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Summary Source rocks contain significant volumes of hydrocarbon fluids trapped in kerogen, but effective recovery is challenged because of amplified fluid/wall interactions and the nanopore–confinement effect on the hydrocarbon–fluid composition. Enhanced oil production can be achieved by modifying the existing molecular forces in a kerogen pore network using custom–designed targeted–chemistry technologies. The objective of this paper is to show that the maturation of kerogen during catagenesis relates to the qualities of the kerogen pore network, such as pore size, shape, and connectivity, and plays an important role in the recovery of hydrocarbons. Furthermore, using molecular–dynamics (MD) simulations, we investigated how the transport of hydrocarbons in kerogen and hydrocarbon recovery can be altered with the delivery of microemulsion and surfactant micelles into the pore network. New 3D kerogen models are presented using atomistic modeling and molecular simulations. These models possess important chemical and physical characteristics of the organic matter of the source rock. A replica of Type II kerogen representative of the source rocks in the Permian Basin in the US is used for the subsequent recovery simulations. Oil–saturated kerogen is modeled as consisting of nine different types of molecules: dimethyl naphthalene, toluene, tetradecane, decane, octane, butane, propane, ethane, and methane. The delivered microemulsion is an aqueous dispersion of solvent–swollen surfactant micelles. The solvent and nonionic surfactant present in the microemulsion are modeled as d–limonene and dodecanol heptaethyl ether (C12E7), respectively. MD simulation experiments include two stages: injection of an aqueous–phase microemulsion treatment fluid into the oil–saturated kerogen pore network, and transient flowback of the fluids in the pore network. The used 3D kerogen models were developed using a representative oil–sample composition (hydrogen, carbon, oxygen, sulfur, and nitrogen) from the region. Simulation results show that microemulsions affect the reservoir by means of two different mechanisms. First, during the injection, microemulsion droplets possess elastic properties that allow them to squeeze through inorganic pores smaller than the droplet's own diameter and to adsorb at the kerogen surfaces. The solvent dissolves in the oil phase and alters the physical and transport properties of the phase. Second, the surfactant molecules modify the wettability of the solid kerogen surfaces. Consequently, the recovery effectiveness of heavier oil fractions is improved compared with the recovery effectiveness achieved with surfactant micelles without the solubilized solvent. The results indicate that solubilized solvent and surfactant can be effectively delivered into organic–rich nanoporous formations as part of a microemulsion droplet and aid in the mobilization of the kerogen oil.

The imbibition of solutions of a model nonionic surfactant into packed beds of crushed reservoir rocks was studied using the Washburn technique. A linear dependence between the equivalent height of capillary rise and the square root of imbibition time was observed at different stages of imbibition experiments. It has been shown that, under the conditions when surfactants did not alter the polarity of rocks, the imbibition rate of surfactant solutions correlated well with the nondispersion (polar) component of the surface free energy of the rocks. It was possible to compare data obtained for different rocks by normalizing the slopes of imbibition curves over the corresponding slopes determined for a completely wetting fluid, hexamethyldisiloxane (HMDS). Such a normalization allowed one to account for substantial differences in the morphology of crushed rock powders. Overall, the observed trends in the imbibition behavior were qualitatively similar to the trends reported previously for the rise of surfactan...

Summary Molecular-dynamics simulation is used to investigate the nature of two-phase (oil/water) flow in organic capillaries. The capillary wall is modeled with graphite to represent kerogen pores in liquid-rich resource shale. We consider that the water carries a nonionic surfactant and a solubilized terpene solvent in the form of a microemulsion, and that it was previously introduced to the capillary during hydraulic-fracturing operation. The water has already displaced a portion of the oil in place mechanically and now occupies the central part of the capillary. The residual oil, on the other hand, stays by the capillary walls as a stagnant film. Equilibrium simulations show that, under the influence of organic walls, the solvent inside the microemulsion droplets enables not only the surfactant but also the complete droplet to adsorb to the interfaces. Hence, delivering the surfactant molecules to the oil/water interface is achieved faster and more effectively in the organic capillaries. After the droplet arrives at the interface, the droplet breaks down and the solvent dissolves into the oil film and diffuses. This process is similar to drug delivery at nanoscale. Using nonequilibrium simulations based on the external force-field approach, we numerically performed steady-state flow measurements to establish that the solvent and the surfactant molecules play separate roles that are both essential in mobilizing the oil film. The surfactant deposited at the oil/water interface reduces the surface tension and acts as a linker that diminishes the slip at the interface. Hence, it effectively enables momentum transfer from the mobile water phase to the stagnant oil film. The solvent penetrating the oil film, on the other hand, modifies flow properties of the oil. In addition, as a result of selective adsorption, the solvent displaces the adsorbed oil molecules and transforms that portion of the oil into the free oil phase. Consequently, the fractional flow of oil is additionally increased in the presence of solvent. The results of this work are important for understanding the effect of microemulsion on flow in organic capillaries and its effect on shale-oil recovery.

Source rocks contain significant volumes of hydrocarbon fluids trapped in kerogen, however their effective recovery is challenged due to amplified fluid-wall interactions and the nanopore confinement impact on fluid composition. Enhanced oil production can be achieved by modifying the existing molecular forces in kerogen pore-network by using custom-designed targeted chemistry technologies. Our objective is to show how the transport of hydrocarbons in kerogen and its recovery can be altered with the delivery of microemulsion nanodroplets into the pore network. This is done by using computational chemistry and molecular dynamics (MD) simulations. Molecular dynamics simulation is used to generate a 3D model replica of a Type II kerogen representative of source rocks located in Delaware and Midland basins in the United States. Oil phase saturated kerogen is modeled as consisting of nine different types of molecules: dimethyl naphthalene, toluene, tetradecane, decane, octane, butane, propane, ethane and methane. The delivered microemulsion is an aqueous dispersion of solvent-swollen surfactant micelles. The solvent and nonionic surfactant present in the microemulsion are modeled as d-limonene and dodecanol heptaethyl ether (C12E7), respectively. Molecular dynamics simulation experiments include two steps: (i) the injection of microemulsion treatment fluid into the oil-saturated kerogen pore-network, and (ii) transient flow-back of the oil-chemical mixture in the pore network. The utilized 3D kerogen models were developed based on a representative oil sample composition (H, C, O, S, N) from the region. Simulation results show that microemulsions can affect the reservoir via two different mechanisms. During the injection, microemulsion nanodroplets that enter the nano-capillaries of pore network disperse in the liquid present in the pore space under the influence of pore walls. The solvent dissolves in the oil phase and alters the physical and transport properties of the phase, while the surfactant molecules modify the wettability of the solid kerogen surfaces. The recovery effectiveness of heavier oil fractions is improved compared to the recovery effectiveness achieved with surfactant micelles without the solubilized solvent. New 3D kerogen models are presented using atomistic modeling and molecular simulations. These models possess important chemical and physical characteristics of the organic matter of the source rock. Molecular dynamic experiments indicate that solubilized solvent and surfactant are delivered as part of a microemulsion droplet and are expected to aid the mobilization of oil present within kerogen.

Characterization studies of organic-rich shale oil reservoirs have revealed significant volumes of hydrocarbon fluids in kerogen. However, the recovery from kerogen pores is challenging due to amplified fluid-solid interactions. New methods can be developed for improved recovery targeting oil from kerogen pore space by modifying the forces of molecular interactions using chemical injection. A highly-developed kerogen pore-network is required for the penetration and delivery of chemical agents that are expected to function in the confined space, such surface active agents. Using advanced computational chemistry tools, the objective of this paper is to show that the maturation (the exposure to high temperature, high pressure) of kerogen during catagenesis relates to the quality of the kerogen pore network such as pore size and shape, and plays important role in the action of added chemicals in the EOR processes. A new molecular dynamics simulation approach is developed applying dramatic changes to the organic chemicals system temperature to mimic varying degree of maturation. Simulation focuses on Type II kerogen, as it is the most common overall source of presently produced hydrocarbons. Two different chemical structures of type-II kerogen (C175H102N4O9S2, C242H219N5O12S2) are used as the building blocks to simulate the solid kerogen. The molar fractions of the elements are controlled to satisfy the overall H/C and O/C ratio of type-II kerogen in the oil window. The simulated hydrocarbon fluid consists of nine different types of molecules: dimethylnaphthalene, toluene, tetradecane, decane, octane, butane, propane, ethane and methane. The simulation box containing these molecules is subjected to a slow quenching process, which continues down to the reservoir temperature and pressure conditions. The effects of maximum temperature and the rate of quenching on the pore morphology of kerogen and the distribution of oil in the pore-network are discussed. We explain how kerogen pore morphology is controlled by the quenching rates. Next, we simulate the interaction of microemulsion droplets with the digital kerogen. Results show that the microemulsion droplets posess elastic properties which allow them to squeeze through the kerogen pores smaller than the droplet's own diameter and to adsorb at pore wall surfaces. One major benefit associated with the use of microemulsions is the ability of the droplets to transport and deliver solvents and surfactants to different parts of the pore network. Our work shows that solvents and surfactants with particular features can be delivered in the form of a microemulsion droplet into oil saturated kerogen pore network and influence the oil mobility.

Resource shales have low permeability matrix with nanoporous features. At nano scale in situ fluids experience strong fluid-wall interactions and confinement effects. The hydrocarbons recovery from the nanopore network is therefore limited. Microemulsions are known to be effective in stimulating oil and gas production from non-conventional reservoirs, however their mechanisms of action, as well as their ability to penetrate into nanosize pores of shale matrix during an operation such as hydraulic fracturing needs to be investigated. In this paper, we investigate the conditions for the microemulsion droplet penetration into model nanopores, identify the penetration mechanisms and, following their penetration, analyze their interactions with model organic and inorganic walls, and study their behavior in confinement. Molecular dynamics simulation is employed to simulate the behavior of a nanodroplet dispersion facing a solid surface. The model nanodroplets comprise swollen micelles of C12E7 nonionic surfactant with the d-limonene solvent solubilized in their cores. An oil-wet solid surface is modeled using graphite to represent hydrophobic kerogen in shale, and a water-wet solid surface is modeled using brucite to represent hydrophilic inorganic materials in shale. These surfaces are considered to have nanocapillaries with varying sizes, available for the microemulsion penetration experiment. Our results indicate that penetration into capillaries with sizes less than 10 nm is strongly influenced by the wettability of the solid surface. In the case of an oil-wet solid surface the droplets adsorb on the surfaces and hence impact the penetration ability. In the case of a water-wet surface, however, microemulsion droplets effectively penetrate into the nanocapillaries. The droplets are capable of penetrating into the capillaries that are smaller than their own size. In both of these two cases, the solubilized solvent and the surfactant are delivered into a tight nanocapillary network and come into contact with the in situ hydrocarbons. This research can be extended to include ionic surfactants, varying salinity, and more complex solid surfaces to develop the next generation microemulsions with superior performance in enhancing oil production from the unconventional reservoirs.

Water-in-oil, oil-in-water and balanced microemulsions made with ethoxylated alcohol surfactant and d-limonene as the oil phase were diluted with distilled water and 2% KCl solutions. The interactions between these diluted microemulsions and Marcellus shale rock were studied by performing adsorption and wettability measurements. Interaction of the balanced microemulsion solutions with sand was studied by monitoring change in the surface tension and droplet size distributions before and after solution exposure to sand. It was established that adsorption of both surfactant and d-limonene took place on both sand and shale. It was discovered that the behavior in adsorption and wetting studies of diluted balanced microemulsion was significantly different as compared to the adsorption from diluted o/w and w/o microemulsions. Balanced microemulsion produced highest value of Γmax and highest contact angle on Marcellus shale. The results of wettability studies were in many aspects similar to those previously established in quartz-surfactant solution system, but with a number of significant differences. Inflection points in adhesion tension–surface tension curves were observed. Such points were previously seen in quartz-cationic surfactants systems, but were not observed in systems with nonionic surfactants.

Colloidal borosilicate and boron-modified colloidal silica sols were studied by 11 B NMR. Formation of B–O–Si chemical bonds is established in both materials. It is shown that boron present in colloidal borosilicate is stable towards the action of complexing agents catechol and tartaric acid. In contrast, the boron in boron-modified silica is readily complexed by these agents. The results presented herein demonstrate that B–O–Si bonds are homogeneously distributed throughout the colloidal borosilicate disperse phase, while in boron-modified colloidal silica they are concentrated at the surface of colloidal silica particles.

Uniform colloidal polypyrrole particles ranging from 17 to 59 nm in diameter were prepared by the oxidation of pyrrole with sodium persulfate in the presence of the nonionic Rhodasurf TB970 polymeric stabilizer and 4-ethyl benzenesulfonic acid. The adhesion of these particles on glass beads was studied as a function of the pH using the packed column technique. Polypyrrole was found to deposit on glass only at pH values below its isoelectric point (i.e.p.), forming a monolayer. The entire amount of the adhered polypyrrole could be rapidly removed by rinsing the column with 1×10-2 mol dm-3 NaOH solution.

The kinetics of deposition of colloidal hematite particles on glass beads in the presence of a cationic surfactant, 1-dodecylpyridinium chloride (DPC), at pH 4.0 and 10.5 was studied using the packed column technique. At the high pH, the surfactant adsorbed both on particles and beads, which influenced the attachment process; depending on conditions either monolayer or multilayer deposition was observed. At pH 4.0, only a monolayer of hematite formed at all surfactant concentrations used. At this pH value, the DPC adsorbed little on particles but significantly on beads, making a part of the collector surface inaccessible for the particle attachment. A modification of the multilayer adhesion theory was proposed which allows one to account for the latter effect. The kinetics of particle release from the monolayer, by rinsing the loaded column with 10 −2 mol dm −3 NaOH solutions, was also investigated. It was shown that the detachment rate was insensitive to the amount of surfactant present in the dispersions used for the deposition.

The kinetics of deposition of colloidal hematite particles on glass beads in the presence of sodium 4-octylbenzenesulfonate (NaOBS) at constant pH and ionic strength were studied using the packed column technique. At low concentrations, the surfactant had little effect on the adhesion phenomena, as compared with the same system in its absence. At sufficiently high concentrations, NaOBS caused charge reversal of hematite particles to negative and a restabilization of the suspension, preventing their deposition under such conditions. It was also established that the adsorption of the surfactant molecules onto particle surface was very fast; thus, no equilibration between NaOBS and hematite was necessary in the adhesion studies. At intermediate surfactant contents, either mono- or multi-layer attachment of hematite on glass took place. The kinetics of particle release from monolayer and multiple layers by rinsing the loaded column with 10−2 mol dm−3 NaOH solutions was also investigated, and the corresponding removal rate constants were estimated. It was shown that the detachment from multilayers was a slower process than the removal from monolayers.

Electrophoretic mobility measurements have been used to characterize monodispersed colloidal particles of silica, silica coated with alumina (cores), of these cores incorporating a dye (pigments), and finally of pigments coated with polymers. The latter consisted of poly(divinylbenzene), of poly(vinylbenzyl chloride), and of their copolymers, synthesized directly on the core or pigment particles, with and without subsequent sulfonation.

Abstract The productivity and economics of horizontal wells are governed by the ability of the transverse fractures to communicate efficiently with the wellbore, which is strongly controlled by the conductivity of the proppant bed and the effectiveness of the fluid additives. These impact the relative permeability, the capillary pressure and the effective conductivity in the proppant bed. If the wellbore is high in the fracture, gravity segregation will cause liquid removal from the lower portion of the fracture to be very difficult. In low conductivity proppant beds, capillary pressure will tend to retain high water saturations, thus lower the effective conductivity even for the portions of the fracture above the wellbore. Laboratory and field studies are presented comparing various sizes and types of proppants and the influence of surfactants used in oil bearing formations including commonly used demulsifiers and a multi-phase complex nano fluid system. Ammot cell and centrifuge tests were used to evaluate imbibition of oil and water. Columns packed with proppant and formation cuttings are used to compare the effectiveness of various additives in allowing the displacement of water and establishing oil flow. Results are correlated with interfacial tension, contact angle, capillary pressures and surface energies of actual formation materials, oils and treating fluids from the Niobrara, Bakken, Granite Wash and Eagleford formations. Simulations are presented that show the impact of capillary pressure and oil viscosity on the displacement of fluids. Field results from various fields including the Niobrara, Bakken, and Marcellus formations are presented. The normalized field data shows that wells with higher conductivity proppants and properly selected surfactant packages result in longer effective frac lengths and greater normalized oil and gas production. Correlations are made between the observed relative perms in the lab vs. the observed field results.

The primary purpose of using surfactants in stimulating hydrocarbon rich gas reservoirs is to reduce interfacial tension, and/or modify contact angle and reservoir wettability. However, many surfactants either adsorb rapidly within the first few inches of the formation, or negatively impact reservoir wettability, thus reducing their effectiveness in lowering capillary pressure. These phenomena can result in phase trapping of the injected fluid adversely impacting oil and gas production. This study describes experimental and field studies comparing various common surfactants used in oil bearing formations including alcohol ethoxylates, EO-PO block copolymers, ethoxylated amines and a multi-phase complex nano fluid system to determine their impact on oil recovery and adsorption tendencies when injected through 5-foot and 1 ft sand columns. Ammot cell tests were used to evaluate imbibition of oil and water and a core flow apparatus was used to evaluate regained relative permeabilities. The results are correlated with surface energies of actual formation materials, oils and treating fluids. The results are used to select formulations containing surfactant, solvents and co-solvents to apply within the fracturing fluid to decrease adsorption, eliminate post treatment emulsions and improve oil and gas recovery in hydrocarbon rich gas wells.

A continuing challenge in hydraulic fracturing of tight gas formations is associated with remediation of formation damage caused by fluid invasion into the porous media. Numerous studies documenting the use of complex nanofluids and surfactants to remediate formation damage have been reported. Recent publications have demonstrated that complex nanofluid additives resulted in lower pressures to displace injected frac fluids over conventional surfactants, and led to greater enhancement of gas and water production. These findings were also confirmed by several recent statistical analyses that took into consideration differences in the properties of treated wells. Many field case studies and supplementary laboratory data have illustrated benefits of complex nanofluid treatment over conventional surfactants. While these publications describe the successes of complex nanofluid treatment, the influence that the formulation composition has on its performance has not been fully investigated. In the present study, we prepared complex nanofluids with different chemical compositions and examined their performance in fluid recovery tests using columns packed with sand, ceramic proppant, and shale, as well as their ability to enhance permeability of sandstone cores to gas. We have established that performance of complex nanofluids in these applications was dependent on the amount of microemulsifed solvent in the original formulations and that optimal performance across all applications was achieved with a complex nanofluid formulation with a near-balanced composition.

The productivity and economics of horizontal wells are governed by the ability of the transverse fractures to communicate efficiently with the wellbore, which is strongly controlled by the conductivity of the proppant bed and the effectiveness of the fluid additives. These impact the relative permeability, the capillary pressure and the effective conductivity in the proppant bed. When time at temperature, stress cycling, embedment, multi-phase flow and non-Darcy effects are considered, the effective conductivity can be reduced 100-fold. Another investigated parameter is the impact of the wellbore location relative to the propagated fracture. If the wellbore is high in the fracture, gravity segregation will cause liquid removal from the lower portion of the fracture to be very difficult. In low conductivity proppant beds, capillary pressure will tend to retain high water saturations, thus lower conductivity even for the portions of the fracture above the wellbore. Laboratory experiments have addressed these issues for proppants with a range of permeabilities from 10 to 100 Darcies; 100 mesh to 20/40 mesh and ceramics. The relative permeability to gas can be as low as 0.01, with as much as a 70-fold improvement when suitable proppants and additives are employed. Evaluation of the production performance of 240 wells, 98 with effective additives and 142 without, and covering a similar range of proppant types and sizes, shows a similar benefit to the wells’ normalized 30 day recovery and gross value. These results clearly demonstrate that economic expediency can be detrimental to a well’s ultimate value and hydrocarbon recovery.

In recent years numerous case studies documenting the use of microemulsions in hydraulic fracturing of tight gas formations have appeared in the literature. Field case studies and supplemental laboratory data have illustrated that microemulsions enhance core permeability to gas and enhance fluid flowback mainly due to lowering capillary pressure and altering wettability in reservoirs and propped fractures. Although proven successful for enhancing gas production, many aspects of the microemulsion behavior in propped fractures are still poorly understood. In the present study we have performed numerous experiments on microemulsion-assisted fluid recovery from columns packed with sand proppant and shale, using microemulsions of different chemical composition. Our results indicate that the extent to which microemulsion additives promote foaming is an important factor in achieving highest possible levels of fluid recovery. By using properly formulated microemulsion compositions one can achieve significant improvement in fluid recovery at commercially viable doses of treatment.

In recent years a number of laboratory studies on the use of surfactants and microemulsions in hydraulic fracturing of shale formations have been reported. These studies mainly focused on such metrics as improvement in permeability regain and enhancement of fluid recovery from packed columns upon the use of surfactant-containing chemicals. Laboratory studies have also been backed by the documented observations from the field illustrating benefits of using microemulsions for the increase in gas production from shale formations. It is a commonly accepted view that these additives benefit gas production by lowering capillary pressure and altering wetability of shale formation. Although it is recognized that the interaction of surfactants and microemulsions with shale is governed by the energetics of solid/liquid, liquid/gas and solid/gas interfaces, there are practically no studies in which surface energies have been determined for different shales. The surface energy, as well as dispersive, non-dispersive, Lifshitz-van der Waals, and Lewis Acid-Lewis Base components of surface energy of several North American shales have been determined from contact angle measurements. It has been discovered that the surface energy of all shale rocks is rather low, typically in the range of 40-50 mJ/m2 a contribution from non-dispersion ("polar") component of about 8-11 dyn/cm. Consequently, shales are capable of interacting with liquids predominantly via dispersion and Lifshitz-van der Waals molecular interactions, which should substantially influence the orientation of surfactant molecules and microemulsion moieties at the shale surface. Furthermore, there was no significant variation in surface energy of shales from different basins, which suggests that individuality of shale surface chemistry should play only a secondary role in the development of shale-specific chemical treatments, and factors other than surface chemistry should be considered first.

Hydraulic fracturing of shale formations presents a well-known challenge due to high entrapment of frac fluids. Typical levels of fluid recovery from shale formations are on the order of 30% or lower. Fluid loss into shale produces liquid blocks that interfere with the gas flow in already tight formations. Friction reducers or other additives present in fluids can cause further formation damage. Search for technologies that would maximize gas flow rate and yield high recoveries is ongoing. In recent years microemulsions have been successfully used for remediating damaged wells and enhancing production from tight gas formations. In the present work we have explored the use of such additives for improving fluid recoveries from columns packed with shale/sand mixtures and for improving permeability of split shale cores to gas.

In recent years the use of microemulsions for remediating damaged wells has been a success story. Microemulsions are unique, thermodynamically stable, optically clear single phase blends of water, biodegradable water-immiscible solvent, co-solvent, and specially designed surfactant. Engineered mixtures of these ingredients can form stable microemulsions over a rather broad range of water to solvent to surfactant ratios. Are all microemulsions equally effective in their performance? The main focus of our study was to prepare various microemulsions and to examine a link between microemulsion composition and their performance. Microemulsion treatments have proved to be effective for improving gas production rates by increasing formation permeability and for enhancing fluid recovery from sand-packed and shale-packed columns. For achieving maximum benefits, microemulsion formulations have to be properly designed. Both microemulsion composition and dose are important for their end performance. The effectiveness of a particular microemulsion may vary depending on the application, and may be sensitive to the type of formation.

Anionic polyacrylamide copolymer friction (or drag) reducers are commonly used in various well stimulation jobs. The effectiveness of friction reduction polymers strongly depends on the compatibility between friction reducing polymers and stimulation liquid to which they are added. Performance of friction reducers can be strongly influenced by the presence of salts, very high or very low pH, or other typical process additives, such as biocides, corrosion and scale inhibitors, hydrogen sulfide (H2S) scavangers, etc. Since most friction reducers are emulsion polymers, an important issue related to their performance is their ability to invert effectively and efficiently upon the addition to the stimulation fluid. A fast enough rate of inversion is especially important, since it determines both the effective onset of optimal friction reduction and its magnitude. Higher friction reduction achieved at the very beginning of a well stimulation will decrease the pumping pressure on a job. We hereby describe novel, fast-inverting friction reducers suitable for effective use in brines and compatible with various additives, such as biocides, clay control agents, and scale inhibitors.

The effects of an anionic (sodium 4-octylbenzenesulfonate, NaOBS) and a cationic (1-dodecylpyridinium chloride, DPC) surfactant on the detachment of colloidal hematite particles adhered to glass beads was studied using the packed column technique. Both additives induced particle removal at concentrations above those necessary for the reversal of charge either on particles or on beads, in order to induce a repulsion between interacting surfaces. The amount of detached hematite was substantially increased as the surfactant concentrations exceeded the corresponding critical micellization concentrations (CMC). Particle removal was shown to follow first order kinetics with two distinctively different rate constants. The value of the constant for rapid removal, k r , was substantially higher than that established in earlier studies for detachment of the same particles with NaOH solutions.

Publisher Summary The difference in the composition and structure of phases in contact, as well as the nature of the intermolecular interactions in the bulk of these phases, stipulates the presence of a peculiar unsaturated molecular force field at the interface. As a result, within the interfacial layer, the density of such thermodynamic functions as free energy, internal energy, and entropy is elevated in comparison with the bulk. The large interface present in disperse systems determines the very important role of the surface (interfacial) phenomena taking place in such systems. In a single component system, two phases (liquid and vapor) coexist in equilibrium only if there is a stable interface present between them. Such an interface is formed only if an increase in the surface area results in an increase in the system free energy. For fluids, the surface tension values are numerically equal to those of the corresponding specific surface energies, while for solids, a tensor quantity related to mechanical stresses that are present at the interfacial layer is considered.

Publisher Summary Disperse systems occupy an intermediate position between macroscopic heterogeneous systems and molecular solutions (homogeneous systems), and thus, can be prepared via two main paths: by dispersing the macroscopic phases (dispersion path) or by condensation, either in true solutions or in homogeneous single component systems (condensation path). In most cases, the formation of disperse systems requires work. This work can either be introduced from the outside, for example, in the form of mechanical work, or be generated within the system through various processes, including chemical reactions. The resulting disperse systems are thermodynamically in disequilibrium, and their prolonged existence is impossible without additional stabilization. In the absence of the latter, the system is unstable and a particular particle size or particle size distribution cannot be maintained, that is, the particles spontaneously coarsen and finally the disperse system collapses. A number of disperse systems may form as a result of the spontaneous dispersion of a macroscopic phase. These systems, referred to as lyophilic, are thermodynamically in equilibrium and do not require any additional stabilization.

Publisher Summary All colloidal systems are subdivided into two large classes-thermodynamically stable systems, referred to as lyophilic, and those characterized by kinetic stability only, referred to as lyophobic systems. The possibility of existence of lyophilic systems in equilibrium with macroscopic phases is determined by the nature of dispersed phase and its interaction with dispersion medium. In systems consisting of simple molecules without strong diphilic features, the formation of equilibrium colloidal systems occurs only within narrow temperature range in a direct vicinity of the critical point. The formation of lyophilic colloidal systems in broad temperature and concentration ranges is typical if molecules of one of the components are of a strong diphilic nature. These are the surfactant molecules with either ionic or large non-ionic polar group, or long hydrocarbon chain. Lyophilic colloidal systems are ultramicroheterogeneous systems formed by spontaneous dispersion of macroscopic phases. These systems are thermodynamically stable with respect to both the growth of dispersed particles and their further refining to molecular level sizes. The equilibrium particle size distribution that does not change with time is the characteristic of these systems.

Publisher Summary Lyophobic colloidal systems are coarse and fine disperse systems that are thermodynamically unstable because of a substantial excess in surface free energy. The latter may be related both to the existence of highly developed interface between the dispersed phase and the dispersion medium, and to relatively high values of interfacial tension, σ. This excess of the surface free energy is the reason for instability of such systems in which various processes leading to a coarsening and finally to complete degradation of systems, that is, separation into individual macroscopic phases take place. General stability of lyophobic colloidal systems, as well as the rate of processes leading to degradation, is determined by nature, phase state, and composition of the dispersed phase and dispersion medium, and also by the degree of dispersion and concentration of the dispersed phase. Stability of disperse systems may vary in a very broad range from complete instability to nearly infinite stability.

Publisher Summary Structural features of disperse systems, particularly, the existence of the electrical double layer (EDL), are responsible for a number of peculiar phenomena related to heat and mass transfer, and electric current propagation in such systems. The methods used to study disperse systems include particle size distribution analysis, studies of the surface structure and of near-surface layers, and the structure of the EDL. Most transfer phenomena are described by the laws of irreversible thermodynamics, which allow carrying out a systematic investigation of different fluxes that originate as a result of the action of various generalized forces. One model example of a transfer process that takes place in a structured disperse system is the transfer of the dispersion medium through a single capillary of radius r c that occurs during filtration or in the processes involving various types of osmosis. The proportionality coefficient between the acting force and the resulting flux is referred to as the conductivity (or permeability) of the capillary, while the reciprocal value is known as the capillary resistance.

Publisher Summary A distinctive force field present at the interface may cause changes in the composition of the near-surface layer: different substances depending on their nature may either concentrate near the surface, or, alternatively, move into the bulk. This phenomenon, referred to as the adsorption, causes changes in the properties of interfaces, including changes in the interfacial (surface) tension. In disperse systems with liquid dispersion medium, adsorption layers present at the surfaces of dispersed particles may significantly influence the interactions between these particles, and hence, affect the properties of disperse system as a whole, including its stability. The laws governing the formation, structure, and properties of the adsorption layers at different interfaces allow to analyze the role such layers play in controlling colloid stability and other properties of disperse systems. Substances that lower the surface tension of the system are referred to as surface active substances, or surfactants. It follows from the Gibbs equation that adsorption of such compounds is positive, that is, their concentration within the surface layer is higher than that in the bulk.

Publisher Summary The laws governing the interfacial phenomena between condensed phases and their vapor (or air) in single- and two-component systems are largely applicable to the interfaces between two condensed phases, which is between two liquids, two solids, or between a solid and a liquid. At the same time, these interfaces have some important peculiarities, primarily related to the partial compensation of the intermolecular interactions. The degree of saturation of the surface forces is determined by the similarity in the molecular nature of the phases in contact. When adsorption of surfactants takes place at such interfaces, it may substantially enhance the decrease in the interfacial energy. The latter is of great importance as surfactants play a major role in the formation and degradation of disperse systems. The intensity of intermolecular interactions at the interfaces between condensed phases is one of the critical factors determining the conditions for wetting and spreading. A large number of important technological processes, such as mineral processing (flotational enrichment and separation), are based on these phenomena.

Publisher Summary This chapter describes the preparation, structure, and properties of different colloidal systems. The chapter reviews the connection among particular properties of disperse systems and the aggregate states of both the dispersed matter and dispersion medium. Among all disperse systems, the aerosols, in which the dispersion medium is a gas, are unique. These systems are principally lyophobic and their stabilization—by introduced surfactants—is ineffective. Aerosols also reveal some specific electric properties. In systems with liquid dispersion medium, that is, in foams, emulsions, sols, and suspensions, there is a broad variety of means to control colloid stability. In these systems, the nature of colloid stability depends to a great extent on the aggregate state of dispersed phase. Similar to aerosols, foams are lyophobic, but in contrast to them, foam can be effectively stabilized by surfactants. Properties of emulsions, and, to some extent, those of sols are quite close to the properties of thermodynamically stable lyophilic colloidal systems. In such systems, a high degree of stability may be achieved with the help of surfactants.

Publisher Summary Aggregation processes that take place in disperse systems lead to the separation of these systems into macroscopic phases or to the development of spacial networks, that is, to the transition from free disperse systems to structured disperse systems. In structured disperse system, the cohesive forces in the contacts are sufficiently high, so these systems are resistant to both the thermal motion and external action. At the same time radical changes in disperse system occur: it acquires new structural and mechanical (rheological) properties that characterize ability of a system to resist deformation and fracture. The system acquires mechanical strength, the most important feature of all solids and materials that determines their role in nature and technology. The laws that govern structure formation in disperse systems regulate mechanical properties of structured systems and of materials based on them, and are referred to as the physical-chemical mechanics. Ensuring fine degree of dispersion as well as maximum uniformity of microheterogeneous structure is the basis for increasing mechanical strength and durability of materials.