Betaines

Betaines are found in plants, animals and microorganisms. Rich sources of betaines in the human diet are seafood, spinach and wheat germ or bran. Research is beginning to indicate that betaines are important nutrients for the prevention of chronic disease. Researchers are also interested in incorporating betaines into polymer brushes used for antifouling and lubrication.

Betaines are compounds with a positively charged functional group linked to a negatively charged functional group with an alkyl chain in between. The alkyl chain is often referred to as an alkyl chain spacer.  The general structure of an N-alkyl betaine is shown below:

The first betaine discovered was found in sugar beets in the nineteenth century. This betaine is (trimethylammonio)acetate, also known as trimethylglycine, and its skeletal structure is shown below:

Another example of a betaine is 2-(trimethylammonio)octadecanoate (also known as hexadecylbetaine) with the skeletal structure shown below:

2-(Trimethylammonio)tetradecanoate, or dodecylbutaine or laurylbutaine, is also a butaine and its skeletal structure is shown below:

Betaines are strongly attracted to water molecules because of these two charged functional groups.
The solubility of betaines in water is dependent on the length of the carbon chain, as well as on temperature and pH. 
In acidic solution, betaines acquire a net positive charge and act like a cationic surfactant. In anionic solutions, betaines acquire a net negative charge and act like an anionic surfactant.

Betaines can also be used in polymer brushes which are polymers bound to a surface. Polymer brushes can be used for antifouling and lubrication because the hydration of the ionic groups reduces the ability of other materials to adhere to the surface. 

Researchers at Kyushu University recently investigated a series of alkly chain spacers of different lengths bound to a silicon surface. They found that the polymer brushes swelled in humid air and water. It is believed that this is due to electrostatic repulsion between charged groups, and not dependent on the length of the alkyl chain.
In deionised water, net positive cations and net negative anions are repelled because of the  electrostatic force which causes the chain dimension to expand, whereas they shrink under high ionic strength by a charge screening effect of the bound ions.

Reference:
https://www.sciencedaily.com/releases/2017/08/170821094302.htm

Further Reading
Introduction to Functional Groups
2-Dimensional Structural Formula
Condensed Structural Formula
Molecular Formula
Amino Acids
Surfactants ( as found in synthetic detergents)
Intermolecular Forces and Solubility

Suggested Study Questions

1. Locate and identify each functional group on the skeletal structural formula of
• general formula N-alkyl betaine 
• (trimethylammonio)acetate
• 2-(trimethylammonio)octadecanoate
• 2-(trimethylammonio)tetradecanoate
2. Draw a 2-dimensional structural formula for each of the following molecules:
• (trimethylammonio)acetate
• 2-(trimethylammonio)octadecanoate
• 2-(trimethylammonio)tetradecanoate
3. Write the condensed structural formula for each of the following molecules:
• (trimethylammonio)acetate
• 2-(trimethylammonio)octadecanoate
• 2-(trimethylammonio)tetradecanoate
4. Write the molecular formula for each of the following molecules:
• (trimethylammonio)acetate
• 2-(trimethylammonio)octadecanoate
• 2-(trimethylammonio)tetradecanoate
5. Compare the structure of betaines to that of 2-amino acids. Can N-alkyl betaines be classified as alpha amino acids (2-amino acids) ? Justify your answer.
6. Write chemical equations to describe what happens to an N-alkyl betaine in:
• acidic aqueous solution
• basic aqueous solution
7. Compare the structure of N-alkyl betaines to the surfactants found in synthetic detergents. In what ways are surfactant molecules 
• similar to N-alkyl betaines
• different from N-alkyl betaines
8. Explain how N-alkyl betaines act like 
• a cationic surfactant in acidic aqueous solution
• an anionic surfactant in basic aqueous solution
9. Consider the structure of (trimethylammonio)acetate and 2-(trimethylammonio)octadecanoate. Which molecule do you expect to be more soluble in water? Justify your answer.
10. Consider the structure of (trimethylammonio)acetate and 2-(trimethylammonio)octadecanoate. Which molecule do you expect to be more soluble in paraffin oil? Justify your answer.