**5. Conclusions**

Supercritical Fluid process can be an excellent alternative to conventional precipitation and encapsulation processes. RESS and SAS processes have been applied on the particle precipitation and co-precipitation of many active ingredient/polymer. Furthermore, SAS has advantages over RESS because SAS is usually operated under mild conditions compared with those of RESS, which is associated to relatively high temperature and high pressure. Anyway, the technical viability of the SAS process requires knowledge of the phase equilibrium of the system; its hydrodynamics (the disintegration regimes of the jet); the mass transfer between the jet generated and the continuous phase; and the mechanisms and kinetics of nucleation and crystal growth. Above MCP the surface tension vanishes before jet break up occurs and the jet evolves as gaseous plume producing nanoparticle. However near MCP the jets atomize into droplets producing spherical microparticles.

At the University of Cádiz, amoxicillin, ampicillin and ethyl cellulose have been sucessfully precipitated by SAS process. The concentration was the factor that had the greatest influence on the PS and PSD. An increase in the initial concentration of the solution led to larger particles sizes with a wider distribution. Moreover, ethyl cellulose- amoxicillin and ethyl cellulose-ampicillin systems have been obtained successfully in our laboratory. A temperature increase of the experiments is traduced to particle size increase. An agglomeration of particles formed by irregular block is observed when the temperature is increased to 333 K. However, an increase of pressure leads to a smaller particle size.

## **6. Acknowledgment**

We are grateful to the Spanish Ministry of Education and Science (Project No. CTQ2007- 67622) for financial support.

### **7. References**

474 Mass Transfer - Advanced Aspects

polymer to encapsulate the AMC particles. In the precipitation over a suspension of particles, the particles behave as nuclei for the precipitation of the polymer, and a polymer

SEM images of these microparticles are shown in Figure 8. SEM images are not accurate enough to observe the distribution of both compounds because all the active substance could be situated on the surface of these microspheres and/or into the core. Thus, X-ray photoelectron spectroscopy (XPS) is one of the main techniques used to determine the success of the encapsulation process by the chemical analysis of the particles on the precipitated surface (Morales et al, 2006). In this case, the elements that differentiate amoxicillin from ethyl cellulose are sulphur (S) and nitrogen (N) atoms. Therefore, these elements can indicate the location of the drug in the precipitated powders. In the coprecipitated the sulphur peak can be identified but there is an absence of this peak in the encapsulate. Moreover, an elemental analysis of the encapsulate is needed to confirm that

matrix of encapsulated particles is produced by agglomeration (Cocero et al., 2009).

Fig. 8. SEM images of amoxicillin ethyl cellulose a) encapsulates and b) co-precipitated

near MCP the jets atomize into droplets producing spherical microparticles.

Supercritical Fluid process can be an excellent alternative to conventional precipitation and encapsulation processes. RESS and SAS processes have been applied on the particle precipitation and co-precipitation of many active ingredient/polymer. Furthermore, SAS has advantages over RESS because SAS is usually operated under mild conditions compared with those of RESS, which is associated to relatively high temperature and high pressure. Anyway, the technical viability of the SAS process requires knowledge of the phase equilibrium of the system; its hydrodynamics (the disintegration regimes of the jet); the mass transfer between the jet generated and the continuous phase; and the mechanisms and kinetics of nucleation and crystal growth. Above MCP the surface tension vanishes before jet break up occurs and the jet evolves as gaseous plume producing nanoparticle. However

At the University of Cádiz, amoxicillin, ampicillin and ethyl cellulose have been sucessfully precipitated by SAS process. The concentration was the factor that had the greatest influence on the PS and PSD. An increase in the initial concentration of the solution led to larger particles sizes with a wider distribution. Moreover, ethyl cellulose- amoxicillin and ethyl

the drug was situated into the core.

**5. Conclusions** 


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**21** 

*Turkey* 

**Electrospark Deposition: Mass Transfer** 

Electrospark alloying (ESA) is one of the surface modification methods to change physical and chemical properties of metal surfaces. It was developed by the Soviet scientists, B.R. Lazarenko and N.I. Lazarenko. The core of this method is the phenomenon of material erosion of both electrodes as a result of the electric discharge between them in a gaseous environment and subsequent mass transfer from one of them to the other, basically from

The anode (treating electrode) usually is a rod with several mm square cross sectional area. Comparing with treating electrode, the cathode (substrate) has significantly larger size and surface area. Both of them are electrically conductive. After the application of a current pulse, a spark discharge takes place between treating electrode and substrate. Following spark discharge, part of the tip of treating electrode and a corresponding spot on substrate melt. Molten spot on the substrate forms a swallow molten pool. Some of the molten tip of treating electrode material transfers to the substrate in the form of molten droplets, mixes with its molten pool and usually solidifies as in the form of splash. By scanning substrate surface, so many splashes could be deposited on it. As a Consequence of single or multiple scanning of substrate surface, a deposit having chemical composition same as treating

According to Lazarenko (1951), the size and sign of electrical erosion at the electrodes, consequently, the mass transfer from the treating electrode to the substrate depends on chemical composition of electrode materials, environment between electrodes and parameters of the electrical pulse. It is obvious that for ESA in air, size of erosion depends basically on chemical composition of electrodes and pulse energy, in turn pulse parameters, pulse amplitude and pulse duration. Therefore, for a given pair of electrodes, mass transfer depends only on electrical parameters of pulses. It was experimentally shown that, the mass gain of substrate is limited, i.e. it is impossible to obtain thick electrospark coating. According to the author (Lazarenko, 1976), the limitation of mass gain of substrate depends on several factors. Lazarenko named them as: change in chemical properties of molten droplet during its transfer to the substrate; change in chemical properties of substrate surface due to mixing with molten droplet ejected from treating electrode and oxidation in air; radical changes arising in alloyed substrate surface - occurrence and accumulation of defects in crystal lattice preventing diffusion; occurrence of residual stress etc. She has also reported that even under the non-oxidizing gas environment–such as argon, helium, hydrogen, there was still limitation on coating thickness. In this case, the processing time till

anode to the cathode (Lazarenko, 1951; Lazarenko, 1976).

limitation was slightly longer than that of the processing in air.

**1. Introduction** 

electrode forms on the substrate.

Orhan Sahin and Alexandre V. Ribalko

*Gebze Institute of Technology* 

