Surfactants

It is extremely important to avoid the discharge of any surfactants into swimming pool water.

The surfactants may react directly with hypochlorite to form trichloramines and THM`s, however the affect the surfactants have on the filtration system performance is potentially very serious.

In order for sand or AFM filters to remove cryptosporidium oocysts, bacteria and dissolved organic matter you require good coagulation and flocculation. The principle chemicals used include PAC, polyelectrolytes and NoPhos. However surfactant chemicals will react with the coagulents, in effect the coagulants will remove the surfactants from solution. If there are sufficient surfactants in the water the performance of the filters will be compromised and the public are placed at risk. The concentration of chlorine reactions products such as THM`s will increase exponentially, bacteria and oocysts of cryptosporidium will pass through the filtration systems. It is therefore absolutely essential that no cleaning agents, especially surfactant chemicals be allowed to enter the swimming pool water.

Surfactants are chemicals that reduce the surface tension of water to help with cleaning. Surfactant chemicals may be used directly but most applications require their mixing (formulation) with other chemicals and solvents to form surfactant preparations.

Anionic surfactants

The most common is linear alkylbenzenesulfonate made from alkyl (especially dodecyl) benzene. Other forms include alkyl sulfates and alkyl ether sulfates using fatty alcohols and other imported hydrocarbons.

Non-ionic surfactants

Non-ionic surfactants are usually produced by reacting ethylene oxide with fatty alcohols, alkyl phenol, amines and/or other chemicals to produce a broad range of surface active chemicals.

Cationic surfactants

Most cationic surfactants are quaternary ammonium salts used in domestic application (shampoos etc.). They are made from chemicals such as tertiary amines and an alkylating agent such as benzyl chloride.

Surfactants

Surfactants have low solubility in water. A typical surfactant has a large lipophile (hydrophobe) which restricts its aqueous solubility. For example, sodium dodecylbenzene sulphonate has a solubility maximum of 0.04.mol l^–1. Beyond this concentration the molecules associate to form colloidal aggregates known as micelles. This concentration is the critical micelle concentration (CMC). Different surfactants have different CMC values.

In a micelle the surfactant orients itself with its lipophiles towards the interior, thus presenting a hydrophilic surface to the water. The simplest micelles are spheres but as surfactant concentration increases the micelles grow and form rods.

At high surfactant concentrations the rods form larger structures such as hexagonally packed rods and palisade arrangements. As these structures increase in size they take on a greater degree of order until, for the biggest structures, they occur as liquid crystals. These structural changes are reflected in the viscosity of the surfactant ‘solution’.

Rheology
The flow characteristics of the surfactant in water is an important feature of many surfactant formulations. For surfactant systems these characteristics are explained in terms of the micelles. Where unassociated molecules are present the viscosity is virtually that of water.

The formation of spherical micelles has little effect, but where the sphere-to-rod changes occur there is a marked increase in viscosity. The hexagonal packing of rods causes viscosity to climb further but, surprisingly, the formation of the largest palisade structures results in a dramatic fall in viscosity.

The latter effect is due to the large layer-like structures being able to slide over each other so that there is little intermolecular friction. In surfactant technology the micelle structure can be manipulated to give a product with the desired properties such as a shampoo gel rather than a runny liquid.

Solubilisation
Micelles can act as solubilisers in which oily molecules of oil/grease/hydrocarbon are taken into the lipophilic core of the micelle and retained by the lipophile-to-lipophile attractive forces. By this means it is possible to make colloidal emulsions or microemulsions –Ťsometimes called swollen micelles. An important application is to make a solvent such as a hydrocarbon ‘dissolve’ in water and surfactant.

Detergency
Washing dirty dishes or clothes involves a complex mechanism comprising many physical and chemical effects resulting from a variety of soil types and a range of substrate materials. However, the most important cleaning action is a result of surface chemistry and surfactants. Detergent action to remove oily/greasy soiling involves wetting, emulsification, solubilisation and micelles.

Emulsification
Adsorption at the water–oil interface results in dispersion of one phase into the other depending on the properties of the system. These dispersions are emulsions and of two types: oil-in-water (o/w) or water-in-oil (w/o).

Wetting
Water is not attracted on an oily/greasy surface, but with a surfactant present wetting occurs because the molecules adsorb at the oil–water interface in a manner similar to that seen for emulsification.

Foaming
A foam is a dispersion of gas in liquid, generally air in water, where there is only a small volume of liquid compared with the large volume of gas. Each gas space, or cell, has walls made up of a thin layer of water with surfactant molecules adsorbed at the surfaces. Adsorption of a suitable surfactant creates a foam that gets its mechanical stability from surface elasticity and just the right amount of drainage in the water between the surfaces of the film.

references.

1. Chemlink

2. Chemical Society

Surfactants

Further reading