Physical - chemistry of surface phenomena

Physical - chemistry of surface phenomena Plan 1. Surface energy and surface tension 2. Classification of sorption’s processes 3. Adsorption. Assistant Kozachok S.S. prepared

Intermolecular forces acting on a molecule gas liquid а, б) – inside the volume of liquid в) – in the surface layer

Surface and interfacial tensions It is well known that short-range forces of attraction exist betweenmolecules, and are responsible for the existence of the liquid state. The phenomena of surface and interfacial tension arereadily explained in terms of these forces. The molecules which arelocated within the bulk of a liquid are, on average, subjected to equalforces of attraction in all directions, whereas those located at, forexample, a liquid-air interface experience unbalanced attractiveforces resulting in a net inward pull. As many moleculesas possible will leave the liquid surface for theinterior of the liquid;the surface will therefore tend to contract spontaneously. For thisreason, droplets of liquid and bubbles of gas tend to attain a sphericalshape.

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 forsurface/interfacial tensions include van der Waals forces (in particular,London dispersion forces, which are universal) and may includehydrogen bonding (as, for example, in water) and metal bonding (as,for example, in mercury). The relatively high values of the surfacetensions of water and mercury (see Table 4.1) reflect the contributionsof hydrogen bonding and metal bonding,respectively.

It is easy to demonstrate that the surface energy of a liquidactuallygives rise to a ‘surface tension’ or force acting tooppose any increasein 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 increasingtemperature

Measurement of surface tension by Stalagmometerdrop-weight methods n0, ρ0, σ0 – number of droplets, density and surface tension of water, n, ρ, σ– …… of investigatedliquid

Capillary rise method For zero contact angle, Where g is gravity = 9,8 m/s2

For the rise of a liquid up a narrow capillary N.B.In practice, the capillary rise method is only used when the contactangle is zero, owing to the uncertainty inmeasuring 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 abalance or by using a torsion-wire arrangement (du Noiiy tensiometer).The detachment force is related to the surface or interfacial tensionby the expressionwhere F is the pull on the ring, Ris the mean radius of the ring andis a correction factor

Involuntary surface phenomena Cohesionis the interaction between moleculars inside one phase (homogeneous system). Adhesionis 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 arbitraryabsorption of the substance by volume Chemisorption - chemical interaction adsorbentwith 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 Absorption

POSITIVE AND NEGATIVE ADSORPTION

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

Types of adsorption

Spreading Adhesion and cohesion The work of adhesion between two immiscible liquids is equal to thework required to separate unit area of the liquid-liquid interface andform two separate liquid-air interfaces (Figure: Work of adhesion (a) and of cohesion (b), and is givenby the Dupre equation

Spreading of one liquid on another When a drop of an insoluble oil is placed on a clean water surface, itmay 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 itis uniformly distributed over the surface as a 'duplex' film. (Aduplex film is a film which is thick enough for the two interfaces -i.e. liquid-film and film-air - to be independent and possesscharacteristic 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 caseof oil on water) as Substituting in the Dupre equation, the spreading coefficient canbe related to the work of adhesion and cohesion

Contact angles and wetting Wetting 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. A wetting agent is a (surface-active) substance which promotes this effect.Three types of wetting can be distinguished: 1. Spreading wetting. 2. Adhesional wetting. 3. Immersional wetting. Spreading wetting In spreading wetting, a liquid already in contact with the solid spreads so as to increase the solid-liquid and liquid-gas interfacial areas and decrease the solid-gas interfacial area.

Yung’s equation Cosθ= γs-g - γl-g /γl-g σ = rhgd/2cos θ

Wetting (A) and unwetting (B) solid by liquid Gas Gas Liquid Liquid θ θ Cos θ = 0÷1 Cos θ = -1÷0 А) B)

Adhesional wetting In adhesional wetting, a liquid which is not originally in contact withthe solid substrate makes contact and adheres to it. In contrast tospreading wetting, the area of liquid-gas interface decreases. Immersionai wetting In immersional wetting, the solid, which is not originally in contactwith the liquid, is immersed completely in the liquid. The area ofliquid-gas interface, therefore, remains unchanged

Introduction to surfactants The name ‘surfactant’ refers to molecules that are ‘surface-active’,usually in aqueous solutions. Surface-active molecules adsorb stronglyatthe water–air interface and, because of this, they substantially reduceits surface energy (Gibbs theorem). This is the opposite behaviour fromthat observed for most inorganic electrolytes, which are desorbed atthe air interface and hence raise the surface energy of water (slightly). Surfactant molecules are amphiphilic, that is, they have both hydrophilic and hydrophobic moieties, and it is for this reason that theyadsorb so effectively at interfaces (note that ‘amphi’ means ‘of bothkinds’ in Greek)

Surface tension of aqueous sodium chloride solutions at 20°C

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

Adsorption of surface-active molecules as an orientated monolayer at air-waterand oil-water interfaces.

The strong adsorption of such materials at surfaces or interfaces inthe form of an orientated monomolecular layer (or monolayer) istermed 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 thefinal state of a surface or interface represents a balance between thistendency towards adsorption and the tendency towards completemixing due to the thermal motion of the molecules.

Figure shows the effect of lower members of the homologous series of normal fatty alcohols on the surface tension of water. Thelonger the hydrocarbon chain, the greater is the tendency for thealcohol molecules to adsorb at the air-water surface and, hence,lower the surface tension. A rough generalisation, known as Traube'srule, is that for a particular homologous series of surfactants the concentration required for an equal lowering of surface tension indilute solution decreases by a factor of about 3 for each additionalCH2 group. If the interfacial tension between two liquids is reduced to a sufficiently low value on addition of a surfactant, emulsification willreadily take place, because only a relatively small increase in thesurface free energy of the system is involved.

SURFACE ACTIVITY OF DRUGS Even small drug molecules are frequently amphiphilic, and therefore also generally surface active. This means that the drug tends to accumulate at or close to an interface, be it a gas/liquid, solid/liquid or liquid/liquid interface. This surface activity frequently depends on the balance between electrostatic, hydrophobic and van der Waals forces, as well as on the drug solubility. Since the former balance depends on the degree of charging and screening, the surface activity, and frequently also the solubility of the drug, it often depends on the pH and the excess electrolyte concentration. the surfaceactivities of drugs may contribute to their biological action, although the relationship between surface activityand biological effect is less straightforward.

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Physical - chemistry of surface phenomena