Chemistry Reference
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Gas
Surface
Liquid
(A)
Gas
Surface
Liquid
(B)
FIGure 3.12
Surface composition: (A) of pure water, (B) of an ethanol-water solution
(shaded = ethanol).
Now let us look at what happens to the surface composition when ethanol is added
to water (Figure 3.12 [A,B]).
The reason why ethanol concentration in the vapor phase is higher than that of
water is its lower boiling point. Next, let us consider the situation when a detergent is
added to water, whereby the surface tension is lowered appreciably (Figure 3.13).
The schematic concentration profile of detergent molecules is such that the con-
centration is homogenous up to the surface. At the surface, there is almost only
detergent molecules plus the necessary number of water molecules (which are in a
bound state to the detergent molecules). The surface thus shows very low surface
tension (ca. 30 mN/m). The surface concentration profile of the detergent is not eas-
ily determined by any direct method. In Figure 3.13, it is shown as a rectangle for
convenience, but one may also imagine other forms of profiles, such as curved.
It can be observed that surface tension decreases due to ethanol. This suggests
that there are more ethanol molecules at the surface than in the bulk phase, which
is also seen in a cognac glass. The ethanol vapors are observed to condense on the
edge of the glass, showing that the concentration at the surface of the solution is
very high.
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