Foaming agents can reduce the surface tension of the liquid and produce a large amount of uniform and stable foam. In fact, foaming agents are very common in everyday life, perhaps while washing the dishes, preparing a load of laundry, or washing your hair. Some surfactants play an important role in the foaming process. When present in small amounts, surfactants can facilitate the formation of a foam, or enhance its colloidal stability by inhibiting the coalescence of bubbles.
It is easy to make a foam, just by mixing air and liquid with some energy and bubbles will form in the mixture. While in most cases it is totally unstable. Surfactants have two parts in their chemical structure, a hydrophilic head (water-loving) and a lipophilic tail (water-hating). If a small quantity of a surfactant is added to the water, possibly a few tenths of a percent, then the surface tension is significantly lowered. This decrease in turn would lower the amount of mechanical energy needed for foam formation and prolong the life of the bubbles. To be specific, surfactants push themselves to the interface between air and water and they would reduce the surface tension of the water to make the surface of this mixture elastic and expandable. The water-loving hydrophilic part protrudes into the water, while the fat-loving hydrophobic end protrudes into the air. This creates a protective layer of surfactant molecules that prolongs the life of the bubbles (Figure 1).
Fig. 1 Foaming process
The stability of surfactant foams is not always satisfactory because of the coalescence and rupture of bubbles. Up to now, it's still a technical challenge to obtain aqueous foam exhibiting long-term stability and high foamability, especially with a little dosage of surfactants. The main influence factors of foamability are as follows:
For the same kind of surfactant, the foaming effect gets better with the increase of carbon chain, but when the carbon chain increases to a certain length, the foaming ability of this surfactant decreases. In general, for straight chain surfactants, C12~C14 are with better foamability, and those with more with more lipophilic branches have better foamability.
The hydrophilic ability of the surfactant may influence its foamability. Generally, those with higher hydrophilic ability would form bubble membrane with higher mechanical properties at the gas-liquid interface. Thus, the stability of foams is improved. For example, the foamability of ionic surfactants is higher than that of non-ionic surfactants.
The type of surfactants is the main determinant of foamability, while the extrinsic factors are important as well. The concentration, temperature, pH, water hardness are factors which would influence the foamability. For example, for some non-ionic surfactants, it is with good foamability when the temperature is lower than the cloud point, while when the temperature is higher than the cloud point, the foamability is reduced.
Surfactant and its foaming properties are very complex, and there is no infallible rules to explain the relationship of the surfactant system and its foamability. The above rules can be used as a reference, and specific analysis is needed in specific cases.
There is no way we can do without surfactants in our daily lives. Whether washing dishes, cleansing your face or doing a load of laundry, surfactants make cleaners work better.
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