1. The Origins of Surface and Interfacial Tension
Chemistry 434
The Origins of Surface and Interfacial Tension

2. 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

3. Surface Tensions of Pure Liquids at 293 K

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 dA dx
expanded
liquid film
f = force needed to move wire
dw = dG = g dA

5. 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 mN/m

6. Capillary Action
The tendency of liquids to rise up in narrow tubes - capillary action.
Due to the phenomenon of surface tension.

7. 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.

8. 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.

9. The Wilhelmy Plate Method
a) detachment g
b) static

10. 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
Water F R

11. 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

12. Drop Weight/Drop Volume Method
A stream of liquid (e.g., H2O) falls slowly from the tip of a glass tube as drops

13. Drop Weight Method The drop weight is found by Vrg= mg = 2p rrgg
Counting the number of drops for a specified liquid volume passing through the tip;
Weighing a counted number of drops
Vrg= mg = 2p rrgg
A correction factor - F
  r/v1/3

14. 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 qe
Surface
Sessile Drop h

15. 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.

16. Drop Profiles
The sessile drop method may also be used to obtain the value of the equilibrium contact angle.
Contact angle, qe < 90° qe

17. 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 b l

18. The Bubble Pressure Technique
The maximum bubble pressure is related to the surface tension of the liquid as follows
P = g l Dr + 2g / b
Dr = 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 = m / s2

19. The Differential Maximum Bubble Pressure Method
Two probes of different diameters.
A differential pressure is generated, DP.
gas stream z1 t z2 b1 b2

20. The Differential Bubble Pressure Equations
The maximum bubble pressure is related to the surface tension of the liquid as follows
DP = g z1 Dr1 + 2g / b1 - g z2 Dr2 + 2g / b2
Dr1 = the density difference between the liquid and the vapour of the first bubble
Dr2 = 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

21. Methods of Measuring Surface Tension

22. Molecular Contributions to an Oil-water Interfacial Tension
g oil
(g oil x g dwater)1/2
Oil Phase
Water Phase
(g oil x g dwater)1/2
gwater

23. The Work of Adhesion
Energy required to reversibly pull apart to form unit surface areas of each of the two substances.
g 12
g 1
g 2

24. 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.
g 1

25. The Definition of the Surface Excess
To obtain a clearer meaning of the surface excess, let’s consider the following system. z Ci zo - +
CJ(1)
CJ(2)

26. 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

27. Three Cases of Spreading
Place a drop of oil on a clean water surface
Define the spreading coefficient

28. 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

29. S > 0, spreading occurs spontaneously
S < 0, formation of oil lenses on surface
Oil qe g
Air wa g oa
Water g ow

30. A third possibility is the a monolayer spreads until spreading is not favourable; excess oil is left in equilibrium with the spread monolayer

31. 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

32. 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

33. Solid Surfaces
Consider the case of a liquid drop placed on a solid surface (non-spreading)
For a liquid drop making a contact angle q with the solid surface

34. Solid Surfaces/Different Contact Angles
Examine the following two surfaces.
A spreading drop  qe < 90° qe

35. A drop with a contact angle << 90qe

36. The Derivation of Young’s Equation
g la qe
g ls qe
g sa 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 qe

37. Young’s Equation
For a liquid (as a drop or at at the surface of a capillary) making a contact angle qc with the solid surface

38. Adhesional Wetting
The ability of the liquid to wet the solid will be dependent on its ability to ‘stick’ to the solid
liquid droplets
g la
g sl
Solid Surface
droplets adhering
to solid surface

39. from the Young Equation
Note: the solid is completely
wetted if qe = 0; it is partially
wetted for finite values of qe.

40. 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
g sa
immersed
g sl

41. Work required to immerse the solid in the liquid
Examine the difference ion the solid-air ‘surface tension’ and the solid-liquid interfacial tension

42. Applying young’s equation
If gsa > gsl, spontaneous wetting while if gsa < gsl, work must be done to wet the surface

43. Degrees of Liquid-solid Interaction

44. Surfactants
What is a surfactant?
Surface active agent
Tail
Headgroup

45. Heads or Tails? Headgroup – hydrophilic functional group(s)
Tail – hydrocarbon or fluorocarbon chain
Typical headgroups (charged or uncharged)
Sulfate
Sulfonate
Trimethylammonium
Ethylene oxide
carboxybetaine

46. 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

47. 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

48. 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

49. 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

50. Using the Gibbs adsorption equation for a 1:1 ionic surfactant
Where Gsurf = nssurf / A

51. Plot of g vs. Log Csurf for Sodium Dodecylsulfate at 298.2 K

52. 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