The ζ-potential of polyamide fiber Nylon-6 was measured in aqueous acidic solution (pH 3) of anionic surfactants-sodium 2-sulfonatofatty acid methyl esters (abbr. SMSF, C
nH
2n+1CH (SO
3Na) COOCH
3; n=12 : SMSM, n=14 : SMSP and n=16 : SMSS) and sodium alkyl sulfates (abbr. SAS, C
nH
2n+1O·SO
3Na; n=12 : SDS, n=14 : STS and n=16 : SHS). The amount adsorbed of surfactants per unit area of the fiber surface was calculated from these ζ-potential values. With increasing the concentration of surfactant, the sign of the ζ-potential value of the fiber changed from positive to negative and the amount adsorbed of surfactant (expressed in mol/cm2-fiber) increased because of the electrostatic attractive force and van der Waals' force between the fiber and those surfactants. The amount adsorbed of SMSF was higher than those of SAS. This result may possibly be attributed to the contribution of the methine group in SMSF molecule to the adsorption and the formation of the hydrogen bond between a carbonyl group of the surfactant and an amide group of the fiber.
The free energy of adsorption for surfactant,
ΔG, was calculated from the slope of the ζ-logC
s (C
s : the concentration of surfactant) curve, etc. The negative values of
ΔG for SMSF increased with rise of temperature and with increase of the chain length of the surfactant. Also the negative values of
ΔG for SMSF were higher than those of SAS. The heat of adsorption
ΔH and the entropy of adsorption
ΔS for SMSM were -2.5kcal·mol
-1 and 14.5cal·mol
-1·deg
-1, respectively. The corresponding parameters for SMSP were -2.0kcal·mol
-1 and 18cal·mol
-1·deg
-1, respectively. The relatively high positive values of
ΔS indicate the importance of entropic interaction and the formation of hydrophobic bond as a driving force to adsorption of SMSF on the fiber surface.
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