Biology Assignment 代写 :Microparticles Of Menthol Palmitic Acid
1.6 Results and Discussion
Theo aim of this research was not just to produce menthol/ palmitic acid microcapsules but rather these capsules should have uniform sizes and certain morphology. The mean diameters of these particles should be between 10 to 20µm and also they must exhibit an irregular morphology. These requirements have become necessary because of the intended use of these particles. As mentioned earlier on, these particles should be heavy enough to be blown onto the tobacco and also big enough to mix with the same tobacco in the blending process. From Table 2, it is evident that PGSS is the right process for this research. But we should not lose sight of the fact that PGSS has its limitations, in that it produces smaller particles together with larger ones. In some of the cases instead of the supercritical fluid dissolving in the mixture, the vice versa occurs. This leads to the production of smaller particles, which is characteristic of the RESS process, in short RESS manifest itself slightly in a predominately PGSS process. The target set was realistic in light of these limitations. Recent researches conducted by Wei et al point to the fact that CO2 - assisted PGSS process produces irregular shaped particles[47]. The mechanism of particle formation that generated these particles was melt crystallization at low pre-expansion pressure [47][32]. The choice of CO2 as the supercritical fluid was not only as result of the reasons elaborated earlier elsewhere in this report but was also based on the above reason.
The main theme of the research was to come out or discover the right conditions of temperature, pressure and composition for the process. The pre-expansion temperature for the PGSS process was maintained at 50oC. This value was the common melting point obtained from the SLG measurement of the various mixtures of menthol/palmitic acid at 8MPa pressure of carbon dioxide. The temperature of the mixer, pre-expansion tubes, nozzle was all maintained at the same temperature throughout the experiments. A nozzle of disc orifice 100µm was used throughout the experiments. There are conflicting results by different authors as to the effect of nozzle diameter on particles size. Alessi et al claim that an increase in nozzle disc orifice diameter produced larger particles due to a greater reduction in pressure and density of the fluid at the exit of the nozzle [48], whiles Li et al assert that the nozzle disc orifice diameter has only a negligible effect on the produced particle size, but rather has a more evident effect on the particle size distribution (PSD) [32]. Li et al further ascertained that large nozzle disc orifice diameters will often produce particles with unimodal distribution [32]. Since larger particles are needed, the 100µm disc orifice was chosen instead of a 25 µm diameter orifice based on Alessi et al assertion.
Of all the factors, the ones that are more likely to affect the menthol/palmitic acid microparticles formation are the pre-expansion temperature, pre-expansion pressure and the composition of the menthol/palmitic mixture. These factors were investigated thoroughly.
1.6.1 Effect of Composition
To ascertain the effect of composition on the various rations menthol to palmitic acid, five different combination of menthol/palmitic acid ratios 1:1,1:2.1:3,1:4 and 1:5 were investigated, while keeping the other conditions constant, that is temperature was kept at 50oC ,pressure at 8MPa and the nozzle size as usual 100µm. Figure 1.10 shows the SEM pictures of the particles produced using carbon dioxide as the compressed gas and various compositions of menthol and palmitic acid.
All the particles produced at the various ratios have similar morphology.
Figure 1.11 depicts the effect of the various amounts of menthol and palmitic acid on the average particle size. Figure 1.11 and fig 1.12 have some things in common in that the two combined elucidate the various particles sizes produced.
It is vividly clear in fig 1.12 that a trimodal particles size distribution (PSD) was obtained. The largest percentage of the particles produced were microparticles (third set of peaks) with sizes in the vicinity of 20 µm, followed by the microparticles ( second set of peaks) with sizes about 3-6 µm and some very few particles with particles sizes in the nanometer range (representing the first set of peaks.)
At higher concentrations of menthol particles produced showed some degree of stickiness than particles with lower amount of menthol. However this phenomenon was at a low level when lower pre-expansion temperature was used. In a research conducted by J. Kerc' et al. similar results were observed at lower felodipine/PEG 4000 ratio of 1:1 and 1:3 [49]. In this particular study, this situation was evident in the 1:1 ratio of menthol/palmitic acid. However it is wealth stating that particles having ratios 1:2,1:3,1:4,1:5 showed much physical strength than the 1:1 ratio.
This could be attributed to the fact that higher loading of the core material reduces the amount of the wall material which in turn reduces the efficiency of the encapsulation process.
The nature of the figure 1.11 suggest clearly that higher concentration palmitic acid produces larger particles as can be seen when the concentration of palmitic acid was 83.3% and that of menthol 16.7
The relatively large size of particles at high palmitic acid concentration may be attributed to a weak atomization when CO2 comes into contact with a viscous solution.
Contrary to expectation the average size of the particles with equal amount of menthol and palmitic acid in fig 1.11 had a large size causing a deviation in the trend. This may be due to the fact that the temperature of 50oC was too high so the particles formed from the melt after expansion through the nozzle were not able to cool before reaching the bottom of the collection unit therefore leading to the formation of slightly sticky masses of agglomerated particles.
Nevertheless, considering that this process frequently produce smaller and bigger particles, the nanoparticles and smaller microparticles which obviously we do not want can be separated using a separator or other techniques and the nanoparticles reused.
(a) (b )
(c) (d)
(e)
Fig 1.10 SEM pictures on effect of composition on particles produced.
(a)1;1, 8MPa, 50oC ; (b ) 1:2,8MP\a, 50oC; (c)1:3,8MPa,5oC; (d) 1:4,8MPa,50oC (e) 1:5,8MPa.50oC
Fig 1.11 : Effect of composition on particles produced.
Fig 1.12 PSD on effect of composition on produced particles
1.6.2 Effect of pre-expansion pressure
Fig, 1.15 shows the SEM pictures of menthol / palmitic acid particles produced by using carbon dioxide (CO2) as the compressed gas with a nozzle orifice of 100 µm, same composition, temperature of 50oC, at different pre-expansion pressure of 8,9,10 and 11MPa.
As depicted by the pictures irregular or distorted shaped particles where produced at the various pre-expansion pressures . It is evident from fig 1.15 that the particles produced are agglomerated that is they tend were stuck together. It can also be visualized that microparticles were produced alongside nanoparticles showing the characteristic traits of the PGSS process especially when the compressed gas is CO2. The scenario in fig 1.15 shows a trimodal particle size distribution (PSD), small to negligible amount of nanoparticles were produced(set of first peaks) alongside some below average microparticles(set of second peaks) and large percentage of bigger microparticle (set of third peaks) that falls into the required size category (10-20 µm). There were no clear difference in particle morphology however; there were marked differences in particle size. Generally large agglomerated particles were formed showing a decrease in size as pressure was increased systematically. This is illustrated by the fig 1.14 and fig 1.15. Both Sameer P. Nalawade et al.[50], and Zhao et. al. [51] showed that during PGSS process with CO2, there was a noticeable temperature drop when high pressure CO2 was sprayed through the nozzle making the CO2 removal from the particles difficult and slow that is there was rapid solidification of the melt and therefore agglomerated particles were predominately produced: whose sizes are bigger at lower pressure and smaller at high pressures. This clearly point to the fact that to obtain larger particles the pressure should be kept low.
Nevertheless, as mentioned earlier considering that this process frequently produce smaller and bigger particles, the nanoparticles and smaller microparticles which obviously we do not want can be separated using a separator or other techniques and the nanoparticles reused.
(a) (b)
(c) (d)
Fig 1.13 SEM pictures on effect of pre-expansion pressure on particles produced
(a) 1:5,8MPa.50oC; (b)1:5, 9MPA,50oC (c) 1:5,10MPA,50oC (d) 1:5,11MPa,50 oC
Fig 1.14 Effect of pre-expansion pressure on particles produced
Fig 1.15 PSD on effect pre-expansion pressure on produced particles
1.6.3 Effect of temperature
Conscious and well conducted experiments were undertaken to investigate the effect of temperature ( at 50oC, 55 oC, 60 oC and 65 oC ) on the particles produced under fixed conditions of pre-expansion of 8MPa, constant composition, nozzle diameter of 100µm. Fig.1.18 shows the particle sizes and particle size distributions of the produced particles by PGSS technique at different temperatures. Fig 1.18 shows both nanoparticles and microparticles were produced at different pre-expansion temperatures indicating as usual the nature of the PGSS technique.
As usual trimodal particles size distribution was obtained which bear resemblance in nature to the ones for the effect of temperature and composition.
Pre-expansion temperature generally influence the morphology of particles formed. At relatively low pre-expansion temperature, just after formation of the particles, solidification begins and a if there is not enough time for all the carbon dioxide escape before formation of the crust, an irregular or (hollow) distorted to sponge like shaped particles are formed depending on the permeability and flexibility of the crust.[51] On the other hand at relatively high pre-expansion temperature solidification time is longer paving the way for more sensible heat to be removed. Also less energy is used to cause the escape of the CO2 since CO2 dissolution in the mixture decreases with increase in temperature [50]. Therefore the delayed solidification allows the formation of near-spherical to spherical particles by visco-elastic relaxation and surface tension [50]. This also promotes agglomeration since wet particles are likely to stick together upon collision [51]. In this study the level or extent of agglomeration is reduced as temperature increases indicating that the operating temperature were mild. It is also clear that no spherical particles were formed because the melt temperatures were near the solid points, no time was available to obtain spherical particles. What is more evident here is that the temperature combined with other factors contributed to the observed morphology. The other factors that might have aided the process include CO2-concentration in the melt which is directly linked to the pressure because at high pressure, the CO2 -concentration is high and vice versa. CO2-concentration influence the morphology as a result of the competition between the solidification rate of the melt and the escape rate of the CO2 [51].
The temperature also played a role in the size of the particles since here it the conscious variable factor. It can be seen from fig. 1.17 that particle size generally decreases as temperature increases and this assertion is supported by fig 1.18. This can be explained from this viewpoint, at lower temperature a slightly higher amount of CO2 was trapped in the particles than at higher temperature leading to the production of bigger porous or hollow distorted particles.
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(c) (d)
Fig 1.16 SEM pictures on effect of temperature on particles produced
(a)1:5,8MPa,.50oC; (b)1:5,8MPa,55oC ; (c)1:5,8MPa,60 oC ; (d) 1:5,8MPa,65 oC
Fig 1.17 Effect of temperature on produced particles
Fig 1.18 PSD on effect of temperature on produced particles
1.7 Conclusion
A series of modifications were made to the original apparatus as and when factor(s) hindering the attainment of the required result was encountered. The completed and certified apparatus was used for the study. The study successfully produced menthol/palmitic acid composite particles of which a larger percentage were in the micrometer range. A study of the essential influencing factors was also carried out. On the premises of the research conducted, the following conclusion can be made:
A greater amount of microparticles and smaller percentage of nanoparticles were produced at different operating conditions of pre-expansion temperature, pre-expansion pressure and compositions of menthol/palmitic acid mixture.
The optimum operating conditions that will ensure the production of higher amount or 100% menthol/palmitic acid composite microparticles are: a nozzle of 100 µm, a pre-expansion pressure of 8MPa, pre-expansion temperature of 50 oC, and menthol/palmitic acid ratio of 1:4 or 1:5.
The production of nanoparticles together with microparticles is to be expected but these particles forms only a small fraction of the particles produced in every case. These nanoparticles or smaller microparticles can be separated from the required particles using a membrane and the nanoparticles reused.
The PGSS technique has all the capabilities of producing the required particle size of 10-20 µm but we still have to deal with the production of too small or too large particles as efforts are made to optimize the process.
The results obtained indicate that there is a probability that a change of raw materials, which comes with their own conditions can help in getting the desired product .
