Multi-Scale Modeling and Rheological Approaches for Understanding the Structure-Property Relationships of Surfactant Solutions.

In this work, we applied multi-scale modeling and rheological measurements to understand the structure-property relationships of surfactant solutions. We used molecular dynamics (MD) and dissipative particle dynamics (DPD) simulations to address behavior extending from the molecular level to the micellar mesoscale, the Cates model to bridge the micellar mesoscale to macroscale rheological behavior, and rheometry to measure rheological behavior and compare it to predictions of the Cates model. Starting with a simple surfactant, sodium dodecyl sulfate, we compared force field effects on micellar properties at various aggregation numbers by MD simulations. We found the parameters that control the shape of large micelles were the Lennard-Jones parameters of Na+ and ionic oxygen atoms, as well as the water model, which controls hydration of Na+ in the presence of surfactants. These parameters control the degree of binding of Na+ to ionic oxygens and head group packing, and resulted in different micellar shapes. We also studied structure-property relationships of a commercial surfactant mixture, polyoxyethylene (PEO) sorbitan oleates, which contains multiple species and were represented as five “typical” structures varying the lengths of EO head groups and the number of tails using MD simulations. We found structures with more than one tail, and with shorter EO head group that attaches the tail to the sorbitan ring, pack more efficiently within micelles and at interfaces. This efficient packing leads to lower interfacial tensions at air–water and oil–water interfaces at the same surfactant interfacial density. Finally to assess the behavior of complex body washes containing cylindrical micelles, we studied the effects of salts (NaCl) and perfume raw materials (PRMs) by combining results from rheology, the micellar Cates model, and DPD modeling. We determined the relationship between viscosity and average micelle length, and elasticity and micellar characteristic time. Salts modify viscoelasticities of body washes by condensing Na+ near micellar surface, changing surfactant head groups packing, and maintaining the cross-section radius constant. PRMs modify viscoelasticities of body washes by partitioning into the micelles according to their octanol/water partition coefficients and chemical structures, adjusting surfactant packing at head and/or tail regions, and possibly changing the cross-section radius.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111497/1/xuemtang_1.pd

Efficient parallel algorithms for solvent accessible surface area of proteins

We present faster sequential and parallel algorithms for computing the solvent accessible surface area (ASA) of protein molecules. The ASA is computed by finding the exposed surface areas of the spheres obtained by increasing the van der Waals ’ radii of the atoms with the van der Waals ’ radius of the solvent. Using domain specific knowledge, we show that the number of sphere intersections is only O(n), where n is the number of atoms in the protein molecule. For computing sphere intersections, we present hash-based algorithms that run in O(n) expected sequential time and O expected parallel time and sort-based algorithms that run in worst-case O (n log n) se-n log n quential time and O p parallel time. These are significant improvements over previously known algorithms which take O � n2 � � � n2 time sequentially and O p time in parallel. We present a Monte Carlo algorithm for computing the solvent accessible surface area. The basic idea is to generate points uniformly at random on the surface of spheres obtained by increasing the van der Waals ’ radii of the atoms with the van der Waals ’ radius of the solvent molecule and to test the points for accessibility. We also provide error bounds as a function of the sample size. Experimental verification of the algorithms is carried out using an IBM SP-2