Molecular properties

Molecular properties

Surface and interfacial tensionSurface tensionis a property of the surface of aliquidthat allows it to resist an external forceIn dealing with multiphase systems, it necessary to consider the effect of the forces at the interface when to immiscible fluid are in contact.When these two fluids are liquid and gas, the term surface tension is used to describe the forces acting on interface.When the interface is between two liquid, the acting force are called interfacial tension.

Surface/Interfacial TensionSurface tension: the contracting force per unit length around the perimeter of a surface if the surface separates gas from liquid or solid phases.Interfacial tension refers to the same contracting force if the surface separates two nongaseous phases, e.g. oil droplet in water.

Unit: dynes/cm or mN/m(1 dyne/cm = 1 mN/m)

Shape of water droplet isdetermined by surface tensionof water on a surfaceSurface tension of waterenables water strider to stayafloat

Surface/Interfacial TensionMore definitions:Surface tension is the reversible work required to create a new area of a liquid.

Unit: energy per unit surface area; i.e. erg/cm2, or mJ/m2Significance:Transport of molecules from bulk to surface region, across phase boundaries, especially in micro-scale phenomena In nature and biological systems

Surface tensions of common liquidsSUBSTANCESURFACE TENSIONwater H(OH)72.7 dyne/cmdiethyl ether (CH3-CH2)2O17.0benzene C6H640.0glycerin C3H2(OH)363mercury (15C)487n-octane21.8sodium chloride solution (6Min water)82.5sucrose solution(85% in water)76.4sodium oleate (soap) solution in water25

The table shows the surface tensions of several liquids at room temperature.

Note especially that hydrocarbons and non-polar liquids such as ether have rather low values

One of the main functions of soaps and other surfactants is to reduce the surface tension of water

Mercury has the highest surface tension of any liquid at room temperature. It is because mercury does not flow in the ordinary way, but breaks into small droplets that roll independently.

Water retention and transport in plantsWater strider walking on water surfaceFoam/Emulsions

Phenomenon of Surface Tension

There is a nett force of attraction between the molecules of water (or any other liquid) holding the molecules together. For a molecule in the middle of the liquid, these forces, acting equally in all directions, more or less balancing out. For a molecule in the surface layer of the liquid, the forces do not balance out, and all the molecules in the surface layer are pulled towards each other and towards the bulk of the liquid. This brings these molecules closer to their neighbors until increasing forces of repulsion create a new balance, and gives rise to the phenomenon of surface tension.

When an object falls onto the surface, it has to push the water molecules apart. If the effect of the weight of the object is insufficient to match the attractive forces between molecules in the surface layer, the object will not enter the surface.Careful observation of the floating needle will show that the water surface is bent down under the weight of the needle, the surface tension causing it to behave as if the needle was supported by a flexible skin

Measurement of surface tensionMethods for measuring surface and interfacial tension three will be discussed:Capillary rise methodRing (Du Nouy) tensiometerDrop 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 desiredThe size of sample.

Capillary rise methodThe difference in pressure across a curved meniscus results in the phenomenon ofcapillarityA liquid which meets the surface of a fine pore or tube will rise in it if the end of the tube dips into a pool of the liquid.The smaller the diameter of the tube, the higher will be the capillary rise.

Capillary rise results from a combination of two effects: the tendency of the liquid to wet the surface of the tube (measured by the value of the contact angle) the action of the liquid's surface tension to minimize its surface area.

h= elevation of the liquid (m) = surface tension (N/m) = contact angle (radians) = density of liquid (kg/m3)g= acceleration of gravity(m/s2)r= radius of tube (m)

Ring (Du Nouy) tensiometerThis method utilizes the interaction of a platinum ring with the surface being tested. The ring is submerged below the interface and subsequently raised upwards. As the ring moves upwards it raises a meniscus of the liquid. Eventually this meniscus tears from the ring and returns to its original position. Prior to this event, the volume, and thus the force exerted, of the meniscus passes through a maximum value and begins to diminish prior to the actually tearing event.

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1 - The ring is above the surface and the force is zeroed.2 - The ring hits the surface and there is a slight positive force due to the adhesive force between ring and surface.3 - The ring must be pushed through the surface (due to the surface tension) which causes a small negative force.4 - The ring breaks through the surface and a small positive force is measured due to the supporting wires of the ring.5 - When lifted through the surface the measured force starts to increase.6 - The force keeps increasing until7 - The maximum force is reached8 - After the maximum there is a small decrease of in the force until the lamella breaks. The process is shown in the diagram below:

Drop weight method (Stalagmometer)Astalagmometeris used for measuring or comparing surface tensions of liquids. This method is based on the direct proportionality between the surface tension and the weight of liquid drops from a vertical capillary tube. The stalagmometer is a pipette-like glass tube with a smooth capillary end C and two calibrated portions of the stemGwith an intervening bulb B In the experiments, the drops of the specific fluid are flowing slowly from the tube in a vertical direction.

The drops hanging on the bottom of the tube start to fall when the volume of the drop reaches the maximum value which is dependent on the characteristic of the solution. In this moment, the weight of the drops is in anequilibrium statewith the surface tension.

Based on the Tates law:

The drop is falling when the weight (mg) is equal to the circumference (2r) multiplied by the surface tension (). The surface tension can be calculated when we know the radius of the tube (r) and the mass of the fluid droplet (m).

SurfactantsThe surface tension of water can be reduced to about one-third of its normal value by adding some soap or synthetic detergent. These substances known assurfactants -generally hydrocarbon molecules having an ionic group on one end.The ionic group, being highly polar, is strongly attracted to water molecules (hydrophilic). The hydrocarbon (hydrophobic) portion is just the opposite

Inserting surfactant into water would break up the local hydrogen-bonding forces and is therefore energetically unfavorable.The surfactant molecules migrate to the surface with their hydrophobic ends sticking out, effectively creating a new surface.Because hydrocarbons interact only through very weak dispersion forces, this new surface has a greatly reduced surface tension.