Anionic and nonionic surfactant phase distribution processes in environmentally relevant matrices
Abstract
The purpose of this research was to further characterize phase distribution processes of anionic and nonionic surfactants under conditions applicable to soil and groundwater remediation precesses. The interactions of: (1) sorption of a specific nonionic surfactant mixture to soil matrices, (2) precipitation of an anionic surfactant with calcium, (3) counterion binding to micelles, and (4) mixed micelle formation were investigated. Chemical speciation models were developed that calculate counterion binding in pure anionic and mixed anionic/nonionic surfactant systems. In addition, the mixed surfactant model allows for the calculation of precipitation boundaries, as well as the mixed cmc. Sorption of a nonionic surfactant mixture, polyoxyethylene 23 lauryl ether (Brij 35), was investigated with five well characterized soils. The results show that preferential sorption of homologues with long ethylene oxide (EO) chains occurs, likely due to hydrogen bonding between the EO groups and soil matrices. Additionally, adsorption isotherms for all soils revealed a plateau in the isotherm at aqueous phase surfactant concentration equal to the cmc. The association reactions involving counterions and micelles composed of the anionic surfactant, dodecylsulfate, were investigated using ultrafiltration experiments. To access this data, a model was developed while considers specific counterion binding within a Stern layer, with binding constant dependent upon the electrical potential as derived by Poisson-Boltzmann equation. The experimental and model results both show that magnitude of counterion binding is approximately the same for Ca$\sp{2+}$ and Mg$\sp{2+}$ and decreases for the monovalent species, Na$\sp{2+}$. However, high concentration of Na$\sp+$ compete for surface area diminishing the ability of the DS$\sp-$ to bind either divalent species. Nevertheless, the change in ionic strength at high concentration of Na$\sp+$ is enough to lower the cmc such that the hardness tolerance actually increases. Based on the anionic surfactant model, a modified mixed surfactant model was developed which included two additional processes of nonideal mixing in the mixed micelles and precipitation of a divalent counterion with the anionic surfactant. The solubility product relationship and regular solution theory are employed to describe these two processes. The predictions for those mixed systems investigated (i.e., precipitation boundary and mixed cmc) show good agreement between the experimental data and model calculations.
Degree
Ph.D.
Advisors
Jafvert, Purdue University.
Subject Area
Civil engineering|Polymers|Environmental engineering
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