Head group packing

In contrast, the phase transition of polymeric liposomes is retained if the polymer chain is more flexible or located on the surface of the vesicles instead within the hydrophobic core. Polymerized vesicles of methacrylamide (29) show a phase transition temperature which is slightly lower than the one for the corresponding monomeric vesicles (Fig. 26). This can be explained by a disordering influence of the polymer chain on the head group packing 15). 

The function a(r) contains the voltage divider effect associated with the spreading resistance external to a pore and the internal pore resistance. We assume that pores have a minimum radius, rmin = 1.0 nm, because head-group packing constraints require rmin to be somewhat greater than the size of the... 

Figure Bl.19.8. (a) STM image (5.7 mu x 5.7 mu) of 10-alkylcyanobiphenyl on graphite (b) model showing the packing of the molecules. The shaded and unshaded segments represent the alkyl tails and the cyanobiphenyl head groups, respectively. (Taken from [38], figure 2.)...

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Traditional amphiphiles contain a hydrophilic head group and the hydrophobic hydrocarbon chain(s). The molecules are spread at molecular areas greater (-2-10 times) than that to which they will be compressed. The record of surface pressure (II) versus molecular area (A) at constant temperature as the barrier is moved forward to compress the monolayer is known as an isotherm, which is analogous to P-V isotherms for bulk substances. H-A isotherm data provide information on the molecular packing, the monolayer stability as de-... 

Scheme 1 gives a representation of an approximately spherical micelle in water, with ionic head groups at the surface and counterions clustered around the micelle partially neutralizing the charges. Counterions which are closely associated with the micelle can be assumed to be located in a shell, the so-called Stern layer, the thickness of which should be similar to the size of the micellar head groups. Monomeric co-ions will be repelled by the ionic head groups. The hydrophobic alkyl groups pack randomly and parts of the chains are exposed to water at the surface (Section 2). 

Addition of 5% ganglioside Gmi into the L-Glu-Bis-3 resulted in the appearance of vesicles along with twisted ribbons, while addition of nonchiral 10,12-docosadiynedioic acid caused the formation of platelets.97 These results affirm the importance of packing geometry, along with head group chirality, for the formation of helical structures. 

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