Headgroup area

As expected, due to the small headgroup area of cholesterol (Achol = 40 A2, while ADOpc = 72 A2) [40, 41], the membrane charge density of DOTAP/DOPC/ Chol-DNA complexes increases with cholesterol content. Exchanging DOPC for cholesterol reduces the total membrane area while the membrane charge, given by dotap = 0-3, remains constant thus cM increases. A particularly strong increase in cM occurs for Choi > 0.4, where part of the cholesterol is not incorporated in the complex. This results in an increased <2>DOTap and thus aM. 

Table 17.4 lists values of the CMC for C12-surfactants with different headgroups. Within the nonionic alcohol ethoxylate surfactants, the headgroup area increases with an increasing number of oxyethylene units. The CMC simultaneously increases, illustrating that steric repulsion increases the CMC (Figure 17.3). 

Polar Headgroup Structure Influence on CMC and Headgroup Area A for a Series of Dodecyl Surfactants (Excerpted from Rosen, 1989). The Solvent is Distilled Water. 

Plot of CMC vs. headgroup area for nonionic (squares), cationic (spheres), and anionic (triangles) surfactants. 

The driving force for formation of rod shaped SDS micelles is the elimination of water from die micellar core/water interface (31). The reduction in average headgroup area reflects the removal of water molecules between the SDS headgroups, and should affect the bands due to the asymmetric S-O stretching vibrations, as indicated in the discussion of the transition moment vectors above. 

Rod micelle formation is promoted in mixtures of surfactants when there is either a large reduction in the headgroup areas, or an increase in the methylene tail volume, and thus, in terms of the surfactant-surfactant interaction parameters, is favored by large negative values of p. The forces which hold micelles together are... 

The spectra clearly show that the adsorption of charged surfactants at the CCLj/H20 interface at nanomolar aqueous phase surfactant concentrations results in a significant modification of the interfacial water behaviour. Wilhelmy balance surface pressure measurements [89] show that at these concentrations (headgroup areas of >4000 molecule ), the water molecules responsible for the observed spectral... 

C H2 i X 1 (CH3)2(CH2), OH decreases the hydrophobicity of the headgroup and the headgroup charge is more shielded by water[183] of solvation or silicate or their anions in solution, thus decreasing the effective cationic headgroup area, a,. 

The effect of temperature on the condensation of silica has been mentioned before (e.g., stabilization of mesoporous silica). The effect of the temperature on the formation of the mesostructure can be understood by considering the g parameter of quaternary ammonium surfactants. As the mixture gel is heated, the conformational disorder of the surfactant tail increases, increasing the surfactant molecular volume and as a result the g value. At the same time, the repulsion of the charged headgroups is also increased with heating, which leads to an increase in the headgroup area a0 value. 

---