Determination of the Surface Tension of Liquid Mixtures.

Submitted by:  Bontoyan, Tom John; Delas Alas, Mary Faith; and Tinaja, Jan Carlo

Determination of the Surface Tension of Water, Oil, and NaCl Solution and Investigation of the Effect of Temperature, and Detergent Additives on the Surface Tension of Water

1. Objectives

1. To quantitatively determine and compare the surface tensions of water, vegetable oil, and saline solution
2. To investigate the effect of temperature on the surface tension of liquids
3. To examine the effect of detergent additives on the surface tension of liquids

1. Background of the Experiment

Surface tension is a fundamental property of a liquid surface which enables it to act as if it were a stretched elastic membrane. It mainly depends upon the forces of attraction between the particles on a given liquid and on the gas, solid, or liquid in contact with it. When the molecules are segregated to the surface, as shown in Figure 1, they go into an unfavorable energy state since half of its attractive interactions are missing. The molecules at the interface with another medium (usually air) do not have other like molecules on all of their sides, so they cohere more strongly to the molecules on the surface and immediately below them [1].

Because of this, the molecules adjust the shape of the liquid to expose the smallest possible surface area. This forms a film of the surface which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

[pic 1]

Figure 1. Surface tension as a result of forces by the liquid molecules acting

in the plane of the surface and minimizing its area

This phenomenon can be observed in the nearly spherical shape of small drops of liquids and soap bubbles. It is responsible for the ability of certain insects and solids (i.e. blades and needles) to float on the surface of water as shown in Figure 2 [2]. If pushed through the surface, these objects will sink and the surface tension will be disrupted. Surface tension is an important concept in various commercial processes like automobile manufacturing where surface preparation is done prior to painting, production of paints, inks and coloring ingredients, formulation of powdered food, and production of creams and other cosmetic products [1].

[pic 2]

Figure 2​: A paper clip and an insect floating on the surface of

water due to surface tension

The energy required to remove the surface layer of molecules per unit area is approximately equal to the energy responsible for surface tension. Therefore, it is often expressed in units of energy per unit area. In this experiment, the surface tension of water, vegetable oil, and saline solution will be determined using a balance beam and a small needle floating in the different media. For a needle floating on the surface of a liquid, as shown in Figure 3, the downward force of the needle’s weight is balanced by the upward forces of surface tension [3].

[pic 3]

Figure 3. Resolution of forces on a “floating needle”

On one side of the balance beam, a needle is hanged and placed carefully on the surface of a liquid. An aluminum box or dish is hanged on the other side. Pre-weighed drops of liquid will be added one at a time to the aluminum dish until the needle is freed from the liquid's surface. Once the total mass of liquid used is measure, the surface tension of can be calculated using the following:

                                            (1)[pic 4]

where  is the surface tension of the liquid,  is the weight of the liquid used to free the needle from the surface, and  is the length of the needle. [pic 5][pic 6][pic 7]

        The surface tension of water, vegetable oil, and saline solution will be calculated using Equation 1. The effect of temperature on the surface tension will also be investigated by calculating the surface tension of water at different temperatures. The dependence of surface tension on the amount of detergent additives on water will also be determined. By the end of the experiment, a comparison on the surface tension of water, vegetable oil, and saline solution will be reported. A plot of surface tension vs. temperature and amount of detergent added will also be presented to observe the effects of these two parameters on the surface tension of the water.

1. Materials & Reagents

1. Materials and Equipment

Apparatus:

                        [pic 8]

                                 Figure 4. Beam Balance

Equipment:

                        [pic 9]

                              Figure 5. Aluminum Box/Dish

                        1. Aluminum Box/Dish

                        2. Hot plate

                        3. Analytical Balance

                        4. Thermometer/Thermocouple

                Materials:

                        1. Aluminum Foil (enough to make a 4x4x2 cu. in box)

                        2. Modeling Clay (more or less 20g)

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