Thermodynamic Modelling of Surfactant Solutions

Hongyuan Cheng

Research output: Book/ReportPh.D. thesis

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In this work, the key physical properties of non-ionic and ionic surfactant solutions, such as the critical micelle concentration (CMC) and the octanol-water partition coefficient (Kow), are studied by molecular thermodynamic methods based on UNIFAC. The mean activity coefficients of the aqueous electrolyte and organic electrolyte solutions are modelled by a modified Achard’s UNIFAC model proposed in this work. Some of important properties in the surfactant solutions, such as the hydrophilic-lipophilic balance (HLB), the Krafft point (KP), the cloud point (CP), the aggregation number (ng), the toxicity (EC50) and the bioconcentration factor (BCF) are also investigated. And suitable correlations have been developed. Surfactant solutions are unique systems because the surfactant molecules form micelles in aqueous and non-aqueous solvents by self-assembly under the hydrophobic interaction with solvent molecules. Surfactant solutions have attracted much attention from academia and industry because they play important role in different industrial areas, e.g. chemical and oil industry, pharmaceutical and bio-industries, paper, food and film industries. In this work, different thermodynamic frameworks for the micelle formation of surfactant molecules in aqueous solution are systematically reviewed and compared. The method proposed by Chen et al. (Chen, C.-C., AIChE J., 42, 3231, 1996; Chen, C.-C., C. P. Bokis, and P. Mathias, AIChE J., 47, 2593, 2001) is selected for studying the micelle formation of surfactant molecules in aqueous solutions. Based on the method of Chen (1996), the CMC of non-ionic surfactant solutions is correlated and predicted successfully with the UNIFAC method. In this step, a new UNIFAC functional group is introduced. The necessary interaction parameters for the new group are obtained from vapour-liquid equilibrium data. In order to correlate the CMC of ionic surfactant solutions, an electrolyte UNIFAC model proposed by Achard et al. (Achard C., C. G. Dussap, and J. B. Gros, AIChE J., 40, 1210 , 1994a; Achard C., C. G. Dussap, and J. B. Gros, Fluid Phase Equilibria, 98, 71, 1994b) is modified and used to correlate the mean ionic activity coefficient of aqueous electrolyte and organic electrolyte solutions. It was found that the structural parameters (Q, R) of ions used in the work of Achard and others do not follow Flory-Huggins assumption (Q/R≤1). Thus, a new method is developed to calculate the Q, R of ions from ionic radii using the condition Q/R<1. The mean activity coefficients of some aqueous electrolyte solutions are correlated with the modified Achard method and compared with the electrolyte NRTL and the extended UNIQUAC models. The mean activity coefficients of five sodium carboxylate systems are correlated simultaneously with the modified Achard model. The correlation results show a good agreement with the experimental values. Based on the work of Chen et al. (2001), the CMC of sodium alkyl sulphates, sodium alkyl sulfonates and potassium carboxylates are successfully correlated using the modified Achard model. Furthermore, the Kow of phthalates and non-ionic surfactants are predicted with different UNIFAC methods and commercial software. The prediction results are compared with the few experimental data available.
Finally, some correlations for properties of surfactant solutions, i.e. HLB, KP, CP, ng, EC50 and BCF are presented.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages203
ISBN (Print)87-91435-00-5
Publication statusPublished - Mar 2004


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