Chemical Reactivity in Colloidal Microheterogeneous Media

José R. Leis 1Luis Garcia-Rio 1Juan C. Mejuto 2

1. University of Santiago (USC), Avda. Ciencias s/n, Santiago de Compostela 15782, Spain
2. University of Vigo (UVIGO), Ourense 32004, Spain

Abstract

Colloids form homogeneous solutions although at microscopic level they are highly structurated microheterogeneous media with domains of very different properties. Good examples are micellar solutions, microemulsions, liposomes and vesicles. The driving force of this particular behaviour is hidrophobicity regulated by the particular properties of surfactants.

Along the last decades models to explain chemical reactivity in those media has been developed. All of them are based on the distribution of reagents among the different parts of the microheterogeneous structure and in general they are based on the pseudophase approach, which assumes equilibrium distribution of reagents among pseudophases (i.e. micellar, aqueous and interphase). This approach is generally valid for chemical reactions slower than rate of exchange of material (monomers and reagents) among pseudophases. This model alow us to obtain information about rate constants and distribution constants and concentrations of each reagent and intermediate in each pesudophase.

Dramatic changes in reaction rates have been reported for very different chemical reactions. We will discuss some of these results focussing in mechanistic changes induced by these systems. Particular properties of each pseudophase dielectric constant and water content are finally responsible for dramatic changes in nucleophilic-electrophilic power of different reagents.

A different approach that will also be discussed is the use of well known chemical reactions as probes to get valuable information about the structure and properties of the pseudophases, in particular about the key structure the interphase.

 

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Presentation: invited lecture at 18th Conference on Physical Organic Chemistry, Plenary session, by José R. Leis
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-06-12 08:18
Revised:   2009-06-07 00:44