Chee2940 Lecture 12 - Surface Chemistry

CHEE2940: Particle Processing Lecture 12: Colloid & Surface Chemistry I This Lecture Covers Introduction to colloid and surface chemistry Surface energy and surface tension Measurement of surface energy and surface tension Chee 2940: Surface Chemistry I

12.1

INTRODUCTION

WHAT ARE THE COLLOIDS? • Are objects in the size range of approx. 1 nm -9 -6 (10 m) to 10 microns (10 x 10 m). .

• Can be particles, droplets, bubbles, macromolecules, flocs, etc. • Examples: aerosols (inhaler), cement, soil, paint, pesticides. • Processes: food, pharmaceuticals, ore flotation, water treatment. Chee 3920: Surface Chemistry I

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COLLOID SCIENCE • Colloid science deals with colloids. • Is an interdisciplinary subject, with some prominent areas of physics (e.g. van der Waals interaction) and physical chemistry (e.g. adsorption). SURFACE/INTERFACE SCIENCE • It deals with surfaces (solid-gas, fluid-gas) and interfaces (more general term). • Focuses on atoms, molecules, and phenomena occurring at the interface (e.g., surface oxidation, surface forces). Chee 3920: Surface Chemistry I

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THE LINK BETWEEN COLLOID AND INTERFACE SCIENCE

• Features of colloidal systems: o Large surface-area-to-volume ratio.

surface area πD 6 o For a sphere, = = . 3 πD /6 D volume 2

o Surface properties (e.g., adsorption & electric double layer effects) are dominant.

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o Surface forces are greater in magnitude than body forces. o Amount of chemicals for surface coverage and modification can be quite small.

• Surface science is closely linked with colloid science. o Colloid science is inevitably part of surface science. o The reverse does not necessarily hold. Chee 3920: Surface Chemistry I

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TOPICS TO BE COVERED IN CHEE 2940

• Surface/Interface characterisation (surface tension, surface energy, hydrophobicity, wettability, surface charge, etc.) • Surface properties (adsorption & surfactants). • Surface and colloid forces (van der Waals forces, electric double layer forces, and nonDLVO forces). Chee 3920: Surface Chemistry I

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12.2

SURFACE ENERGY AND TENSION

SURFACE ENERGY

• Additional energy associated with the creation of a surface – due to the higher energy state of unfulfilled bonds at the surfaces. Extra energy due to these unfilled (broken) bonds

Surface

Individual atom (in solids) or molecule (in liquids)

Chee 3920: Surface Chemistry I

Attractive force holding atoms and molecules together

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• Surface energy of polar (ionic) substances is very high (They are soluble in or wetted by water)

Ionic crystal of NaCl salt. Each atom has six nearest neighbors, with cubic (BCC) geometry. The cubic shape of salt (NaCl) crystals is due to the regular arrangement of atoms forming the crystal. Chee 3920: Surface Chemistry I

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There are two basic cubic crystal structures: - Body centred cubic (BCC) structure and - Faced centred cubic (FCC) structure. In the BCC structures, there is one atom at each corner of the cubic unit cell and one atom in the cell centre. Atom touch along the body diagonal of the cube.

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In the FCC structures, there is one atom at each corner, one atom in each face, and the atoms touch along the face diagonal.

NaCl crystals: Salt has the BBC structure but halite (a NaCl mineral) has the FCC structure. Chee 3920: Surface Chemistry I

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• Surface energy of apolar substances is low (e.g., graphite – van der Waals bonding, layering structure – we can use pencils to write on paper) 0001

10 12 10 11 10 10

Space lattice with structured layers (left) and crystal (right) of graphite Chee 3920: Surface Chemistry I

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Space lattice with structured layers (left) and crystal (right) of diamond

Carbon forms - graphite (layered structures, water repellent) - diamond (hexagonal structures, heavy) - coal (low density – r. d. ~ 1.3). Chee 3920: Surface Chemistry I

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• Chemical bonding (revision from chemistry) o Ionic bonding: - Electrons are completely transferred from one atom to another. - Positively and negatively charged ions are formed. - The oppositely charged ions are attracted to each other by electrostatic forces which are the basis of the ionic bond. Chee 3920: Surface Chemistry I

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- Example: NaCl

Na (left) loses its one valence electron to Cl + (right), producing Na (left) and Cl (right). Chee 3920: Surface Chemistry I

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- Features of ionic bonds & compounds: * Ionic bonds form between metals and nonmetals * Ionic compounds dissolve easily in water and other polar solvents. * Ionic compounds tend to form crystalline solids with high melting temperatures.

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o Covalent bonding: - Occurs when atoms share electrons. - The atoms have a similar tendency for electrons (generally to gain electrons). - Commonly occurs between non-metals. - Examples: H2, O2, H2O.

Water molecule: H2O Chee 3920: Surface Chemistry I

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- Two sub-types of covalent bonds * Non-Polar Covalent Bonding: formed when the bonding electrons are equally shared, e.g. H2. * Polar Covalent Bonding: formed when electrons are unequally shared between two atoms, e.g. H2O.

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- Features of covalent bonds & compounds: * Covalent bonds form between non-metals. * Covalent compounds have low solubility in water and other polar solvents. * Covalent compounds can form various substances (crystalline solids, liquids and gases). Chee 3920: Surface Chemistry I

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SURFACE and INTERFACIAL TENSIONS

• Is the surface (interfacial) energy of the airliquid (liquid-liquid) interface. • Is also equal to the force (per unit length) acting parallel to the surface which tends to cause the surface area to contract. • For fluid-liquid interfaces, interfacial energy and surface tension are two different names for the same thing. Chee 3920: Surface Chemistry I

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• Units: o For surface/interfacial energy: J N⋅m N Surface energy = 2 = = 2 m m m

o For surface/interfacial tension: N Surface tension = m Chee 3920: Surface Chemistry I

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• Examples:

Surface tension

Interface tension

Air

Water

Liquid

Liquid

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12.3 MEASUREMENT OF INTERFACE ENERGY AND TENSION

• Measurement of surface and interfacial tension is easy. • Precise measurement of surface and interfacial energy between the solid and fluid phases can be problematic because solid surfaces are not morphologically and chemically uniform. Chee 3920: Surface Chemistry I

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• The measurement principle is based on the fact that the system of interfaces always minimises its free energy (surface energy and potential energy). o Examples: - Small bubbles and droplets are spherical (Of the objects with the same volume, sphere has the smallest surface area).

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- Contact angle between a solid and a liquid surface presents the geometry with the lowest surface energy. Water Air

γwa

γma

Mineral

θ γwm

Contact angle between an air bubble and a mineral surface in water.

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MEASUREMENT OF SURFACE and INTERFACIAL TENSIONS 1) Capillary Rise Method It uses the rise of the liquid up in a narrow capillary. 2γ Laplace pressure: ∆P = R γ … surface tension R … meniscus radius R = r / cosθ θ … contact angle Chee 3920: Surface Chemistry I

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Hydrostatic pressure: ρ gh ρ … liquid density g … acceleration due to gravity Balancing the pressure gives 2γ = ρ gh R

(Young-Laplace equation)

ρ ghr ∴ γ= 2cosθ

Fully wetted surfaces, γ = ρ ghr / 2 Chee 3920: Surface Chemistry I

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Meniscus corrections: γ =

ρ ghr  2

r  1 + + ....  3h 

It comes from the exact solution to YoungLaplace Equation:

  h′′ h′   ρ gh = γ  + 3/ 2 1/ 2  2 2 ′ ′ x (1 + h )   (1 + h ) where the primes describe the derivatives with respect to the radial coordinate x. Chee 3920: Surface Chemistry I

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Differential Capillary Rise Method for eliminating reference to the flat surface of the liquid reservoir.

γ=

ρ gh1r1

∴γ =

2

=

ρ gh2 r2 2

ρ gr1r2 ∆h

2 ( r1 − r2 )

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2) Wilhelmy Plate Method

- A fully wetted plate is suspended from the balance and dips into the liquid. - The maximum wetting force is measured by detaching the plate from the liquid. Chee 3920: Surface Chemistry I

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Balancing the wetting force, the measured detaching force, and the plate weight gives

Fdet − G 1 γ= plate perimeter cosθ For a fully wetted plate θ = 0. If a liquid with known surface tension is used as a reference, we obtain

γ = γ ref Chee 3920: Surface Chemistry I

Fdet − G Fdet, ref − G 29

3) du Noüy Ring Method

It’s similar to the Wilhelmy method. A ring is used.

Fdet γ= β , where β is a correction. 4π R Chee 3920: Surface Chemistry I

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3) Drop-Volume and Drop-Weight Methods - Drops of the liquid are allowed to detach themselves slowly from the tip of a vertically mounted narrow tube. - Either their weight (m) or volume (V) is measured.

mg V ρg γ= φ= φ 2π r 2π r φ … a correction factor.

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5) Pendant (sessile) Drop (Bubble) Shape Methods

Pendant drop captured to the end of a needle

Sessile drop sitting on a solid surface

The shapes are photographed, digitised, and fitted with the Young-Laplace equation to obtain the surface (interfacial) tension. Suitable for measuring dynamic surface tension. Chee 3920: Surface Chemistry I

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6) Oscillating Jet Method (Adamson)

- A liquid jet emerging from a nozzle is unstable and oscillates about its preferred circular cross section. Chee 3920: Surface Chemistry I

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- Surface tension is calculated from the jet dimensions (obtained photographically). - Suitable for measuring the dynamic surface tension (at very short times – 0.01 s).

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7) Maximum Bubble Pressure Method (Adamson)

- Bubbles are slowly blown through a capillary into the liquid. Chee 3920: Surface Chemistry I

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- The pressure versus time is measured. - The pressure curves passes through a maximum where the bubble radius is equal to the capillary radius – hemispherical shape.

γ=

pmax − phyd r

- Dynamic surface tension is calculated by dividing pmax less hydrostatic pressure by capillary radius. Chee 3920: Surface Chemistry I

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MEASUREMENT OF SURFACE ENERGY

• The contact angle, θ, is measured. • Minimising the free energy gives Liquid Air

γ − γ = γ cosθ sa

ls

la

- Surface tension is measured - State equation links the solidrelated energies (WA Neuman)

γ = f (γ sl

γla

sa

γsa

Solid

θ γls

)

- Solid-fluid surface energy is calculated from the state equation and contact angle. Chee 3920: Surface Chemistry I

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• Possible hysteresis of contact angle

Contact angles between a drop and a titled surface

Hysteresis due to gravity must be eliminated. Chee 3920: Surface Chemistry I

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