Definition of interface Liquid-gas and liquid-liquid interfaces (surface tension, spreading, adsorption and orientation at interfaces)

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1 Definition of interface Liquid-gas and liquid-liquid interfaces (surface tension, spreading, adsorption and orientation at interfaces)

2 Definition of interface How can we define the interface? How we can detect the surface of a condensed phase?

3 Definition of interface If two homogeneous bulk phases meet there is a region of finite thickness where the properties changed. That region is called interface. At a molecular level the thickness of the interfacial region is not zero, and it is significant! The properties of interfacial region can be important for colloid systems, especially for dispersions, where the surface to volume ratio is not negligible.

4 Fluid interfaces The attractive forces acting on molecules at the surface are anisotropic, the net force is oriented toward the liquid phase. As a consequence, liquids tend to reduce their surface. Energy is required to increase the surface to overcome the attraction.

5 Surface tension The energy (G) required to increase the surface (A) isothermally and reversibly by a unit amount is called surface tension (γ). dg da npt,, The unit of surface tension is J/m 2. This definition are applied only for pure liquid. Surface tension value is always positive because of the attraction.

6 Surface tension The surface tension (γ) can also be defined as a force (F) acting to any imaginary line of unit length (l), on the liquid surface if the force is perpendicular to the line. F 2l The unit of surface tension is N/m. This definition is valid for any liquid. =F/2l

7 Factors having influence on surface tension 1. Chemical nature liquid Surface tension (mn/m, 20 o C) Water 72.8 Benzene 28.9 Acetic acid 27.6 Acetone 23.7 Ethanol 22.3 n-hexane 18.4 n-octane 21.8 n-octanol 27.5 Mercury 485

8 Factors having influence on 1. Chemical nature liquid surface tension Interfacial tension: surface tension at the interface of two liquids. It depends on the asymmetry of the two phases. Interfacial tension against the water (mn/m, 20 o C) Benzene 35.0 n-hexane 51.1 n-octane 50.8 n-octanol 8.5 mercury 375 It is only an estimation!!

9 Factors having influence on 2. Temperature surface tension The secondary interactions depend on temperature, at higher temperature the attraction is weaker. Eötvös-law (Hungarian physicist): V 2 3 m Ramsey and Shields law: V 2 3 m const const E ( T T ) Not valid for associating or dissocating compounds! E c ( T c T γ: surface tension (N/m), V m : molar volume (m 3 /mol), T: temperature (K), T c : critical temperature (K), const E : Eötvös constant (2.1 x 10-7 J/(K mol 2/3 ) 6)

10 Factors having influence on 3. Presence of solute surface tension A, Ions, small polar molecules. These compounds prefer being solvated (hydrated), so they tend to move inside the liquid phase where they can be solvated from all direction. Thus more solvent molecule goes toward the surface, which increase the surface tension. Surface inactive (capillary inactive compounds) (N/m) c(mol/m 3 )

11 Factors having influence on 3. Presence of solute surface tension B, Amphiphilic molecules (having polar and non-polar parts). These molecules are oriented on the surface (gas-liquid or liquid-liquid surface. The polar ends are oriented toward the polar solvent, while the non-polar parts are pointed toward the gas, or the non-polar liquid phase. This orientation makes possible the smoothest change of polarity between the phases (Hardy-Harkins rule).

12 Factors having influence on 3. Presence of solute surface tension B, Amphiphilic molecules (having polar and non-polar parts)..

13 Factors having influence on 3. Presence of solute surface tension B, Amphiphilic molecules (having polar and non-polar parts). The interaction between the amphiphiles are weaker compare to the solvent, so the orientation of these molecules decreases the surface tension. Surface active (capillary active) compounds. (N/m) c(mol/m 3 )

14 Effect of solute concentration on the surface excess The Gibbs-isotherm: Describes the relation between the solute concentration (c) and the surface excess(γ) at a given temperature. Γ: Surface excess (mol/m 2 ) A: surface of a molecule occupied: (m 2 /each) R: gas constant (8.314 J/Kmol) T: Temperature (K) c: concentration (mol/m 3 ) B: constant

15 Effect of solute concentration on the surface excess The Gibbs-equation: Describes the relation between the solute concentration (c), the surface tension and the surface excess(γ) at a given temperature. Γ: Surface excess (mol/m 2 ) R: gas constant (8.314 J/Kmol) T: Temperature (K) c: concentration (mol/m 3 ) γ :surfacetension(n/m)

16 Surface tension: the consequences If the gravitational force is smaller than the surface tension acts, the object can float on the surface although the density is higher.

17 Surface tension: the consequences

18 Surface tension: the consequences The Laplace pressure air p 1 The liquid tends to reduce the surface, so: p 2 >p 1 p 2 Laplace equation: 2 p r Consequence: The pressure is always higher at the concave side.

19 Surface tension: the consequences The Laplace pressure p 2 4 p 1 p r Double interface! The pressure difference can be extremely high at small radius! Radius 1mm 0.1mm 1μm 10nm Δp (kpa) What happens if we open the tap between the bubbles? v=kvrsahuvs3m

20 Surface tension: the consequences Meniscus The shape of the fluid surface in a tube depends on the adhesion and cohesion. If the adhesion (liquid-solid attraction) is stronger than the cohesion (interaction of liquid particles) the meniscus is concave, otherwise it is convex. r<0 r= r>0 (the centre is outside) (the centre is inside)

21 Surface tension: the consequences Kelvin equation It has already been seen that the pressure over the curved surface is different compared to the flat one. Thus the vapor pressure of the liquid also depends on the shape of the surface. ln p p r V m RT 2 r p r, p : vapor pressure over the curved and flat surface (Pa), V m :molar volume (m 3 /mol), γ: surface tension (N/m), R: gas constant (J/Kmol), r: radius of the capillary(m), T: temperature (K)

22 Surface tension: the consequences Capillary condensation In case of porous materials (solid phase with capillaries) the vapor can condense even at higher temperature if the fluid (condensed liquid) phase wets the surface. This phenomena can be explained by the Kelvin equation. (Wetting means r<0, so the ln(p r /p ) is negative, therefore p r <p and if p r <p out then the vapor condenses)

23 Surface tension: the consequences Capillary action A, r<0 Concave meniscus The pressure inside the liquid is smaller compared to the flat surface. The fluid phase is pushed into the capillary to balance the pressure difference B, r>0 Convex meniscus The pressure inside the liquid is higher compared to the flat surface. The fluid phase is pushed out from the capillary to balance the pressure difference.

24 Surface tension: the consequences The shape of the meniscus The shape of the liquid surface depends on the ratio of the adhesion and cohesion. If the cohesion is stronger than the adhesion the meniscus is concave (r<0, water, aqueous solutions, polar solvents), while if the adhesion is stronger than the cohesion, the meniscus is convex (r>0, mercury)

25 Measurement of surface tension Wilhelm plate du Nouy ring The difference in pressure (see the Kelvin eq.) is in equilibrium with the fluid pressure. Measuring the capillary rising or depression makes possible to calculate of surface tension 1 2 hgrcap Measurement of force needed to remove a plate or ring from the liquid F 2l

26 Spreading, wetting, contact angle Contact angle (measured in the liquid phase) Θ= Θ1+ Θ2 Θ Perfect wetting (spreading): Θ=0 o Partial wetting: 0 o < Θ < 90 o Non wetting: 90 o < Θ <180 o Perfectly non wetted Θ=180 o

27 Spreading, wetting, contact angle Wettability depends on adhesion /cohesion. When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface (or spread on the other liquid), when the forces of adhesion are less by comparison to those of cohesion, the liquid tends to "refuse" the surface. In this people speak of wettability between liquids and solids. For example, water wets clean glass, but it does not wet wax.

28 Spreading, wetting, contact angle

29 Spreading, wetting, contact angle In equilibrium: 2 1 cos1 12 cos 2 GS LS GL cos Spreading (or wetting) if Θ < 90 o 2 ( 1 12) 0 GS ( ) 0 LS GL S lower ( ) interphase upper 0

30 Adhesion and cohesion Adhesion: γ A + γ B -γ AB Cohesion: 2γ A S=adhesion-cohesion= γ A + γ B -γ AB -2γ A = γ B -(γ A +γ AB )