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 Carbon Tetrachloride 28.8

27.0

Methylene Iodide Water Methanol n-Hexane 50.8

72.8

22.6

18.4

Alternative Explanation of Surface Tension  Suppose we have a thin liquid film suspended on a wire loop as follows

dA l = length of wire liquid film expanded liquid film 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 g r g    gh  gh 2 r

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., H 2 O) 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 = 2 p r  g g  A correction factor - F   r/v 1/3

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 

e Sessile Drop Surface 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,



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 + 2 g / 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 / s 2

The Differential Maximum Bubble Pressure Method  Two probes of different diameters.  A differential pressure is generated, D P.

gas stream z 1 b 1 t b 2 z 2

The Differential Bubble Pressure Equations  The maximum bubble pressure is related to the surface tension of the liquid as follows D P = g z 1 D 1 + 2 g / b 1 - g z 2 D 2 + 2 g / b 2  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 z 1 = the distance from the tip to the bottom, of the first bubble  z 2 = the distance from the tip to the bottom, of the second bubble

Methods of Measuring Surface Tension

Method

Wilhelmy Plate

Pure Liquids

quick and easy to operate

Solutions

Good, suitable when ageing occurs Du Nuöy Ring Satisfactory Sessile Drop Very Good Drop Weight Capillary Height Bubble pressure Suitable Very Good Very Good n/a Good when surface ageing occurs Poor when surface ageing occurs n/a if    Good when ageing occurs

Molecular Contributions to an Oil water Interfacial Tension g

oil

(g

oil x

g

d water ) 1/2 Oil Phase

= Oil (g

oil x

g

d water ) 1/2

g = water

Water Phase

The Work of Adhesion

 Energy required to reversibly pull apart to form unit surface areas of each of the two substances.

W adh

 g 1  g 2  g 12 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.

W coh



2

g

1

g

1

g

1

The Definition of the Surface Excess  To obtain a clearer meaning of the surface excess, let’s consider the following system.

C i C J (1) z o z + C J (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

dG S = dA wo

 g wa  ( g wo  g oa )

dG S = dA wo



W wo



W oo

g

oa Oil

g

wa

g

ow Water Air

 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 g

ow

g

oa Air Oil



Water

S < 0, formation of oil lenses on surface g

oa

g

wa Oil

g

ow



e Water Air

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

wo

g

oa Oil

g

oa

g

ow

g

oa Water Air

g

wo

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 g

sl

dG S = dA wo

 g

wa



(

g

wo

 g

oa

)

g

la Air Solid Liquid

Solid Surfaces  Consider the case of a liquid drop placed on a solid surface (non-spreading) g

la

g

sl Liquid



e

g

sa

 g

sl

 g

la Cos



e

g

sa Air



Solid

For a liquid drop making a contact angle  the solid surface

Cos



e

=

g

sa

g 

la

g

sl

with

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

sa

g

la



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 Cos  c sl  g la Cos  c = g sa  g la g sl

Adhesional Wetting

 The ability of the liquid to wet the solid will be dependent on its ability to ‘stick’ to the solid g

la Solid Surface liquid droplets droplets adhering to solid surface

g

sl

•

from the Young Equation

g

sa

W A

  g  D

G A

sl

 g

sa

 g

la

 g

sl A

 g

la Cos



e W A

 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 ‘ tension’ surface and the solid-liquid interfacial tension W I   D G I A  g sa  g sl

 Applying young’s equation W I   D G I A  g la Cos  e If g sa > g sl , spontaneous wetting while if g sa < g sl , work must be done to wet the surface

Degrees of Liquid-solid Interaction

W adh

adh > coh adh < coh adh < coh D

wet G

< 0 < 0 > 0

S

spont. non-spont. non-spont.

Cos



eq

1 0 -1 

eq

0 90  180 

Surfactants

 What is a surfactant?

Surf ace act ive a ge nt 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

d

g

d lnC surf

 

2RT



surf

Where  surf = n s surf / A

Plot of g vs. Log C surf 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