LECTURE 8: Physical-chemical essence of surface phenomenon. associate. prof. Yevheniya B. Dmukhalska

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

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

Adhesion and cohesion are water properties that affect every water molecule on earth and also the interaction of water molecules with molecules of other substances. Adhesion and cohesion are water properties that affect every water molecule on earth and also the interaction of water molecules with molecules of other substances. Cohesion: Water is attracted to water Adhesion: 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.intermolecular forceshydrogen bondingVan der Waals forces

Adhesive Forces are 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 web Dew drops adhering to a spider web Dewspider web Dewspider web

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 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 cohesive forces within water. 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 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 cohesive forces within water.

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

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

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

Drop method When 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: Cohesion (n. lat. cohaerere "stick or stay together") or cohesive attraction or cohesive force is the action or property of like molecules sticking together, being mutually attractive. Cohesion (n. lat. cohaerere "stick or stay together") or cohesive attraction or cohesive force is the action or property of like molecules sticking together, being mutually attractive. property attractive property attractive Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules. Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules. molecular speciescohesion molecular speciescohesion 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. S Some surfactants can coat the surface of the water to form a layer one molecule thick, a molecular monolayer.

STRUCTURAL COMPONENTS OF A CELL MEMBRANE STRUCTURAL COMPONENTS OF A CELL MEMBRANE LIPID BILAYER OF PLASMA MEMBRANE LIPID BILAYER OF PLASMA MEMBRANE

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/mol 3. 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 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 kJ/mol 3. Surface compounds are formed.

Ion Exchange Resins Ion exchange resins consist of a polymeric matrix and a functional group with a mobile ion which can be exchanged with other ions present in the solution to be treated. The most common synthetic structures are: Ion exchange resins consist of a polymeric matrix and a functional group with a mobile ion which can be exchanged with other ions present in the solution to be treated. The most common 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 – NH 2 or а strongly basic quaternary ammonium group - (NR 3 ) +. 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 - + Н 2 O

Acidic or cation exchange resins – These contain acid groups, е.g. sulfonic acid (SO 3 H), 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 +.) + Ca 2+  (Resin - ) 2 Са Na +

Using of ion exchange resins 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 Salt conversion: The most typical application is softening or decalcification. Salt conversion: The most typical application is softening or decalcification. 2R-Na + Ca 2+ =2R-Ca + 2 Na + 2R-Na + Ca 2+ =2R-Ca + 2 Na + Organic acids can be also converted into their salts by passing them through a cationic resin in the appropriate form. Organic acids can be also converted into their salts by passing them through a cationic resin in the appropriate form.

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

Example of the removal of a metal with a chelating resin

The mechanism can be illustrated as follows On the cationic side: R-SO 3 H + NaCl = RSO 3 Na + HCl 2 R-SO 3 H + CaCl 2 = (R-SO 3 ) 2 Ca + 2 HCl On the anionic side: SBA type R-N(CH 3 ) 3 OH + HCl = R-N(CH 3 ) 3 Cl+ H 2 O WBA type R-N(CH 3 ) 2 + HCl = R-N+H(CH 3 ) 2 Cl-+H+ Regeneration cycle: The reverse reaction takes place by passing an acid (HCl or H 2 SO 4 ) on the cationic resin and an alkaline solution (NaOH, NH 4 OH) on the anionic resin. Regeneration cycle: The reverse reaction takes place by passing an acid (HCl or H 2 SO 4 ) on the cationic resin and an alkaline solution (NaOH, NH 4 OH) on the 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 Sr 90 from milk of cows feeding on pastures containing Sr 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.