16 Surface Active Agents

P. Mageshkumar

Introduction

In English, the term surfactant (short for surface-active-agent) designates a substance which exhibits some superficial or interfacial activity. It is worth remarking that all amhiphiles do not display such activity; in effect, only the amphiphiles with more or less equilibrated hydrophilic and lipophilic tendencies are likely to migrate to the surface or interface. It does not happen if the amphiphilic molecule is too hydrophilic or too hydrophobic, in which case it stays in one of the phases.

In other languages such as French, German or Spanish the word “surfactant” does not exist, 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). This would imply that surface activity is strictly equivalent to tension lowering, which is not absolutely general, although it is true in many cases.

Learning Outcomes

1.      To know the definition, chemistry, synthesis of surfactant

2.      To manufacture a surface active agent

3.      To select the correct surfactant for desired application

1.  AMPHIPHILES AND SURFACTANTS

1.1 Amphiphiles

The word amphiphile was coined by Paul Winsor 50 years ago. It comes from two Greek roots. First the prefix amphi which means “double”, “from both sides”, “around”, as in amphitheater or amphibian. Then the root philos which expresses friendship or affinity, as in “philanthropist” (the friend of man), “hydrophilic” (compatible with water), or “philosopher” (the friend of wisdom or science).

An amphiphilic substance exhibits a double affinity, which can be defined from the physico-chemical point of view as a polar-apolar duality. A typical amphiphilic molecule consists 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 general and hydrocarbon chain of the alkyl or alkylbenzene type, sometimes with halogen atoms and even a few nonionized oxygen atoms.

The polar portion exhibits a strong affinity for polar solvents, particularly water, and it is often called hydrophilic part or hydrophile. The apolar part is called hydrophobe or lipophile, from Greek roots phobos (fear) and lipos (grease). The following formula shows an amphiphilic molecule which is commonly used in shampoos (sodium dodecyl sulfate). Because of its dual affinity, an amphiphilic molecule does not feel “at ease” in any solvent, be it polar or non polar, since there is always one of the groups which “does not like” the solvent environment. This is why amphiphilic molecules exhibit a very strong tendency to migrate to interfaces or surfaces and to orientate so that the polar group lies in water and the apolar group is placed out of it, and eventually in oil.

In the following the word surface will be used to designate the limit between a condensed phase and a gas phase, whereas the term interface will be used for the boundary between two condensed phases. This distinction is handy though not necessary, and the two words are often used indifferently particularly in american terminology

Amphiphiles exhibit other properties than tension lowering and this is why they are often labeled according to their main use such as: soap, detergent, wetting agent, disperssant, emulsifier, foaming agent, bactericide, corrosion inhibitor, antistatic agent, etc… In some cases they are konwn from the name of the structure they are able to build, i.e. membrane, microemulsion, liquid crystal, liposome, vesicle or gel.

2. CLASSIFICATION 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 may arise from that. The most acepted and scientifically sound classification of surfactants is based on their dissociation in water.

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 most commonly 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 nondissociable type, such as alcohol, phenol, ether, ester, or amide. A large proportion of these nonionic 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 or alkylbenzene type, the former coming from fatty acids of natural origin. The polycondensation of propylene oxide produce a polyether which (in oposition to polyethylene oxide) is slightly hydrophobic. This polyether chain is used as the lipophilic group in the so-called polyEOpolyPO block copolymers, which are most often included in a different class, e.g. polymeric surfactants.

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 nitrogen compounds such as fatty amine salts and quaternary ammoniums, with one or several long chain of the alkyl type, often coming from natural fatty acids. These surfactants are in general more expensive than anionics, because of the high pressure hydrogenation reaction to be carried out during their synthesis. As a consequence, they are only used in two cases in which there is no cheaper substitute, i.e. (1) as bactericide, (2) as positively charged substance which is able to adsorb on negatively charged substrates to produce antistatic and hydrophobant effect, often of great commercial importance such as in corrosion inhibition.

When a single surfactant molecule exhibit both anionic and cationic dissociations it is called amphoteric or zwitterionic. This is the case of synthetic products like betaines or sulfobetaines and natural substances such as aminoacids and phospholipids.

Some amphoteric surfactants are insensitive to pH, whereas others are cationic at low pH and anionic at high pH, with an amphoteric behavior at intermediate pH. Amphoteric surfactants are generally quite expensive, and consequently, their use is limited to very special applications such as cosmetics where their high biological compatibility and low toxicity is of primary importance.

The past two decades have seen the introduction of a new class of surface active substance, so-called polymeric surfactants or surface active polymers, which result from the association of one or several macromolecular structures exhibiting hydrophilic and lipophilic characters, either as separated blocks or as grafts. They are now very commonly used in formulating products as different as cosmetics, paints, foodstuffs, and petroleum production additives.

3. RAW MATERIALS FOR SURFACTANTS

3.1Natural oil and fasts: Trigylcerides

Most oils and fats from animal or vegetable origin are triglycerides, i.e., trimesters of glycerol and fatty acids, as for instance the struture indicated in the following formula.

In some cases esterification is uncomplete, leading to mono and diglycerides. Some natural products include polyalcohols which are more complex than glycerol as for instance in C5 and C6 mono-sugar compounds. In all cases, the hydrolysis reaction allows the separation of the polyalcohol from the fatty acids.

3.2 Other natural substances

3.2.1 Wood oils

Some trees like pine and other conifer species contain esters of other carboxylic acids and glycerol (or other alcohols). They are called rosin oils and tall oils. It is worth noting that tall is not related with tallow, but with pine (in Swedish). During the wood disgestion to make pulp, most esters are hydrolized and the acids are released. In a typical conifer wood digestion, fatty acids accounts for about 50%, while other acids are more complex substances such as abietic acid and its derivatives.

3.2.2 Lignin and derivates

Lignin has been said to be the most common polymer on Earth. It accounts for approximately 30   % of dry wood weight. Lignin is a 3D polymer based on 3-hydroxy-4- methoxy-phenyl-propane (guayacyl, coniferyl and similar) units which can reach a high molecular weight (500,000-1,000,000). During wood digestion lignin is fragmented into small pieces and hydrophilic groups (-OH, -COOH, -SO3-) are produced to make it water soluble, particularly at the high pH (11-12) of the pulping licor. Lignin derivatives are polymeric surfactants of the grafted type. They are dispersants for solid particles, as in drilling fluids, among other uses.

3.3 RAW MATERIALS FROM PETROLEUM

Other sources of lipophilic materials such as petroleum refining were considered in order to lower the cost, particularly for detergents. A proper lipophilic group exhibits a hydrocarbon chain containing from 12 to 18 carbon atoms. Such substances are found in light cuts (gasoline and kerosene) coming from atmospheric distillation and catalytic cracking. It is also possible to make such a chain by polymerization of short chain olefin, particularly in C3 and C4.

3.3.1. ALKYLATES FOR ALKYLBENZENE PRODUCTION

After World War II catalytic cracking and reforming processes were developed to produce high octane gasoline. They essentially consist in breaking an alkane chain to produce an alfa-olefin and to reform molecules in a different way. Because of Markovnikov’s rule. The reformation happens with the attachment at the second carbon atom of the alfa-olefin, thus resulting in branching, which is the structural characteristic that confers a high octane number.

These plants were producing short chain olefins which had no use in the early 1950’s, particularly propylene, which was thus quite an inexpensive raw material to produce a surfactant lipophilic chain by polymerization. Because of the 3 carbon atoms difference between the n-mer and the n+1-mer, it is easy to separate by distillation the tetramer, with some amount of trimer and pentamer, to adjust the required chain length.

3.3.3. AROMATICS

Benzene, toluene and xylene are not found in crude oil. They come from dehydrogenation and dehydrocyclization reactions taking place in catalytic reforming and steam cracking plants.

The most valuable substance is benzene and there are several method to dealkylate toluene and xylene which are often carried out in the so-called BTX separation unit.

Benzene enters in the synthesis of the alkyl-benzene sulfonate, the most common surfactant in powdered detergents. It is also used in the synthesis of isopropyl benzene, which is an intermediate to produce both acetone and phenol by peroxidation. Alkyl phenols are synthesized by a Friedel-Craft reaction just as alkyl-benzene. In the 70’s and 80’s ethoxylated alkyl-phenols were the most popular surfactants for liquid dishwashing applications as well as many other. However, in the past few years, toxicity issues have cut down the production of such surfactants, which are likely be displaced by more environmentally friendly alcohol substitutes, although these later are not as good surfactants. Another surfactant application of alkyl-phenol is likely to stay around for a long time however. It is the production of ethoxylated phenol-formaldehyde resins, i.e. low MW bakelite type resins which are the current fashionable additives for crude oil dehydration.

4. ANIONIC SURFACTANTS

4.1 SOAPS AND OTHER CARBOXYLATES

Strictly speaking the term soap refers to a sodium or postassium salt of a fatty acid. By extension the acid may be any carboxylic acid, and the alkaline metal ion may be replaced by any metallic or organic cation.

4.1.1. SOAP MANUFACTURE

Soaps are prepared by saponification of triglycerides from vegetal or animal source. For instance with a triglyceride containing 3 stearic acid (C18:0) units, the reaction with sodium hydroxide produces 3 moles of sodium stearate and 1 mole of glycerol.

3 NaOH + (C17H35COO)3C3H5  à 3 C17H35COONa + CH2OH-CHOH-CH2OH

This type of reaction has been used for centuries to manufacture soap from palm oil, olive oil (from which the brand name “Palmolive”) etc…. and mostly from tallow. The current process takes place in two steps. First the triglyceride is hydrolyzed at high pressure (240 ºC, 40 atm.) with a ZnO catalyst, which is alkaline but not water soluble, and thus does not react with the acids. At the end of the hydrolysis, acids (oil phase) and glycerol (aqueous phase) are separated.

Acids are then distilled under vacuum to separate too short and too long species, to keep the proper cut (C10-C20) and fractionate it into its components, particularly the C12-C14 acids which are scarse and more valuable than their C16-C18 counterparts. This process allows to formulate soaps with the proper mixture of acids, and with the desired hydroxide.

5. NONIONIC SURFACTANTS

5.1. NONIONIC SURFACTANT TYPES

During the last 35 years, nonionic surfactants have increased their market share, to reach about 40 % of the total surfactant production worldwide.

Nonionic surfactants do not produce ions in aqueous solution. As a consequence, they are compatible with other types and are excellent candidates to enter complex mixtures, as found in many commercial products. They are much less sensitive to electrolytes, particularly divalent cations, than ionic surfactants, and can be used with high salinity or hard water. Nonionic surfactants are good detergents, wetting agents and emulsifiers. Some of them have good foaming properties. Some categories exhibit a very low toxicity level and are used in pharmaceuticals, cosmetics and food products.

Nonoionic surfactants are found today in a large variety of domestic and industrial products, such as powdered or liquid formulations. However the market is dominated by polyethoxylated products, i.e., those whose hydrophilic group is a polyethylenglycol chain produced by the polycondensation of ethylen oxide on a hydroxyle or amine group.

Depending on the relative acidity of the RXH molecule and its ethoxylated counterpart RX-CH2-CH2-OH, the polycondensation leads to a different result. However, in all cases the commercial product contains a mixture of different oligomers with a distribution of ethylene oxide number (EON), a characteristic which can be an advantage or a drawback.

At least 4-5 ethylene oxide groups are needed to insure a good solubility in water with a lipophilic group such as a C13 alkyl. However, for some applications, the ethoxylation degree can reach EON=20 and even 40.

It is worth noting that, though the polyethylene-oxide chain is globally hydrophilic, each EO group contains 2 methylene (-CH2-) units which are hydrophobic. This duality becomes evident when it is known that the polypropylene-oxide chain, i.e. the three carbon atoms counterpart, is globally hydrophobic. It can be said that the hydrophilicity conferred by the oxygen atom is thus compensated by approximately 2.5 methylene groups.

This remark is quite important, because it clearly indicates that the polyEO hydrophilic group is not an extremely hydrophilic group, a characteristic that explains why this kind of surfactant is soluble in organic solvents. Moreover, any change in formulation or temperature that affects the interaction between the polyEO chain and the water/oil physicochemical environment is likely to affect the behavior of this kind of surfactant.

6. CATIONIC SURFACTANTS

Cationic surfactants account for only 5-6% of the total surfactant production. However, they are extremely useful for some specific uses, because of their peculiar properties.

 They are not good detergents nor foaming agents, and they cannot be mixed in formulations which contain anionic surfactants, with the exception of non quaternary nitrogenated compounds, or when a cationic complex synergetic action is sought. Nevertheless, they exhibit two very important features.

First, their positive charge allows them to adsorb on negatively charged substrates, as most solid surfaces are at neutral pH. This capacity confers to them an antistatic bahavior and a softening action for fabric and hair rinsing. The positive charge enables them to operate as floatation collectors, hydrophobating agents, corrosion inhibitors as well as solid particle dispersant. They are used as emulsifiers in asphaltic emulsions and coatings in general, in inks, wood pulp dispersions, magnetic slurry etc.

On the other hand, many cationic surfactants are bactericides. They are used to clean and aseptize surgery hardware, to formulate heavy duty desinfectants for domestic and hospital use, and to sterilize food bottle or containers, particularly in the dairy and beverage industries.

6.1 USES OF CATIONIC SURFACTANTS

It can be concluded from the previous sections that the attainment of an amine o alkyl ammonium surfactant requires a chain of chemical reactions which are more or less selective and not necessarily complete. Consequently, only a small part of the original raw material ends up as the desired product. This is why cationic surfactants are in general more expensive than anionics such as sulfonates or sulfates. Hence, cationic surfactants are used only in applications in which they cannot be substituted by other surfactants, i.e. those which require a positive charge or a bactericide action.

They are found as antistatic agents in fabric softeners and hair rinse formulas. They are used in textile manufacturing to delay dye adsorption. In this application they compete with dye and thus slow down their adsorption and help attaining an uniform coloration. Their action as corrosion inhibitor in acid environment is similar, but in this case they compete with H+ ions. Collectors for mineral floatation are often ammonium salts or quats. Asphalts emulsions for roadway pavement and protective coatings and paints are often stabilized by fatty amine salts (at acid pH) or quats (at neutral pH).

Benzalkonium and alkyltrimethyl ammonium chloride or bromide is used as antiseptic agents, desinfectants and sterilizing agents. They are also incorporated as additive in nonionicdetergents formulation for corrosion inhibition purposes, and (in very small quantity) in anionic powdered formulas to synergize detergency.

7 SUMMARY

The surface active agent plays a vital role in textile industry. To modify the characteristics of any textile material (fibre, yarn, fabric, garment), we should treat the surface. For this, we should activate the surface first by using surface active agents. This simply reduces the surface tension (interfacial tension) of the material, which leads to more interaction with water or other chemicals.

REFERENCES and URLs

1. EIRI consultants and engineers,“Textile auxiliaries and chemicals”, Engineers India Research Institute, 2007.
2. Derek Heywood, “Textile Finishing”, Society of Dyers and Colourists, 2003.
3. Kronberg, B., Holmberg, K. and Lindman, B. (2014) Surface Active Polymers, in Surface Chemistry of Surfactants and Polymers, John Wiley & Sons, Ltd, Chichester, UK. doi: 10.1002/9781118695968.ch10
4. Sisley, Jean Paul and Wood, P. J Encyclopedia of surface-active agents. Chemical Pub. Co, New York, 1952.
5. Anthony M.; Perry, James W. And Berch, Julian Schwartz, “ SURFACE ACTIVE AGENTS: Their Chemistry and Technology” , Published by Interscience (1958)
6. https://knowledge.ulprospector.com/3106/pc-surface-active-agents-surfactants/