Chitosan (Cs) is a natural linear polysaccharide generated from the deacetylation of chitin and is composed of β-(1-4)-d-glucosamine units and β-(1-4)-N-acetyl-glucosamine distributed along the polymeric chain. The physicochemical characteristics and functional properties of Cs, such as its polycationic character, biocompatibility, low toxicity, and structural capacity, make it a polysaccharide of interest in different fields. The mechanical and structural properties of chitosan allow for different types of materials to be obtained from chitosan solutions, e.g., nanostructured porous materials. Aerogels are a specific type of nanostructured porous material that are characterized by mesopores (pore diameter between 2 and 50 nm) and large surface areas. Obtaining aerogels of chitosan could improve the availability of the functional groups of chitosan . Together, the aerogel characteristics with the functional properties of chitosan, potentially provides materials with multiple applications, e.g., adsorption, transport, and controlled release of bioactive molecules, toxics, and pollutants removers, among others.
Ionic liquids (IL) are an alternative medium for chitosan dissolution. When compared to the traditional aqueous acid solvents used to dissolve Cs, IL have different physicochemical characteristics because they consist of only ions and water is not needed to dissolve chitosan. Most of the studies of chitosan with IL have been focused on the formation of solutions. Imidazolium based IL, as 1-butyl-3-methylimidazolium acetate (BMIMAc), and 1-ethyl-3-methyl-imidazolium acetate (EMIMAc), have been highlighted because they are able to dissolve chitosan at high concentrations (up to 10% w/w). Other reports have focused on forming materials from chitosan solutions in IL e.g., fibers, blends, films, membranes, hydrogels, and ionogels [2,3,4,5,6,7,8,9,10,11,12]. Aerogels from chitin with BMIMAc solutions has been reported , but the information about chitosan ionogels and aerogels from IL solutions is limited.
Supercritical CO2 drying, unlike other types of drying, keeps most of the internal structure formed at gelation because the effects of surface tension on the three-dimensional macromolecular network are minimized. Therefore, chitosan aerogels that are generated from solutions of Cs in IL (BMIMAc and EMIMAc) were produced in order to study the physicochemical characteristics of these novel materials. Concurrently, observations on the structural features of the aerogels could be related to the network formation and molecular conformation of chitosan in ionic liquids at the physical gelling process.
2. Materials and Methods
Shrimp shells (Pandalus borealis) Cs was acquired from Primex (batch No. TM 1961, Siglufjörður, Iceland). The degree of acetylation (DA) of Cs was 16.3%, as determined by solid-state cross-polarization magic angle spinning 13C nuclear magnetic resonance spectroscopy (CP/MAS 13C-NMR) performed in a Bruker Avance TM 400WB (Bruker Biospin, Wissembourg, France, 9.4 Tesla, 1 ms contact time, 2000 scans) [14,15]. The weight average molecular weight (Mw) of Cs was 2.01 × 105 g/mol, determined by static light scattering, as described previously . Chitosan was purified, as described by Rinaudo et al. (1999), before use it . Commercial grade IL, BMIMAc and EMIMAc (BASF, Steinheim, Germany), were used as received just prior each use they were stored in desiccator and kept at 105° C for 24 h in vacuum to minimize moisture. High grade supercritical drying liquid CO2 (99.99% purity) was supplied by Infra (Hermosillo, México). All of the compounds and solvents that were used were reagent grade acquired from recognized commercial chemical distributors. The water used for the experiments was deionized type I (resistivity of 18.2 MΩ·cm at 25 °C) unless stated otherwise.
2.2. Solutions of Chitosan in Ionic Liquids
The moisture-free IL was added into a round-bottom flask with a sufficient amount of chitosan to obtain a 2% (w/w) concentration. This solution was heated in an oil bath at 105 °C under magnetic stirring at least 6 hours under a nitrogen atmosphere. Upon complete Cs dissolution it was stored in a desiccator at room temperature until use. Two types of solutions were obtained: Cs in EMIMAc (CsEMIM) and Cs in BMIMAc (CsBMIM).
2.3. Formation of Ionogels
The physical gels were formed using approximately 0.1 g of the Cs-IL solution that was casted in cylindrical containers (0.4 cm diameter), with the bottom being covered with plastic paraffin film. Gelation was induced by vapor diffusion of an antisolvent, such as ethanol or water, in a closed chamber . The gels were recovered after 48 h and were transferred to a 70% ethanol aqueous mixture. These gels that contain IL as liquid phase confined within a polymer matrix are defined as ionogels .
2.4. Formation of Aerogels
The ionogels were thoroughly rinsed with aqueous ethanol (70%) until the IL was completely removed. The presence of IL in the rinsing media was monitored with UV/VIS spectroscopy, scanning between 190 to 300 nm using the ethanol-water mixture as reference. Afterwards, the liquid phase was replaced with acetone, which has higher miscibility with supercritical CO2. For this, the gels were repeatedly rinsed with a gradient of acetone-water mixtures that ended with two rinses with pure acetone. The acetogels (gels containing acetone as fluid phase) were dried with supercritical CO2 (>32 °C and 73 atm) in a pressurized reactor. At the end of the process, the dried aerogels were stored in a desiccator at room temperature.
2.5. Characterization of Aerogels
2.5.1. Chemical Identity
The chemical composition of the samples was determined analyzing the characteristic bands in the infrared spectrum obtained by a Fourier transform infrared spectrophotometer (FT-IR, Thermo Scientific, Nicolet iS-50, Madison, WI, USA), using the attenuated total reflection (ATR) mode. All of the measurements were performed at room temperature collecting 32 scans at 4 cm−1 resolution.
2.5.2. Structural Analysis
The surface area, pore volume, and nitrogen adsorption and desorption isotherms of the aerogels were determined with the surface area analyzer Nova 2200e (Quantachrome Instruments, Boynton Beach, FL, USA), and analysis of the data with the software NovaWin version 11.02 (Quantachrome Instruments, Boynton Beach, FL, USA). The morphological characteristics of the aerogels were thoroughly observed by field emission scanning electron microscopy (Hitachi SU8000, Tokyo, Japan). The images were obtained using an accelerating voltage of 1.0 KeV.
2.5.3. Degree of Swelling
3. Results and Discussion
The vapor diffusion of a non-solvent agent was useful to produce ionogels from Cs solutions in IL. By this way, it was possible to obtain three different types of physical chitosan ionogels (Table 1). Using ethanol as non-solvent agent gels were obtained from both Cs-IL solutions. These gels were clear, rigid, and brittle; the only noticeable difference among them was the color tone, darker yellow for the gels from CsBMIM. It has been indicated that the main effect of IL on polysaccharides is to disrupt the hydrogen bonds and promoting their dissolution. Low molecular weight alcohols are miscible with imidazolium based ionic liquids , but chitosan does not dissolve in alcohols. Therefore, when ethanol diffuses into a Cs-IL solution, the solvation effect of the ionic liquid over chitosan decreases, favoring the interactions between chitosan chains and subsequently leading to the generation of a gel. Conversely, water vapor was only useful to produce gels from the CsEMIM solution. It has been noticed that BMIMAc has a lower affinity for water than EMIMAc , this could be related to lower diffusion rates that do not decrease its chitosan solvation capacity in preventing the gel formation. It should be taken in account that chitosan do not dissolve in water, but it is hygroscopic. As result, the obtained gels from water diffusion were weak and difficult to manipulate without compromising their integrity. Hence, subsequent procedures and analysis were performed using only ionogels that are produced by ethanol treatment.
When the ionogels were rinsed with an ethanol-water mixture they became more translucent, reducing their yellow color (Figure 1). The continuous rinsing of the gels gradually eliminated the ionic liquid from inside the gels. The ethanol concentration that was used allowed for keeping the ionogels volume without causing drastic swelling or shrinkage. In the subsequent fluid phase replacement with acetone, the CsE ionogels decrease 25% their volume, and in the case of CsB, the reduction was 42%. Apparently, the chitosan chains underwent rearrangement within the formed network to a more stable configuration as a result of the interaction with acetone .
The obtained aerogels were rigid and brittle cylinders with opaque white color (Figure 1). The volume reduction when compared to the starting ionogels was of 73% for CsE and 82% for CsB. This behavior is similar to that reported for aerogels obtained from chitosan and κ-carrageenan . Such volume reduction has been associated with the rearrangements of chitosan chains due to their lower affinity for acetone. This molecular movement does not cease until all of the acetone has been removed, even when the supercritical CO2 drying reduce the effects of surface tension in the material .
3.1. Characterization of Chitosan Aerogels
The infrared spectra (2000–500 cm−1) of chitosan and the obtained aerogels are shown in Figure 2. The main characteristic bands of chitosan are also observed in the aerogels spectra. At 1652 cm−1 is observed the stretching vibration band of the C=O bond that is associated to the amide I; the amide II –NH2 deformation is related to the band at 1580 cm−1; the band at 1424 cm−1 is associated to the CH2 bending; at 1380 cm−1, the symmetrical vibration deformation of the CH3 group is observed; the band at 1318 cm−1 is associated to the amide III; the antisymmetric tension mode of the COC bridge is observed at 1150 cm−1; finally, the fingerprint zone, between 1075 and 1026 cm−1, is characteristic of the polysaccharides [26,27]. There is no evident modification on the spectra that indicate chemical changes in the chitosan as result of the gel formation or the drying process. Furthermore, there are not absorbance bands that could be related to the presence of residual IL in the aerogels.
The N2 adsorption and desorption isotherms of both types of aerogels are classified as type IV according to the IUPAC conventions (Figure 3). The observed hysteresis of N2 desorption at high relative pressures is indicative of a mesoporous dry material . The specific surface area (SBET) and pore volume that are calculated from the adsorption and desorption isotherms are included in Table 2. The specific surface areas of the aerogels are in the higher rank when compared with other pure polysaccharide aerogels [24,29,30]. The pore size obtained was within the range of mesopores, which is characteristic of aerogels.
The scanning electron microscopy (SEM) images of the aerogels are shown in Figure 4 and Figure 5. Both of the aerogels appear as uniform materials with some imperfections, which could be caused by fracture events. At the highest magnification available (×10,000), the internal structure of the aerogels looks as aggregated clumps forming a compact network with heterogeneous pores. The appearance of the pores is consistent with the mesoporous characteristics of the aerogels. The main difference between both types of aerogels is that the internal structure of CsB appears to be denser. The internal structure of the aerogels produced from ionogels is different to previously reported chitosan aerogels [24,29]. These differences could be related to the solvent-polymer interaction. Electrostatic repulsions dominate in the aqueous acid chitosan solutions. In such conditions, the polysaccharide molecules adopt an extended hydrodynamic volume conformation . Conversely, these repulsive forces are absent in Cs-IL solutions; thus, the chitosan molecules have relatively smaller dimensions, generating more compact structures in their aerogels.
3.2. Diffusion Properties of Aerogels
The physical characteristics of aerogels, such as their large surface area and mesoporosity, endow these materials with a large capacity to adsorb certain compounds. The diffusion properties are key to evaluating the performance of these materials for important applications in pharmacy and biotechnology, among others. For this purpose, dried aerogels that are obtained from chitosan solutions in EMIMAc and BMIMAc, as described previously, were swollen in water at different temperatures between 20 and 40 °C and their kinetics was followed. In Figure 6 and Figure 7, the experimental data is presented.
The CsE aerogels absorbed between three and four times their weight and exhibited a decreasing swelling capacity with an increasing temperature. In contrast, the CsB aerogels showed a greater capacity of absorption (between five and six times their weight), and the effect of temperature on the swelling capacity seems to be less marked. When compared with the aerogels of chitosan that was obtained from aqueous acid solutions, the aerogels from chitosan in IL showed higher W∞. Previous studies with chitosan or chitosan-polyelectrolyte complex matrices that were prepared from aqueous media have shown a similar tendency to decrease the swelling capacity with temperature [20,31].
It is time to learn how to write an analytical essay. This type of academic papers is not the easiest one to deal with. You can analyze book, movie, poem or some certain moment in history. The whole world is there to be analyzed. This paper is about critical and objective observation and detailed description of an object. But enough words, let’s learn what is an analytical essay.
What is an Analytical Essay
To learn how to write an analysis essay let’s start with the term. If it sounds unfamiliar to you, just stop worrying. You have met it before in the newspapers and other sources before without realizing that the format is very similar.
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This piece of writing should be informative and well-thought. This is an observation of some certain object, topic or even idea. You should divide the main thing into pieces and analyze all aspects of it according to your topic. It can be related to the historic event, literature, a piece of art or process like Water Cycle or Cycle of Sleep.
How to Write an Analytical Essay on a Poem
One of the toughest analytical essay topics is one that relates to poems. Not even the most professional writers are aware of how to write an analytical essay on a poem. Fortunately, our experts are great and have an amazing level of experience in this field. Like in a persuasive essay, you need to analyze, research, and collect data.
This topic requires from you to research the content of a certain poem. You need to explore its structure and style in an explanatory manner. And you aim to make your reader understand how great and significant the poem you are analyzing is. You may take any popular poem and make it your own topic. So, what is an analytical essay on a poem? Yes, it is an analysis of its content on various parameters. Here are few the most significant poems ever written:
- “The New Colossus” by Emma Lazarus
- “Ode on a Grecian Urn” by John Keats
- “The Tiger” by William Blake
- “A Psalm of Life” by Henry Wadsworth Longfellow
- “Holy Sonnet 10: Death, Be Not Proud” by John Donne
Analytical Essay Example
Brainstorm Analytical Essay Topics
Here is how you should start your analysis essay. The main thing to start with is to get yourself a nice and winning topic. You need to figure out what you are going to write about. It may sound obvious. This step is crucial if you want to get a better grade and succeed.
The thing is that you have only two ways of how your analysis essay will get a topic. You may get it as a prompt by your teacher, or it can be just a free topic to deal with. Both ways have their strong and weak sides. Your task is to open the topic no matter how did you get it. It is better if the topic is familiar to you or you are passionate about it. Here are few interesting topics suggestions on various subjects.
Topics to suggest
- Sports and drug use. Analyze the reason athletes use drugs.
- Graffiti. Why is it considered art?
- Multiculturalism in the modern society. Explore its significance.
- Analyze body size and fashion. Is it important for models to be the slim or free size?
- Gender roles. Why are gender roles changing?
- Families with one parent. Is there any effect on kids?
- Differences between men and women friendship and relationships.
- Career or family life. Is it possible to have a balance?
- Child-free families. Is it selfish for a couple to have no children?
- Music and health. Is there any effect of music on human health?
- Analyze the main character of Romeo and Juliet.
- Describe the main event in the book “Lord of Rings”.
- Write about cultural impact the book has on the modern society.
- Tell if there is any influence of author’s background on his work.
- Write an analysis of the style poets of XX century used in their works.
- Do horror movies and comics influence human brain?
- Tell why it is important to have limitations for children on TV and in movies.
- Describe true events a movie is based on.
- Tell more about the novel your favorite movie is based on.
- Describe a perfect Christmas movie.
Create an Analytical Essay Thesis
There are two main things you need to learn about analytical essay thesis.
- The main goal of analysis essay is to prove your own point of view as a writer. You need to do a research and link the information together to make a certain statement. Your decision is the thing you need to present your reader through the analytical way. And this decision is called thesis. You should not get any personal features like in a reflective essay. If you are talking about some certain fact in history, you need to get main reasons that caused to that event.
- Your thesis is not just a simple question with a one-word answer. It is a complex sentence and statement that requires certain reaction and similar complex answer. The number of factors in that statement can be huge. It can be very controversial. It should hit the target.
Create an Analytical Essay Outline
One of the most interesting and tough tasks is to create the structure of your future analysis essay. It is called an outline. This step is very important and will help you to deal with the entire paper properly. It doesn’t matter how many paragraphs you have in your paper. Analytical essay outline mainly deals with the topic and thesis statement.
Analytical Essay Introduction
If one wants to learn how to write an analytical essay introduction, he needs to get some background info for his text. It should have a hook to get your reader interested. Or you can use any other way. Just make your Introduction effective. The information you present in your intro should be relevant to your thesis statement. Here is a default Introduction structure for analysis essay:
- Brief introduction or a hook to engage your reader
- Brief background information about the topic
- Slight transition (Use just one sentence)
- Thesis statement
The Body of your analysis essay is the biggest and the main part. Its goal is to prove your thesis statement you’ve presented in the Introduction. Divide your thesis into certain parts and write an individual paragraph for each part. If you are talking about human evolution, you should devise your thing into stages and create a paragraph for each one. You may start with the most ancient period and write about each significant change in human evolution. Here is the proper structure:
- Introduce your main topic with just one or two sentences. It should up to the point and effective.
- Describe your main evidence and tell how it supports your thesis statement. Here you should go just with one or two sentences. Some authors consider this paragraph to be the most important of the entire paper. It will give your reader the feel of how do you understand the topic. These sentences should be creative and effective in the most interesting way.
- Any essay of this type needs supporting evidence. In this part, you will demonstrate to your reader how well you did your research. The entire paper is based on the evidence that you’ve found. Make a nice transition between this part and the previous one.
- It is time to write some conclusion before the main conclusion part. After any strong evidence, you need to get the job done by adding a concluding sentence and describe the significance of the entire topic.
Your Body should be clear and effective. With just a few sentences you need to show the reader your point of view and explain the thing he might never hear before. And if the person can’t understand your point of view, it means you made some mistakes in your text.
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How to Write a Conclusion for an Analytical Essay
And the last part of our guide is on how to write a Conclusion for an analytical essay. One needs to understand that Conclusion has a role that is very important to the entire effect your paper has on your reader. This is your last try to make an impression or improve it after the entire paper was read.
You need to restate your thesis statement and make a summary of your supporting evidence. And the last shot will be the ending concluding the sentence. Make a sound like a statement. You may tell your reader what you have learned from your own paper. Here is a default structure:
- Thesis statement rephrasing
- Summarize main evidence
- Main concluding statement
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