Freebase Nicotine has a higher pH level which in turn affects the alkalinity. It causes a harsher throat-hit. Nicotine salt has lower alkalinity due to the acid which gives you smoother vaping experience.
The nicotine absorption rate into the blood
Nicotine Salt e-liquid has a faster absorption rate.The pH level is more suitable for absorption.
LIQUA 4S offers you the same nicotine delivery as cigarettes in combination with 4S devices and 4S e-liquid.