Lecture 1. The physico-chemical basis of surface phenomenon

Prepared by PhD Falfushynska Halina

Surface tension• The surface of a liquid acts as a tensional film which always tends to contract to a minimum area. It proves that surface of liquid has tension. It is called surface tension.

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface.

Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent—but when referring to energy per unit of area, people use the term surface energy —which is a more general term in the sense that it applies also to solids and not just liquids.

The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.

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To keep the equilibrium, an equal force must be applied to oppose the inward tension in the surface.

Thus SURFACE TENSIONSURFACE TENSION [γ ] is the force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull and has the units of dyne/cm

INTERFACIAL TENSIONINTERFACIAL TENSION is the force per unit length existing at the interface between two immiscible liquid phases and has the units of dyne/cm.

Greater surface tension reflects higher intermolecular force of attraction, thus, increase in hydrogen bonds or molecular weight cause increase in ST

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The work W required to create a unit area of surface is known as SURFACE FREE ENERGY/UNIT AREA SURFACE FREE ENERGY/UNIT AREA (ergs/cm(ergs/cm22))

erg = dyne . cm2 G=*S

Its equivalent to the surface tension γ

Thus the greater the area A of interfacial

contact between the phases, the greater the free energy.

W = γ ∆ AG=*S

For equilibrium, the surface free

energy

of a system must be at a minimum.

Thus Liquid droplets tend to

assume a spherical shape since a

sphere has the smallest surface

area per unit volume.

Factors affecting surface tension

• The presence of impurities either on the surface or dissolved in it, affect surface tension of the liquid. Highly soluble substances increase the surface tension of water, whereas sparingly soluble substances reduce the surface tension of water.

• The surface tension of a liquid decreases with increase in temperature. The surface tension of a liquid becomes zero at its boiling point and vanishes at critical temperature.

The surface tension of water is 72 dynes/cm at 25°C . The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule.

Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.

When compared to many other liquids at room temperature, water exhibits a relatively large value for surface tension-roughly about three times as great.

Surface tension (mN m-1) ofsome liquids

0oC 20oC 40oC 60oC 80oC 100oCH2O 75.64 72.75 59.56 66.18 62.61 58.85Ethanol 24.05 22.27 20.60 19.01 - -Acetone 26.20 23.70 21.20 18.60 16.20Toluene 30.74 28.43 26.13 23.81 21.53 19.39Benzene 31.60 28.90 26.30 23.70 21.30Hg (0oC) 480 Ag (970oC) 800

NaCl (1080oC) 94 AgCl (452oC) 125

A glass slide is in contact with a water surface along one of its sides as shown in the figure below. What is the magnitude of the force required to overcome surface tension as the slide is lifted from the water surface? The surface tension of water, for water at 20 degrees Celsius, is 0.0739 g/cm. The dimensions of the plate are 10 cm, by 2 cm, by 0.02 cm.

Example of problem

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Methods for measuring surface and interfacial tension

1- Capillary rise method

2- Ring (Du Nouy) tensiometer

3- Drop weight method (Stalagmometer)

The choice of the method for measuring surface

and interfacial tension depend on:

Whether surface or interfacial tension is to be

determined.

The accuracy desired

The size of sample.

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Capillary Rise Method

When a capillary tube is placed in a liquid, it rises up the tube a certain distance. By measuring this rise, it is possible to determine the surface tension of the liquid. It is not possible, to obtain interfacial tensions using the capillary rise method.

The Principle

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If a capillary tube of inside radius =r immersed in a liquid

that wet its surface, the liquid continues to rise in the tube

due to the surface tension, until the upward movement is

just balanced by the downward force of gravity due to the

weight of the liquid

a = 2 π r γ cos Ө

The total upward force around the inside circumference of the tube is Where

Ө = the contact angle between the surface of the liquid and the capillary wall2 π r = the inside circumference of the capillary. For water the angle Ө is insignificant, i.e. the liquid wets the capillary wall so that cos Ө = unity

Cont. angle water and

glass

Cont. angle Mercury and

glass

σ = ρghr/(2cosθ).

Capillary Rise Method

whereh = the height the liquid is lifted,

la - = the liquid air surface tension (J/m² or N/m)θ = contact angle ρ = density of liquid (kg/m3 )g = acceleration due to gravity (m/s²)r = radius of tube (m)

Determination Methods

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The downward force of gravity The downward force of gravity (mass x acceleration) is given by

Where: π r 2 = the cross-sectional area h = the height of the liquid column to the lowest point of the meniscus (p – p o) = the difference in the density of the liquid p and its vapor po g = the acceleration of gravity w = the weight of the upper part of the meniscus.

At Maximum height,At Maximum height, the opposing forces are in equilibrium

p o, Ө and w can usually be disregarded Hence the surface tension can be calculated.

π r 2 h (p – p o) g + w

2 π r γ cos Ө = π r 2 h (p – p o) g + w

2 π r γ = π r 2 h p g γ = 1/2 r h p g

Ring (Du Nouy) Tensiometer

the principle of the instrument depends on the fact that: the force necessary to detach a platinum-iridium ring immersed at the surface or interface is proportional to the surface or interfacial tension. The force of detachment is recorded in dynes on a calibrated dial The surface tension is given by:

Where: F = the detachment force R1 and R 2= the inner and outer radii of the ring.

γ = F / 2 π (R1 + R2)

For measuring surface and interfacial tensions.The principle

rack mechanism

Pt ring

Unknown sample of liquid

Balancer

Equipment for surface tension determinationby ring detachment

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If the volume or weight of a drop as it is detached from a tip of known radius is determined, the surface and interfacial tension can be calculated from

Where m = the mass of the drop V = the volume of the drop p = the density of the liquid

r = the radius of the tip g = the acceleration due to gravity

Φ = a correction factor

The correction factor is required as not all the drop leaves the tip on detachment

The tip must be wetted by the liquid so as the drop doesn’t climb the outside of the tube.

γ = Φ mg = Φ V pg 2 π r 2 π r

Drop Weight and drop volume method

Stalagmometric method

The weight of the drops is in an equilibrium state with the surface tension.

To account the surface tension proportional to the weight of the drop, we can use a reference fluid (mostly using water as a reference) to compare with the fluid which we are interested in

Water has a surface tension of 0.4 N/m. In a 3 mm diameter vertical tube if the liquid rises 6 mm above the liquid outside the tube, calculate the contact angle.Data:Surface tension (s) = 0.4 N/mDia of tube (d) = 3 mm = 0.003 mCapillary rise (h) = 6 mm = 0.006 m

Formula:Capillary rise due to surface tension is given by 

h = 4scos(q)/(rgd), where q is the contact angle.Calculations:cos(q) = hrgd/(4s) = 0.006 x 1000 x 9.812 x 0.003 / (4 x 0.4) = 0.11Therfore, contact angle q = 83.7o

Air is introduced through a nozzle into a tank of water to form a stream of bubbles. If the bubbles are intended to have a diameter of 2 mm, calculate how much the pressure of the air at the tip of the nozzle must exceed that of the surrounding water. Assume that the value of surface tension between air and water as 72.7 x 10-3 N/m.Data:Surface tension () = 72.7 x 10-3 N/mRadius of bubble (r) = 1

Formula:p = 2/rCalculations:p = 2 x 72.7 x 10-3 / 1 = 145.4 N/m2

That is, the pressure of the air at the tip of nozzle must exceed the pressure of surrounding water by 145.4 N/m2

Capillarity

The tube with very short diameter is called a capillary tube. When a capillary tube is put in a liquid, the liquid surface in it will change.

If liquid can wet the wall of the capillary tube, the liquid surface in the tube will go up and if the liquid cannot wet the wall, the surface will go down.

This phenomenon is called capillarity.

Cohesive force is the force existing between like mole cules in the surface of a liquid Adhesive force is the force existing between unlike molecules, such as that between a liquid and the wall of a glass capillary tube When the force of Adhesion is greater than the cohesion, the liquid is said to wet the capillary wall, spreading over it, and rising in the tube.

Cohesion and Adhesion

The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.

When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.

convex meniscus

concave meniscus

What is Adsorption?Adsorption is the phenomenon of accumulation of large number of molecular species at the surface of liquid or solid phase in comparison to the bulk.

How Adsorption occurs?The process of adsorption arises due to presence of unbalanced or residual forces at the surface of liquid or solid phase. These unbalanced residual forces have tendency to attract and retain the molecular species with which it comes in contact with the surface.

Adsorption is a term which is completely different from Absorption . While absorption means uniform distribution of the substance throughout the bulk, adsorption essentially happens at the surface of the substance. When both Adsorption and Absorption processes take place simultaneously, the process is called sorption.

http://www.youtube.com/watch?v=djIzXvwIz5U&feature=related

Oxygen molecules (red) adsorb on a bimetallic surface of platinum (purple) and cobalt (green).

Adsorption process involves two components Adsorbent and Adsorbate. Adsorbent is the substance on the surface of which adsorption takes place. Adsorbate is the substance which is being adsorbed on the surface of adsorbent. Adsorbate gets adsorbed.

Adsorbate + Adsorbent gives rise to Adsorption

Some modern techniques have been used to study surface.1.Low energy electron diffraction (LEED).2.Photo electron spectroscopy (PES).3.Scanning Tunneling microscopy (STM).

Adsorption is a spontaneous processFor reaction or process to be spontaneous, there must be decreases in free energy of the system i.e. ΔG of the system must have negative value.Also we know, ΔG = ΔH – TΔSAnd during this process of adsorption, randomness of the molecule decreases which ΔS is negative. We can rewrite above equation as

Types of AdsorptionForces of attraction exist between adsorbate and adsorbent. These forces of attraction can be due to Vanderwaal forces of attraction which are weak forces or due to chemical bond which are strong forces of attraction. On the basis of type of forces of attraction existing between adsorbate and adsorbent, adsorption can be classified into two types: Physical Adsorption or Chemical Adsorption.Physical Adsorption or PhysisorptionWhen the force of attraction existing between adsorbate and adsorbent are weak Vanderwaal forces of attraction, the process is called Physical Adsorption or Physisorption. It takes place at low temperature below boiling point of adsorbate. As the temperature increases in, process of Physisorption decreases.

Chemical Adsorption or ChemisorptionWhen the force of attraction existing between adsorbate and adsorbent are chemical forces of attraction or chemical bond, the process is called Chemical Adsorption or Chemisorption. Chemisorption takes place with formation of unilayer of adsorbate on adsorbent. It has high enthalpy of adsorption

Physical Adsorption vs T and Chemical Adsorption vs T

Comparison between Physisorption and Chemisorption

Physisorption Chemisorption1.Low heat of adsorption usually in the range of 20-40 kJ mol-1

High heat of adsorption in the range of 40-400 kJ mol-1

2.Force of attraction are Van der Waal's forces

Forces of attraction are chemical bond forces

3.It usually takes place at low temperature and decreases with increasing temperature

It takes place at high temperature

4.It is reversible It is irreversible5.It is related to the ease of liquefaction of the gas

The extent of adsorption is generally not related to liquefaction of the gas

6.It is not very specific It is highly specific7.It forms multi-molecular layers It forms monomolecular layers

8.It does not require any activation energy It requires activation energy

Applications of Adsorption1. Charcoal is used as a decoloriser as it adsorbs the coloring matter from the coloured solution of sugar.

2. Silica gel adsorbs moisture from the desiccators.

3. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of rooms.4. Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing.

5 .Adsorption processes are useful in carrying out heterogeneous catalysis.

Natural sorbentsFabricate sorbents

chitin

Pectin

Cellulose

Factors affected AdsorptionTemperature. Adsorption increases at low temperature conditions.Adsorption process is exothermic in nature. According to Le Chatleir principle, low temperature conditions would favour the forward direction.PressureAs depicted by Adsorption Isotherm, with the increases in pressure, adsorption increases up to a certain extent till saturation level is achieved. After saturation level is achieved no more adsorption takes place no matter how high the pressure is applied.Surface Area. Adsorption is a surface phenomenon therefore it increases with increase in surface area.Activation of AdsorbentActivation of adsorbent surface is done so as to provide more number of vacant sites on surface of adsorbent. This can be done by breaking solid crystal in small pieces, heating charcoal at high temperature, breaking lump of solid into powder or other methods suitable for particular adsorbent.