M. Tornblom et U. Henriksson, EFFECT OF SOLUBILIZATION OF ALIPHATIC-HYDROCARBONS ON SIZE AND SHAPE OF RODLIKE C(16)TABR MICELLES STUDIED BY H-2 NMR RELAXATION, JOURNAL OF PHYSICAL CHEMISTRY B, 101(31), 1997, pp. 6028-6035
Multifield H-2 relaxation has been used to quantify the effect of solu
bilization of alkanes on the size and shape of the micelles in aqueous
solutions of hexadecyltrimethylammonium bromide, C(16)TABr. Informati
on about aggregate size and shape was deduced from the reorientation r
ate of the aggregates. The surfactant concentration was chosen so that
the solutions without solubilizate contained moderately long rodlike
micelles (axial ratio similar to 4). It was found that the change in a
ggregation behavior upon solubilization is substantially different for
solubilizates of similar chemical composition but different structure
s. Solubilization of small amounts of cyclohexane, cyclooctane, cyclod
ecane, trans-decalin, n-hexane, 2,3-dimethylbutane, or adamantane caus
es growth of the micelles. Further addition of these solubilizates (ex
cept for adamantane, which has a lower solubility limit than the other
solubilizates) causes a decrease in the aggregate size. On the other
hand, solubilization of n-octane, n-decane, or n-dodecane causes a dec
rease in aggregate size even in small amounts. It was also found that
when the rodlike micelles grow due to the presence of solubilizate mol
ecules, they grow in the axial direction at a constant radius, while t
he decrease in size at higher solubilizate content occurs as a shorten
ing of the rods accompanied by an increase in radius. The differences
in effects are explained by different solubilization sites within the
micelle for alkanes with different molecular volume and rigidity.