The hydrophilic ends of amphoteric surfactants have positive and negative charges, and the double charges cancel each other out, producing a zero net charge, called an amphoteric ion. The way amphoteric surfactants react depends on the pH of any given solution. In acidic solutions, amphoteric surfactants are positively charged and behave like cationic surfactants. In alkaline solutions, they generate negative charges, similar to anionic surfactants.
Fig.1 Some mostly used amphoteric surfactants and their structure. (Sarkar, 2021)
Surfactants have many special properties, and they are highly valued for their different uses in various fields.
The solubility of the surfactant depends on the nature of the alkyl chain, the headgroup, and the presence of additional salt. The solubility of the amphoteric surfactant decreases with an increasing length of the hydrocarbon chain due to its hydrophobic character.
The value of CMC is a prominent property of surfactants, which offers details on its capacity to serve as interfacial agents. The value of the CMC mainly depends on the hydrophobic chain length (m), spacer segment (n), hydrophilic headgroups, and presence of the addition of salt, temperature, and so on.
Krafft temperature (TK) is the temperature at which the solubility and the CMC are equal in an aqueous solution. The solubility of surfactants increases significantly above TK. Hydrophilic headgroups, hydrophobic hydrocarbon chain (m), and the spacer part (n) are the principal determinants of TK.
The surface tension of surfactants is a significant property to determine their interfacial activity.
Amphoteric surfactants with superior dilational modulus have fine dynamic and static stability, brilliant elasticity, foam film stabilization ability, and good micro-stiffness.
The presence of added salt in the surfactant solution screens the ionic atmosphere and therefore decreases electrostatic repulsion considerably to favor micellization.
In nanomaterial dispersion and stabilization, amphoteric surfactants were also of great momentous. Using lauryl dimethylamino acetic acid as an amphoteric template, mesoporous nanostructured average dimensions of around 36 nm, stoichiometric, rod-like pure crystalline hydroxyapatite was effectively developed by microwave irradiation through sequential nucleation-growth route, which is very useful for catalysis, adsorption, biomedicine, and hosts for the synthesis of nanomaterials.
Layered double hydroxides (LDHs) are a family of lamellar ionic compounds consisting of positively charged brucite-like layers, with an interlayer area having solvent molecules and charge recompensing anions. Ion exchange and differing pH can interpose imidazoline amphoteric surfactants into LDH, enlarge the interlayer separation of LDHs, and minimize the interlayer electrostatic interaction and facilitates the exfoliation of LDHs into micrometer-sized, positively charged nanosheets.
Fig.2 Schematic representation of various applications of the amphoteric surfactants. (Sarkar, 2021)
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