Abstract
The elongated shape of linear, wormlike, micelles is among the most common aggregation geometries of amphiphilic molecules in aqueous solutions. The cylindrical body of these uni-dimensional micelles (1D) can be regarded as an intermediate packing geometry, of a higher growth dimensionality than that of a spherical micelle and lower than that of a planar bilayer. In all these three canonical structures, the hydrophobic tails of the constituent amphiphiles form a compact, liquid-like, hydrocarbon core, with their polar headgroups residing on its surface, thus largely shielding the hydrocarbon tails from direct contact with water. The planar bilayer is a two-dimensional (2D) object; it can grow laterally along directions, but its third dimension is always microscopic. Namely, its thickness, or, more precisely, the distance () between its two hydrocarbon-water interfaces cannot exceed, where is the length of the fully extended hydrocarbon tail. Similarly, the growth dimensionality of a wormlike micelle is It can elongate along the cylindrical axis, but its diameter (2b) - and hence the two perpendicular d = 2 2b 2b max b d = 1 2b dimensions - cannot exceed. The growth dimensionality of a spherical micelle (keeping its spherical symmetry) is, of course, since to maintain a compact hydrophobic core all its three dimensions must be smaller than. Note that this last statement is only relevant to systems where a spherical micelle is the intrinsically preferred (or, “spontaneous”) packing environment of the amphiphiles. As briefly discussed later in this chapter, many other amphiphiles, in fact most of those that prefer packing into long cylindrical micelles, first assemble - for entropic reasons - into small spherical micelles. However, above the second critical micelle concentration (second cmc, to be distinguished from the first cmc above which micelles form), driven by the lower packing energy in the cylindrical geometry, the added amphiphiles incorporate into the middle of the aggregate, forming a gradually elongating cylindrical midsection, capped by two approximately hemispherical micellar caps. In these systems the spherical micelle is just the low concentration limit of a linear aggregate.
Original language | English |
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Title of host publication | Giant Micelles |
Subtitle of host publication | Properties and Applications |
Publisher | CRC Press |
Pages | 41-80 |
Number of pages | 40 |
ISBN (Electronic) | 9781420007121 |
ISBN (Print) | 9780849373084 |
State | Published - 1 Jan 2007 |
Bibliographical note
Publisher Copyright:© 2007 by Taylor & Francis Group, LLC.