Laboratory works in colloid chemistry

126 Pages·2020·0.956 MB·English

by  Тажибаева С.М.

AL-FARABI KAZAKH NATIONAL UNIVERSITY

LABORATORY WORKS
IN COLLOID CHEMISTRY

Almaty
«Qazaq University»
2020

1
UDC 544.7 (075.8)
LBC 24.6 я73
L 11
Recommended by the Academic Council
of the Faculty of Chemistry and Chemical Technology
and Editorial and Publishing Council of al-Farabi KazNU
(Protocol No.3 dated 13.03.2020)
Reviewers:
doctor of Chemical Science, professor K.Zh. Abdiyev
doctor of Chemical Science, professor G.A. Mun
Authors:
K.B. Musabekov, S.M. Tazhibayeva, K.I. Omarova,
A.K. Kokanbayev, S.Sh. Kumargaliyeva, A.O. Adilbekova
Zh.B. Ospanova, O.A. Esimova, M.Zh. Kerimkulova
L 11 Laboratory works in colloid chemistry / K.B. Musabekov,
S.M. Tazhibayeva, K.I. Omarova, [et al.]. – Almaty: Qazaq Uni-
versity, 2020. – 126 p.
ISBN 978-601-04-4656-4
The manual presents the guidelines for laboratory works on the main
sections of colloid chemistry: surface phenomena, adsorption of surface-
active substances, molecular-kinetic properties of dispersed systems, stabili-
ty and coagulation of colloids, lyophilic and lyophobic disperse systems.
The manual is intended for students and undergraduates of chemical and
chemical-technological specialties.
DC 544.7 (075.8)
LBC 24.6 я73
ISBN 978-601-04-4656-4 © Musabekov K.B., Tazhibayeva S.M.,
Omarova K.I., [et al.], 2020
© Al-Farabi KazNU, 2020
2
PREFACE

The manual "Laboratory works in colloid chemistry" has been
designed for students of chemical and chemical-technological spe-
cialties of higher educational institutions. The manual is the result of
long-term experience of teachers of colloid chemistry at the depart-
ment of analytical, colloidal chemistry and technology of rare ele-
ments of al-Farabi Kazakh National University.
The purpose of the manual is to study surface phenomena, met-
hodology and properties of dispersed systems.
The manual contains laboratory works on all major items of colloid
chemistry: surface phenomena (adsorption at the liquid-gas interface,
solid-liquid, surface wetting, surface modification), molecular-kinetic
properties of dispersed systems (sedimentation analysis of suspensions),
lyophilic system for determining the critical concentrations of micelle
formation of surface-active substances, stability of lyophobic systems
(study of coagulation and stabilization of sols, preparation of emulsions
and foam), electrical properties of disperse systems (determination of
the electro-kinetic potential by electrophoresis method).
To help students, each laboratory work contains brief theoretical
information and questions for self-control. The practicum is compiled
taking into account prerequisites according to the curriculum of the stu-
dents (higher mathematics, physics, inorganic chemistry, organic che-
mistry). Therefore special methods of measurement and processing of
graphs and tables are not considered in the manual. Methodical textbook
is based on the experience of teachers of department of analytical, col-
loid chemistry and technology of rare elements.
The authors hope that the textbook "Laboratory works in colloid
chemistry" will help the students to gain deeper understunding of sa-
me questions of Colloid Chemistry. Any comments and suggestions
of the readers will be gratefully accepted.

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1.ADSORPTION FROM SOLUTIONS
Adsorption is a spontaneous redistribution of system compo-
nents between the surface layer and the volume of the phase. Desorp-
tion (Г) and related changes in surface tension (dσ) and chemical po-
і
tentials (dμ) of the system components are related to each other by
і
the Gibbs fundamental adsorption equation:
dσ  Гidμi , (1.1)
where Г is an excess of components in the surface layer (per unit
і
surface) in comparison with the initial concentration; μ is the che-
і
mical potentials of the components.
Taking into account that μ = μо + RTlna, a dμ = RTlna, we obtain
i i i
dа  dσ 
Г   i   (1.2)
i  
RT da 
i
At low concentrations of adsorbate in a binary solution, a can be
replaced by с and the relation (1.2) becomes the widely used Gibbs
adsorption equation:
dс dσ
Г   , (1.3)
i
RTdс
where с is the equilibrium concentration of adsorbate in the solution.
In the Gibbs adsorption equation (1.3), the effect of the nature of
dσ
the substances on adsorption is reflected by the derivative . This
dс
derivative also determines the sign of Gibbs adsorption. Thus, the
dσ
quantity can serve as a characteristic of the behavior of sub-
dс
stances during adsorption. To eliminate the effect of concentration on
the derivative, take its limiting value as с → 0. This value was called
surface activity by P.A. Rehbinder:
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dσ  Г
g     RT  (1.4)
 dc  C
c0 c0

Surface activity is the most important adsorption characteristic of
substances, which determines many of their properties and applications.
Units for measuring surface activity in SI (The International System of
Units) are J · m/mol or N · m2/mol, and also in Gibbs (erg · cm/mol).
The equation (1.4) shows that the more the surface tension de-
creases with the concentration of the adsorbed substance, the greater
the surface activity of this substance is. The physical meaning of sur-
face activity is that it represents the force that holds the substance on
the surface and is calculated per unit of Gibbs adsorption. Surface ac-
tivity can be calculated graphically by the surface tension isotherm.
To do this, draw a tangent to the curve before intersection with the
ordinate axis, the negative tangent of the adjacent angle of this
tangent is the surface activity (Fig. 1).

(cid:2026)
ІІ
(cid:2026)
о
∆(cid:2026)
∆С2 С2
Fig. 1.1. Dependence of the surface tension on the concentration of the aqueous so-
lution: I – butyric acid, II – sodium sulfate

Surface activity, like Gibbs adsorption, can be positive and ne-
gative. Its absolute value and sign depend on the nature of both the
adsorbed substance and the medium (solvent). If the surface tension
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at the interface decreases with increasing concentration of the sub-
stance, such a substance is called surface-active. For such substan-
ces
d
g > 0, < 0 and Г > 0
dc
Substances that increase the surface tension at the interface with
increasing concentration are called surface-inactive. For them
d
g < 0, > 0 and Г < 0
dc
Negative Gibbs adsorption Г < 0 means that the concentration of
the adsorbed substance in the volume is larger than in the surface
layer. With an increase in the concentration of the surface-inactive
substance in the volume, its concentration in the surface layer in-
creases more slowly. As a result, with an increase in the concentrati-
on of the surface inactive substance in the volume, the Gibbs adsorp-
tion value is negative (Figure 1.1).
The term "surfactants" is generally applied to specific substances
having very high surface activity with respect to water, which is a
consequence of their special structure. The surfactant molecules have
a non-polar (hydrocarbon) part and a polar part, represented by the
functional groups –СООН, –NH , –O–, –SO OH, etc. Hydrocarbon
2 2
radicals are pushed out of the water to the surface, and their
adsorption is Г > 0. The surfactant type of conventional soaps
(sodium oleate) at a concentration of 10-6 mol/cm3 (1 mol/l) reduces
σ of water at 298K from 72.5·10-3 to 30·10-3 J/m2, which gives
g = 4∙10(cid:2875) Gibbs. This means that for a certain thickness of the sur-
face layer, the surfactant concentration is 3·104 times (i.e. tens of thou-
sands of times) higher than the surfactant concentration in the solu-
tion volume.
An example of surface-inactive substances with respect to water
is inorganic salts, which are strongly hydrated. They interact with
water more than molecules of water between themselves. As a con-
sequence, they have a negative adsorption Г < 0. When adding inor-
ganic salts to water, the surface tension increases. But due to the fact
6
that adsorption is negative, the increase in concentration in the surfa-
ce layer lags behind its growth in volume. Therefore, the surface ten-
sion of the solution with increasing concentration of surface-inactive
substances grows very slowly.
The value of adsorption depends on the nature of the adsorbing
surface, the nature of adsorbent, its concentration, temperature, etc.
The dependence of adsorption on the concentration of the adsorbed
substance in the volume at constant temperature is called the adsorp-
tion isotherm.
The analytical expression for the isotherm of monomolecular ad-
sorption at low concentrations of adsorbate is the Langmuir equation:

Kc
A  A , (1.5)
1Kc

where A is the limiting value of adsorption (monolayer capacity);
∞
K is the equilibrium constant of the adsorption process, expressed in
terms of the ratio of adsorption and desorption rates.
The relation of the Gibbs adsorption equation (1.3) to the Lang-
muir equation (1.5) for the surfactant gives Shishkovsky's equation
showing the change in the surface tension of the solution (two-di-
mensional pressure π) with the concentration of dissolved surfactant
in volume:

π σ σ  A RTln(1Kc), (1.6)
0 

where σ is the surface tension of the pure solvent; A is the limit
0 ∞
number of moles of surfactants per 1 cm2 (limit adsorption).
A joint solution of the Gibbs (1.3) and Shishkovsky (1.6) equa-
tions for highly active surfactants gives an expression for the two-
dimensional pressure in the surface layer (π) equal to the difference
in surface tension of the solvent and solution:

πσ σA RT or πS RT, (1.7)
0  M

where S is the surface occupied by 1 mole of surfactant.
M
7
This equation is valid only in the region of dilute surfactant solu-
tions.
The equation of Gibbs, Langmuir and Shishkovsky from experi-
mental data on the surface tension of solutions allows us to:
1) calculate the adsorption of surfactants at the interface bet-
ween the solution and air;
2) determine the characteristics of the surface monomolecular
layer-limiting adsorption, thickness, linear dimensions of surfactant
molecules.
The most accessible for experimental measurement of surface
tension are liquid-gas and liquid-liquid systems. The σ (c) dependen-
ce determined in this way, in accordance with the Gibbs equation,
makes it possible to calculate the adsorption of the surfactant at the
interphase boundaries.
For solids, the existing methods for determining surface tension
are very few, laborious and not very accurate. Therefore, adsorption
on solids can be measured directly from the difference between the
initial and equilibrium concentrations of the surfactant solution.
Methods for measuring surface tension
The existing methods for determining surface tension are divi-
ded into 3 main groups:
1. Static methods
– method of capillary uplift;
– methods of a recumbent drop (bubble) and a hanging drop;
– measurement of the curvature of the liquid interface;
– method of balancing a ring, plate and other solid in the surfa-
ce layer (Wilhelm);
– method of balancing the barrier, etc.
These methods make it possible to measure σ with a fixed inter-
facial surface in equilibrium with the volume and not changing du-
ring the measurement.
2. Semi-static methods
– The method of greatest pressure formation of bubbles and
droplets;
– The method of tearing off the ring or frame;
– The method of weighing and counting drops is the stalagmo-
metric method.
3. Dynamic methods
8
– method of capillary waves;
– method of oscillating jets and drops.
Dynamic methods are complex in hardware design. In addition,
in the case of solutions, in particular surfactant solutions, a certain ti-
me is required to establish equilibrium in the surface layer.
For practical purposes, static and semi-static methods are more
often used to measure the equilibrium values of the surface tension
of liquids.
In this laboratory practice, it is proposed to use two fairly common
semi-static methods for measuring the surface tension of solutions: the
method of maximum bubble pressure and the stalagometric method.
Method of maximum bubble pressure (Rebinder's method)
It is known that in the presence of a curved phase interface (for
example, a gas bubble in a liquid or a drop of oil in water) some ad-
ditional internal pressure arises. This so-called capillary pressure is
directed from the liquid side and tends to reduce the surface of the
bubble, compresses it. The value of the capillary pressure is
determined by the nature of the liquid (the value of the surface
tension) and depends on the curvature of the surface. According to
Laplace's law for a gas bubble or a droplet of liquid having a
spherical shape, in addition, the internal pressure is expressed as
2σ
ΔP , (1.8)
R
where R is the radius of curvature of the surface.
The center of curvature can be inside the liquid (positive curva-
ture) and outside the liquid (negative curvature). For a plane surface
R = ∞. According to equation (1.8), for a flat surface, for a convex
surface and for a concave surface ∆P < 0.
When a thin glass capillary is lowered into the water, a curved
surface (meniscus) is formed as a result of wetting. The pressure
below this surface is lower than the pressure at the flat surface. As a
result, there is a buoyant force that lifts the liquid in the capillary
until the weight of the column balances the acting force. There is a
quantitative dependence of the height Һ, the radius of curvature of
the surface R, the radius of the tube r, the boundary angle θ and the
boundary tension σ of the sheared layer, called the Jurin equation:
9
2σ cosθ (1.9)
h  ,
rg p  p
1 g
where (cid:1868) −(cid:1868) is the difference in density of two bulk phases.
(cid:3039) (cid:3034)
If p ≫ p , then p can be neglected in this case.
1 g (cid:2917)
In the case of non-wetting cosθ < 0, according to equation (1.9),
h < 0, i.e. the liquid level should drop. In the case of complete
wetting (cosθ = 1), a simplified expression is obtained, which is used
in practice for small edge angles:
h  2σ , (1.10)
rgpg
The equation (1.9) provides the basis for the experimental mea-
surement of surface tension by the method of the greatest bubble
pressure. To find σ, you need to measure the pressure that must be
applied in order to form an air bubble from the capillary with radius r
lowered into the liquid being examined. To form a bubble, it is
2σ
necessary to overcome the capillary pressure ΔP on the surface
R
of the bubble that is concave from the liquid side. As the pressure in-
creases, this bubble grows, changing its shape and radius of
curvature.
In Fig. 1.2 it is shown that at the beginning (position 1) the bub-
ble has a large radius of curvature and its surface is almost flat. In
this case Р < ΔP. Then the radius of curvature decreases, the gas bub-
ble becomes more and more convex. At R = r (position 2), the pres-
sure inside the capillary is equal to the internal pressure Р = ΔP and
reaches its maximum value. Under these conditions, the pressure
on the walls of the bubble on the liquid side is equal to the gas phase.
With further increase in pressure, the radius of curvature again
begins to increase, the pressure from the side of the bubble wall falls,
it cannot balance the air pressure inside the bubble, so the bubble co-
mes to an unstable state – it rapidly expands and breaks away from
the capillary.
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