PHYSICAL-CHEMICAL ESSENCE OF SURFACE PHENOMENON.
COHESION AND ADHESION. Molecules liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute surface tension.
When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.
The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.
COHESION AND SURFACE TENSION. The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.
Introduction: Water has many unusual properties as a result of its ability to hydrogen bond. For example, the density of ice is less than that of the liquid and the predicted boiling point is almost 200 degrees C higher than it would be without hydrogen bonding.
SURFACE TENSION. Liquids sometimes form drops, and sometimes spread over a surface and wet it. Why does this happen, and why are raindrops never a meter wide? A clue to the answer to the second question may be found in pictures of astronauts playing with large blobs of water in their space-craft.
It all comes down to the forces between atoms or molecules, and the forces between them. These particles are unimaginably small. In one gram of water the number of molecules is about 3.3 X 1022, or 33000000000000000000000. If the gram of water were in the form of a 1 cm cube, there would be about 23000000 molecules on a side.
The force between two atoms or molecules is generally repulsive if they are pushed too close together. The force increases so strongly as the distance is reduced that they behave almost as if they were hard objects. Try compressing some water or steel. But at larger distances the force are attractive. Try pulling pulling the bung from a tube which contains only water and no air. Or try pulling a piece of piano wire in two.
A similar effect is a part of the explanation of the finite list of chemical elements found on earth, which terminates at number 92, uranium. The particles in an atomic nucleus experience short range forces analogous to those between molecules in a liquid. So larger and larger nuclei can behave rather like liquid drops. Because the number of nucleons is so small, a large proportion of them lie in or near the surface. In uranium, about 40 or so lie in the surface, which is about one sixth of the total. In a drop of water, the number of molecules is unimaginably large, about 1.4 X 1014, or 140 million million. This is the number in a cube with 52000 molecules on a side. The number in a surface layer about one molecule deep is about 2.7 X 109, which is a very small fraction, 1/52000, of the total.
The diagram below shows how the density of a uranium nucleus varies with the distance from the centre. The horizontal axis is in fermis, which are 10-15 m.
When the big nucleus begins to divide into two smaller ones, enormous tidal effects are set up. Just as the gravitational attraction of earth and moon sets up tides in both, the electrostatic repulsion of the putative nuclei produces huge deformations. This happens because the force varies so strongly with the distance. If you are stretched out by the gravitational field of a black hole, it isn't the strength of the field that gets you: it is the variation with position. The water of the oceans forms a prolate ellipsoid, bulging along the line joining earth and moon. Jupiter produces much bigger forces in its moons. The fleeing nuclei will be momentarily oblate, squashed along the line of flight. As the stable states are spherical, the oval nuclei have a lot of energy to get rid of. They are neutron rich, and some energy may be shed in the form of surplus neutrons. Although nuclear forces can act on time scales as short as 10-23 seconds, some neutrons are delayed long enough to allow a stable feedback loop to be created, enabling energy producing reactors to be built.
Capillary Action. As a result of surface tension acting around the inner circumference of a small-bore tube (or capillary), that is partially immersed in a liquid, there will be a raised or depressed column of liquid inside it. The case of a raised column is shown on the right.
Surfactants are a large group of surface active substances with a great number of (cleaning) applications. Most surfactants have degreasing or wash active abilities. They reduce the surface tension of the water so it can wet the fibres and surfaces, they loosen and encapsulate the dirt and in that way ensure that the soiling will not re-deposit on the surfaces.
Surfactants have a hydrophobic (water repellent) part and a hydrophilic (‘water loving’) part. The hydrophobic part consists of an uncharged carbohydrate group that can be straight, branched, cyclic or aromatic.
Dependent on the nature of the hydrophilic part the surfactants are classified as an-ionic, non-ionic, cat-ionic or amphoteric.
When the hydrophilic part of the surfactant consists of a negatively charged group like a sulphonate, sulphate or carboxylate the surfactant is called anionic. Basic soaps are anionic surfactants. Over the last 50 years many soaps have been replaced with more efficient substances like alkyl sulphates, alkyl sulphonates and alkyl benzene sulphonates.
Anionic surfactants are sensitive to water hardness.
A surfactant with a non-charged hydrophilic part, e.g. ethoxylate, is non-ionic. These substances are well suited for cleaning purposes and are not sensitive to water hardness.
They have a wide application within cleaning detergents and include groups like fatty alcohol polyglycosides, alcohol ethoxylates etc.
For this category the hydrophilic part is positively charged – e.g. with a quaternary ammonium ion. This group has no wash activity effect, but fastens to the surfaces where they might provide softening, antistatic, soil repellent, anti bacterial or corrosion inhibitory effects.
The most typical applications are for softeners and antistatics.
The cationic surfactant called DADMAC was formerly used, but now almost substituted.
Please consult section on ‘fabric softeners’ for further information.
For the amphoteric surfactants the charge of the hydrophilic part is controlled by the pH of the solution. This means that they can act as anionic surfactant in an alkalic solution or as cationic surfactant in an acidic solution.
Most surfactants are more or less toxic to aquatic organisms due to their surface activity which will react with the biological membranes of the organisms.
The biological degradability varies according to the nature of the carbohydrate chain. Generally the linear chains are more readily degradable than branched chains.
Also the toxic effects vary with the chain structure. Generally an increase of the chain length in the range of 10 to 16, leads to an increase in toxicity to aquatic organisms.
The properties of surfactants most often used in laundry detergents are given below.
Specific chemical groups
Alkane sulfonates (anionic),
linear alcohol ethoxylates (non-ionic) and
branched alcohol ethoxylates (non-ionic)
Most of these surfactants are readily degradable with varying eco-toxicity towards aquatic organisms.
Linear alkyl benzene sulphonates - LAS (anionic)
Probably the most frequently used group of surfactants for cleaning and laundering.
Linear alkyl benzene sulphonates (LAS) have been under some debate over the recent years due to the fact that they do not biodegrade under anaerobic conditions. Under aerobic conditions LAS are readily biodegradable.
Eco-toxicity towards aquatic organisms is fairly low.
Alkyl phenol ethoxylates, APEO (non-ionic)
Formerly this group was widely used for cleaning and laundering. Now it has been replaced to a great extent due to the negative environmental effects.
During the biological degradation, alkyl phenol ethoxylates bare transformed to alkyl phenols, e.g. nonyl phenol ethoxylate (NPEO) degrades to nonyl phenol (NP), which is known to be toxic and have hormone like effects.
Pulmonary surfactant is a surface-active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells. The proteins and lipids that comprise thesurfactant have both a hydrophilic region and a hydrophobic region. By adsorbing to the air-water interface of alveoli with the hydrophilic head groups in the water and the hydrophobic tails facing towards the air, the main lipid component of surfactant, dipalmitoylphosphatidylcholine (DPPC), reduces surface tension.
Detergents are a class of chemicals that contain hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group. This general class of molecules are called surfactants. Surfactants can interact with water in a variety of ways, each of which disrupts or modifies the hydrogen bonding network of water. Since this reduces the cohesive forces in water, this leads to reduction in the surface tension and our sulfur sinks.
A typical example of a detergent molecule is sodium lauryl sulfate (read that shampoo bottle of yours!). The structure can be represented in several different ways. Notice that in the models the Na ion has been left off because the anion and cation completely dissociate in water:
If you have the MDL Chime plug-in installed, you can play with this interactive 3-D model of a sodium lauryl sulfate molecule. You can rotate it, change the display features, enlarge/shrink, display solvent accessible surfaces and more...click and play:
When a detergent is placed in water, the long non-polar hydrocarbon tails tend to aggregate because of favorable intermolecular interactions ("like dissolves like" in the interior and ion-dipole interactions at the exterior). The surfactant molecules thereby organize themselves into 3-dimensional spheres called micelles which have a hydrocarbon core and sulfate groups around the outer surface. Here's a 2-D representation:
Without detergent, we can not remove a greasy oily stain from clothing because grease and oil are large, non-polar, hydrophobic molecules. However, the interior core of a micelle is quite greasy just like an oily stain. When we add detergent to our wash water, the oil or grease on our clothes can dissolve in the interior of the micelles and thereby go into solution.
Surfactants can also form other structures. Rather than form a sphere, some surfactants can coat the surface of the water to form a layer one molecule thick, amolecular monolayer. This is shown diagrammatically below:
A good example of a monolayer is oil on water. A small amount of oil can be spread over a large surface of water when the oil is only one monolayer thick! A variety of related structures are also known, particularly in cell walls (lipid bilayers etc.).
There are many, many other Real World examples and applications of surfactants! Here's just one: your body uses surfactants to reduce surface tension in the lungs. The human body does not start to produce lung surfactants until late in fetal development. Therefore, premature babies are often unable to breathe properly, a condition called Respiratory Distress Syndrome. Untreated, this is a serious illness and is often fatal, but administration of artificial surfactants virtually eliminates this health problem.
General. The situation existing at the surface of а liquid or а solid is different from that in the interior. For example, а molecule in the interior of а liquid is completely surrounded by other molecules on all sides and hence the intermolecular forces of attraction are exerted equally in all directions. However, а molecule at the surface of а liquid is surrounded by larger number of molecules in the liquid phase and fewer molecules in the vapour phase i.е. in the space above the liquid surface. As а result, these molecules lying at the surface, experience some net inward force of attraction which causes surface tension. Similar inward forces of attraction exist at the surface of а solid. Alternatively, in case of certain solids such as transition metals (like Ni) there are unutilized free valencies at the surface.
Because of the unbalanced inward forces of attraction or free valencies at the surface, liquids and solids have the property to attract and retain the molecules of а gas or а dissolved substance onto their surfaces with which they come in contact.
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 e. removal of the adsorbed substance from the surface is called desorption. The adsorption of gases on the surface of metals is calledocclusion.
Difference between adsorption and absorption.
1. It is а surface phenomenon i.е. it occurs only at the surface of the adsorbent.
2. In this phenomenon, the concentration on the surface of adsorbent is different from that in the bulk.
3. Its rate is high in the beginning and then decreases till equilibrium is attained.
1. It is а bulk phenomenon i.e. occurs throughout the body of the material.
2. In this phenomenon, the concentration is same throughout the material.
3. Its rate remains same throughout the process.
Examples of adsorption, absorption and sorption.
(i) If silica gel is placed in а vessel containing water vapours, the latter are adsorbed on the former. On the other hand, if anhydrous CaCl2 is kept in place of silica gel, absorption takes place as the water vapours are uniformly distributed in CaCl2 to form hydrated calcium chloride (CaCO3 . 2H2O).
(ii) Ammonia gas placed in contact with charcoal gets adsorbed on the charcoal whereas ammonia gas placed in contact with water gets absorbed into water, giving NH4OH solution of uniform concentration.
(iii) Dyes get adsorbed as well as absorbed in the cotton fibres i.е. sorption takes place.
Positive and Negative Adsorption. In case of adsorption by solids from the solutions, mostly the solute is adsorbed on the surface of the solid adsorbent so that the concentration of solute on the surface of the adsorbent is greater than in the bulk. This is called positive adsorption. However in some cases, the solvent from the solution may be adsorbed by the adsorbent so that the concentration of the solution increases than the initial concentration. This is called negative adsorption. For example, when а concentrated solution of KCI is shaken with blood charcoal, it shows positive adsorption but with а dilute solution of КС1, it shows negative adsorption. To sum up:
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.
Factors affecting adsorption of gases by solids. Almost all solids adsorb gases to some extent. However, the exact amount of а gas adsorbed depends upon а number of factors, as briefly explained below:
(i) Nature and Surface area of the adsorbent. If is observed that the same gas is adsorbed to different extents by different solids at the same temperature. Further, as may be expected, the greater the surface area of the adsorbent, greater is the volume of the gas adsorbed. It is for this reason that substances like charcoal and silica gel are excellent adsorbents because they have highly porous structures and hence large surface areas.
For the same reason, finely divided substances have larger adsorption power than when they are present in the compact form.
Since the surface area of adsorbents cannot always be determined readily, the common practice is to express the gas adsorbed per gram of the adsorbent (The surface area per gram of the adsorbent is called specific area).
(ii) Nature of the gas being adsorbed. Different gases are adsorbed to different extents by the same adsorbent at the same temperature.
(iii) Temperature. Studying the adsorption of any particular gas by some particular adsorbent. It is observed that the adsorption decreases with increase of temperature and vice versa. For example, one gram of charcoal adsorbs about 10 cm3 of N2 at 273 K, 20 cm3 at 244 K and 45 cm3 at 195 K. The decrease of adsorption with increase of temperature may be explained as follows:
Like any other equilibrium, adsorption is а process involving а true equilibrium. The two opposing processes involved are condensation (i.е. adsorption) of the gas molecules on the surface of the solid and evaporation (i.е. desorption) of the gas molecules from the surface of the solid into the gaseous phase. Moreover, the process of condensation (or adsorption) is exothermic so that the equilibrium may be represented as:
Applying be Chatelier’s principle, it can be seen that increase of temperature decreases the adsorption and vice versa.
The amount of heat evolved when one mole of the gas is adsorbed on the adsorbent is called the heat of adsorption.
(iv) Pressure. At constant temperature, the adsorption of а gas increases with increase of pressure. It is observed that at low temperature, the adsorption of а gas increases very rapidly as the pressure is increased from small values.
(v) Activation of the solid Adsorbent. It constant temperature, the adsorbing power of an adsorbent. This is usually done by increasing the surface area (or the specific area) of the adsorbent which can be achieved in any of the following ways:
(а) By making the surface of the adsorbent rough e.g. by mechanical rubbing or by chemical action or by depositing finely dispersed metals on the surface of the adsorbent by electroplating.
(b) By subdividing the adsorbent into smaller pieces or drains. No doubt this method increases the surface area but it has а practical limitation, that is, if the adsorbent is broken into too fine particles that it becomes almost powder, then the penetration of the gas becomes difficult and this will obstruct adsorption.
(с) By removing the gases already adsorbed e.g. charcoal is activated by heating in superheated steam or in vacuum at а temperature between 623 to 1273 К.
Types of adsorption. An experimental study of the adsorption of various types on solids shows that there are two main types of adsorption. These are briefly explained below:
(i) Physical adsorption or van der Waal's adsorption or physicosorption. When а gas is held (adsorbed) on the surface of а solid by van-der-Waal’s forces (which are weak intermolecular forces of attraction) without resulting into the formation of any chemical bond between the adsorbate and the adsorbent, it is called “physical adsorption” or “van-der-Waal’s adsorption” or “physicosorption”. This type of adsorption is characterized by low heats of adsorption i.e. about 40 kJ per mole. Further, physical adsorption of а gas by а solid is generally reversible. Increase of pressure causes more gas to be adsorbed and the release of pressure frees the adsorbed gas. Similarly, decrease of temperature increases adsorption but the gas adsorbed at low temperature can be freed again by heating.
(ii) Chemical adsorption or Chemisorption or Langmuir adsorption. When а gas is held on to the surface of а solid by forces similar to those of а chemical bond, the type of adsorption is called chemical adsorption or chemisorption. This type of adsorption results into the formation of what is called а “surface compound”. That the forces involved are similar to those of chemical bond is confirmed by the fact that the heats evolved during chemisorption are high (i.е. about 400 kJ/mole) which are of the same magnitude as those involved in chemical reactions. Further, as chemisorption is something similar to а chemical change, it is usually irreversible. The efforts to free the adsorbed gas often gives some definite compound instead of the free gas. For example, oxygen adsorbed on tungsten or carbon is liberated as tungsten oxide or as carbon monoxide and carbon dioxide.
Another aspect in which chemisorption differs from physical adsorption is the fact that whereas physical adsorption takes place between every gas and а solid i.е. is not specific in nature (because it involves van der Waal's forces which exist among the molecules of every two substances), the chemisorption is specific in nature and occurs only where there is а tendency towards compound formation between the gas and the adsorbent. Further unlike physical adsorption, the chemisorption like the most of chemical changes, increases with increase of temperature. For this reason, а gas may be physically adsorbed at low temperature but chemisorbed at higher temperature. For example, it happens in case of adsorption of hydrogen on nickel. When chemisorption takes place by raising the temperature i.е. by supplying activation energy, the process is called “activated adsorption”.
Adsorption from solutions. Solid surfaces can also adsorb solutes from the solutions. An application of adsorption from solution is the use of activated charcoal for decolorising sugar solutions. Activated charcoal can adsorb colouring impurities from the solutions of organic compounds. Adsorption from solution can also involve colourless solutions. Adsorption of ammonia from ammonium hydroxide solution and acetic acid from its solution in water by activated charcoal are such examples.
This type of adsorption is also affected by temperature and concentration. The extent of adsorption decreases with increase in temperature and increases with increase in concentration. The isotherm for the adsorption of solutes from solutions (by the solid adsorbents) is found to be similar to that shown in Fig. 2. Hence the relationship between x/m (mass of the solute adsorbed per gram of the adsorbent) and the equilibrium concentration, С of the solute in the solution is also similar i.e:
Taking logarithms of both sides of the equation, we get:
log x/m = log К + 1/n log C
This equation implies that а plot of log x/m against log С should be а straight line with slope1/n and intercept log Х. This is found to be so over small ranges of concentration.
The equation for adsorption from solutions is found to give better results than for adsorption of gases by solids.
Adsorption isobars. As already discussed, adsorption is а case of dynamic equilibrium in which forward process (adsorption) is exothermic while backward process (desorption) is endothermic. Thus applying be Chatelier's principle, increase of temperature will favour the backward process i.е., adsorption decreases.
А graph drawn between the amount adsorbed (x/m) and temperature 't' at а constant equilibrium pressure of adsorbate gas is known as adsorption isobar.
Adsorption isobars of physical adsorption and chemical adsorption show important difference [Fig.3 (а) and (b)] and this difference is helpful in distinguishing these two types of adsorption. The physical adsorption isobar shows а с1есгеаье in х/m throughout with rise in temperature, the chemisorption isobar shows an initial increase with temperature and then the expected decrease. The initial increase is because of the fact that the heat supplied acts as activation energy required in chemisorption (like chemical reactions).
Application of adsorption. Adsorption finds extensive applications both in research laboratory and in industry. А few applications are briefly described below:
In preserving vacuum. In Dewar flasks activated charcoal is placed between the walls of the flask so that any gas which enters into the annular space either due to glass imperfection or diffusion through glass is adsorbed.
In gas masks. All gas masks are devices containing suitable adsorbent so that the poisonous gases present in the atmosphere are preferentially adsorbed and the air for breathing is purified.
In clarification of sugar. Sugar is decolorised by treating sugar solution with charcoal powder. The latter adsorbs the undesirable colours present.
In chromatographic analysis. The selective adsorption of certain substances from а solution by а particular solid adsorbent has helped to develop technique for the separation of the components of the mixture. This technique is called chromatographic analysis. For example, in column chromatography, а long and wide vertical tube is filled with а suitable adsorbent and the solution of the mixture poured from the top and then collected one by one from the bottom.
In catalysis. The action of certain solids as catalysts is best explained in terms of adsorption. The theory is called adsorption theory. According to this theory, the gaseous reactants are adsorbed on the surface of the solid catalyst. As а result, the concentration of the reactants increases on the surface and hence the rate of reaction increases. The theory is also able to explain the greater efficiency of а catalyst in the finely divided state, the action of catalytic promoters and poisons.
In adsorption indicators. Various dyes, which owe their use to adsorption, have been introduced as indicators particularly in precipitation titrations. For example, KBr is easily titrated with AgNO3 using eosin as an indicator.
In softening of hard water. The use of ion exchangers for softening of hard water is based upon the principle of competing adsorption just as in chromatography.
In removing moisture from air in the storage of delicate instruments. Such instruments, which may be harmed by contact with the moist air, are kept out of contact with moisture using silica gel.
Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid (the absorbate) permeates or is dissolved by a liquid or solid (the absorbent). Note that adsorption is a surface-based process while absorption involves the whole volume of the material. The term sorption encompasses both processes, whiledesorption is the reverse of adsorption. It is a surface phenomenon.
Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction.
Adsorption is present in many natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, capturing and using waste heat to provide cold water for air conditioning and other process requirements (adsorption chillers), synthetic resins, increase storage capacity ofcarbide-derived carbons, and water purification. Adsorption, ion exchange, and chromatography are sorption processes in which certain adsorbates are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. Lesser known, are the pharmaceutical industry applications as a means to prolong neurological exposure to specific drugs or parts thereof.
Ion exchange is an exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic 'ion exchangers'.
Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. Ion exchangers are either cation exchangers that exchange positively charged ions (cations) or anion exchangers that exchange negatively charged ions (anions). There are also amphotericexchangers that are able to exchange both cations and anions simultaneously. However, the simultaneous exchange of cations and anions can be more efficiently performed in mixed beds that contain a mixture of anion and cation exchange resins, or passing the treated solution through several different ion exchange materials.
Ion exchangers can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure. This can be dependent on the size of the ions, their charge, or their structure. Typical examples of ions that can bind to ion exchangers are:
Ion exchange is a reversible process and the ion exchanger can be regenerated or loaded with desirable ions by washing with an excess of these ions.