Chitin Microstructure Formation by Rapid Expansion Techniques with

Ind. Eng. Chem. Res. 2009, 48, 769–778

769

Chitin Microstructure Formation by Rapid Expansion Techniques with Supercritical Carbon Dioxide Ricardo Salinas-Hernańdez, F. Alberto Ruiz-Trevinõ, and Ciro-H. Ortiz-Estrada* Departamento de Ingenierıá y Ciencias Quı´micas, UniVersidad Iberoamericana, Prol. Paseo de la Reforma 880, Lomas de Santa Fe, Me´xico D. F. 01219, Me´xico

Gabriel Luna-Ba´rcenas and Yevgeny Prokhorov CINVESTAV del IPN, Unidad Quere´taro, Libramiento Norponiente 2000, Fracc. Real de Juriquilla, Quere´taro Qro. 76060, Me´xico

Juan F. J. Alvarado Departmento de Ingenieria Quimica, Instituto Tecnolo´gico de Celaya, AV. Tecnolo´gico y Garcıá Cubas s/n, Celaya Gto. 38010, Me´xico

Isaac C. Sanchez Department of Chemical Engineering, The UniVersity of Texas at Austin, Austin, Texas 78712

Rapid expansion techniques (of supercritical solution (RESS) and into a liquid solvent (RESOLV)) with supercritical carbon dioxide are used to form chitin microstructures. The effect of the pre-expansion conditions, temperature (40-60 °C), pressure (104-208 bar), chitin concentration (1-6 mg/mL), and nozzle diameter (100-500 µm), on the morphology and the size of the chitin microstructures formed with both RESS and RESOLV are studied. Depending on the experimental conditions, it is found that spherical microparticles, with average diameters of 1.7-5.3 µm, are obtained with RESS, while continuous microfibers, with average diameters of 11.5-19.3 µm, are obtained with RESOLV. For both RESS and RESOLV it was observed that the pre-expansion temperature-pressure conditions studied here have practically no effect on the average diameters of the formed material. On the contrary, it is shown that concentration and nozzle diameter directly influence the morphology and size obtaining that lower concentrations and smaller diameter nozzles lead to the production of smaller diameter microstructures and closed-size distributions. 1. Introduction With the exception of cellulose, chitin is the most abundant natural polysaccharide on Earth, approximately 100 billion tons of chitin per year are produced by crustaceous, mollusks, insects and fungus, it is perhaps the least exploited biomass source.1-3 The waste generated by the processing of crab, lobster, shrimp, clam and oysters represents, in some cases, 75% of the total weight of the shellfish, currently being the main source of chitin production. The chitin production is based upon the waste of the crab’s shell and the shrimp’s skin as well by the canned food industry in the United States and Japan and by several fishing fleets in the Antarctic. Several countries possess great unexplored sources of crustaceous (e.g., Norway, Mexico, and Chile). In fact, the commercial value of chitin has recently increased due to the attractive properties of its miscible derivates, that is, chitosan as a product generated by the N-deacetylation of chitin, which are potentially used in the biomedical, pharmaceutical, and water treatment fields, just to mention a few of the potential applications of this product. The wide range of technological applications is attributed to their excellent properties such as: gelling ability, high adsorption capacity, wide immunological activity, biodegradability, biocompatibility, and nontoxic nature.1,2 In the later years remarkable efforts have been made to use chitin and chitosan partly because of the amount of waste * To whom correspondence should be addressed. Tel.: +52-55-59504168. Fax: +52-55-5950-4269. E-mail: [email protected].

generated annually. On one hand it is recognized that the marine biomass has become an environmental priority, and on the other, it becomes necessary to produce useful biomaterials taking efficient advantages of chitin and chitosan. In this regard, biomaterials are required in the shape of particles (e.g., in the pharmaceutical field for controlled dose liberation medicines and drugs), fibers (used in the biomedical field: sutures, meshes, strings, etc.), or permeable or porous membranes (in the water treatment to eliminate heavy metals, colorants, and enzymes immobilization). In all the cases where microstructures are required, the performance of the biomaterial is favored when the surface/volume ratio is increased since it will improve the mass transfer process of the exploited phenomena. Examples of this are the advances on the formation of micro- and nanoparticles based on chitosan for pharmaceutical applications4 with the use of supercritical fluids5 (SCF). Biomaterials science is a growing area of opportunity that considers natural materials and clean technologies. Knowledge in the formation of biomaterials using supercritical fluid technology as an appealing alternative for the formation of functional structures in the earlier mentioned applications has been explored recently with great interest. Most of the particle production processes assisted by supercritical fluids (SCF) are classified according to the technique in which they are used, whether the SCF acts as a solvent or antisolvent causing the immediate separation of the polymer from the original solution. There are mainly two methods for the formation of particles:

10.1021/ie800084x CCC: $40.75  2009 American Chemical Society Published on Web 12/04/2008

770 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

Figure 1. High-vacuum scanning electron microscopy images of nonprocessed purified chitin. Table 1. Average Diameter of Chitin Microspheres Obtained at Different Experimental Conditions by Rapid Expansion in Air (RESS) of Chitin Solutions Prepared at Different Concentrations. operation conditions Cchitin, mg/mL

nozzle diameter, µm

T, °C

P, bar

diameter average, µm

standard deviation

1 2 4 6

100

40

208 208 208 208

1.40 1.70 2.02 5.81

0.37 0.42 1.12 1.87

2 4 2 4 2 4

175

40

104 104 150 150 208 208

1.66 3.00 1.70 3.12 1.72 3.10

0.80 1.59 0.75 1.10 0.70 0.84

2 4 2 2 4

500

40

104 104 150 208 208

2.03 4.37 1.96 1.94 4.62

0.82 1.58 0.91 0.93 1.25

2 4 2 4

175

50

104 104 208 208

1.82 3.41 1.79 3.12

0.79 1.62 0.71 0.91

2

500

50

104 208

2.13 1.98

0.91 0.94

2 4 2 4 2 4

175

60

104 104 150 150 208 208

1.94 3.66 1.89 3.40 1.87 3.02

0.81 1.61 0.75 1.12 0.64 0.93

2 4 2 2 4

500

60

104 104 150 208 208

2.22 5.34 2.21 2.13 4.65

1.04 2.56 0.95 0.80 3.33

RESS (rapid expansion of supercritical solution) where the SCF is a solvent and SAS (supercritical antisolvent) as antisolvent. These techniques have been described and widely referred in the literature.5-9 In both cases the phenomena taking place is the supersaturation of the solution during the transition from a homogeneous phase to the precipitation of the polymeric material as a result of the action of the supercritical agent.

Figure 2. Effect of pre-expansion temperature and pressure conditions on chitin microsphere particles formed by rapid expansion in air, RESS, of a 2 mg/mL chitin concentration through a 175 µm (open symbols) and 500 µm (solid symbols) nozzle diameter. T ) 40 °C (diamonds); T ) 50 °C (circles); T ) 60 °C (squares).

An interesting variation of the RESS process is the rapid expansion of a supercritical solution into a liquid solvent (RESOLV), consisting of spraying the supercritical solution in a liquid.5,10-15 Operating this way, it should be possible to stop the particles growth in the precipitator, improving the RESS performance, or else to collapse the structures modifying the morphology of the material. Among the supercritical fluids, carbon dioxide (SCCO2), is the preferably used agent as it is: nontoxic, nonflammable, environmentally accepted, low cost for it is abundance, facility of obtaining and recovering. Additionally, its low critical temperature (Tc ) 31.3 °C) allows operation conditions close to the environment temperature, this being an attractive quality in the treatment of thermal unstable materials. The SCCO2 has been used equally as a solvent and antisolvent for different applications, taking advantage of the ability to rapidly vary its solvation force, and therefore, for nucleation or to supersaturate

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 771

Figure 3. Effect of chitin concentration, on the average diameter of chitin microspheres formed by RESS through a nozzle diameter of 100 µm and with pre-expansion conditions of T ) 40 °C and P ) 208 bar.

Figure 4. High-vacuum SEM images of chitin microspheres formed by RESS of chitin concentrations of (a) 1, (b) 2, (c) 4, and (d) 6 mg/mL, through a nozzle diameter of 100 µm and pre-expansion conditions of T ) 40 °C and P ) 208 bar.

dissolved compounds, such aspects are considered key for supercritical technology in the formation of particles. The applications are very diverse: synthesis and processing of polymers, formation of nanomaterials, biomaterials processing, or encapsulating pharmaceutical products on which the size and formation of the particle are relevant for the quality and performance of the product.5-8,16-20 The morphology of the formed solid material, crystalline or amorphous, depends on the chemical structure of the material and the RESS and RESOLV

parameters (pre-expansion T-P and saturation conditions, pressure drop, the geometry of the nozzle, the receiving liquid medium, etc.). Recently RESOLV has been used in the formation of nanostructures of diverse materials, among others, in the formation of silver nanoparticles,10 naproxen,11 tetraphenylporphyrin (TBTPP),12 poly-L-lactic acid (PLA) and poly methyl methacrylate (PMMA),14 paclitaxel15 and poly heptadecafluorodecyl acrylate (PHDFDA) nanofibers.13 Even though recent studies have reported the formation of chitin microfibers21


Figure 5. Effect of the nozzle diameter on the average diameter of chitin microspheres formed by RESS of two different chitin concentrations and with pre-expansion T-P conditions of T ) 40 °C and P ) 208 bar.

and chitosan microparticles22 using SAS, a few cases have RESS being compared to RESOLV, observing that RESOLV displays a reduction in the size of the particles. In this work, the effect of pre-expansion temperature and pressure, chitin concentration in solution, and nozzle diameter on the morphology and particle size distribution of the chitin microstructures formed with both RESS and RESOLV is studied. 2. Experimental Section 2.1. Materials. Hexafluoroisopropanol (HFIP), was acquired from Sigma Aldrich (g99% purity, Mexico). The crab’s shell chitin was acquired from Sigma with a 96% degree of acetylation as reported by the supplier, whereas the carbon dioxide (CO2) was acquired from Air Products (g99% purity, grade 3, Mexico). 2.2. Chitin Purification and Dissolution. To get purified chitin, a typical procedure reported by Louvier-Hernańdez et al.21 was used to remove minerals and proteins from the as-received

chitin sample. Afterward, the chitin was dissolved in HFIP to obtain solutions of chitin/HFIP prepared at different concentrations (from 1 to 6 mg/mL chitin concentrations). The dissolution was carried out at continuous agitation for 120 h, further to this period; almost all the chitin was dissolved, except for a small amount that was filtered before being mixed with SCCO2. 2.3. Chitin Microstructure Formation. The equipment used to get chitin materials with a well-defined microstructure using SCCO2 was that described by Santoyo-Arreola et al.23 It consists of a high pressure cell with temperature and pressure controls, an expansion section, and an additional system to recollect the particles. The expansion section is basically a valve and a PEEK nozzle with different diameters (100, 175, and 500 µm Upchurch), whereas the section to recollect may be adjusted with different configurations of the reception volumes. A rectangular cell was filled with air as the expansion medium for RESS, and a cylindrical cell with water was for RESOLV. The reception volumes are at room temperature (∼23 °C) and ambient pressure (∼0.9 bar). For


Figure 6. High-vacuum SEM images of chitin microspheres formed by RESS of a 4 mg/mL chitin concentration through different nozzle diameters, (a) 100, (b) 175, and (c) 500 µm, and fixed pre-expansion T-P conditions of T ) 40 °C and P ) 208 bar.

this work, the concentrations of chitin in HFIP studied varied from 1 to 6 mg/mL and the levels of temperature and pressure studied were 40, 50, and 60 °C and 104, 150, and 208 bar. A typical procedure to form chitin structures with well-defined microstructures may be described as follows: A 5 mL chitinHFIP solution is confined in the high pressure cell, and then SCCO2 is introduced by means of a manual piston until the CO2 concentration in the solution reaches approximately 30 wt %, since at these conditions the supercritical solution is homogeneous. Once the supercritical homogeneous solution is reached, the pressure is increased until the studied pressure is reached. This homogeneous supercritical solution is stirred for two hours before the expansion valve is opened. The temperature is controlled by a thermocirculating water bath and depending upon the process the particles are recollected in a rectangular reception volume for the case of RESS or in a cylindrical reception volume for RESOLV. 2.4. Particle Characterization. Chitin particles formed in the expansion section were dried for several hours at 30 °C in a convection oven to get rid of HFIP residual solvent. The dried chitin particles were then characterized in terms of their morphology and particles size using images obtained in a highvacuum scanning electron microscope (SEM Phillips ESEM XL30) and the statistical package offered by Digital Micrograph for GMS (Gatan software). The statistical analysis was performed taking enough particle-size data to get their distribution and the average-particle size.

3. Results and Discussion Chitin as received is a flake yellowish material that is hard to convert into a fine powder by physical operations (mills). The original size of the flakes is around 0.2-1.3 mm. Figure 1 shows images of chitin flakes as received. The morphology and size of the micromaterial obtained at different pre-expansion temperatures and pressures (T-P), chitin concentration in solution, and the expansion nozzle diameter is analyzed for both RESS and RESOLV in the following paragraphs. 3.1. RESS. When the chitin/HFIP/SCCO2 is expanded in RESS a rapid chitin precipitation in the shape of spherical microparticles of different sizes and morphologies depending on the study conditions takes place. Table 1 summarizes the results of the average particle diameter at the different conditions studied in this work. Pre-Expansion Temperature and Pressure. Figure 2 shows the effect of the pre-expansion T-P conditions on the average diameter of chitin microspheres formed by RESS, of a 2 mg/ mL chitin concentration through two different nozzle diameters (the dotted lines are just shown as a reference). Observe that a temperature increase causes a slight increase in the size of the chitin microspheres, whereas an increase in pressure slightly diminishes the size of the particles. Generally the pre-expansion T-P conditions do not significantly modify the size and morphology of the microspheres as it as been reported for the cases of poly dehydrofluorooctyl methacrylate (PFOMA),23 poly heptadecafluorodecyl acrylate (PHDFDA),24 cholesterol,25,26

774 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 Table 2. Average Diameter of Chitin Microfibers Obtained at Different Experimental Conditions by RESOLV of Chitin Solutions Prepared at Different Concentrations. operation conditions Cchitin, mg/mL

nozzle diameter, µm

T, °C

P, bar

diameter average, µm

standard deviation

1 2 4 6

100

40

208 208 208 208

11.53 12.78 14.22 18.57

1.32 1.72 1.64 1.07

2 4 2 4 2 4

175

40

104 104 150 150 208 208

14.32 15.61 14.82 15.73 15.59 15.95

1.06 2.04 1.55 1.89 2.19 1.87

2 4 2 4

175

50

104 104 208 208

14.70 15.70 13.66 15.74

2.10 2.13 2.40 2.21

2 4 2 4 2 4 2 4 2 4 2 4 2 4

175

60

500

40

500

60

104 104 150 150 208 208 104 104 208 208 104 104 208 208

15.18 15.04 14.62 15.56 12.98 16.91 18.61 19.29 17.09 18.18 16.51 18.44 16.70 17.63

3.74 1.43 2.81 1.89 1.83 1.19 3.50 3.44 3.42 4.86 2.77 1.02 2.56 4.14

griseofulvin,26,27 β-sitosterol,27 ibuprofen,28,29 aspirin30 and cyclosporine.31 Other studies report that the pre-expansion temperature influences the size of the material structures as in the case of the naphthalene,32 tetraphenylporphyrin (TPTPP),12 benzoic acid,25-27 and salicylic acid33 where it has been observed that their average particle size increases when temperature increases. Just a few studies have reported the opposite effect where the temperature increase reduces the particle size as in the case of naphthalene25 and perfluoropolyether diamide (PFD).34 The variation in the results is probably due to the effect of the low miscibility of some materials in the SCF26,27 and also to the saturation level previous to the expansion. At temperatures and pressures closer to the saturation state, the nucleation and growth mechanism is favored before the material leaves the expansion nozzle, therefore causing effects in the morphology and size. On the contrary, conditions far from saturation will cause the particles to precipitate inside the nozzle or out of it, therefore the tendency in the size and morphology will be influenced by other factors rather than by the preexpansion conditions. In a previous work,23 it has been proven that the saturation level defines the pre-expansion conditions and therefore the morphology and dimensions of the material as it has been observed by other authors.24,35,36 Chitin Concentration. Figure 3 shows the effect of chitin concentration on the average diameter of chitin microspheres formed by expansion in air through a nozzle diameter of 100 µm and fixed pre-expansion T-P conditions (40 °C and 208 bar). For a change in chitin concentration from 1 to 6 mg/mL, the average diameter of the microspheres goes from 1.4 ( 0.37 to 5.81 ( 1.87 µm. In all cases (see Table 1) the increase in the concentration causes an increase in the average particle diameter that is accompanied by a bigger standard deviation due to the wide distribution of obtained particles. These results are in agreement with those reported in the literature where an

increase in concentration leads to a diminished particle size, and even more, also changes in their morphology23,24,34-40 since they can go from microspheres to microfibers. Recently the effect of the PFOMA23 concentration on their particle size has been measured, and the results show that microspheres bigger than 5 µm can be obtained with 20% concentrations, whereas microspheres around 2 µm can be obtained with concentrations in the order of 5%. An example where concentration plays an important role changing the morphology from microspheres to microfibers is the work done by Blasig et al.24 with PHDFDA particles. In this work it is concluded that the concentration (first to saturation grade) is the controlling variable in the morphology, since it is found that microspherical particles are formed with concentrations around 0.5% (and also for nonsaturated conditions), while microfibers are formed with concentrations equal or higher (supersaturated conditions) than 2%. Lele y Shine35 observed that the change in the PMMA concentration affects the morphology since a 0.08% concentrations leads to the formation of a powder and a concentration of 0.263% (conditions close to solution saturation) leads to large fibers. Similar observations have been reported by Tom and Debenedetti,37 Mawson et al.,38 and Aniedobe and Thies.39 The effect of concentration on the size and the morphology of the microparticles can be explained as follows: at high concentrations the nucleation and growth mechanism is favored because of the higher biopolymer content, while at low concentrations the mechanism is limited to particles of smaller size. When carrying out the expansion at high concentration, great volumes of polymer-rich phase are obtained, which favors the coalescence of particle nuclei and therefore the growth until the solution is expanded. At low concentration, the polymerrich phase is surrounded by polymer-lean regions, for which the polymer volume is small and there are no regions close enough to commence the coalescence and growth; thus small nuclei have no chance to grow, so when passing through the nozzle, they maintain their size and spherical structure. Another factor to consider is the change in the properties in the chitin solution as at higher concentrations viscosity increases considerably, which results into a resistance to dispersion of the particles favoring its coalescence and therefore the precipitated growth. Figure 4 displays diverse micrographs at different chitin concentrations. It is remarkable that differences in chitin diameter and particle size distribution increase when the concentration increases. Nozzle Diameter. Figure 5 shows the effect of the nozzle diameter on the size and particle size distribution of chitin microspheres formed using two different chitin concentrations and fixed pre-expansion T-P conditions (T ) 40 °C and P ) 208 bar). The average diameter and particle size distribution, as judged by the standard deviations shown there, observed in the chitin microspheres increases as the nozzle diameter increases. However, the effect is more notorious for higher concentrations since for the case of the 4 mg/mL chitin concentration the microsphere has double its average diameter. Similar results have been observed for the case of PFOMA23 particles produced varying the nozzle diameter from 130 to 794 µm, where particle sizes around 0.2-0.9 µm were formed with a 1% polymer solution and around 0.4-1.7 µm with a 5% concentration. Similarly, Alessi et al.40 using nozzles of 30 and 100 µm obtained average sizes of progesterone particles of 4.1 and 7.5 µm respectively with a closed particle size distribution at minor nozzle diameter. Huang and Moriyoshi41 achieved a significant reduction of lopeamide HCI particles on the order of 0.3-0.5 µm with nozzle clearance of 5 µm, whereas when


Figure 7. Effect of chitin concentration on the average diameter of chitin microfibers formed by RESOLV through a nozzle diameter of 100 µm and with pre-expansion conditions of T ) 40 °C and P ) 208 bar.

Figure 8. High-vacuum SEM images of chitin microfibers formed by RESOLV of chitin concentrations of (a) 1, (b) 2, (c) 4, and (d) 6 mg/mL, through a nozzle diameter of 100 µm and pre-expansion conditions of T ) 40 °C and P ) 208 bar.

using 100 and 200 µm nozzles they obtained particles in an interval of 1-4 µm and 1-7 µm, respectively. Other studies indicate that the size of the nozzle does not cause significant change in the morphology and size of the precipitated, possibly due to the low solubility of the micronized material.29,30 Figure 6 shows micrographs of chitin microspheres formed with different nozzle diameters and maintaining the concentration and pre-expansion T-P conditions steady.

In the processing of materials by RESS, the biopolymer solution has to go through a chocking and an expansion device so the formation of particles takes place on the basis of a sequence of mechanisms such as nucleation, growth, and coagulation. Thus, the nozzle diameter and the concentration of the biopolymer solution play an important role in defining the morphology and size of the micromaterials formed. According to the classic nucleation theory, the rate


Figure 9. Effect of the nozzle diameter on the average diameter of chitin microfibers formed by RESOLV of two different chitin concentrations and with pre-expansion T-P conditions of T ) 40 °C and P ) 208 bar.

of critical size nuclei formation is a strong function of the supersaturation level and in the RESS process it takes place in a region close to the nozzle entrance, while the growth of the particles takes place inside the nozzle and also during the atomization process after the expansion where also the coagulation mechanisms is strongly favored. In RESS, the coagulation mechanism is the critical step that determines the morphology, in this work microspheres under the concentration range studied, and size of the particles. 3.2. RESOLV. In the case of RESOLV, the chitin/HFIP/ SCCO2 solution is expanded into the water as the receiving medium, where surprisingly, a rapid chitin concentration in the shape of flat rolled microfibers takes place. Seemingly, in this case the medium collapses the structures abruptly promoting, for the null miscibility of the chitin in water, the formation of microfibers due to the intermolecular forces of the chitin’s structure because of the presence of hydrogen bonds. In Table 2 the results obtained are reported. It can be seen that fibers around 10 to 20 µm in diameter, with standard deviations between 1 and 5 µm, can be formed under the conditions studied in this work. Pre-expansion Temperature and Pressure. As in the case of RESS, the pre-expansion T-P conditions have little influence

on the morphology and size of the chitin microfibers, as it has been observed in the work carried out by Sane and Thies12 that report the formation of 30 nm diameter microspheres at any of the pre-expansion T-P conditions studied in their work. Unfortunately, it is not possible to get a general conclusion on these observations because they used a stabilizer in water (Pluronic F68). In this work, expansions were carried out in pure water since the objective was to compare the RESS with RESOLV. Chitin Concentration. Figure 7 shows the morphology and size distribution of the chitin microfiber as an effect of the concentration. As in the case of RESS, the concentration is directly proportional to the diameter of the average fibers and, interestingly, the microfibers show a narrower distribution with respect to one shown by the microspheres formed with RESS. For example, for microfibers formed at pre-expansion T-P conditions of 40 °C, 208 bar and nozzle diameter of 100 µm, average fiber diameters go from 11.53 ( 1.32 to 18.57 ( 1.07 µm when chitin concentration is increased from 2 to 6 mg/mL. The same behavior is observed for other tests using different conditions of pre-expansion T-P and nozzle diameters. Figure 8 shows diverse micrographs where the biopolymer concentration of the initial solution changes, maintaining the


Figure 10. High-vacuum SEM images of chitin microfibers formed by RESOLV of a 4 mg/mL chitin concentration through different nozzle diameters, (a) 100, (b) 175, and (c) 500 µm and fixed pre-expansion T-P conditions of T ) 40 °C and P ) 208 bar.

nozzle diameter constant. Sane and Thies12 did not observe significant changes in the size of TBTPP particles with the increase in the concentration; however, at 0.1% concentration, nanoparticles were characteristically well dispersed and at 1% some agglomerates were found. Similarly, Meziani et al.13 did not observe any effect in the PHDFDA nanofibers for the concentration range from 2 to 5% weight, where NaCl solutions were used as the receiving agent. However, in smaller concentrations (0.3% weight), combined formations of nanofibers and nanoparticles were observed.42 For the case of PMMA dispersions from ethanol solutions-SCCO2 to a receiving NaCl solution, it is observed that morphologies at 1.1 mg/mL are nanofibers and at 0.1 mg/mL nanoparticles are formed; the same effect was observed for the PLA.13,14 Nozzle Diameter. Figure 9 shows the tendencies and the effect of the nozzle diameter in relation to the size of the chitin microfibers, where an increase in the diameter of the atomization device is noticeable in the increase of the microfiber diameter as well as its size distribution. Figure 10 shows high-vacuum SEM images of microfibers processed with chitin solutions of 4 mg/ml and different nozzle diameters. For experiments at 40 °C and nozzles diameters from 100 to 500 µm, average fiber diameters from 12.78 ( 1.72 to 17.09 ( 3.42 µm are formed with chitin concentration of 2 mg/mL, whereas for a 4 mg/mL chitin concentration, the average diameters goes from14.22 ( 1.64 to 18.18 ( 4.86 µm. Note the increase in the standard deviation as the nozzle diameter increases. The processing of materials by RESOLV has apparently the same theoretical foundation of the nucleation, growth, and

coagulation mechanisms just as in the case of RESS. However, the low diffusivity into the liquid medium (RESOLV) collapses the condensation process favoring the formation of microfibers instead of microspheres. In the RESS process the air facilitates dispersion, while in RESOLV the mechanisms can be closely related to the concentration of the material in the initial solution13 and also associated to the intermolecular forces linking the molecules, thus avoiding its dispersion in the shape of particles (or spheres). Louvier-Hernańdez et al.21 observed by SAS the formation of chitin nanofibers; they attribute it to the strong intermolecular and intramolecular interactions for the presence of H-bonding. In this work, the microfibers obtained by RESOLV are the result of similar effects as those caused by the collapse in an antisolvent medium similar to the one generated in SAS where the SCCO2 is the antisolvent agent. 4. Conclusions The RESS and RESOLV techniques have been applied to the same pre-expansion T-P, chitin concentration and expansion nozzle diameter, obtaining microspheres with RESS and microfibers with RESOLV, this last, possibly due to the high forces in the hydrogen bonds of the molecular structure of the chitin that collapses with the receiving medium, in this case water, causing an antisolvent effect that favors the intermolecular interactions that in the case of RESS for the high diffusivity of air does not allow the structures to collapse in fibers, obtaining spherical particles.


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ReceiVed for reView January 17, 2008 ReVised manuscript receiVed October 13, 2008 Accepted October 27, 2008 IE800084X

Chitin Microstructure Formation by Rapid Expansion Techniques with

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