Zhang Enren 1 Page Processes At Interfaces Chapter 28

Zhang Enren 2 Page Introductions What is interface Interfaces are boundaries between any two phases. Interfaces are generally composed of just a few layers of molecules.  Solid-liquid interface  Solid-gas interface  Liquid-gas interface

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Zhang Enren 5 Page  phase  phase Interface phase Bulk phase

Zhang Enren 6 Page  phase  phase

Zhang Enren 7 Page 28.1 Surface tension The surface tension of water can support the body of a pond strider. Water above the brim of a cup can’t be overflowed due to surface tension

Zhang Enren 8 Page We can describe surface tension in two ways, physically and thermodynamically. In physical view W 2l2l 28.1 Surface tension

Zhang Enren 9 Page The surface tension at a point is in tangential direction 28.1 Surface tension

Zhang Enren 10 Page 28.1 Surface tension

Zhang Enren 11 Page 12 molecules on surface 14 molecules on surface Area of surface  Number of molecules on surface Work from surrounding is required to increase the surface energy of the system 28.1 Surface tension

Zhang Enren 12 Page In thermodynamic view Work is required to change the area of surface because of the difference of forces putting on molecules inside and on surface Surface tension  is called surface tension, and its value is equal to the reversible non-expansion work at constant temperature and pressure, and compositions.

Zhang Enren 13 Page The work required to change a surface area is , that is .2hl in thermodynamic view or 2  l.h in physical view 28.1 Surface tension

Zhang Enren 14 Page Effects on surface tension 28.1 Surface tension At a given thermodynamic state (constant pressure, temperature, compositions), surface tension of a liquid has a identical value. However tension of liquid generally change with conditions.

Zhang Enren 15 Page Effects on surface tension 28.1 Surface tension Surface tension of solution is related to the types of solutes and concentrations. Concentration c Surface tension  II I III

Zhang Enren 16 Page Gibbs’ isotherm describes the relations among surface tension, concentration of solute and surface excess. Surface phase 28.1 Surface tension

Zhang Enren 17 Page 28.2 Curved surfaces The minimization of the surface area of a liquid may result in the formation of a curved surface, as in a bubble, cavity or droplet

Zhang Enren 18 Page 28.2 Curved surfaces There are two consequences of curvature 1) The vapour pressure of a liquid depends on the curvature of its surface. p out p in = p out p in p out p in P in > p out p in p out

Zhang Enren 19 Page 28.2 Curved surfaces There are two consequences of curvature 1) The vapour pressure of a liquid depends on the curvature of its surface. r p in p out A cavity in liqiud Laplace equation

Zhang Enren 20 Page 28.2 Curved surfaces There are two consequences of curvature 1) The vapour pressure of a liquid depends on the curvature of its surface. plpl pgpg r Kelvin equation

Zhang Enren 21 Page 28.2 Curved surfaces There are two consequences of curvature 2) Curved surface leads to capillary action p p pp pp h  r p pp h fall 

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