Question

A soap is represented as ​$RCO{O}^{-}N{a}^{+}$, where $R=C_{17}{H}_{35}$
When dissolved in water it dissociates into ​$RCO{O}^{-}$ and $N{a}^{+}$ ions. Which of the following is correct?

• A. $CO{O}^{-}$ is the hydrophilic part
• B. $CO{O}^{-}$ is the hydrophobic part
• C. Hydrocarbon chain $\left(R\right)$ is hydrophilic part
• D. Both (b) and (c)

Answer 1

The correct option is A $CO{O}^{-}$ is the hydrophilic part

Soap solution is an example of micelle.
Soap is sodium salt ​of a higher fatty acid. It is represented as ​$RCO{O}^{-}N{a}^{+}$
Eg: Sodium stearate $\left(C{H}_3\right(C{H}_2{)}_{16}CO{O}^{-}N{a}^{+})$

Mechanism of Micelle formation:
When Sodium stearate is dissolved in water it dissociates as $RCO{O}^{-}$ and $N{a}^{+}$.
In $RCO{O}^{-}$, hydrocarbon chain (R) is hydrophobic part and $CO{O}^{-}$ is hydrophilic part.
So, $RCO{O}^{-}$ arranges ​itself in such as way that​ $CO{O}^{-}$ can be close ​to water​ and hydrocarbon chain can stay away from it ​by staying at surface.​

But at higher concentration these are pulled into the bulk of the solution​ and they aggregate in ​a spherical form.​ Spherical form has the hydrocarbon chains pointing towards the centre with $CO{O}^{-}$ part remaining outward on the surface.​ These aggregated form is known as micelle. These micelle may contain 100 such ions.​