Amphiphilic chains

In order to collect information on the qualitative features of the chain conformations, we can directly look at many snapshots of the amphiphilic chain when the solvent quality is progressive worsened. Figure 41 shows a series of typical snapshots obtained for the chain with a 256-unit backbone at the... 

Fig. 41 Snapshot pictures illustrating typical conformations of the amphiphilic chain (poly-A) of length N = 256 for the strong H-P segregation, at different H - H attraction increasing from a to f. Hydrophobic beads are shown as dark gray spheres and hydrophilic beads are presented as light gray spheres. The sizes of all the spheres are schematic rather than space filling...

As the polymer concentration increases, interchain association inevitably occurs, but some amphiphilic chains can undergo a limited interchain association to form a stable mesoglobular phase that exists between microscopic single-chain globules and macroscopic precipitation. As expected, when the solvent quality changes from good to poor, intrachain contraction and interchain association occur simultaneously and there exists a competition between these two processes. Such a competition depends on the comonomer composition and distribution on the chain backbone and also depends on the rate of micro-phase separation. When intrachain contraction happens quickly and prior to interchain association, smaller mesoglobules are formed. A proper adjustment of the rates of intrachain contraction and interchain association can lead to polymeric colloidal particles with different sizes and structures. 

The assignment of the splittings to segments of the alkane chain was based on preliminary experiments with selectively deuterated analogues and is tentative. It indicates an order profile, which is different to that normally expected for amphiphilic chains (4-7). 

Figure 6. Determination of orientation from polarized fluorescence emission experiments. The orientation of the amphiphile chains in neighboring domains is indicated by the sloping parallel lines in the monolayer, and the orientation of the polarizability tensor of the fluorophore is represented by the line segment perpendicular to the chain axis. Long arrows show the direction of the incident light, and small arrows represent the direction of the electric field for p-polarized light. After Moy et al. ...

CJruen, D.W.R. (1981). The packing of amphiphile chains in a small spherical micelle, J. Colloid Interface Scl 84 281. 

It seems in Fig. 2g that all the experimental points are lying on a master surface, which is a first indication that there might be a physical law describing the correlation between the pore size and the molecular architecture of the amphiphile. However, because neither the one-phase nor the two-phase model was appropriate to describe the data (as shown elsewhere)," a new model was needed. It seems that in addition to the hydrophobic core (bright yellow), a certain fraction of the hydrophilic poly(ethylene oxide) (PEO) chain contributes to the size of the mesopore Dc (areas I and H in Fig. 3c). Only the remaining fraction of PEO is imbedded in the pore wall. By considering the total volume given by the number of units in the amphiphile chain and the stabilization of the interface I + II/III, it was finally possible to derive an equation that relates the mesopore size to the molecular composition of the amphiphile expressed as Vvb (see Eq. 1) ... 

A further step in the direction of a more detailed and realistic description of the amphiphile is taken by models in which the amphiphile is modeled by a polymer-like chain of subunits that are water-like at one end and oil-like at the other end [23]. Such models have been studied both in the continuum [24-26] and on a lattice [27 -31]. The advantage of these models is that they allow a calculation of the effect of the amphiphile chain length on the properties of microemulsions. These models have been studied mostly by computer simulations. 

Note that the dependence of the parameters go and c on the amphiphile concentration in Eqs. (17) and (19) is the same. The qualitative behavior as a function of the amphiphile chain length also agrees very well. 

PVGL microgeis with grafted amphiphilic chains were prepared by using reactive macromonomers consisting of a hydrophobic hydrocarbon segment and a hydrophilic PEO... 

Small assemblies that topologically mimic the micelle architecture but that present covalently bound amphiphilic chains have been synthesized as an alternative approach to provide intrinsic stability These nanocarriers are referred to as unimolecular polymeric micelles (UPM) and consist of single macromolecules such that their formation and dissociation are intrinsically independent on polymer concentration (no apparent CMC). UPM can be obtained from both dendrimers and star-block copol)miers, with the latter form involving fewer s)mthetic steps and being simpler to prepare. 

In forming a micelle, amphiphile chains aggregate in the micelle s core to reduce expensive hydrocarbon-water contact. It is possible to imagine that the chains could pack in a frozen array of parallel dXL-trans chains, and indeed this is the packing found in solid bulk R-alkane. However, without considerable expensive hydrocarbon-water contact, it is impossible to see how such an array could exist in a globular micelle. We should expect, therefore, that the interior of a micelle is liquid, and that the alkyl chains are conforma-tionally disordered. This expectation is borne out by experiments, among the most definitive of which are... 

In this model, the single amphiphile chain is treated as accurately as possible. Well established values for the C - C bond length, and for the relative energy and bond angles of trans and gauche conformers [1] are all included in the model. This approach was pioneer-... 

Fig. 1. C - D bond order parameters for different segments in the amphiphile chain. Carbon number 2 is the CHj (or CDj) group bonded to the headgroup while number 10 is the terminal CH, group. Scq = (jCos 6- ), where 0 is the angle between the C - D bond vector and the bilayer normal. , experimental order parameters [24] down the de-canoate chain in a 32 wt% sodium decanoate, 38 wt% deca-nol, 30 wt% water lamellar phase.-----, from the molecular dynamics simulation.-------, from our single chain...

So the Menger and Doll statement is irrefutable but also misleading. The model amphiphile chains have segregated themselves away from the water and their own headgroups almost as much as is physically possible. 94% of the chain volume resides inside the dry core and the hydrocarbon volume fraction falls off rapidly outside the core (fig. 8). There is contact of the chains with water, simply b ause of the large area per amphiphile at the micelle surface (more than 56 A for the micelle of figure 8). 

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