Related Experiment Videos
Capillary electrophoresis of small solutes in linear polymer solutions: relation between ionic mobility, diffusion
1Institute for Analytical Chemistry, University of Vienna, Austria.
Electrophoresis
|December 23, 1999
Summary
Electrophoretic mobility and diffusion coefficients of a small ion were studied in solutions with varying viscosity. Results show dependence on additive size, with separation efficiency remaining constant across viscosity ranges.
Area of Science:
- Physical Chemistry
- Solution Chemistry
- Polymer Science
Background:
- Understanding ion transport in solutions with varying viscosity is crucial for separation science.
- Walden's rule predicts inverse proportionality between mobility/diffusion and viscosity.
- Polymer additives can significantly alter solution properties and ion behavior.
Purpose of the Study:
- To investigate the electrophoretic mobilities (mu) and diffusion coefficients (D) of a small ion in aqueous solutions.
- To determine the influence of viscosity modifiers, such as ethylene glycol and polyethylene glycol (PEG) of various molecular weights, on ion transport.
- To evaluate the validity of Walden's rule and explore alternative models for ion behavior in complex solutions.
Main Methods:
- Measurement of electrophoretic mobilities and diffusion coefficients for a small ion (MW 579).
- Systematic variation of solution viscosity using ethylene glycol and PEG (MW 400, 20000, 100000, 2000000) as additives.
- Analysis of results in relation to viscosity, additive size, and established theories like Walden's rule.
Main Results:
- For small additives (ethylene glycol, PEG400), mu and D are inversely proportional to viscosity, consistent with Walden's rule.
- For large PEG polymers, mu and D approach values seen in pure water, independent of viscosity, supporting fractional free volume and obstruction theories.
- A transition range shows increased mu and D with polymer size (MW 20000-100000) at equal viscosity.
- The ratio mu/D remained nearly constant across all tested conditions (viscosity 1-7 cP).
Conclusions:
- Ion transport behavior is strongly dependent on the size and type of viscosity-modifying additive.
- Walden's rule applies to small molecules but not to large polymers in this context.
- Separation efficiency, indicated by plate number, is largely independent of viscosity due to the stable mu/D ratio.