Discussion
Which of the following is correct ?
(I) CMC of an anionic surfactant is higher than that of a non-ionic surfactant of the same chain length
(II) CMC of an anionic surfactant is lower than that of a non-ionic surfactant of the same chain length
(III) CMC of an ionic surfactant decreases in presence of small quantity of added electrolyte
(IV) CMC of an ionic surfactant increases in presence of small quantity of added electrolyte
Answer: B
The correct statements are (I) and (III).
A surfactant forms micelles when the gain from hiding its hydrocarbon tails outweighs the cost of crowding its head groups together. Ionic head groups repel one another electrostatically, which makes micellization harder, so an anionic surfactant needs a higher concentration to form micelles than a non-ionic surfactant with the same tail. Statement (I) is true and (II) false; for C12 chains the CMC of sodium dodecyl sulfate is about 8 mM, while non-ionic C12 surfactants lie near 0.1 mM.
Added electrolyte screens the repulsion between ionic head groups (and its counter-ions bind to the micelle surface), so micelles form more easily and the CMC of an ionic surfactant decreases. Statement (III) is true and (IV) false. The options that include (II) or (IV) contain one of the false statements.
Post your comment
Please log in or register to post a comment.
Examination Categories
- SSC Exams
- Banking Exams
- Teaching Exams
- Civil Services Exams
- Railways Exams
- Engineering Recruitment Exams
- Defence Exams
- State Government Exams
- Police Exams
- Insurance Exams
- Nursing Exams
- Other Government Exams
- NRA CET
- PG Entrance Exams
- Campus Placement
- MBA Entrance Exams
- CUET and UG Entrance
- Judiciary Exams
- Regulatory Body Exams
- Software Development