Chemistry Reference
In-Depth Information
these two solutions cannot be used for the same application based on their similar
magnitudes of γ.
There are special substances called
soaps
or
detergents
or
surfactants
that exhibit
unique physicochemical properties. The most significant structure of these mole-
cules is manifested by the presence of a
hydrophobic
(alkyl group) and a
hydrophilic
(polar groups, such as -OH, -CH
2
CH
2
O-, -COONa, -SO
3
Na, -SO
4
Na, -CH
33
N-, etc.).
The different polar groups are
Ionic groups
Negatively charged—
anionic
-COONa
-SO
3
Na
-SO
4
Na
-(N)(CH
3
)
4
Br (positively charged—
cationic
)
-(N)(CH
3
)
2
-CH
2
-COONa (Amphoteric)
Nonionic groups
-CH
2
CH
2
OCH
2
CH
2
OCH
2
CH
2
OH
-(CH
2
CH
2
OCH
2
CH
2
O) × (CH
2
CH
2
CH
2
O)yOH
Accordingly, one also calls these substances
amphiphiles
(meaning two kinds, i.e.,
the alkyl part and the polar group):
CCCCCCCCCCCCCCCCCCCC
-
O
ALKYL GROUP(
CCCCCCC
-)—POLAR GROUP(-
O
) = AMPHIPHILE …
(CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
)—POLAR
In representations, the alkyl group is depicted as (.............), while the polar group is
depicted as 0.
For instance, surfactants dissolve in water and give rise to low surface tension
even at very low concentrations (a few grams per liter or 1-100 mmol/L) of the solu-
tion; therefore, these substances are also called surface-active molecules (
surface-
active agents
or substances). On the other hand, most inorganic salts increase the
surface tension of water. All surfactant molecules are amphiphilic, which means that
these molecules exhibit hydrophilic and hydrophobic properties. Ethanol reduces the
surface tension of water, but one will need over a few moles per liter to obtain the
same reduction as when using a few millimoles of surface-active agents.
In comparison, addition of organic molecules such as methanol or ethanol
decreases the magnitude of γ of water from 72 mN/m rather slowly. The value of
γ decreases from 72 to 22 mN/m in pure ethanol. In comparison, the value of γ of
surfactant solutions decreases to 30 mN/m with surfactant concentration around mil-
limoles per liter (range of 1 to 10 g/L). Soaps have been used by humans for many
centuries. In biology, one finds a whole range of amphiphile molecules (bile salts,
fatty acids, cholesterol and other related molecules, phospholipids).
Surfactants are among the most important substances that play an essential role
in everyday life. There also exist many surfactants in nature (such as bile acids in the
stomach) that behave exactly the same way as human-made surface-active agents.
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