Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution.
ABSTRACT Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C16TAX) with different counterions, F-, Cl-, Br-, NO3-, and (1/2)SO4(2-), have been studied. Critical micelle concentration (CMC), degree of micelle ionization (alpha), and enthalpy of micellization (DeltaH(mic)) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the alpha increase in the order SO4(2-) < NO3- < Br- < Cl- < F-, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. DeltaH(mic) becomes less negative through the sequence NO3- < Br- < Cl- < F- < SO4(2-), and even becomes positive for the divalent sulfate. The special behavior of sulfate is associated with both its divalency and its degree of dehydration. Gibbs free energies of micellization (DeltaG(mic)) and entropies of micellization (DeltaS(mic)) have been calculated from the values of DeltaH(mic), CMC, and alpha and can be rationalized in terms of the Hofmeister series. The variations in DeltaH(mic) and DeltaS(mic) have been compared with those for the corresponding series of gemini surfactants. Electron spin resonance has been used to assess the micropolarity and the microviscosity of the micelles. The results show that the microenvironment of the spin probe in the C16TAX surfactant micelles depends strongly on the binding of the counterion.
Article: Effect of 1-alkanols/salt on the cationic surfactant micellar aqueous solutions—A dynamic light scattering study[show abstract] [hide abstract]
ABSTRACT: The effect of 1-alkanols viz., ethanol, 1-butanol and 1-hexanol on five quaternary salts based cationic surfactants in water and in presence of 0.1M salts has been systematically investigated by viscosity and dynamic light scattering (DLS) technique. The concentration for surfactant varying in non-polar tail, polar head and counterion was kept constant (∼50mM) throughout. The viscosity data are less detailed, but confirm the general trend of micellar growth and transitions. Influence of 1-butanol on critical micelle concentration (CMC) and degree of counterion dissociation (() was also investigated conductometrically. These measurements were done to supplement the role of counterions in exploring the binding phenomenon due to the effect of salt and alcohol during micellization. The effect of 1-alkanols upon the viscosity and micelle hydrodynamic diameter (Dh) of micelles in the presence of salt at varying concentration and temperature was also examined in detail to explain the hydrophobic interactions and electrostatic effects.Colloids and Surfaces A Physicochemical and Engineering Aspects 02/2011; · 2.24 Impact Factor
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ABSTRACT: The Hofmeister effect on the critical micelle concentration (CMC), the thin liquid film electrostatic disjoining pressure (Π(el)) and the critical coalescence pressure of emulsion drops (P(CR)) were investigated. For CMC literature data were used, but Π and P(CR) were measured by us. The essence of the theoretical approach was to modify existing theories of CMC and Π(el) by using generalized Gouy equation and dimensionless surface potential (Φ(S)), involving the counterion specific adsorption energy (u(0)). The computational procedure of u(0) does not involve any adjustable parameters. Linear dependences of ln(CMC), Φ(S) and P(CR) on u(0) were found in conformity with Hofmeister series. The experimental slopes of ln(CMC) and Φ(S) vs. -u(0)/k(B)T were negative and very close to the theoretical ones. A hypothesis was put forward for explanation of the positive slopes of P(CR) on u(0). The obtained results suggest that the counterion specific adsorption energy u(0) encompasses all major factors, involved in the Hofmeister effect for the studied phenomena. If this is confirmed by analysis of more phenomena, revealing Hofmeister effect, one could claim that u(0) is the factor controlling the Hofmeister effect and a powerful tool for its study.Advances in colloid and interface science 06/2011; 168(1-2):93-104. · 5.68 Impact Factor
Article: The effect of Hofmeister and alkylcarboxylate anionic counterions on Krafft temperature and Melting temperature of cationic gemini surfactants.[show abstract] [hide abstract]
ABSTRACT: The effect of counterions was investigated to probe the principal ionic effects on the solubility in water and melting behavior of cationic gemini surfactants. We focused on two types of counterions: (1) small and inorganic counterions which are typically taken from the Hofmeister series were studied to focus on the effect of ion type; (2) n-alkyl carboxylate counterions were studied to focus on the effect of the hydrophobicity of counterions. Krafft Temperature (Tk) and melting temperature (Tm) were obtained by conductivity measurements, calorimetric measurements as well as optical microscopy observation. The results clearly indicate that Tk which represents the solubility of surfactants is not determined by one single parameter of ions such as their hydration free energies, as is too often assumed, but rather by the combined effects between the hydrophobicity of anions associated with other effects such as polarizability, dehydrated ion size and ionic morphology. In parallel, our observation demonstrated that all the surfactants showed a transition from a crystalline phase to a thermotropic liquid crystalline phase at around ca. 70°C, which transformed to an isotropic liquid phase at around ca. 150°C, and that the transition temperatures depended strongly on the counterion type. The counterion effects on the solubilization and melting behaviours were then compared with micellization properties that have been reported previously. These results provide new insight in understanding the effect of ions on the delicate balance of forces controlling solution properties and aggregate morphology of charged amphiphilic molecules. Specifically, the solubilization properties of these cationic surfactants with various counterions were determined mainly by the subtle interplay between the hydration of counterions and dissociation energies (stability of crystallinity) of the ion pair.Langmuir 01/2013; · 4.19 Impact Factor