Chimie douce

Metal chalcogenides, apart from their technological significance in industrial applications, have played an important role in the development of new synthetic concepts and methods in the area of solid-state chemistry. A great example is alkali metal intercalation into TiS2 (Chap. 6) first reported three decades ago, which highlighted the then-novel synthetic approach called soft chemistry chimie douce). This low-temperature process allows for new compounds to be obtained while retaining the structural framework of the precursor. Related to this concept is the... 

Rouxel, J., Tournoux, M. and Brec, R. (eds)(1994) Soft Chemistry Routes to New Materials Chimie Douce, Trans Tech Publications, pp. 422. 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

Using coprecipitation methods with a suitable mixture of solutions described above, the resulting LDH materials are often poorly crystallized and exhibit compositional fluctuations due mainly to the difference in the values of the pH at which the precipitation of M(II)(OH)2 and M(III)(OH)3 hydroxides occurs. Consequently, the chemical formula of the final material may not reflect the composition of the solution prior to the precipitation as noted in Chapter 1. Controlling the amount of anion incorporated under such conditions is very difficult. A "chimie douce method has been proposed by Delmas et al. in an effort to overcome this problem [181,182]. The process is illustrated schematically in Fig. 8. Since the synthesis starts from a highly crystalline layered y-oxyhydroxide precursor, it was suggested that this favored the formation of very crystalline LDHs with controllable M(1I)/M(III)... 

Fig. 8 Schematic illustration of the successive reaction steps in the preparation of LDH by chimie douce Reprinted with permission from [182]. Copyright Elsevier...

The solid materials obtained by these methods are always governed by the general thermodynamic laws represented by the phase diagrams. In contrast, the methodologies included in the phrase Chimie Douce involve kinetic control of the solid synthesis when non-reversible reactions are used. Therefore, solids prepared by this route are not the most... 

Thus the sol-gel methodology is an alternative to classical thermodynamic routes for the synthesis of solids. It is also evident that the preparation of the solids through Chimie Douce bridges molecular chemistry and solid state chemistry. This chapter will be focused on material prepared by the sol-gel process, where an organic part is associated with a silica or siloxane Si—O—Si network. Two types of organic-inorganic hybrid materials can be prepared by the sol-gel process and are completely different the nanocomposites and the nanostructured hybrid materials6-9. 

Materials chemistry contains all the elements of modem chemistry. These include synthesis, structure, dynamics, and properties. In synthesis, one employs all possible methods and conditions from high-temperature and high-pressure techniques to mild solution methods (chimie douce or soft chemistry).3 Chemical methods generally tend to be more delicate, often yielding novel, metastable products. Kinetic rather than thermodynamic control of reactions favors the formation of such structures. Supramolecular organization provides new ways of designing materials. 

Polymorphism including also the formation of metastable and amorphous phases is widely observed. The properties of respective phases are strongly dependent on the particular structure and on temperature. Careful investigations of phase transitions may lead to a better understanding of the conditions necessary for the synthesis of new phases. A well-known example comes from the pyrochlore form of FeF3, which was first prepared in the pioneering days of what is nowadays called chimie douce . 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

Crystalline pyr-FeFs , a metastable cubic form of FeFs with pyrochlore-related structure, was obtained in a topotactical oxidation reaction of NH4Fe2F6 with Br2 in acetonitrile. At this example, the designation "chimie douce (soft chemistry) for this type of solid-state reaction under low-temperature conditions was propagated in the 1980s. ... 

Several attempts have been made to stabilize the layered LiMn02 phase by substitution of manganese ions with small amounts of chromium and aluminum by conventional solid state reaction and the introduction of cobalf and nickel on the manganese sites via the chimie douce reaction. Although these substituted layered compounds resulted in improvements in capacity retention in comparison to pure LiMn02, they have been found to be thermodynamically unstable with respect to their transformation to the less desired spinel structure. 

Sol-gel process (Ganguli, 1989 Brinker and Scherer, 1990) is another important method of preparation of glasses. Sol-gel method is essentially a chimie-douce process. A sol by definition is a suspension of colloidal particles, which are of submicron or nanometric size. If these particles have surface active groups such as hydroxyls, interparticle connections are established by a condensation reaction. If the condensation occurs in such as way that the condensation product namely H2O, or the solvent is locked up in the matrix of sol particles, a mildly rigid product is formed, which is known as a gel. A colloidal particle formation can be an intermediate stage and it is not necessary to start with a colloidal suspension only. For example, when sodium silicate is dissolved in H2O, it is hydrolyzed to give silicic acid which forms a gel. Si(OH)4 molecules condense to form Si-O-Si linkages as follows ... 

Specialized low-temperature techniques known collectively as chimie douce ( gentle chemistry ) methods can be used for certain types of compound. The ready formation of intercalation compounds, by insertion of species between layers of a host lattice, is described in Topic D5. 

As explained in Topic B6 the synthesis of solids often requires high temperatures, because of the slow diffusion of atoms. In intercalation compounds and some insertion compounds however, diffusion of guest species is more facile, and such compounds can often be made prepared under fairly mild conditions, sometimes known as chimie douce ( gentle chemistry ). Intercalation compounds of graphite can be made directly by exposure of the solid to Br2 or to alkali metal vapors. 

Another way of obtaining suspensions of anisotropic mineral moieties is by the spontaneous condensation of dissolved molecular species. A typical example of this process is the synthesis of V2O5 ribbons by using chimie douce (soft chemistry) techniques (Fig. 6) [35,36]. At pH=2, V(V) species exist in an octahedral coordination with a V=0 bond pointing along the Oz axis, three V-OH bonds in the xOy plane, and two bonded H2O molecules to fill the coordination sphere. Beyond a concentration threshold of 10 mol 1 , these vanadate species spontaneously condense in the xOy plane by two different reactions respectively called olation and oxolation reactions (olation V-0H-l-V-0H2 V-0H-V-i-H20 oxola-tion V-OH-l-V-OH—>V-0-V-i-H20) to form ribbons 1 nm thick, about 25 nm... 

Gopalakrishnan, J., Chimie Douce Approaches to the Synthesis of Metastable Oxide Materials Chem. 

M. Eiglarz. Chimie-douce - a new route for the preparation of new materials - some examples. Chemica Scripta, 28 3-7, 1988... 

The sol-gel process is one of the so-called chimie douce routes. It has been widely used a as synthetic route to the preparation of new materials, providing very homogeneous samples, even at low temperatures. 

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