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Contact angle of surfactant solutions on precipitated surfactant surfaces. II. Effects of surfactant structure, presence of a subsaturated surfactant, pH, and counterion/surfactant ratio
Author(s) -
Balasuwatthi Piyada,
Dechabumphen Nimit,
Saiwan Chintana,
Scamehorn John F.
Publication year - 2004
Publication title -
journal of surfactants and detergents
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.349
H-Index - 48
eISSN - 1558-9293
pISSN - 1097-3958
DOI - 10.1007/s11743-004-0285-y
Subject(s) - pulmonary surfactant , chemistry , contact angle , surface tension , counterion , wetting , critical micelle concentration , alkyl , aqueous solution , inorganic chemistry , chemical engineering , micelle , chromatography , organic chemistry , thermodynamics , ion , biochemistry , physics , engineering
The contact angle of a saturated aqueous surfactant solution on the precipitate of that surfactant was measured by using the sessile drop method. The sodium and calcium salts of alkyl sulfates (C 12 , C 14 , and C 18 ) had advancing contact angles higher than those of alkyl trimethylammonium bromides (C 14 , C 16 , and C 18 ). The measured advancing contact angles for several surfactant solutions did not substantially change with varying surfactant/counterion ratios; therefore, the precipitating counterion concentration (e.g., water hardness) had little effect on the wettability. The contact angles of fatty acid (C 12 and C 16 ) solutions did not show any dependence on pH between a pH of 4 and 10. The contact angles of saturated calcium dodecanoate (CaC 12 ) solutions containing a second subsaturated surfactant (sodium dodecyl sulfate: NaDS) decreased with increasing NaDS concentrations until reaching the critical micelle concentration of the surfactant mixture. These results show that the second suractant can act as a wetting agent in this saturated surfactant system. Application of Young's equation to contact angles showed that the solid/liquid surface tension can change substantially with surfactant concentration and be important in addition to the liquid/vapor surface tension in reducing contact angles. Application of the Zisman equation results in a “critical” surface tension for the CaC 12 or soap scum of 25.5 mN/m, which is comparable to difluoroethene.