The Origins of Surface and Interfacial Tension
The Molecular Origin of Surface
Tension
 Imbalance of
intermolecular
forces exists at
the liquid-air
interface
 g la= the surface
tension that
exists at the
liquid-air
interface
Surface Tensions of Pure Liquids at
293 K
Substance
g / (10-3 N/m)
Acetone
23.7
Benzene
28.8
Carbon
Tetrachloride
27.0
Methylene Iodide 50.8
Water
72.8
Methanol
22.6
n-Hexane
18.4
Alternative Explanation of
Surface Tension
 Suppose we have a thin liquid film suspended
on a wire loop as follows
l = length
of wire
liquid film
expanded
liquid film
dA
dx
f = force needed to move wire
dw = dG = g dA
Measurement of Surface Tension
 Early 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 Action
 The 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 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
and solutions.
2 gr  gh
gh
g
2r
The Wilhelmy Plate Method
a) detachment
b) static
g
The Du Nüoy Ring Method
 Measure 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
F
R
Water
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
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 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
Vg= mg = 2p rgg
 A correction factor - F
  r/v1/3
Sessile Drop Method
 The surface tension of a liquid may be
obtained from the shape and size of a sessile
drop resting on a horizontal surface
Sessile Drop
e
h
Surface
Sessile Drop Method (Cont’d)
 Three techniques for obtaining the surface tension
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.
Drop Profiles
 The 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
l
b
The Bubble Pressure Technique
 The maximum bubble pressure is related to
the surface tension of the liquid as follows
P = g l D + 2g / b
 D = 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
The Differential Maximum Bubble
Pressure Method
 Two probes of different diameters.
 A differential pressure is generated, DP.
gas stream
t
z1
z2
b1
b2
The Differential Bubble Pressure
Equations
 The maximum bubble pressure is related to the
surface tension of the liquid as follows
DP = g z1 D1 + 2g / b1 - g z2 D2 + 2g / b2
 D1 = the density difference between the liquid and
the vapour of the first bubble
 D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
Methods of Measuring Surface
Tension
Method
Pure Liquids
Solutions
Wilhelmy
Plate
quick and
easy to
operate
Good, suitable
when ageing
occurs
Du Nuöy Ring Satisfactory
n/a
Sessile Drop
Very Good
Good when
surface
ageing occurs
Drop Weight
Suitable
Poor when
surface
ageing occurs
Capillary
Height
Bubble
pressure
Very Good
n/a if   
Very Good
Good when
ageing occurs
Molecular Contributions to an Oilwater Interfacial Tension
g oil
(g oil x g dwater)1/2
Oil Phase
= Oil
(g oil x g
d
1/2
water)
gwater
= water
Water Phase
The Work of Adhesion
 Energy required to reversibly pull apart to
form unit surface areas of each of the two
substances.
Wadh  g 1  g 2  g 12
g 12
g1
g2
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.
Wcoh  2g 1
g1
g1
The Definition of the Surface
Excess
 To obtain a clearer meaning of the surface excess,
let’s consider the following system.
Ci
CJ(1)
+
CJ(2)
zo
z
The Spreading Coefficient
 Substance (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
Three Cases of Spreading
 Place a drop of oil on a clean water surface
 Define the spreading coefficient
dG
S= gwa  (gwo  goa )
dAwo
dG
S =  Wwo  Woo
dAwo
goa
Oil
gwa
Air
gow
Water
 The spreading coefficient (to be defined later) is
indicative of the difference in the adhesive forces
between liquid 1 and liquid 2 (or the solid), and the
cohesive forces that exist in liquid 1
 S > 0, spreading occurs spontaneously
gow
goa
Air
Oil
Water
 S < 0, formation of oil lenses on surface
Oil
e
g wa
g oa
g ow
Air
Water
 A third possibility is the a monolayer spreads until
spreading is not favourable; excess oil is left in
equilibrium with the spread monolayer
Oil
gwo
goa
goa
gow
goa
Water
Air
gwo
Wetting Ability and Contact Angles
 Wetting - 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
Spreading Wetting
 Liquid already in contact with another liquid (or solid) wets
the surface of the second component (liquid or solid) by
spreading across the surface of the second component
 Using the spreading coefficient defined earlier, we find that
the liquid spreads spontaneously over the surface when S
>0
S =
gsl
-
dG
 g wa  (g wo  g oa )
dAwo
g la
Solid
Air
Liquid
Solid Surfaces
 Consider the case of a liquid drop placed on a
solid surface (non-spreading)
g
g
g
la
g sa  g sl  g la Cos e
Liquid
sl
e
Air
sa
Solid
 For a liquid drop making a contact angle  with
the solid surface
Cos e
g sa  g sl
=
g la
Solid Surfaces/Different Contact
Angles
 Examine the following two surfaces.
A spreading drop  e < 90°
e
 A drop with a contact angle << 90
e
The Derivation of Young’s
Equation
g la
g sa
e
g ls
e
dA
change in the liquid-solid
interfacial area = dA
change in the solid-air
interfacial area = - dA
change in the liquid-air
interfacial area = dA Cos e
Young’s Equation
 For a liquid (as a drop or at at the surface of a
capillary) making a contact angle c with the solid
surface
g sa  g sl  g la Cos c
g sa  g sl
Cos c =
g la
Adhesional Wetting
 The ability of the liquid to wet the solid will be
dependent on its ability to ‘stick’ to the solid
g la
liquid droplets
g sl
Solid Surface
droplets adhering
to solid surface
•
from the Young Equation
 DGA
WA 
 g sa  g la  g sl
A
g sa  g sl  g la Cos e
WA  g la (1  Cose )
•
Note: the solid is completely
wetted if e = 0; it is partially
wetted for finite values of e.
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
g sa
Water
g sl
solid particle
immersed
solid particle
 Work required to immerse the solid in the liquid
 Examine the difference ion the solid-air ‘surface
tension’ and the solid-liquid interfacial tension
 DGI
WI 
 g sa  g sl
A
 Applying young’s equation
 DGI
WI 
 g la Cose
A
If gsa > gsl, spontaneous wetting
while if gsa < gsl, work must be
done to wet the surface
Degrees of Liquid-solid Interaction
Wadh
adh > coh
adh < coh
adh < coh
DwetG
<0
<0
>0
S
spont.
non-spont.
non-spont.
Cos eq
1
0
-1
eq
0
90
180
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)
 Sulfate
 Sulfonate
 Trimethylammonium
 Ethylene oxide
 carboxybetaine
Properties of Surfactant Molecules
 Aggregate at various interfaces due to the
hydrophobic effect
 Air-water interface
 Oil-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 Surfactants
 Surfactants are an integral part of everyday life;
they are formulated into a wide variety of
consumer products
 Shampoos
 Dish detergents
 Laundry detergents
 Conditioners
 Fabric softeners
 Diapers
 Contact 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 recovery
 Ore flotation
 Dry cleaning
 Pesticide and herbicide applications
 Water repellency
Interfacial Properties of Surfactant
Molecules
 Surfactants – 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 = g dA
 Using the Gibbs adsorption equation for a 1:1
ionic surfactant
dg
 2RT surf
d lnCsurf
Where surf = nssurf / A
Plot of g vs. Log Csurf for Sodium
Dodecylsulfate at 298.2 K
2
g
dyne/cm

cmc



2
log C s u r f
1
Surfactants and Detergents
 Detergency - the theory and practice of soil
removal from solid surfaces by chemical means
 Early detergents
 Ancient Egypt - boiled animal fat and wood ashes to
make soap
 Past thirty years
 Made significant progress in our understanding of
detergency on a molecular level