Micelles packing parameter

Introduction to the variety of types of surfactants, effect of surfactants on aqueous solution properties. Law of mass action applied to the self-assembly of surfactant molecules in water. Spontaneous self-assembly of surfactants in aqueous media. Formation of micelles, vesicles and lamellar structures. Critical packing parameter. Detergency. Laboratory project on determining the charge of a micelle. 

Figure 9.2 Calculation of the surface packing parameter. Via x I, which determines to some extent the form of the aggregate. For V/(a x 1) c. 1, bilayers are preferentially formed when this ratio is less or more than one, spherical aqueous and reverse micelles are formed, respectively. Self-assembly may be described in terms of the curvature that exists at the hydrocarbon-water interface.

Example 8.3 Packing Parameter for Spherical Micelles. Show that the packing parameter < of a surfactant has to be less than 1/3 for it to form spherical micelles. 

Chapter 8 has been revised to include a discussion of the critical packing parameter of surfactants and its relation to the structure of resulting surfactant aggregates. This simple geometric basis for the formation of micelles, bilayers, and other structures is intuitively easier to understand for a beginning student. 

Figure 2 Micelle curvature in dependence of the packing parameter (left large, right small)...

The packing parameter provides a simple semi quantitative model to interpret the inLuence of the dynamic surfactant structure on the size and shape of the resulting micelles (Israelachvili,... 

Effective Shape of the Surfactant Packing Parameter Aggregate Morphology Cone <1 /3 P = fj Spherical micelles... 

Spherical micelles are formed where the value of surfactant packing parameter is less than 1/3 (single chain surfactants with large head group areas such as anionic surfactants). The spherical aggregates are extremely small and their radius is approximately equal to the maximum stretched out length of the surfactant molecule. 

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

The shape of the micelle formed by a particular surfactant is influenced to a large extent by the geometry of the surfactant molecule, as can be seen if we consider the packing of space-filling models of the surfactants. The dimensionless parameter of use in these considerations is called the critical packing parameter (CPP) and is defined as... 

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