fenomenos superficie

Puu-0.3110 Surface and Colloid Chemistry. Exercise 1

1. The diagrams below represent capillaries of varying construction and arrangement. The diameter of the capillary portion is the same in each case, and all of the capillaries are constructed of glass, unless otherwise indicated. The equilibrium rise for water is shown at the left. Draw meniscuses in each figure to correspond to (a) the level reached by water rising up the clean, dry tube and (b) the level to which the water would recede after having been sucked up to the end of the capillary. The meniscuses in the capillary may be assumed to be spherical in shape. Pay attention to the (qualitative) curvature of the meniscuses.

paraffin

paraffin

Water level

(a )

(b)

(c )

(d) (e)

2. "Sintered glass" is a porous silicate material that can be thought of as a collection of very small capillaries. According to Laplace capillary action, what pressure would be necessary to blow water at 20°C through a sintered glass filter having a uniform pore diameter of 0.20 tension of water at 20°C is 72.8 mN/m. [Answer: 14.4 atm]

m?

Assume a contact angle of zero. The surface

3. If you were to design an absorbent material for use in household cleaning, such as a washcloth or paper towel, list the issues that you would consider and briefly describe how they are relevant to the problem.

4. Mercury makes a contact angle of 140° with a glass surface. Find the capillary depression of Hg in contact with air at 20°C in a glass tube with inside diameter 0.350 mm. For Hg at 20°C, mN/m. [Answer: -3.22 cm]

   =

13.59 g/cm3 and

   =

490

5. At 20°C the surface tensions of water and n-octane are 72.8 and 21.8 mN/m, respectively, and the interfacial tension of the n-octane/water interface is 50.8 mN/m. Calculate a) the work of adhesion between n-octane and water. [Answer: 43.8 mN/m]

 b) the work of cohesion for (i) water and ( ii) n-octane. [Answer: 145.6 mN/m; 43.6 mN/m]

6. Suppose that you want to measure the reduction of the vapor pressure of water caused by the curvature of the surface produced in a small capillary. In order to obtain a 10% reduction of the vapor pressure at 25°C, how small a capillary should be used? The surface tension of water at 25°C is 71.97 mN/m. [Answer 9.9 nm]

7. Given that the surface tensions of n-hexane (C6), n-octane (C8), n-decane (C10), and n-dodecane (C12) are 18.4, 21.7, 23.9, and 25.0 mN/m respectively, estimate to 0.1 mN/m the surface tensions of n-pentane (C5) and n-heptane (C7). [Answer: 16.3 mN/m; 20.2 mN/m]

8. The surface tension of a liquid is determined by the drop weight method. Using a tip with outside diameter 4×10-3  m and inside diameter 2 ×10-5  m, it is found that the mass of 20 drops is 6 ×10-4 kg. The density of the liquid is 950 kg/m3, and it wets the syringe tip. Using the appropriate correction factor, calculate the surface tension of this liquid. [Answer 37.8 mN/m]