Micelle steric repulsion

Certain comb-type silicone surfactants have been shown to stabilize emulsions in the presence of salts, alcohol and organic solvents that normally cause failure of emulsions stabilized using conventional hydrocarbon surfactants and a study by Wang et al. [66,67] investigated the cause of this stability. Interaction forces due to silicone surfactants at an interface were measured using AFM. Steric repulsion provided by the SPE molecules persisted up to an 80% or higher ethanol level, much higher than for conventional hydrocarbon surfactants. Nonionic hydrocarbon surfactants lose their surface activity and ability to form micelles in... 

The size-dependent term ngfkT includes la) the decrease in attractive hydrophobic bonding between hydrocarbon tails of amphiphiles, due to their partial exposure to the aqueous medium, and (b) the repulsive interaction between the hydrophilic head groups, of the amphiphiles. This repulsive interaction is caused by Steric repulsion between head groups in nonionic micelles and by electrical repulsion between the ionic head groups in ionic micelles. ... 

In w/o emulsions the hydrocarbon chains of the adsorbed molecules protrude into the oily continuous phase. Stabilisation arises from steric repulsive forces as described in section 7.2.2. Emulsions are more complex than suspensions, because of the possibility (a) of movement of the surfactant into either the continuous or disperse phase, (b) micelle formation in both phases, and (c) the formation under suitable conditions of liquid crystalline phases between the disperse droplets. 

In the framework of the scaling theory, the corona of a spherical micelle can be envisioned [53-56] as an array of concentric spherical shells of closely packed blobs. The blob size, (r) = r/grows as a function of the radial distance r from the center of the core. Each blob comprises a segment of the chain within the local correlation length of the monomer density fluctuations [57], and corresponds to a contribution to the free energy of steric repulsion between the coronal chains. After calculating the total number of blobs in the micellar corona, one finds fhe free energy (per coronal chain) as ... 

As long as steric repulsions CpVA in the corona dominate over the ionic contributions a, the increase in a does not affect the structure of the micelles. 

The authors explained the observed differences in the rates of such polyion interchange reactions in terms of the collision model [95]. This model considers the transfer of the GPE from the original to the added HPE to proceed via a short-living intermediate ternary complex. Such a ternary complex is formed due to the diffusion-controlled approach and interpenetration of the HPE species occupied by chains of GPE and of free HPE species. Because the electrostatic and steric repulsions upon the approach and interpenetration of the reacting species for the coil/micelle combination (2a, 2b) and, especially, for the micelle/micelle combination (3a, 3b) are obviously much stronger than for the coil/coil combination (la, lb), the rates of the polyion interchange reactions show a pronounced decrease in the abovementioned order. 

Figure 6.14 (a) Schematic of a surfactant molecule, (b) Illustration of surfactant adsorption onto an oxide particle surface by coordinate bonding, (c) Stabilization by steric repulsion between organic tails of the surfactant molecules, (d) Stabilization by micelles. 

Several types of surface forces determine the interactions across thin liquid films. In addition to the universal van der Waals forces, the adsorbed ionic surfactants enhance the electrostatic (double-layer) repulsion. On the other hand, the adsorbed nonionics give rise to a steric repulsion due to the overlap of hydrophilic polymer brushes. The presence of surfactant micelles in the continuous phase gives rise to oscillatory structural forces, which can stabilize or destabilize the liquid films (and dispersions), depending on whether the micelle volume fraction is higher or lower. These and other surface forces, related to the surfactant properties, were considered in Sec. VI. 

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