Jun. 24, 2024
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The word amphiphilewas coined by Paul Winsor 50 years ago. It comes from two Greekroots. First the prefix amphiwhich means &#;double&#;, &#;from both sides&#;, &#;around&#;, as inamphitheater or amphibian. Then the root philos which expresses friendship or affinity, as in&#;philanthropist&#; (the friend of man), &#;hydrophilic&#; (compatible with water), or &#;philosopher&#; (thefriend of wisdom or science).
An amphiphilic substance exhibits a double affinity, which can be defined from thephysico-chemical point of view as a polar-apolar duality. A typical amphiphilic moleculeconsists of two parts: on the one hand a polar group which contents heteroatoms such as O, S, P,or N, included in functional groups such as alcohol, thiol, ether, ester, acid, sulfate, sulfonate,phosphate, amine, amide etc&#; On the other hand, an essentially apolar group which is in generalan hydrocarbon chain of the alkyl or alkylbenzene type, sometimes with halogen atoms and evena few nonionized oxygen atoms.
The polar portion exhibits an strong affinity for polar solvents, particularly water, and it isoften called hydrophilicpart or hydrophile.The apolar part is called hydrophobeor lipophile,from Greek roots phobos (fear) and lipos (grease). The following formula is important for surfactants uses and shows an amphiphilicmolecule which is commonly used in shapoos (sodium dodecyl sulfate).
In English the term surfactant(short for surface-active-agent) designates a substancewhich exhibits some superficial o interfacial activity. It is worth remarking that all amhiphiles donot display such activity; in effect, only the amphiphiles with more or less equilibratedhydrophilic and lipophilic tendencies are likely to migrate to the surface or interface. It does nothappen if the amphiphilic molecule is too hydrophilic or too hydrophobic, in which case it staysin one of the phases.
In other languages such as French, German or Spanish the word &#;surfactant&#; does notexist, and the actual term used to describe these substances is based on their properties to lower the surface or interface tension, e.g. tensioactif(French), tenside(German), tensioactivo(Spanish) which also determines the surfactants uses. This would imply that surface activity is strictly equivalent to tension lowering, whichis not absolutely general, although it is true in many cases.
From the commercial point of view surfactants are often classified according to their use.However, this is not very useful because many surfactants have several uses, and confusions mayarise from that. The most acepted and scientifically sound classification of surfactants is based ontheir dissociation in water. The figures in page 4 show a few typical examples of each class.
Anionic Surfactants are dissociated in water in an amphiphilic anion*,and a cation*,which is in general an alcaline metal (Na+, K+) or a quaternary ammonium. They are the mostcommonly used surfactants. They include alkylbenzene sulfonates (detergents), (fatty acid)soaps, lauryl sulfate (foaming agent), di-alkyl sulfosuccinate (wetting agent), lignosulfonates(dispersants) etc&#; Anionic surfactants account for about 50 % of the world production.
Nonionic Surfactants come as a close second with about 45% of the overall industrial production. They do not ionize in aqueous solution, because their hydrophilic group is of a non-dissociable type, such as alcohol, phenol, ether, ester, or amide. A large proportion of thesenonionic surfactants are made hydrophilic by the presence of a polyethylene glycol chain,obtained by the polycondensation of ethylene oxide.
They are called polyethoxylated nonionics.In the past decade glucoside (sugar based) head groups, have been introduced in the market,because of their low toxicity. As far as the lipophilic group is concerned, it is often of the alkyl oralkylbenzene type, the former coming from fatty acids of natural origin.
The polycondensation ofpropylene oxide produce a polyether which (in oposition to polyethylene oxide) is slightlyhydrophobic. This polyether chain is used as the lipophilic group in the so-called polyEO-polyPO block copolymers, which are most often included in a different class, e.g. polymericsurfactants, to be dealt with later.
Cationic Surfactants are dissociated in water into an amphiphilic cation and an anion,most often of the halogen type. A very large proportion of this class corresponds to nitrogencompounds such as fatty amine salts and quaternary ammoniums, with one or several long chainof the alkyl type, often coming from natural fatty acids.
These surfactants are in general more expensive than anionics, because of a the high pressure hydrogenation reaction to be carried outduring their synthesis. As a consequence, they are only used in two cases in which there is nocheaper substitute, i.e. (1) as bactericide, (2) as positively charged substance which is able toadsorb on negatively charged substrates to produce antistatic and hydrophobant effect, often ofgreat commercial importance such as in corrosion inhibition.
When a single surfactant molecule exhibit both anionic and cationic dissociations it iscalled amphoteric orzwitterionic. This is the case of synthetic products like betaines orsulfobetaines and natural substances such as aminoacids and phospholipids.
The past two decades have seen the introduction of a new class of surface activesubstance, so-called polymeric surfactants or surface active polymers, which result from theassociation of one or several macromolecular structures exhibiting hydrophilic and lipophiliccharacters, either as separated blocks or as grafts. They are now very commonly used informulating products as different as cosmetics, paints, foodstuffs, and petroleum productionadditives.
The world production of soaps, detergents and other surfactants was about 18 Mt (milliontons) in , 25 Mt in and 40 Mt in (not counting polymeric surfactants).Approximately 25 % corresponds to the north american market and 25 % to the european market.
The qualitative evolution of the market in the past 50 years is very significative. In effet,in the world production of surfactants (1.6 Mt) essentially consisted of soaps (fatty acidsalts) manufactured acording to a very old fashioned technology. At the end of World War II, thepetroleum refining market was offering short olefins, particularly C2-C3, as a by-product fromcatalytic craking. In the early &#;s propylene had not yet any use, whereas ethylene started tobe employed in styrene manufacture. The low cost of propylene and the possibility ofpolymerizing it to produce C9-C12-C15 hydrophobic groups, made it a cheap alternative to alkylgroups coming from natural or synthetic fatty acids.
Synthetic detergents of the alkylbenzenesulfonate (ABS) type were born, and they soon displaced soaps for washing machine and otherdomestic uses.
In the early &#;s many rivers and lakes receiving the waste waters from large citiesstarted to be covered by persistent foams, which resulted in ecological damage because the thicklayer curtailed photosynthesis and oxygen dissolution. The culprit was found to be the branchingof the alkylate group of the ABS made from propylene, whose polymerization followsMarkovnikoff&#;s rule. It was found that branching confers to the alkylate group a resistance tobiodegradation. As a consequence environmental protection laws were passed around torestrict and forbid the use propylene-based alkylate in USA and Europe.
Surfactant manufacturers had to find new raw materials and methods to make linearalkylates, e. g., ethylene polimerization, molecular sieve extraction and Edeleanu processthrough the urea-paraffin complex. All new synthetic paths were more expensive, and though thelinear alkylbenzene sulfonates (LAS) are still the cheapest detergents, the difference with othertypes is much less significant than with ABS. This situation favored the development of newmolecules which lead to the current wide range of products.
The developement of steam cracking in the &#;s, essentially to produce ethylene as araw material for various polymers, also contributed to the low-cost availability of thisintermediate in the production of ethylene oxide, the basic building block of nonionic surfactants.
The &#;s displayed a proliferation of new formulas, and a strong increase in the use ofsurfactants not only for domestic use but also for industrial purposes. Nonionic surfactants wereincluded in many products when a good tolerance to divalent cations was required. Cationic and amphoteric surfactants are now offered by several manufacturers, though their use is curbed bytheir high cost. In the - the market shares of the different products stabilize, with aquicker growing of nonionics with respect to anionics, in particular with the introduction of anew type of nonionics, e.g. alkyl polyglucosides. The next table portrays the surfactants uses in the last 20 years:
Polymeric surfactants are often not accounted as surfactants and consequently do notappear in statistics, such as those of the previous table. Their importance is growing however,because they enter in many formulated products (as dispersants, emulsifiers, foam boosters,viscosity modifiers, etc) and could be around 10 % of the surfactant market in , withproducts as polyEO-PolyPO block copolymers, ethoxylated or sulfonated resins, carboxymethylcellulose and other polysaccharide derivatives, polyacrylates, xanthane etc.
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Surfactants are one of many different compounds that make up a detergent. They are added to remove dirt from skin, clothes and household articles particularly in kitchens and bathrooms. They are also used extensively in industry. The term surfactant comes from the words surface active agent.
Figure 1 Surfactants aid the effective washing of dirty rugby kit using low
temperature wash cycles, resulting in environmental benefits.
By kind permission of Stephen Garnett/Wharfedale RUFC.
Surfactants function by breaking down the interface between water and oils and/or dirt. They also hold these oils and dirt in suspension, and so allow their removal. They are able to act in this way because they contain both a hydrophilic (water loving) group, such as an acid anion, (-CO2- or SO3-) and a hydrophobic (water hating) group, such as an alkyl chain. Molecules of water tend to congregate near the former and molecules of the water-insoluble material congregate near the latter (Figure 2).
Soaps were the earliest surfactants and are obtained from fats which are known as glycerides because they are esters formed by the trihydric alcohol, propane-1,2,3-triol (glycerol), with long chain carboxylic acids (fatty acids). The glycerides are hydrolyzed by heating with sodium hydroxide solution to form soaps, the sodium salts of the acids, and propane-1,2,3-triol. The process is known as saponification.
Figure 2 Action of a surfactant.
The glycerides used to make surfactants contain saturated and unsaturated carboxylic acids which have an even number of carbon atoms, generally within the range 12-20, for example, octadecanoic acid (stearic acid), CH3(CH2)16CO2H.
Synthetic surfactants have one very important advantage over soaps. Because soaps form insoluble calcium and magnesium salts with the calcium and magnesium ions in hard water and in the clays which are present in dirt, much of the soap is wasted forming an insoluble scum. However, this is avoided when using a synthetic surfactant. For example, in the anionic surfactants, the carboxylate group in soap is replaced by a sulfonate or sulfate group as the hydrophilic component. The corresponding calcium and magnesium salts are more soluble in water than the calcium and magnesium salts of carboxylic acids.
Surfactants are classified based upon the nature of the hydrophilic "head-groups" as:
In these surfactants the hydrophilic group is negatively charged. They are the most widely used type of surfactants for laundering, dishwashing liquids and shampoos. They are particularly good at keeping the dirt, once dislodged, away from fabrics.
Four anionic surfactants are used:
a) alkylbenzene sulfonates
b) alkyl sulfates
c) alkyl ether sulfates
d) soaps
The most common of the synthetic anionic surfactants are based on the straight chain alkylbenzene sulfonates. Benzene, in slight excess, is mixed with an alkene or chloroalkane in the presence of an acid catalyst, usually a solid zeolite (ion exchange), aluminium chloride (AlCl3) or hydrofluoric acid (HF), to produce an alkylbenzene (sometimes called detergent alkylate).
For example:
The alkylbenzene varies in average molecular mass, depending upon the starting materials and catalyst used and is often a mixture in which the length of the alkyl side chain varies between 10 and 14 carbon atoms. Historically these included branches in the side chains with the result that they biodegrade very slowly and lead to foaming in rivers and sewage plants. By law, in most countries today, the surfactant must have side chains which are not branched so they degrade more rapidly.
The alkylate is sulfonated using an air/sulfur trioxide mixture, and the resulting sulfonic acid is then neutralised with an aqueous solution of sodium hydroxide (often in situ), for example:
Straight chain alkenes for the above process can be produced from ethene using a Ziegler catalyst (triethyl aluminium). Triethyl aluminium reacts with ethene at ca 400 K and 100 atm to form aluminium alkyls, for example:
When heated in excess ethene, straight chain alkenes, with the double bond at the end of the chain (an a-alkene), are produced:
The mixture is then separated into fractions by distillation, the fraction of alkenes containing 10 to 14 carbon atoms being used to make the surfactants.
These are used together with other surfactants in powder and liquid laundry detergents such as Ariel, Daz, Persil and Surf.
Many detergent products, particularly liquids, contain other synthetic anionic surfactants such as alkyl sulfates, esters of linear alcohols (C10-C18) and sulfuric acid. The alkyl sulfates are also used in personal care products such as toothpaste and are manufactured by treating the alcohol with sulfur trioxide. The product is then neutralised with aqueous sodium hydroxide solution to form a sodium alkyl sulfate:
The alcohols are either produced from carboxylic acids obtained from oils, obtained naturally, for example from palm kernel oil or coconut oil, or alternatively from long-chain alkenes, manufactured from ethene.
There are two processes for making the alcohols from ethene. As described above, aluminium triethyl reacts with ethene to produce compounds such as:
where a,b,c are even numbers from 2 to 12. Instead of heating with excess ethene to produce a-alkenes, the aluminium alkyl is treated with oxygen and then water to produce long chain alcohols:
Alternatively, a different process for making the alcohols from ethene is used, known as SHOP (Shell Higher Olefins Process). In the first stage, ethene is passed, under pressure of ca 100 atm, into a solvent (usually a diol, such as butane-1,4-diol) containing a nickel salt at 400 K. It yields a mixture of a-alkenes which are separated by fractional distillation. About 30% are in the range C10-C14.
These are reacted with carbon monoxide and hydrogen (hydroformylation) to yield straight-chain aldehydes, which on reduction form alcohols. For example:
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It is possible to convert the other a-alkene fractions (C4-C10 and C14-C40) into the more desirable C10-C14 fraction.
More widely used than simple alkyl sulfates are various types of sodium alkyl ether sulfates (SLES).
In the manfacture of SLES the primary alkyl alcohol (from a synthetic or natural source and typically a blend based around dodecanol) is first ethoxylated with 1 to 3 molar equivalents of epoxyethane (as described below for the manufacture of nonionic surfactants). The product is then sulfated using sulfur trioxide and neutralized with alkali to form the alkyl ether sulfate:
These materials are preferred by product formulators for many applications (dishwashing liquids, shower gels, shampoo, etc) because they are milder to the skin than alkyl sulfates. They also generate less foam which is an advantage in the formulation of laundry machine products.
Soaps are anionic detergents:
With these surfactants, the hydrophilic head is positively charged.
Although they are produced in much smaller quantities than the anionics, there are several types, each used for a specific purpose.
The simplest quaternary system is the ammonium ion:
An alkyl quaternary nitrogen system has alkyl groups attached to the nitrogen atom. An example is:
They are used as fabric softeners with anionic surfactants, helping them to break down the interface between the dirt/stain and the water.
The directly quaternised fatty acid surfactants described above have been replaced for laundry applications by more complicated structures in which there is an ester linkage between the alkyl chains and the quaternary head-group as these are more biodegradable and less toxic. They are known as esterquats.
An example is:
Esterquats give detergents their fabric softening qualities.
These surfactants do not bear an electrical charge and are often used together with anionic surfactants. An advantage is that they do not interact with calcium and magnesium ions in hard water.
They account for nearly 50% of surfactant production (excluding soap). The major group of nonionics are the ethoxylates made by condensing long chain alcohols with epoxyethane (ethylene oxide) to form ethers, for example:
The long-chain alcohol can come from either a synthetic or natural source.
Although they do not contain an ionic group as their hydrophilic component, hydrophilic properties are conferred on them by the presence of a number of oxygen atoms in one part of the molecule which are capable of forming hydrogen bonds with molecules of water.
As the temperature of the surfactant solution is increased the hydrogen bonds gradually break causing the surfactant to come out of solution. This is commonly referred to as the cloud point and is characteristic for each nonionic surfactant. Nonionics are more surface active and better emulsifiers than anionics at similar concentrations. They are less soluble than anionics in hot water and produce less foam. They are also more efficient in removing oily and organic dirt than anionics. Depending on the type of fibre, they can be active in cold solution and so are useful in countries which lack hot water supplies and in developed countries where there is a desire to lower the wash temperatures either to save energy or because of the type of fabric being washed. Nonionics are used in fabric washing detergents (both powders and liquids), in hard surface cleaners and in many industrial processes such as emulsion polymerization and agrochemical formulations.
Amphoteric (or zwitterionic) surfactants are so called because the head-group carries both a negative and positive charge. A range of methods is used to produce such materials, almost all of which contain a quaternary ammonium ion (a cation). The negatively charged group can be carboxylate, -CO2-, sulfate, -OSO3- or sulfonate, -SO3-. One such well-used class is the alkyl betaines which have a carboxyl group. A long-chain carboxylic acid reacts with a diamine to form a tertiary amine. On further reaction with sodium chloroethanoate, a quaternary salt is formed:
Betaines are neutral compounds with a cationic and an anionic group which are not adjacent to one another.
Amphoteric surfactants are very mild and are used in shampoos and other cosmetics. They are said to be pH balanced.
A detergent is made up of many ingredients, some of which are surfactants. An example of the mixture of compounds in a detergent is shown in Table 1.
In this formulation there are seven surfactants, two anionic, three non-ionic and two soaps.
However, there are other ingredients, each with specific functions:
Bulking agents, such as sodium sulfate and water.
Some detergents need anti-caking agents, for example aluminium silicate, which keep the powder dry and free-flowing.
Builders, usually sodium aluminosilicates, a type of zeolite, remove calcium and magnesium ions and prevent the loss of surfactant through scum formation.
Stains can be bleached with oxidizing agents such as sodium perborate (NaBO3.4H2O) and sodium percarbonate (2Na2CO3.3H2O2) which react with hot water to form hydrogen peroxide which in turn reacts with the stain:
However bleach activators are needed for low temperature washes. Sodium perborate and sodium percarbonate do not liberate hydrogen peroxide in cool water. A compound is added to react with them to liberate a peroxycarboxylic acid, RCO3H, which oxidizes stains readily. The most commonly used activator is:
It is known by its trivial name, TAED, and reacts with the oxidizing agent to form peroxyethanoic acid:
IngredientFunction Sodium silicoaluminate Builder Sodium carbonate Buffering agent Sodium sulfate Bulking agent Sodium carbonate peroxide (sodium percarbonate) Oxidizing agent Sodium
Table 1 Ingredients of a detergent for washing clothes.
Other ingredients which can be added to a detergent include:
Buffering agents - to keep the pH at the appropriate value
Structurants - to give shape to the fabric being washed
Sequestrants - to react with free metal ions which might otherwise cause problems with appearance or scum formation
Optical brighteners - to make the fabrics look brighter and whiter
Antifoaming agents
Enzymes - to remove specific stains: proteases (to remove proteins), amylases (to remove starches), lipases (to remove fats)
Fragrance
Anti-redeposition agents - to prevent dirt being redeposited on fabrics
Skin conditioning agent - to help to keep the skin in good condition
Softness extender - to help keep the clothes 'soft'
Emulsifier - to help keep immiscible liquids as an emulsion
Colorant
Domestic automatic machine laundry liquids are formulated using blends of anionic, nonionic and soap surfactants and various other functional substances. Bleach systems are not compatible with the higher water temperature and cannot be used above ca 315 K.
For hand washing (used for delicate fabrics such as wool or silk), foam-stabilisers are included, to maintain foam. The customer equates the quantity of foam produced with the detergent cleansing action. For the quantity of foam produced the order is:
anionics > soap > nonionics > cationics
The products used in dishwashers are usually powders and contain builders (90-95%), a nonionic surfactant (1-5%), bleach agents with an activator and enzymes. They are formulated with sodium carbonate and sodium silicate to create a very alkaline environment that helps to denature (break down) the fats and proteins left on the used dishes and utensils.
These formulations contain between 13-40% of surfactants which are predominantly alkyl ether sulfates but also include nonionics and amphoterics (betaines).
These tend to be based on alkyl ether sulfates and usually contain small amounts of other surfactants (most typically amphoterics) which help protect the skin from irritation and also condition the hair.
These products are formulated using cationic surfactants (sometimes combined with small amounts of non-ionic surfactants). These are not cleansing products and the cationic surfactant is deposited onto the slightly negatively charged hair or cotton fibre surface, thus giving a lubrication benefit.
In Western Europe all surfactant components of domestic detergents must be biodegradable. This requirement resulted from the fact that the original alkylbenzene sulfonate anionics were based on branched alkenes and these proved resistant to degradation by bacteria at sewage treatment works causing many rivers to suffer from foam. There was also a fear that surfactants could be "recycled" into drinking water. Similar concerns were expressed about nonylphenol ethoxylates and so in the s the industry moved to linear alkylbenzene sulfonates and alcohol ethoxylates as the major ingredients of their formulations. Effective sewage treatment ensures that detergent components which are part of household effluent water are not discharged untreated into rivers and water courses.
The development of compact powders and liquids and refillable packages is designed to reduce packaging waste.
Redesign of washing machines and laundry detergent products (including the addition of bleach activators and enzymes to ensure good stain removal at low temperatures) has resulted in energy savings by reducing water heating and using shorter wash cycles.
Date last amended: 18th March
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