Erica P. Schulz

Supply chain optimization of large-scale continuous processes

In this work, a supply chain model for a petrochemical complex is formulated as a multiperiod mixed integer nonlinear program (MINLP). The model includes production, product delivery, inventory management and decisions such as individual production levels for each product, as well as operating conditions for each plant in the complex. Two multiperiod MINLP are formulated in order to compare models with different levels of rigorousness. The first one includes linear mathematical models derived for the ethylene plants while the second considers semi-rigorous nonlinear models. Simplified models based on rigorous existing models tuned with actual plant data are considered for the natural gas plants, taking into account variations in production with key plant operating variables. Data for chemical transformations and utilities consumption from the literature have been used to model the remaining plants.

Structural and dynamical aspects of water in contact with a hydrophobic surface

Abstract.By means of molecular dynamics simulations we study the structure and dynamics of water molecules in contact with a model hydrophobic surface: a planar graphene-like layer. The analysis of the distributions of a local structural index indicates that the water molecules proximal to the graphene layer are considerably more structured than the rest and, thus, than the bulk. This structuring effect is lost in a few angstroms and is basically independent of temperature for a range studied comprising parts of both the normal liquid and supercooled states (240K to 320K). In turn, such structured water molecules present a dynamics that is slower than the bulk, as a consequence of their improved interactions with their first neighbors.

Behavior of water in contact with model hydrophobic cavities and tunnels and carbon nanotubes.

By means of molecular dynamics simulations we analyze the behavior of water in contact with model hydrophobic cavities and tunnels. We study the hydration and filling propensity of cavities and tunnels carved in alkane monolayers and, for comparison, we also study single-walled carbon nanotubes of similar size. Our results will determine the dependence of the filling propensity as a function of cavity size while revealing the dynamical nature of the process with alternation of filled and dry states. Concerning the tunnels built across the monolayer, we shall show that the minimum diameter in order to get filled is about twice as large as that for the carbon nanotubes, thus evidencing a more hydrophobic behavior. The existence of water-water hydrogen bonds, a necessary condition for penetration, will also be made evident.

Sub-nanoscale surface ruggedness provides a water-tight seal for exposed regions in soluble protein structure.

Soluble proteins must maintain backbone hydrogen bonds (BHBs) water-tight to ensure structural integrity. This protection is often achieved by burying the BHBs or wrapping them through intermolecular associations. On the other hand, water has low coordination resilience, with loss of hydrogen-bonding partnerships carrying significant thermodynamic cost. Thus, a core problem in structural biology is whether natural design actually exploits the water coordination stiffness to seal the backbone in regions that are exposed to the solvent. This work explores the molecular design features that make this type of seal operative, focusing on the side-chain arrangements that shield the protein backbone. We show that an efficient sealing is achieved by adapting the sub-nanoscale surface topography to the stringency of water coordination: an exposed BHB may be kept dry if the local concave curvature is small enough to impede formation of the coordination shell of a penetrating water molecule. Examination of an exhaustive database of uncomplexed proteins reveals that exposed BHBs invariably occur within such sub-nanoscale cavities in native folds, while this level of local ruggedness is absent in other regions. By contrast, BHB exposure in misfolded proteins occurs with larger local curvature promoting backbone hydration and consequently, structure disruption. These findings unravel physical constraints fitting a spatially dependent least-action for water coordination, introduce a molecular design concept, and herald the advent of water-tight peptide-based materials with sufficient backbone exposure to remain flexible.

Colloidal properties of amiodarone in water at low concentration

Amiodarone aqueous systems at concentrations C<or=3mg/mL were studied with different techniques (conductivity, solubility, ion-selective electrodes, pH, viscosity, polarizing microscopy, computational simulation, electron microscopy and fluorescence at various temperatures. The Krafft point (T(K)=70.6 degrees C) was determined. A partial phase diagram was plotted showing the critical micelle concentration (CMC) dependence on temperature, the solubility and a transition between a coacervate and an unstable gel below T(K) as a function of temperature. The nature of aggregates below T(K) at different concentrations was determined. Micellization is an entropy-driven process which is probably caused by a strong dehydration of the amiodarone backbone on micellization. CMC, micelle aggregation number and T(K) given in earlier literature were misinterpreted in the original articles and are re-interpreted in this work.

Aggregation and adsorption behavior of low concentration aqueous solutions of hexadecyltrimethylammonium ortho, meta, and parafluorobenzoate

The aggregation properties of 2-, 3-, and 4-fluorobenzoic acids (2FBA, 3FBA, and 4FBA, respectively) and their salts with hexadecyltrimethylammonium cations (HTA2FB, HTA3FB, and HTA4FB) in water were studied with a battery of techniques. Their activity at the air/solution interface has been also studied. The position of the fluorine atom in the acid affected the solubility, adsorption, and aggregation in the pure acids solutions. The 4FBA is less water soluble, more hydrophobic, and has the lower critical aggregation concentration of the three isomers. The behavior of the HTA2FB compound in aqueous solution is different from that of HTA3FB and HTA4FB. The critical micelle concentration, critical concentration for sphere-to-rod-like micelle transition, and Krafft point of HTA3FB and HTA4FB are lower than those of the other surfactant but their surface activities are higher. The differences between the HTA2FB and the other two surfactants have been explained on the basis of the regular solution theory of mixed micelles and in light of the analysis of the hydration shell of the acids through molecular dynamic simulations. The results of the present work suggest that the different behaviors are due to a combination of different dehydration tendencies and the steric possibility of inclusion of the counterions in the micelle palisade layer. The formation of rod-like micelles by HTA2FB, while the tetradecyltrimethylammonium 2-fluorobenzoate only forms spherical aggregates, is explained on the basis of the packing parameter. The mentioned factors are complementary to others presented in literature. These conditions may be used in the rational design of micelles by means of molecular dynamics simulations, reducing the trial-and-error approach used to date.

Intermediate structures for higher level arrangements: catching disk-like micelles in decane phosphonic acid aqueous solutions.

It has been proposed that disk-like micelles may be precursors to the formation of lamellar liquid crystals. The possibility of obtaining n-decane phosphonic acid (DPA) disk-like micelles in aqueous solution without the addition of a second ionic surfactant led us to study in detail the low-concentration range of this system by both a battery of experimental techniques and molecular dynamics (MD) simulations. The experimental results indicate that premicelles with some capacity to solubilize dyes are formed at 0.05 mM. The critical micelle concentration (cmc) was found to be 0.260 ± 0.023 mM, much lower than that previously reported in the literature. Spherical micelles, which immediately grow, leading to disk-like micelles, are probably formed at this concentration. At 0.454 ± 0.066 mM, disk-like micelles become unstable, giving rise to the formation of an emulsion of lamellar mesophase that dominates the system beyond 0.670 ± 0.045 mM. These experimental results were corroborated by MD simulations which, additionally, allow describing the structure of the obtained micelles at atomic level. The analysis of the MD trajectories revealed the presence of strong intermolecular hydrogen bonds between the surfactant headgroups, producing a compact polar layer with low water content. The formation of such H-bond network could explain the ability of this surfactant to form disk-like micelles at concentrations close to the cmc.

Equation oriented mixed micellization modeling based on asymmetric Margules-type formulations

Abstract The determination of the composition of mixed micellar systems is a major problem since only the composition of the total micellar solution is accessible to the experimenter. A new approach is introduced in the present work based on Equation Oriented Optimization and Margules asymmetric formulations which is not restricted to the number of components and guarantees the applicability of the Gibbs–Duhem relation. The method is validated through its application to systems from literature and the quality of the solutions is tested with already published data and by comparison with other approaches. We do also show how excess properties can be predicted and that the excess entropy is not zero as assumed by the original regular and sub-regular solution theories.

The aqueous Triton X-100 – dodecyltrimethylammonium bromidemicellar mixed system. Experimental results and thermodynamic analysis

Abstract The micellization process of the aqueous mixed system triton X-100 (TX100) – dodecyltrimethylammonium bromide (DTAB) has been studied with a battery of techniques: surface tension, static and dynamic light scattering and ion-selective electrodes. Results have been also analysed with two thermodynamic procedures: the Regular Solution Theory or Rubingh’s model and the recently developed Equation Oriented Mixed Micellization Model (EOMMM). For αDTAB ≤ 0.40 (αDTAB: total molar fraction of the system without considering the water), the micelles are predominantly TX100 with scarce solubilized DTA+ ions, with TX100 acting as a nearly ideal solvent. In the range 0.50 ≤ αDTAB ≤ 0.75, it seems that none of the components acts as a solvent. Above αDTAB ≈ 0.75 there are noticeable changes in the size and electrophoretic mobility of the micelles. These phenomena have been interpreted in the light of the thermodynamic results and literature on some TX100-ionic surfactant mixtures. The case under study is an almost ideal but very asymmetric mixed surfactants system, what is very interesting in view of the very different nature and structures of the components.

Electrochemistry of Surfactants

The application of different electrochemical techniques to surfactant systems, namely polarography and cyclic voltammetry, differential capacitance, chronocoulometry and electrochemical impedance spectroscopy, is reviewed.