colloid chemistry practice

7/30/2019 Colloid Chemistry Practice

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Manual for

Colloid Chemistry Practical Course

Pharmacy program 2012/2013 1st

semester 2nd year

Edited by Mrta Berka, LeventeNovkand

Mnika Kri

University of Debrecen


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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


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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


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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


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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

!


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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


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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


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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


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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


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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


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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)


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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.


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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.


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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


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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


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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


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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


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Du Nouy tensiometer

Chemicals

deionized water, aqueous solution of alcohol or organic acids.


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


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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


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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


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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


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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


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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


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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


c (gcm-3) t (s) t (s) t (s) taverage(s) rel spec spec/c (ln rel)/c

0


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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


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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


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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


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Apparatus

Spectrophotometer with 1 cm cells, volumetric flasks, beakers, volumetric pipettes, burette,

measuring cylinder, laboratory shaker

Chemicals

Adsorbent (aluminum oxide), dye solution (indigo carmine).


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


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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).


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


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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


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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.


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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


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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.


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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)


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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


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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


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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


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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


t (s) r(m) W (%) t (s) r(m) W (%)


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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:


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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


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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?


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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.


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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


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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?


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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


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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


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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


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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.

colloid chemistry practice

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