Patrick Gane

Practical observation of deviation from Lucas-Washburn scaling in porous media

Abstract This work analyses the applicability of the Lucas–Washburn equation to experimental observations of imbibition into real network structures. The experimental pore structures used in this study are constructed from tablets of two finely ground calcium carbonates, with defined differences in particle size distribution. These are compressed under a range of different applied pressures to achieve a controlled series of porosities while maintaining the surface chemical, particulate and morphological pore characteristics constant. The porosities are determined by mercury intrusion porosimetry applying corrections for mercury compression and penetrometer expansion together with a correction for sample skeletal compression (Gane et al., J. Am. Chem. Soc., 35 (1996)). Imbibition studies are made by bringing each porous sample into contact with a supersource of liquid and the dynamic imbibition is recorded gravimetrically. Results follow a long timescale macroscopic absorption rate depending on the square root of time but show a failure to scale according to pore size in the Lucas–Washburn equation even though the constants of surface energy, contact angle and fluid viscosity have been maintained. Furthermore, values of average measured pore radius are shown to be finer than the Lucas–Washburn predicted equivalent hydraulic capillary radius. The predominance of a relevant pore size within a given pore size distribution forming a selective pathway filling based on inertial retardation of larger pores and short-term linear time wetting in finer pores is argued to account for the departure from simple pore size scaling.

Dynamic absorption into simulated porous structures

Abstract A computer model, pore-cor 1 , has been used to simulate the permeation of fluid into porous structures by applying a wetting algorithm for fluids undergoing both inertial and viscous dynamical absorption. These structures comprise cubic pores connected by cylindrical throats on a three-dimensional 10×10×10 position matrix. Previously, pore-cor has been used to simulate the void structure of porous media by fitting as closely as possible the simulated mercury intrusion curve of the model structure to that of the experimentally determined mercury intrusion curve of the actual sample. This refined model structure was then used to simulate the absorption of a number of different fluids and confirmation with experimental absorption data, adopting the same fluid, provided the necessary confidence in the model. Based on this practical experience, the framework is developed from experimental comparison so that pore-cor can now be applied as a ‘predictor’ tool rather than a simulator of existing experimental results. The structures used here have been generated with the aim of isolating certain parameters so their influence on the absorption behaviour of a fluid can be identified. It is shown how, keeping the porosity constant, structures can be represented in a number of different ways and that these structures will have vastly different absorption behaviour. Reducing the value of the fluid density in the simulation has also been investigated to show where the transition occurs in the absorption behaviour from the linear t-dependent short timescale inertial Bosanquet regime to the √t Lucas–Washburn (LW) viscous regime. Such a system of connecting pores is representative of pigmented paper coating structures and, for example, by changing the particle size distribution of the pigment under suitably modified calendering conditions, pore structures of equal porosity could exist in practice and have drastically different effects on fluid absorption. Implications on, say, the way an ink would set on the coating surface, can have dramatic practical significance. To be able to predict these effects and to design optimal coating structures for the fluids and inks used in a variety of printing, lacquering and glueing processes is seen as a significant advance, obviating the need for expensive matrix-designed production trialling.

Influence of Surface Topography on Adhesive and Long-Range Capillary Forces between Hydrophobic Surfaces in Water

We report on the interactions between a hydrophobic probe particle and surfaces with nanoscopic surface features. These surfaces have been prepared by spin-coating of nanoparticles and by polishing. The surface topography was characterized by AFM, using the methods of high-resolution imaging, low-resolution imaging using the probe particle, and by the rolling ball method. The spin-coated surfaces can be characterized as nanostructured due to the high density of nanoparticles that on a short length scale provides a regular pattern of crevices and hills. On these surfaces a larger waviness is also distinguished. In contrast, the polished surfaces display sharp nanoscopic peaks and hardly any crevices. In all cases the dominant force at short separations was found to be a capillary attraction due to the formation of an air/vapor condensate. Our data show that the large-scale waviness of the surface does not significantly influence the range and magnitude of the capillary attraction, but large local variations in these quantities are found. The large variation in adhesion force corresponds to a small variation in local contact angle of the capillary condensate at the surfaces. The report discusses how the nature of the surface topographical features influences the capillary attraction by influencing the local contact angle and by pinning of the three-phase contact line. The effect is clearly dependent on whether the surface features exist in the form of crevices or as extending ridges.

Absorption Rate and Volume Dependency on the Complexity of Porous Network Structures

Results of simulated supersource imbibition into model network structures are compared with experimental observations of real network structures determined by dynamical gravimetric fluid uptake. A computer model, Pore-Cor, has been used previously to simulate the imbibition of fluid into porous structures by applying an imbibition algorithm for fluids undergoing both inertial and viscous dynamical absorption (Schoelkopf et al., 2000). The structures comprise cubic pores connected by cylindrical throats on a three-dimensional 10 × 10 × 10 position matrix. The absorption curves for model structures with monosized pore and throat size ranges and for polydisperse pore and throat size distributions centred around 0.1 µm, increasing from 0.1 µm as a lower limit, and decreasing from 0.1 µm as an upper limit, respectively, are analysed. A relevant observable porosity and 50% volume intrusion radius (r50) are obtained using simulated mercury intrusion. Experimental network pore structures were made using compressed tablets, formed under a series of pressures, of two finely ground calcium carbonates with defined differences in skeletal particle size distribution. The surface chemical, particulate and morphological pore characteristics were maintained constant over a range of porosities using controlled wet grinding and careful use of dispersant levels such that the ratio of dispersant to BET surface area was held constant. The experimental porosities were determined by mercury intrusion porosimetry applying corrections for mercury compression and penetrometer expansion together with a correction for sample skeletal compression (Gane et al., 1996). The applicability of the Lucas–Washburn equation is examined by defining two equivalent hydraulic radii, one based on a Darcy absorption length (rehcDarcy) and the other on a volume uptake (rehc), respectively. The results from the model structures having distributions of pores and throats, which contain either small or large pores, respectively, follow the experimental results qualitatively. Both approaches show a long timescale macroscopic absorption rate depending approximately, but not exactly, on the square root of time. The two experimental series, however, fail to scale with each other via the Lucas–Washburn equation in accordance with pore size, r50. Porosity is shown to be the main factor determining the volume absorption rate, and, when used as a weighting factor, gives linear correlation-scaling between r50 and a derived volume-based r′ehc equivalent hydraulic radius, obtained from an analytical expression of the observed Darcy-based rehcDarcy. The experimental samples showed that the directly observed rehc and the calculated r′ehc, derived from Darcy length, were equal, but this was not the case for the model values. A factor β = r′ehc/r50 is shown to be a possible descriptor of the sample network complexity and an indicator for the probability level of pore filling during the absorption dynamic.

Floating gastroretentive drug delivery systems: Comparison of experimental and simulated dissolution profiles and floatation behavior

INTRODUCTION Gastroretentive drug delivery systems (GRDDS) play an important role in the delivery of drug substances to the upper part of the gastrointestinal tract; they offer a possibility to overcome the limited gastric residence time of conventional dosage forms. AIMS The aim of the study was to understand drug-release and floatation mechanisms of a floating GRDDS based on functionalized calcium carbonate (FCC). The inherently low apparent density of the excipient (approx. 0.6 g/cm(3)) enabled a mechanism of floatation. The higher specific surface of FCC (approx. 70 m(2)) allowed sufficient hardness of resulting compacts. The floating mechanism of GRDDS was simulated in silico under simulated acidic and neutral conditions, and the results were compared to those obtained in vitro. METHODS United States Pharmacopeia (USP) dissolution methods are of limited usefulness for evaluating floating behavior and drug release of floating dosage forms. Therefore, we developed a custom-built stomach model to simultaneously analyze floating characteristics and drug release. In silico dissolution and floatation profiles of the FCC-based tablet were simulated using a three-dimensional cellular automata-based model. RESULTS In simulated gastric fluid, the FCC-based tablets showed instant floatation. The compacts stayed afloat during the measurement in 0.1 N HCl and eroded completely while releasing the model drug substance. When water was used as dissolution medium, the tablets had no floating lag time and sank down during the measurement, resulting in a change of release kinetics. CONCLUSIONS Floating dosage forms based on FCC appear promising. It was possible to manufacture floating tablets featuring a density of less than unity and sufficient hardness for further processing. In silico dissolution simulation offered a possibility to understand floating behavior and drug-release mechanism.

Compaction of functionalized calcium carbonate, a porous and crystalline microparticulate material with a lamellar surface

In the present study, we aimed to characterize the compressibility and compactibility of the novel pharmaceutical excipient, functionalized calcium carbonate (FCC). We studied three FCC modifications and compared the values for compressibility and compactibility with mannitol, microcrystalline cellulose (MCC), and ground calcium carbonate (CC 330) as well as mixtures of paracetamol and MCC or FCC at drug loads of 0%, 25%, 50%, 75%, and 100% (w/w). We used Heckel analysis, modified Heckel analysis, and Leuenberger analysis to characterize the compaction and compression behavior of the mixtures. Compaction analysis of FCC showed this material to markedly differ from ground calcium carbonate, exhibiting properties, i.e. plastic deformability, similar to those of MCC. This effect was attributed to the highly lamellar structure of FCC particles whose thickness is of the order of a single crystal unit cell. According to Leuenberger parameters, we concluded that FCC-based tablet formulations had mechanical properties equal or superior to those formulated with MCC. FCC tablets with high tensile strength were obtained already at low compressive pressures. Owing to these favorable properties (i.e. marked tensile strength and porosity), FCC promises to be suitable for the preparation of solid dosage forms.

Robust hydrophobic surfaces displaying different surface roughness scales while maintaining the same wettability.

A range of surfaces coated with spherical silica particles, covering the size range from nanometer to micrometer, have been produced using Langmuir-Blodgett (LB) deposition. The particles were characterized both in suspension and in the Langmuir trough to optimize the surface preparation procedure. By limiting the particle aggregation and surface layer failures during the preparation steps, well-defined monolayers with a close-packed structure have been obtained for all particle sizes. Thus, this procedure led to structured surfaces with a characteristic variation in the amplitude and spatial roughness parameters. In order to obtain robust surfaces, a sintering protocol and an AFM-based wear test to determine the stability of the deposited surface layer were employed. Hydrophobization of the LB films followed by water contact angle measurements showed, for all tested particle sizes, the same increase in contact angle compared to the contact angle of a flat hydrophobic surface. This indicates nearly hexagonal packing and gives evidence for nearly complete surface wetting of the surface features.

Influence on Pore Structure of Micro/Nanofibrillar Cellulose in Pigmented Coating Formulations

Nano and microfibrillated cellulose (NFC and MFC, respectively, collectively termed MNFC) is known to interact strongly with water, related to its high polarity and surface area. The swelling behaviour acts to form a gel with high water retention properties. The observation that nanocellulose could possibly be used in paper or other coating formulations, as a co-binder, for example, raises a question about the possible effects it could have on coating pore structure. In this study, we analyse the pore structure of pigmented coatings, liquid absorption and permeability, in respect to the influence of partially substituting traditional co-binder carboxymethyl cellulose with MNFC. The contrast between polar water and non-polar liquid, such as alkane, is used to probe the water interactive and extractable in-coating (internal) gel-formation properties of the nanocellulosic materials. These contrasting liquids are important in many processes, such as offset printing, but also in respect to exposure of coatings in general to environmental factors in application. Results show that permeability to liquid water is dramatically reduced when nanocellulosic material is present, though water can permeate by diffusion through the nanocellulose gel network. Long timescale exposure to water during absorption leads to extraction of any soluble salts remaining after the chemical treatment of the fibrillar material during production. Inert alkane, on the other hand, can absorb and permeate freely without interactive hindrance from the nanocellulose, with no extractive effect. Such a construct could in principle be considered for use as an oil-water differential membrane or for slow release concepts in aqueous systems by loading soluble deliverable materials within the nanocellulosic gel.

From colloidal spheres to nanofibrils: Extensional flow properties of mineral pigment and mixtures with micro and nanofibrils under progressive double layer suppression

Suspensions of mineral pigment and cellulose fibrillar derivatives are materials regularly found in the forest products industries, particularly in paper and board production. Many manufacturing processes, including forming and coating employ flow geometries incorporating extensional flow. Traditionally, colloidal mineral pigment suspensions have been considered to show little to no non-linear behaviour in extensional viscosity. Additionally, recently, nanofibrillar materials, such as microfibrillar (MFC) and nanofibrillar cellulose (NFC), collectively termed MNFC, have been confirmed by their failure to follow the Cox-Merz rule to behave more as particulate material rather than showing polymeric rheological properties when dispersed in water. Such suspensions and their mixtures are currently intensively investigated to enable them to generate likely enhanced composite material properties. The processes frequently involve exposure to increasing levels of ionic strength, coming either from the weak solubility of pigments, such as calcium carbonate, or retained salts arising from the feed fibre source processing. By taking the simple case of polyacrylate stabilised calcium carbonate suspension and comparing the extensional viscosity as a function of post extension capillary-induced Hencky strain on a CaBER extensional rheometer over a range of increasing salt concentration, it has been shown that the regime of constriction changes as the classic DLVO double layer is progressively suppressed. This change is seen to lead to a characteristic double (bimodal) measured viscosity response for flocculated systems. With this novel characteristic established, more complex mixed suspensions of calcium carbonate, clay and MNFC have been studied, and the effects of fibrils versus flocculation identified and where possible separated. This technique is suggested to enable a better understanding of the origin of viscoelasticity in these important emerging water-based suspensions.

Influence of wetting and dispersing agents on the interaction between talc and hydrophobic particles.

The interactions between a natural talc surface and a model hydrophobic particle have been investigated in aqueous solutions by employing the atomic force microscopy (AFM) colloidal probe technique. The results demonstrate the presence of long-range attractive forces due to bridging via preadsorbed or induced bubbles/cavities. Due to the natural heterogeneity of talc, and the stochastic nature of the bubble bridging process, the variability in the range and magnitude of the attraction is larger than that for cases when other interactions predominate or than that when only model surfaces are used. Addition of poly(acrylic acid), a common dispersing agent, did not affect the measured forces. Thus, we conclude that poly(acrylic acid) does not adsorb to the basal plane of talc. In sharp contrast, addition of Pluronic PE6400, a nonionic triblock polymer used as wetting agent, resulted in complete removal of the bubble-induced attractive force. Instead, a short-range steric repulsion is the dominating feature. Clearly, Pluronic PE6400 is able to displace air bubbles from the surface and prevent their formation when the particles come into contact. These are suggested to be important features of efficient wetting agents.