Colloid chemistry - Chapter 2: Surface phenomena - Ngo Thanh An

Nội dung text: Colloid chemistry - Chapter 2: Surface phenomena - Ngo Thanh An

1. dG = −SdT +VdP +  idN i + dA G   A T ,P,N i • The surface tension is the increase in the Gibbs free energy per increase in surface area at constant T, P and Ni
2. • In the liquid state, the cohesive forces between adjacent molecules are well developed. For the molecules in the bulk of a liquid • They are surrounded in all directions by other molecules for which they have an equal attraction. For the molecules at the surface (at the liquid/air interface) ▪ Only attractive cohesive forces with other liquid molecules which are situated below and adjacent to them. ▪ They can develop adhesive forces of attraction with the molecules of the other phase in the interface ▪ The net effect is that the molecules at the surface of the liquid experience an inward force towards the bulk of the liquid and pull the molecules and contract the surface with a force F. ❖Cohesive force is the force existing between like molecules. ❖Adhesive force is the force existing between unlike molecules.
3. • The work W required to create a unit area of surface is known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2) (1 erg = dyne.cm) W = γ x ∆A • Its equivalent to the surface tension γ • Thus the greater the area A of interfacial contact between the phases, the greater the free energy. For equilibrium, the surface free energy of a system must be at a minimum. Thus liquid droplets tend to assume a spherical shape since a sphere has the smallest surface area per unit volume.
4. f = f (x, y) f f df = dx + dy x y y x Due to the symmetry of the second derivatives,  f  f = x y y x
5. Schematics of different wetting regimes: (a) Young’s model, (b) Wenzel model, and (c) Cassie model.
6. Applications of superhydrophobic surfaces • Anti-adhesion and self-cleaning. - Self cleaning glasses. - Self cleaning textile. • Anti-biofouling applications. • Corrosion inhibition. • Drag reduction. ▪ Surface roughness increases hydrophobicity ▪ Superhydrophobic if contact angle > 150° ▪ Superhydrophobicity leads to self-cleansing
7. ● When a small cylindrical capillary is dipped in a water reservoir a meniscus is formed in the capillary reflecting balance between contact angle and minimum surface energy. ● The smaller the tube the larger the degree of curvature, resulting in larger pressure differences across the air-water interface. ● The pressure in the water is lower than atmospheric pressure (for wetting fluids) causing water to rise into the capillary until this upward capillary force is balanced by the weight of the hanging water column (equilibrium).
8. Capillary Rise – Example 1 Problem Statement: Calculate the height of capillary rise in a glass capillary tube having a radius of 35 µm. The surface tension of water is assumed to be 72.7 mN/m. Solution: o 2 We use the capillary rise equation with  = 0 , g=9.81 m/s , and w=1000 kg/m3; recall that cos(0)=1: 2 cos 2 0.0727 cos(0) h = h = = 0.423 [m] −5 w g r 9.81 1000 3.5 10 N 2 m kg m 1 s m = = m 2 m kg s m kg m s2 m3 The capillary rise eq. can be simplified by combining constants to yield: 14.84 h[m] = r [m]
9. Adsorption and Capillarity in Soils The complex geometry of the soil pore space creates numerous combinations of interfaces, capillaries, wedges, and corners around which water films are formed resulting in a variety of air water and solid water contact angles. Water is held within this complex geometry due to capillary and adsorptive surface forces. Due to practical limitations of present measurement methods no distinction is made between adsorptive and capillary forces. All individual contributions are lumped into the matric potential. 10  m
10. Surface tension Wettability meniscus Capillarity Pressure deviation