7 ifacapillarytubeofinsideradiusrimmersedinaliquid

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Unformatted text preview: ion of the liquid. It is not possible, to obtain interfacial tensions using the capillary rise method. Cohesive force is the force existing between like mole­cules in the surface of a liquid Adhesive force is the force existing between unlike molecules, such as that between a liquid and the wall of a glass capillary tube When the force of Adhesion is greater than the cohesion, the liquid is said to wet the capillary wall, spreading over it, and rising in the tube. 7 If a capillary tube of inside radius =r immersed in a liquid that wet its surface, the liquid continues to rise in the tube due to the surface tension, until the upward movement is just balanced by the downward force of gravity due to the weight of the liquid The upward component of the force resulting from the surface tension of the liquid at any point on the circumference is given by: a = γ cos Ө Cont. angle water and glass Thus the total upward force around the inside circumference of the tube is a = 2 π r γ cos Ө Where Ө = the contact angle between the surface of the liquid and the capillary wall 2 π r = the inside circumference of the capillary. For water the angle Ө is insignificant, i.e. the liquid wets Cont. angle the capillary wall so that cos Ө = unity 8 Mercury and glass The downward force of gravity (mass x acceleration) is given by πr 2 h (p – p o) g + w Where: π r 2 = the cross­sectional area h = the height of the liquid column to the lowest point of the meniscus (p – p o) = the difference in the density of the liquid p and its vapor po g = the acceleration of gravity w = the weight of the upper part of the meniscus. At Maximum height, the opposing forces are in equilibrium 2 π r γ cos Ө = π r 2 h (p – p o) g + w p o, Ө and w can usually be disregarded Hence the surface tension can be calculated. 2 π r γ = π r 2 hpg γ = 1/2 r h p g 9 Ring (Du Nouy) Tensiometer For measuring surface and interfacial tensions. The principle the principle of the instrument depends on the fact that: the force necessary to detach a platinum­iridium ring immersed at the surface or interface is proportional to the surface or interfacial tension. The force of detachment is recorded in dynes on a calibrated dial The surface tension is given by: (γ = F / 2 π (R1 + R2 Where: F = the detachment force R1 and R 2= the inner and outer radii of the ring. 10 Drop Weight and drop volume method If the volume or weight of a drop as it is detached from a tip of known radius is determined, the surface and interfacial tension can be calculated from γ = Φ mg = Φ V pg π r 2 π r 2 Where m = the mass of the drop V = the volume of the drop p = the density of the liquid r = the radius of the tip g = the acceleration due to gravity Φ = a correction factor The correction factor is required as not all the drop leaves the tip on detachment The tip must be wetted by the liquid so as the drop doesn’t climb the outside of the tube. 11 A surfactant molecule is depicted schematically as a cylinder representing...
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