The hydrophilic head is the ionic end of a soap or detergent particle that dissolves in water.
The hydrophilic head is the ionic end of a soap or detergent particle that dissolves in water. Malay: kepala hidrofilik · Chinese: 亲水头.
The word hydrophilic means “water-loving”. In soap the head is the carboxylate group –COO⁻; in a detergent it is the sulphonate group –SO₃⁻. Both carry a negative charge, so they are attracted to the polar water molecules and dissolve readily in water. This charged head is only one part of the cleaning particle: the other part is the long, non-polar hydrophobic tail that dissolves in grease. During cleaning, the hydrophilic heads all point outward into the water while the tails bury themselves in the grease, so the grease droplet is held in the water and can be rinsed away instead of clinging to the cloth.
Example. When soap surrounds a droplet of oil, the heads face outward and give the whole droplet a negative surface charge. Because like charges repel, neighbouring droplets push each other apart instead of joining back together, so the loosened dirt stays suspended in the water and washes off cleanly. This repulsion between charged heads is the reason the emulsion is stable long enough to be rinsed away.
Confusion to avoid. Do not swap the head and the tail. The hydrophilic head dissolves in water and stays outside the grease; the hydrophobic tail dissolves in grease and stays out of the water. Mixing them up reverses the whole explanation of cleaning, so always attach “water” to the head and “grease” to the tail as you write.
The hydrophilic head and the hydrophobic tail together explain the cleaning action, which is one of the most heavily tested ideas in this chapter. Describing both ends correctly, and the charge on the head, is essential for the full SPM Chemistry explanation mark.
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