Physical - chemistry of Physical - chemistry of surface phenomenasurface phenomena

PlanPlan

1. 1. Surface energy and surface tensionSurface energy and surface tension

2.2. Classification of sorption’s processes Classification of sorption’s processes

3.3. Adsorption. Adsorption.

Assistant Kozachok S.S. prepared

Intermolecular forces acting on a molecule

а, б) – inside the volume of liquid

в) – in the surface layer

gas

liquid

Surface and interfacial tensionsIt is well known that short-range forces of attraction exist between molecules, and are responsible for the existence of theliquid state. The phenomena of surface and interfacial tension are readily explained in terms of these forces. The molecules which are located within the bulk of a liquid are, on average, subjected to equal forces of attraction in all directions, whereas those located at, for example, a liquid-air interface experience unbalanced attractive forces resulting in a net inward pull.

As many molecules as possible will leave the liquid surface for the interior of the liquid; the surface will therefore tend to contract spontaneously. For this reason, droplets of liquid and bubbles of gas tend to attain a spherical shape.

Drops of liquid in a state of weightlessness takes the

form of sphere

Additivity of intermolecular forces at interfacesThe short-range intermolecular forces which are responsible for surface/interfacial tensions include van der Waals forces (in particular, London dispersion forces, which are universal) and may include hydrogen bonding (as, for example, in water) and metal bonding (as, for example, in mercury). The relatively high values of the surface tensions of water and mercury (see Table 4.1) reflect the contributions of hydrogen bonding and metal bonding, respectively.

It is easy to demonstrate that the surface energy of a liquid actually gives rise to a ‘surface tension’ or force acting to oppose any increase in surface area.

Surface tension is the force or tension required to break the film and is defined as the force in dynes acting upon a line cm long on the surface of the liquid.

• Unit of the Surface tension are N/m, J/ m2 , D/cm

• -dW= σ dS, where W- the work is performed against to the forces of internal pressure; S - surface area

Dependence of surface tension on temperature

The surface tension of most liquids decreases with increasing temperature

Measurement of surface tension by Stalagmometer

drop-weight methods

n0, ρ0, σ0 – number of droplets, density and surface tension of water,

n, ρ, σ – …… of investigated liquid

0

00

n

n

Capillary rise method

grh2

1

Where g is gravity = 9,8 m/s2

For zero contact angle,

For the rise of a liquid up a narrow capillary

N.B. In practice, the capillary rise method is only used when the contact angle is zero, owing to the uncertainty in measuring contact angles correctly

Ring methodIn this method the force required to detach a ring from a surface orinterface is measured either by suspending the ring from the arm of a balance or by using a torsion-wire arrangement (du Noiiy tensiometer).The detachment force is related to the surface or interfacial tension by the expression

where F is the pull on the ring, R is the mean radius of the ring and is a correction factor

Involuntary surface phenomena

Cohesion is the interaction between moleculars inside one phase (homogeneous system).

Adhesion is the interaction between moleculars inside of the different phases

Heterogeneous formation of a new phase

Spreading of the liqid on the surface of other liquid

Formation of spherical drops

Sorption Processes

Adsorption – the phenomenon of higher concentration of molecular species on the surface of a solid than in the bulk

Absorption is the process of arbitrary absorption of the substance by volume

Chemisorption - chemical interaction adsorbent with adsorbate

Adsorbent – an adsorptive material, such as activated charcoal

Adsorbate – an adsorbedsubstance

The solid substance on the surface of which adsorption occurs is known as adsorbent.

The substances that get adsorbed on the solid surface due to intermolecular attraction are called adsorbate.

The adsorbent may be a solid or a liquid and the adsorbate may be a gas or a solute in some solution.

Difference between Adsorption and AbsorptionDifference between Adsorption and Absorption

POSITIVE AND NEGATIVE ADSORPTION

In certain cases - solutions of electrolytes, sugars, etc. - smallincreases in surface tension due to negative adsorption are noted. Here, because the solute-solvent attractive forces are greater than the solvent-solvent attractive forces, the solute molecules tend to migrate away from the surface into the bulk of the liquid.

Types of adsorption

SpreadingAdhesion and cohesionThe work of adhesion between two immiscible liquids is equal to the work required to separate unit area of the liquid-liquid interface and form two separate liquid-air interfaces (Figure: Work of adhesion (a) and of cohesion (b), and is given by the Dupre equation

Spreading of one liquid on another

When a drop of an insoluble oil is placed on a clean water surface, it may behave in one of three ways:1. Remain as a lens, as in Figure 4.16 (non-spreading).2. Spread as a thin film, which may show interference colours, until it is uniformly distributed over the surface as a 'duplex' film. (A duplex film is a film which is thick enough for the two interfaces - i.e. liquid-film and film-air - to be independent and possess characteristic surface tensions.)3. Spread as a monolayer, leaving excess oil as lenses in equilibrium, as in Figure 4.17.

Harkins defined the term initial spreading coefficient (for the case of oil on water) as

Substituting in the Dupre equation, the spreading coefficient can be related to the work of adhesion and cohesion

Contact angles and wettingWetting is the displacement from a surface of one fluid by another. It

involves, therefore, three phases, at least two of which must be fluids.

The following account will be restricted to wetting in which a gas(usually air) is displaced by a liquid at the surface of a solid. Awetting agent is a (surface-active) substance which promotes thiseffect. Three types of wetting can be distinguished:1. Spreading wetting.2. Adhesional wetting.3. Immersional wetting.Spreading wettingIn spreading wetting, a liquid already in contact with the solid spreadsso as to increase the solid-liquid and liquid-gas interfacial areas anddecrease the solid-gas interfacial area.

Yung’s equation Cos θ = γs-g - γl-g / γl-g

σ = rhgd/2cos θ

Wetting (A) and unwetting (B) solid by liquid

А) B)

LiquidGas

Gas

Liquid

Cos θ = 0÷1 Cos θ = -1÷0

θθ

Adhesional wettingIn adhesional wetting, a liquid which is not originally in contact with the solid substrate makes contact and adheres to it. In contrast to spreading wetting, the area of liquid-gas interface decreases.

Immersionai wettingIn immersional wetting, the solid, which is not originally in contact with the liquid, is immersed completely in the liquid. The area of liquid-gas interface, therefore, remains unchanged

Introduction to surfactantsThe name ‘surfactant’ refers to molecules that are ‘surface-

active’, usually in aqueous solutions. Surface-active molecules adsorb strongly at the water–air interface and, because of this, they substantially reduce its surface energy (Gibbs theorem).

This is the opposite behaviour from that observed for most inorganic electrolytes, which are desorbed at the air interface and hence raise the surface energy of water (slightly).

Surfactant molecules are amphiphilic, that is, they have bothhydrophilic and hydrophobic moieties, and it is for this

reason that they adsorb so effectively at interfaces (note that ‘amphi’ means ‘of both kinds’ in Greek)

Surface tension of aqueous sodium chloride solutions at 20°C

Surface activityMaterials such as short-chain fatty acids and alcohols are soluble in both water and oil (e.g. paraffin hydrocarbon) solvents. The hydrocarbon part of the molecule is responsible for its solubility in oil, while the polar —COOH or -OH group has sufficient affinity to water to drag a short-length non-polar hydrocarbon chain into aqueous solution with it. If these molecules become located at an air-wateror an oil-water interface, they are able to locate theirhydrophilic head groups in the aqueous phase and allow the lipophilic hydrocarbon chains to escape into the vapour or oil phase

Adsorption of surface-active molecules as an orientated monolayer at air-water and oil-water

interfaces.

The strong adsorption of such materials at surfaces or interfaces in the form of an orientated monomolecular layer (or monolayer) is termed surface activity. Surface-active materials (or surfactants) consist of molecules containing both polar and non-polar parts (amphiphilic). Surface activity is a dynamic phenomenon, since the final state of a surface or interface represents a balance between this tendency towards adsorption and the tendency towards completemixing due to the thermal motion of the molecules.

Figure shows the effect of lower members of the homologousseries of normal fatty alcohols on the surface tension of water. The longer the hydrocarbon chain, the greater is the tendency for the alcohol molecules to adsorb at the air-water surface and, hence, lower the surface tension. A rough generalisation, known as Traube's rule, is that for a particular homologous series of surfactants theconcentration required for an equal lowering of surface tension in dilute solution decreases by a factor of about 3 for each additional CH2 group.If the interfacial tension between two liquids is reduced to asufficiently low value on addition of a surfactant, emulsification will readily take place, because only a relatively small increase in the surface free energy of the system is involved.

SURFACE ACTIVITY OF DRUGSEven small drug molecules are frequently amphiphilic,and therefore also generally surface active. This meansthat the drug tends to accumulate at or close to aninterface, be it a gas/liquid, solid/liquid or liquid/liquidinterface. This surface activity frequently depends on thebalance between electrostatic, hydrophobic and van derWaals forces, as well as on the drug solubility. Sincethe former balance depends on the degree of chargingand screening, the surface activity, and frequently alsothe solubility of the drug, it often depends on thepH and the excess electrolyte concentration.

the surface activities of drugs may contribute to their biologicalaction, although the relationship between surface activity and biological effect is less straightforward.