SURFACE PHENOMENA AND COLLOIDS

SURFACE TENSION

SURFACE TENSION • A molecule of a liquid attracts the molecules which surround it

and in its turn it is attracted by them • For the molecules which are inside a liquid, the resultant of all

these forces is neutral and all them are in equilibrium by reacting with each other.

• When these molecules are on the surface, they are attracted by the molecules below and by the lateral ones, but not toward the outside.

• The resultant is a force directed inside the liquid. In its turn, the cohesion among the molecules supplies a force tangential to the surface.

• So, a fluid surface behaves like an elastic membrane which wraps and compresses the below liquid.

• The surface tension expresses the force with which the surface molecules attract each other.

WETTABILITY

WETTABILITY• Why does one fabric absorb water well while another seems to

refuse it? • Why does water collect into large drops on a greasy surface and

instead form an adherent film on a clean surface? According to the nature of the liquid and the solid, a drop of liquid placed on a solid surface will adhere to it more or less.

• To understand this phenomenon it is necessary to take into account the fact that molecules of a liquid are subject to a cohesive force which keeps them united to one another, but there is also an adhesive force which is the force with which the molecules of the liquid adhere to the surface of materials that they contact.

• When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface, when instead the forces of adhesion are less by comparison to those of cohesion, the liquid tends to "refuse" the surface. In this people speak of wettability between liquids and solids.

• For example, water wets clean glass, but it does not wet wax.

CAPILLARITY

CAPILLARITY • Let us stay in the field of the wettability. • Surely you have noticed that water tends to rise near the walls of a

glass container. • This happens because the molecules of this liquid have a strong

tendency to adhere to the glass. • Liquids which wet the walls make concave surfaces (eg: water/glass),

those which do not wet them, make convex surfaces (eg: mercury/glass).

• Inside tubes with internal diameter smaller than 2 mm, called capillary tubes, a wettable liquid forms a concave meniscus in its upper surface and tends to go up along the tube

• On the contrary, a non-wettable liquid forms a convex meniscus and its level tends to go down.

• The amount of liquid attracted by the capillary rises until the forces which attract it balance the weight of the fluid column.

• The rising or the lowering of the level of the liquids into thin tubes is named capillarity.

• Also the capillarity is driven by the forces of cohesion and adhesion we have already mentioned.

WETTABILITY

SOAPS AND DETERGENTS

SOAPS AND DETERGENTS • How do soaps and detergents work in removing dirt? • Soaps and detergents are formed by special molecules, which

have a hydrophilic head, which therefore loves to remain in water and a hydrophobic tail, which avoids water and loves fat substances

• Because of their hydrophobic tail, a part of the molecules of detergent collects to the water surface forming a monomolecular layer it lowers the surface tension of the water and makes easier its penetration into the fabrics to be cleaned.

• Within the water, the molecules of detergent collect themselves in micelles and membranes, little aggregates of molecules united by their hydrophobic tail

• When they meet dirt, these molecules surround the particles and insert their tail in them.

• The hydrophilic heads attract the dirt toward water and with the agitation of the liquid they contribute to remove the dirt from the fabric.

SOAPS AND DETERGENTS

• The crown of hydrophilic heads carries the particles of dirt in the water where they end up in suspension and then they are rinsed away.

• Hence, the dirt water contains also greasy particles which have been emulsified.

• For the same reason, the detergents aid the formation of emulsions.

• The substances which lower the surface tension of a liquid are called surfactants (from: surface-active agents).

• The lowering of the surface tension of the water allows the formation of soapy membranes, foam and soap bubbles.

CAPILLARITY

SOAP BUBBLES

SOAP BUBBLES • HOW DO THE SOAP BUBBLES FORM?• "How do soap bubbles form? • Why does soapy water produce foam while pure water does

not?". • When water sprays from a tap in a small basin, you can see

bubbles form, but they burst very soon. • This is due to the fact that the surface tension of the normal

water is high and it tends to draw the water molecules into the main body of the water, to the point where the thickness of the bubble wall is too thin to remain intact and quickly bursts.

• Instead, the surface tension of the soapy water is much lower: about a third of the pure water, so the molecules of the bubble are less stressed and it can last longer.

SOAP BUBBLES • Soap and detergents lower the surface tension of water and,

as we have said, they are called surfactants. • As we have said in the paragraph on the soaps and

detergents, the molecules of surfactants have a hydrophilic head and a hydrophobic tail.

• When these molecules are dissolved in water, they tend to collect on the surface with the tails outward, forming continuous layers (figure 12 B).

• The membranes of soapy water are made up by three layers: the external two are formed by surfactant molecules and the internal layer is formed by soapy water (figure 12 C).

• These layers of surfactant molecules are very elastic and they deform easily without breaking. They also slow the evaporation of the water film and so extend the life of the bubbles.

OSMOSIS

OSMOSIS • If you place two solutions of different concentration side by

side, keeping them separated only by means of a membrane, you will see the level of the more concentrated solution increase.

• This happens because the two solutions try to attain the same concentration by diffusion.

• The membrane has to be semipermeable, that is it has to allow the passage of the solvent but not of the solute.

• The molecules of the solvent have to be smaller than those of the dissolved substance.

• In practice, this condition is very frequent given that the molecules of water are very small.

OSMOSIS • • It is necessary to remember that it is possible to make

solutions with other liquids also. • Osmosis is the tendency of the system to reach the same

concentration in both solutions. • It is a phenomenon of great importance in biology and which

is also the basis of the function of the kidney, of the absorption of water by plants and which is used by industries to concentrate or to purify solutions.

• In fact, applying a pressure on the side of the more concentrated solution, it is possible to reverse the process and cause the solvent to pass to the less concentrated solution.

• This is the process of the reverse osmosis. It is used also to purify water, to concentrate solutions, etc.

CAPILLARITY

SOLUTIONS

SOLUTIONS • A solution is a homogeneous mixture of two or more substances.

When placed in water, many substances dissolve and are called soluble, others do not dissolve and are called insoluble.

• Salt and sugar easily dissolve in water. • If instead you put sand in water, you can mix for as long as you

want, but you will not succeed in dissolving the sand. • In fact, sand is insoluble in water. • In a solution, the material present in greater quantity is defined

solvent and that in smaller quantity solute. • What does it mean to say that a substance is soluble in another? • It means that the molecules of the solute separate each other

and they disperse among those of the solvent. • Instead, the insoluble substances keep themselves compact and

their molecules do not disperse into the solvent.

SOLUTIONS • As solvent, we have used the example of water because many

solids are soluble in water, but nearly every liquid can be a solvent.

• And then, why we should limit ourselves to the liquids? • Let us generalize the concept of solvent and concede to all

substances, solid or liquid or gaseous the possibility to be a solvent.

• At this point, even the solutes can belong to all of these three states of matter.

• For example, some solid solutions are the metal alloys such as steel (Fe+C), brass (Cu+Zn), bronze (Cu+Sn).

• Finally, all gases are completely soluble among each other. • Also common are solutions of gases in liquids. • For example, carbon dioxide is added to many beverages to

make them fizz. In the water of ponds, rivers and seas, gases like oxygen, carbon dioxide and others go into solution in a natural way.

CATEGORIES OF SOLUTIONS

SOLUTE SOLVENT EXAMPLEGas Gas air (nitrogen, oxygen, etc.)Liquid Gas moist air (water vapor in air)Solid Gas atmospheric dustGas Liquid CO2 in water (sparkling water)Liquid Liquid wine (water + alcohol)Solid Liquid marine water (salt in water)Gas Solid gas in silicates (pumice stone)Liquid Solid dental alloys (mercury in cadmium)Solid Solid metal alloys (steel, bronze)

MIXTURES

MIXTURES

• By mixing sugar with water, a solution is obtained. • If instead we mix sand into water, we obtain a mixture. • Also by mixing bits of coal and iron filings we obtain a mixture.

With a pair of thin tweezers it is possible to take away sand grains from the water or pieces of coal from the filings, but it is not possible to take away singly molecules of sugar from the water because they are too much small.

• Hence, what distinguishes a mixture from a solution? • In a mixture the particles are enough large to be separated by

mechanical means such as tweezers or sieves, in a solution this is not possible because the particles which form it are so small that they cannot be seen even with an electron microscope.

• To separate the components of a solution it is necessary to use physical method like distillation. So, mixtures are formed by quite big particles, solution are formed by very small particles.

COLLOIDS

COLLOIDS • In the solutions, the molecules of the solute separate

each other and disperse among those of the solvent. • In the mixtures instead, the molecules do not

separate and the particles remain compact. • From the point of view of the sizes, solutions are

formed by very small particles (single molecules) and the mixtures by quite large particles.

• In an intermediate position, between mixtures and solutions, there are the colloids.

• They are dispersions of small particles, but not molecule sized.

• What distinguishes mixtures from colloids and from solutions is therefore the size of the particles which form them.

COLLOIDS

• By convention, a colloid is a dispersion of particles which size is comprised between 0.2 and 0.002 µm (a micrometer, or micron, = 10-6 meters).

• If the particles are larger than 0.2 µm, we have a mixture, if they are smaller than 0.002 µm, we have a solution.

• In general, the components of a colloid are formed by small aggregates of molecules, while the components of a solution are single molecules.

• Anyway, if these molecules are large enough, as it is the case of many macromolecules, their solution will give a colloid.

• So, the criterion of distinction between colloids and solutions cannot be the presence of single molecules, but as we were saying, the size of the particles which form them. 

SOL

SOL

• A sol is a dispersion of very thin solid particles in a liquid. • It has a liquid consistency and resembles a true solution. • An aqueous sol appears clear, very similar to common water. • Anyway, if you shine an intense beam of light across it, a part of

the light will be diffused from the particles which are in suspension.

• These particles are very small, but they are still enough large to obstruct the light and diffuse it.

• This phenomenon is called Tyndall effect. • You can observe it with sols, but not with true solutions.

GEL

GEL • A gel is a dispersion of very thin solid particles in a liquid and it has a

gelatinous consistency. • Increasing the concentration of the particles, a sol can pass to the

state of gel. • On the contrary, by diluting a gel you will obtain a sol. • So, what makes a sol different from a gel is its fluid or gelatinous

consistency. • Also the temperature can determine the passage from sol to gel and

vice versa. • For example, broth gelatin is gelatinous at room temperature, but it

becomes liquid when it is heated. • Animal gelatin is a reversible gel because depending on the

temperature it can pass from gel to sol and vice versa • The albumen of eggs instead is not reversible because when heated

it coagulates and it does not come back to the state of sol. • Silica gel absorbs moisture and keeps its properties with broad

concentrations of water. • Because its affinity for water it is used as dehumidifier. • When left to rest, a sol can spontaneously jell and come back to the

state of sol simply by mixing it (eg: aqueous suspensions of kaolin).

EMULSIONS

EMULSIONS • An emulsion is a dispersion of an insoluble liquid in another

liquid. • For instance, the oil is not soluble in water. • If you pour some oil in a container with water, it will float it and

keeps separate from the water. • Instead, if you vigorously shake the container, you will obtain a

dispersion of small drops of oil in water, however these drops quickly join together, so that in a short time nearly all the oil will return as before.

• To make the emulsion more stable, before shaking the container, add some detergent.

• The surfactant molecules will arrange on the surface of the oil drops with the heads outward.

• As these heads have an electrical charge and as this charge is always the same, the oil drops will repel each other and be unable to return to the homogeneous layer as before.

• So, surfactants can help you to obtain more stable emulsions.

EMULSIONS • • There are special surfactants for emulsions, endowed of a higher

capability to stabilize the oil drops than the detergents. • There are also emulsifying agents for alimentary use such as lecithin

and emulsifiers for industrial purposes which are not edible. • Butter is formed by small water drops suspended in fat. • Cheese and mayonnaise too are considered emulsions. • A lot of creams used both in pharmacy and in cosmetics are

emulsions. • Fuels emulsified with water have been produced. • Emulsified oils are used in machine working to make it easier to cut

metals with machine tools. • In fact, metal cutting can create an intense heat, which has to be

removed if you want to avoid burning the tools. • The oil and water in the cutting fluid help remove the heat and make it

possible to cut metals efficiently. • Milk is another emulsion made up by small greasy drops in an aqueous

phase.

FOAMS

FOAMS • Foam is a dispersion of a gas in a liquid (liquid

foams) or in a solid (solid foams). • Among the liquid foams, we have the ones produced

by soaps and detergents, and various foods such as wine, beer and many others.

• Among the solid foams we have Pumice stone, earthenware, sponges, expanded plastics like expanded polystyrene and expanded polyurethane.

• By dispersing helium in a liquid which produced bubbles with very thin walls and which then solidified, some researchers succeeded in fabricating a solid foam lighter than air

Many thanks

UTS: Kelas A, Rabu 23 April 2008Kelas B, Kamis 24 April 2008

Sifat: Open book