Tuesday 24 November 2009

Surfactants: Surface Active Agents

The information on the properties of water previously, provides background for a discussion of the properties of surfactants. A surfactant is briefly defined as a material that can greatly reduce the surface tension of water when used in very low concentrations. Table Two shows that Softanol 90 reduces the surface tension of water from 73 to 30 dynes per centimetre when used at a concentration of 0.005 percent. Ethanol when used at a concentration of 20 percent, however, only reduced tension of water to 38 dynes per centimetre.

Relationship of Surface Tension and Concentration

Table Two
Percent Concentration required to reduce the surface tension of water to indicated values
Surface tension, dynes per cm 73 50 40 30 22
Softanol 90 0 0.003 0.0008 0.0050 ---
Ethanol 0 9 18 40 100

Schematic Sketch of Surfactant Molecule
A particular type of molecular structure performs as a surfactant. This molecule is made up of a water soluble (hydrophilic) and a water insoluble (hydrophobic) component (Figure Two).

Figure Two Schematic Sketch of Surfactant Molecule

The hydrophobe is usually the equivalent of an 8 to 18 carbon hydrocarbon, and can be aliphatic, aromatic, or a mixture of both. The sources of hydrophobes are normally natural fats and oils, petroleum fractions, relatively short synthetic polymers, or relatively high molecular weight synthetic alcohols. The hydrophilic groups give the primary classification to surfactants, and are anionic, cationic and nonionic in nature. The anionic hydrophiles are the carboxylates (soaps), sulphates, sulphonates and phosphates. The cationic hydrophiles are some form of an amine product. The nonionic hydrophiles associate with water at the ether oxygens of apolyethylene glycol chain polyethylene glycol chain (Figure Three). In each case, the hydrophilic end of the surfactant is strongly attracted to the water molecules and the force of attraction between the hydrophobe and water is only slight. As a result, the surfactant molecules align themselves at the surface and Schematic Sketch of Surfactant Molecules in Waterinternally so that the hydrophile end is toward the water and the hydrophobe is squeezed away from the water (Figure Four).

Figure Three Polyethylene Glycol Chain

Figure Four Schematic Sketch of Surfactant Molecules in Water

This internal group of surfactant molecules is referred to as a micelle (m).

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Because of this characteristic behaviour of surfactants to orient at surfaces and to form micelles, all surfactants perform certain basic functions. However, each surfactant excels in certain functions and has others in which it is deficient.

Foaming agents, emulsifiers, and dispersants are surfactants which suspend respectively, a gas, an immiscible liquid, or a solid in water or some other liquid. Although there is similarity in these functions, in practice the surfactants required to perform these functions differ widely. In emulsification, as an example - the selection of surfactant or surfactant system will depend on the materials to be used and the properties desired in the end product. An emulsion can be either oil droplets suspended in water, an oil in water (O/W) emulsion, water suspended in a continuous oil phase, water in oil (W/O) emulsion, or a mixed emulsion. Selection of surfactants, orders of addition and relative amounts of the two phases determine the class of emulsion.

Each of these three functions is related to the surfactant absorbing at a surface, either gas, liquid or solid with the hydrophilic ends of the molecules oriented to the water phase. The surfactants form what amounts to a protective coating around the suspended material, and these hydrophilic ends associate with the neighbouring water molecules. In addition to surfactant effects the stability of these suspensions is related to the particle size and density of the suspended material.

Solubilisation is a function closely related to emulsification. As the size of the emulsified droplet becomes smaller, a condition is reached where this droplet and the surfactant micelle are the same size.

At this stage, an oil droplet can be imagined as being in solution in the hydrophobic tails of the surfactant and the term solubilisation is used. Emulsions are milky in appearance and solubilised oils, for example - are clear to the eye.

The Function of Detergency

The function of detergency or cleaning is a complex combination of all the previous functions.Simplified Illustration of Detergency The surface to be cleaned and the soil to be removed must initially be wet and the soils suspended, solubilised, dissolved or separated in some way so that the soil will not just re-deposit on the surface in question (Figure Five).

Figure Five Simplified Illustration of Detergency

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Surfactant

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Surfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids.

Contents

Etymology

The term surfactant is a blend of surface active agent[1]. Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their "tails") and hydrophilic groups (their "heads"). Therefore, they are soluble in both organic solvents and water. The term surfactant was coined by Antara products in 1950.

In Index Medicus and the United States National Library of Medicine, "surfactant" is reserved for the meaning pulmonary surfactant. For the more general meaning, "surface active agent" is the heading.

Properties

A micelle—the lipophilic tails of the surfactant molecules remain on the inside of the micelle due to unfavourable interactions. The polar "heads" of the micelle, due to favourable interactions with water, form a hydrophilic outer layer that in effect protects the hydrophobic core of the micelle. The compounds that make up a micelle are typically amphiphilic in nature, meaning that not only are micelles soluble in protic solvents such as water but also in aprotic solvents as a reverse micelle

Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. They also reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface. Many surfactants can also assemble in the bulk solution into aggregates. Examples of such aggregates are vesicles and micelles. The concentration at which surfactants begin to form micelles is known as the critical micelle concentration or CMC. When micelles form in water, their tails form a core that can encapsulate an oil droplet, and their (ionic/polar) heads form an outer shell that maintains favorable contact with water. When surfactants assemble in oil, the aggregate is referred to as a reverse micelle. In a reverse micelle, the heads are in the core and the tails maintain favorable contact with oil. Surfactants are also often classified into four primary groups; anionic, cationic, non-ionic, and zwitterionic (dual charge).

Thermodynamics of the surfactant systems are of great importance, theoretically and practically. This is because surfactant systems represent systems between ordered and disordered states of matter. Surfactant solutions may contain an ordered phase (micelles) and a disordered phase (free surfactant molecules and/or ions in the solution).

Ordinary washing up (dishwashing) detergent, for example, will promote water penetration in soil, but the effect would only last a few days (many standard laundry detergent powders contain levels of chemicals such as sodium and boron, which can be damaging to plants and should not be applied to soils). Commercial soil wetting agents will continue to work for a considerable period, but they will eventually be degraded by soil micro-organisms. Some can, however, interfere with the life-cycles of some aquatic organisms, so care should be taken to prevent run-off of these products into streams, and excess product should not be washed down.

Applications and sources

Surfactants play an important role in many practical applications and products, including:

Pulmonary surfactants are also naturally secreted by type II cells of the lung alveoli in mammals.

[edit] Classification

A surfactant can be classified by the presence of formally charged groups in its head. A non-ionic surfactant has no charge groups in its head. The head of an ionic surfactant carries a net charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic.

Some commonly encountered surfactants of each type include:

Health and environmental controversy

Some surfactants are known to be toxic to animals, ecosystems and humans, and can increase the diffusion of other environmental contaminants.[2][3][4] Despite this, they are routinely deposited in numerous ways on land and into water systems, whether as part of an intended process or as industrial and household waste. Some surfactants have proposed or voluntary restrictions on their use. For example, PFOS is slated for persistent organic pollutant (POP) status by the Stockholm Convention.[5] Additionally, PFOA has been subject to a voluntary agreement by the U.S. Environmental Protection Agency‎ and eight chemical companies to reduce and eliminate emissions of the chemical and its precursors.[6] However, other industries operate outside of the voluntary PFOA program.[7]

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