Surface Phenomena Notes PDF

SECTION B SURFACE PHENOMENA 1 INTRODUCTION When we talk about phases, say in the discussion on phase equilibria, we often define a phase as a system in which the properties are uniform (continuous function) throughout. By this we seem to infer that properties are the same both in the interior and at...

SECTION B SURFACE PHENOMENA 1 INTRODUCTION When we talk about phases, say in the discussion on phase equilibria, we often define a phase as a system in which the properties are uniform (continuous function) throughout. By this we seem to infer that properties are the same both in the interior and at the exterior. This is not usually the case. For example, the molecules (or ions) in the interior of a liquid are "pulled" on all sides by a uniform field of force, whilst those at the surface are bounded only on one side, the bulk of the liquid as illustrated in Figure B1 below. The surface portions of the liquid have a higher free energy than in the bulk. Therefore, in order to bring a molecule from the bulk to the surface (thereby increasing the surface) work must be done against the cohesive forces in the liquid. The extra energy acting parallel to the surface is termed the SURFACE TENSION (γ, N.m-1) Work done (dW) in extending the surface by an amount of dA is: dW = -γ dA = - dG where G is the free energy. The negative sign is because work is done against the force at the surface, i.e. going from a place of lower to higher energy. Remember: If we consider all the different kinds of work (heat absorbed, volume, surface, electrical, etc.) that can be done by the system then the first law can be written as: dU=δQ− pdV+γdA− ℰdq− ...+∑ μ j dN j Since the surface of a liquid does not have the same free energy as the bulk of the liquid, its thermodynamic state is different. Surface tension becomes important in the case of interfacial phenomena, or when the interfacial phenomena must be taken into consideration in the study of a system, for example when considerable fraction of material in a system exists at or near the surface. Surface phenomena play important roles in both science and engineering. In the treatment of water for instance, the coagulation process used in the clarification of the water involves interfacial processes. The effectiveness of certain pesticides in environmental pest control depends on their ability either to spread on the surface of water or to wet the surface of plants and leaves which all involve interfacial processes. Surface phenomena is important in colloid and surface science and knowledge of colloid and surface science is very important in most mineral processing operations notably in flotation and solid-liquid separation. In amalgamation process which involves the collection or recovery of free gold particles in ore concentrates using mercury depends on the ability of the mercury to wet the gold particles’ surface - wetting being a very important surface phenomenon in this case. Surface or Interfacial phenomena also play important roles in materials science, food and drug industries and medicine among others. 32 Five main interfaces can be identified in nature as follows: 1. liquid-gas, 2. liquid-liquid, 3. solid-liquid, 4. solid-gas, and 5. solid-solid. There cannot be a definite gas-gas interface since gases have negligible surface tension or surface energy and therefore completely miscible. Substances that markedly lower the surface tension of water are those that contain both polar hydrophilic group and a non-polar hydrophobic group (e.g. fatty acids, RCOOH). Such substances are referred to as surface-active agents or surfactants. The hydrocarbon (non-polar) section feels uncomfortable in the interior (free energy of state is high) and therefore needs little work to come to the surface, i.e. they tend to accumulate preferably at the surface (positively absorbed). However, solutes such as ionic salts, by virtue of ion-dipole attractions, tend to pull the water molecules into the interior of the solution, increasing the surface tension. The surface layers are poorer in solute than in the bulk (negatively adsorbed). Generally, the surface tension decreases with increase in temperature, as expressed by the empirical equation by Ramsay and Shields: g( Mx 2 where Tc is the critical temperature, T is the temperature, k is a ) 3 = k(Tc - T - 6) constant, M is the molecular mass, x is the degree of association, and r ρ is the density. 2 THERMODYNAMICS OF SURFACES Consider two phases α and ß separated by an interfacial region, S - S'. The choice of the region should be such that no inhomogeneity occurs in the bulk up to A - A' or B - B'. Within the interfacial region, the properties of the system vary continuously from purely α at A - A' to purely ß at B - B'. The region is no more than a few molecular diameters in thickness. The dividing surface (S - S') can be placed such that for any particular component the surface concentration is zero, thus the number of moles of the component in the interfacial film is the same as in the bulk if extended to S-S'. This way the surface concentrations, (cϕi ) of the other components are said to be the concentrations adsorbed at the surface. This adsorption can either be positive or negative. The thermodynamic relation between the extent of adsorption and the change in the surface tension of the solution is known as the Gibbs adsorption isotherm. From the thermodynamic expression: 𝑑𝐺 = −𝑆𝑑𝑇 + 𝑉𝑑𝑃 + Σ𝜇𝑖 𝑑𝑛𝑖 𝑑𝐺 = −𝑆 ∝ 𝑑𝑇 − 𝑆𝛽 𝑑𝑇 + 𝑉 ∝ 𝑑𝑃 + 𝑉𝛽 𝑑𝑃 + 𝜇1 𝑑𝑛1 ∝ + 𝜇2 𝑑𝑛2 𝛽 At constant T, P, ni dG = 0. In considering the surface phase (ϕ), and adding the following terms: dGϕ = -Sϕ dT + γ dAϕ + μ1dn1ϕ + μ2dn2ϕ 33 At constant T, dGϕ = γ dAϕ + μ1dn1ϕ + μ2dn2ϕ Integrating using Euler's theorem, Gϕ = γ Aϕ + μ1n1ϕ + μ2n2ϕ for which the complete differential is: dGϕ = γ dAϕ + Aϕ dγ + μ1 dn1ϕ + n1ϕ dμ1 + n2ϕ dμ2 + μ2 dn2ϕ This means that: Aϕ dγ = - n1ϕ dμ1 - n2ϕ dμ2 or dγ = - c1ϕ dμ1 - c2ϕ dμ2 By choosing the dividing surface (S-S') such that the excess of one component (1), say the solvent vanishes, dg = - G 2 d m2 Γ is the concentration of component (2) at the interface. From the thermodynamic expression, Dm o = - RT ln K a For ideal solutions; , dg = - RT G 2 d lna2 a2 = x2 (mole fraction), i.e. dg = - RT G 2 d (ln x2 ) G =- 1 dg RT d ln x2 For dilute solutions x ≈ c2 (molar concentration), therefore G =- 1 dg RT d ln c2 3 ELECTRICAL PHENOMENA AT THE INTERFACES 3.1 The origins of surface charges Some materials when immersed in an aqueous solution acquire surface charges. The interfacial behaviour of such solids (oxides, sulphides, etc) are then determined by these surface charges. These charges may result from: (1) dissociation of the surface functional groups (2) dissociation of the constituent surface atoms (3) isomorphic substitution of ions (4) selective adsorption/desorption of ions or physical adsorption of charged species onto the surface (5) accumulation or depletion of electrons at the surface 34