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Chemical Formulation
An Overview of Surfactant-based Preparations Used in Everyday Life
By Tony Hargreaves The Royal Society of Chemistry
Copyright © 2003 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-635-5
CHAPTER 1
Formulation Chemicals
Over the past few years the word chemical has taken on some very negative connotations. Apart from being difficult to define, the word all too often generates the wrong images in people's minds and is the source of the current chemophobia.
There are some among the anti-chemicals lobby who create confusion by promoting the idea that chemicals are nasty man-made substances, damaging to humans, animals and the environment and that we should use natural materials that are both healthy and environmentally friendly.
Such messages are unhelpful and foster in the minds of the non-scientists notions that there are natural substances and there are chemicals and the two are totally different things. This is unfortunate but we might expect those with only a basic knowledge of chemistry to see the flaw in it. Sadly this is not the case for even those with a scientific background can be confused.
I asked some A Level chemistry students: What is a chemical? There was an instant response that went something like this: 'Dangerous substances like acids, poisonous gases, pesticides and other stuff that damages the environment'. So, where do we go from there?
It is my view that any text on chemicals or applied chemistry has a duty to acknowledge those fears, put things in a true perspective and hopefully bring about some reassurance. And to do this it is essential to be absolutely clear as to what a chemical is.
Even if this goes only as far as explaining that the whole of the material world is atoms, produced by nature billions of years ago, chemically bonded together to produce the millions of molecules that are us and our world. It does not matter whether the bonding together of those atoms was done by nature or by man.
Good old alcohol is a prime example. The ethanol molecules produced from sugars by the catalytic effect of enzymes in yeast cells, a natural process, are indistinguishable from ethanol molecules made by man in the catalytic reaction between steam and ethene (ethylene). The water molecules exhaled by us as one of the waste products of our metabolizing carbohydrates, fats and proteins are no different from those exhaled by our beloved cars as they gobble up hydrocarbons in their infernal combustion engines.
It has to be accepted that not all man-made chemicals are a good thing just as not all nature-made chemicals are a good thing. Nature's chemical factory can come up with some treacherous substances capable of horrendous damage to the environment and living systems.
Some man-made chemicals have no analogues in nature and these are the ones that need to be watched carefully as, once they are done with and despatched to the environment, they do not always fit happily into nature's recycling mechanisms. The chlorofluorocarbons (CFCs) that damage the ozone layer are a recent example of this. Many of those molecules that are man-made, but are not straight copies of nature-made ones, we would not like to be without. Take for example the modern anaesthetics. What sort of surgery could be carried out without them?
We have gone a long way down the road to synthetic living. In fact our very bodies are now partly man-made. Those protein molecules that form a major part of us have nitrogen and hydrogen with chemical bonds that were made in chemical works in the Haber synthesis of ammonia. The ammonia so made is then used to make fertilizers which then make plant protein which we eat – directly or via animals – and incorporate into our tissue. Yes, we are partly synthetic.
For the purpose of this text I shall regard a chemical simply as a single substance (element or compound) with a definite and fixed composition that can be expressed precisely by means of a formula whether or not the substance is natural or man-made.
In classifying a chemical as natural, man-made or somewhere in between there is a need to examine its origin and any changes it has been subjected to before it is put to use. Take for example gum arabic, a traditional thickener in chemical formulations, which is entirely natural. The lumps of resinous material are a carbohydrate gum that is exactly as produced by the acacia tree and with no chemical modification. Man has simply collected it, picked out some contaminating bits of bark and dispatched it to the user. The product is 100% natural.
At the other extreme there is tetrachloromethane, CCl4. This is as man-made as you can get, not one chemical bond from a natural substance exists. Most chemicals fall somewhere in between these extremes in that they retain at least some of their nature-made structure. An example of one of these 'in between' compounds is sodium oleate, CH3(CH2)7CH:CH(CH2)7 COONa, a type of soap made from vegetable oil and sodium hydroxide, NaOH. In the molecule the carbon chain, CH3 (CH2)7CH:CH(CH2)7CO, is unaltered; it is the ONa that is due to our chemical technology.
The amount of natural content can be quantified by means of simple calculations based on molecular mass. For sodium oleate the natural component amounts to 87%. This is a highly simplified way of assessing substances and no regard is made of the other factors, like the production of the sodium hydroxide, that are essential to the manufacturing process.
It may be that a molecule comes out at over 80% nature-made content but its 20% man-made part involved so many other chemicals that it fares less well than a molecule of a mere 10% nature-made content. This is to say nothing of the energy aspects of the environment/resource equation. The foregoing may be a rather crude instrument but it is a useful exercise in our efforts to focus upon more natural chemicals; it will also be helpful when we consider 'cradle-to-grave' analysis in Chapter 5.
Some chemicals used in formulations are petroleum based and are obtained simply by separation processes such as distillation. As such they retain their natural structure and in this respect they are natural substances but, in contrast to the oleate (above) which is made from vegetable oil, they are not from renewable resources.
In considering the worthiness of a chemical as an ingredient in a preparation the chemist of today must at least pay some attention as to whether it is from renewable or non-renewable resources. Unfortunately, however hard we work to avoid the use of petroleum it will still play a part somewhere in the process – at least for the foreseeable future.
In the discussions on chemicals that follow there will be frequent reference to some relevant chemistry theory. This has been kept to the minimum required for an understanding of the types of formulations discussed in this book.
CHEMICAL NAMES
There are over one hundred thousand chemicals currently in use but this represents only a small fraction of those that are known. In fact, the total number of chemical substances registered with the Chemicals Abstract Service is over 18 million and new ones arrive at a rate of over 300 per hour. With this number of different chemicals, and the need for each one to be completely distinguishable from the rest of the crowd, comes a problem of naming.
Systematic approaches for naming chemical compounds have to be used but, all too frequently, this presents us with names that are virtually unusable in written and spoken communications. Try this one in discussion – and it's by no means one of the complicated ones – N,N'-1,2-ethanediylbis[N-(carboxymethyl)glycine]tetrasodium; it is usually called EDTA tetrasodium. So where does this abbreviation EDTA come from? That is from an earlier naming system: ethylenediaminetetraacetic acid. Unfortunately for the chemist of today there are frequently many names for the same substance.
A look in the Merck Index for our EDTA, which puts it under the heading Edetate Sodium, gives many other names. In addition to systematic names there are trivial names, common names and, if that were not complex enough, commerce comes in with a whole string of trade names. Thus, for the EDTA we have, from the Merck Index: ethylene bis(iminodiacetic acid) disodium salt, edetic acid disodium salt, edathamil disodium, tetracemate disodium, Cheladrate, Chelaplex III, Sequestrene NA, Sodium Versenate. And those are just a few of the ones current in the USA; in Europe there are other names to add to the list.
The trivial names, which are often the older ones, tend to be those that are generally used. Of course some of the much older names may be found from time to time, for example, muriatic acid is the old name for hydrochloric acid. That's not too much of a problem but the oxidation product of this compound was called dephlogisticated muriatic acid gas; its modern name is chlorine. Fortunately, there is international standardization in the form of the IUPAC (International Union of Pure and Applied Chemists) system for assigning names to chemicals.
For cosmetic and toiletries the CTFA (Cosmetics Toiletries and Fragrance Association) and INCI (International Nomenclature of Cosmetic Ingredients) names are used. Generally these names are pretty close to the systematic ones, for example the CTFNINCI name sodium laureth sulfate suggests, to those with a little knowledge of surfactants, sodium lauryl ether sulfate. However, this name falls short of uniquely identifying the chemical in that the number of moles of ethylene oxide (see later) is omitted.
If there are doubts over a name for a particular substance then the only way to be sure is to use the Chemical Abstracts Service registry number, CAS number, a unique identifier. For example, for EDTA tetrasodium (anhydrous), the CAS number is 64-02-8.
WATER
In many preparations, particularly ones based on surfactants, water is the major ingredient and, as such, justifies first place in the list of chemicals. When studying formulations that are water based there is often a temptation to view the water's role as simply one of diluting the preparation to make it go further. But this versatile liquid is not to be thought of lightly. Water plays a vital role in most everyday formulations and so an understanding of it is essential; and without this it is impossible to get a grasp of the way in which many other chemicals work.
In attempting to understand how water behaves we must, just as with other chemicals, consider the structure of its molecules. Two hydrogen atoms covalently bonded to a single oxygen atom give the water molecule its unique and fascinating properties, both physical and chemical. The hydrogen–oxygen covalent bond is highly polar because of the large difference in electronegativity of these two atoms.
Most of us have seen how a jet of water from a burette is bent by the electrostatic attraction of charged plastic rod placed near the jet. The polar bonds make water a polar liquid and this has important consequences as will be discussed in the section on solvents and dissolving.
At any one time a small number of these highly polar molecules will undergo complete polarization, resulting in a covalent bond breaking and dissociation into two ions. This gives rise to an equilibrium from which is obtained the ionic product of water, KW,
H2O [??] H+ + OH- KW = [H+][OH-] (U1)
where [] = concentration.
In any sample of pure water at room temperature the concentration of each of the ions is 10-7 mol l-1. Thus the ionic product is 10-7 x 10-7 = 10-14. An important aspect of this is that it is constant so that whatever is added to the water the ionic product stays the same in accordance with the equilibrium principle (Le Chatelier's Principle). An appreciation of this basic theory is prerequisite to understanding the behaviour in certain aspects of chemical formulation such as acids, bases, buffers and pH.
Another important property arising from the polarity of the water molecule is hydrogen bonding which explains much of how water interacts with other substances. Hydrogen bonding is considered in some detail in the section on dissolving and in the surfactants chapter (Chapter 2).
In some formulation chemicals water of crystallization is present due to the polar water molecules being strongly attracted to the anions and cations and becoming fixed in the lattice. A familiar example is washing soda, sodium carbonate decahydrate, Na2CO3.10H2O, a crystalline solid that is mostly water (62.9%).
Apart from an understanding of some of the theoretical aspects of water, and how these are reflected in its properties, it is also important to know about the practical aspects that affect its use in formulations.
For formulation work the source of water is going to be the public supply provided by the water company. This water will be free from pathogenic micro-organisms but the water treatment processes from which it comes does not remove mineral matter such as dissolved calcium salts. Tap water is therefore a dilute solution of mineral salts. The amount of salts dissolved depends on the catchment area and how much limestone there is in that area. Water from a limestone area is high in calcium salts and is termed hard water whereas from non-limestone areas we get soft water. In general water is classified as:
Up to 100 mg 1-1 CaCO3 = soft water
100–400 mg 1-1 calcium carbonate = medium hard water
400 upwards mg 1-1 calcium carbonate = hard water
mg 1-1 = milligrams per litre; it is numerically the same as parts per million (ppm)
For most chemical preparations tap water falls short in terms of chemical purity and some form of pre-treatment is called for to upgrade it. There are many methods for treating the water to remove the dissolved mineral salts but the most common is ion exchange which involves passing the water through a column of ion exchange resin.
Two types of ion exchange are common: water softening and water deionizing. In the former the calcium ions that cause hardness are exchanged for sodium ions that do not cause hardness. Although water softening plays a small role in preparing formulations it is more usual to opt for deionizing, sometimes called demineralization.
Modern ion exchange resins are based on synthetic polymers. In deionizing there are two types of resin, one for cation exchange and the other for anion exchange. The cation one is acidic and exchanges metal ions for hydrogen ions whereas the anion exchange polymer is basic and exchanges anions for hydroxide ions as shown in Figure 1.1.
These two resins, which are normally supplied as spheres about 1 mm in diameter, may be used in two different columns run in series or, as is more usual, as a 1:1 mixture in a single column. The former arrangement offers the possibility of recharging the resin once it is spent, thus scoring a few environmental points over the mixed resin approach where the whole lot is discarded.
Preparing personal care products imposes greater demands upon the quality of the water. Although the deionization process will mean that dissolved salts are absent it does not guarantee it is free from micro-organisms which can cause product spoilage or, even worse, carry harmful bacteria to the user.
Water from the public supply, because it carries a residual chlorine content, is disinfected when it is drawn from the tap but after passing through a bed of ion exchange resin its chlorine is removed and its disinfecting capacity lost. As such it may need a supplementary treatment to ensure destruction of micro-organisms.
One might argue the irrelevance of this on the grounds that a bactericidal preservative is added to the blend. But, the counter to this is that the preservative is put in to protect the formulation from micro-organisms picked up after its preparation and to give it a reasonable lifetime once its packaging is opened and it enters use. Biocidal preservatives are not intended as remedies to bacterial and fungal contamination from water or dirty mixing equipment.
SURFACTANTS
These are the main chemicals in the everyday formulations that form the basis of this book and, as such, are discussed at length in Chapter 2. Very briefly, surfactants are the most versatile of chemicals found in modern formulations and carry out functions such as: emulsification, detergency, wetting and spreading, solubilizing, foaming and defoaming, lubricity, biocidal, anti-static and corrosion inhibition.
For convenience, surfactants are grouped on the basis of their molecular types rather than functionality. The groups are: non-ionic, anionic, cationic and amphoteric. Within each category we may find that a range of functions is represented and that individual molecules can carry out a variety of tasks but with different degrees of effectiveness to related molecules.
ACIDS AND BASES (ALKALIS)
Strictly speaking an alkali is a base in aqueous solution and so the word is confined to soluble bases. As the word alkali is part of everyday language it is often used instead of the word base. Some texts talk in terms of acids and alkalis, others of acids and bases. The subtle difference between base and alkali need not be a problem in this text but both words must be used as the book sits somewhere between industrial chemistry and academic chemistry. As such, both words are used in a manner that reflects common usage rather than strict chemical definition.
It was seen above that water dissociates into hydrogen ions and hydroxide ions. In terms of simple definitions relating to aqueous chemistry: a substance that produces hydrogen ions as the only positive ions is an acid; a substance that reacts with hydrogen ions in a neutralization reaction is an alkali; usually alkalis react with hydrogen ions because they produce hydroxide ions.
(Continues...)
Excerpted from Chemical Formulation by Tony Hargreaves. Copyright © 2003 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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