SURFACE TENSION

Molecules in the interior of a liquid are attracted equally in all directions by the Fig 3.2 O molecules around it, and are thus subjected to a balanced set of forces, whereas molecules at the surface are attracted only towards the interior as shown in Fig. The attractions pull the surface layer toward the centre, because of the difference in the strength of interactions of the surface molecule with the molecule in the vapour phase and one that is in the bulk below it. As a result of the inward attraction the surface of the liquid experiences an attractive force known as surface tension and surface behaves like a stretched membrane. That is why the surface of any liquid tends to minimize its surface area. A droplet assumes a spherical shape because a sphere has the minimum surface area for a given volume.

The Surface Tension is defined as the force in netwtons acting at right angle on a unit length (Im) along the surface of a liquid. It is denoted by (gamma). The SI unit of surface tension is newton per meter (Nm). Note that the units of Nim, are equivalent to joules per square meter,jm-2.

Surface tension is related to the attractive forces between molecules Liquids with large attractive forces have relatively large surface tensions. The large surface tension of Surface tension is related to the attractive forces between molecules Liquids with Effect of Temperature. The surface tension of a liquid decreases with increasing water is mainly due to more extensive hydrogen bonding in the water structure temperature and becomes zero near the critical temperature.

Capillary Action. The rise or fall of a liquid in a capillary tube is related to the surface depressed, like mercury, depends on the relative magnitude of the forces of cohesion tension of the liquid. Whether a liquid rises in a glass capillary, like water, or is depressed, like mercury, depends on the relative magnitude of the forces of cohesion tension of the liquid. Whether a liquid rises in a glass capillary, like water, or is between the liquid molecules themselves, and the forces of adhesion between the liquid and the walls of the tube These forces determine the contact angle 0, which the liquid makes with the walls of the tube. If a contact angle is less than 90, the liquid is said to wet the surface and a concave meniscus is formed. If the contact angle is greater that 90. the liquid does not wet the surface and a convex meniscus is formed.

The formation of a concave meniscus by a liquid that gets the glass leads to a capillary rise,the formation of a concave meniscus leads to the depression of the liquid (which does not wet the glass) in a capillary tube.

MEASUREMENT OF SURFACE TENSION

 The methods commonly employed for the measurement of surface tension are A fine capillary tube of radius is vertically

The Capillary Rise Method.

A fine capillary tube of radius r is vertically immersed in a test liquid that wets glass. The liquid rises to a certain height 'a' until the force of surface tension pulling the liquid upward is counterbalanced by the downward hydrostatic force.

The force of surface tension (i.e.. upward force) acting along the total circumference of the tube is 2tr y cos 0. The hydrostatic force fie., downward force) is equal to the product of pressure and area of cross-section of the tube (=ghdpir2)

 

But Upward force   = downward force

          2pirYcos0.   =   ghdpi r2

 

           Y = ghdr/2cos0

 

where y is the surface tension, d is the density of the liquid, g is the acceleration due to gravity and o is the contact angle. For most liquids, 0 is essentially zero, and ens 01 Therefore, Eq (1) reduces to

             Y ghdr/ 2

 In order to calculate the value of y, one needs to know the values of g.h.d and r.

DROP FORMATION METHOD

The size and hence the weight of a drop of a liquid falling from the end of a capillary tube depends upon the surface tension of the liquid and the sou the outer circumference of the tube. The weight of drop pulls it. When the two forces are of the capillary end. The drop is supported by the upward force of surface tension acting balanced, the drop breaks. Thus at the point of breaking

 γ. 2pir    =  W  = mg  =  Vdg

where,r  is the radius of the capillary tube. V is the volume of the drop and d is its  density.This equations being a basis of the drop Method is used for the comparison of the surface tensions of different liquids  

Drop Weight Method:

 In this method, the mass of a single drop of liquid, and that of reference liquid (say water) is determined. Then from Eq.(5), and

W1= m1g= 2pirY1

 W₂mg = 2pi r Y2

Therefore Y1/Y2 = m1/m2

 

Knowing the surface tension of reference liquid, that of the experimental liquid can be determined.

 

DROP-NUMBER METHOD:

Drop-Number Method Instead of finding the weights of single drops, it is easier to count the number of drops formed from an equal volume of two liquids If n. and nare the number of drops produced from the same volume V of the two liquids, then

 

.The volume of a single drop of liquid 1 = V/n1

.The mass of a single drop of liquid 1 V/n1 d1 .Similarly, the mass of a single drop of liquid 2 V/n2 d2

Y1/Y2= (V/n1)d1/(V/n2)d2 =n2d1/n1d2 or Y1 = n2d1/n1d2 Y2

The instrument used for determining surface tension is called stalagmometer, which consists of a bulb fused with a capillary tube as shown in Fig.3.7. The stalagmometer is thoroughly cleaned and water is sucked up to the upper mark A. The water is allowed to flow and the number of drops is counted until the lower mark B is reached. Next the experiment is repeated with the other (experimental) liquid and surface tension of the liquid can be determined by using the Eq.(7). For reference liquid water, Eq.(7) can be written as:

Y1= d1 /dw×nw/n1×Yw