Adsorption Calorimetric Study of the Organization of Sodium n-Decyl Sulfate at the Graphite/Solution Interface

• Published in: Langmuir Vol. 17; no. 8; pp. 2420 - 2425
• Main Authors: Király, Z, Findenegg, G. H, Klumpp, E, Schlimper, H, Dékány, I
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17.04.2001
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la001552y

The material and enthalpy balances of the adsorption of sodium n-decyl sulfate from aqueous solutions onto graphitized carbon black were determined between 288 and 318 K by using an automated flow sorption/microcalorimeter system. At low concentrations, the surfactant molecules form a flat monomolecular film on the graphite plane, in consequence of surface-directed ordering. A mechanism is proposed in which two adsorbed phases coexist during the formation of this surfactant monolayer. The enthalpy of adsorption in the monolayer region is ca. −42 kJ mol-1, which does not depend appreciably on the temperature or on the surface coverage. At higher concentrations, the ordered monolayer induces surface aggregation to produce half-cylindrical hemimicelles as the critical micelle concentration is approached. The enthalpies of surface aggregation at 288, 298, and 318 K are −10, −16, and −26 kJ mol-1, respectively. As the temperature is increased from 288 to 318 K, the average number of surfactant molecules in the cross section of a half-cylinder drops from ca. 5.4 to 3.4. Calorimetric evidence is provided that cationic and anionic surfactant adsorption on graphite follow the same mechanism in the low-density and high-density adsorbate regions.