Types of surfactants

Types of surfactants

Surfactants are generally classified according to the nature of the hydrophilic group (Table 10.1). The hydrophilic regions can be anionic (neg-atively charged at certain pH values), cationic (positively charged at certain pH values), or nonionic (not charged at all pH values). In addi-tion, some surfactants possess both positively and negatively charged groups. These surfactants can exist in either or both anionic or cationic states, depending on the pH of the solution and the pK_a of the ionizable groups on the surfactants. Such surfactants are known as ampholytic compounds.

1. Anionic surfactants

• Sodium stearate

• Sodium dodecyl sulfate (SDS)

• Sodium dodecyl benzene sulfonate

• Sodium cholate

2. Cationic surfactants

• Hexadecyltrimethylammonium bromide

• Dodecyl pyridinium chloride

Nonionic surfactants

• Heptaoxyethylene monohexadecyl ether

3. Ampholytic (Zwitterionic) surfactants

• N-dodecyl alanine

• Lecithin

Table 10.1 Classification of surfactants

1. Anionic surfactants

The hydrophilic group of anionic surfactants carries a negative charge, such as R-COO⁻, RSO₄⁻, or RSO₃⁻, where R represents an organic group. Anionic surfactants have high hydrophilicity and are used as detergents and foaming agents, such as in shampoos. Examples of anionic surfac-tants include soap (sodium salt of fatty acids, R-COO⁻Na⁺), sodium dodecyl sulfate (C₁₂H₂₅SO₄Na⁺) (SDS), alkylpolyoxyethylene sulfate (R-[CH₂CH₂O]_nSO₄⁻), and alkylbenzene sulfonate (R-C₆H ₅-SO₃⁻). Some of these surfactants, such as SDS, also known as sodium lauryl sulfate (SLS) (Figure 10.1), are used to create sink conditions during in vitro drug-release studies for new drug product development. It is very water soluble and has bacteriostatic action against gram-positive bacteria. Therefore, SLS also finds use as a preoperative skin cleaner and in medicated shampoos.

2. Cationic surfactants

Cationic surfactants have a cationic group, a functional group that can be positively charged at certain pH values, as the hydrophilic portion of the molecule. For example, primary (RNH₂), secondary (R₂NH), or tertiary amines (R₃N) are positively charged at low pH values. However, the quaternary amines (R4N+) are permanently positively charged irre-spective of the solution’s pH. 

Figure 10.1 Structures of some surfactants.

Most cationic surfactants are quaternary derivatives of alkylamines, for example, alkyl trimethyl ammonium salts, dialkyl dimethyl ammonium salts, and alkyl benzyl dimethyl ammonium salts.

Cationic surfactants are used in fabric softeners and hair conditioners. In addition, cationic surfactants can destabilize biological membranes due to the interaction of their cationic groups with the negatively charged phospholipids on the cell membranes. This results in their germicidal activ-ity. Thus, the quaternary ammonium and pyridinium cationic surfactants have bactericidal activity against a wide range of gram-positive and some gram-negative organisms and are commonly used as preservatives in phar-maceutical formulations. They may also be used on the skin for cleans-ing of wounds. For example, solutions containing 0.1%–1% cetrimide (Figure 10.1) are used for cleaning the skin, wounds, and burns, as well as for cleaning contaminated vessels. Benzalkonium chloride (Figure 10.1) is a mixture of alkyl benzyl dimethyl ammonium chlorides. Its dilute solu-tion may be used for the preoperative disinfection of the skin and mucous membranes, for application to burns and wounds, and for cleaning poly-ethylene tubing and catheters. Benzalkonium chloride is also used as a preservative in eye drops.

3. Nonionic surfactants

Nonionic surfactants contain ether [–(CH₂CH₂O)_nOH] and/or hydroxyl [–OH] hydrophilic groups. Thus, these surfactants are nonelectrolytes; that is, their hydrophilic groups do not ionize at any pH value. Nonionic surfactants are commonly used for stabilizing oil-in-water (o/w) and water-in-oil (w/o) emulsions. Since the nonionic surfactants do not contain an ionizable group, their properties are much less sensitive to changes in the pH of the medium and the presence of electrolytes. In addition, they have fewer interactions with cell membranes compared with the anionic and cationic surfactants. Thus, nonionic surfactants are preferred for oral and parenteral formulations because of their low tissue irritation and toxicity.

Most commonly used nonionic surfactants include Spans and Tweens. Sorbitan fatty acid esters (Spans), such as sorbitan monopalmitate (Figure 10.1), are oil-soluble emulsifiers that promote the formation of w/o emulsions. Polyethylene glycol sorbitan fatty acid esters (Tweens) are water-soluble emulsifiers that promote the formation of o/w emulsions. The Spans and the Tweens come in different molecular weight or size ranges, which differ in their physical properties.

4. Ampholytic surfactants

Ampholytic surfactants possess both cationic and anionic groups in the same molecule. Their ionization state in solution is dependent on the pH of the medium and the pK_a of ionizable groups. For example, the acidic func-tional groups, such as carboxylate, sulfate, and sulfonate, are negatively charged (ionized) at pH > pK_a, while the basic functional groups, such as amines, are positively charged (ionized) at pH < pK_a. The extent of ioniza-tion of functional groups, that is, the proportion of molecules in solution that bear the positive or the negative charge, at a given pH is governed by the Henderson–Hasselbalch equation, discussed elsewhere in this book. Lecithin (Figure 10.1), for example, is an ampholytic surfactant and is used for parenteral emulsions.