ABSTRACT
The concept of stability applies to virtually everything that surrounds us: the geographical terrain, snow and mud banks in the mountains, buildings and various other civil structures, currency exchange rates, the political situation in the world, etc� In colloid science and physical-chemical mechanics, we study the stability of disperse systems, that is, of heterogeneous systems consisting of two or more phases in which the constituent particles are small (<1 μm), that may participate in Brownian motion but still retain the physicochemical and mechanical properties of the phase and the interface� We distinguish diluted free-disperse systems and concentrated connected-disperse systems (Figure 4�1)�
In connected-disperse systems consisting of coagulation structures, gels, and sediments, the cohesive forces between the particles are sufficiently strong to withstand both the slacking effect of the thermal motion and external mechanical impacts� In the latter case, we are interested in the stability of the connected-dispersed state of the system, that is, the stability of the structure and its resistance to transition into a free-disperse state as a result of the peptization process� The response of connected-disperse systems to applied stress is the subject of rheological studies� We have discussed the principal concepts of rheology in Chapter 3 and have described the role of entropy in the change in rheological behavior upon the coorientation of anisometric particles�
Free-disperse systems comprise dilute emulsions, sols, and suspensions in which the participation of particles in thermal Brownian motion plays a dominant role over the cohesive forces between them� In these systems, we are particularly interested in the stability resisting the transition from the free-disperse state to the connected-disperse state via aggregation, flocculation, or sedimentation (Figure 4�2)�
It is worth pointing out again that in both free-disperse and connected-disperse systems, the interaction and cohesion between particles play the determining role� The principal difference between the two cases is as follows� Stability and its loss in connected-disperse systems (structures) are governed by the competition between cohesive forces and applied mechanical stresses, while in the free-disperse system, they are governed by the competition between the kinetic energy of particles participating in the Brownian motion and the energy of cohesion� These, however, are the two elementary approaches: the loss of stability in a connected-disperse system may, to a large extent, be driven by thermal fluctuations, while in a free-disperse system, the loss of stability can be initiated by external forces, such as gravity (in sedimentation) or a velocity gradient (in orthokinetic coagulation)�
In this chapter, we will address the thermodynamic and kinetic aspects of colloid stability in free-disperse systems� We will discuss the concept of the factors for weak and strong stabilization, the possibility of spontaneous dispersion, and the conditions necessary to form thermodynamically stable colloidal systems� Furthermore, we will discuss the necessary conditions for the coagulation-peptization (dispersion) transition and the equilibrium between a coagulate comprising the connected-disperse system and the free-dispersed system formed in the course of dispersion� The fundamentals of colloid stability have been partially discussed in Chapters 1 and 2 and are covered to a great detail in textbooks on colloid and surface science [1-29]� We will address here the subject of colloid stability to the extent appropriate to the general scope of this book�
A general approach to the analysis of the conditions of stability in free-disperse systems is the analysis of the interactions in the contact established between the particles in a given dispersion medium� This is essentially the analysis of the properties of a thin film of dispersion medium in the gap between the particles (see Section 1�2)� The quantitative thermodynamic description of the properties of the film is based on the analysis of the change in free energy of interaction, Δσf(h), when the surfaces are brought close together from an infinite gap to a given width of the gap, h (Figure 4�3)�
The derivative –dΔσ(h)/dh = Π(h) is the force per unit cross-sectional area, also known as Derjaguin’s disjoining pressure [4,5]�Within the context of this definition, both Δσ(h) and Π(h) are positive in the case of a repulsion and negative in the case of an attraction� Molecular attraction forces prevail at long distances, while repulsive forces prevail at very short distances (the so-called Born repulsion)� The principal theory that describes the interactions in a thin film is the well-known DerjaguinLandau-Verwey-Overbeek (DLVO) theory, which focuses on the analysis of the competitive contribution of molecular (dispersion) attractive forces and electrostatic repulsion to the interaction between surfaces separated by a liquid film�
The dispersion component of the molecular attractive force per unit cross-sectional area in a flat-parallel gap of thickness h is characterized by the free potential energy of interaction, U h h A hmold ( ) ( ) * ,= = -Ds pfd /12 2 where the complex Hamaker constant, A*, takes into account the molecular interactions in the solid phase and the liquid medium and between the two (Figure 4�4)� This is the molecular component of the free energy of interaction�Its derivative -P - Ds - pmold fd / /( ) ( ) *h h h A h= =d d 6 3 is the molecular component of the disjoining pressure� Due to the natural character of the dispersion forces and the use of the Lennard-Jones “6-12” potential [2,11,26], the attraction at very short distances corresponding to a contact between electron shells is replaced by rapidly increasing repulsion-the so-called Born repulsion�
The value of |Δσf(h0)| = Δσf = F characterizes the extent of cohesion between the surfaces in direct contact at the primary free energy minimum�
The electrostatic component of the disjoining pressure is given by the positive term describing the interaction between the electrical double layers of the two surfaces in contact� The detailed description of the electrical double layer can be found in textbooks on colloid science and in specialized textbooks on electrokinetic phenomena at interfaces [5,8-13]� We will restrict ourselves to a very brief revision of the basic concepts associated with the electrical double layer�
The electrical double layer at the interface between the solid and liquid phases can be viewed as a capacitor formed by the combination of a charged surface, which is a carrier of fixed potentialdetermining ions, and the equivalent number of counter ions distributed in a volume of the dispersion medium adjacent to the surface�The distribution of a potential, U(h), within the region of interest (a diffuse part of electrical double layer) is described by an exponential function decreasing with distance, U(h) = const × exp(−h/δ), where δ = (εε0kT/2z2e2n0)½� Here, k is the Boltzmann constant, T is the absolute temperature, ε is the dielectric constant, ε0 is the electric constant, e is the elementary charge, and n0 is the number concentration of ions� The concentration of ions has a strong influence on the value of δ, which is the effective thickness of the diffuse double layer� The meaning of parameter δ is similar to that of the Gibbs’ discontinuity surface� In the Debye-Hückel theory of strong electrolytes, δ is known as the Debye length�
The value of the preexponential constant, const = (4kT/ze) tanh (zeϕ0/4kT), includes the dependence of a potential on these parameters and on the surface potential, ϕ0� At high electrolyte concentrations, c ~ 0�1-1 mol/dm3, the double layer is compressed to a thickness on the order of fractions of a nm, while at low electrolyte concentrations, c ~ 10−3-10−4 mol/dm3, the diffuse double layer thickness is on the order of tens of nanometers�
According to DLVO theory, the repulsion of the surfaces across the gap is determined by the overlap between the double layers of both surfaces at a distance h ~ 2δ� The essence of the calculations discussed in the traditional literature on colloid science is presented in Figure 4�5� In the middle of the (film filling the) gap between the two identical surfaces, the potential of electric field is 2U(h/2) × const1 × e−h/2δ� The charge density in the middle point, given by the derivative of the potential with respect to distance, is given by ρ(h/2) × const2 × e−h/2δ� The product of the charge density and the potential given by const3 × e−h/δ characterizes the work of concentrating the charge (the density of the free energy excess) and thus the pressure force per unit area (J/m3 = N/m2)� In this way, we have come up with an exponential function for the dependence of the electrostatic component of the disjoining pressure on the distance in a flat-parallel gap of thickness δ
el const( ) /h e h= ´4
where the constant const4 = 64n0kTγ2 and γ = tanh(zeϕ0/4kT)� Integrating this expression over distance yields the ion-electrostatic component of the free energy of interaction
Ds d - d
fel const( ) /h e h= ´5
The main concept of DLVO theory is the summation of the positive (exponential) and negative (hyperbolic) terms of the free energy of interaction, Δσf(h), and the disjoining pressure, Π(h),
Ds g d -
f k( ) */h n T e A h h= ´64
P g -
p - d( ) */h n T e A
h h= ´64
3k
For a given concentration of a monovalent 1:1 electrolyte (e�g�, c ~ 10−3 mol/dm3), the free energy of interaction and the disjoining pressure are shown in Figure 4�6�The function Δσf(h) reveals two minima separated by a potential barrier� These minima are referred to as the primary minimum and the secondary minimum� Even a qualitative analysis of the Δσf(h) functions points to the relation between the height of the potential energy barrier and the ability of particles engaged in thermal motion to overcome it and thus coagulate in the primary minimum� In Section 1�2, the description of the interactions between surfaces was related to the thickness of the flat-parallel gap between the particles�This is not sufficient for a quantitative assessment of the comparison of the potential energy barrier to the value of thermal energy, kT� The latter requires knowledge of the interaction energy between particles of a particular size� Within the first approximation, we can multiply Δσf(h) and Π(h) by some effective area, Seff, characterizing the zone of particle contact� Alternatively, one can utilize the main concept of DLVO theory, which defines the necessary condition for coagulation as the disappearance of potential energy barrier� Figure 4�7 shows the Δσf(h) curves obtained by varying the electrolyte concentration, n0 (number of ions per unit volume, or c, mol/dm3), which is the principal factor determining the shape of these curves� At very high electrolyte concentrations, the electrical double layer is compressed, and consequently, the electrostatic repulsion is completely
suppressed, leaving only the molecular attraction described by the hyperbolic term and the primary minimum� At very low n0, electrostatic repulsion can play a predominant role at substantial distances, yielding a potential barrier and a secondary minimum� At some critical value of concentration, nc, the positive potential barrier can disappear� At this point, the function Δσf(h) and its derivative Π(h) have the same value, which allows one to obtain the value of the critical coagulation concentration (c�c�c�), nc:
n
k T A zc
6 6 ( ) ( )
*
ee g e
This expression is the principal result of DLVO theory�The steep z−6 dependence of the critical coagulation concentration has been observed experimentally for strongly charged surfaces and is commonly known as the Schulze-Hardy rule� In the case of weakly charged surfaces, a less steep dependence of the critical coagulation concentration on the valence (z−2) is observed� In fact, it was the empirical dependence of the c�c�c�on the valence established in the course of experimental studies on coagulation that inspired the development of DLVO theory�
The typical Δσf(h) isotherms (force-distance curves) calculated using the DLVO theory have been experimentally verified in a large number of studies using specialized experimental techniques� In contrast to the measurement of the contact forces discussed in Sections 1�2 and 1�3, the determination of the long-range DLVO forces requires simultaneous measurements of both the forces and the distances� The latter is a rather complex experimental task� Modern instrumentation for such studies has been developed and improved over the several past decades [10-13]� The surface force apparatus developed by Israelashvili [10] for measuring the interaction forces between crossed cylindrical surfaces of mica utilized an interferometric technique for measuring the distance� Another technique that is commonly used nowadays is atomic force microscopy (AFM)� This technique involves the scanning of the surface with a cantilever tip of diameter of ~20 Å or measuring the interaction force between the cantilever and the surface while keeping the distance between the surface and the cantilever tip constant� Another technique originally developed at Sandia National Laboratory is interfacial force microscopy (IFM)� It functions similar to AFM
but has a force sensor of zero compliance� In IFM, the induction system is used to control the distance between the cantilever tip and the surface� In the Russian Institute of Physical Chemistry, the author and his colleagues developed a capacitor-based sensor for measuring the distance between two particles [30]� This device is shown in Figure 4�8�
While describing the basic mechanisms of the coagulation and stabilization of aqueous sols in the presence of electrolytes, DLVO theory has limitations when working with dilute sols of hydrophobic particles� Such sols are weakly stabilized and readily undergo coagulation� DLVO theory is also not applicable to the description of the stability of concentrated colloidal systems that remain stable at high electrolyte concentrations and freezing, that is, strongly stabilized systems� In this sense, the electrostatic component of the disjoining pressure can be referred to, following Rehbinder’s terminology, as the “weak factor of the colloid stability�” This factor has a peculiar dual nature: while the origin of the potential barrier is thermodynamic, the ability of particles to overcome it in the course of their thermal motion is a phenomenon of kinetic nature�
In addition to the ion-electrostatic factor of colloid stability, which we have discussed, there are also other factors leading to the appearance of a positive disjoining pressure, Π(h) > 0, that prevents particles from approaching each other under the action of the attractive (negative) component of the disjoining pressure, Õ <mold ( )h 0�The most essential factors are as follows (Figure 4�9):
1� The stretch of a thin soap film requires that more surfactant molecules enter the surface from the bulk, and consequently, the bulk of the solution become deficient in surfactant� At the same time, with respect to the new equilibrium state, both of these factors lead to a decrease in the surface surfactant concentration, as compared to the initial one, that is, they result in an increase in the surface tension (in Section 1�1, we did not discuss the possibility of this effect)� The increase in surface tension as film stretching increases is known as the characteristic of effective elasticity, the Gibbs effect� The latter is a purely thermodynamic phenomenon that results in a resistance to film thinning�
2� As the soap films stretch and become thinner, there is also a kinetic phenomenon, referred to as the Marangoni-Gibbs effect� When drainage of liquid from the film into the channels bordering the film takes place under the action of negative capillary pressure (without film stretching), the surfactant concentration in the film decreases, resulting in an increase in the surface tension�
3� Hydrodynamic factor: According to Reynolds, at a given compression force, the rate of fluid drainage from the gap between round flat-parallel plates drops as the cube of gap thickness, h� Such a steep drop in the fluid drainage rate makes drainage slow and, when the gaps are very small, prevents a complete approach between the surfaces and limits film thinning�
These three phenomena, which prevent a decrease in film thickness and retard the drainage of fluid from the gap between particles, may also be classified as weak factors of colloid stability� In contrast to these, Rehbinder introduced the concept of a factor of strong stabilization of disperse systems� This factor ensures colloid stability in sols and emulsions against coagulation and coalescence at high concentrations of electrolytes, in concentrated disperse systems, under substantial changes in temperature or due to the action of mechanical forces� This factor, referred to as the structure-mechanical barrier, arises from a combination of the structural-mechanical (rheological) properties of the interfacial adsorption layer, the ability of such a layer to resist deformation and destruction, and the extreme lyophilization of the outer part of the adsorption layer facing the dispersion medium� This is schematically illustrated in Figure 4�10 and is discussed in detail further in this chapter�
The original teachings of Rehbinder and his school on colloid stability were based on some qualitative concepts that were known with regard to protective colloids� As one of the factors of colloid stability, Rehbinder’s teachings included the concept of the so-called steric stabilization, which was fully developed at a later time� The principal achievements and contribution of Rehbinder and
his coworkers are the development of experimental methods for the investigation of the structuralmechanical barrier, that is, the methods that allow one to study the strength of the adsorption layers and understand their critical role in controlling the coalescence of emulsion droplets [1,31-38]� It has been shown that strong colloid stability is achieved as a result of the high lyophilicity of the adsorption layer facing the dispersion medium� The importance of these teachings for the improvement of various industrial processes and a deep understanding of the stability of living cells and live tissues should not be underestimated�
The concept of the strong stabilization of colloidal systems may be illustrated by a simple experiment-adding a teaspoon of table salt into a glass of milk� If we address colloid stability strictly from the standpoint of DLVO theory, the system should be stable at 10−3-10−4 M salinity and should coagulate at ~10−1 M of salt� A teaspoon contains 5-7 g of salt, which would produce an electrolyte concentration well exceeding the critical coagulation concentration� Nevertheless, coagulation does not occur-there is no deposit forming on the glass walls� As is also well known, milk is stable at both high and low temperatures: it can be both boiled and frozen� At the same time, squeezing a little bit of lemon juice into the same glass of milk results in immediate coagulation and a drastic loss of colloid stability�
Rehbinder was the first one to formulate the principals of the thermodynamic and “nonthermodynamic” factors of colloid stability� The thermodynamic factors are referred to as the factors of the stability of thin films that can be described in terms of the thermodynamics of reversible equilibrium processes� Such are the factors described by DLVO theory and Gibbs elasticity� The nonthermodynamic factors include the kinetic and hydrodynamic factors described in the previous section (e�g�, Marangoni effect and Reynolds effect), but also, and even more so, the mechanical resistance of films to rupture and displacement� There are numerous examples of systems in which the factors of both kinds determine stability� Those belonging to the first group include not too thin electrolyte-stabilized wetting films and foam films, as well as dilute sols� Examples of common systems representing the second group include milk, natural latex dispersions, the cell walls in living tissue, and crude petroleum�
Appealing to concepts regarding thermodynamic and nonthermodynamic stabilization factors, we can state that strong stabilization against both high electrolyte concentration and high concentration of the disperse phase is due to the mechanical resistance of film to rupture and resistance to a displacement from the gap between droplets, bubbles, and particles� In the case of solid particles, this can be achieved by the firm attachment of the adsorption layer to the surface (i�e�, by chemisorption), whereas in the case of emulsions (mobile fluid-fluid interface), it is necessary for the interface itself to have sufficient mechanical stability� The latter include high (nonlinear) viscosity,
enhanced elasticity, and strength, which is the main characteristic of the stabilizing layer responsible for strong stabilization� Nevertheless, this is not it� Mechanical strength provides stability against the coalescence caused by the action of external forces� Resistance to coagulation requires that the outer part of the stabilizing layer is lyophilic, that is, must have a nature close to that of a dispersion medium� Consequently, one can talk about a lyophilic structural-mechanical barrier. Within the framework of the concepts dealing with protective colloids, such a complex approach can be regarded as quantitative development of these concepts in the light of the physicochemical mechanics of disperse systems�
There are a number of experimental methods that can be used to evaluate these two factors of a lyophilic structural-mechanical barrier separately� We will briefly review these methods in the succeeding text and then focus on specific experimental data that have been collected by employing these methods�
1� A torsion pendulum device was developed by the scientists at the Department of Colloid Chemistry of Moscow State University, that is, by Izmaylova et al� [35-37]� This device, shown schematically in Figure 4�11, allows one to evaluate the mechanical characteristic of thin-film behavior at both the liquid-air interface and the liquid-liquid interfaces� Nowadays, similar studies can be conducted with commercial high-sensitivity shear rheometers using a special bicone tool�
Figures 4�12 and 4�13 show some results obtained with the torsion pendulum instrument by Izmaylova et al� [35-37]� These figures show the typical rheological and deformation curves of the 2D interfacial layers formed at the interface between an aqueous gelatin solution and benzene� The deformation curves show the force as a function of time at a constant rate of deformation, and the rheological curves show the steady-state rate of the deformation as a function of the shear stress� These experimental data allow one to obtain direct quantitative characteristics of the elasticity in the slow and fast regions and, most importantly, the critical stress values that correspond to strength (by extrapolation to the x-axis, Figure 4�13)�
Like any other experimental method, the torsion pendulum method has its advantages and disadvantages� While the method has the advantage of making it possible to carry out studies under pure stress conditions, it has a limitation due to the nonuniformity of the stress in the gap between the pendulum and the cuvette�
2� The Langmuir trough allows one to study film behavior under conditions of one-sided compression or one-sided tension� In both physical-chemical mechanics and material science, one needs to compare different stressed states� Tens of dynes per cm in compression or tension applied to the adsorption layer determine the critical stress, that is, the conditions corresponding to the mechanical collapse of the adsorption layer in one-sided compression�
3� A neat direct method for studying droplet rupture and coalescence, allowing one actual visual observation of individual droplets, was developed by Parfenova et al� [38,39]� A very small droplet (about 0�3 mm in diameter) is immersed into another liquid phase and stretched under controlled conditions (see Figure 4�14)� The interfacial tension and rupture force are measured at the point when the droplet assumes cylindrical shape due to deformation� This corresponds to uniaxial tension resulting from asymmetric uniaxial stretching of the film (membrane) at the interface between the polar and nonpolar phases (which can represent both the dispersed phase as well as dispersion medium)� In subsequent compression of two half droplets, one measures the critical force causing coalescence, fcoal, that is, the rupture of a double-sided emulsion film� These results correspond to a symmetrical double-sided axisymmetric stretch of the membrane�
The study of droplet rupture and coalescence by direct visual observation has been utilized in numerous essential studies [39-43]� Of principal importance are the experimental studies by Amelina et al� on the analysis of colloid stability in artificial blood substitutes [40-43]� These studies involved the use of various nonpolar phases, including perfluorinated systems, such as perfluorodecalin (PFD), perfluorotributylamine (PFTBA), perfluoromethylcyclohexylpiperidine (PFMCHP), and “conventional” hydrocarbons, such as heptane� Stabilizing agents included Pluronic surfactants (ethylene oxide (EO)/propylene oxide (PO) block copolymers), as well-fluorinated surfactants, such as perfluorodiisononylene with 20 mol of EO (ϕ-PEG)� Tables 4�1 and 4�2 show some very characteristic results�
These tables indicate that in the presence of Pluronic surfactant the strength of the adsorption layer at the interface between fluorinated phases and water was two orders of magnitude higher than the strength of the layer formed by the same surfactant at the waterheptane interface� In the case of an adsorption layer formed with the fluorinated surfactant, the opposite effect was observed: the strength of this adsorption layer at the water/heptane interface was an order of magnitude higher than that of an adsorption layer formed at the fluorinated hydrocarbon/water interface�
When discussing the mechanical properties of interfacial adsorption layers, one should clearly identify the role of the so-called steric factor among other factors responsible for colloid stability� The concept of a “steric factor” was introduced at a much later time than Rehbinder’s concept of the lyophilic structural-mechanical barrier� The steric factor predominantly addresses the contribution of the configuration flexibility of the loops and tails of the lyophilic portions of the adsorbed macromolecules� These loops and tails penetrate the dispersion medium as flexible “tentacles�” This steric factor has an osmotic origin and represents only the entropic contribution to the elastic strength� The magnitude of this contribution is rather small, and hence the steric factor alone cannot be responsible for strong colloid stability� This is illustrated by the data shown in Tables 4�1 and 4�2, which can’t be explained in terms of the steric factor concept alone� It is especially evident from a comparison of the Pluronic adsorption layer strength at the interface with nonpolar PFD and more polar PFTBA� Most likely, due to an unfavorable interaction with the dispersed phase, the “tentacles” are forced into the adsorption layer and participate in the formation of a dense mechanically strong layer� In the general case, within such a layer, both polar and nonpolar groups interact with each other and are responsible for the strength, which is a nonthermodynamic characteristic�
The method described represents a rather attractive tool� A number of other known experimental approaches are as follows:
TABLE 4.1 Stability of Droplets Formed by Various Nonpolar Liquids in Aqueous Solutions of Pluronic F-68 (5 × 10−9 mol/dm3)
TABLE 4.2 Stability of Droplets Formed by Various Nonpolar Liquids in Aqueous Solutions of Fluorinated Surfactant Perfluorodiisononylene-Polyethylene Glycol, (ϕ-PEG) (5 × 10−6 wt %) Media Heptane PFD
4� Methods based on the analysis of the interaction between a bubble or a drop with a flat surface�
5� Methods based on the analysis of the behavior of foam columns, foam caps, and especially of the flow of foams�
6� Methods based on studies of individual foam and emulsion films, for example, using a Mysels-Sheludko cell�
7� Various optical methods focusing on the study of the surface layer� These include ellipsometry or the observation of capillary waves� Usually, these methods are restricted to linear characteristics, such as constant viscosity (serving as a dissipation factor) or surface elasticity�
One does indeed need so many different experimental approaches because each of these tools provides a new equation, which allows one to estimate new parameters� The more independent equations are available, the better� Some of these methods allow one to conduct experiments not only with liquid phases but also with solid particles, and hence new parameters specific to the interactions with solid phases become available� Here, it is worth mentioning an eighth technique that focuses on the analysis of contact interactions between solid particles:
8� The measurement of contact forces between plastic particulates immersed in different media [31,44]� In this method, a particle is compressed against another particle with a given force, f, over a given period of time and then separated, as schematically shown in Figure 4�15� The magnitude of the measured contact strength, p1, at a given compression, f, serves as an indication of whether or not the adsorption layer has been ruptured�
The results of the cohesive force measurements between two silver chloride crystals compressed against each other and immersed in different media are shown in Figure 4�16� These are histograms reflecting the probability of getting a particular strength of contact, p1,
between crystals� As one can see, there are either very weak coagulation contacts or very strong phase contacts� Strong contacts emerge when a particle attaches to another particle as a result of plastic deformation in the zone of contact at sufficiently high contact stresses� The higher the applied force, f1, the greater the fraction of these strong (phase) contacts reflecting the situation of local coalescence, that is, the crystals become fused together� The left column (a) in Figure 4�16 shows the results of measurements conducted in air with untreated crystals, while the right column (b) shows the results of measurements between crystals coated with a complete monolayer of octadecylamine� In the latter case, even the maximum compression applied was unable to rupture the adsorption layer: only weak contacts were observed� At the microscopic level, this means that the nearsurface dislocations in the zone of contact are blocked by the adsorption monolayer of octadecylamine�
Since it is difficult for one to assess the absolute strength values, in reality, one can only talk about the probability of rupturing the adsorption layer� This probability varies significantly in cases involving nonspecific (physical) and specific adsorption (chemisorption)� Both of these cases are presented in Figure 4�17, which illustrates the fraction of the phase contacts formed in solutions of alcohols in heptane (physical adsorption) and in the
presence of octadecylamine (chemisorption)� This figure clearly shows the protective role of the adsorption layer of octadecylamine� The formation of the phase contacts is significantly influenced by the degree of the completeness of the adsorption layer� Model studies conducted by this method, in addition to being of an academic value, are also significant for understanding such areas of technology as friction and wear and specifically the role of greases with chemisorbing additives�
9� Finally, it is worth mentioning another method that can be used to study the structural-rheological (mechanical) properties of film� Sclera is the opaque fibrous outer layer of the eye, which contains collagen and elastic fiber� Changes in the sclera properties are responsible for a very strong nearsightedness that develops with ageing� In terms of material science, we talk here about the creep of the sclera under the action of internal eye pressure� The creep of the sclera results in an increase in the thickness of the eye, causing a distortion in focus and resulting in myopia� Every extra millimeter results in three extra diopters in the lens strength� Over many years, the action of a pressure of 20 mm of Hg on a relatively thin film that makes up the sclera constitutes a fairly strong mechanical action� It is, indeed, somewhat surprising, in this case, that such strong nearsightedness affects only 10% of people�
There are three basic questions that need to be addressed here: first, what kind of rheological behavior does sclera exhibit and what types of measurements and stress regimes are needed to study it? The second question is related to which factors (substances present in the media or enzymes) catalyze sclera creep? Finally, the third question is how to find a solution that inhibits sclera creep?
