The origin of surfactants uses. The Amphiphiles.
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).
What are surfactants?
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.
Types of Surfactants
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.
Surfactants uses and production
The world production of soaps, detergents and other surfactants was about 18 Mt (milliontons) in 1970, 25 Mt in 1990 and 40 Mt in 2000 (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 1940 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 1950’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 1960’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 1965 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 1960’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 1970’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 1980-1990 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 2000, withproducts as polyEO-PolyPO block copolymers, ethoxylated or sulfonated resins, carboxymethylcellulose and other polysaccharide derivatives, polyacrylates, xanthane etc.
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