Article

Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution.

CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People's Republic of China.
Journal of Colloid and Interface Science (impact factor: 3.07). 06/2005; 286(2):755-60. DOI:10.1016/j.jcis.2005.01.064 pp.755-60
Source: PubMed

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.

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Keywords

alpha increase
 
C16TAX surfactant micelles
 
cationic surfactants hexadecyltrimethylammonium
 
Cl-
 
conductivity measurements
 
corresponding series
 
Critical micelle concentration
 
DeltaH(mic)
 
different counterions
 
divalent anion sulfate
 
divalent sulfate
 
enthalpy
 
gemini surfactants
 
Gibbs free energies
 
Hofmeister series
 
isothermal titration microcalorimetry
 
microenvironmental properties
 
NO3-
 
sequence NO3-
 
special behavior
 

Nan Jiang