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Menthol is a monocyclic, saturated, secondary terpene alcohol is found in high concentrations in oils of peppermint, corn mint, occurs widely in nature and can also be made synthetically[1, 3]. Whiles d-menthol has an undesirable taste, l-Menthol has a characteristic peppermint flavor and refreshing coolness. Due to its flavor and refreshing coolness, l-menthol is widely used in foods (such as candy, beverages etc), peppermint oil, toothpaste, cigarettes, over-the-counter medications, local anesthetics, and cosmetic products [2, 3].
Menthol is unique because it is the only cigarette additive that tobacco companies actively market to consumers . Secondly, it is the only aspect of cigarette design that is unambiguously marketed due to its physiological effects, as an anti-irritant and a cooling agent . It is also the only cigarette additive about which consumers consciously choose to buy . Menthol flavoring provide cooling, Subjective effects of smoothness and also mask the undesirable taste of tobacco cigarettes [4-5]. The coolness sensation is experienced because of the ability of menthol to chemically trigger the cold-sensitive TRPM8 receptors in the skin which is responsible for the well known cooling sensation that it provokes when inhaled, eaten, or applied to the skin  with the resulting sensation of coolness perceived not only in the mouth and pharynx, but also in the lungs [7-8]. This has the tendency to lure smokers to opt for menthol cigarettes . Menthol levels in cigarette can also be manipulated to attract first-time smokers .
Most of the cigarettes available on the market contain inconspicuous amounts of menthol (approximately 0.03% of cigarettes' tobacco weight), but most tobacco cigarette companies advertise individual brands as mentholated. These specific brands, contain between 0.1% and 1.0% of their tobacco weight as menthol, and therefore exert an appreciable cooling sensation and mint like flavor when inhaled.
Fig.1.1: Tobacco Cigarette
Fig. 1.1 is a diagram showing that cigarette is made composed of the following parts : 1. Filter made of 95% cellulose acetate; 2. Tipping paper to cover the filter; 3. Rolling paper to cover the tobacco; 4. Tobacco blend. Menthol is often applied to tobacco to form the tobacco blend by spraying the tobacco with a dilute menthol solution. This method, however, does not produce a uniform product, since the spray is in the form of discrete droplets which do not contact all of the tobacco. Also, part of the menthol may be lost, using this method, in the course of processing the tobacco.  Another method of applying menthol to tobacco is given by U.S. Pat. No. 3,548,838. This method subject the tobacco to an alcohol-menthol vapor as the tobacco is blown through a conduit. A disadvantage of this method is that alcohol is an extra cost. Secondly, there is the fear that the alcohol vapors might reach explosive concentrations 
Recent modifications to these methods involve the addition of finely dry particles produced by spray drying, RESS, PGSS and other methods. The particles which consist of the flavor encapsulated in a wall material is then blown onto the tobacco and further processed to get the perfect tobacco flavor mix.  The method and apparatus for addition of menthol flavor to tobacco cigarette should be such that 
Cleaning of apparatus
exposing personnel to irritating vapors
use of other solvents which adds to cost of production should be minimized.
These particles should meet some specific specification before it can be successfully blended with the tobacco or incorporated into a filter.
Therefore, there is an increasing interest and utmost need to developing new technologies, particularly in the case of the Tobacco industry , that allow the production of microparticles with specific particle-size distribution and morphology to be produced under mild, non-polluting operating conditions [12-13]. These microparticles should have an inner core which could be either a liquid droplet, solid particle, or gas cell with a continuous thin protective layer or wall material [15-16].
There are a host of wall materials available for microencapsulation but the choice should suit specific purposes, including optimum concentration of the active ingredient, preservation of the properties and activity of the active substance, ease to process with the selected precipitation technique, good mechanism of release, final particle size, compatibility, lack of toxicity, final physical form, and cost  [17-18].
Palmitic acid is one of the most common saturated fatty acids found in animals and plants .
It is a saturated fatty acid used. Like all fatty acids the hydrocarbon tail is lipophilic or hydrophobic and the carboxylic acid head is lipophobic or hydrophilic.
Fig 1. 2. Chemical structure of Palmitic acid
Palmitic acid has a hydrocarbon hydrophobic or lipophilic tail and a carboxylic acid hydrophilic or lipophobic head and properties of a surface active organic substance (Seidl, 2000). Ellison et al.,1999, reported that there has been the suggestion that palmitic acid could form a reverse micelle around a salt core. Indeed, field studies conducted have found palmitic acid favorably on the surface of aerosols (Tervahattu et al., 2002; Peterson and Tyler, 2003) [23,24,52]. This two unique findings and a host of others proofs that palmitic acid can be used as a wall material. To buttress this claims, palmitic acid has a low melting point, therefore the heat from cigarette can be used as a trigger to release the encapsulated menthol. Last but not the least and smell good when it burns. The encapsulation of menthol in palmitic acid will not only provide coolness and refreshing taste from menthol but will also ensure controlled release of menthol. This will improve the effectiveness, broaden the time range of menthol flavor and ensure optimal dosage . With carefully fine-tuned controlled release properties, microencapsulation of menthol in palmitic acid will not just be an added value, but is also a source of a totally new approach with matchless properties .
Various encapsulation techniques are available for improving the efficiency of aroma chemicals in fragranced consumer products like cigarette. Encapsulation methods are still being optimized in terms of fragrance performance, scaling up and cost . Conventionally, various techniques such as milling, grinding, spray drying, spray chilling or spray cooling, Centrifugal extrusion , fluidized bed coating, liposome entrapment, coacervation, inclusion complexation, air suspension coating, extrusion, Centrifugal suspension-separation and rotational suspension separation can be employed to form the microcapsules [1,27]. However, mechanical treatment most as times results in damage of products or performance degradation as a result of particle distress, frictional heat or wide particle size distribution (PSD) . Spray drying or chemical precipitation can eliminate some of the problems with mechanical treatment methods but once again, it is difficult to obtain particles with the desired particle size and distribution due to limitations in reaction rate that is controlled by mass transfer or droplet sizes formed during spraying . The use of supercritical fluids is one method to overcome some of these inherent limitations of conventional methods . For example Carbon dioxide becomes a supercritical fluid when both the temperature and pressure equal or exceed the critical point of 31Â°C and 7.3MPa (see Fig 1.3 ) . In its supercritical state, CO2 has both gas-like and liquid-like qualities, and it is this dual characteristic of supercritical fluids that provides the ideal conditions for production of fine particles in a short period of time.
Fig 1.3 CO2 Phase Diagram
The specific features of a supercritical fluid allows for the control and manipulation of operating temperature and pressure . Some advantages of supercritical fluids-based processes are: the possibility of avoiding (or at least minimizing) the use of organic solvents; the flexibility or ease to apply to a wide range of materials, including thermo labile substances; the ability to produce particles with a wide range of sizes, shapes and morphologies; fig 1.4 illustrates the possibility of loading the particles with an active substance , which can be dispersed in a matrix (composites or microspheres) or surrounded by a shell (microcapsules or encapsulates) .
Fig 1.4. 
a) Composites or microspheres b) Encapsulate or microcapsules
For the case of supercritical CO2, additional advantages are mild operating temperature and the ease of solvent separation, recovery and recycle since depressurization causes the supercritical to revert to its gaseous standard state and to provide a solvent-free product .
Among all the supercritical fluid particle formation techniques in Table 1, Particles from gas-saturated solution (PGSS) process is a promising technique that uses fluids at supercritical conditions to produce fine particles under mild operating conditions .
Supercritical particle formation techniques
Precipitation driving force
Drug or drug mixture
Simple(if no cosolvent), does not use organic solvent
High pressure; consumption of fluid is high; low solubility of molecules
Drug or drug mixture
Liquid organic solvent
Antisolvent effect + solvent evaporation
Drug or drug mixture
Liquid organic solvent
Drug or drug mixture
Organic solvent with/
Antisolvent effect + solvent evaporation
The PGSS process makes good use of the advantage that a compressed gas is more soluble in a liquid than the corresponding liquid is in the same compressed gas . In this particular process, the SCF is dissolved in a melted substrate (or substrates), or a solution of the substrate(s) in a solvent, or a suspension of the substrate(s) in a solvent, then followed by a rapid expansion, at moderate pressures, of the saturated solution through a nozzle [33,37].The expanding gas supports the formation of fine droplets. Owing to the Joule-Thomson effect the gas cools down extremely rapidly below the solidification temperature, removing heat from the droplets of the molten solute[14,35]. The time for the solidification process is in the range between some 10ms up to a few100ms . Depending on the type of system, fine solid particles or liquid droplets are formed [36-37].
Solubility of either the carrier or core material in the SCF is not a requirement. However, the SCF used must be very soluble in the liquid phase. This technique is particularly suited for the encapsulation of drugs into polymer or lipid matrices  which generally absorb a large amount of carbon dioxide. CO2 is generally used as a supercritical fluid in PGSS for several polymers mainly due to its high solubility. One of the key features of PGSS is the strong reduction of the melted substance viscosity once it is mixed with the supercritical fluid. A second effect of mixing the melted substance with the supercritical fluid is the melting point temperature depression; this phenomenon prevents the solidification inside the nozzle as well. Through the choice of the appropriate combination of solvent and operating conditions for a particular compound, PGSS can eliminate some of the disadvantages of other SCF and conventional methods. Some other well known advantages are that the process is versatile process; applicable for several substances (example mixtures and water-soluble ingredients to form composite microparticles ); uses moderate pressures; has low gas consumption; and uses no harmful organic solvents ; gives solvent-free powders; is suitable for highly viscous or sticky products; gives fine crystalline and amorphous powders with a narrow, controllable size-distribution, thin films, and is easy to scale-up . Because to the low operation costs PGSS can be used for a wide variety of substances not only for highly valuable, but also for commodity products.
Since other individual cigarette brand preferences tend to diminish fairly early with time but the menthol brand can capture smokers because it provides the same enjoyment of normal cigarettes plus giving freshness feel with a cooling effect [4,29]. The need to for further research to produce new and improved menthol flavor will continue.
Therefore, the primary aim of this research is to produce menthol/ palmitic acid composites using the novel PGSS process, investigates the effect of operation conditions such as composition, pressure and temperature on the particles formed and last but not the least undertake flavor release studies to determine the factors that could lead to menthol flavor loss.
Menthol (2-Isopropyl-5-Methylcyclohexanol, C6H9OHCH3C3H7) is a waxy, crystalline substance, clear or white in color. Figure below shows the l- or -(-) form of menthol. It is this form that has characteristic minty smell. For this research menthol was selected as the core material, its purity Ëƒ99%, melting point 43.5oC and was supplied by
Fig. 1.5 Chemical Structure of menthol 
1.2.2 Palmitic acid
The carrier/wall material, palmitic acid (C16H32O2) (purityËƒ99%, ) was obtained from Sinopharm Chemical Reagent Co., Ltd and had melting point 63oC. Palmitic chemical structure shown in fig 1.2, is a white crystalline solid with Molecular weight of 256.42 g/mol.
1.2.3 Carbon dioxide
The compressed gas commonly used in supercritical particle technology is carbon dioxide, because it is environmentally benign, cheap, non-flammable and non-toxic solvent that has relatively mild critical temperature (Tc) and pressure (Pc) of 31oC and 7.38 MPa, respectively.
Carbon dioxide ( 99.9 % purity) was supplied by Linde Gas, Xiamen, China
1.2.4 Surfactant - Span 20
Span 20, Sorbitan monododecanoate, C18H34O6, is amber to brown oily liquid, non-toxic, odorless, slightly soluble in isopropanol, tetracarp, xylene, cotton seed oil, mineral oil, slightly soluble in liquid paraffin, insoluble in water, density of 1.032Â g/mLÂ at 25Â Â°C, HLB = 8.6. Span 20 obtained for used in this work was supplied Shantou Xilong Chemical Factory, Guangdong (China).
Fig.1.6: The chemical structure Span 20
The water for this research obtained from a water purification plant in the
College of Chemistry and Chemical Engineering, Xiamen University, China, purified using reverse osmosis technique. The water had an electrical conductivity of 0.5-1.5 Î¼S/cm.
1.3 Solid-Liquid-Gas Equilibrium for the Menthol-Palmitic acid-CO2
Since the reason behind this research is to get right conditions for attaining particles of specific size and morphology, this work would be incomplete without taking a look at one important phenomenon that aid the process, that is the solid-liquid- gas (SLG) phase behavior of the system at stake. SLG equilibrium measurement for this system, first and foremost was the key to determining the melting point - composition (T-w) data at 8MPa pressure for the various ratios of menthol/palmitic acid that were investigated.
The SLG equilibrium may also serve a very purpose by assisting in maintaining a high or low viscosity of the menthol-palmitic acid-CO2 systems . Knowing the malting point would help in choosing the right operation temperature at the appropriate pressure. This avoids the issue of too high or too low temperature which is used as the main factor causing changes in particle morphology.
1.3.1 SLG Measurement
220.127.116.11.1 Setup and Procedure.
Figure 1.7 shows the schematic diagram of the equipment used for the determinations (or experiments) . The setup consists of a high-pressure view cell having an interior volume of about 11cm3, a pre-saturated vessel having an interior volume of about ? cm3, a compressor (G447-400, acquired from Beijing HuiZhi M&E Facilities Co., Ltd. Beijing, China), a vacuum pump (2XZ-2, from Linhai Tanshi Vacuum Equipment Co., Ltd., Zhejiang, China) and an air bath controlled with a thermostat . The pressure was controlled by a back-pressure regulator (BY-3, made by Yantang Equipment Co. Beijing, China). The range of temperatures used during the experiment was from room temperature to about 90 Â°C and the pressure for the study was 8MPa.
Fig 1.7 Schematic diagram of experimental apparatus
1, BPR, Back Pressure Regulator; V-1, check valve; V-2, exhaust valve; V-3, valve; 1, CO2 cylinder; 2, compressor; 3, thermostat air bath; 4, pre-saturation vessel; 5, high pressure view cell; 6, capillary; 7, vacuum pump; 8, pressure indicator; 9, temperature indicator; (a), initial loading state; (b),first melting state; (c), last melting state
The first and last melting points (FLMP) was employed and values obtained formed the solid-liquid-gas (SLG) equilibrium data for the menthol-palmitic acid-CO2 system; in other words the melting point - composition (T-w) data at 8MPa was investigated. For the pure substance only the first melting point was used.
A capillary tube was filled to a depth of about 3-4mm with well dried mixture of the sample or pure substance. To ensure accuracy in the reading other strict measures such as calibration of thermocouple and pressure transducers to have minimal uncertainties were also done.
The SLG-coexistence data for the menthol/palmitic acid/CO2 system is presented in Table1. 2 and the plot in fig1. 8
SLG measurement from experiment
mass fraction of menthol
0 49.7 --
16.7 47.1 48.4
20 45.0 47.5
25 42.8 44.8
33.3 41.1 43.0
50 36.4 39.4
60 32.5 35.9
66.7 29.2 32.8
75 22.4 25.8
100 - -
Fig 1.8 Plot of SLG data
From diagram one thing is certain that the system resemble a solid solution, but certainly not an eutectic system. From the SLG plot a temperature of 50oC was identified as the ideal temperature to carry out the particle production to get good results.
1.4.1 Setup and procedure
Fig1. 9: Schematic diagram for the research.
A, CO2 cylinder ; B, compressor; C, pre-heater; D, mixer; E, water bath; F, nozzle; G, precipitator; P, pressure indicator (manometer); T, temperature indicator; V#, valve; TC, temperature controller; LF#, filter; BPR, back-pressure regulator.
Figure 1.9 indicates the schematic diagram of the PGSS equipment used for the research. The setup for the process consists of compression unit (gas cylinder, filter, compressor, back-pressure regulator), mixing unit (pre-heater, pump, mixer), and particle formation and collection unit (nozzle, precipitator). The mixing unit is put in a water bath and the temperature controlled with a temperature controller.
As shown in fig 1.9 CO2 was delivered to the system by the compressor B ( GZ-5/30 - 400 , Beijing Huizhi Mechanical and Electric Equipment Co., L td), and then the gas was divided into two parts: one part goes through the pre-heater C. The other part though V1 to the mixer. V4 leads directly into the outer tube of the nozzle system and is opened during particle production. The compressed gas was dissolved in the molten mixture of menthol/palmitic acid in the mixer . Finally, V3 was opened and the gas-saturated solution was passed through the inner pipe of the nozzle and expanded through the orifice by the compressed gas from the outer tube of the nozzle into the precipitator to form fine particles.
In the 400 mL precipitator, solid particles precipitated were collected at ambient conditions. A filter was installed just at the exit of the precipitator to retain particles. The compressed gas flow rate was measured by a wet flowmeter ( ML-2, Changchun Instrument & Meter Co. Ltd ) and was vented. The flow of the gas-saturated solution is controlled by the pre-expansion pressure and the nozzle size .
1.5 Analysis methods
Particle size and particle size distributions (PSDs) were determined by a laser diffraction spectrometer (LS908, OMEC Technology Co. Ltd, China) with the minimum detected particle size of 0.05Âµm. SEM pictures were taken with HITACHI, S-4800, Scanning Electron Microscope from Japan.
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 20Âµm 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. The mechanism of particle formation that generated these particles was melt crystallization at low pre-expansion pressure . 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 100Âµm 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 , 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) . Li et al further ascertained that large nozzle disc orifice diameters will often produce particles with unimodal distribution . Since larger particles are needed, the 100Âµm disc orifice was chosen instead of a 25 Âµm 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 100Âµm. 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 Âµm, followed by the microparticles ( second set of peaks) with sizes about 3-6 Âµm 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 . 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 )
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 Âµm, 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 Âµm). 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., and Zhao et. al.  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.
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 100Âµm. 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. 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 . Therefore the delayed solidification allows the formation of near-spherical to spherical particles by visco-elastic relaxation and surface tension . This also promotes agglomeration since wet particles are likely to stick together upon collision . 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 .
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.
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
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 Âµm, 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 Âµm 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 .