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LAB.5.Surface Tension LAB.5.Surface tension Key words: Types of interface, defining the interfacial region, surface tension, wetting, adsorption, measurement of surface tension, adhesion, cohesion, surface tension of pure liquids, hydrophobic – hydrophilic interaction, surfactants, micelles Literature: A. Martin, P. Bustamante, Physical Pharmacy. Physical Chemical principles in the pharmaceutical science, Lippincott Williams & Wilkins, Chapter 14, 362 – 379. http://en.wikipedia.org/wiki/Oil-drop_experiment http://en.wikipedia.org/wiki/Capillary_rise http://physics.about.com/od/physicsexperiments/a/surfacetension.htm http://www.attension.com/surface-tension Theoretical Background Fig. 1. Interactions in the inner layers and at a liquid-air interface of liquid. Molecules in the inner layers of a liquid experience the same average force of attraction in all directions by surrounding molecules, while molecule in the surface layer (see Fig. 1) experience different forces of attraction because molecules on the surface have neighboring molecules only on one side (the side facing the interior). As a result attractive forces will tends to pull them into the interior. The overall result: the surface area will be minimized, the surface molecules will be somewhat more ordered and resistant to molecular 1 disruptions, the surface will seem to have a “skin”. We shall designate the surface area by σ. Its dimension is mN/m. At room temperature surface tension of water is 72.8 mN/m. Table 1 gives the surface tension of some liquids at 20oC. The values differ widely. Table 1. Surface tension σ (mN/m) of selected liquids. Liquid Temperature °C Surface tension, σ Acetic acid 20 27.6 Acetic acid (40.1%) + Water 30 40.68 Acetic acid (10.0%) + Water 30 54.56 Acetone 20 23.7 Diethyl ether 20 17.0 Ethanol 20 22.27 Ethanol (40%) + Water 25 29.63 Ethanol (11.1%) + Water 25 46.03 Glycerol 20 63 n-Hexane 20 18.4 Hydrochloric acid 17.7M aqueous solution 20 65.95 2-propanol 20 21.7 Mercury 15 487 Methanol 20 22.6 n-Octane 20 21.8 Sodium chloride 6.0 M aqueous solution 20 82.55 Sucrose (55%) + water 20 76.45 Water 0 75.64 Water 20 72.8 Water 25 71.97 Water 50 67.91 Water 100 58.85 Surface tension of various liquids in dyne/cm against air mixture, dyne/cm is also called mN/m (milli-Newton per meter) in S.I. units; %'s are by mass. There are many methods for determining surface tension of a liquid. In Fig. 2 one of the methods is presented. U - shaped metal ring Liquid layer Sliding metal bar L Force Fig. 2. Wireframe apparatus used to demonstrated the principle of surface tension Note: A force is needed to pull the sliding metal bar against surface tension. This force F is equal: F = 2 σ L (1) where: σ is the surface tension; L is the length of sliding bar. We can calculate surface tension σ from the equation: F (unit N/m) (2) 2 L The surface tension can also be determined by the rise of liquid in glass capillary if attractive forces exist between glass material and liquid (e.g. water). This is the basis of “capillary” action, where water can move up a thin capillary, against the force of gravity. Surface tension “pulls” neighboring water molecules along. Polar liquid, such as water, have strong intermolecular interactions and thus high surface tension. Any factor which decreases the strength of this interaction will lower surface tension. Thus: an increase in the temperature of this system will lower surface tension e.g. Surface tension water (20O C) 72.8 mN/m water (80O C) 62.6 mN/m contamination of water by a surface – active substance (emulsifier, surfactants etc.) will lower surface tension. When a surface – active (surfactant) is placed in a water system it adsorb at the surface and lowers the surface tension between the water and air. Since the surfactant is adsorbed at the surface it is logical that the concentration of surfactant at the surface would be greater than the concentration in the bulk solution. The molecules which contain highly polar hydroxyl (alcohols), carboxyl acids or phosphate (phospholipids) groups bound to a large non-polar hydrocarbon chain are called surfactants. Chemicals such as sodium stearate, sodium oleate, high molecular fatty alcohols, acids with both a hydrophilic (water – attracting) and hydrophobic (water repelling) parts can behave as surfactant. Purpose 1. To measure the surface tension of pure liquids: methanol, 2. To measure the surface tension of water – acids solutions and compare them with water surface tension. 3. To measure the surface tension of water solution of highly effective surfactant sodium salt of oleic acid. 4. To measure the surface tension of water solution of high concentration of sodium chloride. EXPERIMENTAL PART Procedure The measurement of the surface tension is based on counting of droplets slowly discharged from the end of a vertical glass tube (stalagmometer). 1. Fill the stalagmometer with pure, distilled water. Slowly discharge from stalagmometer known – constant volume of water and count how many droplets are in this volume. Results record in Table 2. 2. Repeat the procedure using 6.0; 3.0; 1.5 M solution of sodium chloride. Results record in Table 2. 3. Fill the stalagmometer with 3 mole water solution of methanol. Slowly discharge solution from stalagmometer, counting number of droplets. 4. Repeat the procedure described in 3, using water solution of other alcohols following concentration from Table 3. Results record in Table 4. Table 2. Surface tension vs. number of droplets. Liquid Concentration Number of droplets Surface tense [M] [mN/m] 1 2 3 Average Distilled water - 72.8 Sodium chloride 6.0 3.0 1.5 On the base of the data presented in Table 2, prepare a graph: surface tension of solution vs. concentration of sodium chloride. Calculate the surface tension from the relationship: n solution water (3) water nsolution where: σwater the surface tension of water, σsolution the surface tension of the solution, n – number of droplets. Table 3. Concentration of alcohols solutions. Alcohol Concentration [M] Methanol 3.0 1.0 0.3 - Ethanol 3.0 1.0 0.5 0.25 Propanol 1.0 0.3 0.1 - Table 4. Surface tension vs. number of droplets. Alcohol Concentration Number of droplets Surface tense [M] 1 2 3 Average [mN/m] Methanol 3.0 1.0 0.3 Ethanol 3.0 1.0 0.5 0.25 Propanol 1.0 0.3 0.1 On the base of the data presented in Table 4, prepare a graph: surface tension of solution vs. concentration. Compare the values of surface tense liquids from Tables 2 and 4 with its values find in literature. Compare the values of surface tense liquids from Table 2 with values of surface tense liquids from Table 4. Compare the values of surface tense liquids from Tables 4. .
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