Synthetic zeolites were studied in order to investigate their ability to encapsulate and to release drugs. In particular, a zeolite X and a zeolitic product obtained from a cocrystallization of zeolite X and zeolite A were examined. These materials were characterized by chemical analyses (ICP-AES), X-ray diffraction, nitrogen adsorption isotherm, scanning electron microscopy, laser diffraction, and infrared spectroscopy. Since ketoprofen was chosen as a model drug for the formulation of controlled-release dosage forms, it was encapsulated into these two types of synthetic zeolites by a soaking procedure. Drug-loaded matrices were then characterized for entrapped drug amount and thermogravimetric behavior. In both types of activated zeolites, the total amount of ketoprofen (800 mg) was encapsulated in 2 g of matrix. By using HPLC measurements, ketoprofen release studies were done at different pH conditions so as to mimick gastrointestinal fluids. The absence of release in acid conditions and a double phased release, at two different pH values (5 and 6.8), suggest that after activation these materials offer good potential for a modified release delivery system of ketoprofen.

Summary Syneresis is commonly believed to be incompatible with the application of polymer gels to reduce the permeability of porous media. In this paper it is shown that although the permeability of a gel-treated porous medium does increase as syneresis proceeds, the degree of permeability reduction in cores remains technologically useful even when 95% syneresis is observed in bulk samples. Arguments are presented for the preferential shrinking of syneresed gel into pore throats; this model explains why permeability reduction is maintained and accounts for several other experimental results. The absence of performance penalties for syneresis has significant implications for the applicability of gels for water shut off treatments in matrix formations. For example, it raises the possibility that polymer gels can be applied successfully in situations previously considered unfeasible because of the difficulty of maintaining gel stability, e.g. at high temperatures or in the presence of hard formation brines. Introduction The controlled transformation from solution to gel is the basis for water and gas shutoff treatments using cross-linked polymers. In these applications, an appropriately formulated cross-linker/polymer solution is pumped into the desired part of the reservoir. After the transition to gel occurs, the treated rock is rendered essentially impermeable, and so subsequently produced or injected fluids will be blocked by or re-directed around the treated rock volume. This technology is becoming increasingly important for reducing the production of water and gas from oil wells and for improving the distribution of injected fluids. The polymer solution-to-gel transformation is mediated by a chemical cross-linker. As the cross-linker bonds to reactive groups on adjacent polymer chains, the effective molecular weight of the polymer increases. Above some threshold the solution becomes a viscoelastic solid. Where too much cross-linker is present, cross-linking continues well past the point of gelation. This causes the polymer gel to contract in volume, expelling water as it does so. The phenomenon of gel contraction is known as syneresis. Depending on the composition, a syneresed gel may occupy as little as 5% of the initial solution volume. Syneresis can result not only from excessive cross-linking but also from chemical modification of the polymer during aging. The acrylamide-based polymers employed for water shut-off treatments are prone, to a greater or lesser degree, to hydrolysis at elevated temperature. Hydrolysis converts acrylamide groups on the polymer backbone to the acrylate functionality, whose interaction with divalent cations can lead to syneresis of the polymer gel. This form of syneresis is particularly relevant to the use of polymer gels in seawater or hard formation brines at elevated temperature. The search for polymers suitable for such applications has focused primarily on reducing the tendency toward hydrolysis. It seems obvious that syneresis would fatally compromise the usefulness of a gel for fluid-blocking applications. If the final gel volume were only a fraction of the initial solution volume, then part of the pore space of the treated rock would remain open for fluid flow. The intuitive appeal of this argument perhaps explains why it has scarcely been tested experimentally.

No abstract

No abstract

This paper describes an experimental investigation of the effect of various commercial anionic polyelectrolyte thinners on the stability of bentonite dispersions contaminated by mono and divalent salts. Combined adsorption and electrophoresis studies lead to the definition of a model for the mechanism of interaction between the polyelectrolyte and clay. The data presented suggest that, with the exception of FeCr-lignosulphonate, the thinners adsorb preferentially onto the edges of the clay particles. In the case of FeCr-lignosulphonate significant adsorption onto the faces of the bentonite is attributed to the bridging effect of the polyvalent metal ions. The different adsorption mechanisms have consequences for the amount of polyelectrolyte required to deflocculate and stabilise the dispersions. Flocculation tests confirm that polymers adsorbing selectively on the "edge" sites are more effective in stabilising the dispersion against flocculation than those which adsorb less selectively onto the bentonite surface. Interestingly, the rheological properties of bentonite dispersions improve even where the thinner produces a scarce deflocculation effect. At least two mechanisms appear to describe the rheology-reducing properties of the thinners studied. Thinners like FeCr-lignosulphonate, which do not deflocculate the dispersions, can be distinguished from others such as the polyacrylates, which are true dispersing agents for bentonite in the presence of high electrolyte concentration, by means of scaling analysis of the rheological results. Introduction Aqueous drilling fluids generally contain bentonite clay, a low cost additive which imparts the desired rheological characteristics to the fluid. However, in conditions of high salinity or at elevated temperature bentonite dispersions tend to flocculate and therefore a dispersing, "thinning", agent, usually an anionic polyelectrolytes may be added to reduce the viscosity and control the dispersion properties. These polymeric electrolytes include natural and modified natural products (lignite and lignosulphonate) as well as fully synthetic materials (e.g., either homo- and copolymers of the acrylic acid, or maleic anhydride-styrene sulphonate copolymers). Lignite and lignosulphonates are more effective dispersing agents when formulated with polyfunctional metals (Fe3+, Cr3+). It is known in general that the stability of a colloidal dispersion may be improved by modifying the surface charge of the bentonite or by providing a steric barrier against aggregation. The presently limited insight into bentonite dispersions does not provide a clear indication of the mechanisms by which the different polyelectrolytes affect the dispersion stability. Our interest in this question has led us to investigate the interaction between the polymers and clay by means of several colloidal techniques including adsorption isotherms, electrokinetic determinations, flocculation tests and rheological measurements. In order to achieve the greatest possible relevance to the phenomena that take place in drilling fluids, these studies have been carried out on dispersions of bentonite in the presence of polyelectrolyte thinners employed by the drilling industry and at alkaline pH. On the basis of the results obtain a fully consistent description of the basic phenomena underlying the stabilisation of bentonite dispersions has been developed. These studies also point the way to procedures by means of which the effectiveness of different dispersing agents can be evaluated. As a first step, we outline the colloidal properties of bentonite dispersions and then introduce an interpretative model for describing their rheological properties. P. 291^