Micelle critical packing

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. 

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. 

Lipophilic and hydrophilic calculations are performed at -0.6 and -3.0kcalmol", respectively. Critical packing is a good parameter to predict molecular packing such as in micelle formation, and may be relevant in solubility studies in which the melting point plays an important role. 

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... 

Fig. 4 Critical packing parameters and characteristic structure typical to mesoporous silica films. The top shows a cross-section of a micelle, displaying the parameters used to calculate the CPP. The spherical head group for the surfactant represents the effective cross-sectional area that the head group occupies. (View this art in color at www.dekker.com.)...

A useful concept for characterizing micelle geometry is the critical packing parameter (CPP) [2]. The aggregation number N is the ratio between the micellar core volume, V and the volume of one chain, v. 

Molecular dynamics simulations are consistent with calculations based on the critical packing parameter p, which indicate that the structure of the surfactant controls the shape of the micelle at the cmc. Esselink et al. [16] show that the surfactants / 2/5, hihts, and h thts form bilayers, cylindrical micelles, and spherical micelles, respectively, as expected. However, /14/4, expected to form micelles of low curvature based on p, instead forms sphere-like structures due to the coiling of the headgroup. If this increased effective headgroup area is accounted for in the calculation of the packing parameter, then a spherical shape is predicted, in agreement with the result of the simulations. 

The various phases produced by a surfactant can be related to its structure. An important parameter that can be used to predict the phase behaviour of surfactants is the critical packing parameter (CPP) described above. (CPP = v/la, where v is the volume of the hydrocarbon chain with a length I and a is the cross sectional area of the hydrophilic head group.) For spherical micelles, CPP for cylindrical micelles 1 > CPP > and for lamellar micelles CPP 1. 

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