Manual

Estimating Equilibrium Times:

The Equilibrium Time Estimation module can be loaded from the Equilibrium Menu or the Simulation Menu in the Main Menu section of UltraScan.

Whenever you are planning or designing an equilibrium experiment, the question comes up of how long it takes to reach equilibrium at a certain speed. The time it takes depends on several factors:

• The diffusion coefficient. The smaller the diffusion coefficient, the longer it will take to reach equilibrium, since the sample will diffuse slower. Asymmetric and random coil molecules will take longer to equilibrate than globular molecules with the same molecular weight.

• The sedimentation coefficient. The larger the sedimentation coefficient, the faster the molecule will equilibrate, provided the diffusion coefficient stays constant, which is rarely the case. As molecular weight and sedimentation coefficient increase, the diffusion coefficient will generally decrease, which has the opposite effect.

• The position inside the rotor. The further to the outside of the rotor the sample is positioned, the faster the sample will equilibrate, since the centrifugal force is larger at the outside.

• The column length. The longer the column (the more volume is loaded), the longer it will take to equilibrate.

• Finally the rotor speed. The larger the rotorspeed, the faster the sample will approach equilibrium.

• How many components are in the system. When more than one component is in the system, always simulate the component with the smallest diffusion coefficient, since this molecure with generally take the longest to equilibrate. Once this molecule is equilibrated, all other molecules will have equilibrated already.

• It also matters if the system is at chemical equilibrium, and if that equilibrium can be established rapidly enough (on the time order of the experiment). If a monomer-dimer equilibrium experiment takes a long time to equilibrate (i.e., the equilibrium kinetics are not diffusion controlled), then it may take substantially longer than predicted by this program. THis program will only predict the experiment of a diffusion controlled kinetics environment.

How to use this module:

When designing an equilibrium experiment, you should plan to acquire equilibria at multiple speeds (4-5 speeds, ranging from sigma=1 - sigma=5, use this program to predict the correct speeds), and therefore it would be nice to estimate before you start the experiment how long it will take to reach equilibrium. Using the finite element simulator programmed in this module, you can predict the time it takes to reach equilibrium for all practical purposes, i.e., the change over time is less than a certain tolerance value. You can then pre-program the speeds into the XLA and are not required to check for equilibrium evertime before you change to the next speed.

When simulating these times, you should simulate a molecule that is sedimenting slightly slower than the actual molecule, perhaps by simulating a system with a larger frictional ratio or larger axial ratio, and perhaps a nonglobular model rather than a spherical model. While the equilibrium times may be longer than needed, at least they won't be too short. In this module you can change the parameters that influence the time it takes to reach equilibrium.