Current – Electric potential/current induced by motion of electrolyte solution over a charged surface.

Forced movement of electrolyte solution

displaces the counter ions from the double layer thus inducing a potential

or current

+-


13

Zeta Potential of Nanoparticles and Large Surfaces

1 nm – few 1000s nm Macroscopic cm

Nanoparticle Size Electrophoresis

Surface Zeta PotentialStreaming Potential


14

Zeta Potential of Nanoparticles UsingLaser Doppler Velocimetry

Light wave couples for an instant

𝛿+

𝛿–

Coupled light is randomly remitted in any direction

Rayleigh Scattering by stationary particle

λ

λ

λλ

Doppler Effect


15

Incident LightFrequency v (nu)

V = 0

V > 0

Scattered Light

ν (Same as incident)

Scattered ν + Δν or

Increase in Freq

Basis of Laser Doppler Velocimetry

Observer/Detector

Observer/Detector

What about V < 0 ?

Particle moving away from observerν - Δν

Incident LightFrequency v (nu)

V is Velocity of Nanoparticle


16

Basis of Laser Doppler VelocimetryInterferometry

Scattered ν + Δν

Reference ν

Interference

ResultantBeat Frequency


17

Basis of Laser Doppler VelocimetryInterferometry

Interference

ResultantBeat Frequency

The Frequency of the Scattred light is directly linked to velocity of Particles

How do we get the velocity of Particle from Doppler shift?


18

From Doppler Shift to Electrophoretic Mobility

Electrophoretic Mobility mcm/Vs

How to measure Electrophoretic Mobility UE ?We know Field Strength (V/cm) because we apply it Need to find Velocity of ParticleDoppler Effect: Frequency increase or decrease Δfreq

Wavelength of Incident lightΔfreq – Doppler shift (+Ve or –Ve)E – Applied Field Stregthn – Refractive index of solventθ – Angle of Scattering


19

UE – Electrophoretic Mobility – Zeta Potential – Dielectric Constant – ViscosityF(ka) – Henry’s Function

From Electrophoretic Mobility to Zeta Potential

Helmholtz Smoluchowski equation


20

Setup for Zeta Potential Nanoparticle Measurement

Sample CellProvision for applying voltage while measuring Velocity of particlesFrom Velocity and Voltage we can calculate UE

From UE we can calculate Zeta potential


Nanoparticle Zeta Potential MeasurementSample Cell

Observer/Detector


22

Stern

pla

ne

Ψ0 DebyeLength (Low)

Ψd

ζ

Distance from charged surface

Electrica

l pote

ntial

High Electrolyte

Low Electrolyte

Diffuse layerHigh ionic strength

Or High electrolyte

DebyeLength (High)

ζElectrolyte

Shear Plane


Double Layer Model – Effect of Ionic Strength


23

Double Layer Model – Ionic Strength


Debye Length (1/κ) – The distance over which the potential 1/e of surface potential Ψ0

1/ κ=( ε 0 ε 𝑟 𝐾𝑇2𝑁𝐴𝑒2 𝐼 )

1/2

ε0 – Permittivity of spaceεr – Permittivity of mediumΚ – Boltzmann ConstantT – TemperatureNA – Avagadro NumberI – Ionic Strength

The Zeta Potential Measured will decline with increasing ionic strengthSo?

If you are comparing surface charge of two surfaces, make sure the I are same.


Effect of pH on Zeta potentialDissociation or ionization of a surface group depending on

Isoelectric pH

(NH 3)+

COOHCOO

-

Neutral AnionicpH > PI

COOHCOOH

COO-

COO-

NH 2 NH 2 NH 2

(NH 3)+

(NH 3)+

Neutral CationicpH < PI

Cationic surface Anionic surface

ExamplePI 9.0

ExamplePI 4.0

pH = 0 - 8pH = 9 - 14 pH = 0 - 4 pH = 5 - 14

NH 2 NH 2 NH 2

COOHCOOH

COOH

Discussion: Predict the sign of Zeta potential

2.0 COOH, NH3+ + Ve

6.0 COO –, NH3+ + or – Ve depending

on relative conc.

11.0 COO –, NH2 - Ve

pH Zeta sign Groups


25

Zeta Potential of Nanoparticles and Large Surfaces

1 nm – few 1000s nm Macroscopic cm

Nanoparticle Size Electrophoresis

Surface Zeta PotentialStreaming Potential


26

Streaming Potential

Negatively Charged Surface

Shear Plane

Solvent Flow

Potential Develops

A flow of fluid over charged surface removes the counter-ions at and above the shear planeImbalance of charge in double layer causes a potential


27

Streaming Potential: Parallel Plate Capillary System

Streaming Current IS

Conduction Current IC

ΔP

V / IC

Apply Pressure (ΔP) to make water flow

Counterions from ’diffuse layer’ above shear plane are removed (Streaming Current IS )

Streaming potential causes a flow of ions in Stern Layer in direction opposite to flow of liquid constituting the Conduction Current IC

Creates a potential (Streaming Potential V)


28

Streaming Potential: Rotating Disk System


29

Zeta Potential and Streaming Current/Potential

Electrophoresis 2004, 25, 187–202

Zeta Potential from Streaming Current

𝜁 – Zeta PotentialV = streaming potential (V)ΔP = pressure difference across the channel 𝜂 = viscosity of the solution (kg/m/s) εr = relative permittivity of the solution (-) ε0 = electric permittivity of vacuum (F/m) σ = conductivity of polyelectrolyte solution (S/m)

𝜁=𝑉 𝜂σ

Δ P ε 0 ε r 𝜁=I S 𝜁σ RΔ P ε 0 ε r𝜁 – Zeta Potential

IS = streaming CurrentR = Resistance of channelΔP = pressure difference 𝜂 = viscosity of the solution εr = relative permittivity of the solution ε0 = electric permittivity of vacuum σ = conductivity of polyelectrolyte solution

Zeta Potential from Streaming Potential

These equations hold only for thin double layers


30

Ways to Find Surface ChargeNot to be confused with Surface Potential

Titrations:Anionic – Cationic TitrationEstimating anionic surface by titrating against cationic polymer

Acid Base TitrationEstimating Carboxyl surface surface by titrating against Base NaOH


31

Example of Zeta Potential and Surface Charge

Soft Matter, 2008, 4, 2238–2244 | 2239

Cationization of Cellulose NanoCrystals (CNC)Negatively Charged due to Sulfate groups

e – Sulfate groups

e – Chloride

The number of anionic groups is higher than the cationic groups introduced


32

SUMMARY of Part I Origin of Surface Charge

Surface Charge and Colloidal Stability

Surface Charge and Surface Potential

Electrical Double Layer Model

Shear/Slipping Plane

Electrokinetic Phenomena

Phoresis, Osmosis, Streaming

Nanoparticle Zeta potential (Laser Doppler Velocimetry)

Surface Potential - Streaming Potential/Current

Surface Charge – Titrations

Example of Surface Charge and Potential


33

Short Break

Surface energy

Atoms in bulk

Atoms at surfaceUnsatisfied

Bonding interactions

What are bonding interactions?

34

Intermolecular interactions Lifschitz-van der Waals-forces (LW).

These occur between all atoms and molecules. They are due to correlation between the electromagnetic fields created around molecules due to the distribution and motions of electrons within them (permanent and fluctuating dipole moments)

Acid-Base interactions, AB)

Interactions between electrophilic and nucleophilic groups

*Only the LW forces are of importance at distances larger than a few Å

Electrostatic interactions and electrical double layer forces are also long range interactions

35

36

Lifshitz- van der Waals interaction between neutral molecules 1 and 2

The interaction is electromagnetic, it is always attractive and it occurs between all molecules

r12

A, B = coefficients that depend on molecular dipole moments, polarisabilities and temperature

r12 = intermolecular distance

U = interaction energy

A general equation for the interaction energy is

When r is larger than a few molecular diameters, the interaction becomes negligible

U A

r126

B

r1212

37

Acid-base interactions

Lewis’ acids and bases• Acid: a group that tends to receive electron• Base: a group that tends to donate electron

Acid-base interactions are of importance only at very short distances (0,1 - 0,5 nm) but are of essential importance for the adhesion between

surfaces Example: plasma treatment of cellulose

Base Acid

Acid Base

38

Surface energy

Origin: Inter-molecular/atomic interactions

Quantification:Liquid surface – Suface tensionSolid surface – Surface energy

Surface energy of materials

• For solids, actual surface energy often depends on the history of the surface

• Surface energy = the work of the cleavage, it can depend on how material is cleaved

• Most common units of surface tension:

r

obsolete) ,cm

dyn 1 (=

m

mN 1

m

mJ 1

2

Fdr

0

r

39

SURFACE ENERGY MEASURING METHODS

40

Methods to measure surface tension of liquid

F = force, mN

L = wetted length, mm

Liquid

air

Pt plate

g

http://www.kruss.de/

Wilhelmy plate method Du Noüy ring method

Pt/Ir ring

air

F (force)

g

41

Contact angle measurement

Methods to measure surface energy of Solid

http://www.kruss.de/

Video microscope

Lightsource

Syringe

Drop

Lenses

42

43

Contact Angle

The contact angle can be measured relatively easily, but both absolute

values of angles and reproducibility of measurements are influenced by

porosity and surface heterogeneity

The most common way to measure the surface energy of solids

A liquid that does not completely wet a surface forms a finite contact angle on it

At equilibrium

gsg = gls + glgcosq (Young’s equation)

glg

gsg

q = contact angle

44

θ – contact angle. g -total surface tension,gd-Lifshitz van der Waals component, g+-Electron acceptor component, g- Electron donor component. The subscript ‘S’ and ‘L’ stand for solid surface and liquid respectively.

C. J. Van Oss et al. , Chemical Reviews 1988, 88, 6, 927

Liquid gsd mJ/m2 gs+ mJ/m2 gs

- mJ/m2

Water 21.8 25.5 25.5Formamide 39 2.28 39.6

a-Bromonapthalene 44 0 0

Surface energy componenets from CA It is often useful to formally divide surface tension into contributions from

LW and AB interactions

g = gLW + gAB additionally, gAB = g + & g -

45

Thermodynamics of adhesion

ABLWG 121212

LW12

interaction due to Lifschitz-van der Waals’ forces (dispersion, dipole/dipole)

12LW 2 1

LW2LW

AB12

interaction due to acid/base interactionsAcid: acceptor of electrons (e.g. -NH3, -OH)

Base: donor of electrons (-C=O. p-electron systems)

))((2 212112 AB

12G -ve adhesion+ve no adhesion

46

Solid Surface chemical interactions

Quantification:

Components can be determined experimentally by• Contact angle measurements (different liquids)

• Measurements of adsorption from liquid• Inverse gas chromatography

gLW g + & g -

Cationic, anionic charge

Surface charge can be experimentally determined by• Conductometric titrations

• Zeta potential measurement (Potential)

Other properties: Magnetic, Gravity


47

Lotus FlowerMechanism of Water Repellance

What about physical properties of surface?Topography

48

How do we make use of surface energy information?Example: Bacterial Adhesion to Surface

Zhang et. al. RSC Adv. , 2013, 3, 12003-12020

Designing a surface that will resist bacterial adhesion!

49

Designing a non-sticky surface

http://www.wpi.edu/Pubs/ETD/Available/etd082206162049/unrestricted/Arzu_Atabek_MSthesis.pdf

50

Designing a non-sticky surface

WDS'10 Proceedings of Contributed Papers, Part III, 25–30, 2010.

ABLWG 121212

12LW 2 1

LW2LW

))((2 212112 AB

12G -ve adhesion+ve no adhesion

51

Examples of surface energy manipulation of renewable materials

Lignin

52

How to change surface chemistry and surface topography simultaneously?

53

Coating paper with hydrophobic nanoparticlesChemistry & topography

PAH – Poly(allylamine hydrochloride)Polycation

+

--

--

--

SiO2 SiO2

PAH

Anionic surface Cationic surface

++

++

AKD – Alkyl ketene dimer

Silica

SiO2

AKD

HydrophobicCationic

Paper

-ve charge

54

0

20

40

60

80

100

120

140

160

180

0 5 10 15 20 25

Time (s)

Co

nta

ct

an

gle

PAH/AKD-coated silica

PAH-coated silica

Contact angle of water on fine paper surface coated with hydrophobically modified silica nanoparticles

The rough paper surface becomes superhydrophobic !!!

55

Contact Angle 650±20 Contact Angle 1500±20Contact Angle 1100±20

Lignin surface Lignin surface coated with 1g/100ml Aerosil R972

Lignin surface coated with 2g/100ml Aerosil R972

Effect of hydrophobic silica on the surface structure and contact angle (lignin model surface)


56

AdvancingContact Angle

RecedingContact Angle

Contact Angle Hysterisis: Dynamic Vs Static Contact Angle

Add/Remove Volume Method

Tilted Plane Method


57

AdvancingContact Angle

RecedingContact Angle

Contact Angle Hysterisis

Hysterisis is the difference between Advancing and Receding Contact Angle

H = θa - θr Mechanism: Pinning of Liquid FrontCauses of Pinning: High rougness

Chemical Inhomogeneity

Low H High H

Example of Chemical

Homogeneity


58

Summary of Part II

• Origin of Surface Energy

• Definition of Surface Energy and Surface Tension

• Methods to Measure Liquid Surface Tension

• Method to Measure Surface Energy of Solid Surface

• Surface Energy Components

• Thermodynamics of Adhesion

• Physcial property of Surface and Surface Energy

• Examples – Bacterial Adhesion

- Lignin Hydrophobization

• Dynamic Contact Angle - Hysterisis

Surface Charge Surface Potential Surface Energy. Course 3130, Dr. Lokanathan Arcot 2 Origin of surface charge 1. Dissociation or ionization of a surface.

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