The mean activity coefficients (γ
±) of sodium 4-(
p-dialkylaminophenylazo) naphthalene-1-sulfonates [MN dyes, alkyl=methyl (MN), ethyl (EN), and
n-butyl (BN)] in water were determined by means of vapor pressure osmometry (VPO). The γ
± values for the dyes were much smaller than unity and decreased with increases in the concentration of dye and in the alkyl chain length of the molecule (MN > EN > BN). The γ
± values for MN dyes over a wide range of the concentration were smaller than those for the methyl orange series with corresponding alkyl substituents. These results suggested that the γ
± values for MN dyes were also affected by the naphthalene nucleus. The aggregation numbers (n) for the dyes were predicated by the relation between the mean activities and concentrations, and furthermore by the analysis of visible absorption spectra. At 50°C, MN dye existed in aqueous solutions as nearly a trimer (n=3) and EN dye as a tetramer (n=4), while BN dye formed much larger aggregates than the others. The formation of large BN aggregates was confirmed by the solubilization of a hydrophobic cationic dye (methylene blue)-BN complex into a concentrated BN aqueous solution.
View full abstract