SURFACE CHEMISTRY (ADSORPTION)

  

                 

SURFACE CHEMISTRY

             
       Surface tension

• Molecules of the liquid at the surface are in a different situation than those of the liquid.

• A molecule lying inside the liquid is surrounded by other molecules and so is attracted equally in all directions.

• Hence the net force of attraction acting on the molecule is zero. But a molecule lying at the surface is attracted down words by the molecules lying in the bulk of the liquid. 

• Thus a molecules lying at the surface experiences a net inward attraction.

• As a result of this inward pull on all the molecules lying on the surface behaves as if it were under tension. This property of liquids is called surface tension.

• The surface tension of a liquid is defined as the force acting at right angles to the surface along one cm.length of the surface.

• Thus the units of surface tension are dynes per cm or newtons per meter.

• As a result of the inward pull on the molecules at the surface the surface of the liquid tends to contract to the smallest possible area for a given volume of liquid. It is for this reason that the drops of liquid are spherical because  for a given volume a sphere has a minimum surface area.

• To increase the area of surface some work has to be done against the inward pull.
• Consider a soap solution film contained in rectangular wire frame ABCD in which the side CD is movable. To extend the surface area of the film the movable wire has to be pulled from position CD to EF. Thus some work has to be done against surface tension.
• The work(in erg) required to be done to increase the surface area by 1sq.cm is called surface energy.
• Surface energy=work per sq.cm=(force x length)per sq.cm=dynes x cm/cm x cm =dynes/cm

     

• Unit is  same as the unit of surface tension so surface energy=surface tension   

 CAPILLARY ACTION 

  OR   CAPILLARITY

    

• When one end of capillary tube in put in to a liquid that wets [adhere] glass the liquid rises into the capillary tube to a certain height. This rise is obviously due to the inward pull of surface which pushes the liquid  in to capillary tube.

           

• In case of liquids which do not wet glass .e.g.mercury the level inside the capillary falls below the level outside.

• Whereas the upper surface of a liquid that wet the glass is concave and that of mercury is convex .such a surface is called meniscus. The angle which the curved surface [meniscus]makes with the wall of the tube is called the contact angle.

• For concave meniscus the angle of contact θ is less than 90 degrees.

  

• Pressure of flat surface is p1 and p2 is the pressure of concave curvature.

• However, the capillary radius is small  p2 will be lower than p1 means p2<p1.  so the liquid from bulk migrates into the capillary.

• The pressure difference across the meniscus is given by
               ∆p =p₁ –p₂ = 2γ/r  -----------1
     
  Where r=radius of meniscus eq -1 is known as Laplace equation. This equation shows that the difference in pressure decrease to zero as the radius of curvature becomes infinite (when the surface is flat)
  Since the pressure difference across a planar surface must vanish. we see from the figure that
  
• ∆p =p₁ –p₂ =ρgh
• Where ρ=density of liquid
• g=acceleration due to gravity
• h=height of the liquid column
• and   r = R/cosθ
• From eq. 1 and 2
• ρgh=2γ/r
• γ  = ρghr/2-------------3
• Since the radius of the capillary is easily measured, it is more convenient to relate surface tension to it than to the radius of curvature. Hence equation—3 can be expressed as,
• γ  = ρghR/2cosθ----------4 
• Where θ =angle of contact R=Radius of capillary