# Etymology and organization of surfactants

## Etymology

Technical terms (neologisms) are formed by combinations of prefixes and suffixes. English meanings are not literal translations, but interpretations of how the words are understood in this branch of science.

English Greek Latin
oil lipo- oleo-
water hydro- aqua-
solvent lyo- solvo-
affinity -philic
lack-of-affinity -phobic
nature -pathic
science -logy
flow rheo-

hydrophilic = with affinity for water

lipophilic = with affinity for oil

lyophilic = with affinity for the solvent

lyophobic = lack of affinity for the solvent

amphipathic = combining both natures (oil and water understood)

amphiphilic = with affinity for both (oil and water understood)

## Common surfactant molecules

Witten, Fig. 7.1

Five common surfactant molecules.

Top left: SDS also called sodium lauryl sulfate (a leading ingredient in house-hold cleaning products lie soap, detergent, and shampoo, a anionic surfactant.

Top right: the cationic cetyl trimethyl ammonium bromide (CTAB).

Bottom left: the phospholipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC)

Center right: sodium bis(2-ethylhexyl)sulfosuccinate (AOT) AOT is a rare surfactant - it is soluble and active in both oil and water.

Botton right: pentaethylene glycol monodecyl ether (C12EO5), a non-ionic surfactant.

## Large volume aqueous surfactants

Surfactant Structure
Fatty alcohols and alkylphenol ethoxylates
Alkanolamides
Alkylbenzene sulphonates
Fatty alcohol and fatty alcohol ether sulphates

## Oil soluble surfactants

Surfactant Structure
Sorbitan mono-oleate (Span 80)
Span 80
Solsperse 17000
Solsperse 17000
Polyisobutylene succinimide(OLOA 11000)
OLOA 11000

## Graded series of solutes - HLB scale

The HLB scale

HLB stands for hydrophile / lipophile / balance. The scale measure the affinity of non-ionic surfactants for oil as opposed to water. The method introduced by Griffin in 1954 assigns an index

HLB = 20 * Mh / M,

where Mh / M is the proportion of the molecular mass that is hydrophillic. A higher HLB indicates a higher water solubility.

## Pulmonary Surfactant

Pulmonary Surfactant

Surfactant is a complex substance containing phospholipids and a number of apoproteins. Surfactant reduces surface tension throughout the lung, thereby contributing to its general compliance. Pulmonary surfactant is a surface-active lipoprotein complex formed by type II alveolar cells. The proteins and lipids that comprise surfactant have both a hydrophilic region and a hydrophobic region. By adsorbing to the air-water interface of alveoli with the hydrophilic headgroups in the water and the hydrophobic tails facing towards the air, the main lipid component of surfactant, dipalmitoylphosphatidylcholine, reduces surface tension.

Alveoli can be compared to air bubbles in water, as the alveoli are wet and surround a central air space. The surface tension acts at the air-water interface and tends to make the bubble smaller (by decreasing the surface area of the interface). The gas pressure (P) needed to keep equilibrium between the collapsing force of surface tension (T) and the expanding force of gas in an alveolus of radius r is expressed by the law of Laplace:

$P = \frac{2T}{r}$

Lung compliance is defined as the volume change per unit of pressure change across the lung. Measurements of lung volume obtained during the controlled inflation/deflation of a normal lung show that the volumes obtained during deflation exceed those during inflation, at a given pressure. Compliance is calculated using the following equation, where ΔV is the change in volume, and ΔP is the change in pressure.

$C = \frac{ \Delta V}{ \Delta P}$

The difference in inflation and deflation volumes at a given pressure is called hysteresis and is due to the air-water surface tension that occurs at the beginning of inflation. However, surfactant decreases the surface tension part of elastic recoil observed by von Neergaard. If the lungs did not secrete surfactant, this surface tension would be much higher preventing the lungs from inflating normally, as is the case in premature infants suffering from infant respiratory distress syndrome.
The effect of surfactants on alveoli
The normal surface tension for water is 70 dyn/cm (70 mN/m) and in the lungs it is 25 dyn/cm (25 mN/m); however, at the end of the expiration, compressed surfactant phospholipid molecules decrease the surface tension to very low, near-zero levels. Pulmonary surfactant thus greatly reduces surface tension, increasing compliance allowing the lung to inflate much more easily, thereby eliminating the work of breathing. It reduces the pressure difference needed to allow the lung to inflate. The reduction in surface tension also reduces fluid accumulation in the alveolus as the surface tension draws fluid across the alveolar wall. As the alveoli increase in size, the surfactant becomes more spread out over the surface of the liquid. This increases surface tension effectively slowing the rate of increase of the alveoli. This also helps all alveoli in the lungs expand at the same rate, as one that increases more quickly will experience a large rise in surface tension slowing its rate of expansion. It also means the rate of shrinking is more regular as if one reduces in size more quickly the surface tension will reduce more so other alveoli can contract more easily than it.

Synthetic pulmonary surfactants: