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Surfactants..

Posted by andriantoangkadirjo85 on May 12, 2011

Hello!

This time I choose this topics, SURFACTANTS.., because this is a really fascinating topic and while in laboratory, it is really amusing to do activity with them. In cosmetic, surfactants are really the cornerstone of nearly all formulations.. to have a great formulation, we have to know the chemistry of surfactants. In industry, surfactant is multi-billion pounds business (Figure 1) with markets everywhere from household detergents to explosives (Table 1).

Fig. 1 The global surfactant market

Table 1. The major surfactant markets

Value (£m)

 Quantity (kt)

Household detergents

2800

4000

Industrial and institutional cleaning

420

530

Personal care

940

860

Crop protection

290

200

Oilfield

390

440

Paints and coatings

140

160

Textile spin finish

200

160

Textile auxiliaries

450

500

Construction

190

470

Emulsion polymerisation

240

290

Food

190

200

Leather

30

60

ORE/mineral

60

150

Plastic additives

60

40

Pulp and paper

100

120

Explosives

10

10

Other

630

380

Total

7140

8570  

Surfactants (surface active agents) can be broadly defined as compounds which, when dissolved in water, concentrate at surfaces (interfaces) such as water-air or water- oil. All these surfactant molecules have in common the same basic molecular structurea hydrophile attached to a lipophile– and it is this nature that makes them adsorb at surfaces. The hydrophile is attracted to water in preference to lipid-like substanoes (hydrocarbons such as oil and grease) whereas the lipophile is attracted to these in preference to water (see fig. 2)

Fig.2 Surfactant basic molecular structure

It is the amphiphilic nature of surfactant molecules that makes them bifunctional. This can be seen when oil/grease and water come together. No matter how much energy is expended in getting them to mix, the oil and water will always separate into two distinct phases. The intermolecular forces between water molecules and between oil molecules are stronger than the forces between water and oil molecules. Added surfactant molecules adsorb at the oil-water interface, where they orient themselves such that the hydrophile is in the water and the lipophile is in the oil. With a little agitation the oil becomes dispersed/removed in the water and the surfactant acts as an emulsifying agent (fig.3).

Fig.3 How surfactant work 

The first surfactant, namely soap, was and still is, made by the alkaline hydrolisis of animal fats (tallow) or vegetable oils.. a process known as saponification ( Fatty acid + Base = Soap ). The next surfactants to be developed were the sulphates and sulphonates of vegetable oils eg. reaction of castor oil with sulphuric acid. Later, the development of sulphonation and sulphation processes using other oils as reactants led to a move away from natural and renewable plant oils and animal fats to the sulphonation of petroleum products eg. alkyl benzene sulfonatis (ABSs)linear alkylbenzene sulphonates (LABSs), changing the traditional soap powders to detergent powders for household laundry.

Progress was not confined to the sulphonation of different oils, but was soon accompanied by ethoxylation, in which a few or many ethylene oxide (EO) molecules react with a fatty alcohol – which may be synthetic or plant derived – to make the surfactant molecule eg. alcohol ethoxylates, alcohol ether sulphates and alkyl phenol ethoxylates. Along with the development of ether technology came the polymerisation of ethylene oxide with propylene oxide(PO) to give EO-PO copolymers – surfactants that are totally reliant on petrochemicals as raw materials. Recently, developed surfactants are an attempt to satisfy the modern consumers’ desire for products to be ‘more natural’.  These are surfactants derived from the carbohydrates sorbitol, sucrose, glucose and from plant oils such as coconut or palm kernel eg. sorbitan esters, sucrose esters, alkyl polyglucosides (AGs) and alkyl glucamides.  Sorbitan esters are used as emulsifiers in cosmetics and the sucrose esters in food manufacture. The alkyl polyglucosides find application as detergents rather than as emulsifiers and are making inroads into some everyday products.

Lipophiles are usually similar from one surfactant to another but hydrophiles show a range of chemical types and this is the basis for surfactant classification: anionic, cationic, non-ionic and amphoteric(Fig 4).

Fig 4. Structures of some common surfactants

anionic surfactant

cationic surfactant

non-ionic surfactant

amphoteric surfactant      

Anionic surfactants, which include soap, are the most widely used for cleaning processes because many are excellent detergents. In anionic surfactants the hydrophile comprises some highly electronegative atoms, making these molecules strongly polar. The counterion is usually a small cation such as sodium but occasionally may be a larger cation such as ammonia or amines. Anionic surfactants also tend to be good solubilizers and are relatively nontoxic. Examples of anionic surfactant groups include  sulfonic acid salts (taurates, isethionates, olefin sulfonates, sulfosuccinates)alcohol sulfates (sodium lauryl sulfate/SLS, ammonium lauryl sulfate/ALS, sodium laureth sulfate/SLES) , alkylbenzene sulfonatesphosphoric acid esters, andcarboxylic acid salts (stearic acid/sodium stearate).

Cationic surfactants, in contrast, comprise a long chain hydrocarbon as the lipophile with a quaternary amine nitrogen as hydrophile, and a halide ion as counterion. An important property of cationics is that they are attracted to surfaces carrying a negative charge, upon which they adsorb strongly. Proteins and synthetic polymers can thus be treated with cationics to provide desirable surface characteristics(ie. hair conditioners & fabric softeners are cationic surfactants). Cationic surfactants tend to be toxic and are therefore not widely used in environmental applications at this time. Ex.   include polyamines and their salts, quaternary ammonium salts (Quats), and amine oxides.  

Non-ionic surfactants are characterized by hydrophilic head groups that do not ionize appreciably in water. They second to anionics in cleaning applications and are frequently used in conjunction with them (i.e., used as cosurfactants). Nonionic surfactants tend to be good solubilizers and are relatively nontoxic. An important group of non-ionics includes those where the hydrophile comprises a chain of ethoxy groups and is known as the ethoxylates. Varying the number of ethoxy groups in the chain adjusts the amount of hydrophilic character in the final products. Examples include polyoxyethylenated alkylphenols, alcohol ethoxylates, alkylphenol ethoxylates, and alkanolamides .

Amphoteric surfactants comprise a long hydrocarbon chain (lipophile) attached to a hydrophile containing both positive and negative charges, which give it the properties of a zwitterion. The simplest amphoterics can therefore behave as a cation or anion depending on pH. Mild and with low irritancy, amphoterics are widely used in shampoos. Examples include Sodium Lauriminodipropionate and Disodium Lauroamphodiacetate.

When surfactants are put into solutions, the molecules have a tendency to line up in a certain way depending on the solution composition, the concentration of the surfactant, and the temperature. In a water solution with extremely low surfactant concentrations, the molecules tend to bounce around randomly without forming structures. But at the Critical Micelle Concentration (CMC) they arrange themselves in spherical structures called micelles. On the outer layer of the spheres are the hydrophilic parts of the surfactant molecule and on the inner layer are the lipophilic parts. It’s a bit like a cream filled donut (fig.5)

Fig.5 Micelle formation

This interfacial activity of surfactants (CMC = Critical Micelle Concentration) gives rise to a wide range of surface chemistry functions of surfactants such as:  wetting,  emulsifyingsolubilising, foaming/defoamingrheology-modifying,  antistatic,  ‘glossing‘, lubricity and surface conditioning. In cosmetics, surfactants are useful for the following application :

1. Cleansing Surfactants

The useful thing about micelles is that they can help suspend oil in water. When a small amount of oily materials is put into an aqueous solution of surfactants, it will migrate into the center portion of the micelle. So, when you put a surfactant solution on a surface like hair or skin, the oil that is there will be drawn away from the surface and into the micelles. When the surfactant solution is rinsed away, the surface is clean.

2. Foaming

Foam is another characteristic of surfactant solutions so you’ll need surfactants if you want your product to foam. Essentially, foam is the entrapment of air in liquids and the alignment of the surfactant molecules helps keep the foam stable. It should be noted that foam itself is not related to the ability of a product to clean. But consumers expect cleansing products to foam so as a cosmetic formulator, you’ll have to add foaming surfactants.

3. Emulsification

While cleansing cosmetics remove oils, many cosmetic products are design to add oily materials to the skin and hair. These ingredients usually can’t be applied directly because they have undesirable aesthetic characteristics in their concentrated form. For this reason, cosmetic chemists create emulsions using surfactants. A full exploration of emulsion science is beyond the scope of this entry so suffice it to say, emulsions are semi-stable mixtures of oils and water. A surfactant, or emulsifier, is used to help blend and stabilize the mixtures. In the simplest case, an emulsion formula is made by mixing an oil phase with a water phase and a surfactant. The micelles created by the surfactant entrap the oil in their centers and it remains suspended throughout the mixture. Products like creams and lotions are typically emulsions. When the product is applied to the skin or hair, the surfactant micelles break open and deliver the oily materials.

4. Solubilization

The problem with most emulsions is that they usually create opaque products. However, there are times when a cosmetic chemist wants a clear formula but still wants to blend an oil in a mostly water formula. Fortunately, there are surfactants that have the ability to create particles so small that light passes through them and the solutions remain clear. Molecules that do this are solubilizing surfactants. They are used to blend oily materials like fragrances or natural ingredients into clear solutions. An example would be a surfactant like Polysorbate 20.

5. Conditioning

Since surfactants often contain an “oily” part on their molecule, they have conditioning properties that can improve the feel and look of the surfaces of skin and hair. For them to work this way, the surfactants have to be left behind and also be non-irritating. This can be achieved through a leave-on cosmetic product or by using surfactants that can bond to surfaces through an electrostatic charge (more on this later).

6. Special effects

In addition to the four properties discussed, surfactants have a number of other special effects that are useful to formulators. For example, some surfactants have anti-microbial effects so they can be used as a preservative. Certain surfactants can be used to create an elegant, pearly effect in cosmetics to increase their aesthetic appeal. They can be used for thickening systems, reducing irritation and improving formula stability.

The hydrophile-lipophile balance (HLB) number is an indication of the relative strength of the hydrophilic and hydrophobic portions of the molecule and can be used to characterize the relative affinity of surfactants for aqueous and organic phases. A high HLB number generally indicates good surfactant solubility in water, while a low HLB number indicates a lower aqueous solubility and higher relative affinity for the organic phase (e.g., NAPL). A surfactant with a low HLB number can partition significantly into the NAPL phase and form reverse micelles having hydrophilic interiors and lipophilic exteriors (fig.6).  For a particular organic contaminant, optimum aqueous phase solubilization will generally occur at a specific HLB number. Less hydrophobic contaminants (those that are more water soluble) generally require a higher HLB number surfactant to bring about sufficient solubilization.

Fig.6 RM=Reverse Micelle M=Micelle

Note. Blue is aqueous phase & yellow is oily phase

So, that’s it for surfactant discussion! If you have comment, write down below.

andrianto angkadirjo bottom line

– Based on their cleansing power surfactants are classified into primary and secondary or co-surfactants. Based on the chemical structure there are anionic, amphoteric, non-ionic, and quaternary agents.

– There are thousands of different types of surfactants and it can be difficult to know which to use for any specific application.  We need to work with surfactants, talk with our surfactant suppliers and experiment with different blends. Only then will we get a better understanding of these molecules and what we can do with them.

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