Zwitterionic amphiphiles

Non-ionic and zwitterionic amphiphiles typically have lower values of the cmc than otherwise similar amphiphiles, because there is no formal coulom-bic repulsion between the head groups. 

We shall deal with ionic and zwitterionic amphiphiles. These can take on a confusing variety of different shapes and sizes some aggregate into small spherical or globular micelles, others appear to form long cylindrical micelles, while others coalesce spontaneously into vesicular or lamellar bilayers. 

For zwitterionic amphiphiles there will be a small additional electrostatic contribution to g%—gx of the form e /(e r+—r ). 

Numerous micellar systems of both ionic and nonionic surfactants in water have been investigated [55,56], occasionally with an added electrolyte or another amphiphile as a third component. The electrical birefringence dynamics observed reflect the significant differences in the structure and thermodynamic behavior of ionic and nonionic systems, while zwitterionic amphiphiles [57] show their ambivalent nature. 

Hence, for a la 1, the plot of nsaIkT versus a la should be a straight line of intercept 1. However, in some cases, an intercept close to j can be found see Ref. 27. This can be attributed to the formation of doublets of adsorbed molecules due to the component of their dipole moments oriented laterally to the interface. This effect can be observed with zwitterionic amphiphiles like some lipids [27]. A doublet represents a couple of adsorbed molecules with antiparallel orientations of their lateral dipole moments. 

Most of the SAS have been reported in amphiphilic polymers, which show the ability to self-assemble due to the desolvation, collapse, and intermolecular association of the hydrophobic portions of monomers. In the case of charged (anionic, cationic, or zwitterionic) amphiphiles, further stability and structural specificity can be designed using intermolecular polar interactions, such as electrostatic and hydrogen bonding. Various biologically inspired materials... 

At the same time, these researchers believed that the oblate spheroid micelles formed by ionic and zwitterionic amphiphiles were unacceptable. As their peripheral regions have very high curvature while the central regions are too thick, the oblate spheroid micelle is energetically unfavorable due to electrostatic repulsion between the polar groups of surfactants. In the CTAB/KBr micellar system, the addition of neutral salt reduces the electrostatic interaction between the head groups in the micelle. 

Fig. 3 a-c. Summary of data from different laboratories, obtained by surface force measurement, on the average layer thickness L as a function of tethered chain length for flat, tethered layers constructed by adsorption of amphiphilic polymers on mica. Adapted from Ref. 21. (a) Data of reference 20 on poly-tert-butylstyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (b) Data of references 11 and 12 on polystyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (c) Data of references 13 and 14 on polystyrene chains anchored by adsorbing zwitterionic groups [13] or by small adsorbing blocks of polyethyleneoxide [14]... 

Amphiphile-oil-water system, temperature of, 16 424-426 Amphiphiles, 16 420 Amphiphile strength, 6 424 Amphiphilic chemicals, 17 56 Amphiphilic copolymers, 20 482 behavior of, 20 483 well-defined, 20 485-490 Amphiphilic molecules, 15 99-101 Amphiphilic plasticizers, 14 480 Amphiphilic polymer blend, 23 720 Amphiphilic polymers statistical, 20 484-490 stimuli-responsive, 20 482-483 Ampholytes, 9 746-747 Amphoteric cyclocopolymers, water-soluble, 23 721 Amphoteric starches, 4 722 Amphoteric (zwitterionic) surfactants, 24 148... 

One characteristic feature of surfactants is their amphiphilic nature. These molecules present two moieties the hydrophobic moiety (usually a hydrocarbon chain) interacts with the nanotube sidewalls, while the hydrophilic part, called polar head group, is generally charged or has zwitterionic character. It has the double function of helping solubility in aqueous solvents and of providing additional stabilization towards tubes aggregation by coulombic charge repulsion. 

Performance of surfactants is closely related to surface activity and to micelle formation. Both these are due to amphiphilic nature of the surfactant molecule. The molecule contains a nonpolar hydrophobic part, usually, a hydrocarbon chain, and a polar hydrophilic group, which may be nonionic, zwitterionic, or ionic. When the hydrophobic group is a long straight chain of hydrocarbon, the micelle has a small liquid like hydrocarbon core (1,2). The primary driving... 

Fig. 8. Two pairs of a polymerizable zwitterionic dienoyl lipid and a cleavable disulfide amphiphile derived from cysteine. In each pair, one amphiphile has a hydrocarbon tail and the other a fluorocarbon tail.

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

It is thus clear that a treatment of the micellization process of ionic amphiphiles must include a discussion of electrostatic effects. Furthermore, even for zwitterionic and nonionic surfactants, the electrostatic effects play a role. The favorable interaction between the polar groups of these amphiphiles and the solvent water is probably mainly of an electrostatic origin. 

Structure Formation in Surfactant Solutions. Surfactants, also referred to as soaps, detergents, tensides, or surface active agents, are amphiphilic molecules possessing both hydrophilic and hydrophobic regions. They can be classified as anionic, cationic, zwitterionic, or nonionic (neutral) depending upon the nature of the polar... 

CRP is a powerful tool for the synthesis of both polymers with narrow molecular weight distribution and of block copolymers. In aqueous systems, besides ATRP, the RAFT method in particular has been used successfully. A mrmber of uncharged, anionic, cationic, and zwitterionic monomers could be polymerized and several amphiphilic block copolymers were prepared from these monomers [150,153]. The success of a RAFT polymerization depends mainly on the chain transfer agent (CTA) involved. A key question is the hydrolytic stability of the terminal thiocarbonyl functionaHty of the growing polymers. Here, remarkable progress could be achieved by the synthesis of several new dithiobenzoates [150-152]. 

Hydrophobically modified polybetaines combine the behavior of zwitterions and amphiphilic polymers. Due to the superposition of repulsive hydrophobic and attractive ionic interactions, they favor the formation of self-organized and (micro)phase-separated systems in solution, at interfaces as well as in the bulk phase. Thus, glasses with liquid-crystalline order, lyotropic mesophases, vesicles, monolayers, and micelles are formed. Particular efforts have been dedicated to hydrophobically modified polyphosphobetaines, as they can be considered as polymeric lipids [5,101,225-228]. One can emphasize that much of the research on polymeric phospholipids was not particularly focused on the betaine behavior, but rather on the understanding of the Upid membrane, and on biomimicking. So, often much was learnt about biology and the life sciences, but little on polybetaines as such. 

The implications so far are that egg PC vesicles should be fairly homogeneous in size, and that this size is effectively independent of the total amphiphile concentration even down to extremely low concentrations. In general, the factors that are expected to affect vesicle size are changes in the optimal surface area Oq and the ratio y//c. For anionic amphiphiles the surface area Qq may be expected to be changed by variations in pH, ionic strength, [Ca +], while zwitterions may be expected to be less sensitive to such changes. On the other hand, p/4 may be expected to vary with the degree and type (cw, trans) of unsaturation. 

Isocyanides bearing ammonium side-chains 51 and 52 have been polymerized in the presence of nickel catalysts [72, 73]. The amphiphilic isocyanide 51 forms vesicles on dispersion in water. The isocyano groups located in the vesicle bilayers were polymerized by nickel capronate to form polymerized vesicles. The isocyanide 51 was also used in the preparation of polymerized vesicles containing metalloporphyrin components within the bilayer membrane [74]. The redox behavior of this membrane-bound cytochrome P-450 mimic has been investigated in detail. In addition to those bearing cationic side chains, isocyanides 53 and 54 bearing zwitterionic side chains were successfully used [75]. 

---