1.2.2 Palmitic acid
The carrier/wall material, palmitic acid (C16H32O2) (purity˃99%, ) 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 1.032 g/mL at 25 °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
1.2.5 Water
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 μS/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 [45]. 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
1.3.1.1 Experimental
1.3.1.1.1 Setup and Procedure.
Figure 1.7 shows the schematic diagram of the equipment used for the determinations (or experiments) [46]. 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 °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
Table1. 2
SLG measurement from experiment
mass fraction of menthol
T1
T2
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 EXPERIMENTAL
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 0.05µm. SEM pictures were taken with HITACHI, S-4800, Scanning Electron Microscope from Japan.
