The Structure And Properties Of Surfactants

Surfactants are compounds that lower the surface tension of a liquid, allowing easier spreading, and lowering of the interfacial tension between two liquids, or between a liquid and a solid. Surfactants may act as:

Detergents

Wetting agents

Emulsifiers

Foaming agents

Dispersants

STRUCTURE OF SURFACTANT

Figure 1Surfactant molecule possess a dualistic character.This arises from the combination of a hydrophobic (water-rejecting) and a hydrophilic (water-preferring) group in one molecule.In a classical surfactant,the hydrophobic part usually consists of one or more hydrocarbon chains sometimes with various degrees of unsaturation and branching.However,the apolar part may also be partly or completely fluorinated or may be composed of a siloxane chain.The size and length of the hydrocarbon may vary considerably,but it must consists of atleast 8 carbon atoms.For the hydrophilic part(usually called the headgroup),there is a wide range of possible structures(Table1.1).The hydrophilic part can either be ionic or dipolar depending on whether the headgroup has a net charge or not. [1]

PROPERTIES OF SURFACTANTS

They enable the cleaning solution to fully wet the surface being cleaned so that dirt can be readily loosened and removed.

They clean greasy, oily, particulate-, protein-, and carbohydrate-based stains.

They are instrumental in removing dirt and in keeping them emulsified, suspended, and dispersed so they don’t settle back onto the surface being cleaned.

Surfactants are one of the major components of cleaning products and can be regarded as the ‘workhorses’: they do the basic work of breaking up stains and keeping the dirt in the water solution to prevent re-deposition of the dirt onto the surface from which it has just been removed. Surfactants disperse dirt that normally does not dissolve in water.

As anyone who uses oil based dressings in the kitchen knows, oil and water do not mix unless shaken vigorously in the bottle. They separate almost immediately afterwards. The same is true when washing your dishes or clothes. With the addition of surfactants, oil, which normally does not dissolve in water, becomes dispersible and can be removed with the wash water.

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CLASSIFICATION OF SURFACTANTS

It is generally classified in 4 basic types depending upon the nature of the head group:

Anionic

Non anionic

Cationic

Amphoteric or zwitter-ionic

Anionic Surfactants

In solution, the head is negatively charged. This is the most widely used type of surfactant for laundering, dishwashing liquids and shampoos because of its excellent cleaning properties and high sudsing potential. The surfactant is particularly good at keeping the dirt away from fabrics, and removing residues of fabric softener from fabrics. Anionic surfactants are particularly effective at oily soil cleaning and oil/clay soil suspension. Still, they can react in the wash water with the positively charged water hardness ions (calcium and magnesium), which can lead to partial deactivation. The more calcium and magnesium molecules in the water, the more the anionic surfactant system suffers from deactivation. To prevent this, the anionic surfactants need help from other ingredients such as builders (Ca/Mg sequestrants) and more detergent should be dosed in hard water.

The most commonly used anionic surfactants are alkyl sulphates, alkyl ethoxylate sulphates and soaps.

Non anionic Surfactants

These surfactants do not have an electrical charge, which makes them resistant to water hardness deactivation. They are excellent grease removers that are used in laundry products, household cleaners and hand dishwashing liquids.

Most laundry detergents contain both non-ionic and anionic surfactants as they complement each other’s cleaning action. Non-ionic surfactants contribute to making the surfactant system less hardness sensitive.

The most commonly used non-ionic surfactants are ethers of fatty alcohols

Cationic Surfactants

In solution, the head is positively charged. There are 3 different categories of cationics each with their specific application:

In fabric softeners and in detergents with built-in fabric softener, cationic surfactants provide softness. Their main use in laundry products is in rinse added fabric softeners, such as esterquats, one of the most widely used cationic surfactants in rinse added fabric softeners.

An example of cationic surfactants is the esterquat.

In laundry detergents, cationic surfactants (positive charge) improve the packing of anionic surfactant molecules (negative charge) at the stain/water interface. This helps to reduce the dirtl/water interfacial tension in a very efficient way, leading to a more robust dirt removal system. They are especially efficient at removing greasy stains.

An example of a cationic surfactant used in this category is the mono alkyl quaternary system.

In household and bathroom cleaners, cationic surfactants contribute to the disinfecting/sanitizing properties.

Amphoteric Surfactants

These surfactants are very mild, making them particularly suited for use in personal care and household cleaning products. They can be anionic (negatively charged), cationic (positively charged) or non-ionic (no charge) in solution, depending on the acidity or pH of the water.

They are compatible with all other classes of surfactants and are soluble and effective in the presence of high concentrations of electrolytes, acids and alkalis.

These surfactants may contain two charged groups of different sign. Whereas the positive charge is almost always ammonium, the source of the negative charge may vary (carboxylate, sulphate, sulphonate). These surfactants have excellent dermatological properties. They are frequently used in shampoos and other cosmetic products, and also in hand dishwashing liquids because of their high foaming properties.

An example of an amphoteric/zwitterionic surfactant is alkyl betaine.

Figure 2

Figure 3

SURFACE TENSION

Water has many unusual properties as a result of its ability to hydrogen bond. For example, the density of ice is less than that of the liquid and the predicted boiling point is almost 200 degrees C higher than it would be without hydrogen bonding.

The water molecules at the surface of water are surrounded partially by air and partially by water. These surface molecules would be much more stable if they could be in the interior of the liquid where all their hydrogen bonds could be fulfilled (cohesion). Therefore, water normally tends to have the smallest surface possible, i.e. it has a high surface tension, in order to achieve the lowest possible energetic state.

If a solid material more dense than water is placed on the surface of water, what happens next depends on the nature of the material. If the material is hydrophilic (“water loving”) it has a surface to which water is attracted. The adhesion of water to the surface of this material coats the surface of the object with water, reduces the surface tension, and causes the object to sink.

If the solid object is hydrophobic (“water fearing”),the unfavorable interactions between the water surface and the object make it difficult to wet the surface. Two forces now come into play — the energy it would take to overcome this repulsion and the force of gravity. If the force of gravity is strong enough, it will prevail and the object will sink (assuming that the object has a density greater than water). If the gravitational force is less than the surface tension then the object will float on the surface of the water.

Surface tension is what permits water striders and other insects to walk across the surface of water and what enables a needle to float. Of course, the critical feature here is the amount of force per unit area — put a needle into water end-on instead sideways and the needle will immediately sink.

SURFACTANT FUNCTION

Surfactants are also referred to as wetting agents and foamers. Surfactants lower the surface tension of the medium in which it is dissolved. By lowering this interfacial tension between two media or interfaces (e.g. air/water, water/stain, stain/fabric) the surfactant plays a key role in the removal and suspension of dirt. The lower surface tension of the water makes it easier to lift dirt and grease off of dirty dishes, clothes and other surfaces, and help to keep them suspended in the dirty water. The water-loving or hydrophilic head remains in the water and it pulls the stains towards the water, away from the fabric. The surfactant molecules surround the stain particles, break them up and force them away from the surface of the fabric. They then suspend the stain particles in the wash water to remove them.

Surfactants can work in three different ways: roll-up, emulsification, and solubilization.

Roll-up mechanism

The surfactant lowers the oil/solution and fabric/solution interfacial tensions and in this way lifts the stain of the fabric.

Figure 3

Emulsification

The surfactant lowers the oil-solution interfacial tension and makes easy emulsification of the oily soils possible.

Figure 4

Solubilization

Through interaction with the micelles of a surfactant in a solvent (water), a substance spontaneously dissolves to form a stable and clear solution

How can surfactants prevent dirt from being re-deposited?

Surfactants have a vital role to play in preventing the re-deposition of soils like greasy, oily stains and particulate dirt on the surface or fabric from which they have just been removed. This works by electrostatic interactions and steric hindrance.

Electrostatic interactions

Anionic surfactants are adsorbed on both the surface of the dirt which is dispersed in the detergent solution, and the fabric surface. This creates a negative charge on both surfaces, causing electrostatic repulsions. This repulsion prevents the soil from re-depositing on the fabric.

In the presence of hardness, however, this mechanism acts like a ‘bridge’ between the suspended soil and the fabric. This is another reason why hardness sequestrants (a chemical that promotes Ca/Mg sequestration) are often used in detergents.

Steric hindrance:

Non-ionic surfactants like alcohol ethoxylates also adsorb on the dirt. Their long ethoxylated chains extend in the water phase and prevent the dirt droplets or particles from uniting and from depositing onto the fabric surface.Dirt is present in solution. The non-ionic surfactants adsorb to the dirt particles.Their long hydrophilic heads extend in the water phase and as a result prevent the dirt particles/droplets from uniting and from re-depositing onto fabrics. [2]

SURFACTANT USES

There are many uses of surfactant in different industries and different fields.

Some of them are discussed below:

Use of Surfactant in Detergents

Surfactants are literally “Surface Acting Agents”. They are called this because they act to reduce the surface tension of a liquid, especially water. They are large molecules with two distinct parts. First there is a head which is hydrophilic. This means that it is attracted to water and soluble in water, usually because it has a positive or negative charge. The other part of the surfactant is the tail which is hydrophobic, meaning it is repelled by water. The tail is also “lipophilic” which means that it is soluble in organic solvents particularly oils and fats or lipids.

It is this mixed structure which gives a surfactant its properties. When added to water the surface tension of water is reduced. The surface tension of water is caused by the hydrogen bonds which form between the slight charges on different parts of the water molecule (for further information see the water article). Surfactants break up these hydrogen bonds by remaining at the surface, their heads dissolved in the water but their tails extending out of the water, repelled by it. When the surfactant concentration increases sufficiently, micelles are formed. These are spheres of surfactant with all the heads on the outside protecting all the tails within.[3]

Use of Surfactant in Cosmetics

Shampoos and soaps clean by the use of surfactants (surface active agents). Surfactant molecules have both fat soluble (lipophilic) and water-soluble (hydrophylic) parts. The lipophilic part of the molecule sticks to oil and dirt, and the hydrophilic part allows water to then carry away the otherwise water-insoluble grime. Washing-up detergents work in the same way, although it isn’t generally advisable to wash your hair with dishwashing liquid – they are formulated to remove thick grease from plates, not to gently clean your hair! [4]

Use of Surfactants to remove solvent-based inks from plastic films

There is substantial economic and environmental incentive to remove the ink (deink) from heavily printed plastic films so that the plastic can be reused to produce clear films. In this study, polyethylene films printed with solvent-based ink were deinked using surfactants under a variety of conditions. Water without surfactant does not deink the solvent-based ink from plastic films over a pH range of 3 to 12. In common with earlier studies of water-based inks, it is found that solutions of cationic surfactants are the most effective for deinking of solvent-based ink but a pH of at least 11 is required. Presoaking of plastic film in aqueous solution of cationic surfactant increases the level of deinking. Limited studies performed with a pilot-size paper deinking apparatus on solvent-based ink removal indicates that the deinking of plastic film using surfactant solutions is technically feasible. [5]

Pulmonary Surfactants and Therapeutic uses, including Pulmonary Lavage

The present invention discloses useful surfactant molecules including polypeptides, proteins, and a variety of other organic molecules, as well as methods of making and using same. Surfactant compositions, including liposomal surfactant compositions, are also disclosed. Use of the surfactant molecules of the present invention in pulmonary lavage procedures for a variety of therapeutic applications is also disclosed, including the treatment of respiratory distress syndrome; the removal of inflammatory exudate from inflamed lung tissues; and the treament of meconium aspiration syndrome in infants. [6]

Adsorption of non-petroleum base Surfactant on reservoir rock

SURFACTANT molecules adsorb well to solid interfaces such as the porous media found in petroleum reservoirs.The adsorbed surfactant layer represents both an additional resistance to flow as well as loss of surfactant properties and is therefore, of fundamental importance in the enhanced oil recovery (EOR) process that involves the flow of surfactant solution through porous media.According to Austad and Milter1, chemical flooding of oil reservoirs is one of the most successful methods to enhance oil recovery from depleted reservoirs at low pressure. [7]

Use of Surfactant in Neonates

Surfactant replacement therapy has become an established treatment for neonatal respiratory distress syndrome (RDS). This article reviews the current evidence for the various practices regarding the use of surfactant in this condition, looking at surfactant type, timing of the first dose, size and frequency of the dose and the need for further doses. As the use of surfactant is expanding into other lung diseases in adults as well as children, we also review those neonatal conditions where surfactant may be of benefit and summarize the evidence published to date supporting its use in these conditions. [8]

Surfactant as Antifog

Many of the cheaper anti-fogs are made of surfactants like glycerin (soap) and water or alcohol combinations. That works for a very short period. Better anti-fogs usually contain advanced silicones but suffer from poor spreading. You have to rub them in and wipe off excess. [9]

Surfactant as Fabric Softner

Fabric softeners have long been used to soften fabric and prevent static cling. Available in dryer sheets or liquid form, fabric softeners are made up of surfactants or “surface acting agents.” It is these chemicals that create a softer, fluffier feel to your laundry.

Surfactants contain chemicals with lubricating properties. These chemicals coat fabrics with a waxy film that “lubricates” them, causing them to feel smoother or fluffier. It’s suggested that these lubricating chemicals also make ironing easier and reduce drying time and wrinkling.

The lubricating properties of surfactants are a result of positive charges affecting negative charges. Surfactants are generally acidic and made up of positively charged particles. These positively charged particles attract the negatively charged particles within scratchy fabric. The negative charge is neutralized, creating a lower frictional resistance. Thus, there is less static cling and the fabric feels softer and less scratchy to touch. [10]

Surfactant in Ski Waxes

Once the ski is in motion, you are applying pressure and exerting friction, melting the snow, and creating a fine layer of water between your ski base and the snow. Control and maneuverability in skiing is derived from altering the structure of this water. Hertel wax systems perform a special process using an encapsulation process; tiny bits of surface-active agents formulated into the waxes interact with water, decreasing surface tension and friction, plus increasing control. The wax breaks up the water (snow) resulting in easier running, better control, added safety, and more fun. When commanded to turn, skis slide with ease. [11]

Effect of oil soluble Surfactant in emulsions stabilised by clay particles

Although surfactants and particles are often mixed together in emulsions, the contribution of each species to the stabilisation of the oil-water interface is poorly understood. We report the results of investigations into the formation of emulsions from solutions of surfactant in oil and aqueous suspensions of laponite. Depending on the salt concentration in the aqueous suspensions, the laponite dispersed as individual disc-shaped particles, 30 nm in diameter, or flocculated into aggregates tens of micrometres in diameter. At the concentrations studied, the flocculated particles alone stabilized oil-in-water emulsions. Synergistic interactions between the particles and octadecylamine at the oil-water interface reduced the average emulsion drop size, while antagonistic interactions with octadecanoic acid enhanced coalescence processes in the emulsions. The state of particle dispersion had dramatic effects on the emulsions formed. Measurements of the oil-water interfacial tension revealed the origins of the interactions between the surfactants and particles. [12]

Surfactant in process for deinking of recycled paper

A process for deinking recycled paper using a pressurized deinking module to separate ink from paper pulp stock. The addition of salts of imidazolinium based compounds with alkyl, alkenyl and amidoethyl side chains to the pulp slurry at the beginning of the pressurized deinking module cycle removes ink more effectively and results in a brighter recycled paper and an increase in yield of final paper stock. [13]

Surfactant in Spermicides (NONOCYNOL-9)

The most common active ingredient of spermicides is Nonoxynol 9. Spermicides containing Nonoxynol-9 are available in many forms, such as jelly (gel), films, and foams. Nonoxynol-9, sometimes abbreviated as N-9, is an organic compound that is used as a surfactant. It is a member of the nonoxynol family of nonionic surfactants. N-9 and related compounds are ingredients in various cleaning and cosmetic products. Its use is controversial because it may cause genital lesions.

Nonoxynol-9’s ability to kill microbes in vitro was initially taken as evidence that it might be effective at preventing STD transmission. However, more recent findings indicate that it may actually increase a person’s risk of contracting STDs, especially if used frequently. This is because the chemical causes tiny abrasions inside the sensitive vaginal and anal walls. [14]

Surfactant in Ferro Fluid:Magnetic Liquid Technology

A ferrofluid is a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier. The particles, which have an average size of about 100Å (10 nm), are coated with a stabilizing dispersing agent (surfactant) which prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid. The surfactant must be matched to the carrier type and must overcome the attractive van der Waals and magnetic forces between the particles. The colloid and thermal stabilities, crucial to many applications, are greatly influenced by the choice of the surfactant. A typical ferrofluid may contain by volume 5% magnetic solid, 10% surfactant and 85% carrier. [15]

Surfactant as Alkali Surfactant Polymers

In the Alkaline Surfactant Polymer (ASP) process, a very low concentration of the surfactant is used to achieve ultra low interfacial tension between the trapped oil and the injection fluid/formation water. The ultra low interfacial tension also allows the alkali present in the injection fluid to penetrate deeply into the formation and contact the trapped oil globules. The alkali then reacts with the acidic components in the crude oil to form additional surfactant in-situ, thus, continuously providing ultra low interfacial tension and freeing the trapped oil. In the ASP Process, polymer is used to increase the viscosity of the injection fluid, to minimize channeling, and provide mobility control.

Oil Chem Technologies’s patented ORS and ORS-HF series surfactants are specifically formulated for each field to optimize the oil recovery. Crude oil characteristics, brine characteristics, bottom hole temperature, alkali, well history, and treatment design are considered to maximize the treatment results. [16]

Other applications of surfactants are:

In biochemistry, the practical as well as theoretical importance of surfactants may be illustrated with the following examples: Surfactants have allowed the investigation of molecular properties of membrane proteins and lipoproteins, acting as solubilizing agents and as probes for hydrophobic binding sites. The properties of surfactants, as well as further facts relevant to the particular experiments, must be carefully considered. Surfactants have successfully contributed to the purification of receptors in their active forms, such as the neuropeptide receptors and opiate receptors. All holoreceptor- complex and reaction- center isolations require the use of a surfactant in order to separate the integral protein systems from the rest of the membrane.

Surfactants have been used in the investigation of the denaturation of bacteriorhodopsin and in thermal stability experiments of rhodopsin.

The operations of exchange and removal of surfactants bound to membrane proteins are crucial and have been successfully applied to a wide variety of these proteins.

The effects of surfactants on the function of membrane-bound enzymes such as cytochrome P-450 and (Na+ + K+)-ATPase have also been determined.

Integral membrane proteins can be separated from hydrophilic proteins and identified as such in crude surfactant extracts of membrane or cells .

Methods for the solubilization of low-density lipoproteins have advanced the understanding of the assembly, interconversion and molecular exchange processes with plasma lipoproteins.

In electrophoresis, various techniques require the use of surfactants. The popular techniques of SDS-PAGE for the identification and subunit molecular weight estimation of proteins is based on a specific type of surfactant-protein interaction. 2D-PAGE uses SDS in one direction and Triton X-100 in the other. This technique has been used to identify proteins containing long hydrophobic regions and relies on the different binding ability of non-ionic surfactants to water-soluble and intrinsic membrane proteins. Isoelectric focusing, native electrophoresis and blotting are other electrophoretic techniques which may need surfactants for the solubilization or transfer of membrane proteins.

In high performance liquid chromatography, common techniques such as ion-exchange HPLC, reversed-phase HPLC and sizeexclusion-HPLC may require surfactants to solubilize membrane proteins. Ionpair HPLC requires surfactants as reagents in order to achieve the separation conditions (ionpairing).

Affinity surfactants have been used as reversibly bound ligands in high performance affinity chromatography.

Crystallization of membrane proteins was achieved using short chain surfactants, which are believed to shield the hydrophobic intermembrane part of the molecule. Thus the polar interactions betvveen individual molecules come into play, providing the stabilizing force in crystallization.

Surfactants are also employed to promote the dissociation of proteins from nucleic acids on extraction from biological material.

Further applications of surfactants in biochemistry are the solubilization of enzymes in apolar solvents via reversed micelles and the isolation of hydrophobic proteins. [17]

CAPTION TO FIGURES:

Figure 1-http://www.scienceinthebox.com/en_UK/glossary/popup-roll-up-mechanism.html

Figure 2-http://www.scienceinthebox.com/en_UK/glossary/popup-emulsification.html

Figure 3-http://www.caes.uga.edu/applications/publications/files/html/B1319/images/Figure%202.jpg

BOOK REFERREDFigure 4-http://www.inkline.gr/inkjet/newtech/tech/dispersion/surfactants.gif

APPLIED SURFACTANTS:PRINCIPLES AND APPLICATIONS

-BY THARWAT F. TADROS

WILEY-WCH PUBLICATION