Investigations on zwitterionic alkylsulfobetaines and nonionic triton X-100 in mixed aqueous solutions: Effect on size, phase separation and mixed micellar characteristics

Zwitterionic sulfobetaine surfactants are widely applied in industries dealing with personal care products, mainly due to an advantage of minimal irritating effects to skin as compared to ionic surfactants. We investigate on the aqueous solution behaviour of zwitterionic surfactant, namely alkyl N,N-dimethyl-3-ammonio-1-propanesulfonate (C_nPS, n = 10, 12, 14, 16) in pure and mixed state with Triton X-100 (non-ionic surfactant) by using surface tensiometry, dynamic light scattering (DLS) and cloud point measurements. The critical micelle concentration (CMC) for aqueous C_nPS solutions decreases with corresponding increase in alkyl chain length and is also accompanied by an increase in aggregation number. The calculated interfacial parameters for binary mixed aqueous systems involving zwitterionic surfactant and TX-100 depict almost independent behaviour of π_CMC and Γ_max, indicating a non-associative ideal mixing. The calculated interaction parameter (β^m) for TX-100 + C_nPS mixed system reveals insignificant deviation from ideal behaviour as confirmed by calculated values in proximity to zero. As concluded from size measurements using DLS, the apparent hydrodynamic size of the mixed micelles decreases with gradual addition of alkylsulfobetaines due to demicellization of formerly present ellipsoidal aggregates of TX-100. The clouding temperature of formed mixed micelles increases on gradual addition of C_nPS due to corresponding increase of ionic charge and accordingly improved solubility. It is assumed that electrically neutral nature of sulfobetaines confines its electrostatic interactions with non-ionic TX-100 and accordingly deters feasible interactions leading to synergism.