A widely accepted model in obtaining the mixed micelle composition is Rubingh's thermodynamic model, in which CMC data from surface tension and conductivity measurements of mixed surfactant solutions are used to calculate compositions of mixed micelles. The validity of this model has not been previously challenged. In this study, the behaviour of binary mixtures of an anionic; sodium dodecylsulfate (SDS) and a cationic; dodecyltrimethyl ammonium bromide (DTAB) with non-ionic surfactants; N -dodecyl-N ,N -di mcthyl-3-ammoni o-l-prop<mesulfonatc (SB), N-dodecyl maltoside (DM), Octaethyleneglycol monododccylcthcr (C12Es) and 7,7-bis([( 1,2,3,4,S pentahydroxyhexanamido) methyl]-n-tridecane (di-GLU) in aqueous solutions has been studied using surface tensiometry, conductometry and small-angle neutron scattering (SANS) techniques. Firstly we investigated the critical micelle concentrations (CMCs) of aqueous solutions of single and binary mixtures of the surfactants and found large discrepancies in the CMCs obtained from surface tension and conductivity measurements even when those of the individual components are fairly constant. CMCs obtained from conductivity measurements are found to be larger, in fact, in some mixtures, about twenty times higher than those obtained from surface tension measurements. SANS measurements suggest that the size of mixed micelles vary with surfactant concentration and composition above CMC. Therefore, it is possible that surface tension measurements detect CMC at the point of maximum surface coverage above which mixed micelle size and composition continue to change. This is in agreement with the conductivity measurements, which suggest that only the presence of micelles with an appreciable size that is able to influence the conductivity of the mixed solution is detected. We conclude that the CMC of surfactant mixtures is not a well defined thermodynamic parameter as previously thought. Secondly, mixed micelle compositions calculated from the CMCs of the single and mixed surfactants using Rubingh's thermodynamic model and a new conductivity model developed in this work arc compared with those obtained from SANS model. The results from SANS model and the conductivity model show that the compositions of mixed micelles are close to ideal compositions in contrasts to those calculated from Rubingh's thermodynamic model which deviate widely from ideality. Mixed micelle compositions obtained from conductivity measurements using the new model show very good agreement with SANS results while those from surface tension and conductivity data applied to Rubingh's model differ appreciably from both. Thirdly. the values of Rubingh' s interaction parameter between the mixing surfactants calculated using CMCs determined from conductivity (CMC.J and surface tension (CMCy) measurements also show discrepancies. Taken together, our results in all the eight different binary mixed surfactant systems studied suggest that Rubingh's model cannot correctly predict the compositions of surfactant mixtures and thus its thermodynamic foundation needs further re-examination.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:619452 |
| Date | January 2013 |
| Creators | Cookey, Grace Agbizu |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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