Condensation methods for obtaining dispersed systems. Obtaining dispersed systems. Methods for obtaining dispersed systems, their classification and brief description. Which method of obtaining dispersed systems is the most advantageous from the thermodynamic point of view
DISPERSION
Dispersion can be spontaneous and non-spontaneous. Spontaneous dispersion is characteristic of lyophilic systems. With regard to lyophobic systems, spontaneous dispersion is excluded, dispersion in them is possible by the cost of a certain amount of work.
Dispersion is characterized by the degree of dispersion ( a ) ... It is determined by the ratio of the sizes of the initial product and the particles of the dispersed phase of the resulting system.
a = d n / d k, (7.1)
d n, dк - particle diameter before and after grinding.
Work W required to disperse a solid or liquid is spent on deforming the body W e and on the formation of a new interface W a, which is measured by the work of adhesion. Deformation is a necessary prerequisite for the destruction of the body. The dispersion work is determined by the formula:
W = W but + W d = * DB + kV (7.2)
* - a value proportional to or equal to the surface tension at the interface,
DB- an increase in the interface of phases as a result of dispersion,
V- the volume of the original body before dispersion,
kis a coefficient equivalent to the work of deformation of a unit volume of a body.
Using methods colloidal chemistry the energy required for dispersion can be reduced. These methods include adsorptive strength reduction. As a result of surfactant adsorption on the outer and inner surfaces of the solid, the interfacial surface tension decreases, and the deformation of the solid is facilitated.
Reducing the energy of dispersion can be achieved by the following methods: carrying out the process in a liquid medium, grinding with simultaneous vibration, the use of an ultrasonic method.
OBTAINING DISPERSED SYSTEMS DUE TO CONDENSATION PROCESSES
Condensation methods : condensation, desublimation, crystallization. They are based on the formation of a new phase under conditions of a supersaturated state of matter in a gaseous or liquid medium. In this case, the system goes from homogeneous to heterogeneous. Condensation and desublimation are typical for a gas medium, and crystallization for a liquid medium.
Necessary condition for condensation and crystallization - supersaturation and uneven distribution of matter in a dispersion medium and the formation of condensation centers (embryos).
Supersaturation b for solution and steam can be expressed as follows:
b w = s / s s, b n = p / p s (7.3)
р, с - pressure of supersaturated vapor and concentration of a substance in a supersaturated solution, р s- equilibrium pressure of saturated vapor over a flat surface, s s- equilibrium solubility corresponding to the formation of a new phase.
The smallest particles contribute to condensation. For example, the products of combustion of aircraft fuel, soil particles, etc. can serve as nuclei of condensation of water vapor.
When condensation nuclei are absent, the droplets can exist in a supercooled state. When vapors condense under these conditions, not drops, but crystals will form. The process of transition of a gaseous substance into a solid, bypassing the liquid state, is called desublimation.
Sublimation -the transition of a solid to a gaseous, bypassing liquid.
Condensation methods are based on spontaneous processes, which are accompanied by a decrease in the Gibbs energy.
During the nucleation and formation of particles from a supersaturated solution or gas phase, the chemical potential changesm, a phase interface appears, which becomes a carrier of excess free surface energy.
Condensation is physical and chemical.
Physical condensation - carried out with a decrease in the temperature of the gaseous medium containing vapors various substances.
Isothermal distillation : decrease in the size of small particles until they disappear completely and growth of large particles.
MEMBRANES AND MEMBRANE PROCESSES
Membranes- semi-permeable partitions, with the help of which osmosis is carried out. Osmosis- a spontaneous process of transfer of a solvent (dispersion medium) through a membrane from a solvent (less concentrated or colloidal solution) into a solution (or into a more concentrated solution).
Membranes are thin porous films; they are two-dimensional highly dispersed systems.
Most often, membranes are used to purify liquids from impurities using reverse osmosis(movement of dissolved impurities through the membrane under the influence of external pressure)
Figure 7.2. Scheme of osmosis (a), reverse osmosis (b), ultrafiltration and dialysis (c)
1.dispersive medium (pure liquid) F, 2.colloidal S / F or true solution, 3.membrane, 4.flow of pure liquid (solvent), 5.flow of impurities.
If pressure P is applied from the side of dispersed system 2, then the liquid flow from region 2 will move to region 1. Only solvent molecules pass through membrane 3 (due to their greater mobility). The contents of area 1 will be enriched with pure liquid, and impurities will concentrate in area 2.
The direction of movement of a liquid in reverse osmosis is opposite to its movement in the case of osmosis.
The work required to carry out reverse osmosis is spent forcing the liquid through the pores:
W os = D R V (7.4)
Dp is the pressure drop on both sides of the membrane,
V- the volume of liquid that has passed through the membranes.
D p = P - p (7.5)
P - excess pressure above the solution,
p- osmotic pressure.
Equality 7.5 implies that P> p... This condition determines the overpressure required for reverse osmosis.
With help dialysis(7.2, c) the dispersed system is purified from impurities in the form of ions or molecules. The dispersed system is placed in the right part 2 of the vessel, separated from the left part 1 by membrane 3. The membrane is permeable to molecules and ions, but retains particles of the dispersed phase. Impurities as a result of diffusion from the area of higher concentration 2 will spontaneously move to the area of lower concentration 1.
It is possible to intensify the purification of colloidal solution using dialysis by applying external pressure p (7.2, c). In this case, the process is called ultrafiltration.
Reverse osmosis, dialysis, ultrafiltration are used for various purposes, but they have much in common; similar equipment is used.
The basic principle of membrane action is selective permeability, which is determined by the pore size, the properties of the systems to be cleaned, and external pressure.
In addition to cleaning solutions, membranes promote the equilibrium of electrolytes in the presence of particles or ions, the size of which does not allow them to penetrate through the pores, the so-called membrane equilibrium, which is of practical importance for IUD solutions, in the processes of swelling of substances and in various physiological processes.
Membrane technology is much more efficient than other similar technologies and requires less energy consumption.
CAUSE OF MOLECULAR-KINETIC PROPERTIES
All molecular kinetic properties are caused by the chaotic thermal motion of the molecules of the dispersion medium, which consists of the translational, rotational and vibrational motions of the molecules.
Molecules have different kinetic energies. However, at a given temperature, the average kinetic energy molecules remains constant. Fluctuation of the kinetic energy of the molecules of the dispersion medium is the cause of the molecular kinetic properties.
Molecular kinetic properties are manifested in liquid and gaseous dispersion media.
BROWNIAN MOTION
The smallest particles of insignificant mass experience unequal impacts from the molecules of the dispersion medium, the figure shows the resulting force F which makes the particles move.
Figure 7.3 The action of the molecules of the dispersion medium on the particle of the dispersed phase.
The direction and momentum of this force is constantly changing, so the particles are in a chaotic motion.
Einstein and Smoluchowski succeeded in determining the direction of the resulting force and connecting it with the molecular-kinetic properties of the medium in 1907 independently of each other.
Their calculations were based not on the true path of the particles, but on the shift of the particles (Figure 7.4).
The path of the particle is defined by a broken line, and the shift NS characterizes the change in the coordinate of the particle over a certain period of time. The average shift will determine the root mean square displacement of the particle:
(7.6)
x 1, x 2, x i- particle shifts over a certain time.
The theory of Brownian motion is based on the concept of the interaction of a random force f( t) , which characterizes the impacts of molecules and, and forces Ft, which depends on the time and the friction force when the particles of the dispersed phase move in the dispersion medium at a speed v. Brownian motion equation(Langevin equation) has the form:
m(dv/ dt) + hv = Ft + f( t) (7.7)
where m- particle mass,h- coefficient of friction when particles move.
For long periods of time, the inertia of the particles, that is, the term m(dv/ dt) can be neglected. After Integration 7.7. provided that the average product of impulses of random force is equal to zero, find the average shift:
(7.8)
where t- time, h- the viscosity of the dispersion medium, r is the radius of the dispersed phase particles.
Brownian motion is most pronounced in high dispersed systems... Clarification of the reasons and development of the theory of Brownian motion is a brilliant proof of the molecular nature of matter.
DIFFUSION
Diffusion- the process of spontaneous spread of a substance from an area with a higher concentration to an area with a lower concentration.
Diffusion types:
1. molecular;
2. ionic;
3. diffusion of colloidal particles.
Ionic diffusion is associated with the spontaneous movement of ions. The formation of a diffuse layer of counterions on the surface of the dispersed phase particles occurs by the mechanism of ion diffusion.
The diffusion of highly dispersed colloidal particles is shown in Fig. 7.5.n 1 > n 2 ... That is, diffusion goes from bottom to top. Diffusion is characterized by a certain speed of movement of the substance through the cross section B, which is equal to dm/ dt.
On distance Dx the concentration difference will ben 2 - n 1 , this value is negative.
dn/ dx - concentration gradient.
Material movement speed:
dm = – D·( dn/ dx) · Bdt (7.9)
Dis the diffusion coefficient.
Equation 7.9 - basic diffusion equation in differential form. It is valid for all types of diffusion. In integral form, it is applicable for two processes: stationary and non-stationary.
For a stationary process, the concentration gradient is constant. Integrating 7.9., We get:
m = – D(dn/ dx) Bt- Fick's first law (7.10)
The physical meaning of the diffusion coefficient : if- dn/ dx= 1, B = 1, t= 1, then m = D, that is, the diffusion coefficient is numerically equal to the mass of the diffusing substance when the concentration gradient, the cross-sectional area of the diffusion flow, and time are equal to one.
Colloidal particles are characterized by a minimum diffusion coefficient.
Diffusion is quantified diffusion coefficient which is related to the average shift:
x -, 2 = 2 Dr, r= x -, 2 / (2 Dt) (7.11)
D= kT/ (6 phr) (7.12)
k= R/ N BUT .
It can be seen from this formula that the diffusion coefficient also depends on the shape of the particles, thus, knowing the diffusion coefficient, one can determine the size of the particles of the dispersed phase.
OSMOSIS
When two solutions of different concentrations are separated by a semi-permeable partition, a solvent flow occurs from a lower concentration to a higher one. This process is called osmosis.
1 - a vessel with a solution, 2 - a container with a clean liquid, 3 - a semi-permeable septum (membrane).
Thermodynamic explanation of osmosis:
Chemical potential of a pure liquidm 2 exceeds the chemical potential of the same liquid in solutionm 1 The process goes spontaneously towards a lower chemical potential until the chemical potentials are equalized.
As a result of the movement of liquid in container 1, excessive pressure is createdpcalled osmotic... The solvent entering region 1 raises the liquid level to a height H, which compensates for the pressure of the pure solvent.
Osmotic pressure - excess pressure above the solution, which is necessary to exclude the transfer of the solvent through the membrane.
The osmotic pressure is equal to the pressure that the dispersed phase would produce if it in the form of a gas at the same temperature occupied the same volume as the colloidal system (solution). Osmotic pressure arises spontaneously as a consequence of the molecular-kinetic properties of the dispersion medium.
Osmotic pressure for ideal non-electrolyte solutions:
pV = RTln(1 – x) (7.13)
Vis the molar volume of the solvent, x is the molar fraction of the solute.
In the case of dilute solutions of non-electrolytes:
pV = nRT (7.14)
where n- the number of moles of the solute.
If the mass of the solute = q, mass = M, then n = q/ M, then:
p = n(RT/V) = (q/V)(RT/V)(7.15)
M = mN BUT, m = 4/3 pr 3 r (7.16)
r- particle density, m- molecular weight of dispersed phase particles, r- the radius of the particles of the dispersed phase.
Then:
(7.17)
It follows from this formula that the osmotic pressure is directly proportional to the concentration of the dispersed phase and inversely proportional to the size of these particles.
The osmotic pressure of colloidal solutions is negligible.
SEDIMENTATION
Sedimentation- sedimentation of particles of the dispersed phase, reverse sedimentation - floating of particles.
Each particle in the system is affected by the force of gravity and the buoyant force of Archimedes:
F g = mg= vgr and F A = vgr 0 (7.18)
where r, r 0 - the density of the particles of the dispersed phase and the dispersion medium, m- particle mass, v- particle volume, g- acceleration of gravity.
These forces are constant and directed in different directions. The resultant force causing sedimentation is equal to:
F sed = F g -F A = v( r - r 0 ) g (7.19)
If r> r 0 , then the particle settles, if on the contrary, then floats up.
With the laminar motion of a particle, resistance arises - the friction force:
F tr = B u (7.20)
B - coefficient of friction, u- the speed of the particle.
Force acting on a particle during movement:
F = F gray - F tr = vg(r - r 0) - B u (7.21)
With an increase in speed with a sufficiently large coefficient of friction, a moment comes when the friction force reaches the force causing sedimentation and the driving force will be equal to zero. After that, the speed of the particle becomes constant:
u = vg(r (7.23)
Knowing the quantities included in the equation, one can easily find the radius of the particles of the dispersed phase.
The sedimentation capacity is expressed in terms of sedimentation constant:
S sed = u/g (7.24)
The phenomenon of sedimentation is widely used in various industries, including often used for the analysis of dispersed systems.
Sol is a dispersed system with a solid-particle dispersed phase. Aerosol corresponds to a gaseous dispersed medium, and lyosol (hydrosol) - to a liquid dispersed medium.
Dispersion of liquids is usually called atomization if it occurs in the gas phase, and emulsification, when it is carried out in another liquid that is immiscible with the first.
Dispersion- fine grinding of solids or liquids, resulting in powders, suspensions, emulsions ( emulsification, or emulsification). When solid bodies are dispersed, their mechanical destruction occurs.Dispersion methods
– mechanical dispersion- carried out under the influence of external mechanical work. Methods: abrasion, crushing, splitting, spraying, bubbling (passing a stream of air through a liquid), shaking, explosion, action of sound and ultrasonic waves. This method produces flour, powdered sugar, cocoa powder, spices, ground coffee and others. The size of the particles obtained by this method, efficiency. quite large, at least 100 nm. Equipment: mortars, mills, crushers of various types, millstones.
To improve efficiency, mechanical dispersion is carried out in a liquid medium. Liquids (solutions of surfactants, electrolytes), wetting a solid, are adsorbed on it and reduce the strength during mechanical processing. This is called adsorptive degradation of solids or Rebinder effect(founded in 1982 by P.A.Rebinder).
– electrical dispersion- based on the formation of a volt arc between the electrodes of the sprayed metal, placed in a cooled DS. Metals evaporate at the temperature of the volt arc and then condense in a cold DS. This method is mainly used to obtain metal hydrosols (the dispersion medium is water), for example, silver, gold and platinum.
– ultrasonic dispersion- based on exposure to ultrasonic vibrations with a frequency of more than 20 thousand per 1 sec., Not captured by the human ear, is effective only for substances with low strength. These include sulfur, graphite, starch, rubber, gelatin, etc.
Physical and chemicaldispersing refers method peptization. It consists in transferring freshly prepared loose sediments into a colloidal solution under the action of special stabilizing additives (peptizing agents - electrolytes, surfactant solutions). The action of the peptizer is that the sediment particles are separated from each other and become suspended, forming a sol. This method can be used to obtain, for example, a hydrosol of iron hydroxide (III). The peptization method can only be used for freshly prepared sediments, since during storage, recrystallization and aging processes occur, leading to the adhesion of particles with each other. Particle sizes obtained by this method are about 1 nm.In terms of particle size, highly dispersed systems - sols - occupy an intermediate position between coarsely dispersed systems and true solutions (atomic-molecular dispersion of a solute). Therefore, the methods of obtaining such systems can be conventionally divided into dispersion - crushing of large particles to particles of colloidal size and condensation - the combination of atoms, molecules or ions into larger particles.
Dispersion- fine grinding of a solid or liquid, as a result of which dispersed systems are formed: powders, suspensions, emulsions, aerosols. Dispersion of a liquid in a gaseous medium is called spraying, dispersion of another liquid, immiscible with the first, - emulsification... During the dispersion of solids, their mechanical destruction occurs, for example, using mills of various types. The fragmentation of a substance can also occur under the action of ultrasound.
Dispersion conditionally includes the method peptization... It consists in transferring freshly prepared loose sediments into a colloidal solution under the action of special stabilizing additives - peptizing agents (electrolytes, surfactant solutions). The peptizer promotes the separation of sediment particles from each other
from a friend and their transition into a suspended state with the formation of a sol.
Condensation- the process of formation of a dispersed phase from substances in a molecular or ionic state. Necessary requirement with this method - the creation in a dispersion medium of a supersaturated solution (above the solubility limit) of a dispersible substance, from which a colloidal system should be obtained. This can be achieved under certain physical or chemical conditions.
Physical condensation - condensation of vapors of a substance when the equilibrium vapor pressure is exceeded as a result of changes in temperature or pressure, for example, the formation of fog - liquid droplets in gas. The addition of a liquid to the solution that mixes well with the solvent but is a poor solvent for the solute leads to the formation of a sol (solvent replacement).
Electrical dispersion... An electric arc is created between the sputtered metal electrodes placed in a cooled dispersion medium. Metals evaporate at high temperatures and then condense in a cold dispersion medium. This method is mainly obtained by metal hydrosols, for example by dispersing silver, gold and platinum in water.
Chemical condensation. Chemical condensation can be based on exchange, redox reactions, hydrolysis, etc., as a result of which an insoluble substance is formed, which is precipitated from a supersaturated solution.
test questions
1. Disperse systems - signs, main characteristics, properties.
2. Classification of dispersed systems by state of aggregation and size.
3. Free and connected dispersed systems.
4. Methods for obtaining dispersed systems.
Surface phenomena
Surface phenomena are associated with spontaneous processes leading to a decrease in the energy of the system (Δ G =
= Δ H − TΔ S + σ S) mainly due to a decrease in the surface tension (σ) of the condensed phase. These include adsorption, adhesion, wetting, capillary phenomena.
Adsorption
Adsorption- an increase in the concentration of a substance at the interface as a result of spontaneous redistribution of system components between the volume of the phase and the surface layer. Distinguish between the adsorption of solute molecules by the surface of a liquid solution and the adsorption of absorption of gases or liquids by the surface of a solid.
2.1.1. Solute adsorption
solution surface
In the volume of the solution, the molecules of the solute are evenly distributed. Depending on their effect on the surface tension of the solvent, the surface concentration of the solute may differ from the volume concentration.
With a decrease in the surface tension of the solvent with an increase in the concentration of the solute (Fig. 2.1), its surface concentration increases - adsorption occurs. Such substances are called surface-active(Surfactant). If the surface tension increases, the surface concentration decreases accordingly. Such substances are called surface-inactive(PIV), derivative - surface activity... Substances for which - surface-inactive (PNV). The surface activity of a substance depends on the solvent. One and the same substance can be surfactant for one solvent and surfactant for another.
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Rice. 2.1. Dependence of surface tension at the "solution-gas" interface
on the concentration of the solute
For water, surfactants are substances whose molecules have a diphilic structure, i.e. contain hydrophobic and hydrophilic groups of atoms. The hydrophobic part is usually a non-polar hydrocarbon radical CH 3 - (CH 2) n-, with a relatively long chain length. The hydrophilic part is a polar group, for example the functional groups of carboxylic acids - COOH; sulfonic acids - SO 2 OH; amines - NH 2; ethers - O- and others.
Hydrophilic groups ensure the solubility of surfactants in water, and hydrophobic ones - in non-polar media. In the adsorption layer, surfactant molecules are oriented in an energetically favorable way: hydrophilic groups - towards a polar medium (water), and hydrophobic ones - towards a non-polar medium (gas, hydrocarbon) (Fig. 2.2).
Distinguish between ionic and non-ionic surfactants. The former in solution dissociate into ions, one of which is surface-active (anionic and cationic surfactants). The latter do not dissociate.
All inorganic soluble substances (acids, alkalis, salts) are surface-inactive (SIS) relative to water. Examples of inactive surfactants (PNS) can be glucose, sucrose.
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Rice. 2.2. Orientation of surfactant molecules on the surface of an aqueous solution
Solid adsorption
Upon contact of a solid with a gas or liquid, adsorption occurs - the absorption of substances by the surface of the phase. A solid with a large specific surface area (for example, microporous bodies) is called adsorbent(AD). The absorbed substance, which is in the gas or liquid phase, is called the adsorptive (S), and after it has passed into the adsorbed state, it is called the adsorbate (ADS) (Fig. 2.3). The reverse process of the transition of a substance from the surface layer into the volume of the gas or liquid phase is called desorption.
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Rice. 2.3. Adsorption process diagram
By the nature of the forces that hold the adsorptive molecules on the surface of a solid, adsorption is generally divided into two main types: physical adsorption and chemical (chemo-sorption).
Physical adsorption is determined by the forces of intermolecular interaction (van der Waals forces). The main contribution is made by dispersion forces, which do not depend on the nature of the adsorbed molecules; orientational and induction forces can play a certain role. The interaction energy is relatively low - 8 ... 25 kJ / mol. Physical adsorption forces have the property of long-range action, although they rapidly decrease with distance (~ 1 / r 6). Physical adsorption is a spontaneous process (Δ G < 0), экзотермический (ΔH< 0), с уменьшением энтропии (ΔS < 0), так как сопровождается упорядочение системы. Поэтому количество сорбируемого вещества при физической адсорбции растет с уменьшением температуры. Соответственно десорбция происходит при относительно high temperatures.
Chemical adsorption (chemisorption) is associated with the formation of strong chemical bonds. When a substance is absorbed by a surface, the electron density is redistributed with the formation of a chemical bond, i.e. a chemical reaction occurs between the sorbent and the sorbent at the interface. During chemisorption, the adsorbed substance is localized on the surface of the adsorbent. The energy of interaction is about an order of magnitude higher than that of physical sorption. Chemical sorption can be efficient at high temperatures. The absorption capacity varies greatly depending on the nature of the interacting substances.
The sorption capacity of an adsorbent is characterized by a value equal to the amount of adsorbate (mol, g, etc.) absorbed by a unit of surface (surface concentration). It is called adsorption (G) and is measured, respectively, in mol / cm 2; g / cm 2, etc. Specific adsorption - the amount of adsorbate sorbed by a unit of mass of the adsorbent (mol / g; eq / g, etc.).
Equilibrium adsorption depends on nature
sorbent and sorbed substance. In addition, it depends on the molar concentration of the sorbed substance ( C) or the partial pressure of the sorbed gas ( R), as well as from temperature
tours ( T):
G = f(C, T); G = f(p, T).
For a process carried out at a constant temperature, the dependence Г = f(C) is called isotherm of adsorption.
One of the models describing the adsorption process is the Langmuir monomolecular adsorption model based on the following assumptions:
- adsorbate molecules fill the surface of the adsorbent in one layer, forming monomolecular layer(monolayer);
- the surface of the sorbent is uniform;
- the sorbed molecules are immobile.
The adsorption process can be represented as a quasi-chemical reaction between the molecules of the sorbed substance, the concentration of which is C, and sorption centers AD on the surface of the adsorbent:
The equilibrium state of the reaction is characterized by an equilibrium constant, which in this case is called the sorption constant ( TO with).
- the concentration of the sorbed substance on the surface of the sorbent is equal to adsorption - = G (C);
- the concentration of sorption centers on the surface - Г ¥, in the case of sorption in one layer, it corresponds to the maximum number of molecules that can be sorbed (monolayer capacity);
- the number of free places on the surface of the sorbent - =
= Г ¥ - Г ( WITH);
- the concentration of the sorbed substance in the volume of liquid or gas - [S] = C.
Consequently, 
and correspondingly,
; 
.
This equation is called Langmuir adsorption isotherm. It represents the dependence of the amount of a substance absorbed by the adsorbent at a constant temperature on the concentration in the liquid ( WITH) or partial pressure in the gas ( p) (Fig. 2.4).
At low concentrations ( K c C<< 1) количество вещества, поглощенного сорбентом, растет линейно с ростом концентрации. При больших концентрациях (K c C>> 1), Г ( WITH) = Г ¥ the sorbent surface is completely occupied by the molecules of the sorbed substance. The amount of absorbed substance is equal to G ¥ and does not depend on the concentration of the sorbed substance in the volume of liquid or gas. The quantity Г ¥ is called sorption capacity and characterizes the maximum possible amount of a substance that the sorbent can absorb.
In the sorption of vapors of a substance by porous adsorbents, the process of monomolecular adsorption can turn into capillary condensation... At the first stage, vapor molecules fill the surface of the pore walls (capillaries) in one layer, then the number of layers increases, a liquid phase is formed, which fills the pore volume. The adsorption isotherm in this case is S-shaped. At low pressures, the curve is the Langmuir adsorption isotherm, and when approaching the value of the limiting sorption, it rises sharply, the process turns into capillary condensation (Fig. 2.5).
Solid porous adsorbents are widely used in various fields to remove unwanted impurities from gases and liquids - purification of substances. For example, in a filtering gas mask, poisonous gases are removed from the air.
Here are some examples of porous adsorbents.
Active carbons- porous carbon adsorbents, which are obtained by thermal treatment of organic raw materials (for example, wood materials) without air access, followed by physicochemical treatment to create the required microporous structure. The surface of coal sorbents is electrically neutral, and adsorption is mainly determined by dispersive interaction forces. Active carbons absorb well non-polar substances from the gas phase and aqueous solutions... They have a specific surface area of up to 1000 m 2 / g.
Depending on the purpose, coal sorbents are subdivided into gas, recovery and clarifying coals. Gas coals are designed to trap poorly sorbed substances contained in gases in low concentrations, as well as to purify water from impurities of substances with a small molecular size, in particular, deodorize drinking water. Recuperative coals designed to capture vapors of organic solvents from the air. Lightening coals serve for the absorption of relatively large molecules and microsuspensions from a liquid medium, in particular, they are used for pharmaceutical purposes and for clarifying food products.
Silica gel- mineral adsorbent (hydrated amorphous silica), formed by spherical particles 10 ... 100 nm in size, which are linked together, forming a rigid silicon-oxygen framework. Specific surface area 300 ... 700 m 2 / g. The adsorption properties of silica gel are largely determined by the surface Si-OH groups. It is usually used for the absorption of water vapor from gases (desiccant) and organic solvents, for the adsorption purification of non-polar liquids.
Alumogel- active alumina, which is obtained by calcining aluminum hydroxide (). It is a hydrophilic adsorbent with a highly developed porous structure. It is used for drying gases, for cleaning transformer oils, gases and liquids containing fluorine compounds.
Zeolites- crystalline framework aluminosilicates,
containing ions of alkali and alkaline-earth metals (). The main "building block" for the creation of various forms of natural and synthetic zeolites is the crystal structure, which is a cuboctahedron, the volume of which is the adsorption cavity. On the hexagonal faces there are “entrance windows” into the adsorption cavities, the size of which is strictly fixed and depends on the parameters of the crystal lattice. Depending on the brand of synthetic zeolites, the diameter of the entrance windows can be from 2 to 15 Å. Therefore, zeolites can be used to separate substances not only on the basis of selective adsorption, but also on the basis of the difference in the size of the molecules - molecular sieves.
Note. The adsorption of different substances by the same sorbent is not the same. This property is based on the method of separation of a mixture of gases, vapors, liquids or solutes, called chromatography... Passing a mixture of gases or a solution (mobile phase) through a fixed bed of adsorbent, it is possible to separate the mixture into individual substances.
Colloidal systems in terms of the degree of dispersion occupy an intermediate position between true solutions (molecular or ionic dispersed systems) and coarsely dispersed systems. Therefore, there are two groups of methods for obtaining dispersed systems: Group 1 - dispersion, i.e. grinding of particles of a dispersed phase of coarsely dispersed systems, group 2 is based on aggregation (condensation) processes, in which molecules under the action of cohesion forces combine and give first a nucleus of a new phase, and then real particles of a new phase
Another necessary condition obtaining sols, in addition to bringing the particle size to colloidal, is the presence in the system of stabilizers - substances that prevent the process of spontaneous enlargement of colloidal particles.
Dispersion methods
Dispersion methods are based on crushing solids to colloidal particles and thus forming colloidal solutions. The dispersion process is carried out by various methods: mechanical grinding of the substance in colloidal mills, electric arc spraying of metals, crushing of the substance using ultrasound.
Condensation methods
A substance in a molecularly dispersed state can be converted into a colloidal state by replacing one solvent with another - those. by replacing the solvent. An example is the preparation of a rosin sol, which is insoluble in water, but readily soluble in ethanol. With the gradual addition of an alcoholic solution of rosin to water, a sharp decrease in the solubility of rosin occurs, as a result of which a colloidal solution of rosin in water is formed. Similarly, a sulfur hydrosol can be prepared.
Colloidal solutions can also be obtained by the method chemical condensation, based on carrying out chemical reactions, accompanied by the formation of insoluble or slightly soluble substances. For this purpose are used Various types reactions - decomposition, hydrolysis, redox, etc. So, a red gold sol is obtained by reducing the sodium salt of golden acid with formaldehyde:
NaAuO 2 + HCOH + Na 2 CO 3 ––> Au + HCOONa + H 2 O
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The first law of thermodynamics is a postulate. This means that this law cannot be proven logically, but follows from the sum of human experience. Just
The first law of thermodynamics under isobaric, isochoric, isothermal and adiabatic conditions for ideal gas systems
The equation of the first law of thermodynamics, as already mentioned above, for isobaric (p = const) conditions in ideal gas system has the form: QP = DН = DU + р
Hess's law. Consequences from Hess's Law
Thermochemistry is a branch of physical chemistry that studies the thermal effects of chemical reactions. The thermal effect of a chemical reaction is the heat that
Standard thermal effects
For the convenience of comparing thermal effects, as well as other thermodynamic functions, the concept of the standard state of matter is introduced. For solids and liquids as standard with
The first consequence of Hess's law
This consequence is associated with the heats of formation of compounds. The heat (enthalpy) of formation of a compound is the amount of heat released or absorbed during the formation of 1 mol
Second corollary from Hess's law
In some cases, it is more convenient to calculate the heat effect of a reaction from the heats (enthalpies) of combustion of the substances participating in the reaction. The heat (enthalpy) of combustion of a compound is called those
Kirchhoff's equation. Dependence of the thermal effect of the reaction on temperature
Differentiating with respect to temperature (at constant pressure) the equality DН = Н2 - Н1, we obtain ¶ (
The concept of entropy. Statistical thermodynamics and the physical meaning of entropy
All processes occurring in nature can be divided into spontaneous and non-spontaneous. Spontaneous processes proceed without the expenditure of energy from the outside; for pro
Change in entropy as a criterion for the spontaneous course of a process in an isolated system
Spontaneous processes take place without external energy consumption. The spontaneous course of the process is associated with irreversibility. Irreversible in thermodynamics
Planck's postulate. (Third Law of Thermodynamics)
Unlike internal energy and enthalpy, absolute values can be determined for entropy. This possibility appears when using the Planck postulate, which
Thermodynamic potentials
The mathematical apparatus of thermodynamics is based on the combined equation of the first and second laws of thermodynamics for reversible processes: dU = T d
Gibbs energy change in chemical reactions
DG calculation for chemical processes can be done in two ways. The first method uses the relation (27): DG = D
Chemical potential
Consider systems in which the amounts of substances change. These changes can occur as a result of chemical reactions or phase transitions. At the same time, change
Gibbs Phase Rule
Component - a chemically homogeneous substance contained in the system that can be isolated from the system and can exist in an isolated form for a long time
One-component systems
At kн = 1, the equation of the phase rule takes the form: С = 3 - Ф, If there is 1 phase in equilibrium, then С = 2, say
Phase diagram of water
The phase diagram of water in p - T coordinates is shown in Fig. 8. It is composed of 3 phase fields - regions of different (p, T) -values, at which
Phase diagram of sulfur
Crystalline sulfur exists in the form of two modifications - rhombic (Sр) and monoclinic (Sм). Therefore, it is possible that there is
Clausius - Clapeyron equation
Movement along the lines of two-phase equilibrium in the phase diagram (C = 1) means a consistent change in pressure and temperature, i.e. p = f (T). The general view of such a function for one-component
Evaporation entropy
The molar entropy of evaporation DSevap = DHevap / Tboil is equal to the difference Svap - Sliquid. Since Sп
Chemical equilibrium
Thermodynamic equilibrium is a state of the system, the characteristics of which (temperature, pressure, volume, concentration) do not change with time at constant
The law of action of the masses. Equilibrium constants
The quantitative characteristic of chemical equilibrium is the equilibrium constant, which can be expressed in terms of the equilibrium concentrations of Ci,
Isobar and isochore of a chemical reaction
To obtain the dependence of the equilibrium constant Кр on temperature, we use the Gibbs-Helmholtz equation:
Thermodynamics of solutions
The existence of absolutely pure substances is impossible - any substance necessarily contains impurities, or, in other words, any homogeneous system is multicomponent. Solution - homogeneous system
Formation of solutions. Solubility
The concentration of a component in solution can vary from zero to a certain maximum value, called the solubility of the component. Solubility - the concentration of a component in a saturated
Solubility of gases in liquids
The solubility of gases in liquids depends on a number of factors: the nature of the gas and liquid, pressure, temperature, concentration of substances dissolved in the liquid (especially the strength
Mutual solubility of liquids
Depending on the nature, liquids can be mixed in any ratio (in this case, they speak of unlimited mutual solubility), they can be practically
Solubility of solids in liquids
The solubility of solids in liquids is determined by the nature of the substances and, as a rule, depends significantly on temperature; solubility information for target solids
The relationship between the composition of a liquid solution and a vapor. Konovalov's laws
The relative content of the components in the vapor, as a rule, differs from their content in the solution - the vapor is relatively richer in the component, the boiling point of which is lower. This fact
Saturated vapor pressure of dilute solutions. Raoult's law
Imagine that in equilibrium system liquid A - vapor A introduced some substance B. When the solution is formed, the molar fraction of the solvent XA of the mills
Deviations from Raoult's Law
If both components of a binary (consisting of two components) solution are volatile, then the vapor above the solution will contain both components. Consider a binary solution, sos
Crystallization temperature of dilute solutions
A solution, unlike a pure liquid, does not completely solidify at a constant temperature. At a certain temperature, called the temperature of the onset of crystallization
Boiling point of dilute solutions
The boiling point of solutions of a non-volatile substance is always higher than the boiling point of a pure solvent at the same pressure. Consider a p - T diagram with
Solute activity concept
If the concentration of the solute does not exceed 0.1 mol / L, then the non-electrolyte solution is usually considered dilute. In such solutions, the interaction between the molecule
Colligative properties of solutions
Some properties of solutions depend only on the concentration of dissolved particles and do not depend on their nature. Such properties of the solution are called colligative. At the same time, sales
The theory of electrolytic dissociation. Dissociation degree
Electrolytes are substances whose melts or solutions conduct electric current due to dissociation into ions. To explain the features of the properties of electrolyte solutions, S. Arrhenius proposed
Weak electrolytes. Dissociation constant
The dissociation of weak electrolytes is reversible. A dynamic equilibrium is established in the system, which can be quantitatively estimated by the constant pa
Strong electrolytes
Strong electrolytes in solutions of any concentration completely dissociate into ions and, therefore, the regularities obtained for weak electrolytes cannot be applied to strong electrolytes b
Electrical conductivity of electrolyte solutions
Electricity there is an ordered movement of charged particles. Electrolyte solutions have ionic conductivity due to the movement of ions in electricity
Electric potentials at phase boundaries
When a metal electrode (a conductor with electronic conduction) comes into contact with a polar solvent (water) or an electrolyte solution, a double
Galvanic cell. EMF of a galvanic cell
Consider the simplest Daniel-Jacobi galvanic cell, consisting of two half-cells - zinc and copper plates, placed in solutions of zinc and copper sulfates, respectively, which are connected
Electrode potential. Nernst equation
The EMF of a galvanic cell E is conveniently represented as the difference of some quantities characterizing each of the electrodes - electrode potentials; O
Reference electrodes
To determine the potential of the electrode, it is necessary to measure the EMF of a galvanic cell composed of the tested electrode and an electrode with a precisely known potential.
Indicator electrodes
Electrodes reversible with respect to a hydrogen ion are used in practice to determine the activity of these ions in a solution (and, therefore, the pH of a solution) by a potentiome
Redox electrodes
In contrast to the described electrode processes, in the case of redox electrodes, the processes of obtaining and returning electrons by atoms or ions occur
Chemical reaction rate
The basic concept of chemical kinetics is the rate of a chemical reaction. The rate of a chemical reaction is the change in the concentration of reactants per unit of time. Mathematic
Basic postulate of chemical kinetics
(the law of action of masses in chemical kinetics) Chemical kinetics is based on the basic postulate of chemical kinetics: The rate of a chemical reaction is directly proportional to
Zero order reactions
Let us substitute expression (71) into equation (74), taking into account that the calculation is carried out for the initial substance A (which determines the choice of the minus sign):
First order reactions
Let us substitute expression (71) into equation (75): Integration
Second order reactions
Let us consider the simplest case when the kinetic equation has the form (76). In this case, taking into account (71), we can write:
СН3СООС2Н5 + Н2О ––> СН3СООН + С2Н5ОН
If this reaction is carried out at close concentrations of ethyl acetate and water, then the general order of the reaction is two and the kinetic equation is as follows:
Methods for determining the order of reaction
To determine the partial orders of the reaction, the method of excess concentrations is used. It consists in the fact that the reaction is carried out under conditions when the concentration of one of the reagents varies greatly.
Parallel reactions
The starting materials can simultaneously form various reaction products, for example, two or more isomers:
Chain reactions
These reactions consist of a series of interconnected stages, whereby particles resulting from each stage generate subsequent stages. As a rule, chain reactions proceed with the participation of free
Van't Hoff and Arrhenius equations
The reaction rate constant k in equation (72) is a function of temperature; an increase in temperature tends to increase the rate constant. The first attempt to take into account the effect of temperature was made
Photochemical reactions
Overcoming the activation barrier during the interaction of molecules can be carried out by supplying energy to the system in the form of light quanta. Reactions in which particle activation
Catalysis
The rate of a chemical reaction at a given temperature is determined by the rate of formation of an activated complex, which, in turn, depends on the amount of energy
Michaelis equation
Enzymatic catalysis - catalytic reactions involving enzymes - biological catalysts of protein nature. Enzymatic catalysis has two characteristics.
Molecular kinetic properties of dispersed systems
Crushed particles are characterized by Brownian motion. It is the more intense, the smaller the particle diameter and the lower the viscosity of the medium. With a particle diameter of 3-4 μm, the Brownian motion is
Optical properties of colloidal systems
For colloidal systems a matte (usually bluish) glow is characteristic, which can be observed against a dark background when a beam of light is passed through them. This glow on
Adsorption. Gibbs equation
Adsorption is the phenomenon of spontaneous thickening in the surface layer of a mass of a substance that lowers its surface tension by its presence. Adsorption value (G, mol / m
Adsorption at the solid-gas interface
When gases are adsorbed on solids the description of the interaction of adsorbate molecules (a substance that is adsorbed) and an adsorbent (a substance that adsorbs) is very complex
Adsorption from solutions
Surfactants (Surfactants) Surfactants (surfactants) reduce surface tension. Surfactant molecules adsorbed at the water p border
Micelle formation
Like adsorption, the phenomenon of micelle formation is associated with molecular interactions of its polar molecules (parts of molecules) and hydrophobic linkages of the hydrocarbon chain. Higher
Electric double layer and electrokinetic phenomena
When considering the structure of the micelle, it was shown that an electric double layer (EDL) is formed on the surface of colloidal particles. The first theory of the DES structure was developed by Helmholtz and Perret
purpose of work: get acquainted with various methods of obtaining dispersed systems.
Brief theoretical introduction.
Methods for preparing dispersed systems can be divided into two groups: dispersion methods and condensation methods.
Dispersion methods are based on crushing large pieces of material to the required degree of dispersion. These methods are more often used to obtain suspensions and emulsions. Systems with particle sizes of 10 -6 - 10 -7 cm are obtained by condensation methods. Condensation methods are the combination of molecules or ions to the size of colloidal particles, resulting in the appearance of a phase boundary.
To obtain dispersed systems by any of these methods, the following conditions must be met:
a) insolubility or limited solubility of the dispersed phase in the dispersion medium;
b) the presence of a stabilizer in the system, which should ensure the stability of suspended particles and stop their growth.
Dispersion methods.
By exerting work against the molecular forces of adhesion, the desired degree of dispersion can be achieved in various ways.
1. Mechanical dispersion.
The method consists in vigorous and prolonged grinding, grinding or spraying the substance of the dispersed phase and mixing it with a liquid that serves as a dispersion medium. Large particles are crushed using mortars, colloidal mills, paint grinders. Pharmaceuticals, lubricants, food products are obtained by mechanical dispersion.
2. Dispersion by ultrasound.
The method is based on the use of ultrasonic vibrations (more than 20,000 vibrations per second). Dispersion using ultrasound is effective only for substances with low strength: sulfur, graphite, paints, starch, rubber, gelatin. Emulsions are very easily obtained by this method, for example, cocoa emulsions, high-quality creams, etc.
Condensation methods.
Condensation methods are based on the processes of formation of dispersed phase particles from substances in a molecular or ionic state. These processes can be both physical and chemical in nature.
Physical condensation.
1. Method of replacing the solvent.
The essence of the method lies in the fact that the solvent in which the substance dissolves, forming a true solution, is replaced by a solvent in which this substance is insoluble. For example, if an alcoholic solution of sulfur, phosphorus or rosin is poured into water, then the solution becomes saturated, condensation occurs, and particles of the dispersed phase are formed. This is because these substances are poorly soluble in a water-alcohol mixture.
2. Condensation during cooling of steam.
The most obvious example of vapor condensation is the formation of fog or smoke. Another example of the appearance of colloidal particles as a result of vapor condensation is the Wilson chamber used in nuclear physics.
Chemical condensation.
The production of dispersed systems by chemical condensation methods is reduced to the formation of molecules of insoluble substances as a result of a chemical reaction, followed by their enlargement to the size of colloidal particles. Chemical condensation methods are classified according to the type of chemical reaction underlying the preparation of the sol. Among the reactions, as a result of which, under appropriate conditions, substances in a colloidal state can form, include the reactions of oxidation, reduction, exchange, hydrolysis.
1. Oxidation reactions.
An example of an oxidative reaction is the oxidation of hydrogen sulfide in an aqueous medium:
H 2 S + O 2 = 2S + 2H 2 O
2. Reactions of exchange.
An example of such a reaction is the formation of an arsenic (III) sulfide sol:
As 2 O 3 + 3H 2 S = As 2 S 3 + 3H 2 O
3. Hydrolysis reactions.
Hydrolysis is most often used to obtain metal hydroxide sols:
FeCl 3 + 3H 2 O = Fe (OH) 3 + 3HCl
Peptization method.
Peptization is the process of transition into a colloidal solution of sediments formed during coagulation. Peptization can be caused by washing the coagulum with a solvent, as well as by exposure to peptizing agents (electrolytes, non-electrolytes, surfactants, high molecular weight compounds). Only freshly obtained sediments in which crystallization phenomena have not passed and the particles have not lost their individuality can be peptized.
Experimental part.
I. Physical condensation methods.
Test 1... Sulfur sol production by solvent replacement method.
Pour 10 ml of distilled water into a test tube, add 5 drops of a solution of sulfur in ethanol and vigorously mix the contents of the test tube. A transparent opalescent sol is formed. Sulfur is soluble in alcohol but insoluble in water. When alcohol is replaced with water, the molecules of the solute are combined into aggregates of colloidal sizes.
To observe the Faraday-Tyndall effect, a test tube with colloidal solution is placed in the path of the light beam of the projection lamp. Consider the tube at an angle of 90 ° to the direction of the incident beam.
