The Origins of Surface and Interfacial Tension Chemistry 434 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

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

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.

The Wilhelmy Plate Method a) detachment g b) static

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

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 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

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

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, qe < 90° qe

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

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 = 9.807 m / s2

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

Methods of Measuring Surface Tension

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

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

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

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)

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

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 S < 0, formation of oil lenses on surface Oil qe g Air wa g oa Water g ow

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

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

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

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

A drop with a contact angle << 90 qe

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

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

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

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

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

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

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

Degrees of Liquid-solid Interaction

Surfactants What is a surfactant? Surface active agent Tail Headgroup

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 Where Gsurf = nssurf / A

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

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