Surfactant packing parameter

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. 

To date, there are many synthesis recipes for MCM-48 available. The basic concept for these syntheses is to control the effective surfactant-packing parameter g between the limits 1/2 and 2/3 in other words, to increase the palisade-layer volume of micelles. The following are some successful synthesis strategies. 

If we have the estimated values of Ah, Vt and Lc, then it is possible to define a surfactant packing parameter... 

At moderate surfactant concentration, the micelle shape is determined by the value of the surfactant packing parameter P=v/aol, where v and I are the volume and length of the hydrophobic moiety (alkyl chain), and Uq is the optimal surface area occupied by one surfactant at the micelle-water interface. - It is important to realize that the value of is determined by the cross-sectional area of the surfactant head group and also by the various interactions at play in micelle formation.Surfactants characterized by values of P<113 give rise to sphericall... 

Mechanistically, the reaction generated triethanolamine (TEA) molecules, which were the by-product from the hydrolysis of silatrane. TEA can also act as a structuredirecting agent for the system, resulting in a lower requirement of CTAB concentration. Moreover, TEA could also improve the surfactant-packing parameter that encourages MCM-48 formation. 

The surfactant packing parameter v /aolc can be used to predict what types of nanostructures in microemulsions can form using specific surfactants. Amphiphilic molecules with 0.5 < v/aok < 1 tend to form three-component o/w and... 

A surfactant packing parameter between 3 and 1 is likely to lead to bilayer structures [22,23], These tend to be either lamellar bilayers or vesicles, which are closed spheroidal packages of surfactant [5], A n/uolc between 3 and 1 is found in double-chain surfactants such as dialkyl-dimethylammonium salts or phosphatidylcholines of chain length 12-20 (Fig. 2). Such surfactants are insoluble in water and form bilayers when cast as films. Common single-chain surfactants such as sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) have n/ao/c < i, are soluble in water, and form micelles. 

With this type of nonionic surfactant, the presence of a cosurfactant is not needed to obtain a large microemulsion single-phase domain at a fixed temperature. Moreover, by varying the relative volumes of the polar and apolar parts of the surfactant, it is easy to modify the hydrophilic/hydrophobic balance (expressed as the spontaneous curvature //q, surfactant packing parameter Pq, or HLB balance R). These quantities can be modified by temperature variations only because the hydration of the polar head is temperature-dependant. Therefore, the spontaneous curvature turns from oil to water as the temperature is increased. Shinoda, Kunieda and co-workers, as well as Kahlweit s group, have given a general description of the phase behaviour obtained with C/Ey species (49). 

The input information needed for a given sample is the polar volume fraction 0, and the specific surface E. Knowing the surfactant packing parameter and the scattering peak position, converted into real space as distance D, the conductivity can be deduced or, by using the analytic model in the reverse way, knowing... 

The method of deducing solute location from the scattering peak shift in the case of spherical reversed-micelles can be generalized to the case of connected structures (56). For instance, consider the model case of a bicontinous structure, close to the electrical antipercolation threshold. Take the polar volume fraction to be in the range 0.3 to 0.4 and the specific area to be E = 0.02 A /A, which corresponds to a 0.5 M solution of a classical surfactant. In addition, consider a chain length of 15 A and an initial surfactant packing parameter of Po = 1.25. 

Figure 4.14 A spherical micelle. The packing of amphiphilic molecules is controlled by the effective cross-sectional area of the head group, a, and the hydrophobic chain of length / and volume V. These quantities define a surfactant packing parameter (see Eq. 4.53)...

Table 4.3 Surfactant packing parameter range for various surfactant aggregates...

The nature of the lyotropic liquid crystal phase formed by amphiphiles in solution is described at a molecular level by the surfactant packing parameter model, introduced in Section 4.9. Consider the situation where the head group has a larger effective cross-sectional area than the chain. This is the usual situation, and the resulting structures are termed normal structures. If there is a large difference in cross-sectional area between the head group and chain (Ns < ), spherical micelles are formed (Fig. 4.24a). For molecules with less of a mismatch between the effective head and tail cross-sectional areas, rod-like micelles provide a more... 

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