Non-ionic Surfactants (1)

General introduction

Non-ionic surfactants are surfactants that do not have a charged group. The hydrophilic group is provided by a water-soluble group that does not ionize. The most common are the hydroxyl group (R-OH) and the ether group (R-O-R’). Other groups are the oxide (amine oxide) and triple unsaturated bond (acetylenic alcohols).

The water-solubilizing properties of a hydroxyl group or an ether group are low compared to the sulfate or sulfonate groups. If only one hydroxyl or one ether group is present, the chain length of the hydrocarbon R will be only 6-8 before the product becomes insoluble and has poor surfactant properties. Thus, dodecyl alcohol is practically insoluble, and aqueous solutions show poor foaming, poor detergency, poor wetting, etc. Surfactants showing desirable properties are obtained using multi hydroxyl groups or multi-ether groups to increase water solubility. Inpractice, the most versatile method of using ether groups is by the addition of ethylene oxide to the hydrophobe. Ethylene oxide will react with a hydrogen atom attached to a hydrophobic group. The amount of ethylene oxide in the molecule can be controlled by varying the amount which is added to the hydrophobe. The reaction is almost quantitative. Any free ethylene oxide can easily be removed at the end of the reaction (it is a gas at room temperature). The larger the amount of ethylene oxide the more water-soluble the product. The properties of the ethoxylates will depend mainly upon the hydrophobe used but can also be affected in a minor way by the method of ethoxylation because the value of n is only an average value and the actual number of ethoxy groups will be a distribution around the average value. The latter can, on occasions, be important to the formulator so that the empirical formula given above will not truly reflect the structure of the surfactant.

The alternative method of using multi hydroxyl groups is not utilized to the same degree in practice because there is no easy cheap method of attaching multiple hydroxyl groups onto a hydrocarbon. Nevertheless, many surfactants are based on this principle because of the widespread occurrence of multi-hydroxyl products in natural products, i.e. the saccharides, multi-saccharides and carbohydrates. However, the chemistry is complex and the intermediates are often high melting solids which can degrade on heating. A very large amount of research has been carried out and many surfactants based on multi-hydroxyl groups are on the market but they do not offer the formulator the same variety of properties obtained from the ethoxylated derivatives. Nevertheless, there are significant problems surrounding ethylene oxide and its derivatives and more efforts will probably be made in the future to find alternatives to ethylene oxide. Some space is therefore devoted to various alternatives to ethylene oxide. Examples of ethoxylates (from ethylene oxide reacting with a hydrophobe)

are:

• Alcohol ethoxylates

• Mono alkanolamide ethoxylates

• Fatty amine ethoxylates

• Fatty acid ethoxylates

• Ethylene oxide/propylene oxide copolymers

• Alkyl phenol ethoxylates

Examples of multi hydroxyl products (from reaction of a hydrophobe with a Multi-hydroxyl product by esterification) are:

• Glucosides

• Glycerides

• Glycol esters

• Glycerol esters

• Polyglycerol esters and polyglycerides

• Polyglycosides

• Sorbitan esters and sorbitan ester ethoxylates

• Sucrose esters

The description of non-ionic surfactants will follow closely that of anionics, i.e. by describing groups of surfactants which have a similar hydrophilic group. However, ethoxylated products share many common characteristics which are independent of the hydrophobe and it is simpler and avoids repetition if these common characteristics are described in this section.

The chemistry of ethoxylation

An examination of the chemistry of the reaction between ethylene oxide and the various hydrophobes will give considerable insight into the properties of the various ethoxylates. The mechanism of ethoxylation depends upon the catalyst used but most common ethoxylates are made using an alkaline catalyst. Using alkaline catalysts, the rate of ethoxylation is dependent upon the ionization of the active hydrogen. The acid ionization constants in water are: alcohol, 10^-15; phenol, 10^-9; carboxylic acid, 10^-5.

There are three different situations:

1. Where the active hydrogen on the starting material is equal in reactivity to that of the ethoxylate which is formed, e.g. starting material is an alcohol, an amide or water. As more ethylene oxide is added to the alcohol ethoxylate, the ethylene oxide adds on in a random manner to any hydroxyl group and therefore free alcohol remains until a very large amount of ethylene oxide has been added.

2. Where the active hydrogen on the starting material is more acidic than that of the ethoxylate which is formed, e.g., starting material is a phenol, mercaptan, or carboxylic acid. As ethylene oxide is added to the alkyl phenol, it is preferentially added to the hydroxyl group attached to the benzene ring, i.e., the starting material. Thus, all the phenol group reacts before any EO goes on to the hydroxyl group on the product. The difference is shown on comparing the ethoxylation of an alcohol or alkyl phenol with 3 moles of ethylene oxide (Table 1).

Table 1: Ethoxylation of alcohols and phenols with 3 moles of EO

3. Where the active hydrogen on the starting material is less acidic than that of the ethoxylate which is formed, e.g. starting material is an amine. Using basic catalysts, very little reaction with ethylene oxide would take place because ionization of the hydrogen on the amine in the presence of a base does not take place to any practical extent. However, amines will react readily with ethylene oxide if either water or an acid is present to give an ethanolamine. The ethanolamine formed can then be reacted with further ethylene oxide using a basic catalyst.

The ethoxylation of an alcohol or alkyl phenol gives a distribution of chain lengths. This distribution is dependent upon the catalyst used and the conditions of ethoxylation. In the majority of applications, the exact distribution is not that critical but there have been specific applications where the distribution can be critical. The easiest way to change the distribution is by using a blend of two ethoxylates but this can only broaden the distribution. There have been attempts to commercialize ethoxylated alcohols with a narrow distribution.

Impurities in the finished ethoxylate include:

1. Polyglycols:

The hydroxyl group on water can take part in all the reactions as ethylene oxide will react with water, particularly under basic catalyst conditions, to form polyglycols. Thus it is necessary to remove water from the starting materials (except for amines where the level must be controlled) or the formation of polyglycols is inevitable. Polyglycols can be formed, however, in the absence of water. Polyglycols are often insoluble in the non-ionic and show as a haze or even separate out. The addition of water to the finished non-ionic will often clear the haze, so a clear ethoxylate is no guarantee that it does not contain polyglycols. For the majority of detergent applications, small quantities of polyglycols are not detrimental, but there are applications where they can affect surfactant functional performance.

• Ethylene oxide:

Unreacted ethylene oxide is left at the end of the reaction and is readily removed by vacuum and heating. However, very small traces (1-25ppm) remain in the ethoxylate and this quantity slowly reduces with time. Very small levels of EO are demanded for cosmetic use.

• I,4-Dioxane:

Formed in small amounts (< 50 ppm) in most ethoxylates; can be removed by steam distillation; very low levels required for cosmetic use.

General properties of non-ionics

Solubility in water

The solubility of EO derivatives is due to the hydrogen bond between water and the EO group. Energy of hydrogen bond (Table 2) Cloud points of ethoxylates is approx. 7 kcal/mol and heating can impart enough energy to destroy the bond. Dehydration takes place and the product comes out of solution; the temperature at which this takes place is known as the cloud point. On cooling, the product dissolves. A 1 % solution is usually used for determination of the cloud point as, at low concentrations, the cloud point is dependent upon the concentration (Table 2). The water solubility increases as the amount of ethylene oxide increases. There is a simple rule of thumb relating the amount of ethylene oxide with the number of carbon atoms (N) in the hydrophobe to achieve water solubility; water solubility just achieved at N/3 moles of EO; fairly good water solubility at N/2 moles of EO; very good water solubility at 3N/2 moles of EO.

Non-jonics tend to have maximum surface activity near to the cloud point. The addition of alkalis and/or inorganic salts generally lowers the cloud point but there is no consistent pattern on the addition of inorganic acids. The addition of large quantities of inorganic salts can cause precipitation at room temperature (salting out). The addition of mineral acids does not usually cause reduction in cloud point and solubility. The addition of non-polar liquids generally increases the cloud point but there are exceptions (Table 3). The addition of aromatic and polar aliphatic compounds generally reduces the cloud point, particularly aliphatic alcohols, fatty acids and phenols.

Table 3: Effect of added non-polar liquids on cloud points

Double cloud points are sometimes observed with mixtures of non-ionics and some EO/PO co-polymers. On increasing the temperature the solution first becomes turbid then less turbid, hazy and then turbid again. Non-ionics with hydrophobic groups with two or three side branches have decreased cloud points compared to linear products and do not form spherical micelles. Most water-soluble ethoxylates form very viscous solutions or even gels at concentrations of 40-70% in water. In order to prepare dilute solutions, ethoxylates must be added to well-stirred water. To prepare concentrated solutions, water must be added to well-stirred ethoxylates.

Compatibility with other surfactants

Ethoxylates are compatible with all other surfactants. This does not mean that they are inert to other surfactants. In fact, synergy is very strong with anionics and mixed micelles with other surfactants are well known.

Chemical stability of the polyoxyethylene chain

Non-ionics show excellent chemical stability in a very large number of aqueous formulations, particularly household products. Nevertheless, the polyethoxy chain shows behavior similar to that of the simple ethers and undergoes oxidation very readily. Oxidation can cleave the polyethoxy chain which will then change the surfactant properties as the degree of hydrophilicity has been reduced. The chemical stability of products where the polyethoxy chain is attached to the hydrophobe by an ester or amide group is described in the relevant section dealing with ethoxylated esters or amides. In the majority of cases, oxidation will attack the polyethoxy chain before the hydrophobic chain unless unsaturation is present in the hydrophobic chain. When free radicals are introduced into a system containing polyethoxy groups, they initiate oxidation so long as oxygen is present. A chain reaction is set up which is propagated by the regeneration of new free radicals.

Hydroperoxy groups are intermediates which accumulate as they are more stable than the free radicals. Catalysts can have a profound influence on the formation and the decomposition of the hydroperoxides. Transition metal ions (e.g., copper, cobalt, manganese), even at very low concentrations (a few parts per million), can induce decomposition of the peroxide. Reducing or oxidizing agents (e.g., ferrous ions, bleach) can accelerate peroxide decomposition. Acid catalysts can accelerate peroxide decomposition to form aldehyde groups which can give rise to colored compounds.

On oxidation, the peroxy concentration first increases to a maximum and then decreases as the peroxy compounds degrade into the final product. A variety of final product groups can be found which depend upon the conditions of oxidation, the temperature, the catalyst, etc. The principal organic groups to be found are: carboxylic acids; aldehydes; alcohols; lactones; esters. Some of these groups may have a better water solubility than the ether group but scission of the polyoxyethylene chain commences almost from the start of the oxidation, whatever the product groups formed. The overall effect will change the surface active properties, but the interpretation of the chemical effect by the observed physical effect is extremely difficult if not impossible. An example is viscosity, which should fall if chain scission takes place but nothing else except the molecular size changes. However, a change in the water solubility may change the size of the micelle, which will then influence the viscosity.

As the cloud point is a measure of water solubility of the hydrophilic chain, oxidation and scission of the chain would be expected to give decreased cloud points which has been shown to be true in practice. Thus, the cloud point can be used as a measure of oxidation, but again, care should be taken in interpretation as there can be many reasons for reduction in cloud point.

The most common methods for determining hydroperoxide are iodine or arsenite titrations. However, in the presence of non-ionic surfactants, as much as 40% of the iodine liberated from the potassium iodide by the hydroperoxides was found to be unavailable for titration. Hydroperoxide formation is increased by:

• Decreasing the surfactant concentration

• Increasing exposure to light

• Increasing temperature (e.g. sterilization by autoclaving)

• Bleaching with hydrogen peroxide

• Decrease in pH below 6

• Presence of transition metal ions

Stabilization of polyoxyalkylene derivatives requires the following:

• Store in the dark

• Minimal air access, store under nitrogen if possible

• Temperature as low as possible

• Buffer aqueous solutions to neutrality

• Low concentrations should be avoided wherever possible

• Add an antioxidant but check that the non-ionic already contains an antioxidant; get non-ionic manufacturers recommendation on the antioxidant to use. The great majority of aqueous surfactant formulations containing polyoxyethylated surfactants will not need any further stabilizer added other than that added by the non-ionic manufacturer. However, the formulator should be aware of the tendency of such products to oxidize as many formulations are used in oxidizing conditions.

References

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