Molecular-Thermodynamic Framework to Predict the Micellization Behavior of Mixtures of Fluorocarbon-Based and Hydrocarbon-Based Surfactants

We present a molecular-thermodynamic (MT) framework to predict the micellization properties of mixtures of fluorocarbon-based and hydrocarbon-based surfactants. Practically, this mixing reduces the use of fluorinated surfactants in the surfactant formulation, thereby addressing environmental concerns associated with the non-biodegradability and toxicity of fluorinated surfactants. The micellization properties of these mixtures are affected by the enthalpic interactions between the fluorocarbon and hydrocarbon surfactant tails. Consequently, the MT framework incorporates an enthalpy of mixing contribution estimated using regular solution theory (RST). The RST interaction parameter is estimated on the basis of phase equilibrium data. The MT framework also makes allowance for the coexistence of two types of micelles in solution to account for experimental findings which suggest that mixtures of fluorocarbon-based and hydrocarbon-based surfactants can form two types of micelles. Furthermore, the model used to calculate the packing free energy of binary mixtures of surfactant tails is generalized to incorporate the difference in the tail volumes, tail lengths, and conformational energies of the fluorocarbon and hydrocarbon tails. The MT framework is then used to predict micelle population distributions, critical micelle concentrations, and optimal micelle compositions for various mixtures of fluorocarbon-based and hydrocarbon-based surfactants, and the predictions are compared with the corresponding experimental values. While many of the predictions compare well with experiment, some of the experimentally observed trends are not reproduced by the MT framework. Ways to eliminate the discrepancy between theory and experiment are discussed. We also find that prediction of the micelle population distribution is very sensitive to the magnitude of the RST interaction parameter used to calculate the enthalpy of mixing, where an increase in the RST interaction parameter results in sharper peaks in the predicted bimodal micelle population distribution. In addition to the quantitative prediction of micellization properties, the MT framework provides useful physical insight about the reasons behind the differences in the micellization properties of various surfactant mixtures

Are Ellipsoids Feasible Micelle Shapes? An Answer Based on a Molecular-Thermodynamic Model of Nonionic Surfactant Micelles

The existence of ellipsoidal micelles in aqueous solution has been debated in the literature. Although a number of experimental studies suggest that certain surfactants form ellipsoidal micelles, many theoretical studies have claimed that micelles with an ellipsoidal shape cannot exist. To shed light on this topic, in this paper, we develop a curvature-corrected, molecular-thermodynamic model for the free energy of micellization of nonionic surfactant biaxial ellipsoidal micelles. We subsequently use this model to evaluate the feasibility of forming ellipsoidal micelles, compared to forming spherical, spherocylindrical, and discoidal micelles, and conclude that ellipsoidal micelles can exist in solution. Utilizing the model developed here, we also establish theoretical limits on the size of the ellipsoidal micelles. These limits depend solely on the chemical structure of the surfactant molecule

Impact of Chemical and Mechanical Processes on Leakage from Damaged Wells in CO₂ Storage Sites

The perceived risk of CO2 leakage through wells has been considered a potential limitation to commercial scale deployment of geologic CO2 storage. However, chemical and mechanical alteration of cement can reduce the permeability of leakage pathways. We conducted 100s of simulations spanning realistic operating conditions and well-damage characteristics to understand (1) under what conditions and time frames do fractures seal and (2) for fractures that do not seal, how quickly and to what extent is the permeability reduced. For the conditions simulated, fractures with apertures in the tens of microns seal while those greater than hundreds of microns may exhibit long-term leakage. Fractures with apertures between 10 and 500 μm took a few days to a couple of years to seal. For non-sealing fractures mechanical deformation of altered asperities can rapidly reduce permeability. A sealing criterion was developed to relate fracture aperture with the cemented length required for self-sealing. Longer cemented intervals can seal large fractures; however, they take longer to seal and leak larger volumes before sealing. While the results presented here are subject to uncertainties, the manuscript provides a framework in which a model can be used to quantitatively answer questions regarding well integrity to facilitate decision making

Computer Simulation–Molecular-Thermodynamic Framework to Predict the Micellization Behavior of Mixtures of Surfactants: Application to Binary Surfactant Mixtures

We present a computer simulation–molecular-thermodynamic (CSMT) framework to model the micellization behavior of mixtures of surfactants in which hydration information from all-atomistic simulations of surfactant mixed micelles and monomers in aqueous solution is incorporated into a well-established molecular-thermodynamic framework for mixed surfactant micellization. In addition, we address the challenges associated with the practical implementation of the CSMT framework by formulating a simpler mixture CSMT model based on a composition-weighted average approach involving single-component micelle simulations of the mixture constituents. We show that the simpler mixture CSMT model works well for all of the binary surfactant mixtures considered, except for those containing alkyl ethoxylate surfactants, and rationalize this finding molecularly. The mixture CSMT model is then utilized to predict mixture CMCs, and we find that the predicted CMCs compare very well with the experimental CMCs for various binary mixtures of linear surfactants. This paper lays the foundation for the mixture CSMT framework, which can be used to predict the micellization properties of mixtures of surfactants that possess a complex chemical architecture, and are therefore not amenable to traditional molecular-thermodynamic modeling

Microbial Carbonation of Monocalcium Silicate

Biocement formed through microbially induced calcium carbonate precipitation (MICP) is an emerging biotechnology focused on reducing the environmental impact of concrete production. In this system, CO2 species are provided via ureolysis by Sporosarcina pasteurii (S. pasteurii) to carbonate monocalcium silicate for MICP. This is one of the first studies of its kind that uses a solid-state calcium source, while prior work has used highly soluble forms. Our study focuses on microbial physiological, chemical thermodynamic, and kinetic studies of MICP. Monocalcium silicate incongruently dissolves to form soluble calcium, which must be coupled with CO2 release to form calcium carbonate. Chemical kinetic modeling shows that calcium solubility is the rate-limiting step, but the addition of organic acids significantly increases the solubility, enabling extensive carbonation to proceed up to 37 mol %. The microbial urease activity by S. pasteurii is active up to pH 11, 70 °C, and 1 mol L–1 CaCl2, producing calcite as a means of solidification. Cell-free extracts are also effective albeit less robust at extreme pH, producing calcite with different physical properties. Together, these data help determine the chemical, biological, and thermodynamic parameters critical for scaling microbial carbonation of monocalcium silicate to high-density cement and concrete