Tails groups

Bain C D, Evall J and Whitesides G M 1989 Formation of monolayers by the ooadsorption of thiols on gold— variation in the head group, tail group, and solvent J. Am. Chem. Soc. Ill 7155-64... 

The abiHty to tailor both head and tail groups of the constituent molecules makes SAMs exceUent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular—substrate and molecule—solvent interactions, such as ordering and growth, wetting, adhesion, lubrication, and corrosion. Because SAMs are weU-defined and accessible, they are good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. 

Figure 20 provides important information on the activities of individual molecules, considering the fact that the performance of tail groups is a manifestation of molecule motion. The periodic jumps observed in Fig. 20(a) indicate that the alkane chains are plucked by the opposite mono-layer at the moment of slip. Looking at Fig. 20(b), however, it is difficult to tell whether or not the plucking mechanism also involves in an incommensurate sliding. 

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

Fig. 18. LFER between the rate constants k for bleaching Safranine 0 and ki for intramolecular inactivation in the series of head -substituted Fem-TAML catalysts with methyl tail groups. Conditions [H202] 0.012 M, 25°C, pH 11.0. The data point for la was ignored in the linear regression. From Ref. (52).

The involvement of the carboxy terminal in the G protein coupling of the mGluR is perhaps best illustrated by the significant differences in agonist potencies, signaling kinetics, and basal activities between the long-tailed and short-tailed Group I... 

Cleavage between the hydrophobic head group and the hydrophilic tail group (a) would generate the products 3 and 4, shown in Fig. 5.5.3, whilst cleavage at the terminal methyl group (b) would give rise to... 

Figure 5.5 Architecture of a layered SAM consisting of a rigid biphenyl unit (BP) on top of an aliphatic spacer chain of m methylene units. The surface-terminating tail group can be either chemically inert (e.g., X = CH3, H) or active (e.g., X = CN).

SAM-coated and clean substrates can easily reach 20 j,C/cm, which means that a simple coverage calculation from charge associated with the desorption wave is flawed, as pointed out by Schneider and Buttry [137]. In this context it is worth noting that the diflerence in capacity between SAM-coated and dean electrodes is not only due to the introduction of a dielectric layer but depends also on the surface of the SAM and its interaction with the environment, that is, whether the surface exhibits some additional charges either due to ionic tail groups or due to speciflc interaction of the SAM with the electrolyte. 

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