Transverse ionic mobility measured in a dynamic light scattering device

• A chiral measurement of electrophoretic mobility is introduced.
• Chiral molecules are observed to have differential mobility with right-vs-left handed light.
• Achiral molecules can display this effect in the presence of l-alanine.
• The effect depends on the ‘floppiness’ of the molecule, rigid molecules show little affect.

We describe here some novel experiments with a commercial dynamic light scattering device. By inserting a quarter-wave plate in the light beam of the HeNe laser used in the Malvern DLS ‘Zetasizer’, one can obtain right handed (RH) or left-handed (LH) circularly polarized light from the incoming horizontally polarized laser light. This RH vs LH light is used in the ionic mobility (ζ-potential) measuring mode to detect what we believe are phenomena related to transverse ionic mobility, i.e. speed of a particle (or portions of the particle) as a function of applied static electric field, in directions transverse to those fields, and which, we suggest, arise from surface impedence phenomena related to the (1) parity-biased mechanical flexing of charged molecular moieties at the surface of a chiral particle or of an achiral particle + chiral co-solvents, possibly driven by the electrophoresis field and (2) electro-optic effects (induced currents) arising from the interaction of chiral co-solvents upon the surface of charged colloid particles in the presence of a (high frequency) electric field. Fluctuations of structure induce currents which are chirally biased either in themselves (in a chiral particle) or which ‘borrow’ chirality from chiral co-solvents conditioning the local high frequency E-field, and advance or retard the scattered phase of RH or LH polarized light. In either case the ‘differential mobility’ observed is related to the relative extent of motion in internal portions of the colloid particle – i.e. ‘floppiness’ in the particle.

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