Interfacial Phenomena

Interface:  boundary between 2 immiscible phases. NB: it is not present between miscible liquids  properties of molecules at the interface differs from molecules in the bulk. Surface  boundary between solid/ gas or liquid/ gas.

Surface tension:  force per unit length acting parallel to the surface to counter balance the net inward pull. Units of surface tension:  = F/L  dyne/cm Interfacial tension:  tension existing at the interface between 2 immiscible liquids. N.B. = zero in case of completely miscible liquids e.g. alcohol and water)

Polar Ex: Water, glycerin Non Polar Ex: oils, lipids Polar head Non Polar tail Surfactant = surface active agent

Critical Micelle Concentration (CMC) CMC

2- The Drop Fall method: The drop weight method: The drop number method: using Stlagmometer 1- fill with water, count drops between the 2 marks  n water 2- 20ml in beaker, fill the liquid, count drops  n liquid

Determination of critical micelle concentration (CMC) Surface tension method: 0.02%0.04%0.08%0.16%0.2%0.24% Determine surface tension of each concentration using stlagmometer

CMC  water= 72.8 dyne/cm

Precautions: Try the stlagmometer with the highest concentration of SAA solution, If stream  change stlagmometer Avoid air bubbles (withdraw slowly from deep inside the beaker) Put the stalagmometer in vertical position. Receive fallen drops in your beaker Use the same stlagmometer for both liquid and water ( if broken????) Don’t wash the stlagmometer when moving from low to high concentraions. Your finger is away while counting.