Headgroup area effect

When spread from dilute hexane solution, acid-dependent enantiomeric discrimination was observed in the 11/A compression isotherms of the monolayer at 25°C (Fig. 12). It is interesting to note that at higher subphase acidities, both racemic and enantiomeric film systems become more highly expanded, and the surface pressures where enantiomeric discrimination commences occur at high (85-90 A2/molecule) average molecular areas. This may be taken as direct evidence of headgroup ionization effects. The surface... 

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. 

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. 

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