A surfactant is a compound that reduces surface tension between two phases, such as two liquids, a liquid and gas, or even a liquid and solid. For example, adding detergent reduces surface tension in oily water, making it easier to remove soiling from fabrics or cookware. The term “surfactant”, coined around 1950, is a contraction of surface-active agent.
How Surfactants Work
Most surfactants are organic compounds that contain hydrophilic or water-loving “heads” and hydrophobic or water-fearing “tails”. In other words, the molecules are amphiphilic. The hydrophilic head is polar and may (or may not) carry an electrical charge. The hydrophobic tail is a hydrocarbon, siloxane, or fluorocarbon.
In the interface between air and water, the hydrophobic tail orients toward air, while the hydrophobic portion stays in water. In oil and water, the hydrophobic tails extend into the oil, while the hydrophilic heads are in water. In either case, the surfactant interrupts the normal cohesive forces between water molecules. Intermolecular forces between water molecules are lower, so surface tension decreases. At the same time, surfactants stabilize the interface between the two phases. Above a critical concentration, surfactant molecules form micelles that physically separate the two phases.
Examples of Surfactants
Chemists classify surfactants based on whether they are anionic (negative charge), cationic (positive charge), non-ionic (neutral), or zwitterionic (can be either positive or negative). Surfactants are responsible for the functions of soap, dishwashing liquid, laundry detergent, shampoo, fabric softener, anti-static treatments, wetting agents, foaming agents, and emulsifiers in foods and cosmetics.
| Type of Surfactant | Examples | Uses |
|---|---|---|
| anionic | soap, alkyl sulfates, Texapon, Calsoft | dishwashing liquid, laundry detergent, shampoo |
| cationic | quaternary ammonium salts | fabric softener, anti-static products |
| non-ionic | Triton X-100, Span, Tergitol, ethoxylated aliphatic alcohols, polyoxyethylene surfactants | wetting agents, food ingredients |
| Zwitterionic | amphoacetates, betaine | cosmetic |
Pulmonary Surfactant
In the lungs, type II alveolar cells produce a mixture of compounds that act as pulmonary surfactant. Pulmonary surfactant is a mixture of phospholipids and proteins that adsorbs onto the alveoli at the interface between water-based liquid and air. The hydrophilic heads face the mucous membrane, while the hydrophobic tails of the molecules orient toward the air.
Pulmonary Surfactant Composition
Pulmonary surfactant is a mixture of phospholipids and proteins that adsorbs onto the alveoli at the interface between water-based liquid and air. The hydrophilic heads face the mucous membrane, while the hydrophobic tails of the molecules orient toward the air.
- ~40% dipalmitoylphosphatidylcholine (DPPC)
- ~40% other phospholipids (phosphatidylcholine or PC)
- ~10% surfactant proteins (SP-A, SP-B, SP-C and SP-D)
- ~10% neutral lipids (cholesterol)
- Traces of other substances
The lamellar bodies that are the organelles in type II alveolar cells that secrete pulmonary surfactant first appear around 20 weeks gestation. Artificial surfactants given to preterm infants consists of a mixture of DPPC with other compounds or is an extract from cow, calf, or pig lung.
Pulmonary Surfactant Functions
The wet alveoli encompass an air space. Without surfactant, surface tension compresses the size of the air bubble within an alveolus. Surfactant reduces the air pressure necessary to maintaining equilibrium between the contraction from surface tension and the expansion from air.
Pulmonary surfactant serves several functions in the lungs.
- It increases the thorax’s and lung’s ability to expand. This is called pulmonary compliance.
- Pulmonary surfactant helps prevent alveoli collapse (atelectasis) after exhalation.
- It helps repair collapsed airways.
- It regulates alveolar size. During inhalation, alveoli increase in size and the surfactant spreads across their inner surface. This increases surface tension and slows the rate of alveoli expansion, helping all alveoli expand at approximately the same rate. During exhalation, surfactant controls alveolar shrink rate. As alveoli shrink, surfactant becomes more concentrated and surface tension lowers more readily.
- Surfactant prevents fluid accumulation in the lungs and keeps airways dry.
- Pulmonary surfactant contributes to innate immunity. The proteins SP-A and SP-D bind sugars on pathogen surfaces so they are more easily engulfed by phagocytes. Surfactant also regulates lung inflammation.
References
- Bernhard, W. (November 2016). “Lung surfactant: Function and composition in the context of development and respiratory physiology”. Annals of Anatomy. 208: 146–150. doi:10.1016/j.aanat.2016.08.003
- Rebello, Sharrel; Asok, Aju K.; Mundayoor, Sathish; Jisha, M. S. (2014). “Surfactants: Toxicity, remediation and green surfactants”. Environmental Chemistry Letters. 12 (2): 275–287. doi:10.1007/s10311-014-0466-2
- Rosen, M.J.; Kunjappu, J.T. (2012). Surfactants and Interfacial Phenomena (4th ed.). Hoboken, New Jersey: John Wiley & Sons. ISBN 978-1-118-22902-6.
- Schurch, S.; Lee, Martin; Gehr, Peter (1992). “Pulmonary surfactant: Surface properties and function of alveolar and airway surfactant”. Pure and Applied Chemistry. 64 (11): 1745-1750. doi:10.1351/pac199264111745