8/16/2019 Module 3_Theory Farfis

1/2

Theory

Cohesion phenomenon of intermolecular forces holding particles of a substance together.Cohesion differs from adhesion in being the force of attraction between adjacent particleswithin the same body; adhesion is the interaction between the surfaces of different bodies.

The force of cohesion in gases can be observed in the liquefaction (condensation) of a gas,which is the result of a number of molecules being pressed together to produce forces ofattraction high enough to give a liquid structure.Cohesion in liquids is reflected in the surfacetension caused by the unbalanced inward pull on the surface molecules, and also in thetransformation of a liquid into a solid state when the moleculesare brought sufficiently close together. Cohesion in solids depends on the pattern ofdistribution of atoms, molecules, and ions, which in turn depends on the stateofequilibrium(or lac of it) of the atomic particles. !n many organiccompounds, which form molecularcrystals,for e"ample, the atoms are bound strongly into molecules, but the molecules are

bound wea ly to each other. #dhesion, attraction between the surfaces of two bodies. The

term is sometimes used to denote the tendency of two adjacent surfaces, which may be ofdifferent chemical compositions, to cling to each other, whereas cohesion is used to refer tothe attraction between portions of a single body. $or e"ample, if a sheet of glass is loweredinto water and withdrawn, some water will cling to the glass (adhesion) but the rest will be

pulled bac into the main body of water (cohesion).The force of attraction is attributed toelectromagnetic interactions produced by fluctuations in the distribution of electrons in themolecules of the facing surfaces. The distance between the molecules of the facing surfaces isa determining factor in the amount of force e"erted. # surface that may appear smooth to thena ed eye actually may be too rough to hold its molecules close enough to a facing surface to

produce an electromagnetic bond. %auge bloc s, pieces of metal used for ta ing accuratemeasurements, have such smooth surfaces that their facing surfaces can be made to stic toeach other by twisting them together.!n the human body, when tissues or organs that arenormally separated unite or grow together, the process is called adhesion. This abnormaloccurrence may come about after inflammation or during healing after a surgical operation.

(&oe, ' )

*eduction of surface+interfacial tension in aqueous solutions is one of the ey functions ofsurfactants. !n a number of practical applications such as oil e"traction and fire e"tinctionfoams the minimal surface tension, lim, is a property of interest. The lim is the limiting

surface tension that can be achieved by adding surfactant to a solvent. The smaller the valueof lim, the better is the surfactant -effectiveness- . evelopment of modeling capability to

predict lim as a function of the composition and structure of the surfactant molecule wouldenable rapid development and optimi/ation of new surfactants. 0e believe that this modelingcapability would clarify the thermodynamic equilibrium between surfactants in the bul ,micelle and at the surface and reveal the impact of surfactant structure and composition onthe limiting surface tension. The surface tension, , of the surfactant solution decreases withincrease of the concentration of surfactant molecules in the system. The concentration ofsurfactants in the system, C, is defined as the total number of moles of thesurfactantmolecules divided by the total system volume. #s C increases, decreases. #limitation for decrease is imposed by the surfactant solubility limit. 1owever, for themajority of surfactants the system undergoes a phase separation long before C reaches thesolubility limit. The surfactants form a disperse phase comprised of micelles along with the

8/16/2019 Module 3_Theory Farfis

2/2

solution of individual surfactant molecules in the bul . The concentration at which themicelles formation starts is called critical micelle concentration (cmc). $or C lower than thecmc, surfactants are distributed between the bul of the solution and the surface. The bulconcentration of surfactants, C bul , and the surface concentration, 2, both increase with Cuntil C reaches cmc. #fter C reaches cmc, all added 3e"cess4 surfactants form micelles so that

the bul concentration of the individual surfactant molecules is constant and equal to the cmc.The surface concentration also reaches the saturation level, 2cmc. 5ince the surface tension isgoverned by 2and is not sensitive to the micelle concentration in the bul , it also reaches thesaturation level that is equal to at cmc, cmc. Thus, cmc is the limiting surface tension thatcan be archived by using the specific surfactant, i.e. cmc 6 lim.

(Burlatsky, 2013)

5urface tension and surface free energy. Consider a drop of a one component liquid inequilibirium with its vapour and in the absence of any e"ternal forces. !t can therefore beinferred that wor must be done on the drop to increase its surface area, and hence that thesurface molecules are in a state of higher free energy than those in the bul liquid. This is in

part e"plained by the fact that surface molecules have fewer nearest neighbour and as aconsequences, fewer intermolecular interaction than bul molecules. 5urface molecules aresubject to an inward attraction normal to surface. This is equivalent to saying that the surfaceitself is in a state of lateral tension and leads to the concept of surface tension.

(#veyard, 789)

The surface tension is another measure of the internal cohesive forces within a solvent. #llliquids tend to adopt shapes that minimi/e their surface area, because this leads to thema"imum number of molecules in the bul interacting with their neighbours. #t the surfaceof a solution the solvent molecules cannot have the normal number of intermolecular

interactions because these molecules are at an interface with air. 5olvents with a high surfacetension require the greatest energy to increase their surface area, and will tend to minimi/etheir e"posed surface the most. 5olvents with low cohensive forces will have a low surfacetension and less of a driving force to minimi/es e"posed surface area.

(:ric, ' )

The molecules present in the surface of a liquid e"perience a resultant downward pull because the number of molecules below the surface, i.e, in the bul of the liquid is muchlarger than the number of molecules above it, i.e, in the vapour phase. Consequently, the

molecules in the surface tend to be drawn inside, with the result that the liquid surface tendsto contract and behaves as if it were in a state of tension. This tension or force which actsalong the surface of the liquid, uniformly in all directions, is nowns as surface tension.