surface tension elearning
TRANSCRIPT
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Surface Tension
Compiled By: Bertha Yonata
From several sources
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Objectives
Students are able to explain
The surface tension phenomena
The methods to measure surface tension
Factors affecting surface tension
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Video 1 (why the water still in the
turning glass?)
http://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/SURFACE%20TENSION%20-%20ENGLISH%20-%2018MB.flv -
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Discussion
Explain why does water still in the turning
glass
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Definition
The surface tension of a liquid resultsfrom an imbalance of intermolecularattractive forces, the cohesive forcesbetween molecules:
A molecule in the bulk liquid
experiences cohesive forces with othermolecules in all directions.
A molecule at the surface of a liquidexperiences only net inward cohesive
forces.
Surface tension is measured as the energy required toincrease the surface area of a liquid by a unit of area.
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Video 2 (cohesion, adhesion, surface
tension)
http://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/The%20Cohesion,%20Adhesion%20and%20Surface%20Tension%20of%20Water.flv -
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A microscopic view of water illustrates the difference between
molecules at the surface of a liquid and water molecules within a
liquid.
The molecules at the surface of thissample of liquid water are notsurrounded by other watermolecules. The molecules inside thesample are surrounded by othermolecules. The average lifetime of amolecule at the surface of a liquid is
10-6s
The unbalanced attraction of molecules at
the surface of a liquid tends to pull themolecules back into the bulk liquid leavingthe minimum number of molecules on thesurface. It required energy to increase thesurface area of a liquid because a largersurface area contains more molecules inthe unbalanced situation
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As many molecules aspossible will leave theliquid surface willtherefore tend tocontract spontaneously.For this reason, dropletsof liquid and bubbles of
gas tend to attain asperical shape
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Video 3 (36 drops of water on penny)
http://localhost/var/www/apps/conversion/tmp/scratch_9/36%20Drops%20of%20Water%20on%20a%20penny.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/36%20Drops%20of%20Water%20on%20a%20penny.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/36%20Drops%20of%20Water%20on%20a%20penny.flv -
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Discussion
What is the difference between surface and
interface?
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Surface vs Interface
There is no fundamental distinction between
the terms surface and interface
Surface: the boundary between two phases
one of which is gaseous phase
Interface: the boundary between two non-
gaseous phases
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Surface tension and interfacial tension against water for
liquid at 20C
Liquid 0 i
Water 72.8 -
Benzene 28.9 35.0
Acetic acid 27.6 -Acetone 23.7 -
CCl4 26.8 45.1
Ethanol 22.3 -n-octanol 27.5 8.5
n-octane 21.8 50.8
Mercury 485 375
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Additivity of Intermolecular Forces at Interface
The short-range intermolecular forces which areresponsible for surface/interfacial tension include Vander Waals forces and hydrogen bonding (in water) andmetal bonding (in mercury)
The relatively high values of the surface tensions ofwater and mercury reflect the contributions of hydrogenbonding and metal bonding respectively
The surface tension of water
For mercury:
hW
dWW
m
Hg
d
HgHg
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Oil-Water Dispersion
The interfacial tension for oil-water dispersion
is given by:
2/1).(2)( dOd
W
h
W
d
W
d
OOW
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Curved Interfaces
(The Kelvin Equation)
As a consequence of surface tension,
there is a balancing pressure difference
across any curved surface
The pressure being greater on the
concave side
Young-Laplace equation
For spherical surface:
21
11
rrp
r
p 2
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Contd
If the radius of droplet increase from r to r +
dr, the surface area increase from 4r2to
4(r+dr)2and the surface free energy will be
8...r.dr
Assuming ideal gaseous behaviour:
drrp
p
TRdn r
....8ln...0
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Since
so
A liquid which wets the wall of capillary will have aconcave liquid-vapour interface and lower vapour
pressure in the capillary than in the bulk phase. So
Kelvin equation will be written:
Mdrrdn
....4 2
r
V
r
M
p
pRT mr
22ln
0
Kelvin equation
r
V
p
pRT mr
cos2ln
0
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Measurement of Surface and Interfacial
Tensions
1. Capillary Rise Method
This method is only used when the contactangle is zero, owing to the uncertainty in
measuring contact angles correctly (=liquidvapour)
For zero contact angle:
cos2
grh
2
grh
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Capillary Action
The tendency of liquids
to rise up in narrow
tubes - capillary action. Due to the
phenomenon of surface
tension.
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The Complication of Contact Angles
The balance of forces
that results in a contact
angle, c. The contact angle gives
information on the
wettability of a
surface.
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Capillary Rise
The pressure exerted by
a column of liquid is
balanced by the
hydrostatic pressure.
This gives us one of the
best ways to measure
the surface tension of
pure liquids andsolutions. r2
gh
ghr2
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2. Wilhelmy Plate Methods
A thin mica plate or microscope slide is suspended from
the arm of a balance and dips into the liquid.
Detachmentthe container holding the liquid is
gradually lowered and the pull on the balance at the
point of detachment is noted
Staticfor measuring changes in surface tension. Thechange in the force required to maintain the palte at
constant immersion as the surface tension alters is
measured
)(2.det yxWW X =length of the plate
Y= breadth of the plate
W=weigh (at zero contact angle)
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The Wilhelmy Plate Method
a) detachment
b) static
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3. Ring Method
The force required to detach a ring from asurface or interface is measured either by
suspending the ring from the arm of a balance
or by using torsion-wire arrangement (du
Noy tensiometer)
R
F
4F= the pull of the ringR=the mean radius of the ring
=correction factor
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The Du Noy Ring Method
Measure the force required to pull the ring fromthe surface of the liquid or an interface bysuspending the ring from one arm of a sensitive
balance
Water
F
R
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The Correction Factor
The correction factor takes into account of the
small droplets that are pulled up by the ring
when it detaches from the surface
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4. Drop Weight/Drop Volume Method
A stream of liquid (e.g., H2O) falls
slowly from the tip of a glass tube as
drops
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Drop Weight Method
The drop weight is found by Counting the number of drops for a specified
liquid volume passing through the tip;
Weighing a counted number of drops
Vg= mg = 2 rg
A correction factor - F
r/v1/3
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5. Sessile Drop Method
The surface tension of a liquid may be
obtained from the shape and size of a
sessile drop resting on a horizontalsurface
e
Surface
Sessile Drop
h
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Sessile Drop Method (Contd)
Three techniques for obtaining the surfacetension from the image of the sessile drop
Measure the height of the top of a large sessile
drop above its maximum diameter. Estimate the shape factor of the drop from the
coordinates of the drop profile.
Fit the drop profile to ones that are generated
theoretically.
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Drop Profiles
The sessile drop method may also be used to
obtain the value of the equilibrium contact
angle.
Contact angle,e < 90
e
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The Maximum Bubble Pressure
Method
The maximum pressure required to force abubble through a tube is related to the
surface tension of the liquid.gas stream
b
l
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The Bubble Pressure Technique
The maximum bubble pressure is related to the
surface tension of the liquid as follows
P = g l + 2/ b = the density difference between the liquid and
the vapour
b = radius of curvature at the apex of the bubble
l = hydrostatic height to the bottom of the bubble
g = 9.807 m / s2
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The Differential Maximum Bubble
Pressure Method
Two probes of different diameters.
A differential pressure is generated, P.
b2
gas stream
b1
z2z1
t
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The Differential Bubble Pressure
Equations
The maximum bubble pressure is related to thesurface tension of the liquid as follows
P = g z11+ 2/ b1 - g z22+ 2/ b2 1= the density difference between the liquid
and the vapour of the first bubble 2= the density difference between the liquid
and the vapour of the second bubble
z1= the distance from the tip to the bottom, of
the first bubble z2= the distance from the tip to the bottom, of
the second bubble
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Methods of Measuring Surface Tension
Method Pure Liquids Solutions
Wilhelmy
Plate
quick and
easy to
operate
Good, suitable
when ageing
occurs
Du Nuy Ring Satisfactory n/a
Sessile Drop Very Good Good when
surface
ageing occurs
Drop Weight Suitable Poor when
surface
ageing occurs
Capillary
Height
Very Good n/a if
Bubble
pressure
Very Good Good when
a ein occurs
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Molecular Contributions to an Oil-
water Interfacial Tension
= Oil = water
Oil Phase
Water Phase
oil
water
oilx d
water)1/2
oilx
dwater)1/2
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Surface Tension Examples
Walking onwater: Smallinsectssuch as the water stridercan walkon water because their weight isnot enough to penetrate thesurface.
Floating a needle: If carefullyplaced on the surface, a smallneedle can be made to float on thesurface of water even though it isseveral times as dense as water. Ifthe surface is agitated to break upthe surface tension, then needlewill quickly sink
http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/surten.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/surten.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html -
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Contd
Don't touch the tent!: Common tent materials aresomewhat rainproof in that the surface tension ofwater will bridge the pores in the finely wovenmaterial. But if you touch the tent material with your
finger, you break the surface tension and the rain willdrip through.
Washing with cold water: The major reason for usinghot water for washing is that its surface tension is
lower and it is a better wetting agent. But if thedetergent lowers the surface tension, the heatingmay be unneccessary
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A spreading drop e < 90
e
Solid Surfaces/Different Contact
Angles
Examine the following two surfaces.
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A drop with a contact angle
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The Derivation of Youngs Equation
la
sa
lse
change in the liquid-solid
interfacial area = dA
dA
e
change in the solid-air
interfacial area = - dA
change in the liquid-air
interfacial area = dA Cose
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Youngs Equation
For a liquid (as a drop or at at the surface of
a capillary) making a contact angle cwith
the solid surface
claslsa Cos
=Cosla
slsa
c
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Adhesional Wetting
The ability of the liquid to wet the solid will
be dependent on its ability to stick to the
solidliquid droplets
Solid Surface
la
droplets adhering
to solid surface
sl
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from the Young Equation
WG
AA
A
s a la s l
sa sl laCos e
W C o s A l a e
( )1
Note: the solid is completely
wetted ife= 0; it is partially
wetted for finite values ofe.
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Immersional Wetting
Immerse a solid substance in a pure
liquid or solution
area of the solid-air interface decreases interfacial contact between solid and
liquid is increased
solid particle
Water
sa
immersed
solid particle sl
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Work required to immerse the solid in theliquid
Examine the difference ion the solid-airsurfacetension and the solid-liquid interfacialtension
slsaI
IA
GW
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Applying youngs equation
elaI
I CosA
GW
If sa> sl, spontaneous wetting while
if sa< sl, work must be done to wet
the surface
Obj i
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Objectives:
students are able to
Describe the action of surfactant
Explain the effect of detergent in dirt removing
Explain the effect of temperature in surface
tension and interfacial tension
Vid 4 (th ff t f d t t/
http://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flv -
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Video 4 (the affect of detergent/soap
adding to surface tension)
http://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/Does%20soap%20affect%20surface%20tension%20of%20water.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flv -
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How surfactant work
http://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_9/How%20Surfactants%20Work.flv -
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53
What are surfactants?
Emulsifier, Dispersant, Wetting agent
Common examples:cleaning dirty clothes and kitchenware,writing on paper with a pen,greasing of cooking surfaces
Use of surfactants in industry:emulsion polymerisation, paper coatingfood, pharmaceuticals, cosmetics
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The Function of Detergency
The function of detergency or cleaning is a
complex combination of all functions. The
surface to be cleaned and the soil to be
removed must initially be wet and the soilssuspended, solubilised, dissolved or separated
in some way so that the soil will not just re-
deposit on the surface in question
Explain these pictures
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Explain these pictures