Fig. 1.2 - Surfactant in action.
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Nonionic surfactants have covalently bonded oxygen-containing hydrophilic groups, which are bonded to hydrophobic parent structures. The water-solubility of the oxygen groups is the result of hydrogen bonding. Hydrogen bonding decreases with increasing temperature, and the water solubility of nonionic surfactants therefore decreases with increasing temperature. The characteristic feature of cloud point (CP) of nonionic surfactants is the temperature at which the surfactant separates out from an aqueous solution due to the weakening of hydrogen bonds between the surfactant and water molecules. Nonionic surfactants are less sensitive to water hardness than anionic surfactants, and they foam less strongly.
Most anionic and nonionic surfactants are nontoxic, having LD50 comparable to sodium chloride. The toxicity of quaternary ammonium compounds, which are antibacterial and antifungal, varies. Prolonged exposure to surfactants can irritate and damage the skin because surfactants disrupt the lipid membrane that protects skin and other cells. Skin irritancy generally increases in the series nonionic, amphoteric, anionic, cationic surfactants..
Surfactants play an important role in personal care products such as cosmetics, shampoos, shower gel, hair conditioners, and toothpastes. The property of providing cleaning, wetting, and dispersing, emulsifying, foaming and anti-foaming effects are used in many practical applications and products. Surfactants with different HLB are used in detergents, fabric softeners, soaps, paints, adhesives, inks, emulsions, anti-fogs, ski waxes, snowboard wax, deinking of recycled papers, in flotation, washing and enzymatic processes. Agrochemical formulations such as some herbicides, insecticides, biocides (sanitizers), and spermicides also contain Surfactants. Surfactants find use in firefighting and pipelines as liquid drag reducing agents. Alkali Surfactant polymers are used to mobilize oil in oil wells for exploration.
Nonionic surfactant refers to the surfactant molecules, which do not undergo ionization when being dissolved in water. The Nonionic surfactant are not in the ionic state in the solution, thereby having high stability and being less susceptible to the effect of strong electrolyte inorganic salts as well as acid and alkalis. Nonionic surfactants have excellent compatibility with other types of surfactants and have excellent solubility (which vary depending on different structures, HLB etc) in both water and organic solvents.
Nonionic surfactants have covalently bonded oxygen-containing hydrophilic groups, which are bonded to hydrophobic parent structures. These Nonionic surfactants, are not ionized in water, and contain both hydrophilic groups (e.g. oxyethylene-CH2CH2O-, ether groups, hydroxyl group -OH or -CONH2 amide group, etc.) and lipophilic group (e.g., hydrocarbons which can be natural fatty alcohols or synthetic alcohols, acids or glyceryl esters/oils). The water-solubility of the oxygen groups is the result of hydrogen bonding. Hydrogen bonding decreases with increasing temperature, and the water solubility of Nonionic surfactants therefore decreases with increasing temperature. This result in formation of a milky/cloudy emulsion called the cloud point of surfactants. This property is very essential for determining the optimum use of Nonionic surfactant in formulations at elevated temperature especially in cleaning formulations like detergents, CIP etc.
As discussed above Nonionic surfactants have a unique property called a cloud point. The cloud point is the temperature at which the Nonionic surfactant begins to separate from the cleaning solution, called phase separation. When this occurs, the cleaning solution becomes cloudy. This cloud point is therefore considered the temperature for optimal detergency. For low foaming cleaners, optimal detergency is at the cloud point; for foaming cleaners optimal detergency is either just below the cloud point or at the start of the cloud point. The agitation of low foaming cleaners is sufficient to prevent phase separation. The temperature of the cloud point depends upon the ratio of the hydrophobic and hydrophilic portions of the Nonionic surfactant. Some cloud points are at room temperature while others are very high. Some Nonionic surfactants don&#;t have a cloud point because they have a very high ratio of hydrophilic to hydrophobic moieties.
Nonionic surfactants are less sensitive to water hardness than anionic surfactants, and they foam less strongly. The differences between the individual types of Nonionic surfactants are slight, and the choice is primarily governed having regard to the costs of special properties (e.g., effectiveness and efficiency, toxicity, dermatological compatibility, biodegradability) or permission for use in food. In areas with hard water (high mineral content), Nonionic surfactants are more heavily marketed, as they are less likely to form a soap scum. The Nonionic surfactants are less likely to cause skin irritation, but this is associated with a less potent cleaning ability. Most cleaning products are manufactured as a blend of anionic and Nonionic surfactants to balance out the cleaning potential with the risk of skin irritation.
The aqueous solution of Nonionic surfactants has poor foaming capability with the foam being not stable as well. This is due to that each molecule of the Nonionic surfactant has relatively large surface area and the interface being in uncharged foam. Polyoxyethylene has long chain and uniform molecular weight distribution. The lipophilic group has long chain and also contains branched chain. The presence of the polyoxyethylene -polyoxypropylene copolymer both has a great impact on the foaming of the Nonionic surfactants. Owing to the presence of the polar portion and non-polar portions existing in their molecular structure, they have large surface activity. Such kind of active agents can be divided into the ester type (e.g. polyoxyethylene fatty acid esters, sorbitan fatty acid esters anhydrides), ether type (e.g., polyoxyethylene alkyl ether, polyoxyethylene alkyl phenol ether), amine type (such as polyoxyethylene fatty amine), amide type (such as polyoxyethylene alkyl amide) and mixing type (such as sorbitol anhydride fatty acid esters, polyoxyethylene ether). In the field of oiling, Nonionic surfactants are mainly used in foaming, emulsifying, anti-wax, anti-corrosion, retarder, production increase of oil wells, intensified injection of injection wells as well as improving oil recovery and so on.
Trade/Common name Name Applications Product-35-AJ Polyoxyethylene glycol octylphenol ethers: C8H17&#;(C6H4)&#;(O-C2H4)1&#;25&#;OH Wetting agent &#; coatings Product-30-AI Polyoxyethylene glycol alkylphenol ethers: C9H19&#;(C6H4)&#;(O-C2H4)1&#;25&#;OH Spermacide Polysorbates Polyoxyethylene glycol sorbitan alkyl esters Food ingredient Span® Sorbitan alkyl esters Polishes, cleaners, fragrance carriers PEGs, PPGs Product L-61, L-62, F-108 Block copolymers of polyethylene glycol and polypropylene glycol VariousAnionic surfactants contain anionic functional groups at their head, such as sulfonate, phosphate, sulfate and carboxylates. Alkyl sulfates include sodium lauryl and the related alkyl-ether sulfates sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES), and sodium myreth sulfate. Sodium stearate is a good example of a surfactant. It is the most common surfactant in soap. The stearates comprise >50% of the global usage of surfactants. Many of these find utilization in emulsion polymerization. Other anionic surfactants include dioctyl sodium sulfosuccinate (DOSS), linear alkylbenzene sulfonates (LABs) and alkyl-aryl ether phosphates.
Abbreviation Name Applications DOSS Dioctyl sodium sulfosuccinate (DOSS) Wetting agent &#; coatings, toothpaste LABSA Linear alkylbenzene sulfonates Laundry detergents, dishwasher detergents SLES Sodium lauryl ether sulfate Shampoos, bath products N/A Sodium stearate Handsoap, HI&I productsCationic surfactants are comprised of a positively charged head. Most of cationic surfactants find use as anti-microbials, anti-fungals, etc. in HI&I (Benzalkonium chloride (BKC-80 and BKC-50). The cationic nature of these surfactants disrupts the cell membranes of bacteria and viruses.
Zwitterionic (amphoteric) surfactants have both cationic and anionic centers attached to the same molecule. The anionic part can be variable and include sulfonates, as in the sultaines (Foamer HS). Betaines such as cocamidopropyl betaine (CAPB, coco betaine) have a carboxylate with the ammonium. The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium cations. Zwitterionic surfactants are often sensitive to pH and will behave as anionic or cationic based on pH. Fast dry (&#;coacervation&#;) latex traffic paints are based on this concept, with a drop in pH triggering the latex in the paint to coagulate.
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.
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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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