why we must modify pH value in sol-gel method? What is the benefit of modify pH in this way, and the materials which are used to adjust the pH?
Dear Manar,
Modification of the pH during the sol-gel process, is necessary to catalyse the hydrolisys of the alkoxy moieties (for example: CH3CH2O- groups) which is then followed by condensation of the silica network. Moreover, the pH influences the rate of the gelation process. Strongly acidic values of pH (2-3) induce a slow gelation which is ideal for the formation of extended network while basic values of pH (8-9) induce a fast gelation, ideal for the formation of smaller aggregates (i.e. NPs).
You can find a very good explanation of each step of the process and the influence of the different parameters in the following book: "Sol-gel materials: Chemistry and Applications" by J. D. Wright and Nico A. J. M Sommerdijk.
Hi Manar,
Usually, pH value is modified in co-precipitation or Sol-gel method to control the particle size of the material.
Dear Manar,
Modification of the pH during the sol-gel process, is necessary to catalyse the hydrolisys of the alkoxy moieties (for example: CH3CH2O- groups) which is then followed by condensation of the silica network. Moreover, the pH influences the rate of the gelation process. Strongly acidic values of pH (2-3) induce a slow gelation which is ideal for the formation of extended network while basic values of pH (8-9) induce a fast gelation, ideal for the formation of smaller aggregates (i.e. NPs).
You can find a very good explanation of each step of the process and the influence of the different parameters in the following book: "Sol-gel materials: Chemistry and Applications" by J. D. Wright and Nico A. J. M Sommerdijk.
Dear colleague Manar Mustafa,
Have a nice time and good day.
Peace be up on you [ Al-Salamo Alicom wa rahmato Allah wa barakatoh]
There are many factors affecting the PH. Value; temperature, exposure to air and Heat.. Also taking in mind that the PH value represents the nature of the solution; acidic or basic (alkaline). In the sol-gel method one of the mentioned parameters may be changed. so the properties of the produced material will be different like; amorphousness, crystallization consequently grain size, particle size, porosity, homogeneity, density and the morphology, ....etc . The aforementioned parameters controls the structural, optical and electrical properties. So it is important to modify the pH value to get the properties stability of the produced material.
I wish this is useful and help you.
Yours
A. El-Denglawey
The pH is an important factor in sol-gel method. The controlling of pH value will assit to reach the compound phase and also the stability of the sol.
I found that this article/tutorial was very helpful for understanding the behaviours of silica sol-gel syntheses under different pH.
I can't comment on metal sol-gel reductions, but for silica the acid catalysed reactions favour the exposed ends of the silica chains, leading to long stringy matrixes and gels rather than mono disperse, spherical sols like those produced from the base catalysed reactions.
http://macro.lsu.edu/CoreCourses/MSweb4/IE/PreparingAerogels.pdf
Hi Manar,
The pH value is modified in co-precipitation or Sol-gel method to control the particle size and also morphology of the material.
Hello, Dear Manar,
What a sol-gel technology you are dealing with? It is getting "a chemical polymer gel"? or "physical gel"?
"Physical gels" can be obtained by destabilization and aggregation of sols. In other words, the formation of "physical gel" - the process is usually reversible. Freshly chemical precipitates amenable to peptization.
"Chemical gel" can not be followed by disaggregation. In this case the chemical reactions take place (e. g. in the case of alkoxides - hydrolysis of alkoxysilanes and condensation processes).
The hydrolysis rate minimum is at pH 7, and significantly increases with increasing concentration of protons or hydroxyl. As for the second step - the condensation reaction, then (in contrast to the hydrolysis reaction) proton concentration increasing does not increase the rate of condensation.
Sols - are thermodynamically unstable systems with high free surface energy and can exist only in the presence of stabilizing colloids. One of the most effective ways to stabilize - the electrostatic controlled by pH of system.
For the sol-gel process as substances that can be used to adjust the pH, there can be mentioned organic carboxylic and inorganic acids, including polymers (e.g., polystyrenesulfonic acid), organic and inorganic bases (alkalis), and salts.
Choice of the substance type for adjusting the pH of the system affects the kinetics of the individual stages of the multistage process (rate, the order of reaction), etc., and the characteristics and morphology of the products obtained.
Respectfully,
Svetlana V. Chuppina
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