Journal of Microencapsulation Micro and Nano Carriers
ISSN: 0265-2048 (Print) 1464-5246 (Online) Journal homepage: http://www.tandfonline.com/loi/imnc20
Microencapsulation of pheromone-analogue and measurement of the sustained release Shinzo Omi, Nobuo Umeki, Hiroaki Mohri & Mamoru Iso To cite this article: Shinzo Omi, Nobuo Umeki, Hiroaki Mohri & Mamoru Iso (1991) Microencapsulation of pheromone-analogue and measurement of the sustained release, Journal of Microencapsulation, 8:4, 465-478 To link to this article: http://dx.doi.org/10.3109/02652049109021870
Published online: 27 Sep 2008.
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Date: 06 November 2015, At: 02:40
J. MICROENCAPSULATION,
1991, VOL. 8,
NO,
4, 465-478
Microencapsulation of pheromone-analogue and measurement of the sustained release SHINZO OMIT, NOBUO UMEKI, HIROAKI MOHRI and MAMORU I S 0 Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo 184, Japan
Downloaded by [Deakin University Library] at 02:40 06 November 2015
(Received .?I October 1990; accepted 30 January 1991)
A model study was conducted to establish 2 feasible production and application systems for the long-term, sustained release of pheromone into the atmosphere of targeted areas. The desired goal of effective release was set at least half a year. 2Ethylhexyl acetate (EHA) was selected as a pheromone analogue due to its similar structure and easier access for quantitative analysis. At first EHA was impregnated in wax particles, which were then encapsulated employing the complex coacervation of a gelatin-gum arabic system. The release period of EHA through the gelatin wall, however, turned out to be too short-only a week at most. As a second attempt, a modification of the two-phase emulsion technique was employed to encapsulate multiple numbers of wax particles in hydrated networks of gelatin. Though the initial release rate of EHA was still too high, 60 per cent of encapsulated EHA underwent sustained release over six months after absorbed moisture had completely evaporated. A two-step mechanism of mass transfer was proposed and the related parameters in terms of the capacity coefficient and effective diffusion coefficient were estimated.
Introduction According to the statistics (Trimnell et al. 1982), almost 30 per cent of agricultural products have been said to be damaged by harmful insects. Conventional insecticides, numerous kinds of which have been developed and applied for practical use, have sometimes caused unfavourable after-effects to t h e environment and ecosystems; such as, for example, residual toxicity in land, indiscriminate annihilation of insects, gradual immunity gained by generations of target insects, a n d so forth. Pheromone is a new type of insecticide, being very specific and effective in trace amounts ( p p m o r even ppb), and leaves n o harmful after-effects in the surrounding environment. For example nearly 50 pheromones have been identified as belonging to the lepidopterae. Being classified as esters, alcohols, aldehydes a n d ketones, they have in common a long carbon chain with one o r two double bonds. Against t h e advantages mentioned above, a minor disadvantage is the considerable cost of synthesis, owing to the absolute necessity of incorporating double bonds in the exact location of a carbon chain. Since pheromone is so effective in a very small amount, controlled release of t h e drug from microcapsules seems to provide a promising solution to prevent unnecessary waste in practical use. However, the time-scale required to obstruct t h e mating of target insects is at least half a year, o r even a full year, a n d this has remained an insurmountable obstacle for conventional encapsulation techniques.
t T o whom correspondence should be addressed. 0265-2048191 83.00 (Q 1991 ‘Taylor & Francis Ltd
Downloaded by [Deakin University Library] at 02:40 06 November 2015
466
S . Omi et al.
Herbig and Smith (1990) have reported the encapsulation of gossyplure (pheromone for the pink bollworm (PBW)) in particular asymmetric microporous beads. They also give a release profile of the drug from the beads, which were distributed from an aeroplane. T h e effective duration of the release was a month at most, necessitating repeated distribution once every few weeks. As a compromise solution, Shin-etsu Chemical Co., Japan, is now supplying a particular polyethylene tube and a roll of string, the core part of which is impregnated with pheromone, with a microporous shell or wall to achieve sustained release. T h e major problem is high distribution cost compared with conventional insecticides. Being fully aware that there were difficult problems to be solved, the authors tried to produce microcapsules capable of pheromone release over the long period required. A pheromone-analogue, 2-Ethylhexyl acetate (EHA) was encapsulated, and the release profile as well as the rate were observed to evaluate the effectiveness of the encapsulation technique. Some chemical engineering aspects associated with sustained release: in particular, the modelling of the mass transfer process and the estimation of related parameters were also discussed.
Experimental Reagents 1 . Pheromone syrup and its analogue. PBW Pheromone syrup (Reference No. 26390) was donated by Shin-etsu Chemical Co., and was encapsulated as supplied. 2-Ethylhexyl acetate (EHA) was a reagent grade supplied by Tokyo Chemical Co. T h e chemical structure of EHA was compared with those of Lepidopterae pheromones, as shown in figure 1.
c2 H5 2-Ethyl-hexylacetate
(E)-S-decene-l-ol
(Z)-S-Dodecene-l-ol
(EHA)
acetate
acetate
Figure 1. Comparison of chemical structure between EHA and pheromones.
Microencapsulation of pheromone analogue
467
Downloaded by [Deakin University Library] at 02:40 06 November 2015
2. Wall materials. Gelatin (Wako Pure Chemical Co.) and gum arabic (Kokusan Chemical Co.) were used as the agents of complex coacervation. Beeswax, white, (mp. 337-339"K, Wako Pure Chemical Co.) was used to prepare wax particles impregnated with EHA or pheromone. 3. Crosslinking agent. 30 wt. per cent aqueous solution of formaldehyde (Kokusan Chemical Co.) was used to crosslink gelatin chains. 4. Other reagents. 10wt. per cent aqueous solution of acetic acid (Wako Pure Chemical Co.), and the same concentration of sodium hydroxide (Koso Chemical Co.) were used to control the p H of solutions. Liquid paraffin (Kokusan Chemical Co.) was used as a medium of the aqueous suspension (S/W) in oil (0)dispersion.
Preparation of wax microparticles 4.5 g of melted white beeswax was mixed with 0.5 g of EHA, and then added to 200g of 1 wt. per cent gelatin solution stirred in a 500ml separable flask equipped with a teflon-blade stirrer and four baffle plates. T h e agitation rate was 250min-', and the temperature was kept at 343°K. After five minutes the composition was quenched in an ice water bath, and the wax particles were filtered out. Complex coacervation and microencapsulation Equal portions (30 g) of gelatin and gum arabic aqueous solution were mixed in a 300ml separable flask equipped with a stirrer blade and a thermometer. T h e temperature was kept at 313°K during agitation. Then 140g of warm water was added, and the p H of the ingredient was subsequently adjusted to 4.0 by the dropby-drop addition of 1Owt. per cent of acetic acid. T h e generation of coacervate droplets was observed. After 30 minutes elapsed, the ingredient was quenched to 278°K. 3 ml of 30 wt. per cent formaldehyde aqueous solution was added to crosslink the gelatin chains, the p H being adjusted to 9.0, using 10wt. per cent sodium hydroxide. Crosslinking continued for 2 hours, and then the temperature was raised to 3 13°K. Microcapsules were separated by means of a centrifuge, and preserved for further experiments. T h e same procedure was adopted for the encapsulation of wax particles, which were added as a suspension in 140 g of water. Weight per cent of gelatin and gum arabic were varied from 2.5 to 10.0. An alternative encapsulation was tried to suppress the fast release of EHA from the microcapsules prepared using the above method. I n this procedure, wax particles were dispersed in 2 0 8 of 20wt. per cent aqueous solution of gelatin, and then this suspension (S/W) was redispersed in 200g of liquid paraffin. After 30 minutes' agitation at 250min-', (W/O) emulsion composed of 4 g of formaldehyde aqueous solution and 50g of liquid paraffin was added to the (S/W)/O dispersion. Crosslinking reaction was allowed to proceed for 2 hours; the microcapsules were then filtered, and washed with benzene, cold water and ethanol. Characterization of microcapsules 1. Feature and dimension of microcapsules. General features of the surface and the cross-section of the wall were observed on photographs taken by a scanning electron microscope (SEM) (JSM-35CF, J E O L Ltd. Co.), average diameters of wax particles and microcapsules were measured from photographs taken through an optical microscope.
S. Omi et al.
468
Downloaded by [Deakin University Library] at 02:40 06 November 2015
Average wall thicknesses of the microcapsules were calculated from the mathematical formula given below by measuring the amount of gelatin. Elemental analysis (CHN analyser Model CHN-1 A, Shimadzu Co.) was conducted to measure the nitrogen content of the microcapsules. Since only the gelatin contained nitrogen, the amount of gelatin was easily obtained. A correction for the small amount of gum arabic incorporated during the complex coacervation process was made based upon the composition of crosslinked coacervates. T h e average wall thickness of microcapsules can be obtained as follows. From the weight ratio between the wax and wall materials, we obtain the following relation
where, P = wt. per cent of wall material incorporated, d,=average diameter of wax particles, 6 =average wall thickness, and pc, pw= density of wax particles and wall material, respectively.
Then, the average diameter of microcapsules can be obtained dpm= d,
+ 26.
(3)
As mentioned later, microcapsules obtained from the (S/W)/O emulsion technique contained large numbers of wax particles. T h e approximate numbers of wax particles in one microcapsule can be estimated from the following equation
In this equation, dpmcis the average diameter of dried microcapsules, and is related to that of wet ones (containing moisture soon after preparation) as follows
100-W-D dpm=(
)
'I3
dpmcw,
where, d,,, = average diameter of wet microcapsules, W, D = wt. per cent of moisture and drug in microcapsules, respectively. Measurement of drug content T h e encapsulated EHA content was determined by gas chromatograph (Model802, Ohkura Giken Co.) with a thermal conductivity cell detector. Stainless steel column (3 mm diameter and 3 m length, Silicone SE-30 adsorbed on Chromosorb carrier) was used with 463°K column temperature and 523°K at injection port. nDodecyl acetate (Tokyo Kasei Co.) was employed as an internal standard substance and chloroform (Kokusan Chemical Co.) as a solvent.
Downloaded by [Deakin University Library] at 02:40 06 November 2015
Microencapsulation of pheromone analogue
469
Figure 2. Schematic diagram of the system for release experiment. 1. Nitrogen cylinder, 2. Sample chamber, 3. Microcapsules, 4. Gas sampler, 5. Flow meter, 6. Gas chromatograph.
E H A release experiment Apparatus. A schematic diagram of an experimental system to measure the release rate of EHA is shown in figure 2. No. 2 is a particular glass tube, 2.1 cm diameter and 17-5cm length. Microcapsules of known weight were extended on a filter paper placed in the tube. Released EHA was mixed with continuous flow of nitrogen, the flow rate being 1 ml s-l at room temperature and was introduced to the No. 4 gas sampler with a certain time interval for GC analysis. T h e release rate of EHA was directly obtained from this procedure.
Results and discussion Microcapsules prepared using complex coacervation W a x particles. A SEM photograph of a wax particle, impregnated with pheromone, is shown in figure 3, and an enlarged surface feature is shown in figure 4. No pin-holes or cracks were observed apart from the fine creases which characterize the surface of wax materials. Particle size was distributed between diameters of 40 to 400pm,the average value being around 250pm and very few agglomerated particles were observed. EHA content in the particles was around 70 mg g - microcapsule, approximately 80 per cent yield based on the feed EHA.
Composition of coacervates from geletan-gum arabic system The composition of gelatin in coacervates was 75wt. per cent, and remained constant regardless of the feed ratio. I n the existing conditions of p H = 4 0 and 313"K, maximum amount of coacervates was observed at the feed ratio of gelatin/gum arabic = 60/40 (wt.). This result was slightly different from that of Bungenberg, de Jong and Dekker (1935,1936), whose data showed the maximum at 40wt. per cent gelatin. In the subsequent encapsulation, the majority of procedures were carried out with a feed composition of 50/50 (wt.). Microcapsules of wax particles All the microcapsules obtained contained only a single core. No doublets or further multiple cores were observed. T h e characteristics of the microcapsules
Downloaded by [Deakin University Library] at 02:40 06 November 2015
470
S. Omi et al.
Figure 3. SEM photograph of wax particle impregnated with EHA. (160-fold, dF = 233 pm).
Figure 4. SEM photograph of enlarged surface. (2000-fold).
prepared together with one example of uncapsulated wax particles are listed in table 1. EHA content was sufficient for the present purpose. However, the thickness of the wall was of the order of 1 pm at most, causing concern that the wall thickness might be too thin. In this context, M C 107 was encapsulated twice to double the wall thickness. SEM photographs of general features of the microcapsule and its surface are shown in figures 5 and 6, respectively. One might feel that there are not many differences between the features of figures 3 and 5, but the magnified view of figure 6 revealed that the uniform and somewhat soft structure of a gelatin wall had developed on the surface of the wax particles.
101 102 103 104 105 106 107 Wax P.
58.5 64.8 761 78-6
14.1 6.9 1.5 500
302 237 176 212
201 202 203 204
Based on dried capsules. Based on calculations.
Gelatin" [wt. per cent]
EHA
[mg/g-MC]
dF
No.
a
-
3.0 4.4 1.8 0-9 1.3 02 8.7
Gelatin [wt. per cent]
17.8 15.8 10-6 17-6 17.6 13-0 17-9
Water [wt. per cent]
-
1.3 1.5 0-4 0.2 06 02 3.0
6
-
338 271 169 200 330 203 262
d,m
Diameter of MC [Pnl
52.6 59.8 68.8 62.8
Water (wt. percent]
28 11 12 70
1192 724 61 8 1366
dpmw
1545 987 914 1904
6 133 95 107 247
N b
dPm
bml
bml
bml
Wet diameter
Thicknessb
Dry diametep
Characterization of microcapsules prepared with (S/W)/O multiple phase emulsion technique.
MC
Diameter of core [pm]
Table 2.
335 268 168 200 329 203 256 233
No. 66.0 71.9 67.6 60.2 65.2 57.0 71.6 78.1
EHA [mg/g-MC]
dF
MC
Thickness bml
Characterization of microcapsules prepared with complex-coacervation technique.
Diameter of core b m ]
Table 1.
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f
f
G
2
0
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472
Figure 5.
S. Omi et al.
SEM photograph of MC 107, twice encapsulated. ( 1 SO-fold, d,,, = 262 pm).
Figure 6. SEM photograph of the surface of MC 107. (2000-fold).
Release profile of EHA from microcapsules ( M C 107) A typical release profile together with the release rate is shown in figure 7. As mentioned before, MC 107 was twice encapsulated, and has a maximum wall thickness of 3.0 pm. T h e fraction of EHA was plotted against the elapsed time and the release rate, also plotted in the figure, appeared very fast during the early stages. Unexpectedly, indeed disappointingly, 50 per cent of total EHA was released within three days. T h e release profile of EHA from unencapsulated wax particles is also shown in the figure (broken line). A comparison between two curves of the release rate reflects
473
Microencapsulation of pheromone analogue
I
I
I
No. 107 MC 0 59
-1.0
\
F0.81 -
-0.8
I
8 -0.6
ISSN: 0265-2048 (Print) 1464-5246 (Online) Journal homepage: http://www.tandfonline.com/loi/imnc20
Microencapsulation of pheromone-analogue and measurement of the sustained release Shinzo Omi, Nobuo Umeki, Hiroaki Mohri & Mamoru Iso To cite this article: Shinzo Omi, Nobuo Umeki, Hiroaki Mohri & Mamoru Iso (1991) Microencapsulation of pheromone-analogue and measurement of the sustained release, Journal of Microencapsulation, 8:4, 465-478 To link to this article: http://dx.doi.org/10.3109/02652049109021870
Published online: 27 Sep 2008.
Submit your article to this journal
Article views: 14
View related articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=imnc20 Download by: [Deakin University Library]
Date: 06 November 2015, At: 02:40
J. MICROENCAPSULATION,
1991, VOL. 8,
NO,
4, 465-478
Microencapsulation of pheromone-analogue and measurement of the sustained release SHINZO OMIT, NOBUO UMEKI, HIROAKI MOHRI and MAMORU I S 0 Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo 184, Japan
Downloaded by [Deakin University Library] at 02:40 06 November 2015
(Received .?I October 1990; accepted 30 January 1991)
A model study was conducted to establish 2 feasible production and application systems for the long-term, sustained release of pheromone into the atmosphere of targeted areas. The desired goal of effective release was set at least half a year. 2Ethylhexyl acetate (EHA) was selected as a pheromone analogue due to its similar structure and easier access for quantitative analysis. At first EHA was impregnated in wax particles, which were then encapsulated employing the complex coacervation of a gelatin-gum arabic system. The release period of EHA through the gelatin wall, however, turned out to be too short-only a week at most. As a second attempt, a modification of the two-phase emulsion technique was employed to encapsulate multiple numbers of wax particles in hydrated networks of gelatin. Though the initial release rate of EHA was still too high, 60 per cent of encapsulated EHA underwent sustained release over six months after absorbed moisture had completely evaporated. A two-step mechanism of mass transfer was proposed and the related parameters in terms of the capacity coefficient and effective diffusion coefficient were estimated.
Introduction According to the statistics (Trimnell et al. 1982), almost 30 per cent of agricultural products have been said to be damaged by harmful insects. Conventional insecticides, numerous kinds of which have been developed and applied for practical use, have sometimes caused unfavourable after-effects to t h e environment and ecosystems; such as, for example, residual toxicity in land, indiscriminate annihilation of insects, gradual immunity gained by generations of target insects, a n d so forth. Pheromone is a new type of insecticide, being very specific and effective in trace amounts ( p p m o r even ppb), and leaves n o harmful after-effects in the surrounding environment. For example nearly 50 pheromones have been identified as belonging to the lepidopterae. Being classified as esters, alcohols, aldehydes a n d ketones, they have in common a long carbon chain with one o r two double bonds. Against t h e advantages mentioned above, a minor disadvantage is the considerable cost of synthesis, owing to the absolute necessity of incorporating double bonds in the exact location of a carbon chain. Since pheromone is so effective in a very small amount, controlled release of t h e drug from microcapsules seems to provide a promising solution to prevent unnecessary waste in practical use. However, the time-scale required to obstruct t h e mating of target insects is at least half a year, o r even a full year, a n d this has remained an insurmountable obstacle for conventional encapsulation techniques.
t T o whom correspondence should be addressed. 0265-2048191 83.00 (Q 1991 ‘Taylor & Francis Ltd
Downloaded by [Deakin University Library] at 02:40 06 November 2015
466
S . Omi et al.
Herbig and Smith (1990) have reported the encapsulation of gossyplure (pheromone for the pink bollworm (PBW)) in particular asymmetric microporous beads. They also give a release profile of the drug from the beads, which were distributed from an aeroplane. T h e effective duration of the release was a month at most, necessitating repeated distribution once every few weeks. As a compromise solution, Shin-etsu Chemical Co., Japan, is now supplying a particular polyethylene tube and a roll of string, the core part of which is impregnated with pheromone, with a microporous shell or wall to achieve sustained release. T h e major problem is high distribution cost compared with conventional insecticides. Being fully aware that there were difficult problems to be solved, the authors tried to produce microcapsules capable of pheromone release over the long period required. A pheromone-analogue, 2-Ethylhexyl acetate (EHA) was encapsulated, and the release profile as well as the rate were observed to evaluate the effectiveness of the encapsulation technique. Some chemical engineering aspects associated with sustained release: in particular, the modelling of the mass transfer process and the estimation of related parameters were also discussed.
Experimental Reagents 1 . Pheromone syrup and its analogue. PBW Pheromone syrup (Reference No. 26390) was donated by Shin-etsu Chemical Co., and was encapsulated as supplied. 2-Ethylhexyl acetate (EHA) was a reagent grade supplied by Tokyo Chemical Co. T h e chemical structure of EHA was compared with those of Lepidopterae pheromones, as shown in figure 1.
c2 H5 2-Ethyl-hexylacetate
(E)-S-decene-l-ol
(Z)-S-Dodecene-l-ol
(EHA)
acetate
acetate
Figure 1. Comparison of chemical structure between EHA and pheromones.
Microencapsulation of pheromone analogue
467
Downloaded by [Deakin University Library] at 02:40 06 November 2015
2. Wall materials. Gelatin (Wako Pure Chemical Co.) and gum arabic (Kokusan Chemical Co.) were used as the agents of complex coacervation. Beeswax, white, (mp. 337-339"K, Wako Pure Chemical Co.) was used to prepare wax particles impregnated with EHA or pheromone. 3. Crosslinking agent. 30 wt. per cent aqueous solution of formaldehyde (Kokusan Chemical Co.) was used to crosslink gelatin chains. 4. Other reagents. 10wt. per cent aqueous solution of acetic acid (Wako Pure Chemical Co.), and the same concentration of sodium hydroxide (Koso Chemical Co.) were used to control the p H of solutions. Liquid paraffin (Kokusan Chemical Co.) was used as a medium of the aqueous suspension (S/W) in oil (0)dispersion.
Preparation of wax microparticles 4.5 g of melted white beeswax was mixed with 0.5 g of EHA, and then added to 200g of 1 wt. per cent gelatin solution stirred in a 500ml separable flask equipped with a teflon-blade stirrer and four baffle plates. T h e agitation rate was 250min-', and the temperature was kept at 343°K. After five minutes the composition was quenched in an ice water bath, and the wax particles were filtered out. Complex coacervation and microencapsulation Equal portions (30 g) of gelatin and gum arabic aqueous solution were mixed in a 300ml separable flask equipped with a stirrer blade and a thermometer. T h e temperature was kept at 313°K during agitation. Then 140g of warm water was added, and the p H of the ingredient was subsequently adjusted to 4.0 by the dropby-drop addition of 1Owt. per cent of acetic acid. T h e generation of coacervate droplets was observed. After 30 minutes elapsed, the ingredient was quenched to 278°K. 3 ml of 30 wt. per cent formaldehyde aqueous solution was added to crosslink the gelatin chains, the p H being adjusted to 9.0, using 10wt. per cent sodium hydroxide. Crosslinking continued for 2 hours, and then the temperature was raised to 3 13°K. Microcapsules were separated by means of a centrifuge, and preserved for further experiments. T h e same procedure was adopted for the encapsulation of wax particles, which were added as a suspension in 140 g of water. Weight per cent of gelatin and gum arabic were varied from 2.5 to 10.0. An alternative encapsulation was tried to suppress the fast release of EHA from the microcapsules prepared using the above method. I n this procedure, wax particles were dispersed in 2 0 8 of 20wt. per cent aqueous solution of gelatin, and then this suspension (S/W) was redispersed in 200g of liquid paraffin. After 30 minutes' agitation at 250min-', (W/O) emulsion composed of 4 g of formaldehyde aqueous solution and 50g of liquid paraffin was added to the (S/W)/O dispersion. Crosslinking reaction was allowed to proceed for 2 hours; the microcapsules were then filtered, and washed with benzene, cold water and ethanol. Characterization of microcapsules 1. Feature and dimension of microcapsules. General features of the surface and the cross-section of the wall were observed on photographs taken by a scanning electron microscope (SEM) (JSM-35CF, J E O L Ltd. Co.), average diameters of wax particles and microcapsules were measured from photographs taken through an optical microscope.
S. Omi et al.
468
Downloaded by [Deakin University Library] at 02:40 06 November 2015
Average wall thicknesses of the microcapsules were calculated from the mathematical formula given below by measuring the amount of gelatin. Elemental analysis (CHN analyser Model CHN-1 A, Shimadzu Co.) was conducted to measure the nitrogen content of the microcapsules. Since only the gelatin contained nitrogen, the amount of gelatin was easily obtained. A correction for the small amount of gum arabic incorporated during the complex coacervation process was made based upon the composition of crosslinked coacervates. T h e average wall thickness of microcapsules can be obtained as follows. From the weight ratio between the wax and wall materials, we obtain the following relation
where, P = wt. per cent of wall material incorporated, d,=average diameter of wax particles, 6 =average wall thickness, and pc, pw= density of wax particles and wall material, respectively.
Then, the average diameter of microcapsules can be obtained dpm= d,
+ 26.
(3)
As mentioned later, microcapsules obtained from the (S/W)/O emulsion technique contained large numbers of wax particles. T h e approximate numbers of wax particles in one microcapsule can be estimated from the following equation
In this equation, dpmcis the average diameter of dried microcapsules, and is related to that of wet ones (containing moisture soon after preparation) as follows
100-W-D dpm=(
)
'I3
dpmcw,
where, d,,, = average diameter of wet microcapsules, W, D = wt. per cent of moisture and drug in microcapsules, respectively. Measurement of drug content T h e encapsulated EHA content was determined by gas chromatograph (Model802, Ohkura Giken Co.) with a thermal conductivity cell detector. Stainless steel column (3 mm diameter and 3 m length, Silicone SE-30 adsorbed on Chromosorb carrier) was used with 463°K column temperature and 523°K at injection port. nDodecyl acetate (Tokyo Kasei Co.) was employed as an internal standard substance and chloroform (Kokusan Chemical Co.) as a solvent.
Downloaded by [Deakin University Library] at 02:40 06 November 2015
Microencapsulation of pheromone analogue
469
Figure 2. Schematic diagram of the system for release experiment. 1. Nitrogen cylinder, 2. Sample chamber, 3. Microcapsules, 4. Gas sampler, 5. Flow meter, 6. Gas chromatograph.
E H A release experiment Apparatus. A schematic diagram of an experimental system to measure the release rate of EHA is shown in figure 2. No. 2 is a particular glass tube, 2.1 cm diameter and 17-5cm length. Microcapsules of known weight were extended on a filter paper placed in the tube. Released EHA was mixed with continuous flow of nitrogen, the flow rate being 1 ml s-l at room temperature and was introduced to the No. 4 gas sampler with a certain time interval for GC analysis. T h e release rate of EHA was directly obtained from this procedure.
Results and discussion Microcapsules prepared using complex coacervation W a x particles. A SEM photograph of a wax particle, impregnated with pheromone, is shown in figure 3, and an enlarged surface feature is shown in figure 4. No pin-holes or cracks were observed apart from the fine creases which characterize the surface of wax materials. Particle size was distributed between diameters of 40 to 400pm,the average value being around 250pm and very few agglomerated particles were observed. EHA content in the particles was around 70 mg g - microcapsule, approximately 80 per cent yield based on the feed EHA.
Composition of coacervates from geletan-gum arabic system The composition of gelatin in coacervates was 75wt. per cent, and remained constant regardless of the feed ratio. I n the existing conditions of p H = 4 0 and 313"K, maximum amount of coacervates was observed at the feed ratio of gelatin/gum arabic = 60/40 (wt.). This result was slightly different from that of Bungenberg, de Jong and Dekker (1935,1936), whose data showed the maximum at 40wt. per cent gelatin. In the subsequent encapsulation, the majority of procedures were carried out with a feed composition of 50/50 (wt.). Microcapsules of wax particles All the microcapsules obtained contained only a single core. No doublets or further multiple cores were observed. T h e characteristics of the microcapsules
Downloaded by [Deakin University Library] at 02:40 06 November 2015
470
S. Omi et al.
Figure 3. SEM photograph of wax particle impregnated with EHA. (160-fold, dF = 233 pm).
Figure 4. SEM photograph of enlarged surface. (2000-fold).
prepared together with one example of uncapsulated wax particles are listed in table 1. EHA content was sufficient for the present purpose. However, the thickness of the wall was of the order of 1 pm at most, causing concern that the wall thickness might be too thin. In this context, M C 107 was encapsulated twice to double the wall thickness. SEM photographs of general features of the microcapsule and its surface are shown in figures 5 and 6, respectively. One might feel that there are not many differences between the features of figures 3 and 5, but the magnified view of figure 6 revealed that the uniform and somewhat soft structure of a gelatin wall had developed on the surface of the wax particles.
101 102 103 104 105 106 107 Wax P.
58.5 64.8 761 78-6
14.1 6.9 1.5 500
302 237 176 212
201 202 203 204
Based on dried capsules. Based on calculations.
Gelatin" [wt. per cent]
EHA
[mg/g-MC]
dF
No.
a
-
3.0 4.4 1.8 0-9 1.3 02 8.7
Gelatin [wt. per cent]
17.8 15.8 10-6 17-6 17.6 13-0 17-9
Water [wt. per cent]
-
1.3 1.5 0-4 0.2 06 02 3.0
6
-
338 271 169 200 330 203 262
d,m
Diameter of MC [Pnl
52.6 59.8 68.8 62.8
Water (wt. percent]
28 11 12 70
1192 724 61 8 1366
dpmw
1545 987 914 1904
6 133 95 107 247
N b
dPm
bml
bml
bml
Wet diameter
Thicknessb
Dry diametep
Characterization of microcapsules prepared with (S/W)/O multiple phase emulsion technique.
MC
Diameter of core [pm]
Table 2.
335 268 168 200 329 203 256 233
No. 66.0 71.9 67.6 60.2 65.2 57.0 71.6 78.1
EHA [mg/g-MC]
dF
MC
Thickness bml
Characterization of microcapsules prepared with complex-coacervation technique.
Diameter of core b m ]
Table 1.
Downloaded by [Deakin University Library] at 02:40 06 November 2015
f
f
G
2
0
Downloaded by [Deakin University Library] at 02:40 06 November 2015
472
Figure 5.
S. Omi et al.
SEM photograph of MC 107, twice encapsulated. ( 1 SO-fold, d,,, = 262 pm).
Figure 6. SEM photograph of the surface of MC 107. (2000-fold).
Release profile of EHA from microcapsules ( M C 107) A typical release profile together with the release rate is shown in figure 7. As mentioned before, MC 107 was twice encapsulated, and has a maximum wall thickness of 3.0 pm. T h e fraction of EHA was plotted against the elapsed time and the release rate, also plotted in the figure, appeared very fast during the early stages. Unexpectedly, indeed disappointingly, 50 per cent of total EHA was released within three days. T h e release profile of EHA from unencapsulated wax particles is also shown in the figure (broken line). A comparison between two curves of the release rate reflects
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Microencapsulation of pheromone analogue
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No. 107 MC 0 59
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