In our previous blog post, we introduced concepts from organic chemistry, such as esters, commercial uses of esters, and the process of diffusion. Before we continue in our discussion about esters and fragrances, it is probably important to learn about how smell actually occurs.
How Smell Occurs (the Chem Way!)
Everything that we smell, whether it is food, smoke, wood, soap or shampoo, emits molecules that reach our nose. The molecules that are emitted are all volatile and relatively light. As we have learned, to be a volatile substance, it must be easily to evaporate. Objects that have no smell, such as NaCl, behave this way because they are non-volatile solids.
Behind the inside of one’s nose lie a set of neurons. Unlike many other sets of neurons in our body, they are regularly exposed to oxygen. Also, these neurons have cilia, or tiny hairs, that are special to the nose. the molecule emitted from our volatile substance attach to the cilia in our nostrils and trigger a sensation in the neurons of our nose. The neurons send a signal to our brain that translates it into a smell.
But, the inquisitive mind may ask, what molecules are emitted from our volatile objects that enter our nostrils? Many natural objects and plants emit molecules that are present when the object undergoes esterification. A banana, for instance, emits the ester isoamyl acetate (CH3COOC5H11.) Oranges produce Octyl Acetate (CH3COOC8H17) when they undergo the same process.
How Perfume Works
As you may or may not be aware, perfume is very dilute. Someone may think it is because the producer’s of the perfume are trying to minimize their costs, possibly to the detriment of the product, but this is not the case. In mixtures such as perfume, there are a variety of different alcohols in the same liquid. Alcohols all are very strong-scented; they emit many molecules that are received by the cilia in your nostrils. If it were a very concentrated substance, perfume would give off all the smells of the different alcohols within it simultaneously. Although they might be sweet on their own, the smells together would not be nearly as enjoyable, as we can no longer distinguish one from the other.
Perfumes contain different alcohols mostly due to the way they are supposed to work. There are three stages to perfume smells; top notes, heart notes, and base notes. Top note smells occur within 15 minutes of spraying the perfume on your skin. These chemicals are the first to evaporate, and thus the first molecules to be received by your nostrils. These scents are strange or exotic, and are made that way to interest consumers, yet not stay too long to be sick of them. Heart note smells occur 3-4 hours after application. These are generally the floral smells of a perfume. Base note smells occur 5-8 hours after application, and are usually musky or mossy. The relation to kinetics is the rate it takes for each individual scent to be activated. This can be represented using rate laws and reaction coordinate diagrams, to indicate exactly which scents are being activated at what time. The rate for the top notes will be the fastest, followed by the heart notes and base notes.
One must be careful with perfume, as light has enough energy to speed the decay of top note chemicals. Air also corrodes the fragrance through oxidation, and this occurs quicker after application. Also, top notes will evaporate faster on warmer skin that is less oily than on cold and oily skin. It is because of this that one must store their perfume in a dark, room temperature area to maximize its shelf life.
In the previous section, we mentioned how the duration of perfumes can help in classifying a perfume. Another way for classifying perfumes is by their smell. As we know, esters give the perfume its fragrance. A consultant in the fragrance industry, Michael Edwards, devised a qualitative way of describing fragrances for perfumes. This classification can be seen in the fragrance wheel, as shown in the picture below.
We know that perfumes can have scents, and that esters have the ability to cover up scents. But, how do we actually create a perfume? That question will be answered in the next section.
How Perfumes Are Made
First, all ingredients necessary for a perfume must be obtained. A perfume can have over 100 different ingredients, so this step is essential. This process may of extracting oils from plants, extracting scents from fatty substances of animals, or using synthetic fragrances developed by chemists. For example, the citrus tree blossom or the myrrh resins can be extracted and used in perfumes.
These ingredients are grouped into 4 categories: primary scents, modifiers, blenders, and fixatives. The primary scents are the most important scents required to give a perfume its scent. The modifiers are often esters, such as having a fruity ester and floral ester to make the scent fruity floral. Modifiers essentially replace one scent with one more geared towards the perfume scent, just like how an ester replaces an odor. The blenders and fixatives add some more scent to the primary scent to make it easier to transition between the three stages (top, heart, and base notes).
After all the scents are gathered, the scents are blended together to create the perfume. After the blending of ingredients, ethyl alcohol and water is added. The amount of alcohol added is based on what strength of the perfume is desired. The concentration of the perfume mixture is often mentioned in units of either volume percent or weight/volume percent. The perfume is then aged in tanks for several weeks and filtered before being put into bottles.
The fragrance industry is a large, growing industry in the world, and new formulas are constantly being developed for new fragrances. It is important to remember that alcohol is added to fragrances to give them scents that come in three notes or phases, the top note, heart note, and base note. Fragrances can be grouped by their qualitative scents by the fragrance wheel as well as by their duration. The process of making fragrances can involve many ingredients of various types, and can be altered to create new fragrances for commercial sale.
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