{"id":449,"date":"2018-12-04T08:30:32","date_gmt":"2018-12-04T08:30:32","guid":{"rendered":"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=449"},"modified":"2018-12-04T09:26:34","modified_gmt":"2018-12-04T09:26:34","slug":"experimental-crystal-growth-ii-gel-technique","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/chapter\/experimental-crystal-growth-ii-gel-technique\/","title":{"rendered":"Experimental crystal growth\u2013II (gel technique)"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n
\r\n\r\n \r\n\r\n \r\n\r\n27.1 Introduction.<\/strong>\r\n\r\n \r\n

The gel technique of crystal growth is the most simple and versatile technique compared to other methods of crystal growth under ambient conditions. Since the method involves minimum technological sophistication it has been exploited for the growth of useful crystals suitable for research and technology. Crystal growth in gel media yields well facetted crystals in several cases which exhibit many crystallographic features. Gel growth has also provided opportunities for the experimentalists to conduct studies under microgravity conditions in space ships. The method has gained attention owing to its suitability for growing crystals of biological macromolecules and in studies involving crystal deposition diseases in human beings.<\/p>\r\n \r\n

It was in 1896, that a German chemist Edward Liesegang, who was a collide chemist and a photographer, first observed the periodic precipitation phenomenon. Mechanism of the phenomenon of periodic precipitation of rings was described by German chemist Oswald at the end of 19thcentury.These discoveries led many investigators to concentrate their studies on colloids and observe this particular phenomenon .The idea of utilisation of gel as a medium of crystal growth was put forward by Fisher and Siemen in 1926. The art and science of growing crystals in gels remained largely dormant during the period 1930 \u2013 1960. Henisch has described the technique of crystal growth in gels in his book\u201d Crystal Growth in Gels \u201c. Crystal growth in gels is an intermediate process between growth from solid phases and growth from liquid solutions.<\/p>\r\n \r\n\r\n27.2<\/strong>\u00a0\u00a0 The Structure and Properties of Gel.<\/strong>\r\n\r\n \r\n

The gel is defined as a highly viscous two-component semi-solid system rich in liquid and having fine pores. These fine pores may allow the free passage of electrolytes and sustain nucleation. The gel medium works as a \u201csmart material\u201d that is sensitive to the minutest changes in the ambience. Gels are broadly divided into two: Organic and Inorganic. If water is in the place of liquid it is called hydro-gel.<\/p>\r\n \r\n

The various types of gels used in crystal growth experiments are hydro-silica gel (sodium meta silicate), agar-agar gel , carbohydrate polymer gelatin gel (resembling protein structure) , clay gel , soap fluid , poly-acrylamide , hydroxide in water, Oleates , stereates etc. The silica gel made out of sodium meta silicate is often used because of its easy availability and better performance in growing\u00a0many crystal compounds. In some particular cases, organic gels are preferred and the selection of gel depends entirely on the nature of the electrolytes involved.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n\r\n27.3<\/strong>\u00a0\u00a0\u00a0 Preparation of hydro-silica gel.<\/strong>\r\n\r\n \r\n

The water glass (sodium meta silicate) powder is dissolved in doubly distilled water and by changing the hydrogen ion concentration (pH) of the solution; the desired gel can be prepared. The pH factor is an important parameter which determines the rate of polymerization and the speed of gel setting. For maintaining the acidity or the hydrogen ion concentration, an acid in requisite concentration is added to the system. During gelation the pH of the mixture varies and the gelation period varies from few minutes to hours or days. For growing the crystals of a good number of materials, the pH ranging from 3 to 6 are found suitable. Acid commonly used to acidify the gel are nitric acid, hydrochloride acid, tartaric acid, acetic acid, oxalic acid, selenous acid etc. Fresh gels of pH ranging from 6 to 8 are highly transparent in nature. Gel ageing is another parameter which is required to be monitored. The ageing hardens the gel and decreases its transparency and easiness of diffusion.<\/p>\r\n \r\n

One of the most important factors affecting the hardness of the gel medium is its density. Dense gels produce poor quality crystals. On the other hand gel of insufficient density takes a long time for formation and it is mechanically unstable. Range of densities in between 1.03 to 1.06g\/cc yield better results in many systems. The optimum density allows the growth of reasonably bigger crystals by this technique.<\/p>\r\n \r\n\r\n27.4<\/strong>\u00a0\u00a0\u00a0 The Mechanism of Gelling<\/strong>.\r\n\r\n \r\n

Gels are two-phase systems comprising a porous solid with liquid-filled pores. The solid separating the pores is quite thin, typically less than 2 x 10\u25006 mm. The size of the pores depends on concentration and pH of the gel. While the median dimensions of pores are about 0.2\u03bcm in gels of high density , the dimensions of pores in dilute gels gets bigger which may be taken as around as around 2\u03bcm. There is a large spread of dimensions in gels; the ratio of maximum to minimum diameters of pores being a factor of about 10 to 50 or even more. The pore size can be manipulated by varying the growth parameters like gel density, concentrations of reactants, pH and the surrounding\u00a0temperature.<\/span><\/p>\r\n \r\n

Structures of many of the gels are complicated and still not fully understood. However, the structure and properties of the hydro silica gel is described in great detail in the literature. It is worth noting that the hydro silica gel is the polymerised form of silicic acid. When sodium metasilicate is dissolved in water, mono-silicic acid is produced due to the reaction:<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n\r\nNa2<\/sub>SiO3<\/sub>+ 3H2<\/sub>O \u2192 H4<\/sub>SiO4<\/sub> + 2NaOH\r\n\r\n \r\n

This is a reversible process and the by-product, which is strong alkali (NaOH) remains in the solution. This is the reason that the solution remains alkaline. The mono-silicic acid liberates the hydroxyl ions and polymerises as shown below:<\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\"\"\r\n\r\n \r\n

This process continues until\u00a0\u00a0the entire molecule becomes part of of three dimensional network. The extremely strong and which is irreversible. A section of the cross -linked\u00a0polymer is shown below:<\/p>\r\n \r\n\r\n\"\"\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n

The by-product resulting from the reaction is water and it accumulates on top of the gel surface because it is lighter than the gel. This phenomenon is called syneresis. Much of the water has its origin in the above condensation process and some may arise from purely mechanical factors connected with a small amount of gel shrinkage. The well- known stability of the silicon-oxygen bonds is responsible for the fact that the polymerization is largely irreversible .The period of gelation\u00a0is controlled by pH value of the solution, though it is very difficult to control the total period of gelation precisely.<\/span><\/p>\r\n \r\n

In the above structure H<\/span>3<\/sub>SiO<\/span>4<\/sub>\u2500<\/sup> and H<\/span>2<\/sub>SiO<\/span>4<\/sub>2\u2500<\/sup> are also formed during the process of gelation.. The relative abundance of these products depends on the pH value. When the pH is high H2SiO42\u2500 ions are abundant and more active. The H3SiO4\u2500 is favoured by low pH and they are believed to be responsible for triggering the polymerization. In due course, cross linkages are formed between the chains and these contribute to sharp increase of viscosity that is clearly visible in gelation. The first result of such a linking process would be the production of sol particles, and the extent to which such particles then continue to associate to form macroscopic gel depend on their surface charge.<\/span><\/p>\r\n \r\n

The important feature of gel is its abundance of pores. The gel matrices contain fine pores having different dimensions. The pore is usually of the order of a micro- meter in size. The pores may behave as capillary for the transport of ions .X-ray studies of silica gel show that it has close resemblance with silica glass but with some in-homogeneities.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n\r\n27.5<\/strong>\u00a0\u00a0\u00a0\u00a0 Basics of gel technique<\/strong>\r\n\r\n \r\n

Gel growth is defined as a method in which two solutions of soluble salts are brought in contact by diffusion through gel. The precipitation of an insoluble phase within the gel results in subsequent nucleation followed by crystal growth. The gel performs the following role:<\/p>\r\n \r\n\r\n(i) It limits the number of critical size nuclei formed in the medium.\r\n\r\n \r\n

(ii) It decreases the rate of growth by controlling the rate of diffusion of reacting ions and by governing the<\/p>\r\n

speed of reaction at the crystal growing surface.<\/p>\r\n \r\n\r\n(iii)\u00a0\u00a0\u00a0 It acts as a three \u2013 dimensional crucible holding the crystal in space within the medium without\r\n

exerting strain on the growing crystal.<\/p>\r\n \r\n\r\n27.6<\/strong>\u00a0\u00a0\u00a0\u00a0 Crystallisation Process In Gel Medium.<\/strong>\r\n\r\n \r\n

Based on the nature of the chemical reactions and physical changes involved during the growth processes, gel medium growth may be classified into four groups:<\/p>\r\n \r\n\r\ni) Growth by chemical reaction,\r\n\r\nii) Growth by chemical reduction,\r\n\r\niii) Growth by complex dilution method,<\/span>\r\n\r\niv) Growth by solubility reduction method<\/span>\r\n\r\n<\/div>\r\n

\r\n\r\nLet us briefly describe each one of them:\r\n\r\n \r\n\r\n27.6.1<\/strong>\u00a0\u00a0 Growth by chemical reaction.<\/strong>\r\n\r\n \r\n

The method can be taken as the basis of all methods of gel growth and is actually a crystal growth by chemical reaction. The crystals, which are insoluble or slightly soluble in water and decompose before reaching their melting points, can be grown by adopting this procedure. The basic requirements of using this method for the growth of crystals are:<\/p>\r\n\r\n