Determination of Surface Tension
Purpose
To get information about surface tension and surface adsorption, determination of surface tension by drop weight method.
Theoretical Knowlodge
The surface of a liquid may be surrounded by solid, gas or a liquid that does not mix with itself. The interaction forces on the molecule are in equilibrium as the molecule in the interior of the liquid is symmetrically and spherical enveloped by other molecules. For the molecule on the surface, the forces of interaction on the molecule are not equalized because they are under the influence of molecules of different types and are not wrapped by equal numbers of molecules on side by side. The interaction force between molecules of the same type (kohesion) is usually different from the interaction force between molecules of different types (adhesion). The unequal force on the surface molecules causes the surface to be stretched. As a result, the system arrange to have the smallest surface area to minimize the free energy that is coming from the balance. The elastic force on the surface of a liquid is the measure surface tension. Surface tension (y), acts parallel to the surface of the liquid, pulls the liquid inward and thus avoiding the expansion of the liquid surface. In other words, surface tension is the amount of energy required to expand the surface area of the liquid or on the surface is the force per unit length which is the inverse of the surface expansion of the liquid. SI unit of surface tension is N/m or J/m^2. The surface tension between two liquid phases formed in liquid-liquid systems, which are partially mixed with one another, is called inter-surface tension.
Surface tension of liquids decreases with temperature but there are two different states of dissolution of another substance in the liquid. If the concentration of the solute on the solution surface is higher than its concentration in the interior, the surface tension is reduced. These substances that reduce surface tension are called capillary actived substances. If the concentration of the solute on the solution surface is lower than its concentration in the interior, the surface tension is increased. The substances that increase the surface tension of the liquid are called capillary inactivated substances. The amount of decrease or increase in surface tension varies with the concentration of the solute.
There are many different methods for surface tension measurement. Capillary rise, stallagmometer or drop weight, wilhelmy plate or ring, maximum bulk pressure are some of them. Drop weight and drop count methods are frequently used because of their convenience in application.
The weight of drops falling from a pipe will be equal to the surface tension of the fluid around the pipe at the time of full fall, according to the drop weight method.
If the directions of gravity and surface tension forces are taken into consideration,
may be written. In the equation, g is accelaration of gravity, y is surface tension, r is outer radius of capillary tube and m is weight of liquid drop. Since it is difficult to determine the radius of the capillary tube, surface tension is measured with the help of a known mixing fluid. If the equation for both fluids is written and proportioned separately,
equality is obtained. weighing n drops are done to reduce the error and the average mass of a drop is caluculated. A simple device called Traube stalagmometer is used while the surface tension is determined in the drop counting method because the error is minimized and rapid results are obtained. Stalagmometer has a certain volume. The number of drops of this liquid in the fixed volume is counted and the surface tension of the liquid whose surface tension is unknown is calculated by the following equation,
| n1, n2 | Drop number of liquids |
|---|---|
| p1, p2 | Densities of liquids |
| y1, y2 | Surface tensions of liquids |
Surface Adsorption Isotherm
Surface adsorption is called the concentration difference between the surface and the inner part. The concentration difference per unit area of the surface at the test temperature is given by the equation Gibbs adsorption isotherm, where c is the concentration of solute.
The gas constant (R) should be taken as 8,314x10^7 erg/mol.K because the surface tension is in dyn / cm. The surface stresses of the solutions in the different concentrations are determined to find the change in the surface tension with the concentration of the solute. A graph of concentration-surface tension is drawn. The slope of the drawn tangent curve obtained for the selected concentration (c) will change for the concentration.
Gibbs adsorption values for each concentration are calculated from the equation using the tangent and concentration values plotted for different concentrations. The c-q values form graphical surface adsorption isotherms. The value of m, which remains constant at concentrations of high solute concentration, is the value the surface covers for the molecules of the solute. 1/qnM value gives the surface area of a molecule of the adsorbed material on the surface.
If the dy/dc value is positive indicates that the solute is inactive for capillaries.
References:
- https://en.wikipedia.org/wiki/Surface_tension
- https://chem.libretexts.org
- Eskisehir Osmangazi University, Department of Chemical Engineering, Physical Chemistry Lab. Brochure
A very useful post about surface tension.
Nice to see someone reading the article I wrote 4 months ago. Thank you.
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