Download - Colloid Chemistry Practice
-
7/30/2019 Colloid Chemistry Practice
1/48
Manual for
Colloid Chemistry Practical Course
Pharmacy program 2012/2013 1st
semester 2nd year
Edited by Mrta Berka, LeventeNovkand
Mnika Kri
University of Debrecen
-
7/30/2019 Colloid Chemistry Practice
2/48
2
OPERATION OF THE LABORATORY
THE COURSE
In the seven-week period assigned to the Colloid Chemistry Practical Course. There are six
experiments obligatory, each of which is designed to be completed in four hours.
Practicals can be finished for data processing in the laboratory. All experiments are preceded
by ashort written test or debrief will be given at the start of the practice.
ATTENDANCE
The Colloid Chemistry laboratory is open from 14.00 to 18.00 on Monday and 8.00 am to
12.30 pm on Tuesday. You must arrive on time. Coats and bags should be checked in the
checkroom. The department does not accept responsibility for personal values left in the
corridor of the laboratory.
DO NOT LEAVE VALUABLE ITEMS IN THIS AREA!
If you are absent from a practical class (e.g. you are ill) you should report to administration
(room D205, Mihly Szatmri) at the earliest opportunity, to see whether the missed
experiment(s) can be re-scheduled for an alternative date. Lack of attendance without good
ill lt i k f
-
7/30/2019 Colloid Chemistry Practice
3/48
3
PRE-LABORATORY WORK
You must complete pre-laboratory work, before coming to the laboratory! Particularly, you
should write manually a short introduction to the experiment you will perform, along with
your name and the date of the lab practice. On arrival in the laboratory you must show this
work to a Demonstrator and get him/her to sign it. Failure to carry out the pre-laboratory work
will result in the loss of marks. PRE-LABORATORY WORK SHOULD BE CARRIED OUT
IN YOUR LABORATORY NOTEBOOK.
SHORT WRITTEN TEST
A short test will be written at the beginning of each lab. The test consists of two questions,
one pertaining to the topic of the actual work, the other being chosen randomly from the
following:
Kelvin-equation
Einstein-Stokes equation
Gibbs-equation
Langmuir isotherm equation
Stern eq ation
-
7/30/2019 Colloid Chemistry Practice
4/48
4
explanation has to be given concerning possible differences between calculated and expected
values.The criteria used for assessing your reports are based on:
theoretical preparation, experimental skill, the quality of your data;
the correct processing of data (e.g.correctness in calculations, accuracy in plotting
graphs, inclusion of the appropriate units, labelling of axes etc., estimation of
experimental uncertainties); correct answers to questions, conclusion, the overall coherence of the report.
Failure to hand in work on time without good reason will result in your being awarded a
downgraded mark. Exceptions will only be made if you have made prior arrangements with
the academic staff member in charge of the laboratory (or have reported an illness to theadministration). At the end of the six-week session you MUST hand reports during the
following week. Reports handed in after this deadline will receive no more than half marks.
PLAGIARISM
Plagiarism means trying to pass off someone elses work as your own. The University takes a
-
7/30/2019 Colloid Chemistry Practice
5/48
5
experiment was carried out. The record of each experiment should begin with a brief
statement of the experiment to be performed, and a brief plan of the experimental procedure.This should be done before you come to the lab. You must plot data in your lab notebook as
you are going along. Your laboratory notebook must be signed and dated by the demonstrator
or superviser when you have finished the experiment.
SAFETY INFORMATIONEmergency telephone numbers: First aid medical doctor number: 23009; Receptionist: 22300
There are First Aid Boxes in each Teaching Laboratory. In the event of a minor accident call
the staff. External Numbers Fire 9-105, Police 9-107, Ambulance 9-104
FIRE ALARMS
The alarms sound if a sensor detects flame, heat or smoke or if the break-glass alarm button is
activated.
If you are in a practical class when the alarms sound, spend a few second for making your
experiment safe, then leave the building by the nearest marked exit. DO NOT USE THE
ELEVATORS! Do not attempt to enter the building until you have been told it is safe to do
!
-
7/30/2019 Colloid Chemistry Practice
6/48
6
You should make yourself familiar with the location of fire extinguishers and fire blankets.
Training in the use of this equipment is given at the beginning of the year. A safety shower islocated by the main door to the lab.
SAFETY PRECAUTIONS
Dress like a chemist. Wear clothing in the laboratory that will provide the maximum body
coverage. When carrying out experimental work you MUST wear a fastened lab coat. In case
of a large chemical spill on your body or clothes, remove contaminated clothing to preventfurther reaction with the skin. Every student has to have own safety glasses and to wear them
when it is neccessary.
If the fire alarms sound, make your working area safe and leave the building as directed by
the laboratory staff.
Eating and drinking is strictly PROHIBITED. This includes chewing gum. Keep long hairtied back and ensure that long or large necklaces are safely tucked away. Mobile phones
personal stereos etc. must be switched off when entering the laboratory. Anything that
interferes with your ability to hear what is going on in the laboratory is a potential hazard.
You are strongly advised not to wear contact lenses in the laboratory, even under safety
i ll d
-
7/30/2019 Colloid Chemistry Practice
7/48
7
EXPERIMENTS
1. Rheological characterization of concentrated emulsions (creams)
Theory
Rheology is the science of flow and deformation of body and describes the interrelation
between force, deformation and time. Rheology is applicable to all materials, from gases to
solids. Viscosity is the measure of the internal friction of a fluid. This friction becomes
apparent when a layer of fluid is made to move in relation to another layer. The greater the
friction is the greater amount of force is required to cause this movement, which is called
shear. Shearing occurs whenever the fluid is physically moved or distributed, as in pouring,
spreading, spraying, mixing, etc. Highly viscous fluids, therefore, require more force to move
than less viscous materials.
Isaac Newton defined viscosity by considering the model: two parallel planes of fluid of equal
area A are separated by a distance dy and are moving in the same direction (x) at different
velocities v1 and v2. Newton assumed that the force required maintaining this difference
in speed was proportional to the velocity gradient.
T thi N t t d
dv
A
F
h i t t f i t i l d i
-
7/30/2019 Colloid Chemistry Practice
8/48
8
Classification of materials
Fluids are normally divided into three different groups according to their flow behaviour:
Newtonian fluids
The viscosity of a Newtonian fluid is dependent only on temperature but not on shear rate and
time.
Examples: water, milk, sugar solution, mineral oil.
Non-Newtonian fluids, time independent
The viscosity of a Non-Newtonian time independent fluid is dependent not only on
temperature but also on shear rate.
Depending on how viscosity changes with shear rate the flow behaviour is characterized as:
shear thinning or pseudoplastic - the viscosity decreases with increased shear rate
shear thickening or dilatant - the viscosity increases with increased shear rate
plastic - exhibits a so-called yield value, i.e. a certain shear stress must be applied before flow
occurs
Examples of shear thinning fluids: paint, shampoo, slurries, fruit juice concentrates, ketchup,
Examples of shear thickening fluids: wet sand, concentrated starch suspensions
Examples of plastic fluids: tomato paste, tooth paste, hand cream, some ketchups, grease
-
7/30/2019 Colloid Chemistry Practice
9/48
9
Non-Newtonian fluids. A non-Newtonian fluid is broadly defined as for which the
relationship / D is not constant. In other words, when the shear rate is varied, the shear stressdoesnt vary in the same proportion (or even necessarily in the same direction). The viscosity
of such fluids will therefore tend to change as the shear rate is varied. Thus, the experimental
parameters of a Viscometer model, spindle and speed all have an effect on the measured
viscosity called apparent viscosity of fluid, which is accurate only when explicit experimental
parameters are furnished and adhered to.Non-Newtonian flow is probably a mechanical proposition. As non-symmetrical objects pass
by each other, as happens during flow, their size, shape, and cohesiveness will determine how
much force is required to move them. At another rate of shear, the alignment of the objects
may be different and more or less force may be required to maintain motion. By non-
symmetrical objects, we refer to large molecules, colloidal particles, and other suspended
materials such as clays, fibre, and crystals.
There are several types of Non-Newtonian flow behaviour, characterized by the way a fluids
viscosity changes in response to variations in shear rate. The most common types of Non-
Newtonian fluids you may encounter include:
Pseudoplastic. This type of fluid will display a decreasing viscosity with an increasing shear
-
7/30/2019 Colloid Chemistry Practice
10/48
10
from Figure2. Rheopectic. This is essentially the opposite of thixotropic behaviour, in that
the fluids viscosity increases with time as it is sheared at a constant rate. See Figure1. Both
thixotropy and rheopexy may occur in combination with any of previously discussed flow
behaviors, or only at certain shear rates. The time element is extremely variable; under
conditions of constant shear, some fluids will reach their final viscosity in a few seconds,
while others may take up to several days. Rheopectic fluids are rarely encountered.
Thixotropy is frequently observed in materials such as greases, heavy printing inks, and
paints. When subjected to varying rates of shear, a thixotropic fluid will react as illustrated in
Figure 2. A plot of shear stress versus shear rate was made increased to a certain value, then
immediately decreased to the starting point. Note that the up and down curves do not
coincide. This hysteresis loop is caused by the decrease in the fluids viscosity with
increasing time of shearing. Such effects may or may not be reversible; some thixotropic
fluids, if allowed to stand undisturbed for a while, will regain their initial viscosity, while
others never will.
Dispersions and emulsions, which are multiphase materials consisting of one or more solid
(or liquid) phases dispersed in a fluid phase, can be affected rheologically by a number of
factors. One of the major characteristics to study is the state of aggregation of the sample
-
7/30/2019 Colloid Chemistry Practice
11/48
11
commercial medical creams, shampoos, and a childrens cough syrup are generally tested by
rheometry.
(Pa)0
D(s
-1)
(Pa)0
D(s
-1)
(Pa)0
D(s
-1)
1
2
3
4
5
6
(Pa)0
(Pas)
1
2
3
4
5
6
Figure 1. Flow and viscosity curves of newtonian and non-newtonian time-independent
fluids
1. ideal (newtonian), 2. shear thinning or pseudoplastic, 3. shear thickening or dilatant,
4. ideal plastic (Bingham type), 5. plastic (pseudo-plastic type), 6. plastic (dilatant type)
-
7/30/2019 Colloid Chemistry Practice
12/48
12
Chemicals
Type ungventum emulsificans nonionicum of basic cream, water
Experimental procedure
In this practice, basic rheological terms and an interpretation on the relationship between
rheological response and material structure have to be presented. A flow curve (shear rate D
versus shear stress ), and a viscosity curve (viscosity versus shear stress ), across a wide
range of shear rates can provide important information about storage stability; optimal
conditions for mixing, pumping, and transferring; and end-user applications. It also provides
important information regarding the ways in which the structure changes to comply with the
applied shear in different conditions, such as storage, processing, and application. The
rheological behavior of the material may change as a result of these forces. If the shear ratechanges during an application, the internal structure of the sample will change and the change
in stress or viscosity can
then be seen.
-
7/30/2019 Colloid Chemistry Practice
13/48
13
spindle/speed combination should be used for all tests. The spindle should be immersed up to
the middle of the indentation in the shaft. We recommend inserting the spindle in a different
portion of the sample than the one intended for measurement. The spindle may then be moved
horizontally to the center of the sample container.
1. Immerse the spindle into the sample, and turn the rectangular srew on maximum speed
(the actual speed is on the top of the screw), and allow it to run for 15 minutes then let
it run at minimum speed until a constant reading is obtained (or for about 10 minutes).
(Homogenization) For measurement use every speed for 30 s.
2. Start the run with 6 RPM and read and write the value of display after 30 s running.
3. Increase the speed of rotation without stopping the motor and read the display again as
before. If there is no dial or readable display or it is less than 5%, change spindle or
speed. Repeat the measurement decreasing the speed too.
4. After measurement at all 4 speeds dilute the emulsion with 5 cm3
water and
homogenize thoroughly with a glass rod. Repeat the measurement from 1 point then
dilute the sample with deionized water 5 cm3
again and repeat again the procedure
from 2 to 3 twice.
-
7/30/2019 Colloid Chemistry Practice
14/48
14
Fill the next tables.
RPM(initial sample)
(+5 cm
3water)
(+5 cm
3water)
(+5 cm
3water)
6
6
6
1212
12
30
30
30
60
60
60
30
30
-
7/30/2019 Colloid Chemistry Practice
15/48
15
30
126
Determined shear stress (calculated from the average torques using z andD)
RPM (Pa)
(initial sample)
(Pa)
(+5 cm
3
water)
(Pa)
(+5 cm
3
water)
(Pa)
(+5 cm
3
water)6
12
30
60
3012
6
Plot the rheology (D) and viscosity () curves. Compare your 2 figures to the figure 1
d d i h h l l f A i t h th d h t i l
-
7/30/2019 Colloid Chemistry Practice
16/48
16
2. Measurement of surface tension of solutions by Du Nouy tensiometer
Theory
The molecules at the surface of a liquid are subjected to an unbalanced force of molecular
attraction as the molecules of the liquid tend to pull those at the surface inward while the
vapor does not have as strong an attraction. This unbalance causes liquids to tend to maintain
the smallest surface possible. The magnitude of this force is called the surface tension.When
this lowest possible energetic state is achieved the surface tensionacts to hold the surface
together where the force is parallel to the surface. The symbol for surface tension is
"gamma". Conventionally the tension between the liquid and the atmosphere is called surface
tension while the tension between one liquid and another is called interfacial tension.
The specific surface free energy or surface tension of surface is equal to the expenditure of
work required to increase the net area of surface isothermally and reversibly way by unit of
area, in J m2
(Joule per meter); if the increase in the surface area is accomplished by moving a
unit length of line segment in a direction perpendicular to itself, is equal to the force, or
tension, opposing the moving of the line segment. Accordingly, it is usually expressed in
i f N
-1
(N ) S f b f f ( d i
-
7/30/2019 Colloid Chemistry Practice
17/48
17
toward the water, and the nonpolar group(s) are oriented toward the oil. This orientation of
amphiphilic molecules is described by Hardy-Harkins principle of continuity. The surfactant
is adsorbed or oriented in this manner, consequently lowering interfacial tension between the
oil and water phase. When a surfactant is placed in a water system it adsorbs at the surface
and lowers the surface tension between the water and air. When it is place in a mixture of
solid and liquid it adsorbs on the surface of the solid and lowers the interfacial tension
between the solid and the water. Since the surfactant is adsorbed at the surface it is logical
that the concentration of surfactant at the surface would be greater than the concentration in
the bulk solution. Mathematically such a relationship has been derived by Willard Gibbs. It
relates lowering of surface tension to excess concentration of surfactant at the surface.
The Gibbs equation can write for a dilute solution in two forms:
dc
d
RT
cc
where c is the concentration (in mol m-3
) in the solution, T (K) the
absolute temperature,R the gas constant (8.314 JK-1
mol-1
), (Nm-1
) the surface tension and
c (mol m2) is the surface excess concentration. It follows from Equation 1 that c is positive
if d /dc is negative, that is the surface tension decreases with increasing solute concentration.
On the basis of experimental surface tension vs. solute concentration function the d /dc can be
-
7/30/2019 Colloid Chemistry Practice
18/48
18
Du Nouy tensiometer
Chemicals
deionized water, aqueous solution of alcohol or organic acids.
Experimental procedure
This tensiometer is capable of measuring the surface tension of liquids with the du Nouy ring.
The du Nouy tensiometer consists of a platinum-iridium ring supported by a stirrup attached
to a torsion balance. The force that is just requiring breaking the ring free of the liquid/liquid
or liquid/air interface is proportional to the surface tension. The surface tension is displayed
-
7/30/2019 Colloid Chemistry Practice
19/48
19
c, mol/L c,1, mN/m c,2, mN/m c,3, mN/m c,average c, corrected
00.05
0.1
0.2
0.4
11.5
2
Calculation of results
Calibration with water: Correction Factor. It is necessary to apply a correction factor that
takes into account the shape of the liquid held up by the ring (the ring is not perfectly flat and
circular. It is bent and tilted. The liquid film (surface) will not be perfectly flat and some
liquid will flow downwards making the surface tension not even over the surface. We
calibrate by measuring for water. Take 5 measurements and take the average to be water.
] h 72 0 N/ i h li l f f
-
7/30/2019 Colloid Chemistry Practice
20/48
20
Plot the Gibbs isotherm (Fig1b) and the linear representation of the isotherm, c/ = f(c) and
determine the value of from the slope (1/slope= ) and calculate the area per molecule
A
m
N
1. Discuss the result and compare with literature results.
40
45
50
55
60
65
70
75
0 0.5 1 1.5 2
c, mol/l
,mN/m
0.0E+00
1.0E-06
2.0E-06
3.0E-06
4.0E-06
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
c, mol/l
c,mol/m
2
Figure 1. Graphical differentiation of the measured surface tension vs. concentration plot, =f
(c). Gibbs adsorption isotherm, = f(c).
5.0E+05
-
7/30/2019 Colloid Chemistry Practice
21/48
21
3. Polymers relative molecular masses from viscosity measurements
Theory
Rheology is the science of flow and deformation of body and describes the interrelation
between force, deformation and time. Rheology is applicable to all materials, from gases to
solids. Viscosity is the measured of the internal friction of a fluid. This friction becomes
apparent when a layer of fluid is made to move in relation to another layer. The greater the
friction is the greater amount of force is required to cause this movement, which is called
shear. Shearing occurs whenever the fluid is physically moved or distributed, as in pouring,
spreading, spraying, mixing, etc. Highly viscous fluids, therefore, require more force to move
than less viscous materials.
Isaac Newton defined viscosity by considering the model: two parallel planes of fluid of equal
area A are separated by a distance dy and are moving in the same direction at different
velocities v1 and v2. Newton assumed that the force required maintaining this difference
in speed was proportional to the velocity gradient.
To express this, Newton wrote:dy
dv
A
F where is a constant for a given material and is
-
7/30/2019 Colloid Chemistry Practice
22/48
22
Viscosities of dilute colloid solutions. For most pure liquids and for many solutions and
dispersions is a well defined quantity for a given temperature and pressure which is
independent ofand d/dy, provided that the flow is streamlined (i.e. laminar), these are so-
called Newtonian liquids orsolutions. For many other solutions and dispersions, especially
if concentrated and/or if the particles are asymmetric, deviations from Newtonian flow are
observed. The main causes of non-Newtonian flow are the formation of a structure throughout
the system and orientation of asymmetric particles caused by the velocity gradient. Capillary
flow methods, the most frequently employed methods for measuring viscosities are based on
flow through a capillary tube. The pressure under which the liquid flows furnishes the
shearing stress. The relative viscosities of two liquids can be determined by using a simple
Ostwald viscometer. (Figure 1). 10 mL liquid is introduced into the viscometer. Liquid is then
drawn up into the right-hand limb until the liquid levels are above A. The liquid is then
released and the time, t(s) for the right-hand meniscus to pass between the marks A and B is
measured. For a solution the relative viscosity:
00t
trel
(1)
h d 0 i h fl i i f l i d l i l 0 d i h
-
7/30/2019 Colloid Chemistry Practice
23/48
23
and degree of solvation. However, if the shape and/or solvation factor alters with particle size,
viscosity measurements can be used for determinining the particle size (for molar mass). The
intrinsic viscosity of a polymer solution is, in turn, proportional to the average solvation factor
of the polymer coils. For most linear high polymers in solution the chains are somewhat more
extended than random, and the relation between intrinsic viscosity and relative molecular
mass can be expressed by a general equation proposed by Mark and Houwink:
viscMK
(3)
where K and are characteristic of the polymer-solvent system. Alpha depends on the
configuration (stiffness) of polymer chains. In view of experimental simplicity and accuracy,
viscosity measurements are extremely useful for routine molar mass determinations on a
particular polymer-solvent system. Kand for the system are determined by measuring the
intrinsic viscosities of polymer fractions for which the relative molecular masses have been
determined independently, e.g. by osmotic pressure, sedimentation or light scattering. For
polydispersed systems an average relative molar mass intermediate between number average
(alpha=0) and mass average (alpha=1) usually results:aa
visc
dNM
dNMM
1
1
-
7/30/2019 Colloid Chemistry Practice
24/48
24
0
50
100
150
200
250
0 0.02 0.04 0.06c, g/mL
spec/c
ln rel/c
Figure 1 Figure 2
Ostwald viscometer Determination of the limiting viscosity number
by extrapolation c0 of reduced viscosity concentration curves
Calculation of results
c (gcm-3) t (s) t (s) t (s) taverage(s) rel spec spec/c (ln rel)/c
0
-
7/30/2019 Colloid Chemistry Practice
25/48
25
4. Adsorption from solution
Theory
Adsorption is the enrichment (positive adsorption, or briefly, adsorption) or depletion
(negative adsorption) of one or more components in an interfacial layer. The material in the
adsorbed state is called the adsorbate, while that one which is present in the bulk phase may
be distinguished as the adsorptive. When adsorption occurs (or may occur) at the interface
between a fluid phase and a solid, the solid is usually called the adsorbent. Sorption is also
used as a general term to cover both adsorption and absorption. Adsorption from liquid
mixtures occurs only when there is a difference between the relative composition of the liquid
in the interfacial layer and in the adjoining bulk phase(s) and observable phenomena result
from this difference. For liquids, accumulation (positive adsorption) of one or several
components is generally accompanied by depletion of the other(s) in the interfacial layer;
such depletion, i.e. when the equilibrium concentration of a component in the interfacial layer
is smaller than the adjoining bulk liquid, is termed negative adsorption and should not be
designated as desorption. Equilibrium between a bulk fluid and an interfacial layer may be
-
7/30/2019 Colloid Chemistry Practice
26/48
26
molecule in a complete monolayer):maa/ . Micropore filling is the process in which
molecules are adsorbed in the adsorption space within micropores. The micropore volume isconventionally measured by the volume of the adsorbed material, which completely fills the
micropores, expressed in terms of bulk liquid at atmospheric pressure and at the temperature
of measurement. Capillary condensation is said to occur when, in porous solids, multilayer
adsorption from a vapour proceeds to the point at which pore spaces are filled with liquid
separated from the gas phase by menisci. The concept of capillary condensation loses itssense when the dimensions of the pores are so small that the term meniscus ceases to have a
physical significance. Capillary condensation is often accompanied by hysteresis.
Adsorption from solution is important in many practical situations, such as those in which
modification of the solid surface is of primary concern (e.g. the use of hydrophilic or
lipophilic materials to realize stable dispersions in aqueous or organic medium, respectively)
and those which involve the removal of unwanted material from the solution (e.g. the
clarification of sugar solutions with activated charcoal). Adsorption processes are very
important in chromatography, too.
The theoretical treatment of adsorption from solution is in general complicated since this
adsorption always involves competition between solute(s) and solvent. The degree of
-
7/30/2019 Colloid Chemistry Practice
27/48
27
where a is the specific adsorbed amount, g/g , at constant volume , V is the volume of
solution, dm3, m is the mass of adsorbent, g in volume V, c0 and c are the initial and
equilibrium concentrations , g/dm3
of dissolved substance, respectively. In many practical
cases it is found that the adsorption obeys an equation known as the Langmuir isotherm. This
equation was derived from adsorption of gases on solids and assumes that:
1. the adsorption is limited to monolayer
2. and occurs on a uniform surface; i.e. all sites for adsorption are equivalent,
3. adsorbed molecules are localized,
4. there is no interaction between molecules in a given layer; independent stacks of
molecules built up on the surface sites.
The Langmuir equation may be written as follows:
cb
caa
s
m
/1(2)
whereasm is the monolayer capacity or the amount adsorbed at saturation,c is the equilibrium
concentration of solute and b is constant. The monolayer capacity can be estimated either
directly from the actual isotherm or indirectly by applying the linear form of the Langmuir
-
7/30/2019 Colloid Chemistry Practice
28/48
28
Apparatus
Spectrophotometer with 1 cm cells, volumetric flasks, beakers, volumetric pipettes, burette,
measuring cylinder, laboratory shaker
Chemicals
Adsorbent (aluminum oxide), dye solution (indigo carmine).
Experimental procedure
Prepare a concentration series of indigo carmine solution from the stock solution with a 100
cm3
volumetric flask for the calibration curve and the adsorption experiments. For the
calibration about 10-20 cm3, for the adsorption measurements 50-50 cm
3of solutions are
necessary. Suggested concentrations are 1.010-4, 1.510-4, 2.010-4, 2.510-4, 3.010-4,
4.010-4 mol/dm3.
Clean six 250-ml Erlenmeyer flasks which should either have glass-stoppered tops or are
fitted with rubber stoppers. Put 0.100.01 g of aluminum oxide (weighed accurately between
0.11g and 0.09g to the forth decimal place in a weighing dish or on a weighing paper) into
each flask. To each of these flasks add with a pipette exactly 50-50 cm3
of indigo carmine
solution from the concentration series and shake the flasks for at least 1 hr in order to reach
-
7/30/2019 Colloid Chemistry Practice
29/48
29
0
0.5
1
0.0E+0 2.0E-4 4.0E-4c0, mol / L
A
A(ce)
ce
Figure 1. Determination of the equilibrium concentration c or ce of the dye in solution in
presence of the adsorbent from the calibration curve (absorbance vs. concentration).
Calculation of results
Calculate the adsorbed amounts with the equation 1. Plot the original (equation 2) and the
linearized form of Langmuir equation (equation 3). Determine the slope of the line and
-
7/30/2019 Colloid Chemistry Practice
30/48
30
0.E+00
1.E-05
2.E-05
3.E-05
4.E-05
0.E+00 1.E-04 2.E-04 3.E-04 4.E-04
c(mol dm-3)
a(molg-1)
Figure 2. Langmuir isotherm.
5
6
7
8
9
10
gdm-3)
cb
caa
m
/1
-
7/30/2019 Colloid Chemistry Practice
31/48
31
5. Solubilization
Theory
Soluble amphipathic substances, both ionic and nonionic, show sharp changes in a variety of
colligative physical chemical properties at a well-defined concentration which is characteristic
of the solute in question. These phenomena are attributed to the association of solute
molecules into clusters known as micelles. The concentration at which micellization occurs is
known as the critical micelle concentration, generally abbreviated CMC. Figure 1 shows the
superpositioned curves for a variety of properties (such as surface tension, electrical specific
and molar conductance, freezing-point depression, osmotic pressure, turbidity) versus
concentration for sodium dodecyl sulfate solutions. Another interesting property of micelles is
their ability to solubilize materials which otherwise are insoluble in aqueous solutions.
Insoluble organic matter, for example, may dissolve in the interior of the micelle even though
it shows minimal solubility in water. Certain oil-soluble dyes barely color water, but give
vividly colored solutions above the cmc. This solubilization of organic molecules in micelles
is known to play an important part e.g. in the process of emulsion polymerization.
-
7/30/2019 Colloid Chemistry Practice
32/48
32
In discussing micellization, we have intentionally restricted attention to concentration near to
cmc where the micelles are fairly symmetrical in shape and far enough from each other to be
treated as independent entities. At higher concentration, neither of these concentrations is met
and more complex phase equilibria must be considered.
Figure 2. Pictorial represantation of the conformation of surfactant molecules (monomers) in
solution. Above the CMC (a) micelles are formed. At higher surfactant concentration, the
amphiphiles form a variety of structures, here illustrated with (b) a liquid crystalline lamellar
single-phase and (c) reversed micelles, an isotropic single-phase.
The critical micelle concentration depends on the solvent, the structure of the surfactant
molecules, the salt concentration and the temperature. For charged micelles, the situation is
-
7/30/2019 Colloid Chemistry Practice
33/48
33
the organic matter. The organic molecule, depending on its structure, takes place in the
micelle according to the equalization of polarities (Hardy-Harkins principle).
Apparatus
Volumetric flasks, beakers, volumetric pipettes, burette, magnetic stirrer or shaker.
Chemicals
Non-ionic surfactant (e.g. Tween 20 or 40), organic acids (e.g. salicylic or benzoic acid),
0.1 M NaOH solution, propanol.
-
7/30/2019 Colloid Chemistry Practice
34/48
34
Calculation of results
Calculate the quantity of acid solubilized (m) in g/10 cm3
of surfactant solution. Plot the
calculated amounts m as a function of the surfactant concentration c. Calculate the slope and
the intersection of the best fit straight line. The slope of this line gives amount of acid
solubilized by 1 g of surfactant (if the concentration is plotted in units of g/10 cm3).
Surfactant concentration % 0 0,1 0,2 0,5 1,0 2,0
Surfactant g/10 ml solution
Added NaOH ml /10 ml solution
Titrated acid (g /10ml solution
10ml)
-
7/30/2019 Colloid Chemistry Practice
35/48
35
6. Determination of size distribution of a sedimenting suspension
Theory
The size of a spherical homogeneous particle is uniquely defined by its diameter. For irregular
particles derived diameters are determined by measuring a size-dependent property of the
particle and relating it to a linear dimension. The most widely used of these are the
equivalent spherical diameters. If an irregular shaped particle is allowed to settle in a liquid,
its terminal velocity may be compared with the terminal velocity of a sphere of the same
density settling under similar conditions. The size of the particle is then equated to the
diameter of the sphere. That is to say we use the diameter of some consubstantial sphere that
has the same sedimentation rate as the measured particle to represent the dimension of the
actual particle. In the laminar flow region, the particle moves with random orientation, the
free-falling diameter becomes the Stokes diameter.
Average diameters. The purpose of the average is to represent a group of individual values in
a simple and concise manner in order to obtain an understanding of the group. It is important,
therefore that the average should be representative of the group. All average diameters are a
measure of central tendency which is unaffected by the relatively few values in the tails of
-
7/30/2019 Colloid Chemistry Practice
36/48
36
0
0.005
0.01
0.015
0.02
0.025
0.03
0 20 40 60 80 100 120 140 160 180 200
x
d
/dx
33.5 ; P 1.3
15 ; P 1.065
mean = mode = meadian = 100mean =68.26%mean 2 =95.5%
Figure 2. Relative percentage frequency curves for normal distribution.
The principle of the sedimentation is the determination of the rate at which particles settle out
of a homogeneous suspension. This may be determined by allowing the sediment to fall on to
a balance pan and weighing it. In a sedimentation cylinderh is the height of suspension above
-
7/30/2019 Colloid Chemistry Practice
37/48
37
.00
.25
.50
.75
1.00
0 5 10 15 20 25
r, micron
W(r)
dW/dr
Figure 3. Determination of weight percentage oversize, W(r) from a graph of weight of
powder sedimented,P(t) against time,t.;
Cumulative W(r) and differential dW/dr (r) distribution functions
Apparatus
Sedimentation cylinder, sedimentation balance
Chemicals
4 g of silica powder, deionized water
-
7/30/2019 Colloid Chemistry Practice
38/48
38
Save the data and open it in excel.
h (m) P(g) medium kg/m3 solid kg/m
3 (Pas)
2600 1000 0.001
Calculation of results
t (s) r(m) W (%) t (s) r(m) W (%)
-
7/30/2019 Colloid Chemistry Practice
39/48
39
7. Zeta potential measurements of dyes by paper electrophoresis
Theory
When a colloid particle emerges in a polar solvent or an electrolyte solution, a surface charge
will be developed through preferential adsorption of ions or dissociation of surface charged
species. In order for the interface to remain neutral the charge held on the surface is balanced
by the redistribution of ions close to the surface. The overall result is two layers of charge (the
double layer). The liquid layer surrounding the particle exists as two parts; an inner region
(Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are
less firmly associated. Within this diffuse layer is a notional boundary known as the slipping
plane, within which the particle acts as a single entity. The potential at the slipping plane is
known as the zeta potential and its value can be determined by measurement of
electrophoretic mobility. The movement of a charged particle relative to the liquid it is
suspended in under the influence of
an applied electric field:
-
7/30/2019 Colloid Chemistry Practice
40/48
40
Scheme of a horizontal electropheresis
apparatus.
The apparatus and experimental procedure
The apparatus consists of air-tight housing, a horizontal plastic surface, which keeps the
paper, two electrodes in the pool, high voltage power supply, volt meter and ampere meter.
Filter paper is used as a supporting medium for the solvent bulk.
Indicate the start place in the center of the suitable sized strip of chromatographic paper
with a dashed line by a pencil.
Drop some micro liter from each solution of different dyes with capillary tubes at the start line
-
7/30/2019 Colloid Chemistry Practice
41/48
41
dyes distance, m time, s velocity, m/s mobility, m /Vs zeta pot., V
Discuss the results
Review questions
Definition of the zeta potential. What kind of relation is between of the colloid stability and
the zeta potential of lyophobic colloids? What type of compounds are the used dyes? What is
the origin of the dyes charge?
-
7/30/2019 Colloid Chemistry Practice
42/48
42
8. Steric and electrostatic stabilization mechanisms of colloidal dispersions
Theory
DVLO theory suggests that the stability of a colloidal system is determined by the sum of the
van der Waals attractive (VA) and electrical double layer repulsive (VR) forces that exist
between particles as they approach each other due to the Brownian motion they are
undergoing. This theory proposes that an energy
barrier resulting from the repulsive force prevents
two particles approaching one another and
adhering. But if the particles collide with sufficient
energy to overcome that barrier together
DVLO theory assumes that the stability of a
particle in solution is dependent upon its total
potential energy function VT.
-
7/30/2019 Colloid Chemistry Practice
43/48
43
where 0 surface potential , a activity of potential determining ion, apzc concentration at point
of zero charge. pAgpzc=4.5 (Agpzc =2.910-5
M) and pClpzc=5.2 (Clpzc =6.310-6
M) the
solubility product for AgCl Ksp=1.810-10
.
Chemicals
0.05% PVA solution, 0.005 M AgNO3and 0.005 M KCl solution, deionized water.
The apparatus and experimental procedure
Cary spectrophotometer, glass tubes, pipettes, optical cuvette.
Follow the basic instructions of the Manual for the use of the CARY 50 UV-Vis
Spectrophotometer and select : Dwell Time: 0s; Cycle Time: 0.5 min; Stop Time: 15 min
Prepare 5 ml A and 5 ml B solutions according the table:
A solution, ml B solution, ml
sol preparation PVA 0.05 % Cl-0.005 M water Ag
+0.005 M water
1 pour B into A and back 0 1 4 1 4
-
7/30/2019 Colloid Chemistry Practice
44/48
44
4
5
6
Discuss the results
Compare and discuss the colloid stability of the sols.
Review questions
Definition of the point of zero charge. What kind of relation is between of the colloid stability
and the zeta potential of lyophobic colloids? What is the origin of the charge of silver halide
sols? How charged is a stoichiometric silver chlorides?
-
7/30/2019 Colloid Chemistry Practice
45/48
45
9. Determination of the critical micelle concentration of SDS
Theory
Surfactants are amphiphilic molecules that possess both hydrophobic and hydrophilic
properties. A typical surfactant molecule consists of a long hydrocarbon tail that dissolves in
hydrocarbon and other non-polar solvents, and a hydrophilic headgroup that dissolves in
polar solvents (typically water). Amphiphilic molecules can be found in a wide range of
chemical structures, e.g. one or two hydrophobic chains as well as cationic, anionic or neutral
polar heads. Anionic head groups are commonly found in soaps, as the strongly hydrophilic
carboxyl and sulphonate groups increase their solubility in water. On the other hand, cationic
head groups exhibit anti-bacterial properties, which find use in mild antiseptics and
disinfectants.
If these molecules are dissolved in water, a large amount of solvent molecule is needed to
keep them in the solution since the secondary interaction between the hydrophobic part and
the water molecules is quite week. As a consequence less water molecules remain in the bulk
-
7/30/2019 Colloid Chemistry Practice
46/48
46
Conductometric Determination of the CMC
Below the CMC, the addition of surfactant to an aqueous solution causes an increase in the
number of charge and consequently, an increase in the conductivity. Above the CMC, further
addition of surfactant increases the micelle concentration while the monomer concentration
remains approximately constant (at the CMC level). Since a micelle is much larger than a
monomer it diffuses more slowly through solution and so it is a less efficient charge carrier. A
plot of conductivity against surfactant concentration is, thus, expected to show a break around
CMC.
Apparatus:
Consort multiparameter analyser,
magnetic stirrer rod,
pipettes, tall vessel
Chemicals:
0.1M SDS (Sodium dodecyl sulphate)
stock solution, MilliQ deionized water
-
7/30/2019 Colloid Chemistry Practice
47/48
47
Calculation:
Calculate the derived conductivity (), the molar conducitivity () using the following
equations:
watermeasured
'
c
'
(Calculate in SI units!)
Plot the c-and c - (Lottermoser-diagram) curves, find the breakpoints on the curves and
calculate the CMC values from the two figures.
5.0E-02
1.0E-01
1.5E-01
2.0E-01
(Sm-1)
1.0E-02
1.2E-02
1.4E-02
(Sm2mol-1)
nVincrements Vtotal added Vtotal c c measured c
-
7/30/2019 Colloid Chemistry Practice
48/48
n(cm
3) ( cm
3) ( cm
3) (mol dm
-3) (mol m
-3) (S cm
-1) (S cm
-1) (S m
-1)
c(Sm
2mol
-1)
0. 0 0 50 0 0
1.
2.
3.
4.
5.
6.
7.8.
9.
10.