Surface and Interface Chemistry  Liquid/gas Interface Valentim M. B. Nunes Engineering Unit of IPT 2014

The concept of phase and interface: From a technological standpoint, the heterogeneous processes involve surfaces: precipitation, diffusion, flocculation, detergents effects, etc. The concept of phase and interface: Phase  Interface,  Few molecular layers Phase 

Surface Tension

Surface molecule is "pulled" to the interior Molecular explanation: imbalance in Intermolecular forces that exists on the surface. Gas Liquid R 0 R = 0 Molecule inside the liquid is subject to Intermolecular forces, but the resultant is null. Surface molecule is "pulled" to the interior

To increase the surface area is necessary to perform work against the intermolecular forces of liquid. The work required to change the surface area of an infinitesimal quantity dσ is given by: The coefficient  is the surface tension (SI units: J.m-2 or N.m-1). At constant pressure and temperature dw = dG , then:

Liquids with greater Intermolecular forces are those that presents larger surface tensions. /mN.m-1 (T = 293 K) Isopentane 13.72 Ethyl ether 17.10 Hexane 18.43 Ethyl bromide 24.16 Benzene 28.86 Carbon tetrachloride 26.66 Water 72.75 Mercury 472.0

There is no fundamental distinction between surfaces and interfaces, although it is customary to designate the boundary between two phases in which one of them is gaseous as surface, and between two non-gaseous phases as an interface. At the interface between two liquid's there is again an imbalance in intermolecular forces in the interior and along the interface. Intermolecular forces responsible for surface or interfacial tension include London dispersion forces (Universal), hydrogen bonds and metallic bond.

Interfacial tension: contact angle The imbalance of intermolecular forces on the liquid/gas interface and solid/gas interface are always bigger than between condensed phases, then interfacial tensions for two liquids or between solid and liquid systems are always smaller than the biggest of the surface tensions. The work of adhesion between two phases α and β is expressed by the equation of Dupré: At a solid/liquid interface it comes:

Consider the following system: solid liquid Gas F F’ F’’  F – surface tension of liquid, L F’ – interfacial tension, SL F’’- surface tension of solid, S  - contact angle Condition of equilibria (Young’s equation): And we obtain the Young-Dupré equation:

Since WLL is 2 L :  = 0 S = L WSL = WLL Liquid completely wets the solid 0 <  < /2 S > SL WLL/2<WSL<WLL Liquid partially wets the solid /2< <  S < SL 0 < WSL < WLL/2 Solid is difficulty wet by the liquid  =  SL = L WSL = 0 Liquid does not wet the solid

As a consequence of surface tension there is a difference of pressure through any curved surface; consider a drop of liquid ~ Spherically: z Pe Pi a  r

The existence of surface tension prevents the liquid to spread across the surface. The outside pressure (Pe) will be lower than the internal pressure (Pi). At equilibrium, the resultant of the forces that are due to the difference in pressure, Pi-Pe, will be equal to the forces due to ϒ, along the zz axis (spherical symmetry)

This is the Laplace equation For non spherically surfaces (two curvature radii): Laplace's equation shows that the pressure inside a curved surface (concave side of the interface) is greater than the pressure outside (drops, bubbles,......)

Vapor pressure of a pressurized liquid Pressure of liquid is increased in dP, then dp represents the change of vapor pressure..

This equation shows that the vapor pressure increases when the pressure exerted over an condensed phase increases! Kelvin’s equation

Vapor pressure of water drops with 10-3 mm to 10-6 mm is 1. 001 to 2 Vapor pressure of water drops with 10-3 mm to 10-6 mm is 1.001 to 2.95 times the vapor pressure of the water "flat"!, preventing condensation, and favoring the formation of clouds

The surface tension of most liquids decreases with increasing temperature, being fairly low near the critical point (except for liquid Cu and Fe!) Eötvos equation: Ramsay and Shields: k  2.1 for normal liquids (benzene, carbon tetrachloride, S2C, etc…) Katayama: Guggenheim:

Capillarity The tendency of fluids to climb in a capillary tube is a consequence of surface tension and is called capillarity or capillary rise.

R 2r 