Surfactants critical packing parameter

TABLE 4.3. Expected Aggregate Characteristics in Relation to Surfactant Critical Packing Parameter, vlaJc... 

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. 

The critical packing parameter can be used as a guide to the aggregate architecture for a given surfactant (as shown in Figure 4.6). Typical values and their corresponding aggregate structures are ... 

Describe how the critical packing parameter for surfactant self-assembly can be used to describe the structure of typical biological lipid membranes. 

Explain the link between the critical packing parameter and the interaction forces between surfactant molecules in water. 

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. 

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... 

Figure 6.25 Influence of the critical packing parameter, CPP = vl(l a], on the type of aggregate formed by surfactants in solution.

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.)...

The thermodynamic modeling of microemulsions has taken various lines and gave conflicting results in the period before the thermodynamic stability and microstructure were established. It was early realized that a maximal solubilization of oil and water simultaneously could be discussed in terms of a balance between hydrophilic and lipophilic interactions the surfactant (surfactant mixture) must be balanced. This can be expressed in terms of the HLB balance of Shinoda,Winsor s R value, and a critical packing parameter (or surfactant number), as introduced to microemulsions by Israelachvili et al. [37], Mitchell and Ninham [38], and others. The last has become very popular and useful for an understanding of surfactant aggregate structures in general. 

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. 

As an example of the effects of an amphiphilic drug on the structure of surfactant self-assemblies. Figure 1.4 shows part of the phase diagram of monoolein, water, lidocaine base and licocaine-HCl (21). As can be seen, the cubic phase (c) formed by the monoolein-water system transforms into a lamellar liquid crystalline phase on addition of lidocaine-HCl, whereas it transforms into a reversed hexagonal or reversed micellar phase on addition of the lidocaine base. Based on X-ray data, it was inferred that the cubic phase of the monoolein-water system had a slightly reversed curvature (critical packing parameter about 1.2). Thus, on addition of the... 

The ratio u//max which gives a geometric characterization of a surfactant molecule, is very useful when discussing the type of structure formed by a given amphiphile. This is called the critical packing parameter (CPP) or the surfactant number. 

A simple but effective concept described by Israelachvilli et al. predicts the aggregation of surfactants in solution. This model is based on the molecular shape of the surfactant molecules. In this model, the ratio of the size of the hydrophobic and the hydrophilic portion of the molecules is expressed in terms of a critical packing parameter P [9] ... 

FIGURE 15.11. The critical packing parameter, allows one to quickly determine the general type of aggregate structure to be expected for a given surfactant molecular composition. 

Surfactant-like lipids adopt either normal (type 1) or inverted (type 2) self-assembled phases, resulting in either oil-in-water (o/w) phases with convex curvature lipid/water interface or water-in-oil (w/o) phases with a concave interface, respectively. The formation of a normal or an inverted self-assembled nanostructure in water mainly depends on the lipid s molecular shape, as discussed in the seventies by Israelachvili and co-workers [78], In this regard, the geometric shape of the lipid can be a useful tool for predicting the water-lipid interface curvature and also can be helpful in imderstanding the phase behavior of binary, ternary, and even multi-component systems [79], For this purpose, the shape factor or more commonly known in the literature as the critical packing parameter CPP) was defined [78] as ... 

The critical packing parameter, CPP, is described by eq 2, where v is the average volume of the amphiphile, a is the effective head group area, and / is the effective chain length of the surfactant in the molten state. The CPP can be used to predict the aggregate structures and to correlate stmctural clmges of the surfactant (or PIL) with changes to the self-assembly phases. 

FIGURE 16.1 Surfactant molecules self-assemble into various aggregate shapes, depending on the surfactant molecular structure as described by the critical packing parameter (CPP), which is the ratio of the molecular volume (v) divided by its length (/) times the cross-sectional area of the head group (a) v/al. 

---