Surfactants hydrophilic groups

The contemporaneous presence of different solubilizates sometimes involves competition for the micellar binding sites [31], For instance, from an analysis of the heats of solution of benzene and water in solutions of reversed micelles of tehaethylene glycol dodecyl ether in decane, a competition between water and benzene for the surfactant hydrophilic groups was shown [32],... 

The adsorption of mixed surfactants at the air—water interface (monolayer formation) is mechanistically very similar to mixed micelle formation. The mixed monolayer is oriented so that the surfactant hydrophilic groups are adjacent to each other. The hydrophobic groups are removed from the aqueous environment and are in contact with other hydrophobic groups or air. Therefore, the forces tending to cause monolayers to form are similar to those causing micelles to form and the thermodynamics and interactions between surfactants are similar in the two aggregation processes. 

A micelle that is formed in a non-aqueous medium, thus having the surfactants hydrophilic groups oriented inward away from the surrounding medium. 

In the case of nonionic surfactants, hydrophilic groups orient in the water phase and lipophilic groups orient in the oil or solid phase. 

The conclusions drawn on the basis of the dielectric loss analysis of liquid samples, support the interpretation that a very gradual confluence of the different types of dispersions takes place.Such an interpretation could explain the instauration of polydispersed samples in terms of the coexistence, at equilibrium, first, of micellar aggregates with w/o microemulsion droplets and, successively, of a microemulsion with l-I O-per hydrophilic group monolayer, in equilibrium with a hydrated type of microemulsion (U-water molecule per polar head of the surfactant hydrophilic groups monolayer). The latter interpretation is in accordance with Steinbach and Sucker findings that the two types of structures ( 1-HpO and U-HgO molecule), may coexist at equilibrium (23.). 

EON is the average number of oxyethylene groups in the surfactant hydrophilic group,... 

Micelles are colloidal aggregates that form above some concentration, the so-called critical micelle concentration (CMC), according to the pattern shown in Fig. I. In a normal micellar aggregate (left), (he surfactant hydrhilic groups are in contact with the aqueous solvent, while the hydrt hobic "tails are located in the micelle core away from the aqueous environment—a favorable situation from an energetic point of view. Inverse micelles (right) are similar in structure but this time the solvent is an oil phase and the surfactant hydrophilic groups arc located inside (3). 

The enthalpy values associated with the water endotherm allow evaluation of the free water fraction (i.e., melting at 273 K) as well as of the amount of water bound to the surfactant hydrophilic groups. The latter information results Irom the difference between the a priori known water content of the sample, Irom which the expected enthalpy value (A// ) can be estimated, and the amount of water that was foimd to melt at 273 K. The latter is obtained from the measured enthalpy (A// ) value associated with the experimentally recorded DSC peak. Worked examples are given at the end of the next paragraph. 

As surfactant composition increases, the water/surfactant ratio decreases. The heat required to destroy the crystal increases as this ratio decreases because the contribution of hydration energy to the process (which is stoichiometry limited) also decreases. Such data could in principle serve to define the intrinsic hydrophilicity of surfactant hydrophilic groups [37], but this analysis has yet to be done. 

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

As esters of sulfuric acid, the hydrophilic group of alcohol sulfates and alcohol ether sulfates is the sulfate ion, which is linked to the hydrophobic tail through a C-O-S bond. This bond gives the molecule a relative instability as this linkage is prone to hydrolysis in acidic media. This establishes a basic difference from other key anionic surfactants such as alkyl and alkylbenzene-sulfonates, which have a C-S bond, completely stable in all normal conditions of use. The chemical structure of these sulfate molecules partially limits their conditions of use and their application areas but nevertheless they are found undoubtedly in the widest range of application types among anionic surfactants. 

On the other hand, in.the case of the nonionic surfactants C-15, NP-15 and 0-15 (the nonionic surfactant/cyclohexane system), mono-dispersed silicalite nanocrystals were obtained as shown in Fig. 1(c), 1(d) and 1(e), respectively. The X-ray diffraction patterns of the samples showed peaks corresponding to pentasile-type zeolite. The average size of the silicalite nanocrystals was approximately 120 nm. These results indicated that the ionicity of the hydrophilic groups in the surfactant molecules played an important role in the formation and crystallization processes of the silicalite nanocrystals. 

M. Dahanayake, J. Li, R. L. Reierson, and D. J. Tracy. Amphoteric surfactants having multiple hydrophobic and hydrophilic groups. Patent EP 697244, 1996. 

In mixtures of nonpolar solvents with little water, surfactants form spherical reverse micelles. They have a reversed orientation of the molecules with the hydrophilic groups in the interior and a drop of enclosed water in the middle. Starting from a precursor material, metal oxides in the form of uniform nanosized spheres can be obtained by hydrolysis under controlled conditions (pH, concentration, temperature). For example, titanium oxide spheres are obtained from a titanium alkoxide, Ti(OR)4 + 2 H20 —t Ti02 + 4 ROH. 

The hydrophobic group is usually a hydrocarbon chain and is generally called the tail, while the hydrophilic group, called the head, is an ionic group and very polar. Depending on the nature of the hydrophilic group, the surfactants are classified as ... 

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