Chapter 25
Thermodynamic Study of Adsorption of Anionic—Nonionic Surfactant Mixtures at the Alumina—Water Interface
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Edward Fu, P. Somasundaran, and Qun Xu Langmuir Center for Colloids and Interfaces, Henry Krumb School of Mines, Columbia University, New York, NY 10027
Mechanisms of adsorption of an anionic surfactant, sodium octylbenzenesulfonate (C øS), and a nonionic surfactant, dodecyloxyheptaethoxyethylalcohol (C EO ), and their mixtures on alumina were investigated by adsorption and microcalorimetric studies. Adsorption of anionic surfactant alone on alumina was initially highly exothermic due to the electrostatic interaction with the substrate. Further adsorption leading to solloid (hemimicelle) formation is mainly an entropy driven process. The entropy effect was found to be more pronounced for the adsorption of anionic-nonionic surfactant mixtures than that for anionic C øS alone. High surface activity of the nonionic C EO and its hydrophobic interaction with adsorbedQ8øSis proposed to be the main mechanism for the marked entropy effect for mixture adsorption. 8
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Mixtures of anionic and nonionic surfactants have shown enhanced surface activity and salt tolerance (1-3) which are highly advantageous for applications in industrial processes such as enhanced oil recovery, detergency and flotation (4-5). However, an overwhelming majority of basic studies of surfactant adsorption have been performed with systems containing single surfactants. Although the solution behavior of ionic/nonionic surfactant mixtures have been extensively studied in the past (1-3, 6-10), only very limited amount of work has been done on adsorption at solid/liquid interface (11-13). The mechanisms by which ionic/nonionic surfactant mixtures adsorb have not been well understood, in part due to the lack of thermodynamic data for such adsorption. Regular mixing theory, which is usually adequate for fitting CMC data of ionic/nonionic surfactant mixtures, was found to be inadequate for correlating the data for adsorption of anionic/nonionic surfactant mixtures at alumina/water interface (11). It is clear that direct measurement of thermodynamic parameters is needed for understanding the mechanisms and for developing better theoretical models for adsorption of surfactant mixtures at solid/liquid interfaces. 0097-6156/92/0501-O366$06.00/0 © 1992 American Chemical Society
In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
25. FU ET AL.
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Adsorption of Anionic—Nonionic Surfactant Mixtures
In this study, adsorption at alumina/water interface was conducted with isomerically pure anionic and nonionic surfactants and their mixtures. Enthalpy of adsorption was measured using a microcalorimetry system. The adsorption mechanism was discussed on the basis of adsorption and microcalorimetric results.
Downloaded by NORTH CAROLINA STATE UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch025
Materials and Methods Alumina: Linde A alumina was purchased from Union Carbide Co., USA. It had a mean diameter of 0.3 microns and a surface area of 14 m /g as measured by N BET adsorption using a Quantasorb system. Anionic surfactant: Sodium para-octylbenzenesulfonate (Q^S) was synthesized in our laboratory. High performance liquid chromatography data of this compound showed it to be more than 97% isomerically pure. Nonionic surfactant: Dodecyloxyheptaethoxyethylalcohol (C EO ) was purchased from Nikko Chemicals, Japan. This surfactant was specified to be monodispersed and at least 97% pure. NaCl, HC1 and NaOH used for regulating ionic strength and pH were of A.R. grade. Triply distilled water (conductivity 1-2x10"* mhos) was used throughout the experiments. Adsorption: Adsorption experiments were conducted in capped 50 ml centrifuge tubes at a constant ionic strength of 0.03 M NaCl and a solid/liquid ratio of 0.1 w/w. The samples were kept in a water bath set at 50 °C for three days during which time the pH was adjusted using 0.1 N HC1 or NaOH. The sample was centrifuged for 30 minutes at 4500 rpm inside an incubator set at 50 °C and 20 ml of supernatant was pipetted out for analysis. Alcohol concentrations were measured by high performance liquid chromatography using 90:10 v/v solvent mixtures of acetonitrile and water, a reverse phase column, and a refractive index detector. Sulfonate concentrations above 2x1ο kmol/m were measured by a two phase titration technique (14). Dilute solutions below 2x1ο kmol/m were analyzed by U V absorbance at 223 nm using a Beckman DU-8 UV-visible spectrophotometer. Microcalorimetry: Calorimetric experiments were performed using an L K B 2107 differential isoperibol microcalorimetry system. The actual temperature in the air bath was 49.3 ± 0.1 °C for the experiments. Within the calorimeter were two 18 carat gold vessels, one for sample solutions and the other for reference. Heat was generated when the materials in the vessels were mixed by rotating the cylindrical calorimetrical body. Each mixing action consists of two complete revolutions, one in each direction, and was repeated for a sufficient number of times to ensure complete mixing. The heat was passed through a thermopile and was transduced into voltage signal. The area under the voltage-time curve was integrated using a digital read-out system and was given as 2
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