STRUCTURAL-MECHANICAL PROPERTIES OF DISPERSE SYSTEMS
| Parameter name | Meaning |
| Article topic: | STRUCTURAL-MECHANICAL PROPERTIES OF DISPERSE SYSTEMS |
| Rubric (thematic category) | Chemistry |
The formation of structures in colloidal and microheterogeneous systems is a consequence of the coagulation of these systems, and as the concentration of the dispersed phase increases, a wide “spectrum” of states passes through - from truly liquid (sols) through structured liquids, gels, to solid systems.
Colloidal and microheterogeneous systems with a liquid and solid dispersion medium, like all condensed systems, have certain mechanical properties - viscosity, often plasticity, elasticity and strength. These properties are associated with the structure of systems; therefore, they are called structural-mechanical properties, or rheological.
Colloidal and microheterogeneous disperse systems are divided into freely dispersed and coherently dispersed. If the dispersion medium is a liquid, then there are also transitional systems, the individual particles of which are connected with each other into loose aggregates, but do not form a continuous structure - structured liquids.
The type of system is greatly influenced by the concentration of the dispersed phase. Colloidal systems in which the particles are located at sufficiently large distances from each other and practically do not interact are called freely dispersed. By introducing a stabilizer into such a system, which prevents the convergence of particles and the manifestation of molecular forces between them, it is possible to significantly increase the critical concentration at which bonds arise between the elements of the structural network. It should be noted that in their properties such colloidal systems are very similar to ordinary liquids; their viscosity differs little from the viscosity of the dispersion medium and increases slightly with increasing content of the dispersed phase.
In connection dispersed systems ah the concentration of the dispersed phase can reach high values. Particles in such a system are connected to each other by intermolecular forces and, as a result, are not capable of mutual movement; they form a spatial network or structure. Cohesively dispersed systems have, to a certain extent, the properties solids– the ability to maintain shape, some strength, elasticity, elasticity. But due to the low strength of the connection between the individual elements of the structure, they are easily destroyed and these systems acquire the ability to flow.
Structured liquids are systems with a low concentration of the dispersed phase, but with a pronounced tendency of particles to stick together. They have intermediate properties; these systems are capable of flow, but they do not obey the laws of flow of ordinary unstructured liquids.
STRUCTURAL-MECHANICAL PROPERTIES OF DISPERSE SYSTEMS - concept and types. Classification and features of the category "STRUCTURAL-MECHANICAL PROPERTIES OF DISPERSED SYSTEMS" 2017, 2018.
Structure refers to the relative arrangement of body parts. The structure of dilute aggregation-stable lyosols is similar to the structure of true solutions. An increase in the concentration of particles leads to their aggregation and then to coagulation. The appearance of structure in disperse systems is always associated with the concept of coagulation. The formation of the structure goes through the following stages:
sol structured liquid gel solid systems.
Structuring leads to a change in the nature of the flow or complete solidification of the liquid and a change in all its properties. Dispersed systems acquire the ability to resist load, the nature of their flow changes, etc. Features of behavior various systems During flow and deformation, they are studied by rheology - the science of deformation and flow of bodies. The structural and mechanical properties of dispersed systems are studied by physical and chemical mechanics, which is a section of the course on surface phenomena and dispersed systems. Structural and mechanical properties include: viscosity, plasticity, elasticity and strength.
Freely dispersed state of lyosols. If the particles do not interact with each other and are able to move freely in a dispersion medium, then this state of lyosols is called freely dispersed. Freely dispersed systems flow like any liquid. Resistance to external pressure during flow is characterized by viscosity. But the nature of their flow differs from Newtonian fluids. Newtonian fluids are those that obey Newton's law. Newton discovered that force internal friction(F) equal in magnitude, but opposite in direction to the force applied from the outside, is proportional to the area of the layer (S) to which this force is applied and the change in deformation over time (strain rate dх/dτ):
F = η S(dх/dτ) = ηSγ
The proportionality coefficient η is called the viscosity coefficient or viscosity of the liquid. The ratio F/S = P is called shear stress. Freely dispersed systems and Newtonian fluids flow at any shear stress.
η = Р/γ. (XIII.1)
For Newtonian liquids, viscosity is a constant value at a given temperature and does not depend on shear stress (Fig. XIII.1).
Lyosols have a number of features. They do not obey Newton's law. The viscosity of the sol is always greater than the viscosity of the dispersion medium. Due to the presence of dispersed phase particles, the flow of sols is characterized by early turbulence (i.e., the Reynolds number Re for them is less than for Newtonian liquids). The viscosity of sols depends on the measurement method and the velocity gradient, i.e. is not a constant value. Therefore, colloidal systems are characterized by effective viscosity η*. Newton's law for them will be written in the form
P = η*γ. (XIII.2)
The dependence of the viscosity of freely dispersed sols on the concentration of the dispersed phase is described by various forms of the Einstein equation
(η – η о)/η о = Кφ;
η/ η o = 1+ Kφ;
η = η o (1+ Кφ),
where η is the viscosity of the sol;
η о – viscosity of the dispersion medium;
η/ η о – relative viscosity of the sol; K is a coefficient depending on the shape of the particles;
φ is the volume fraction of the dispersed phase (V dis) in the total volume of the system (V) (φ = V dis /V).
Figure XIII.1 - Dependence of strain rate on shear stress for Newtonian liquids (1) and freely dispersed lyosols (2).
For spherical particles with a volume fraction of the dispersed phase ≤6%, Einstein’s equation takes the form: η = η о (1+ 2.5φ), with a volume fraction of the dispersed phase ≤30% this equation is written in the form
η = η o (1+ 2.5φ +14.7 φ 2). (XIII.3)
The dependence of the sol viscosity on the concentration of the dispersed phase is shown in Fig.
Structuring of sols . With increasing concentration or as a result of coagulation, a spatial structure is formed in the sols. The structure is a spatial framework formed by dispersed phase particles that are interconnected. Such structured disperse systems are called connected dispersed systems. They are characterized by a new set of properties, showing strength, ductility, elasticity, and fragility.
Classification of structures according to P.A. Rebinder . Depending on the nature of the forces acting in a structured system, P.A. Rebinder proposed to distinguish between two main types of structures: coagulation (reversibly collapsing) and condensation-crystallization (irreversibly collapsing).
Figure XIII.2 - Dependence of the viscosity of a Newtonian liquid (curve 1), freely dispersed lyosol (curve 2), structured sol (curve 3) on the concentration of the dispersed phase
Coagulation structures arise as a result of the loss of aggregative stability of the system and the interaction of particles in the far energy minimum of the energy curve. In this case, the particles do not stick together completely, but only weakly interact with each other in certain parts on which the stability factor is removed. The particles form a spatial network, and gelation occurs in the system. In this case, the solution changes its mechanical properties. The diagram of the resulting structure is shown in Fig. XIII.3.
Dispersed systems in which a coagulation structure has formed are called gels. Gelation is a reversible process. It is facilitated by increasing the concentration of the dispersed phase, increasing the degree of dispersion, adding electrolytes, asymmetry of particles of the dispersed phase, lowering the temperature, and adding a surfactant.
Figure XIII.3- Scheme of the gel structure
Gels exhibit a number of characteristic properties. The spontaneous restoration of the gel after its mechanical destruction is called thixotropy. There are strength thixotropy, which is associated with the destruction and formation of a spatial network, and viscous thixotropy, which is associated with the destruction and formation of particle aggregates.
Gels are characterized by the phenomenon of syneresis. This is a spontaneous reduction in the size of the gel with the simultaneous release of a dispersion medium from it. The essence of this phenomenon is that during storage, a rearrangement of particles in the gel occurs, the bonds between them increase and they come closer to each other. This causes the dispersion medium to be squeezed out.
Gels tend to dry to form a xerogel and swell when a dispersion medium is added.
Gels are characterized by structural viscosity. In the presence of a coagulation structure, the flow of the gel begins only after its destruction. In this case, the voltage R exceeds the critical shear stress Θ necessary to destroy the structure. Magnitude Θ is called the yield stress, and the flow of gels is called plastic flow. To describe the properties of such systems, the Bingham-Shvedov equation is used:
Р- Θ = η’γ,
where η’ is plastic viscosity.
The rheological curve for the gel is shown in Fig.
Figure XIII.4- Rheological curve of the gel
Condensation-crystallization structures arise as a result of chemical interaction between particles and the formation of a rigid volumetric structure. This process corresponds to coagulation at the near potential minimum of the energy curve. These are typical structures for bound disperse systems. Their destruction is irreversible. They do not swell and exhibit elastic-brittle properties.
Section “Structural and mechanical properties of disperse systems”
1. Structuring in colloidal and polymer systems. Gels and jellies. Their properties, mechanism of formation and practical significance. Thixotropy and synteresis
According to A.I. Rabinerson and G.I. Fuchs, structures formed in highly dispersed systems can be classified according to their density:
1. Spatial- structures are characteristic of dispersed systems with anisodiametric particles;
2. Compact- structures often arise in systems with isodiametric particles.
In true coagulation, when the particles completely lose their stability factor, they stick together to form component aggregates. Having reached a certain size, these aggregates form a dense coagulum. If incomplete astabilization of the system occurs, then the stability factor will be removed only from some areas of the surface of the particles, and not completely, and as a result of this, the particles, sticking together in such places, form a spatial network, in the loops of which there is a dispersed medium. Gelation occurs.
Gelation is called a transition colloidal solution from a freely dispersed state (sol) to a bound-dispersed state (gel).
A number of factors influence gelation:
· concentration of dispersed medium;
· reduction of particle size;
· temperature;
· mechanical impact.
A similar transition of the IUD solution into jelly is called gelation. It can occur spontaneously, as a result of a change in temperature when the solution is concentrated or when not too much is added to it. large quantity electrolyte.
Jellies have such properties as viscosity, osmotic pressure, elasticity, fluidity, the ability to scatter light, thixotropic properties, and synthesis.
Thixotropy- the ability of structures, after their destruction as a result of some mechanical action, to spontaneously recover over time.
Synthesis- spontaneous reduction in the size of the gel with the simultaneous release from it of a dispersed medium contained in the loops of the gel.
Jelly and the process of gelation have great importance in medicine, biology, technology, baking industry. Formation of an adhesive layer when gluing, gelling pyroxylin, obtaining artificial fiber, tanning leather.
2. Coagulation and condensation-crystallization structures according to P.A. Rebinder
According to Rehbinder, structures in colloidal and microheterogeneous systems can be divided into:
· coagulation (thixotropic-reversible) - structures that arise as a result of a decrease in the aggregative stability of disperse systems, when the particles completely lose the stability factor, they stick together, forming compact aggregates.
· condensation-crystallization (irreversible - destructible structures) - bonds between particles are formed due to chemical forces. These structures arise either as a result of the formation of strong chemical bonds between particles, or due to the merging of crystals during the crystallization of a new phase.
3 Normal Newtonian fluids, structured fluids. Viscosity. Viscosity anomaly. Newton, Poiseuille, Bingham equation. Rheological dependencies. Einstein's equations for determining viscosity colloidal systems
Liquid bodies are classified into:
1. Newtonian fluids - systems whose viscosity does not depend on shear stress and is a constant value in accordance with Newton’s law;
2. structured - the flow of which does not follow Newton’s law, their viscosity depends on the shear stress;
2.1 stationary - the rheological properties of which do not change over time;
2.2 non-stationary - for which these characteristics depend on time.
Viscosity is the ability of a liquid substance to resist movement. In liquids, viscosity is determined by internal pressure and with increasing temperature the viscosity decreases. In gases, viscosity is caused by the thermal movement of molecules; with increasing temperature, viscosity increases.
The viscosity coefficient is the resistance force that arises between layers of a fluid body with surfaces of area and spaced apart when they move relative to each other at speed.
Dynamic viscosity
The properties of a substance opposite to viscosity are called fluidity, and the value opposite to the viscosity coefficient is called the viscosity coefficient.
Kinematic viscosity takes into account the density of the substance and is related to dynamic viscosity:
Liquids capable of flow, but not obeying Newton's law, are usually called anomalous.
According to Newton's definition of viscosity, the internal friction force, equal in value but opposite in direction to the externally applied force, is proportional to the area of the layer to which this force is applied and the velocity gradient between the layers:
Relating force to area, then the equation would look like this:
where is the shear stress that maintains fluid flow.
Laminar flow of liquid through tubes is described by the Poiseuille equation:
where is the volumetric flow rate;
Tube radius and length;
Pressure difference at the ends of the tube;
Viscosity of the liquid.
Bingham expressed plastic viscosity with the equation:
where is the angle formed by the straight line with the abscissa axis.
However, for most structured colloidal systems the dependence on is expressed not by a straight line, but by a curve.
The reason for this phenomenon is that when the yield point is reached, the structure does not collapse immediately, but gradually as the fluid velocity gradient increases.
There are three critical stress shift:
1. - the first, or minimum, yield strength corresponding to the beginning of the flow.
2. - Bingham yield strength, corresponding to the segment on the abscissa axis, cut off by the continuation of the straight section of the curve.
3. - maximum yield strength, corresponding to the value at which the curve becomes a straight line.
The first axiom of rheology: under all-round uniform compression, material systems behave the same way - like ideal elastic bodies.
Second axiom of rheology: any material system has all rheological properties.
Einstein established a connection between the viscosity of a disperse system and the volume fraction of the dispersed phase:
where is the viscosity of the dispersed medium.
It was found that the coefficient at depends on the shape of the particles, so Einstein’s equation can be given a more general form:
where is a coefficient depending on the shape of the dispersed phase particle.
For the relative and specific viscosity of a disperse system, the Einstein equation transforms into the following relations:
The structure of bodies is usually understood as spatial mutual arrangement constituent parts of the body: atoms, molecules, small particles. The structure of dilute aggregation-stable disperse systems is very similar to the structure of true solutions in a number of properties. The main difference is that in dispersed (heterogeneous) systems, the particles of the dispersed phase and the molecules of the dispersion medium differ greatly in size. An increase in the concentration of the dispersed phase leads to the interaction of its particles. The change in the properties of dispersed systems with increasing concentration occurs gradually until particle coagulation occurs. In colloid chemistry, the concepts of structure and structure formation are usually associated specifically with coagulation. During the coagulation process, a spatial structural network is formed from particles of the dispersed phase, which sharply increases the strength of the system.
Thus, structure formation in freely dispersed systems is the result of the loss of their aggregative stability. As the strength of the structure increases, the freely dispersed system transforms into a coherently dispersed system.
A wide range of structural and mechanical properties reflects the diversity of natural and synthetic bodies, most of which are dispersed systems with every possible combination of phases that differ in nature and state of aggregation, particle size and interactions between them. Therefore, the structural and mechanical properties of dispersed systems appear to be a continuous and endless series of not only intermediate, additive properties, but also qualitatively new ones that are not inherent in individual components. The ability to control processes occurring in dispersed systems opens up unlimited possibilities for obtaining materials with desired properties.
During the formation of coagulation structures, the interaction of particles carried out through the layers of the dispersion medium is, as a rule, van der Waals, and therefore the spatial framework of such a structure cannot be highly durable. The mechanical properties of coagulation structures are determined not so much by the properties of the particles forming the structure, but by the nature and characteristics of interparticle bonds and layers of the medium.
Coagulation structures usually have a liquid dispersion medium. They are characterized by the ability to restore the structure over time after its mechanical destruction. This phenomenon is called thixotropy . Accordingly, such structures are often also called coagulation-thixotropic.
The spontaneous restoration of the coagulation structure indicates that it has the greatest mechanical strength with a relative minimum of Gibbs energy.
In practical activities, people use real bodies with various structures. As a rule, materials and products made from them are solids with condensation-crystallization structures (metals, alloys, ceramics, concrete, etc.), and raw materials and intermediate products are most often liquid or solid systems with a coagulation structure. The latter are very convenient in materials technology, since they provide the ability to regulate composition and homogeneity, and in product technology - regulation of molding processes, etc.
Fig.39. Typical flow curves for liquid bodies:
1- Newtonian fluids; 2- pseudoplastic liquids; 3- dilatant liquids
Fig.40. Typical flow curves for solids:
1 - Bingham body; 2 - pseudoplastic solid body; 3 - plastic dilatant body
The variety of structures in real dispersed systems does not allow them to be clearly separated. Of course, there are many intermediate states of systems. And yet, the proposed P.A. Rebinder's classification of the structures of dispersed systems helps to connect the mechanical properties of bodies with their structure.
There are classifications of bodies based on their rheological properties. In accordance with these properties, all real bodies are usually divided into liquid-like(yield strength is zero, P*= 0) and solid-like(P*>0).
Liquid bodies are classified into Newtonian And non-Newtonian fluids .
Newtonian fluids are systems whose viscosity does not depend on shear stress and is a constant value in accordance with Newton’s law.
The flow of non-Newtonian fluids does not follow Newton's law; their viscosity depends on shear stress. In turn, they are divided into stationary, whose rheological properties do not change over time, and non-stationary, for which these characteristics depend on time. Among non-Newtonian stationary fluids there are pseudoplastic And dilatant. Typical dependences of the rate of relative deformation of liquid-like bodies on shear stress (flow curves, or rheological curves) are presented in Fig. 39.
Experimental studies have shown that graphical relationships between shear stress and strain rate, presented in logarithmic coordinates, for stationary liquid-like systems often turn out to be linear and differ only in the tangent of the straight line. Therefore, the general dependence of shear stress on the rate of relative deformation can be expressed as power function:
Where k And P- constants characterizing a given liquid-like system.
The two-parameter equation (XIV.7) is known as the Ostwald-Weyl mathematical model.
If n = 1, the fluid is Newtonian and constant k coincides with the value of Newtonian viscosity η (straight line 1 in Fig. 39). So the deviation P from unity characterizes the degree of deviation of the properties of the liquid from Newtonian ones. For pseudoplastic liquids ( P< 1) characterized by a decrease in viscosity with increasing shear strain rate (curve 2 in Fig. 39). For dilatant liquids n>1 and viscosity increases with increasing shear strain rate (curve 3 in Fig. 39).
Dilute disperse systems with evenly axis particles are usually Newtonian fluids. Pseudoplastic liquids include suspensions containing asymmetric particles and polymer solutions. As the shear stress increases, the suspension particles gradually orient their major axes along the flow direction. The chaotic movement of particles changes to ordered, which leads to a decrease in viscosity. Dilatant liquids are rare; their properties are characteristic, for example, of some ceramic masses. Dilatant behavior is observed in dispersed systems with a high solid phase content. When such dispersed systems flow under the influence of small loads, the dispersion medium plays the role of a lubricant, reducing the friction force and, accordingly, viscosity. As the load increases, the dense packing of particles is disrupted (looses), the volume of the system increases slightly (the interparticle volume increases), which leads to the outflow of liquid into expanded areas and its insufficiency to lubricate the particles rubbing against each other, i.e., the viscosity increases.
Solid dispersed systems are divided into Bingham And non-Bingham ones. Their behavior is described general equation:
At P= 1 the equation follows a Bingham body, n> 1 - plastic dilatant body and P < 1- псевдопластическое твердообразное тело (рис.40).
It should be noted that solid and liquid bodies differ not only in the presence or absence of a yield stress, but also in a certain behavior during the development of deformation. With increasing load, structured liquids are characterized by a transition to Newtonian flow, corresponding to an extremely destroyed structure; for solid bodies, an increase in load leads to a break in the continuity of the body and its destruction. There are many systems with intermediate structural and mechanical properties.
In terms of rheological properties, washing liquids, sludges, oil paints, etc. are very similar to Bingham solid systems. They are characterized by a low yield strength, and when deformation develops, they behave like structured liquids. Such systems are classified as non-Newtonian fluids.
Typical solids have a significant yield strength. A brittle body collapses under a load less than the yield point (elastic limit). In most real solids, plastic deformations develop at all loads, but often in the region of small loads they can be neglected.
Thus, the division of solids into elastic, plastic and brittle is also to a certain extent arbitrary, since the nature of the deformation depends on the conditions, type of stress, duration of their action and other factors. Brittle solids include inorganic materials such as concrete, ceramics, etc. Metals and alloys have plastic properties. Highly elastic and viscous flow states are more typical for organic plastics.
Non-stationary systems, the rheological properties of which change over time, are characterized by the phenomenon thixotropy. Thixotropy is a specific property of coagulation structures. The destruction of the structure is expressed in the rupture of contacts between particles of the dispersed phase, and its thixotropic restoration is expressed in the resumption of these contacts due to the mobility of the medium and Brownian motion particles. The restoration of the structure is usually controlled by an increase in the viscosity of the system, therefore the phenomenon of thixotropy can be defined as a decrease in the viscosity of the system over time when a load is applied and a gradual increase in viscosity after the load is removed.
Suspensions of bentonite clay with a dispersed phase concentration of more than 10% have pronounced thixotropy. In a quiet state, this system is a plastic solid body that does not flow under the influence of gravity. After shaking, the suspension becomes so thin that it can easily flow out of the container. After a certain time of keeping the suspension in a calm state, it again turns into a non-fluid structured system. This circumstance must be taken into account when pumping suspensions, which may solidify in the event of a possible stop of the pumps.
