Zhang Enren 1 Page Processes At Interfaces Chapter 28.

Zhang EnreZhang Enrenn

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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



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The surface tension at a point is in tangential direction



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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



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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



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Effects on 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


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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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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rpin

pout

A cavity in liqiud

in out

2 p p

r

Laplace equation


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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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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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Zhang Enren 1 Page Processes At Interfaces Chapter 28.

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