ChE Physicochemical Systems Laboratory EXPERIMENT 2: SURFACE TENSION

Transcription

1 ChE Physicocheical Systes Laboratory EXPERIMENT 2: SURFACE TENSION Before the experient: Read the booklet carefully. Be aware of the safety issues. Object To deterine the surface tension of water and to observe the change with respect to teperature. Theory Within the body of a liquid, a olecule is acted upon by olecular attractions, which are distributed ore or less syetrically about the olecule. At the surface, however, a olecule is only partially surrounded by other olecules, and as a consequence it experiences only an attraction towards the body of the liquid. This latter attraction tends to draw the surface olecules inward and in doing so akes the liquid behave as if it were surrounded by an invisible ebrane. This behavior of the surface, called surface tension, is the effect responsible for the resistance; a liquid exhibits to surface penetration, the nearly spherical shape of ercury particles on a flat surface, and the rise of liquids in capillary tubes [1]. Fro a therodynaic point of view surface tension ay be thought of as due to the tendency of a liquid to reduce its surface to a point of iniu potential surface, a condition requisite for stable surface equilibriu. Since a sphere has the sallest area for a given volue the tendency of a liquid particle should be to draw itself into a sphere due to the action of surface tension, as is actually the case. In ters of the Helholtz and Gibbs energy concepts, surface tension (γ) can also be defined as the ratio of the work for needed to change the area of the liquid (dw) by a given aount to the change of the surface area (dσ) [1]: (1) Surface tension is not observed only in liquids but on interfacial surfaces between different phases as well. Solutions exhibit this surface activity. The effect of dissolved substances on the surface tension of the solution is exeplified by the three types of curves shown in Figure 1. In solutions of

2 type I, addition of solute leads to an increase in surface tension, but the increase is generally not large. Such behavior is exhibited by strong electrolytes, sucrose, and ainobenzoic acid in water. On the other hand, with non-electrolytes or weak electrolytes in water, the behavior ost often encountered is that given by curves of type II. Here the solutions exhibit surface tensions, which decrease regularly and ore or less gradually with increase in solute concentration. Finally type III curves are given by aqueous solutions of soaps, certain sulfonic acids and other types of organic copounds. These substances, called surface active reagents possess the ability to lower surface tension of water to a low value even at very low concentrations. Surface Tension Concentration Figure 1. Concentration versus surface tension graphs.

3 Experiental Work Apparatus and Cheicals: Water Figure 2. Du Nouy Ring Apparatus. Procedure 1. The easuring ring is rinsed in water and dried. 2. The torsion dynaoeter s indicator is set to 0 and the weight of the ring copensated using the rear adjusting knob so that the lever ar is in the white area between the arks. 3. The liquid under investigation is poured into the carefully cleaned dish and the ring is copletely suberged. 4. The liquid is wared using the heating unit of the agnetic stirrer and stirred. As soon as the required teperature has been nearly reached, switch off the heating and allow the teperature to stabilize. 5. Switch off the stirrer and allow the liquid to coe to rest. 6. Let liquid slowly allowed to runoff through the iersion tube fro the dish on the agnetic stirrer into the dish located adjacent to the stirrer. To achieve this, open the one-way stopcock, which is connected to the iersion tube via a rubber hose. Fill the iersion tube (siphon) with liquid before beginning the easureent by briefly applying suction with a pup. 7. Continuously readjust the torsion dynaoeter while the liquid runs out to keep the lever ar in the white area between the two arks. 8. Stop the easureent at that oent when the liquid fil tears fro the ring, and read off the last value set on the torsion dynaoeter. Record this value together with the teperature of the

4 liquid. (Throughout the entire easureent procedure, ensure that the apparatus is not subject to vibration.) 9. Repeat the experient at different teperatures. Calculations A olecule in a liquid is subject to forces exerted by all olecules surrounding it; pressure p is isotropic. The resultant force acting on a olecule in a boundary layer of a liquid surface is not zero but is directed towards the interior of the liquid. In order to enlarge the surface of a liquid by an aount A, a certain aount of work E ust be perfored. E (2) A ε is the specific surface energy. It is identical with the surface tension. F (3) l where force F acts along the edge of length I, tangential to the surface in order to aintain the liquid fil. When a ring of radius r used, the length of the edge is l 2 2 r (4) There is no need to correct the easured force to copensate for the weight of the liquid lifted if the ring has a sharp botto edge like in this case. The surface tension of alost all liquids drops linearly with increasing teperature. ' ( T k ' T ) (5) where T k is a teperature near the critical teperature T k. The olar surface tension can be defined with reference to the olar volue V. (6) V

5 which together with equation (4) results in The teperature coefficient ' V ( T ' T ) (7) k k ' V (8) is equal for alost all liquids (Eötvös equation): 7 k J / K (9) Values below this indicate association in the olecules in the liquid, larger values indicate dissociation. The easureent results obtained for olive oil have an inverse linear relationship to teperature. Safety issues: Only water is the cheical used in this experient, there is no toxic cheical usage. During the experient, pay attention to dealing with hot water and use of heating unit. Make sure all the beakers are eptied, and all the electronic devices are unplugged at the end of the experient. Calculations 1. Draw surface tension versus teperature values on a graph. 2. Discuss the graph you obtained fro the experient. 3. Discuss the effect of teperature on surface tension. 4. Do error calculation for the teperature coefficient k and surface tension of water at roo teperature. 5. Explain what other ethods are used to deterine surface tension. References [1] Atkins, P. and Paula J.D., 2006, Physical Cheistry, 8 th edition, Oxford University Press, New York, , (T) 1016.