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Processes At Interfaces

Chapter 28Chapter 28

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IntroductionsWhat is interface

Interfaces are boundaries between any two phases. Interfaces are generally composed of just a few layers of molecules.

Solid-liquid interface

Solid-gas interface

Liquid-gas interface

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phase

phase

Interface phase

Bulk phase

Bulk phase

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phase

phase

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28.1 Surface tension

The surface tension of water can support the body of a pond strider.

Water above the brim of a cup can’t be overflowed due to surface tension

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We can describe surface tension in two ways, physically and thermodynamically.

In physical view

W

2l

tangential -1 N mF

l

28.1 Surface tension

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The surface tension at a point is in tangential direction

28.1 Surface tension

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28.1 Surface tension

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12 molecules on surface

14 molecules on surface

Area of surface

Number of molecules on surface

Work from surrounding is required to increase the surface energy of the system

28.1 Surface tension

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In thermodynamic view

Work is required to change the area of surface because of the difference of forces putting on molecules inside and on surface.

d d d

d d d

d d d

d d d

U T S p V

H T S V p

A S T p V

G S T V p

d

d

d

d

B BB

B BB

B BB

B BB

n

n

n

n

d

d

d

d

28.1 Surface tension is called surface tension, and its value is equal to the reversible non-expansion work at constant temperature and pressure, and compositions.

, , , , , , , ,B B B BS V n S p n T V n T p n

U H A G

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-2

-1

The unit of surface tension can be expressed as J m ,

which is equal to N m

The work required to change a surface area is , that is

.2hl in thermodynamic view or

2 l.h in physical view

28.1 Surface tension

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Effects on surface tension

28.1 Surface tension

At a given thermodynamic state (constant pressure, temperature, compositions), surface tension of a liquid has a identical value. However tension of liquid generally change with conditions.

2 3

6.0

(Ramsay-Shields empirical formula)

c

m

k T T

V

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Effects on surface tension

28.1 Surface tension

Surface tension of solution is related to the types of solutes and concentrations.

Concentration c

Sur

face

tens

ion

II

I

III

I <0. Non-ionic low

molecule organic matter

d

dc

II >0. Organic or

inorganic electrolytes

d

dc

dIII 0. Surface-active

dc agent, generally containing

amphiphilic groups.

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Gibbs’ isotherm describes the relations among surface tension, concentration of solute and surface excess.

0d

da

Surface phase

0d

da

Surface phase

28.1 Surface tension a d

RT da

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28.2 Curved surfaces

The minimization of the surface area of a liquid may result in the formation of a curved surface, as in a bubble, cavity or droplet

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28.2 Curved surfaces

There are two consequences of curvature

1) The vapour pressure of a liquid depends on the curvature of its surface.

pout

pin= pout

pin

pout

pin

Pin > poutPin > pout

pin

pout

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28.2 Curved surfaces

There are two consequences of curvature

1) The vapour pressure of a liquid depends on the curvature of its surface.

rpin

pout

A cavity in liqiud

in out

2 p p

r

Laplace equation

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28.2 Curved surfaces

There are two consequences of curvature

1) The vapour pressure of a liquid depends on the curvature of its surface.

pl

pg

r

olp

ogp

g m,l0g

2 2ln

Where is surface tension of liquid

is density of liquid

is molar mass of liquid

is radius of droplet

p V MRT

p r r

M

r

Kelvin equation

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28.2 Curved surfaces

There are two consequences of curvature

2) Curved surface leads to capillary action

p

pp p

p p

h

r

p

p p

hfall

in outh h h

2 cos

gr

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