The selection of solvent for preparing a working electrode (and to act as the electrolyte) is known to influence the efficiency of dye-sensitized solar cells. In this topical review, results taken from a systematic study are presented from the authors’ own lab examining how protic and aprotic solvents, as well as solvent polarity, affect adsorption of carboxylic dyes on the titanium dioxide nanoparticle surface and electron injection from the dye to the semiconductor. Adsorption of dye molecules on nanoparticle surfaces is measured through second harmonic light scattering and electron injection through ultrafast transient mid-infrared absorption. It is revealed that protic solvents do not allow direct adsorption of the dye onto the semiconductor surface, due to hydrogen bonding with the dye and competitive binding to the semiconductor surface. Aprotic solvents, on the other hand, support solvation of the dye molecules but also facilitate dye adsorption on the semiconductor nanoparticle. Among aprotic solvents, it is found that solvents with higher polarity result in larger adsorption free energy for the dye and faster electron injection. Overall, these studies reveal that aprotic solvents with high solvent polarity (such as acetonitrile) yield more efficient solar cell devices.

    The world of dyes and pigments is vast and there are innumerable varieties of these colorants to fulfill the requirements of varied industrial and commercial sectors. Acid dyes, basic dyes, solvent dyes, lake colors, pigment colors are just to name a few from the vast ocean of colors. This article will talk in brief about the solvent dye.

    Solvent dye is a dye that is soluble in plastics or organic solvents. When it goes with an organic solvent the dyeing process occurs in a solution. As the molecules of solvent dyes have a very small polarity or none at all there is no ionization involved in the dyeing process as it does, say, with acid dyes. Solvent dyes are normally water insoluble. One commonly used organic solvent with solvent dyes that is non-polar is petrol.

    As for the naming of solvent dyes a standardized pattern is followed. In the pattern, the first word is always ‘solvent’ which is followed by the dye color and then a distinguishing number. For example, the varied shades of red are segregated by the distinct number that comes after the shade name like ‘Solvent Red 49’, ‘Solvent Red 1’, and ‘Solvent Red 24’ and so on.  Another example of the shade red occurring in another type of dye is Pigment Red 48 which is an azo derivative from naphthalene.

    Solvent dyes are pretty versatile and have found their way into a number of applications. One of their common uses is in the automotive sector to impart color to petrol fuel and other lubricants. Varied hydrocarbon based non-polar materials such as waxes and candles, coatings and wood stains are colored with the aid of solvent dyes. In the printing industry they go towards marking inkjet inks, inks and glass coloration. Textile printing is followed by the media industry where the solvent dyes are used for magazines and newspapers.

    Dyeing of plastics is another application which uses solvent dyes because of its chemical compatibility. In the plastics industry these dyes lend color to a number of solid materials like nylon, acetates, polyester, PVC, acrylics, PETP, PMMA, styrene monomers, polystyrene and other fiber. They are also increasingly being used for smoke signaling in the pyrotechnics industry. A mention has to be made of its application in scientific research and medical diagnostics. Here, the solvent dye is used as an important component to produce stains that help in identification of varied components in a cell structure.

    There are several advantages offered by solvent dyes that have led to its wide use in varied applications. Color shade consistency, superior light fastness, resistance to migration, good thermal stability, extremely dissolvable in plastics and lack of precipitation even after extensive storage are just to name some of its superior attributes.

    However, sourcing solvent dyes from reputed solvent dyes manufacturers is highly important. This guarantees you the quality of the product and its effectiveness in the application it shall be used for. There are several reputed manufactures of these dyes and the names can be easily obtained from the online yellow pages.

    At the heart of every drop of paint, every thread of cloth, every bit of your brightly colored phone case is a pigment. Pigments are the compounds added to materials to give them color. This deceptively simple application has shaped our perception of the world via art, fashion, and even computer displays and medicine. Pigments are used in paints, inks, plastic applications, fabrics, cosmetics, and food.

    Some of the earliest chemistry was to make and isolate pigments for paints, and pigment conservation is a focus for many modern researchers who identify and preserve artwork.

    Get to know pigments

    But what is a organic pigment, exactly? Pigments are brightly colored, insoluble powders (brightly colored liquids are called dyes). In most cases, the bright color is a result of the material absorbing light in the visible spectrum. In inorganic pigments, this absorption is the result of charge transfer between a metal (transition metals are really good at this); organic pigments tend to have conjugated double bonds that absorb visible wavelengths.

    Pigments are mixed with binders to attach them to a substrate. The resulting suspension—a paint—is used to coat materials and impart color onto them. In industry, there are three pigment classes: absorption pigments (used in watercolor paints), metal effect pigments (used to create surface luster), and pearlescent pigments.

    Pigments are found in nature, such as ochre (a blend of iron oxides and hydroxides) and indigo (C16H10N2O2). They can also be synthetic pigments such as mauve (an aniline derivative) or white lead. White lead, one of the earliest synthetic pigments, is made by treating sheets of lead with vinegar. They are often more robust than dyes, which dissolve in the material they are coloring. Pigments can keep their color for many centuries and withstand high heat, intense light, and exposure to weather or chemical agents.  

    Cataloging colors

    Because of their prominence in art, pigments have an important place in history. The Forbes Pigment Collection, housed in the Straus Center for Conservation and Technical Studies at the Harvard Art Museums, catalogs and preserves more than 2,500 pigments. Its founder, Edward Forbes, started the collection by gathering pigment samples from his travels all over the world, including colors like mummy brown, made from ground-up mummies, and carmine red (C22H15AlCaO13), obtained from cochineal insects.

    The Forbes Pigment Collection is often used as a reference library to standardize colors and identify pigment samples from artworks, which can confirm or disprove the piece’s origins. For instance, in 2007, a painting supposedly by Jackson Pollock was discovered to be a forgery when chemical analysis revealed the presence of pigments that weren’t available until decades after his death (Custer, 2007).

    Furthermore, many artists had personal preferences and favored certain pigments over others. Thus, knowing which pigments were used, and whether they were in character for the artist or not, can help art historians determine an art piece’s authenticity.

    The Forbes Pigment Collection, which boasts more than 60 natural samples, also highlights one of the challenges with natural pigments. Natural pigments were gathered from nature, for example, ore deposits, minerals, and flowers. But tiny shifts in chemical composition or particle growth cause specific shades to vary significantly due to impurities present in the sample.

    Analyzing and understanding the high performance pigment used in paintings is also vital to artwork restoration and preservation. Many pigments chemically, like coating and paints, react with ambient light and humidity, as well as harsher substances like soot and smoke from cigars or fireplaces. Pigments may oxidize, dissolve in acid or water, undergo phase transitions, react with the binders in the paint, or degrade.

    For example, eosin Y was a pigment historically favored by many artists, most notably Vincent van Gogh. Initially a vibrant red, exposure to light gradually turns eosin white as UV radiation excites the pigment molecules and leads to the production of OH radicals. This breaks down the structure of the pigment, and eventually turns it white. Knowing such information allows art historians to better conserve art.

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