LECTURE 8: Physical-chemical essence of

surface phenomenon.

associate. prof.associate. prof. Yevheniya B. DmukhalskaYevheniya B. Dmukhalska

•The surfaceThe surface is the place of is the place of contact among different substances.contact among different substances.

The properties of surfaces are so The properties of surfaces are so special and important that there is a special and important that there is a branch of science, the physics of branch of science, the physics of surfaces, devoted to the study of surfaces, devoted to the study of surface phenomena.surface phenomena.

Adhesion and Cohesion of Water

• Water drops on pine needles, showing Water drops on pine needles, showing the effects of gravity, adhesion, and the effects of gravity, adhesion, and cohesion on water.cohesion on water.

• Adhesion and cohesion Adhesion and cohesion are water properties are water properties that affect every water that affect every water molecule on earth and molecule on earth and also the interaction of also the interaction of water molecules with water molecules with molecules of other molecules of other substances.substances.

Cohesion: Water is attracted to waterAdhesion: Water is attracted to other substances

Cohesive forces • are the intermolecular forces (such as those

from hydrogen bonding and Van der Waals forces) which cause a tendency in liquids to resist separation. These attractive forces exist between molecules of the same substance. For instance, rain falls in droplets, rather than a fine mist, because water has strong cohesion which pulls its molecules tightly together, forming droplets.

Adhesive Forcesare the attractive forces between unlike molecules. They are caused by forces acting between two substances, such as mechanical forces (sticking together) and electrostatic forces (attraction due to opposing charges). In the case of a liquid wetting agent, adhesion causes the liquid to cling to the surface on which it rests.

• Dew drops adhering to a spider webDew drops adhering to a spider web

• Meniscus is the curvature of a liquid's surface within a container such as a graduated cylinder. Meniscus is the curvature of a liquid's surface within a container such as a graduated cylinder. Water, for example, is a polar molecule that consists of a partial positive charge on the hydrogens Water, for example, is a polar molecule that consists of a partial positive charge on the hydrogens and a partial negative charge on the oxygen. Thus, within liquid water, each molecule's partial and a partial negative charge on the oxygen. Thus, within liquid water, each molecule's partial positive charge is attracted to its neighbor's partial negative charge. This is the origin of the positive charge is attracted to its neighbor's partial negative charge. This is the origin of the cohesive forces within water. cohesive forces within water.

• Surface tension could be defined as Surface tension could be defined as the property of the surface of a liquid the property of the surface of a liquid that allows it to resist an external that allows it to resist an external force, due to the cohesive nature of force, due to the cohesive nature of the water molecules.the water molecules.

Water molecules want to cling Water molecules want to cling to each other. At the surface, to each other. At the surface, however, there are fewer however, there are fewer water molecules to cling to water molecules to cling to since there is air above (thus, since there is air above (thus, no water molecules). This no water molecules). This surface layer (held together surface layer (held together by surface tension) creates a by surface tension) creates a considerable barrier between considerable barrier between the atmosphere and the the atmosphere and the water.water.

The work done by increasing a unit area of liquid surface is called surface energy.The surface of a liquid acts as a tensional film which always tends to contract to a minimum area. It proves that surface of liquid has tension. It is called surface tension.

Surface tension F can be described by the surface tension coefficient , L is the length of a line on the liquid surface.LFtension

)( 1 mNL

Ftension

Walking on water: Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

Why bubbles are round: The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes.

Using torsion balance or Using torsion balance or tensiometer (Du Nouy’ tensiometer (Du Nouy’ s).s).In this method а light metal ring is set

on the surface of the liquid. When the ring is raised, а film of the liquid clings to it. The amount of force required to pull the ring and break the film is measured and gives the surface tension.

Tensiometer EasyDyneTensiometer EasyDyne• The EasyDyne instrument The EasyDyne instrument

is a tensiometer for is a tensiometer for measuring surface and measuring surface and interfacial tension of interfacial tension of liquids using the liquids using the Wilhelmy Plate or the Du Wilhelmy Plate or the Du Noüy Ring method. In Noüy Ring method. In addition, it is able to addition, it is able to determine the density of determine the density of liquids.liquids.

Drop methodDrop methodWhen а liquid is allowed to flow very slowly through а

capillary tube, а drop will form which will increase to а certain size and then fall down. For finding out the surface tension of а liquid relative to that of water, the number of drops produced by а given volume of the two liquids is found out. The apparatus used in these determinations consists of а bulb fused with а capillary tube and is called а drop pipette or stalagmometer

number of water drops x σ water

σ liquid = ------------------------------------------

number of drops of liquid

Surface phenomenon:Surface phenomenon:• CohesionCohesion ( (nn. lat. . lat. cohaererecohaerere "stick or stay together") or "stick or stay together") or

cohesive attractioncohesive attraction or or cohesive forcecohesive force is the action or is the action or property of like molecules sticking together, being mutually property of like molecules sticking together, being mutually attractive.attractive.

• AdhesionAdhesion is any attraction process between dissimilar is any attraction process between dissimilar molecular species that can potentially bring them in close molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar contact. By contrast, cohesion takes place between similar molecules.molecules.

• The phenomenon of attracting and retaining the molecules of а substance on the surface of а liquid or а solid resulting into a higher concentration of the molecules on the surface is called adsorption.

Surfactants • The substances which lower the surface tension of a liquid are called

surfactants (from: surface-active agents)• Class of molecules that contain hydrophobic (non-polar) hydrocarbon

"tails" and a hydrophilic (polar) "head" group are called surfactants. • А surfactant accumulates at the interface, and modifies its surface

tension.• is а surface which separates а liquid from air or other gases or which

separates one liquid from another. • Soaps, detergents, phospholipids, bile salts and proteins

If the material is hydrophilic ("water loving") it has a surface to which water is attracted. If the solid object is hydrophobic ("water fearing"),the unfavorable interactions between the water surface and the object make it difficult to wet the surface.

• The surfactant molecules thereby organize themselves into 3-dimensional spheres called micelles which have a hydrocarbon core and polar groups around the outer surface.

• SSome surfactants can coat the surface of the water to form a layer one molecule thick, a molecular monolayer.

• STRUCTURAL COMPONENTS OF A CELL STRUCTURAL COMPONENTS OF A CELL MEMBRANEMEMBRANE

• LIPID BILAYER OF PLASMA LIPID BILAYER OF PLASMA MEMBRANEMEMBRANE

• The phenomenon of attracting and retaining the molecules of а substance on the surface of а liquid or а solid resulting into a higher concentration of the molecules on the surface is called adsorption.

• The substance thus adsorbed on the surface is called the adsorbate and the substance on which it is adsorbed is called adsorbent. The reverse process removal of the adsorbed substance from the surface is called desorption.

• The adsorption of gases on the surface of metals is called occlusion.

• The process of adsorption involves separation of a substance from one phase accompanied by its accumulation or concentration at the surface of another.

Adsorption:• It is а surface phenomenon i.е. it occurs only at the surface

of the adsorbent.• In this phenomenon, the concentration on the surface of

adsorbent is different from that in the bulk.• Its rate is high in the beginning and then decreases till

equilibrium is attained.Absorption:• It is а bulk phenomenon i.e. occurs throughout the body of

the material.• In this phenomenon, the concentration is same throughout

the material. • Its rate remains same throughout the process.

•When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk. it is called positive adsorption.

• If the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption.

Physical adsorption:1. The forces operating in these cases are weak van-der-

Waal’s forces.2. The heats of adsorption are low viz. about 20 – 40 kJ/mol3. No compound formation takes place in these cases.4. The process is reversible i.е. desorption of the gas occurs

by increasing the temperature or decreasing the pressure.5. It does not require any а activation energy.б. This type of adsorption decreases with increase of

temperature.7. It is not specific in nature i.е. all gases are adsorbed on all

solids to some extent.8. The amount of the gas adsorbed is related to the ease of

liquefaction of the gas.9. It forms multimolecular layer.

van der Waals Bondingvan der Waals Bonding •dipole moment

•dipole electric field

Chemisorption:1. The forces operating in these cases are similar to those of а chemical bond.2. The heats of adsorption are high viz. about 400-400 kJ/mol3. Surface compounds are formed.

Ion Exchange Resins Ion Exchange Resins • Ion exchange resins Ion exchange resins consist of a consist of a

polymeric matrix and a functional group polymeric matrix and a functional group with a mobile ion which can be with a mobile ion which can be exchanged with other ions present in the exchanged with other ions present in the solution to be treated. The most common solution to be treated. The most common synthetic structures are: synthetic structures are:

• Cross-linked polystyrene Cross-linked polystyrene

• Cross-linked polymethacrylate Cross-linked polymethacrylate

• Phenol-formaldehyde Phenol-formaldehyde

Strong base anionic resin polyacrylic type

Strong acid cationic resin polystyrenic type

• Anion exchange resins - These contain а weakly basic group like – NH2 or а strongly basic quaternary ammonium group - (NR3)

+. One exchange is Dowex-1. These resins can bind negatively charged groups like hydroxyl, halide, citrate, su1fate, etc. An example of the use of anion exchange resin in medicine is its administration by mouth to bind gastric HCl in the treatment of peptic ulcer.

• Resin+ ОН- + НСl Resin+Cl- + Н2O

• Acidic or cation exchange resins – These contain acid groups, е.g. sulfonic acid (SO3H), carboxyl group (СООН) or phenol group (ОН). One example is Dowex-50. Their acid groups dissociate as do other acid groups. For example Resin.СООН which may also be written as Resin-Н+ will dissociate as Resin- Н+ Resin-+ Н+.

• These resins may occur as free acids or as one of their salts, е.g. Resins-.Na+. An example of the use of а salt of а cation exchange resin is given below:

• 2(Resin- .Na+.) + Ca2+ (Resin-)2Са2+ + 2Na+

Using of ion exchange Using of ion exchange resinsresins• Decalcification Decalcification • Salt conversion Salt conversion • Decationation Decationation • Deacification Deacification • Demineralization Demineralization • Selection Selection • Decolorization Decolorization • Separation Separation • Catalysis Catalysis • Enzyme mobilization Enzyme mobilization

Exchange Reactions Exchange Reactions • Salt conversionSalt conversion: The most typical : The most typical

application is softening or application is softening or decalcification. decalcification.

• 2R-Na + Ca2R-Na + Ca2+2+=2R-Ca + 2 Na=2R-Ca + 2 Na++

• Organic acids can be also converted Organic acids can be also converted into their salts by passing them into their salts by passing them through a cationic resin in the through a cationic resin in the appropriate form. appropriate form.

DemineralizationDemineralization• The use of a The use of a cationic exchanger cationic exchanger in in

hydrogen form hydrogen form followed by an followed by an anionic anionic exchanger exchanger in in hydroxyl form removes hydroxyl form removes all ionic species present in a feed all ionic species present in a feed solutionsolution. The following figure illustrates . The following figure illustrates a simple pass deashing process. a simple pass deashing process.

Example of the removal of a metal with a chelating resin

The mechanism can be illustrated The mechanism can be illustrated as followsas follows

On the cationic side:On the cationic side:R-SOR-SO33 H + NaCl  =  RSO H + NaCl  =  RSO33Na + HClNa + HCl2 R-SO2 R-SO33H + CaClH + CaCl22  = (R-SO  = (R-SO33))22Ca + 2 HCl Ca + 2 HCl On the anionic side: On the anionic side: SBA type R-N(CHSBA type R-N(CH33))33OH + HCl  =  R-N(CHOH + HCl  =  R-N(CH33))33Cl+ HCl+ H22OOWBA type R-N(CHWBA type R-N(CH33))22 + HCl  = R-N+H(CH + HCl  = R-N+H(CH33))22 Cl-+H+ Cl-+H+

• Regeneration cycle: The reverse reaction takes place Regeneration cycle: The reverse reaction takes place by passing an acid (HCl or Hby passing an acid (HCl or H22SOSO44 ) on the cationic ) on the cationic resin and an alkaline solution (NaOH, NHresin and an alkaline solution (NaOH, NH44OH) on the OH) on the anionic resin. anionic resin.

Uses of ion exchange resins • 1. Removal of excess of Na+ and К+ from body fluids in

congestive heart failure and renal failure respectivly; а cation exchange resin is given by mouth or by enema.

• 2.Production of low sodium milk for special dietary needs.

• 3. Removal of radioactive Sr90 from milk of cows feeding on pastures containing Sr90

• 4. In the separation and purification of amino acids, vitamins and. hormones.

• 5. А very important technique based on the selective adsorption of chemical compounds by various ion exchange resins at specific pH values is called column chromatography.