Transcript 04. The Physical-chemical essence of surface phenomenon

LECTURE 4:
Physical-chemical essence of
surface phenomenon.
associate. prof. Yevheniya B. Dmukhalska
• 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.
• 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.
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
• 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.
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.
Ftension  L
Surface tension F can be described by the
surface tension coefficient , L is the length of a
line on the liquid surface.
Ftension

L
1
(N  m )
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.
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.
• 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.
• 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
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liquid are called surfactants (from: surface-active
agents)
Class of molecules that contain hydrophobic (nonpolar) 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.
• 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
• 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-derWaal’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.
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/mol
3. Surface compounds are formed.
van der Waals Bonding
• dipole moment
• dipole electric field
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:
• Cross-linked polystyrene
• Cross-linked polymethacrylate
• Phenol-formaldehyde
Strong acid cationic resin polystyrenic
type
Strong base anionic resin polyacrylic type
• Anion exchange resins - These contain а
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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
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acid groups, е.g. sulfonic acid (SO3H), carboxyl group
(СООН) or phenol group (ОН). One example is Dowex50. 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 resins
• Decalcification
• Salt conversion
• Decationation
• Deacification
• Demineralization
• Selection
• Decolorization
• Separation
• Catalysis
• Enzyme mobilization
Exchange Reactions
• Salt conversion: The most typical
application is softening or decalcification.
• 2R-Na + Ca2+=2R-Ca + 2 Na+
• 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.
Example of the removal of a metal
with a chelating resin
The mechanism can be illustrated as
follows
On the cationic side:
R-SO3 H + NaCl = RSO3Na + HCl
2 R-SO3H + CaCl2 = (R-SO3)2Ca + 2 HCl
On the anionic side:
SBA type R-N(CH3)3OH + HCl = R-N(CH3)3Cl+ H2O
WBA type R-N(CH3)2 + HCl = R-N+H(CH3)2 Cl-+H+
• Regeneration cycle: The reverse reaction takes
place by passing an acid (HCl or H2SO4 ) on the
cationic resin and an alkaline solution (NaOH,
NH4OH) on the anionic resin.
Uses of ion exchange resins
• 1. Removal of excess of Na+ and К+ from body
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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.