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The role of organic additives in changing the sign of standard enthalpy of micellization

Rabah A.Khalil*, Alaa M.Hashim

The phenomenon of changing the sign of standard enthalpy ofmicellization ( o Hm) from negative to positive for anionic surfactant [sodium dodecyl sulfate (SDS)]; due to presence of hydrocortisone acetate (HA) as an additive has been recently detected by Khalil and Hassan (J. Dispersion Science&Technology, 31:1195–1201, 2010). No such observation for cationic cetyltrimethylammonium bromide (CTAB) surfactant has been noticed by those authors. The reason for this phenomenon has been doubtfully attributed to the hydrogen bond between anionic polar head group of SDS with hydrogen of HA hydroxyl group. In order to investigate a certain reason for this problem, the effect of four organic additives (methyl salicylate (MS), benzene, toluene and cyclohexane) on the critical micelle concentration (CMC) of SDS and CTAB has been examined. The results exhibit that the effect of the presented additives on the sign of o m H was in a similar manner to that of HA. It was concluded that the hydrophobic effect through iceberg formation which resulted from the presence of hydrophobic additives plays a major role in changing the sign of o m H and increasing the CMC of both anionic and cationic surfactants. CTAB surfactant does not show such changes in the sign of o m H which attributed to competitive between the hydrophobic effect with its high tendency towards forming micelle in contrast with that of SDS.