surface tension
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PREPARED BYAFAQ WAJID
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The Molecular Origin of Surface Tension
Imbalance of intermolecular forces exists at the liquid-air interface
la= the surface tension that exists at the liquid-air interface
Surface Tensions of Pure Liquids at 293 K
Substance / (10-3 N/m)
Acetone 23.7
Benzene 28.8
CarbonTetrachloride 27.0
Methylene Iodide 50.8
Water 72.8
Methanol 22.6
n-Hexane 18.4
Alternative Explanation of Surface TensionSuppose we have a thin liquid film
suspended on a wire loop as follows
liquid film
expanded liquid film
dx
dA
f = force needed to move wiredw = dG = dA
l = length of wire
Measurement of Surface TensionEarly measurements – even pure liquids has
been described as a ‘comedy of errors’Today – possible to routinely measure the
surface tension of liquids and solutions to an accuracy of + 0.05 mN/m
Capillary ActionThe tendency of
liquids to rise up in narrow tubes - capillary action.
Due to the phenomenon of surface tension.
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.
Capillary RiseThe 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 and solutions. r2
ghghr2
The Wilhelmy Plate Method
The Du Nüoy Ring MethodMeasure the force required to pull the
ring from the surface of the liquid or an interface by suspending the ring from one arm of a sensitive balance
Water
F
R
The Correction FactorThe correction factor takes into account of the
small droplets that are pulled up by the ring when it detaches from the surface
Drop Weight/Drop Volume Method
A stream of liquid (e.g., H2O) falls slowly from the tip of a glass tube as drops
Drop Weight MethodThe drop weight is found by
Counting the number of drops for a specified liquid volume passing through the tip;
Weighing a counted number of dropsVg= mg = 2rg
A correction factor - F r/v1/3
Sessile Drop MethodThe surface tension of a liquid may be
obtained from the shape and size of a sessile drop resting on a horizontal surface
e
Surface
Sessile Drop
h
Sessile Drop Method (Cont’d)Three techniques for obtaining the surface
tension from the image of the sessile dropMeasure 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.
Drop ProfilesThe sessile drop method may also be used to
obtain the value of the equilibrium contact angle.
Contact angle, e < 90°
e
The Maximum Bubble Pressure Method
The maximum pressure required to force a bubble through a tube is related to the surface tension of the liquid.
gas stream
bl
The Bubble Pressure Technique The maximum bubble pressure is related to
the surface tension of the liquid as followsP = g l + 2 / b
= the density difference between the liquid and the vapour
b = radius of curvature at the apex of the bubblel = hydrostatic height to the bottom of the bubbleg = 9.807 m / s2
The Differential Maximum Bubble Pressure Method
Two probes of different diameters. A differential pressure is generated, P.
b2
gas stream
b1
z2z1
t
The Differential Bubble Pressure Equations The maximum bubble pressure is related to
the surface tension of the liquid as followsP = g z1 + 2 / b1 - g z2 + 2 / b2
= the density difference between the liquid and the vapour of the first bubble
= 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
Methods of Measuring Surface Tension
Method Pure Liquids Solutions
WilhelmyPlate
quick andeasy tooperate
Good, suitablewhen ageingoccurs
Du Nuöy Ring Satisfactory n/a
Sessile Drop Very Good Good whensurfaceageing occurs
Drop Weight Suitable Poor whensurfaceageing occurs
CapillaryHeight
Very Good n/a if
Bubblepressure
Very Good Good whenageing occurs
Molecular Contributions to an Oil-water Interfacial Tension
= Oil = water
Oil Phase
Water Phase
oil
water
oil x dwater)1/2
oil x dwater)1/2
The Work of Adhesion Energy required to reversibly pull apart
to form unit surface areas of each of the two substances.
1221 adhW
121
2
1
1
Wcoh 2 1
The Work of Cohesion Defined in terms of the energy required to
reversibly separate a column of a pure liquid to form two (2) new unit surface areas of the liquid.
The Spreading CoefficientSubstance (usually liquid) already in contact
with another liquid (or solid) spreads increases the interfacial contact between the
first and second liquid (or the liquid and the solid)
decreases the liquid-vapour interfacial area
Wetting Ability and Contact AnglesWetting - the displacement of a fluid (e.G.,
A gas or a liquid) from one surface by another fluid
Wetting agent - a surfactant which promotes wetting
Three types of wetting Spreading wetting Immersional wetting Adhesional wetting
A spreading drop e < 90°
e
Solid Surfaces/Different Contact AnglesExamine the following two surfaces.
A drop with a contact angle << 90
e
The Derivation of Young’s 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 Cos e
Young’s EquationFor a liquid (as a drop or at at the surface
of a capillary) making a contact angle c with the solid surface
claslsa Cos
= Cosla
slsac
Immersional WettingImmerse a solid substance in a pure
liquid or solutionarea of the solid-air interface decreasesinterfacial contact between solid and
liquid is increased
solid particle
Water
sa
immersedsolid particle
sl
Surfactants What is a surfactant?
Surface active agent
Headgroup Tail
Heads or Tails? Headgroup – hydrophilic functional group(s)Tail – hydrocarbon or fluorocarbon chain Typical headgroups (charged or uncharged)
SulfateSulfonateTrimethylammoniumEthylene oxidecarboxybetaine
Properties of Surfactant MoleculesAggregate at various interfaces due to the
hydrophobic effectAir-water interfaceOil-water interface
Form aggregates in solution called micelles at a specific concentration of surfactant called the critical micelle concentration (the cmc)Micellar aggregates are known as association
colloids
Applications of SurfactantsSurfactants are an integral part of
everyday life; they are formulated into a wide variety of consumer products ShampoosDish detergentsLaundry detergentsConditionersFabric softenersDiapersContact lens cleaners
Applications of Surfactants (Cont’d)Surfactants are also widely used in
industry due to their ability to lower surface and interfacial tensions and act as wetting agents and detergents Heavy and tertiary oil recoveryOre flotationDry cleaning Pesticide and herbicide applicationsWater repellency
Interfacial Properties of Surfactant MoleculesSurfactants – used in a large number of
applications due to their ability to lower the surface and interfacial tension
Gibbs energy change to create a surface of area dA
dG = dA
Using the Gibbs adsorption equation for a 1:1 ionic surfactant
surfsurf
2RTlnC dd
Where surf = nsurf /
A
Surfactants and Detergents Detergency - the theory and practice of
soil removal from solid surfaces by chemical means
Early detergentsAncient Egypt - boiled animal fat and wood
ashes to make soapPast thirty years
Made significant progress in our understanding of detergency on a molecular level