zhang enren 1 page processes at interfaces chapter 28
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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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