High-energy Solids Research Articles

An analytic solution for Counter-Current spontaneous imbibition in porous media by the perturbation method

Following primary production of hydrocarbon from naturally fractured reservoirs, secondary recovery by waterflooding relies on Spontaneous Imbibition (SI) as the primary mechanism for oil expulsion to the fracture network, where it can be easily transported to neighboring production wells. Thus, a more complete understanding of this phenomenon is warranted and essential for process improvement. The imbibition process, taking advantage of inherent preferential affinity for water contact over oil of high energy solids, is the rate determining step in recovery and can proceed in either co-current or counter-current fashion, depending upon entrance and exit boundary conditions. Due to the prominence of linear flow normal to extensive planar fracture faces, the phenomena can be largely described as one-dimensional displacement. In this study, the nonlinear one-dimensional diffusivity equation, derived by combining Darcy’s law for both wetting and non-wetting phases with the mass conservation equation, is solved for counter-current spontaneous imbibition using a direct regular perturbation scheme. Saturation and phase pressure profiles in time and space were found to agree with literature solutions. In its analytic formulation, the method constitutes a simpler, more accurate, and more direct way to compute spontaneous imbibition performance than previously available. We claim generality with regard to functional forms for relative permeability and capillary pressure and applicability over the entire range of wettability, provided these effects are reflected in the parameterization of capillary pressure and saturation normalization. In particular, the present method avoids the difficulty in solving nonlinear integral equations. Instead, we reduce computations to a set of simple numerical integrations of the diffusion coefficient. Such a streamlined solution method can more easily provide a compute engine for inverse modeling or optimization schemes to recovery rock and fluid properties from displacement experiments. The solution method is benchmarked against oil–water and air–water solutions by alternative techniques and experimental data. The case of spontaneous imbibition into dry diatomite represents extreme contrast in phase mobility and a rigorous test of the perturbation method as an easier and more direct way to model 1D SI. Following method construction and benchmarking of the new streamlined countercurrent spontaneous imbibition solution method, a subsequent parametric study contributes to a greater understanding of controlling factors in fracture-matrix exchange in porous media.

Supersaturation Potential of Salt, Co-Crystal, and Amorphous Forms of a Model Weak Base

High energy solids, such as salts, co-crystals, or amorphous solid dispersions, have been widely used to generate supersaturated aqueous solutions and improve drug bioavailability. However, most research on solubility enhancing strategies has focused on the kinetics of dissolution, and there is relatively little comparison of the different degrees of supersaturation achieved by using different solid state forms of the same compound. Recent studies from our group have demonstrated that the maximum achievable supersaturation is dictated by the aqueous solubility of the amorphous form of the drug. Liquid–liquid phase separation (LLPS) occurs at concentrations above this value. Herein, it was hypothesized that the upper limit of supersaturation that can be achieved from dissolution of various high energy solids is also governed by the amorphous solubility. To test this hypothesis, the dissolution and supersaturation behavior of different solid forms of a model compound, CRH1, were investigated using a variety...

Trends in the Precipitation and Crystallization Behavior of Supersaturated Aqueous Solutions of Poorly Water-Soluble Drugs Assessed Using Synchrotron Radiation

Amorphous materials are high-energy solids that can potentially enhance the bioavailability of poorly soluble compounds. A major impediment to their widespread use as a formulation platform is the tendency of amorphous materials to crystallize. The aim of this study was to evaluate the relative crystallization tendency of six structural analogues belonging to the dihydropyridine class, in an aqueous environment in the absence and presence of polymers, using wide-angle X-ray scattering synchrotron radiation and polarized light microscopy. The crystallization behavior of precipitates generated from supersaturated solutions of the active pharmaceutical ingredients was found to be highly variable ranging from immediate to several hours in the absence of polymers. Polymers with intermediate hydrophilicity/hydrophobicity were found to substantially delay crystallization, whereas strongly hydrophilic or hydrophobic polymers were largely ineffective. Nuclear magnetic resonance spectroscopy experiments supported the supposition that polymers need to have affinity for both the drug-rich precipitate and the aqueous phase in order to be effective crystallization inhibitors. This study highlights the variability in the crystallization tendency of different compounds and provides insight into the mechanism of inhibition by polymeric additives.

On the solubility advantage of a pharmaceutical’s glassy state over the crystal state, and of its crystal polymorphs

At equilibrium, the saturation solubility and vapor pressure of a material in a state of high free energy are greater than in its state of low free energy. This knowledge from classical thermodynamics is currently used for increasing the solubility of crystalline pharmaceuticals by producing them in their glassy state, or in other solid states of high free energy. The ratio of the apparent saturation solubility of these solids to that of a crystal, calculated from the thermodynamic data of the pure solute, ϕcal, is called the solubility advantage, and it is used as a guide for increasing the solubility of a pharmaceutical. We argue that the ϕcal differs from the measured solubility ratio, ϕmeas, because, (i) ϕcal is independent of the solvent, but ϕmeas is not so, (ii) ϕcal would increase with the dissolution time monotonically to a constant value, but ϕmeas would first reach a maximum and then decrease, and (iii) approximations are made in estimating ϕcal and the effect of thermal history on high free energy solids is ignored. On the other hand, ϕmeas is affected by, (a) another chemical equilibrium in the solution, e.g., hydrogen-bond formation and ionic dissociation, (b) the production method and thermal history of a glass or an amorphous samples, and (c) mutarotation in the solution, isomerization or tautomeric conversion in the solid. We also discuss the effects of structural relaxation and crystallization on ϕmeas. The ϕmeas value of a (crystal) polymorph would be affected by all the three, and further if the polymorph is orientationally disordered. We provide evidence for these effects from analysis of the known data. The ϕmeas value is preferable over ϕcal.

Amorphous drugs and dosage forms

The transformation to an amorphous form is one of the most promising approaches to address the low solubility of drug compounds, the latter being an increasing challenge in the development of new drug candidates. However, amorphous forms are high energy solids and tend to recrystallize. New formulation principles are needed to ensure the stability of amorphous drug forms. The formation of solid dispersions is still the most investigated approach, but additional approaches are desirable to overcome the shortcomings of solid dispersions. Spatial separation by either coating or the use of micro-containers has shown potential to prevent or delay recrystallization. Another recent approach is the formation of co-amorphous mixtures between either two drugs or one drug and one low molecular weight excipient. Molecular interactions between the two molecules provide an energy barrier that has to be overcome before single molecules are available for the formation of crystal nuclei, thus stabilizing the amorphous form.

To evaluate the effect of addition of an anionic surfactant on solid dispersion using model drug indomethacin

Formation of solid dispersion also known as high energy solids is one of the most successful concepts to improve dissolution profile of poorly water-soluble drugs. Use of surfactants in formulation is one of the methods to increase solubility profile. In this research, we have used model drug, a weak acid (indomethacin) together with polymer (PVP) and anionic surfactant (sodium lauryl sulfate (SLS)) in different concentrations to study the effect of incorporation of SLS in solid dispersion. Three ratios and control were prepared. Physical characterization was performed using modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Critical micelle concentration (CMC) measurements were conducted to see the effect of SLS on dissolution media. Dissolution studies were performed in hydrochloric acid buffer (pH 1.2 buffer), purified water and phosphate buffer (pH 7.4), respectively. Interestingly, depending upon addition of SLS into the system, release profiles were changed. SLS incorporated internally in a solid dispersion gave the highest release.

An overview of the basic aspects of polymer adhesion. Part I: Fundamentals

AbstractAdhesion between two substrates is a complex phenomenon which at present is still not well understood. The important existing adhesion models (electrical, diffusion, thermodynamic adsorption, chemical, etc.) are reviewed in order to try to explain their mechanisms. Thermodynamic adsorption is now believed to be one of the most importnat mechanisms by which adhesion is achieved. Difusion and wetting are kinetic means in attaining good adsorption of a polymer at the interface. In the case of this model (thermodynamic adsorption), the notion of surface energy is developed and the importance of this property in the understanding of adhesion phenomena is emphasized. The methods of determining the surface characteristics of low and high energy solids are presented. The role played by acid‐base interactions in adhesion is also mentioned.

Surface tension of talc and talc-chlorite mixtures

It is possible to estimate surface tension of high-energy solids combining the immersion microcalorimetry thermodynamics and Van Oss' model. In this study we have applied this method on talc and talc-chlorite samples in order to obtain thermodynamic values which permit to understand surface properties useful in the industrial applications of these solids. Some talcite samples are preferentially used in specific industrial applications because they are less hydrophobic or more lamellar. This method seems to be reliable to classify the solids and predict some properties.

Numerical Computation of Two-Dimensional Unsteady Detonation Waves in High Energy Solids

We are concerned with theoretical modelling of unsteady, two-dimensional detonation waves in high energy solids. A mathematical model and a numerical method to solve the associated hyperbolic system of equations are presented. The model consists of the Euler equations augmented bby extra conservation laws and source terms to accoxint for chemical reaction and tracking of materials. Both the thermodynamics and the chemistry are treated in a simple way. Using a detonation analogue due to Fickett, we test several numerical methods and assess their performance in modelling the essential features of detonation waves. The numerical method selected for the full model is an extension of the conservative, shock capturing technique of Roe, together with an adaptive mesh refinement procedure that allows the resolution of fine features such as reaction zones. Results for some typical tests problems are presented. Starting in 1946 as the College of Aeronautics, the Cranfield Institute of Technology was granted university status in 1969. In 1993 it changed its name to Cranfield University.

Numerical computation of two-dimensional unsteady detonation waves in high energy solids

We are concerned with theoretical modelling of unsteady, twodimensional detonation waves in high energy solids. A mathematical model and a numerical method to solve the associated hyperbolic system of equations are presented. The model consists of the Euler equations augmented by extra conservation laws and source terms to account for chemical reaction and tracking of materials. Both the thermodynamics and the chemistry are treated in a simple way. Using a detonation analogue due to Fickett, we test several numerical methods and assess their performance in modelling the essential features of detonation waves. The numerical method selected for the full model is an extension of the conservative, shock capturing technique of Roe, together with an adaptive mesh refinement procedure that allows the resolution of fine features such as reaction zones. Results for some typical tests problems are presented.

Nonuniformity in Thin Polymer Films

Abstract Thin polymer films solidified from solutions can display a broad range of barrier and mechanical properties, depending on the quality of the solvent and the initial polymer concentration. These findings apply specifically to polymers containing polar groups, exemplified in this work by polymethyl methacrylate and a styrene-butyl acrylate copolymer, but not to polymers like polystyrene. In the former two cases it has also been shown that very different surface tensions exist at the air and the substrate interface, provided the films were solidified against high surface energy solids. These observations have been attributed to the ability of polar-group-containing polymers to adopt various chain conformations at and near interfaces, these conformations reflecting interactions between polymer and solvents and between polymer and substrate. An additional contribution is attributed to the degree of chain entanglement persisting in solutions. The measured properties often pertain to metastable states of the films, and the findings thus indicate the existence of a thermodynamic driving force for the time-dependent change (aging) in properties of protective films.

Thermodynamics of displacement wetting

A thermodynamic analysis is presented of the various types of immersion heat measurements which can be carried out on systems having two immiscible phases. The analysis is applied to the available data. It is shown that the film pressure and film energy are negligible for the adsorption of a hydrocarbon component at the interface between water and several high-energy solids. The analysis also permits the calculation of the corresponding quantities for the adsorption of water at the interface between liquid hydrocarbon and high-energy solid phases. Possible calorimetric tests of the Harkins duplex film hypothesis for the adsorption of water at such interfaces are discussed.

Surface properties of high-energy solids: II. Determination of the nondispersive component of the surface free energy of mica and its energy of adhesion to polar liquids

A method of determining the polar term of the adhesion energy of several liquids to a high-energy solid, I SL P, has been developed, based on the measurement of the contact angle of water on a solid in a liquid medium. The I SL P values for mica are found to be a linear function of the square root of the polar term of the surface free energy of liquids. This finding agrees with the suggestion that the polar term of the energy of adhesion may be represented by the geometric mean of the polar term of the surface free energy of a solid and a liquid. The slope of the straight line provides the value of γ S P = 90 ergs/cm 2 for the polar term of the surface free energy of mica. The results were compared with those obtained by a cleavage method and also discussed in terms of each component of the surface free energy of mica. The present method is useful for the determination of the polar part of the energy of adhesion of a high-energy solid to liquids, and its surface free energy.

Surface properties of high-energy solids: I. Determination of the dispersive component of the surface free energy of mica and its energy of adhesion to water and n-alkanes

A method for measuring the dispersive part of the surface free energy γs D of a high-energy solid, and its interaction energy with water and n-alkanes, W SL, has been developed. It is based on the measurement of the contact angle of water on the solid under n-alkanes. Muscovite mica was chosen as a model high surface energy solid. The results obtained for γs D and W SL of mica are in good agreement with the results obtained by other techniques. The present method can be considered to be applicable for other solids.

Effect of Relative Humidity on the Wettability of Steel Surfaces

Abstract Metals and metal oxides are usually classed as high-energy solids since in the ultra-clean state they have surface free energies typically greater than 500mJ/m2. However, Bernett and Zisman1 measured the critical surface tension, yc , of wetting of various hydrophilic, high-energy, metal oxides and pure metals at two extreme humidities namely 0.6% r.h. and 95% r.h., and found that the surfaces were converted to ones of low yc (i.e. about 46 and 38 mJ/m2 respectively) and, further, the yc values were almost independent of the solid surface. They ascribed these observations to the adsorption of water and proposed that the surface free energy of any clean, high-energy surface after exposure to an atmosphere containing water vapour is mainly dependent upon the surface concentration of adsorbed water. Recent studies2,3 using surface-specific analytical techniques have indicated that the surfaces of high-energy solids are not only covered by adsorbed water but also by other contaminants such as nitrogen and organic adsorbates such as hydrocarbons.

The spreading of liquids on low-energy surfaces. V. Perfluorodecanoic acid monolayers

The wettability of more than 90 pure liquids on oriented close-packed monolayers of perfluorodecanoic acid adsorbed on platinum, copper, brass, and glass has been studied. Comparison with results reported earlier for solid polyethylene, oriented monolayers of octadecylamine, and solid polytetrafluoroethylene shows that oriented, close-packed monolayers of perfluorodecanoic acid are the most nonwettable surfaces ever found and that for most liquids in contact with these surfaces the contact angle (θ) decreases in the order θ CF 3 > θ CF 2 > θ CH 3 > θ CH 2. Values of the work of adhesion, the initial and final spreading coefficients, and the free energy decrease on immersion, f SL , were computed from contact angle measurements and surface tensions using the assumption justified by experiment that f SV ∘ = 0. From the values of f SL , it has been possible to estimate the free surface energy of the surfaces prepared from films of perfluorodecanoic acid. Structural differences determining the wetting properties of liquids are presented, and the effect of increase in surface tension of higher members in a homologous series is discussed. Observed modifications of the wetting properties of high-energy solids by adsorption of a single monolayer, demonstrate beyond doubt the short-range nature of the forces involved in wetting. In view of the remarkable resistance to wetting shown by surfaces of close-packed perfluoromethyl groups, surface-active compounds with —CF 3 groups and hydrophilic groups at opposite ends of the molecule should have unusual and valuable properties, and they deserve to be treated as a special class of surface-active compounds.