No abstract

A random-walk simulation of microdialysis is used to examine how a reaction that consumes analyte in the medium external to the probe affects the extraction and recovery processes. The simulations suggest that such a reaction can promote the extraction process while simultaneously inhibiting the recovery process, which appears to be consistent with recent experimental evidence of asymmetry in the extraction and recovery of the neurotransmitter, dopamine, during brain microdialysis. This suggests that quantitative microdialysis strategies that rely on the extraction fraction as a measure of the probe recovery value, such as the no-net-flux method, will produce an underestimate of the analyte concentration in the external medium when that analyte is consumed by a reaction in the external medium. Furthermore, if experimental conditions arise under which the kinetics of the reaction are changed, then changes in the extraction and recovery processes are likely to occur as well. The implications of these theoretical findings for the quantitative interpretation of in vivo microdialysis results obtained for the neurotransmitter dopamine are examined.

Using scaling analysis and a self-consistent field (SCF) theory, we compress two copolymer-coated surfaces and isolate conditions that yield multiple, distinct minima in the interaction profile. We focus on planar surfaces that are coated with ABC triblock copolymers. Tethered to the surface by the last monomer in the C block, the copolymers are grafted at relatively low densities. The surrounding solution is a poor solvent for both the A and C blocks, and is a good solvent for the B blocks. Through scaling theory, we pinpoint the parameters that yield two minima in the interaction profile. The SCF calculations reveal the changes in the morphology of the polymers as the layers are compressed. Through both studies, we determine how the morphological changes give rise to the observed surface interactions. The results provide guidelines for creating polymer-coated colloidal systems that can form two stable crystal structures. Such systems could be used for bistable, optical switches. The findings also yield a prescription for creating systems that exhibit additional minima in the free energy of interaction.

Using Langevin dynamics simulations, we model the dynamics of a polymer in a fixed network of random obstacles containing a spherical cavity. We define the partition coefficient K as the time-averaged ratio of the number of monomers inside/outside the cavity and calculate this quantity as a function of polymer length N. Our results show that ln K(N) increases with N until the polymer’s radius of gyration is approximately equal to the size of the cavity Lh. Further increase of N leads to a decrease in ln K. The linear regime of this curve can be understood by comparing the free energy of a polymer confined through the spherical cavity to the corresponding free energy of the polymer in the mesh of the random network (this is the origin of the phenomenon of entropic trapping). The decrease in ln K when N is large results from imperfect confinement of the polymer inside the cavity. The number of monomers confined inside the cavity is limited by the size of the cavity and thus ln K decreases with N, roughly logarithmically, when N is very large.

No abstract