Modeling the micellization behavior of fluorosurfactants using molecular-thermodynamic theory

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 267-281).Fluorinated surfactants are an important class of surfactants because they possess properties that are far superior than those of their hydrocarbon analogs. As a result, they are used in a wide variety of applications including in paints, polishes, fire-fighting foams, and emulsion polymerization processes. However, concerns regarding the non-biodegradability and toxicity of fluorinated surfactants have prompted the search for new, benign alternative surfactant formulations that possess micellization properties comparable to those of traditional fluorinated surfactants. With this need in mind, this thesis focuses on gaining a molecular-level understanding of the micellization behavior of traditional fluorinated surfactants, and then using the acquired knowledge to design novel surfactant formulations that can reduce the use of fluorinated surfactants. Molecular-thermodynamic (MT) models were developed to calculate the various contributions to the free energy of micellization for discoidal and biaxial ellipsoidal micelles; two important micelle shapes in the context of fluorocarbon-based surfactants. These models explicitly incorporate the effect of the position-dependent curvature associated with discs and biaxial ellipsoids. Comparison between the models developed here with those that do not explicitly account for the varying curvature shows that accounting for the position-dependent curvature is extremely important in modeling these two micelle shapes. The new MT model for the free energy of micellization is also used to demonstrate the feasibility of realizing biaxial ellipsoidal micelles, a result refuted in the past in many theoretical studies on the basis of average geometrical properties of the micelle. A new computer-simulation-molecular-thermodynamic (CSMT) framework was developed to predict the micellization behavior of mixtures of fluorocarbon-based surfactants. To facilitate the practical implementation of the mixture CSMT framework, which involves the computationally intensive task of simulating several mixed micelles, an approximation to the mixture CSMT model was developed. In this approximation, relevant properties for a mixed micelle are estimated using a micelle-composition based weighted average of the analogous properties obtained from simulations of the single-component surfactant micelles for each of the surfactants comprising the mixture. Therefore, in this approximation, the need for simulating mixed micelles is eliminated. The approximation was found to compare well with the mixture CSMT model for various binary surfactant mixtures considered, except for those containing alkyl ethoxylate surfactants. A rationalization of this finding is presented. CMC predictions made using the mixture CSMT model were found to compare very well with the experimental CMCs for several binary mixtures of linear surfactants, thereby laying the foundation for using the CSMT model to predict micellization properties of mixtures of surfactants that have a more complex chemical architecture. Finally, an MT framework was also developed to predict the micellization properties of mixtures of fluorocarbon-based and hydrocarbon-based surfactants. This mixing reduces the use of fluorinated surfactants in the surfactant formulation, thereby addressing the non-biodegradability and toxicity concerns associated with fluorinated surfactants. An enthalpy of mixing contribution resulting from the interactions between the fluorocarbon tails and the hydrocarbon tails, estimated using the Regular Solution Theory, was included in the MT framework. The ability of the MT framework to predict the coexistence of two types of mixed micelles in solution was demonstrated. The MT framework predictions of micelle population distributions, CMCs, and optimal micelle compositions were compared with the experimental values for various mixtures of fluorocarbon-based and hydrocarbon-based surfactants. The models developed in this thesis provide a molecular level understanding of the micellization behavior of fluorocarbon-based surfactants and their mixtures. The models are able to predict several important micellization properties of surfactants and their mixtures that can guide surfactant formulators in the synthesis, characterization, design, and optimization of surfactant formulations that exhibit desirable properties.by Jaisree Iyer.Ph.D

Optimal Temperature Profiles within a CCS Absorber

It is well known that absorber intercooling may be used to reduce column height and reboiler heat duty (Freguia et al.2003, Rezazadeh et al. 2017, Plaza et al. 2010, Le Moullec et al. 2014, Walters et al. 2016). However, to date mostresearch has focused on relatively simple intercooling geometries, and little work has been done to establish upperlimits on the improvements more advanced intercooling techniques could provide. In this work, we develop and validate a numerically efficient and flexible model of an absorption column containingmonoethanolamine (MEA), similar to that developed by Saimpert et al. (2013). By coupling this model with adifferential evolution optimization algorithm, we calculate the optimal temperature profile along the column’s lengthwhich minimizes the column height required for 90% capture of CO2 . The optimal temperature profile represents atradeoff at each point in space between increasing the temperature to increase reaction kinetics and decreasing thetemperature to increase the driving force for CO2 consumption. In general, the optimal profile is not isothermal (seeFigure 1), however for some simple cases analytical expressions for the optimal profile may be derived. By comparing these optimized columns to the adiabatic case, it is possible to put an upper limit on the reduction incolumn height intercooling can provide. In general, this is a strong function of the liquid flowrate: at flowrates close tothe minimum, the temperature bulge tends to be large and near the top of the column (Kvamsdal and Rochelle 2008),and under these circumstances the adiabatic column height is substantially larger than the minimum possible height(see Figure 1). For example, for 90% capture from a CO2 stream containing 10mol% CO2 , when the temperaturebulge is near the top the adiabatic case is typically 10-20% larger than the minimum column height. On the other hand,at larger liquid flowrates (typically around 1.5Lmin - 2Lmin ) the temperature bulge tends to shrink in size and movetowards the bottom of the column (Kvamsdal and Rochelle 2008) and under these circumstances the differencebetween the adiabatic and minimum possible column height tends to be marginal – typically less than 5%. Themaximum possible improvement is also dependent on the inlet CO2 concentration: for gaseous streams containing20mol% CO2 , the adiabatic column may be more than 30% taller than column with optimized temperature profile, whilefor capture from a 5mol% CO2 stream the maximum improvement intercooling could provide is correspondinglysmaller. The model was also used to investigate a range of heat integration strategies within the column. It was shown thatunder a wide range of circumstances the column height could be reduced to close to the minimal value simply byoptimally rearranging heat within the column – moving heat away from regions where increasing the driving force ismore important, and towards regions where higher temperatures are beneficial to improve reaction kinetics. Thetransfer of heat was consistent with both the first and second laws of thermodynamics. Such sophisticated heatintegration may be possible through the use of active packings, which incorporate a heat exchange fluid within thecolumn packing itself. For a number of cases of interest, it was also shown that traditional in-and-out intercooling(Rezazadeh et al. 2017) was able to reduce the column height to close to the minimal value, though approaches whichincorporate heat redistribution within the column itself would have the benefit of reduced cooling loads.</p

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

Heat transfer and pressure drop characteristics of heat exchangers based on triply periodic minimal and periodic nodal surfaces

The popularity of additive manufacturing has increased interest in the use of triply periodic minimal surfaces (TPMS) in engineering applications due to their potential for superior mechanical, heat and mass transfer properties. Periodic nodal surfaces (PNS) are a class of periodic continuous surfaces that also divide the space into non-intersecting, smooth and continuous domains like TPMS and can potentially have superior mechanical, heat and mass transfer properties. To evaluate the potential for superior performance, in this manuscript we characterize the flow and heat transfer properties of seven TPMS and PNS based structures by numerically computing the friction factors and Nusselt numbers in the laminar flow regime. This is the first study to evaluate the use of PNS as heat exchangers. In addition, it adds to the limited quantitative data available on the flow and thermal properties of TPMS. The friction factors associated with most of the TPMS and PNS based structures in this study are about an order of magnitude higher than that for laminar flow in tubes. These structures also had higher Nusselt numbers compared to tubes, with the enhancement increasing with increase in Reynolds number. Among the TPMS and PNS studied, Schwarz-D had the best heat transfer performance while Schwarz-P was the poorest performing structure. Basic heat exchanger design calculations showed that to remove the same amount of heat and operate under the same pressure conditions, the Schwarz-D based heat exchanger was 3–10 times smaller than a tubular heat exchanger. This shows how TPMS or PNS can be used to design heat exchangers with superior performance, especially in applications where space and weight are at a premium.</p

Advanced absorber heat integration via heat exchange packings

A rate-based model of an absorption column was developed and used to analyze several intercooling strategies utilizing “heat exchange packings.” These packings are capable of removing heat from the column and transferring it to a cooling fluid within the packing. For absorption of CO2 into aqueous monoethanolamine under industrial conditions, intercooling via heat exchange packings placed along 10–20% of the column could reduce the column height by ∼15%. The height of these columns was close to the minimum theoretical value, calculated by numerically optimizing the temperature profile. Effective intercooling could also be achieved by using the cool, rich solvent as the cooling fluid. This reduces the cooling load and facilitates recovery of waste heat. Heat exchange packings could also be used to redistribute heat within the column, reducing the column height with no net cooling load. However, this approach requires larger heat transfer coefficients than have been experimentally observed.</p

Impact of a Dedicated Cardiac Anesthesiology Team on Peri-Operative Outcomes in Children with Congenital Heart Disease Undergoing Non-Cardiac Procedures

Children with Congenital Heart Disease (CHD) undergoing noncardiac surgery are at higher risk for adverse perioperative (Ramammorthy, 2010, Faraoni, 2016) and less likely to survive cardiac arrest (Ramammorthy, 2010). The perioperative team is often confronted with the question about who should care for these patients when undergoing non cardiac surgery. Our objective was to identify those patients at highest risk for adverse outcomes and to examine if using a risk stratified approach to allocate care of these patients would mitigate the risk of adverse events under anesthesia. We conducted a single center retrospective cohort study of children with CHD who underwent non cardiac surgery between June 2014 - December 2015. Perioperative outcomes and survival data were reviewed and compared to that reported in the literature. We identified 131 patients with CHD undergoing a total of 171 non-cardiac surgical procedures during the study period all cared for by a cardiac anesthesiologist. The majority of patients taken care of by the cardiac anesthesia team had either major (45%) or severe (45%) CHD when utilizing the ACSNSQIP classification system. Patients with severe CHD represented the highest risk for perioperative events accounting for all intra-and post- operative cardiac arrest events at 0.6% (CI 0.0-3.2) and 1.7% (0.4-5.0) respectively. Moreover, patients with severe CHD accounted for the majority (83.3%) of the 3.5% (CI 1.3-7.5) of patients requiring post-operative reintubation. There was no correlation between age, sex, type of surgery and perioperative arrest/30 day mortality. Children with severe CHD are at increased risk of perioperative complications including cardiac arrest, death, and reintubation. At Children’s National Medical Center these patients undergo a comprehensive risk stratification and multidisciplinary planning which includes intraoperative care by a dedicated cardiac anesthesia team and the incidence of intraoperative cardiac arrest is below what has been reported in the literature. References: Faraoni, D., Zurakowski, D., Vo, D., Goobie, S. M., Yuki, K., Brown, M. L., & DiNardo, J. A. (2016). Post-operative outcomes in children with and without congenital heart disease undergoing noncardiac surgery. Journal of the American College of Cardiology, 67(7), 793-801. Ramamoorthy C Haberkern CM Bhananker SM, et al. Anesthesia-related cardiac arrest in children with heart disease: data from the Pediatric Perioperative Cardiac Arrest (POCA) registry, Anesth Analg, 2010, vol. 110 (pg.1376-82

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

Geochemical Narrowing Of Cement Fracture Aperture During Multiphase Flow Of Supercritical CO2 And Brine

For carbon capture and storage operations, wells are used as a necessary conduit for injecting CO2 at depth, but they can also act as leakage conduits if the cement seals are compromised. The specific objective of this research was to experimentally investigate the nature of self-healing of a fracture within cement under multiphase flow of CO2 and brine, and to compare the findings with the predictions of a recently developed model. This was accomplished by flowing a multiphase mixture of supercritical CO2 and brine through a cement fracture. The influent end of the fracture was degraded as evidenced by the formation of cracks across the surface. At the effluent end of the fracture, the initial aperture of 137 μm was reduced to 50 μm. This reduction by 87 μm compared well with an aperture reduction of 80 μm predicted by a recently developed model tested in this study. Self-healing of the fracture contributes to permeability reduction through the potential leakage pathway