particular with respect to time-resolved data and mechanistic models for kinetics. The
fundamentals of scattering techniques and their application to micellar systems will
be reviewed together with fundamentals of thermodynamics and kinetic models for
block copolymer self-assembly. In the literature there is often an unclear terminology
regarding the use of “kinetics of micelles”. The terms “micelle dynamics” or “micelle
kinetics” are often used for equilibrium kinetics (exchange kinetics), relaxation
kinetics (micelle–micelle relaxation kinetics) and micellization kinetics
(unimer–micelle transition). Here, we will attempt to clarify this subject and give a
broader overview of the different processes encountered in micellar solutions. A
particular focus will be on equilibrium kinetics and non-equilibrium micellization
kinetics, although other processes will be covered as well.
2 Theoretical Background
In this section, we give a brief review of important selected theories for surfactant
and block copolymer micelles. First, the classical thermodynamic theories covering
both mean-field and scaling approaches are briefly reviewed before discussing
kinetics. Classical theories for equilibrium and near-equilibrium surfactant and
block copolymer micelle kinetics will be briefly reviewed before covering nonequilibrium kinetics in the final part.
2.1 Structure and Thermodynamics
The molecular organization and morphologies of micellar structures depend intimately on a delicate balance between hydrophobic and hydrophilic interactions as
well as, if charges are present, electrostatic interactions. In addition, translational
entropy may play an important role. Thermodynamics permits the determination of
the structural micellar parameters, cmc (the concentration above which a significant
aggregation occurs), micellar size distributions, etc. via molecular parameters. The
earliest, most extensive descriptions of micellization were presented by Hill [20], who
developed a molecular thermodynamic theory in which the individual micelles were
considered as “small” phase-separated entities in equilibrium with other micelles,
unimers, and solvent. This microscopic phase-separation picture was further developed by Hall and Pethica [21] and provides the most fundamental basis for many later
developments. Tanford later made seminal contributions to an understanding of the
effect of hydrophobicity and its driving force for micellization in aqueous solutions
[22–24]. He showed that the hydrophobicity of hydrocarbons and organic molecules
is mainly related to the entropy of water associated with hydrogen bonds.
In order to calculate the free energy of micellization accurately, one needs to
take into account the enthalpic terms describing the interactions between the block
copolymer and solvent, the free energy of the micelle (F micelle ), the mixing free
energy (F mix ), as well as the translational entropy associated with micelles and
free chains (S m ).
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R. Lund et al.
fundamentals of scattering techniques and their application to micellar systems will
be reviewed together with fundamentals of thermodynamics and kinetic models for
block copolymer self-assembly. In the literature there is often an unclear terminology
regarding the use of “kinetics of micelles”. The terms “micelle dynamics” or “micelle
kinetics” are often used for equilibrium kinetics (exchange kinetics), relaxation
kinetics (micelle–micelle relaxation kinetics) and micellization kinetics
(unimer–micelle transition). Here, we will attempt to clarify this subject and give a
broader overview of the different processes encountered in micellar solutions. A
particular focus will be on equilibrium kinetics and non-equilibrium micellization
kinetics, although other processes will be covered as well.
2 Theoretical Background
In this section, we give a brief review of important selected theories for surfactant
and block copolymer micelles. First, the classical thermodynamic theories covering
both mean-field and scaling approaches are briefly reviewed before discussing
kinetics. Classical theories for equilibrium and near-equilibrium surfactant and
block copolymer micelle kinetics will be briefly reviewed before covering nonequilibrium kinetics in the final part.
2.1 Structure and Thermodynamics
The molecular organization and morphologies of micellar structures depend intimately on a delicate balance between hydrophobic and hydrophilic interactions as
well as, if charges are present, electrostatic interactions. In addition, translational
entropy may play an important role. Thermodynamics permits the determination of
the structural micellar parameters, cmc (the concentration above which a significant
aggregation occurs), micellar size distributions, etc. via molecular parameters. The
earliest, most extensive descriptions of micellization were presented by Hill [20], who
developed a molecular thermodynamic theory in which the individual micelles were
considered as “small” phase-separated entities in equilibrium with other micelles,
unimers, and solvent. This microscopic phase-separation picture was further developed by Hall and Pethica [21] and provides the most fundamental basis for many later
developments. Tanford later made seminal contributions to an understanding of the
effect of hydrophobicity and its driving force for micellization in aqueous solutions
[22–24]. He showed that the hydrophobicity of hydrocarbons and organic molecules
is mainly related to the entropy of water associated with hydrogen bonds.
In order to calculate the free energy of micellization accurately, one needs to
take into account the enthalpic terms describing the interactions between the block
copolymer and solvent, the free energy of the micelle (F micelle ), the mixing free
energy (F mix ), as well as the translational entropy associated with micelles and
free chains (S m ).
58
R. Lund et al.