We prescribe an analytical form of the interaction potential between rigid water and a rigid platinum metal surface, which takes into account the surface symmetry and corrugation. Using this potential we perform a molecular dynamics computer simulation on water lamina restricted by two PtC 111) surfaces and investigate the structure and dynamics of water at the Pt interface. At 300 K the water layer adjacent to the metal surface displays solid-like properties. Patches of ice-like structure embedded in this layer are observed in the simulation. The next two layers of water display ordering similar to ice-I. Beyond these three layers the structure and dynamics of water are bulk-like.

We report the results from a molecular dynamics study of water between dilauroylphosphatidylethanolamine (DLPE) bilayers. The simulations were carried out with the head groups of DLPE treated as flexible as well as rigid. We studied the orientational properties of the phospholipid head groups and of water, hydrogen bonding, and polarization of water between the molecular surfaces. The simulations show that the thermal motion of the polar head groups has no influence on the orientational polarization of water but has a large influence on the dynamics of the intersurface water.

The properties of a dilute Lennard-Jones vapor in contact with an adsorbing microporous medium are investigated using grand canonical ensemble Monte Carlo and molecular dynamics techniques. The bulk structure of the microporous system is modeled as an assembly of randomly distributed interconnected solid spheres, and vapor/surface interactions are treated in two ways: (i) using a smooth continuous interaction potential and (ii) using a molecular model for the surface structure of the solid. The microporous solid representation employed in these simulations is chosen to conform in realistic manner with the bulk and surface properties of silica gel. The results obtained from the simulations include equilibrium partition coefficients, diffusivities, and related microscopic properties. By comparing these results with available experimental data it is shown that the properties of simple nonpolar gases in microporous silica may be predicted with reasonable accuracy. This is particularly true when the molecular structure of the silica surface is taken into consideration.

The conductance catheter technique could be improved by determining instantaneous parallel conductance (G(P)), which is known to be time varying, and by including a time-varying calibration factor in Baan's equation [alpha(t)]. We have recently proposed solutions to the problems of both time-varying G(P) and time-varying alpha, which we term "admittance" and "Wei's equation," respectively. We validate both our solutions in mice, compared with the currently accepted methods of hypertonic saline (HS) to determine G(P) and Baan's equation calibrated with both stroke volume (SV) and cuvette. We performed simultaneous echocardiography in closed-chest mice (n = 8) as a reference for left ventricular (LV) volume and demonstrate that an off-center position for the miniaturized pressure-volume (PV) catheter in the LV generates end-systolic and diastolic volumes calculated by admittance with less error (P < 0.03) (-2.49 +/- 15.33 microl error) compared with those same parameters calculated by SV calibrated conductance (35.89 +/- 73.22 microl error) and by cuvette calibrated conductance (-7.53 +/- 16.23 microl ES and -29.10 +/- 31.53 microl ED error). To utilize the admittance approach, myocardial permittivity (epsilon(m)) and conductivity (sigma(m)) were calculated in additional mice (n = 7), and those results are used in this calculation. In aortic banded mice (n = 6), increased myocardial permittivity was measured (11,844 +/- 2,700 control, 21,267 +/- 8,005 banded, P < 0.05), demonstrating that muscle properties vary with disease state. Volume error calculated with respect to echo did not significantly change in aortic banded mice (6.74 +/- 13.06 microl, P = not significant). Increased inotropy in response to intravenous dobutamine was detected with greater sensitivity with the admittance technique compared with traditional conductance [4.9 +/- 1.4 to 12.5 +/- 6.6 mmHg/microl Wei's equation (P < 0.05), 3.3 +/- 1.2 to 8.8 +/- 5.1 mmHg/microl using Baan's equation (P = not significant)]. New theory and method for instantaneous G(P) removal, as well as application of Wei's equation, are presented and validated in vivo in mice. We conclude that, for closed-chest mice, admittance (dynamic G(P)) and Wei's equation (dynamic alpha) provide more accurate volumes than traditional conductance, are more sensitive to inotropic changes, eliminate the need for hypertonic saline, and can be accurately extended to aortic banded mice.

A classic problem in traditional conductance measurement of left ventricular (LV) volume is the separation of the contributions of myocardium from blood. Measurement of both the magnitude and the phase of admittance allow estimation of the time-varying myocardial contribution, which provides a substantial improvement by eliminating the need for hypertonic saline injection. We present in vivo epicardial surface probe measurements of electrical properties in murine myocardium using two different techniques (a digital and an analog approach). These methods exploit the capacitive properties of the myocardium, and both methods yield similar results. The relative permittivity varies from approximately 100,000 at 2 kHz to approximately 5000 at 50 kHz. The electrical conductivity is approximately constant at 0.16 S/m over the same frequency range. These values can be used to estimate and eliminate the time-varying myocardial contribution from the combined signal obtained in LV conductance catheter measurements, thus yielding the blood contribution alone. To study the effects of albumin on the blood conductivity, we also present electrical conductivity estimates of murine blood with and without typical administrations of albumin during the experiment. The blood conductivity is significantly altered (p < 0.0001) by administering albumin (0.941 S/m with albumin, 0.478 S/m without albumin).