## 4 Operating Parameters  
Particle generation procedures comprise a large number of variables that can be set up to optimize production with regards to their functionality. Such variables have an impact on two essential parameters. On the one hand, crystallization rate and, on the other hand, precursors and generated products solubility. With regards to crystallization rate, Professor [PERSON] proved that the rate-determining step in the recrystallization global kinetics is the superficial reaction rate, since diffusivity at conditions near the critical point is very high [16]. With regards to solubility, high values are required for the precursors, while low values are required for the products. Therefore, variations in operating variables should focus on improving the surface reaction rate and the solubility of the reagents and precursors. The variables are as follows:  
* Surface modifier: organic molecules such as formic acid, octanoic acid, or hexane, which by forming ligands with metals have an impact on the growth of metal and specifically change the morphology of the particles, generally by reducing their size. * Operating temperature: Temperature is a variable with an impact on conversion and particle size. Generally, an increment in temperature makes the conversions grow, but also particle size increases since both reaction and crystallization rates are also augmented. There is, therefore, an optimum temperature level that lets us obtain a large number of particles of the desired size. * Reactor heating rate: In general, a rapid heating rate results in a greater number of crystals cores in a shorter time. This is due to a greater solubility of the precursors in the reaction medium. However, low heating rates, generally result in a gradual generation of crystals cores, which favors a greater distribution of particle sizes. Depending on the kinetics of each specific nanoparticle, a high heating rate may be a limiting factor, i.e., it may control the process when the kinetics are fast, but it would not affect the process when the kinetics are slow. The heating rate is usually reduced by using microreactors or different heating systems. Both aspects will be explained in more detail in Section 5.1. * pH: pH is an essential factor to consider in hydrothermal synthesis of particles. Acids and basis are normally used to alter pH depending on the solubility of the specific precursors to be used. The pH value can affect the appearance of impurities, as can be observed in the article by [PERSON] et al. [98], or even be an influent parameter in the modulation of the particles morphology, as can be seen in the article by [PERSON] et al. [99]. That is, depending on the particle to be synthesized, there is a specific optimum pH. * Residence time: When working under subcritical conditions, reactions are slow. Therefore, the required residence times may be as long as several hours. When working under supercritical conditions the reactions are extremely rapid, and, therefore, residence time is reduced to minutes or even seconds. Exceeding optimum residence time may lead to particle size increments. * Precursor concentration: Generally, when the concentration of the reagent is low, the sizes of the particles obtained are more regular. However, if the precursor's concentration is really high, a larger number of particles will be produced, although with more irregular sizes since agglutinations may lead to greater size particles. * Pressure: Pressure, a priori, has no direct effect on particle forming rate. It may, though, affect thermophysical properties, such as solubility, density, etc. It is, therefore, important to work under pressure levels that allow water to remain in its liquid phase or supercritical state at the required operating temperatures. It is important to specify, therefore, that no optimum value or range of values can be specified for each one of the above explained parameters, since their optimum value will depend on the final particle to be obtained as well as on the precursors used.