Experimental precipitation techniques

In this experimental section, the production of polysulfone microcapsules containing vanillin is described. Our method is based on the phase inversion by immersion precipitation technique. The herein described process has been successfully employed for the encapsulation of vanillin into polysulfone microcapsules and, in addition, vanillin release from those capsules has been characterized. Polysulfone/vanillin microcapsules could have an application in laundry industry and also in medical applications. ... 

It is even more difficult to estimate not only one but four parameters (nucleation rate, growth rate, agglomeration kernel and disruption kernel) simultaneously from a particle size distribution. The errors are likely to be unacceptably high and it might be impossible to distinguish between the mechanisms involved. Therefore, an alternative sequential technique has been developed to obtain the kinetic parameters nucleation rate, growth rate, and agglomeration and disruption kernels from experimental precipitation data. 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

The separation between substrates in batch-produced CBD CdS is also a likely important factor for reproducibility. Arias-Carbajal Readigos et al.29 studied thin-film yield in the CBD technique as a function of separation between substrates in batch production. Based on a mathematical model, scientists proposed and experimentally verified that, in the case of CdS thin films, the film thickness reaches an asymptotic maximum with an increase in substrate separation. This behavior is explained on the basis of a critical layer of solution that exists near the substrate, within which the relevant ionic species have a higher probability of interacting with the thin-film layer than of contributing to precipitate formation. The critical layer depends on the solution composition and the temperature of the bath, as well as on the duration of deposition. 

When the aluminium halide solution is added to a solution of monomer, only the aluminium present as cations can initiate (disregarding any active cations that may have been formed by reaction of the initiator with impurities in the solvent) and the unionised aluminium halide becomes complexed with monomer and thus formation of further ions from it stops or becomes at best a very slow process. This is what was called the Esso technique [1] and it was the commonest method of experimentation. If the system is sufficiently free from terminating impurities and if the propagating ions are not occluded in precipitated polymer, all the monomer should be consumed eventually, and so the bound aluminium halide should in the end become free by the shifting of equilibrium (ii). However, these conditions are generally unfavourable for the reaction going to completion, and it comes virtually to a stop at incomplete conversion. 

Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-a-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecular-level probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. 

Surface spectroscopic techniques must be separated carefully into those which require dehydration for sample presentation and those which do not. Among the former are electron microscopy and microprobe analysis, X-ray photoelectron spectroscopy, and infrared spectroscopy. These methods have been applied fruitfully to show the existence of either inner-sphere surface complexes or surface precipitates on minerals found in soils and sediments (13b,30,31-37), but the applicability of the results to natural systems is not without some ambiguity because of the dessication pretreatment involved. If independent experimental evidence for inner-sphere complexation or surface precipitation exists, these methods provide a powerful means of corroboration. 

A very reactive form of a finely divided metal is a so-called Rieke powder [79]. These materials are produced as fine powders by chemical precipitation during the reduction of various metal halides ivith potassium metal in refluxing tetrahydrofuran. Obviously this is a potentially hazardous laboratory procedure and ultrasound has provided an alternative method of preparation of these extremely valuable reagents [80]. The sonochemical technique involves the reduction of metal halides with lithium in TH F at room temperature in a cleaning bath and gives rise to metal powders that have reactivities comparable to those of Rieke powders. Thus powders of Zn, Mg, Cr, Cu, Ni, Pd, Co and Pb were obtained in less than 40 min by this ultrasonic method compared with reaction times of 8 h using the experimentally more difScult Rieke method (Tab. 3.1). 

Chapter 10 provides an exhaustive description of how these techniques can be applied to a large number of industrial alloys and other materials. This includes a discussion of solution and substance databases and step-by-step examples of multi-component calculations. Validation of calculated equilibria in multi-component alloys is given by a detailed comparison with experimental results for a variety of steels, titanium- and nickel-base alloys. Further selected examples include the formation of deleterious phases, complex precipitation sequences, sensitivity factor analysis, intermetallic alloys, alloy design, slag, slag-metal and other complex chemical equilibria and nuclear applications. 

A variety of experimental conditions have been used for oxidations of alcohols by Cr(VI) on a laboratory scale. For simple unfunctionalized alcohols, oxidation can be done by addition of an acidic aqueous solution containing chromic acid (known as Jones reagent) to an acetone solution of the alcohol. Oxidation normally occurs rapidly, and overoxidation is minimal. In acetone solution, the reduced chromium salts precipitate, and the reaction solution can be decanted. Entries 2,3 and 4 in Scheme 12.1 are examples of this technique. 

Systems in which surfactant precipitate is present in substantial quantities in equilibrium with micelles and monomer are of interest. For example, in a technique for improving mobility control in oil reservoirs, surfactant is purposely precipitated in the permeable region of a reservoir to plug it (44). When substantial precipitate is present, crystals of different composition can be in simultaneous equilibrium. Experimental study and modeling of these systems where several Ksr- relationships are simultaneously satisfied will be a challenging task. 

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