Hydrocarbon-water repulsion

A qualitative description of the factors responsible for micelle shape has been put forward by a number of different workers. " At the micelle surface there exists a balance of forces between head group repulsions and hydrocarbon-water repulsions. The former are particularly prevalent for surfactants containing ionic head groups. Head group repulsions serve to increase the average surface area per molecule (a). The... 

A surface term that reflects the opposing tendencies of the molecules to crowd closely together minimizing the unfavorable hydrocarbon-water interactions and to spread apart as a result of the electrostatic (head group) repulsion, hydration and steric hindrance,... 

These equations imply that A132 will exceed A12 if A33 is larger than A13 + A23. This effect, termed lyophobic bonding, occurs if the solvent-surface interaction is weaker than that between the solvent molecules. More interestingly, the dispersion interaction will be repulsive (A 132 < 0) when An and/or A23 are sufficiently large. Israelachvili [1] tabulates a number of Am values Awhw Ahwh 0-4X 10 erg, Apwp 1 x 10" erg, and Aqwq = O.SX -IO erg, where W, H, P, and Q denote water, hydrocarbon, polystyrene and quartz respectively. 

The set of parameters, i.e. the force-field parameters used in the SCF calculations, are listed in Table 1. We will not discuss all of them. The most important one is the repulsion between water and hydrocarbon. The value of this FH parameter is set to Xh2o, c = 0-8. One should remember however that in... 

The influence of the addition of cetyl trimethyl ammonium chloride, CTAC, to the reverse micellar solution affects the droplet size and micellar interactions, as demonstrated by the DQLS experiment (64). Addition of CTAC to micellar system at a given water content leaves the droplet size unchanged, whereas a decrease in the intermicellar attraction has been observed. This decrease is more important for high CTAC concentrations. This has been interpreted to steric repulsion induced by the long hydrocarbon tail of CTAC (C ft). Thus, the addition of this compound to CdS synthesis could modify the nucleation and/or growth process. The experiments were performed by solulization of CTAC in the micellar solution containing either sodium sulfide or Cd(AOT)2. 

Formation of hydrophobic bonds between nonpolar hydrocarbon groups on the drug and those in the receptor site is also common. Although these bonds are not very specific, the interactions take place to exclude water molecules. Repulsive forces that decrease the stability of the drug-receptor interaction include repulsion of like charges and steric hindrance. 

Repulsive van der Waals forces occur in a number of practically important cases, such as for different types of polymers in organic solvents (van Oss et al. 1980) and for certain hydrocarbon films on water (as mentioned in Vignette X). For example, the use of repulsive van der Waals forces has been suggested as a way to dissociate antigen-antibody complexes by van Oss et al. (1979). A much more detailed and quantitative introduction to repulsive van der Waals forces may be found in Israelachvili (1991). 

The most common agents to stabilize an emulsion are surfactants. Different effects contribute to the stabilization of emulsions. Steric repulsion between those parts of the surfactant, which are in the continuous phase, is an important effect. For a water-in-oil emulsion the hydrocarbon chains are hindered in their thermal movements if two water drops approach each other too closely. For an oil-in-water emulsion there is an additional effect the hydrophilic head groups have to be dehydrated to come into close contact. The resulting hydration repulsion stabilizes the emulsion. 

A typical spherical micelle contains 30-100 surfactants and has diameter of 3-6 nm. Micelles form because of two competing factors transfer of hydrocarbon chains out of water into an oil-like interior and repulsion between the head groups. 

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