Answer:  

Dr. Izumi Yamada kindly answered the question above:
   
 
The difference between anionic and nonionic surfactant
 
Surfactant is a substance which has both a hydrophilic group and a hydrophobic group.

1. Concerning the name, a surfactant which dissociates in water and releases cation and anion (or zwitterions) is termed ionic (cationic, anionic, zwitterionic) surfactant. On the other hand, a surfactant which does not dissociate is called a nonionic surfactant.

2.  An anionic surfactant has an anionic hydrophilic group. Examples of anionic surfactants are generally called “soap”(fatty acid soap), alkylsulfonic acid salts (the main component of synthetic detergent, such as linear alkyl benzene sulfonate (LAS)), fatty alcohol sulfate (the main component of shampoo or old neutral detergents), etc.
     

3. Because fatty acid soap is a salt of fatty acid and alkali metal (a salt of a weak acid and a strong base), it hydrolyzes in water and the solution becomes slightly basic. However, the solutions of other anionic surfactants are neutral. The solution of synthetic detergent is adjusted to slightly basic, but this is not because of the detergent itself (it is neutral) but because of the effect of auxiliary agents (sodium carbonate, etc). This is the main difference between soap and synthetic detergent.
     

4. It has been known since early times that dirt is washed off by alkali, such as lye and washing soda. Detergents on the market (both powder soap and synthetic detergent) would have the same effect, since the liquids are alkaline when they dissolve in water. It is thought that fibers (laundry) are softened by alkali, causing the release of dirt. So it is ineffective to wash poor alkali-proof animal fiber, such as silk and wool with alkali. It is also thought that alkali cleans off oily dirt by saponification reaction (formation of water-soluble soap), which is the same as the synthesis of soap. However, it is doubtful whether this reaction really occurs during home washing. 
     

5. Animal fibers, such as silk and wool, are called “amphoteric fibers”, which means that the fibers can become both cationic and anionic, depending on the property of the liquid. If alkaline detergents (both powder soap and synthetic detergent) are used for washing these fibers, anionic surfactant would adsorb on the cationic groups (amino groups) on the fiber. It would be possible to wash poor alkali-proof fiber by maintaining a neutral pH. However, we cannot do anything about the ionic adsorption of surfactant on fibers. This is one of the advantages of non-ionic surfactants. There is a detergent that can wash clothes which have the “dry cleaning sign” (the picture sign for recommending that dry cleaning be done, found on silk and wool products) by water at home. As you may already realize, the main component of this type of detergent is non-ionic surfactant. Since electrostatic force does not work for non-ionic surfactants, the amount of detergent remaining after washing would be low for other kinds of clothes as well as for silk and wool.
     

6. There is a possibility that anionic surfactants combine with cationic ions, for example, calcium ion in hard water. Especially in the case of powder soaps, fatty acids combine with calcium ions and form scum, which is not water-soluble and precipitates, decreasing the cleaning effect. Other anionic surfactants also combine with calcium, but these amounts would be low. Anyway, no precipitation happens if non-ionic surfactant is used. So this point is also one of the advantages of non-ionic surfactants.
     

7. “Two in One (Conditioner & Shampoo)” was popular a short time ago. This was a shampoo, which did not require the use of another conditioner. Conditioner serves a similar function as “softener” for washing clothes, so the main component of a conditioner is cationic surfactant, which resembles fabric softener. This means that it is impossible to mix conditioner agents with regular shampoo. If you mix shampoo and conditioner in the bathroom, you may see precipitation like scum. This is caused by combination of the cationic surfactant with the anionic surfactant. Of course, this precipitation does not do anything.
    
   There were three kinds of “Two in One (Conditioner & Shampoo)”:
     1) Just conditioner whose cationic surfactant has some detergency.
     2) Just shampoo in which the oil content is blended.
     3) Real “Conditioner & Shampoo”, i.e., the main surfactant of shampoo does not combine with the cationic surfactant of the conditioner.
    
   You could easily guess that the surfactant used in 3) is non-ionic surfactant. The same concept was applied to a detergent for washing clothes, and now the detergents containing fabric softeners are widely used. The main component of this type of detergent has to be non-ionic surfactant. Non-ionic surfactant can be used with many kinds of auxiliary agents, making non-ionic surfactants very advantageous.

 
Application of non-ionic surfactant to detergent for clothes
 

8. Let’s consider why non-ionic surfactants, which seem to have many advantages, have not been used for detergents up to now. All surfactants which can be used for food (according to food hygiene law in Japan) are non-ionic surfactants, except for soybean phospholipid (lecithin, an amphoteric surfactant). It is thought that they are harmless because they are fatty acid esters of polyalcohol such as sorbitan, sucrose, and glycerin. So it would be wonderful if we could apply these surfactants to laundry. I did an experiment – however, hardly any dirt was cleaned off by sucrose fatty acid ester. Thus, the detergency of the non-ionic surfactant was weak. I did a further experiment and found a non-ionic surfactant which does possess a certain level of detergency. However, this surfactant was liquid, making it difficult to use. This surfactant could be usefully applied to liquid detergent, but it was not the trend to use liquid detergent, and additionally it dripped, making it difficult to handle, and so it did not become common. Since non-ionic surfactants can be easily synthesized, they are used in many fields now. However, non-ionic surfactants were not used as the main surfactant of synthetic detergent for laundry until the powder form of non-ionic surfactant (polyoxyethylene) was invented.
    

9. I found that liquid crystal was formed during the washing process of oily dirt and the liquid crystal contributed to removing the dirt. And I established the new method to indicate the effectiveness of washing (kinetics of wash). I measured the speed of liquid crystal formation and the washing speed. When I used the non-ionic surfactant (polyoxyethylene) for the experiment, I found that the liquid crystal was formed at lower concentration and lower temperature (compared to anionic surfactant). This experiment was published, and around the same time a`manufacturer invented the powder form of non-ionic surfactant (polyoxyethylene). So powder compact detergent, whose main component was non-ionic surfactant, was released by a major detergent manufacturer with the motto “liquid crystal cleans off dirt”. However, the detergent was discontinued a few years after its release probably because consumers at that time were not very environmentally conscious.
     

10. The use of detergent whose main components are non-ionic surfactants is increasing. This increase is related not only to the advantages mentioned above but also to the effect of its good image. Since in recent times most clothes do not get as dirty as they once did, and at the same time that environmental concern is increasing, an environmentally friendly detergent, even one with decreased dirt cleaning power, can be accepted by modern society.

  
Acknowledgment
We would like to thank Dr. Izumi Yamada for his kind answer.
 
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.