Biology Reference
In-Depth Information
tensiometer
tensiometer
tensiometer
tensiometer
DU NOUY RING
tensiometer
WILHELMY PLATE
FIGURE 3.6
Du Nouy Ring and Wilhelmy Plate methods to measure surface tension.
Plate in place of the Du Nouy Ring (
Figure 3.6
). With this method, the plate (usually made of
platinum) is slowly lowered to the aqueous interface whereupon the surface 'jumps' onto the
plate and the weight of the 'jumped' solution is measured. It is remarkable that the simple
century-old technique to measure surface tension, developed before the concept of a lipid
bilayer had even been imagined, has changed very little through the years.
Anything that disrupts the surface packing decreases surface tension. One class of mole-
cules that is very effective at reducing the surface tension of water is known as surfactants
(surface active reagents). Surfactants are amphipathic molecules, having both a polar and
a non-polar end (see
Figure 3.7
). Surfactants accumulate at the air
water interface where
their non-polar tails stick into the air, avoiding unfavorable interaction with water, while
the polar end of the molecule sits comfortably in the water. The surfactants replace some
of the surface interface waters, decreasing the net surface tension. For example, in an early
study, the high surface tension of water (72.8 dynes/cm) was reduced to ~7
e
15 dynes/cm
by adding membrane phospholipids to the interface. The implication is that a membrane
surface would have a low surface tension.
Surfactants are often detergents (see Chapter 13) that employ reduction in surface tension
in order to function. In one simple demonstration, metal needles are carefully floated on the
air
e
water interface. Although metal is denser than water, the needles can float due to the
surface tension of the water. Upon the addition of a small drop of detergent to the aqueous
sub-phase, the needles sink to the bottom. The heavier needles sink before the lighter needles
as the surface tension drops. Detergent surfactants play a critical role in disrupting or even
dissolving membranes. They are essential for extracting proteins from membranes in
a non-denatured form for subsequent reconstitution studies. Detergents are a vital tool in
membrane studies and are discussed in detail in Chapter 13.
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