The ability of getting wet

Rodolfo Marin Rivera

1Have you ever wondered why when the rain stops, water drops remaining on the surface of the leaf do not flow or roll? Well that’s because of the surface tension of the water and the hydrophobicity of the leaf… but wait a minute! What’s surface tension and what’s hydrophobicity?

Both terms can be explained by considering Young’s principle about thermodynamics of wetting. Young described the ability of a drop to spread or not on a surface as the wettability of the surface, and he correlated this phenomenon by measuring the contact angle formed between the liquid, the solid and the gas around these two phases, as it is presented in this diagram:

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If a drop of a liquid is put it on a surface (as the leaf shown in the picture) there are two possibilities:

  1. The liquid spreads on the surface completely, so that the red arrow squash the angle θ formed between the green and the red line (see the drop of liquid at the left), which means the contact angle θ becomes equal to zero, or
  2. A “contact angle” is formed between the liquid, the solid and the gas phase, so that these three phases are in equilibrium and the contact angle θ tends to 180°, as  described by the drop of liquid in the right.

In other words, one can assume that the drop will wet the surface as long as the contact angle θ tends to zero, as the handsome drop of liquid in the left. Therefore, if the drop of water remains on the surface without spreading, then it is said that the surface is hydrophobic (no wet), but if the drop spread into the surface then the surface is called hydrophilic. It may occur that a liquid can be partially adsorbed on a specific surface because the wettability of a solid surface also depends on the surface tension between the liquid, solid and gas phases.
On the diagram on the left, the surface (or interface) tension is defined by the intermolecular forces that attract the liquid particles together. Along the surface, the 3particles are pulled toward the rest of the liquid. Surface tension (denoted with red arrows and described by the greek letter gamma, γ) is defined as the ratio of the surface force F to the length d along which the force acts:

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The term “surface tension” is used when a liquid in contact with the gas phase acts like a thin-elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air around the leaf, or the foam formed when you’re taking a very nice bath), but If the surface is between two liquids (such as water and oil), then the term “interface tension” is used.

Young correlated the contact angle with the surface tension between the different phases with this very simple equation

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where γlg represents the surface tension between the liquid and the gas phase, γsl represents the surface tension between the solid and the liquid phase and, γsg describes the surface tension between the solid and the gas phase. The location of these surface/interface tension are shown in the picture above with the two drop of liquids.

Therefore, wettability of a surface is controlled by the first term of the equation, which is known as the “adhesion tension”. If θ becomes equal to 0°, and, for instance, the left term in Young’s equation becomes equal to γlg, which means that the liquid wets the solid surface perfectly. But, if the adhesion tension becomes smaller than γlg, it means that cos θ < 1 , and therefore θ > 0°, which means partial wetting of the solid.

For more info about wettability you can check the following links:
https://www.sciencedirect.com/science/article/abs/pii/S0301751617300510

https://www.spec2000.net/09-wettability.htm

http://web.mit.edu/nnf/education/wettability/wetting.html

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What does make a radioactive element radioactive?

Rodolfo Marin Rivera

1The periodic table as we know contains 118 elements,

which are organised according to their atomic number, electron configuration, and chemical properties thanks to discovery done be Dmitri Mendeleev (1869). However, all of them may have some level of radioactivity.

As we know, atoms are made up of small, positively charged nucleus surrounded by a relatively large space (orbitals) occupied by tiny, fast-moving, negatively charged electrons, which are 10,000 to 100,000 times smaller than the atoms. Despite its relatively small size, the nucleus contains over 99.9% of the mass of the atom. A very close approximation about what is going on during the interaction between the electrons and nucleus was given by Bohr (1913) who depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus with attraction provided by electrostatic forces.

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The nucleus is made of positively charged protons and neutral neutrons. The number of protons is equal to the atom atomic number and determines, for instance, what element it is. For example, an atom with 8 protons in its nucleus is an oxygen atom, the eight-element listed in the periodic table. The number of neutrons will simple affect the mass of the atom and determine which isotope of oxygen is. Therefore, every chemical element has one or more isotopes that, for a given element, the number of protons and neutrons may change or not. Isotopes are better known as nuclide. Nuclide refers to any isotope of any element, or in other words, a nucleus with any number of protons and neutrons. Therefore, the radioactivity of a certain element is given by the transformation of a certain nuclide, or by the intensity at which the nuclide may decay. This means that a radioactive nuclide is one that spontaneously undergoes nuclear decay.

3During nuclear reactions (as the big bang), the interaction between a particle (like a neutron) or a photon with a nucleus leads to the formation of another nuclide. This transformation is known as nuclear decay. A nuclear decay stars with an unstable nuclide (parent isotope) that typically spits out a particle and/or a photon while turning itself into something more stable. Therefore, energy is released as part of all nuclear decay. There are three different kinds of decay: alpha (i.e. ejection of neutrons and protons at once, formation of the helium atom), beta (ejection of electrons) and gamma (no particles are emitted or absorbed, and both the numbers of protons and neutrons remains the same). In this last type of decay, at the beginning, neutrons and protons are arranged poorly, in terms of energy. That is to say one or more neutrons/protons occupy a higher energy state than they need to, and that the nucleus is said to be in an excited state. Rearrangement allows the neutrons/protons that are in unusually high-energy states to drop down into lower ones. The lowest energy configuration would be called the ground state. Rearrangement from a higher energy state to a lower one requires the release of energy, which can be done in the form of a high-energy photon, or gamma ray.

Nowadays, about 20 radioactive nuclides are recognised, which will take a very long time to decay. One of the most notable is 40K. Since all isotopes of an element have the same chemical behaviour, and because we humans need potassium in our bodies, we all have some 40K in our bodies. Anything that contains potassium (bananas, Gatorade®, your friends) is naturally radioactive, i.e. of natural radioactive origin (also known as Naturally-Occurring Radioactive Materials, NORM).

All but three of these 20 long-lived radioactive nuclides decay to stable nuclides. The three that don’t are 232Th, 235U and 238U. They all eventually decay to stable isotopes of lead, but along the way, they produce a number of other naturally occurring radioactive nuclides (e.g., 222Rn).

4You must know that from all the ionizing radiation that exist around us, not all of them is related to our activities as human being, because radionuclides exist everywhere in nature (in soil and rocks, air and water, in plants and in our bodies). Some of the isotopes have been there since the creation of Earth, others are being constantly produced or brought in from outer space as cosmogenic radiation.

Like this topic? You can  find more information in the following links:

 

 

3D-Printing human tissues using cellulose: will science fiction become reality?

Carmen Piras

imagesMost of you have probably heard about the Frankenstein’s monster, a novel written by the English author Mary Shelley. For those who don’t know it, it is the story of Victor Frankenstein, a scientist who invented a secret technique to impart life to unanimated matter. This technique allowed him to create a monstrous creature with human emotions and sensations.

This is of course only science fiction. However, nowadays, the on-demand growth of human tissues and organs using artificial instruments is becoming reality. One emerging manufacturing technique with promise in tissue engineering and regenerative medicine is 3D printing, which is based on the 3D deposition of material in specific shapes. This technology can be applied in biology (3D bioprinting) to produce 3D scaffolds of our tissues. The cells are directly embedded inside the scaffolds and different cell types can be distributed in different locations. The material loaded with the cells is then 3D printed into the desired shape. This method allows the fabrication of bone and cartilage tissues, skin and cardiac constructs and the regeneration of hepatic tissue.

One of the most important factors in 3D bioprinting is the choice of the material used as ink (or, more specifically, bioink), which should be biocompatible and allow cell survival during the printing process. Beside this, the ink should have an optimal viscosity to allow printing and to avoid the 3D printed shape to collapse. Water based gels (also called hydrogels) are ideal inks for 3D bioprinting. These materials are mainly composed of water (> 99 %) and therefore can closely mimic the natural environment of cells.

Hydrogels can be obtained from a wide range of molecules including natural derivatives such as gelatine, alginate, collagen, hyaluronic acid and cellulose. Being an abundant, renewable, low-cost resource, cellulose represents an ideal candidate for the production of hydrogels for 3D bioprinting. This molecule is formed of long glucose chains and it is obtained by extraction from plants or can be produced by bacteria. Mechanical and chemical treatments of raw materials allow the extraction of cellulose in the form of nanofibers or nanocrystals. The suffix nano- refers to the fact that the extracts have one dimension (length or width) in the nanometre range (1.000.000 smaller than 1 mm).

UnknownA variety of bioinks based on cellulose nanofibers and nanocrystals has been created by a number of research groups. These could be applied for the regeneration of human tissues (such as cartilage) or to obtain 3D printed drug delivery systems and wound dressings. Although very promising, this research field is still new and growing. We hope that in the future cellulose will be further exploited to develop new bioinks. Will this be the route towards a new Frankenstein’s monster? Stay tuned!

Want to know more about this? Read this article: http://pubs.rsc.org/en/content/articlepdf/2017/bm/c7bm00510e

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Challenges in tissue engineering: are hydrogels the answer?

Carmen Piras

Gels are materials with several applications in everyday life. They can be defined as coherent systems composed of a liquid as the majority component, and a solid as the minority component. In these systems, the liquid is the dispersion medium, with the solid acting as the dispersed constituent. These materials can be classified in different ways depending upon their origin, composition, constitution and the kind of interactions that are responsible for the formation of the gel. One of the main classifications is based on the medium they encompass and, in this case, further subdivisions can be made, into organogels, hydrogels, xerogels and aerogels.

  • In organogels, organic solvents represent the disphydrogel-wound-care-fillerersion medium.
  • In hydrogels, water is the dispersion medium, making them highly absorbent and flexible materials. Due to these amongst many other properties, they can be used for several applications, such as drug delivery, tissue engineering, regenerative medicine, diagnostics and clinical practice.
  • Xerogels are formed after evaporation by drying with unhindered shrinkage of the solvent in a hydrogel or an organogel. The main properties of these gels are high porosity and enormous surface area. These characteristics make them suitable for various optical, acoustic and thermal applications.
  • Aerogels are highly porous, low-density materials with excellent insulation properties, formed by evaporation of the solvent in supercritical conditions. They are utilised for several applications in the field of cosmetics and insulation.

PROPERTIES AND APPLICATIONS OF HYDROGELS

id35162Despite being around for only a few decades, hydrogels are now a class of versatile materials that have become a sine qua non in everyday life e.g. hair gel, nappies, contact lenses, wound healing products and medicines. They can be defined as three-dimensional, cross-linked networks of water-soluble molecules. These aggregates can assume different morphologies (micelles, vesicles, fibres, ribbons or sheets), depending on their molecular structure. Due to their composition, but also due to the conditions under which the hydrogels are formed, they present lots of interesting properties. One of these is the ability to absorb water, which is caused by the presence of hydrophilic groups within the molecules that constitute the gel. The presence of water inside the hydrogels, together with their swelling and elasticity properties, permits some solute molecules entrapped in the structure to diffuse, which is very important for the applications of these materials in drug delivery. Another important characteristic related to this application is their porosity that can be modified by controlling the density of cross-links in the gel matrix. The porosity of these materials is responsible for the release of drugs loaded into the matrix. Hydrogels can also show massive changes of their volume in response to the variation of external conditions, such as pH, temperature, solvent quality, etc. Furthermore, these materials show physicochemical similarity with the native extracellular matrix. This characteristic, accompanied by the high water content, makes them considerably biocompatible and permits their utilization in tissue engineering and in biomedical and pharmaceutical fields. Hydrogels derived by natural molecules, such as polypeptides and carbohydrates, also show natural biodegradability, high biocompatibility and low toxicity that make these materials especially suitable for this kind of application.

HYDROGELS IN TISSUE ENGINEERING

Tissue engineering has been of great interest to scientists for many years, as it holds great promise for tissue and organ regeneration. The high amount of water that hydrogels can incorporate, the presence of pores large enough to host living cells, and the similarity of these materials to the extracellular matrix, make them suitable candidates as matrices for tissue engineering. Many publications over the decades showed how they can be used for cartilage regeneration, bone and skin repair, and recently also for cardiac tissue repair (especially as injectable materials).

cuoreHydrogels used for such purposes are predominately based on natural polymers or derivatives, such as collagen, hyaluronic acid, chitosan and alginate, which allow high viability and proliferation rates for cells, due to the abundance of chemical signals. This also helps the formation of neo-tissues, which is fundamental for organ regeneration and repair.

Although hydrogels based on natural polymers show interesting properties and a wide range of biomedical applications, synthetic chemistry can also be used to produce similar materials from synthetic polymers such as poly(ethylene glycol) and pluronics. Hydrogels obtained from these polymers, however, behave as inert scaffolds for cells, due to the absence of active binding sites. To overcome this problem, peptides, growth factors and other bioactive compounds can be added to the network.

The use of hydrogels in tissue engineering has been facilitated by bio-fabrication techniques, which enable the industrial production of biomaterials. Different methods can be used for this purpose:

  1. Laser-induced forward transfer technology, which allows the precise deposition of materials and cells in small 3D structures.
  2. Inkjet printing, which permits the deposition of bioink (i.e. a hydrogel) on a substrate from a very small orifice.
  3. Robotic systems, which are dispensing systems that use disposable plastic syringes to release hydrogels with suspended cells onto a building platform.

Although hydrogels have now been studied for a number of years and their characteristics and applicability have been massively improved and tuned to many different purposes, the development of effective, low-price, non-toxic materials with high rates of water absorption, biocompatibility and biodegradability for tissue engineering still remains a challenge. However, research on hydrogels is one of the most rapidly developing fields of material science, showing their enormous potential in tissue engineering and organ regeneration.

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Chocolate, why it is so irresistible? The chemistry behind pleasure and addiction

Carmen Piras

Chocolate is one of the most cheap and popular antidepressants available in the market. What is in chocolate and why is it so good and irresistible? After you will read this articleyou will realise it is all about chemistry! All the properties of chocolate, in fact, depend on different molecules that are predominantly present in cocoa, one of the main components of chocolate.

HISTORY

Cocoa comes from the seeds of a plant with very ancient origins. The ancient civilisations of Olmecs, Mayas and Aztecs used to 1cultivate it as a food and as currency trading. The last led to the first name of the plant “Amygdalae Pecuniariae” (“Money Almond”), later replaced by the Swedish botanist Carl Von Linne with “Theobroma Cacao” (“Food of Gods”), which referred to the religious cults of these populations.

The consumption of cocoa was reserved mainly to upper classes (nobles, warriors and priests) that used to drink it for its energising and aphrodisiac properties with the addition of chilli, hot spices, anise, cinnamon and vanilla to mitigate the bitter taste. In 1528 it was imported to Europe, when the Spanish Conquistador Cortez offered it as a gift to the Spanish Royal Family. From Spain, the recipe of chocolate spread throughout Europe: Italy first, then France, England, Austria, Switzerland, Germany and then Scandinavia.

THE PLANT

Cocoa plant (Theobroma Cacao L.) is an evergreen tree that can reach up to 20 meters height. It can be cultivated only in certain areas, as it requires hot humid climates and mild temp2eratures ideally between 20 and 30°C. The main producing countries are, for this reason, situated in Africa (Ghana, Cameroon, Nigeria, Ivory Coast, Madagascar), Asia (Indonesia, Malaysia, Sri Lanka), Oceania (New Guinea, Papua) and Central and South America (Mexico, Brazil, Colombia, Ecuador, Venezuela).

3The fruits are collected a couple of times a year and the beans are extracted after natural fermentation. The fermentation takes place outdoors, in a 2 to 12 days period of time depending on weather conditions and quality of the beans. This process regards the sugars contained in the mucilage (glucose and fructose) and it is vital for the chemical composition and the biochemical characteristics. Once extracted, the seeds are then dried in order to stop the fermentation and decrease the moisture content to avoid the formation of mould. The drying process takes from 7 to 15 days when the seeds are exposed to the sun. The dry beans can be then processed to produce a variety of derivatives (e.g. chocolate, cocoa powder, pralines, snacks).

CHEMICAL COMPOSITION OF COCOA

The chemical composition of cocoa, thus the percentages of main components, varies depending on the geographical origin. The main components are:

  • water (5-7%)
  • lipids (45-53% – especially fatty acids such as palmitic acid, arachidic acid, linoleic acid, linolenic acid, stearic acid and sterols)
  • proteins (10-15% – g. albumin, prolamin, glutelin, globulin and enzymes)
  • starch and carbohydrates (2-4% – i.e. fructose, sucrose, lactose, cellulose, lignin)
  • tannin (6%)
  • gums (2-3%)
  • polyphenols (g. catechins, anthocyanins, proantocyanins)
  • organic acids
  • vitamins (vitamin A, B1, B2, B6, biotin, folic acid, nicotinamide)
  • minerals (e. sodium, potassium, magnesium, iron, chlorine, fluorine, iodine, chromium, nickel, zinc)
  • natural bioactive molecules.

The last are responsible not only for the aroma of cocoa, but also for the effects it induces. The characteristic aroma of cocoa can be attributed to different fermentation products (firstly vicilin, but also pyrazines, esters, aldehydes, ketones, alcohols, hydrocarbons, furans and phenols), whereas other natural components cause the effects that are normally associated with cocoa consumption. These include:

  • Theobromine and caffeine (methylxantines)

4These two molecules are present in different percentage (2-2.7% theobromine and 0.6-0.8% caffeine) and they induce:

  • increased concentration and attention
  • improved waking state
  • improved skeletal muscle contractility
  • vasodilatation
  • diuresis.

Theobromine has a lower stimulant effect compared to caffeine, which however is present at a lower concentration. Both molecules act as antagonists of adenosine purinergic P1 receptors. Adenosine has normally an inhibitory effect on central nervous system; consequently the antagonist action is responsible for the psychostimulant effects of these substances, but also for the side effects related to abuse, such as insomnia, tachycardia and agitation.

  • Biogenic amines (tyramine, tryptamine, histamine, 2-phenylethylamine)

5These are vasoactive molecules that, at a high concentration or in combination with MAO inhibitors (drugs with an antidepressant effect), may cause the so-called “cheese reaction”, which is characterised by redness, headache, pressure changes and even circulatory shock.

Among these molecules, 2-phenylethylamine is considered responsible for chocolate desire. Its structure is very similar to amphetamine, therefore it interacts with the same receptors, inducing various stimulant effects such as:

  • improved waking state
  • psychostimulation
  • reduction of fatigue
  • reduced sense of hunger.
  • Anandamide

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This molecule interacts with the cannabinoid CB1 receptors inducing:

  • behavioural effects
  • effects on cognitive functions
  • improved mood
  • improved sensory perceptions
  • euphoria
  • sense of accomplishment and satisfaction.
  • Salsolin and salsolinol

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These are alkaloids dopamine-derivatives, which are also present in the brain as endogenous molecules. They are responsible for the antidepressant effect of chocolate and the psychological dependence due to two actions on central nervous system:

  • inhibition of MAO enzymes and tyrosine hydroxylase, which results in increased serotonin levels with consequent improvement of mood
  • inhibition of catecholamines re-uptake (e. norepinephrine and dopamine), with consequent antidepressant effect and psychological dependence.
  • Clavamide, which has a considerable antioxidant activity.9
  • Tetratetrahydro-b-carboline, which act as neuromodulators on MAO enzymes, thus contributing to the antidepressant action.10

EFFECTS OF CHOCOLATE

 All these molecules together with others are responsible for the different effects of chocolate.

  1. Antidepressant effect12

Serotonin, associated with methylxanthines and 2-phenylethylamine, is responsible for the induction of endorphins production, opioids that are naturally synthesized by the brain and that have an exhilarating effect.

  1. Feeling of pleasure and addiction

13The feeling of pleasure induced by chocolate can be correlated to various substances, including theobromine, caffeine and anandamide, which act on the brain reward circuit. This is modulated by the neurotransmitter dopamine, whose release is induced by natural rewarding stimuli (e.g. food, and water), or drugs. It acts by facilitating the establishment of a positive memory related to cocoa assumption, with consequent sensation of pleasure after chocolate consumption and desire of more chocolate.

  1. Antioxidant effect

This is related to the presence of flavonoids, a class of polyphenols present at a higher concentration than in other commonly consumed foods. The antioxidant activity is important to fight free radicals, which are formed as a result of oxidative stress and are responsible for damage to cells and cellular components.

  1. Protective action on cardiovascular system

Due to the fact that dark chocolate promotes increased levels of HDL cholesterol, the “good cholesterol”. This improves the functioning of blood vessels (by increasing the ability of the endothelium to dilate) and prevents heart diseases.

Although chocolate has many positive effects, it is not for everyone! Are indeed excluded diabetics, people with allergies, gastroesophageal reflux (as chocolate consumption causes an increased gastric acidity), hyperuricemia, headache (due the presence of tyramine, that could trigger crises), obesity or liver diseases (due to the high lipid content).

Anyway, after reading this article, next time you will be in a bad mood, try to have some chocolate and it might act as a magic medicine!

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Chemistry on the crime scene

Carmen Piras

1One of my favourite TV series is Criminal Minds, an American police series set primarily at the FBI’s Behavioural Analysis Unit, Quantico (Virginia). It focuses on a skilled team of FBI profilers who catch serial killers through behavioural profiling. Besides Criminal Minds, other series recently captured the interest of the audience, such as CSI, Law and Order and Cold Case. In all these TV programs the police solve the cases quickly and efficiently, thanks to the talent of the agents but also thanks to the evidence that is collected on the crime scene and analysed by the chemists in a crime laboratory, who provide essential information to reconstruct the story. Although the role of forensic chemistry is fundamental in case solving, this is not often recognised. However, chemistry plays a key role to put together the pieces of the puzzle in all investigations.

WHAT DOES A FORENSIC CHEMIST DO?

In few words, a forensic chemist analyses physical evidence collected from the crime scene and provides the obtained information to the detective who is working on the case to solve the crime. The analysis of physical and chemical properties of substances is a crucial part of the process. The physical properties (e.g. appearance, texture, colour, solubility, polarity and odour) can be observed and measured without altering the 2composition of the matter. By contrast, the chemical properties may only be observed by performing a chemical reaction or change, characteristic of a particular substance. For example, cocaine is a drug that can be described as a fine white powder (physical property); it also reacts with cobalt thiocyanate to give a blue-coloured product (chemical property). These different properties of cocaine help the investigators to identify it.

Chemical and physical properties, however, can only be used for presumptive analyses, as sometimes these characteristics are not enough by themselves to identify a substance. Methamphetamine, for example, gives a deep blue-coloured product when it reacts with sodium nitroprusside in the presence of sodium bicarbonate. Nevertheless, other substances with a similar molecular structure will react in the same way, giving the same product. For this reason, confirmatory analyses, which are based upon unique properties of substances, are required to identify a sample with certainty.

THE TECHNIQUES

 Different techniques can be applied for this purpose and most of them are spectroscopic techniques. The ultraviolet-visible-near infrared spectroscopy, for instance, is ued to test certain drugs of abuse to confirm their property to absorb UV light at a certain wavelength. This property depends on the presence of chromophores in the structure of the compound, which are functional groups that can absorb UV light and are responsible for its colour. Another technique is represented by Fourier Transform infrared spectroscopy (FTIR), which gives a characteristic spectrum for each compound depending on which functional groups are present in the molecule structure. The analysis of the obtained spectrum depends on the recognition of these functional groups in the molecule, thus allowing its identification. These techniques are often used in combination with others, especially if the analysed sample is in fact a mixture of two or more compounds. In this case, the compounds in the mixture can be separated and individually analysed using a gas chromatograph-mass spectrometer (GC-MS). This instrument allows the separation of the different components in the mixture and their recognition by breaking them into fragments whose mass can be measured. The generated fragments are exclusive foreach compound, enabling specific identification.

Another technique used to separate different compounds in a mixture is thin layer chromatography (TLC), which can be applied to separate drugs, fibre dyes, poisons and inks. This method exploits the different polarity of the compounds in th3e mixture, permitting their separation by the different interaction with a solvent system (mobile phase) and a flat glass plate (stationary phase). Polar compounds tend to interact more with the stationary phase, therefore they “run less” on the TLC plate; whereas less polar compounds interact more with the mobile phase, therefore they “run more” on the plate and can be separated from compounds that have a different polarity.

FORENSIC ANALYSES 

All the above techniques are applied to recognize and identify specific substances. However, the work of a forensic chemist also involves other types of analysis. The colour tests, for example, are utilized to probe drug samples for their chemical properties. These reactions are carried out on ceramic or plastic dishes c4alled spot plates, which contain several wells. Each well is filled with a small amount of the questioned drug and a chemical known to produce a coloured product in the presence of the drug. This allows for presumptive identification of many drugs. Some drugs can also be recognized through microcrystalline tests, which analyse representative crystalline structures under the microscope with transmitted illumination. These tests, though, are more complicated than the colour tests as few forensic chemists are sufficiently trained to recognize the characteristic crystalline structures.

Different analyses can be performed to verify the properties of soil relating to a specific location where a crime was committed. This can help to confirm, for example, if the soil 5found under the shoes or on the car of a suspect is the same as that of the crime scene. Soil is a mixture of organic and inorganic materials whose properties can be analysed (pH, density, composition, colour, texture) through microscopic techniques, as well as sieves to determine the size distribution of a sample, and density gradients to evaluate the distribution of densities comprised in the soil mixture.

Other common items of evidence collected on a crime scene are fingerprints. All fingerprints are unique for each individual (even twins have different fingerprints!) and they can be very useful to identify a suspect, especially if their fingerprints had been previously recorded in databases collected by law enforcement agencies. Latent fingerprints cannot be visualized by eye but development techniques can be used to observe them. One of these is called powder dusting, which involves the use of powders (dark, light or fluorescent) to see contrast a7nd to highlight the fingerprints, which can then be lifted and preserved using a fingerprint tape. Another substance that can be used to colour fingerprints is called ninhydrin, which is used in biochemistry for qualitative and quantitative determination of a-amino acids. This substance gives a purple-coloured product upon reaction with the amino acids contained in fingerprint residues. Further techniques that can be used for fingerprint development include the silver nitrate reaction (reaction between silver nitrate and soluble sodium/potassium salts to give a white solid product called silver chloride, which produces silver and chlorine purple-black gas when exposed to ultraviolet light) an6d the iodine sublimation (based on the absorption of iodine vapours by the fingerprint’s components to give an amber-coloured product). Finally, if the fingerprints contain traces of blood, these can be observed by employing luminol, a derivative of phthalic acid, which allows the detection of blood traces through reaction with metal cations (e.g. the iron-shaped cation present in the heme group of haemoglobin) to give a blue luminescencent product called 3-aminophthalate.

CONCLUSIONS

 Many different techniques are used to analyse evidence, which are essential to solve a case. Therefore, although it is not well known, forensic chemistry gives a vital contribution to all investigations. Think about it next time you watch a police TV series…

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Soap:properties and production….So much chemistry in just a bubble

Carmen Piras 

Soap is a sine qua non in everyday life that has a very ancient tradition.

HISTORY

Itmount sapos use dates back to the Babylonians and the Egyptians, who used to combine animal and vegetable oils with alkaline salts to produce it. According to the roman legend, the word “soap” comes from Mount Sapo, where the animals were sacrificed. The fat from sacrificed animals together with wooden ashes were washed by the rain to the Tiber River, where people used to wash their clothes and found out that this mixture could help to clean the clothes. A similar word was also present in German language for a mixture of ash and fats that was utilized to dye the hair red. Gallic women have been the first to realize that this mixture could easily remove stains. This recipe didn’t change for centuries and had a great diffusion all over the world, however it evolved during the years, finally allowing the industrial production of soap from the mid-nineteenth century. Since then soap, which was initially considered a luxury, became widely available to everyone.

CHEMISTRY OF SOAP

From a chemical point of view, soaps are water-soluble sodium or potassium salts of fatty acids. They are obtained by mixing a strong base with fats and oils from animal or plant sources. When these are heated up together, they react in the so-called “saponification reaction”. In the past, this reaction was performed using fat and lye, which was obtained from leaching ashes through a purification process by filtration and heating. Nowadays, the principle is the same, but some variations have been introduced to make the process easier and quicker.reaction

In the saponification reaction fats and oils comes usually from animal or plant sources. These are used in the form of triglycerides, whose structure is composed of a glycerol molecule to which are linked three molecules of fatty acids. When these react with a base, the alkaline conditions hydrolyse the triglyceride allowing the formation of the corresponding salt (which constitutes the soap) and glycerin, which is reused for other purposes in industrial processes.

Each soap molecule is constituted by two moities:

fatty-acid-structure1An hydrophobic moiety or “tail”, which is a long hydrocarbon chain

An hydrophilic “head”, which is represented by the carboxylate group that forms the sodium or potassium salts and is negatively charged.

This hydrophilic part of the molecules interacts with water molecules through hydrogen bonding and ion-dipole interactions, whereas the hydrophobic parts attract dirt and do not interact with water molecules. However, this long hydrocarbon chains interact with each other by dispersion forces and form structures called micelles. In each micelle, the molecules are oriented in a way that allows the hydrophobic moieties of the molecules to hide from water, giving rise to a spherical structure. The hydrophilic heads of the molecules, instead, interact with water, but since they are negatively charged, they repel other micelles, which remain dispersed in water.

micelle

THE INDUSTRIAL PRODUCTION

The industrial manufacture of soap has a long tradition and it can be performed through discontinuous or continuous processes.

The discontinuous processes include different phases:

  1. Saponification, which is carried out in boilers heated with direct fire or steam, by stirring the fat mixture and the base that is gradually added. This operation takes about 3-4 hours keeping the temperature around 100°C. However, the same process can be carried out in few minutes under pressure and at higher temperatures (250°C).
  2. Salting: from the saponification a colloidal solution is obtained and it contains soap, glycerin and base residues. These two components can be separated from the lye simply by adding sodium chloride (kitchen salt). During this stage, also soap and glycerin are separated and the glycerin can be recovered and reused for other industrial purposes (cosmetic and pharmaceutical industries).
  3. Strong change: This process allows removing the fat that has not saponified by adding a strong caustic solution. In this way, all the fat is converted into soap. This step can be followed by a second salting treatment.
  4. Pitching: During this process, the soap is boiled again with water, to form two different layers

– “Neat soap”, which is formed by 70% soap and 30% water

– “Nigre”, which contains most of the impurities (salts and dirt) and most of the water

The neat soap is collected and cooled and finally treated with other substances such as emollients, colorants and perfumes.

The continuous processes are the most common applied production systems and also the most convenient, as the prices are consistently reduced and the obtained products have consistent characteristics.

soaps

MAKE YOUR OWN SOAP AT HOME

Soap can be easily made at home with some nice experiments that will help you to understand the chemistry and to create your own unique soap.

Some nice recipes are described in this website: http://candleandsoap.about.com/od/soaprecipes/tp/basicsoaprecipes.htm

Or you can simply follow this youtube video: https://www.youtube.com/watch?v=qGfXLznJJY0

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