Chapter 10

Chapter 10

Surface Interactions

Interfaces• Why does the coating on non-stick

frypans stick to the pan but not to food?

Interfaces• Surface chemistry is concerned with the

nature and properties of the interfaces, or boundary regions, between solids, liquids and gases.

• It deals with systems where surface properties, rather than bulk properties, control the behaviour of the system.

• 5 different types of interface possible• List them

Surface Energy• A measure of bond strength within a substance• Particles are held together by attractive forces

• Solids may be strong ionic, covalent or metallic• Liquids may be hydrogen bonding and dispersion

forces• If you cut a substance into two so that new

surfaces are created, we need to provide energy to break the bonds between particles within the substance.

Surface Energy• Diamond requires a large amount of

energy to break the bonds (3-dimensional covalent network lattice) – high surface energy

• Candle wax – molecules are held together by weak dispersion forces, therefore easy to separate them – low surface energy

• Energy required to break bonds and create new surface is surface energy.

Surface Energy• When a new surface is formed, the

particles at the surface have the ability to bond to other substances.

• The nature of these bonds determines the properties of the surface an the type of interface it will form.

• Work through the sample problem page 219

Review• Complete the revision questions pages

219, 220 (1 – 5)

The behaviour of liquid at interfaces

• Water and air have an important liquid/gas interface.• Many everyday activities rely on the features of this

interface.• Features include;

• Surface tension• Droplet shape• Wetting• Menisci• Capillary action• Solubility

Surface Tension• Surface energy of a liquid

• Water has a higher surface tension than most liquids due to the hydrogen bonding between water molecules.

• Water molecules at the interface cannot bond with air, hence they are drawn into the centre of the liquid.

• The hydrogen bonding of the interface water molecules in the surface layer cause them to be in a state of tension, minimising the surface area and behaving like a stretched membrane.

Droplet Shape• Surface tension of a liquid tends to hold

a drop of liquid in a spherical shape• Liquid with high intermolecular

attractions have high surface tension.• The higher the surface tension, the

more spherical the droplet

Review• Complete the revision questions page

222 (6 – 7)

Wetting• The contact between a liquid and a solid

surface• Water does not wet all surfaces• Degree of wetting depends on the

contact angle between the drop and the surface

Wetting• The ability of a liquid to wet a surface depends largely

on two factors• The attraction of the liquid particles to the bulk of the liquid• The attraction of the liquid particles to the solid surface with

which they are in contact.• Wetting occurs if the surface energy of the solid is

greater than the surface energy (tension) of the liquid.

Wetting• Surface tension of water may be

decreased by adding a ‘wetting agent’ such as soap or detergent (surfactants)

• The detergent molecules reduce the hydrogen bonding between water molecules – surface tension reduced, beads collapse.

Review• Work through the sample problem page

223• Complete the revision questions page

224 (8 – 9)

Menisci• The curved upper surface of

a liquid in a container is called a meniscus.

• Most liquids have a meniscus, depending on how well the liquid wets the container.

• Water forms a concave meniscus in a glass container

• Mercury forms a convex meniscus in a glass container.

Capillary Action• A capillary is a very thin tube• If a glass capillary is placed in

water, the water will be seen to rise slowly up the tube.

• The level to which a liquid rises or falls in a capillary depends on• The nature of the bonding within

the liquid and within the capillary tube

• The diameter of the capillary tube


226

• Complete the revision questions page 226 (10 – 11)

Solubility – hydrophobic and hydrophilic substances

• Hydrophobic substances – do not dissolve in water

• Hydrophilic substances – dissolve in water – contain polar groups that are attracted to either of the polar ends of water molecules

The Dancing Grape• When the two grapes are dropped

into soda water, the peeled grape will sink to the bottom of the beaker.

• The unpeeled grape will quickly accumulate a coating of shining bubbles, and will rise and fall repeatedly, seeming to dance in the sparkling soda water.

The Dancing Grape• Tiny Bubbles.

• The bubbles clinging to the grape are the cause of the effect. • The peeled grape's surface is hydrophilic: it bonds to water,

giving the bubbles no place to latch on. • The unpeeled grape has a hydrophobic surface: it repels

water, leaving a dry surface for the carbon dioxide bubbles to cling.

• The bubbles are lighter than the water, and act like little balloons lofting the grape up through the mineral water. As it reaches the surface or as the bubbles burst and break loose, the grape will drop down in the beaker again until it accumulates a new collection of bubble-balloons.

• This is why the peeled grape sinks but the unpeeled grape dances and floats.

Surfactants• Some large molecules are both hydrophilic

and hydrophobic in different parts of the same molecule

• If such a substance is added to water, the hydrophilic end tends to stay in the water but the hydrophobic ends will tend to stick out from the water into the air.

• The former ‘air-water’ interface becomes covered with an ‘oily’ skin which lowers the surface tension.

• Molecules that lower the surface tension of a liquid are called surface-active agents (surfactants).

Breathing and surface tension

• Alveoli would collapse completely without a surfactant.


229 (13 – 14)

Soaps and Detergents• Water, although it is a good solvent,

does not by itself remove dirt effectively from substances such as cloth.

• High surface tension means that drops of water do not readily spread through cloth.

• Dirt is insoluble in water• Detergents are needed to help separate

dirt from cloth and carry it away in the water.

Soap• Sodium salts of long-

chain carboxylic acids known as fatty acids.

• Represented as RCOO Na+ (R represents the hydrocarbon chain)• This chain may be

saturated (single C bond) – palmitic acid

• It may be unsaturated (double or triple C bonds) – oleic acid

Saponification – making soap• Common sources of fatty

acids – animal and plant fats• All fats and oils mainly

contain large molecules of a type of ester call a triglyceride.

• Soaps are manufactured from fats or oils that are broken down with NaOH to produce glycerol and soap molecules


231 (15 – 17)

Soap and skin• Soaps made from fatty acids containing

less than 12 carbon atoms will irritate the skin and those made from fatty acids containing more than 20 carbon atoms will not lather well.

Synthetic Detergents• Can be made using oils but is very

expensive therefore made synthetically.• Using products obtained from refining of

petroleum• Structure of detergents is similar to

soaps – both have an ionic head joined to a hydrocarbon tail• In detergents the hydrocarbon tail must be

joined to an ionic head.

How soaps and detergents work• Surface active agents (surfactants) –

wetting agents• They assist water to remove grease, oil

dirt and other water insoluble substances that adhere to surfaces

• When soaps and detergents dissolve in water, negatively charged surfactant ions are produced in solution (as well as a Na+ion.

How soaps and detergents work• Polar charged particles tend to dissolve

in polar solvents• Non-polar substances tend to dissolve

in non-polar solvents.• Hydrocarbons are therefore insoluble in

water but soluble in oil or grease.• Surfactant molecules contain both a

hydrocarbon part and an ionic part, they are able to dissolve in both water and oil or grease.

Getting clothes clean• Washing detergents help to get clothes

clean in three ways• Reduce the surface tension of water so

that it wets the cloth• Separate dirt from the fibres of the cloth• Keep dirt suspended in water so that it

can be rinsed away• In the spherical micelles, the hydrophobic

tails pack inside the core of the structure to escape water, so that only the hydrophilic heads are in contact with the solution.

• Laundry detergents clean clothes by capturing hydrophobic dirt inside micelles. Dirt is thus solubilised and then removed from the clothes by water.

Mixing Oil and Water• Emulsion –

• temporary mix of oil and water when the two are simply shaken

• Add a surfactant for a more permanent emulsion

• Oil floats on water. Surfactant molecules align with their hydrophilic heads in the water and their hydrophobic tails in the oil

• Oil micelles form as surfactant molecules surround the oil droplets.

Emulsions• Oil-in-water emulsion – consist of tiny droplets of oil

suspended in water.• The type of emulsion with which you are most familiar

is probably the oil in water emulsion (e.g. salad dressing or milk). This is sometimes called a normal or o/w emulsion.

Emulsions• Water-in-oil emulsion – consist of tiny droplets of water

suspended in an oil.• It is possible to form an inverse or water in oil (w/o)

emulsion. In an inverse emulsion, the water droplets are dispersed in a continuous phase of oil.

• Many medicinal creams and butter are water in oil emulsions.


236• Complete the revision questions page

237 (18 – 22)

Surface Chemistry applications in nanotechnology

• Nanotechnology• Technology that works at the scale of single

atoms or small molecules• Nano – one billionth of a metre, nm = 10-9m

Applications of surface chemistry in nanotechnology

• Complete the revision questions page 244 (25 – 29) and page 246 (30 – 31) to address these applications.

• Complete the multiple choice questions pages 250 – 251 (1 – 13)

• Complete the review questions 2, 3, 5, 6, 8, 11, 13, 14, 17, 22, 23, 27, 32

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Chapter 10