Polyoxyethylene chain hydration

The classical model, as shown in Figure 1, assumes that the micelle adopts a spherical structure [2, 15-17], In aqueous solution the hydrocarbon chains or the hydrophobic part of the surfactants from the core of the micelle, while the ionic or polar groups face toward the exterior of the same, and together with a certain amount of counterions form what is known as the Stern layer. The remainder of the counterions, which are more or less associated with the micelle, make up the Gouy-Chapman layer. For the nonionic polyoxyethylene surfactants the structure is essentially the same except that the external region does not contain counterions but rather rings of hydrated polyoxyethylene chains. A micelle of... 

For pure nonionic EO adducts, increase in the number of oxyethylene groups in the molecule results in a decrease in the tendency to form micelles and an increase in the surface tension of the solution at the critical micelle concentration (1 ) (l. ) This change in surface activity is due to the greater surface area of the molecules in the adsorption layer and at the micellar surface as a result of the presence there of the highly hydrated polyoxyethylene chain. The reduction in the tendency to form micelles is due to the increase in the free energy of micelle formation as a result of partial dehydration of the polyoxyethylene chain during incorporation into the micelle ( 1 6) (17). 

Therefore, the hypothesis of an increasing nonionic character of alkyl ether sulfates with increasing number of oxyethylene groups is not tenable. Some time ago (30), it was suggested that a certain hydrophobic nature can be attributed to the polyoxyethylene chain of alkyl ether sulfates. At first, this appears to be in contradiction to the decidedly hydrophilic character of the polyoxyethylene chain for nonionic surfactants. However, the possibility of EO group hydration impairment by the sulfate group cannot be excluded. 

For compounds with one and two oxyethylene groups in their molecule, the increase of the micelle formation tendency can be explained by a contribution of these groups to the hydrophobic part of the molecule. With a longer polyoxyethylene chain in the molecule, however, the increased tendency to form micelles is primarily caused by the increased distance between the charged groups due to increased hydration of the ether groups. 

In the same study Attwood et al. [55] investigated the effect of increasing the surfactant concentration on the overall viscosity of an o/w ME system and obtained values for the viscosity constant a of 3.19-4.17. The authors concluded that allowance for the hydration of the polyoxyethylene chain of the used surfactant reduced the value of the viscosity constant a toward the theoretical value of 2.5 for a solid sphere. They also concluded that changing the ratio of the nonionic surfactants did not significantly affect the viscosity of the system. 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

Figure 7.7 Enthalpic stabilisation representation of en-thalp ic stabilisation of particles with adsorbed hydrophilic chains. The hydrated chains of the polyoxyethylene molecules —(0CH2CH2) 0H protrude into the aqueous dispersing medium. On close approach of the particles to within 26 (twice the length of the stabilising chains), hydrating water is released, resulting in a positive enthalpy change which is energetically unfavourable.

Temperature increase generally causes a slight decrease in the extent of adsorption of ionic surfactants. The effect is not pronounced and is insignificant compared with the effects of electrolyte and pH. Adsorption of polyoxyethylated non-ionic surfactants on to Graphon has been reported to increase with increase in temperature [43]. This effect has been attributed to a decrease in the hydration of the polyoxyethylene chain. The opposite effect was, however, noted for the adsorption of cetomacrogol on griseofulvin [46]. 

The PIT method can be nsed when polyoxyethylene-type nonionic surfactants are present in the system, as their solubility changes with temperatnre [44]. At low temperatures, these surfactants are hydrophilic their polyoxyethylene chains are hydrated acqniring a relatively big volnme, as compared to the lipophilic part of the molecnle. Conseqnently, the surfactant monolayer has a large positive spontaneous curvature, forming O/W microemnlsions (Wn at equilibrium, which may coexist... 

Figure 5.10 shows changes in zeta potential and turbidity of iron(III) oxide hydrate sols flocculated with NFIOO. The optimum flocculation concentration was about 3 X 10 mM NFIOO. The sols were redispersed by NF7 or NP7.5, a hydrocarbon-type nonionic surfactant (polyoxyethylene nonylphenyl ether with a polyoxyethylene chain of average 7.5 EO). The turbidity increased sharply. The zeta potential changed only a little, as expected for a nonionic surfactant. Sols flocculated by NFIOO were not redispersed by SDS. The inability of SDS, an anionic hydrocarbon surfactant, to redisperse the sols was attributed... 

In the case of a nonionic surfactant, there is no electrostatic interaction between the particle surface and the hydrophilic groups of the surfactant. Furthermore, because of the hydrophobic property of Ag particles, adsorption of hydrophobic groups of the smfactant onto the particle surface takes place and the polyoxyethylene chains form the steric barrier. The highly hydrated polyoxyethylene chain in Tween 20 is extended into the aqueous phase in the form of a coil that acts as an effective barrier against aggregation. 

FIG. 3 Schematic diagram of the stabilization effect of hydrophobic particles by a surfactant with a polyoxyethylene chain in an aqueous solution, (a) Surfactant with polyoxyethylene in an aqueous solution, (b) Particle stabilization mechanism by steric hindrance due to surfactant with hydrated polyoxyethylene in an aqueous solution. 

The EO/PO copol5uners are good at stabilizing hydrophobic particles. The polyoxypropylene chain will adsorb onto the hydrophobic particle surface and allow the polyoxyethylene chain to form a steric barrier because of its highly hydrated nature. The polyoxyethylene chain should be more than 20 units long to provide efQcient stabilization. 

In typical nonionic micelles the shell surrounding the hydrocarbon core also resembles a concentrated aqueous solution, this time a solution of polyoxyethylene. The ether oxygens in these chains are heavily hydrated, and the chains are jumbled into coils to the extent that their length and hydration allow. 

C Eg has eight ethylene oxide (EO) residues and nine oxygen molecules. While polyoxyethylene glycol (PEG) can acquire nearly two molecules of water per EO group, the EO group in an organized assembly, such as in reversed micelles, is associated with four molecules of water [61-64]. Almost all the water molecules are hydrated to the EO chains of Ci2Eg up to 30 [67]. Thus, in the case of C12 Eg reversed micelles the free water is assumed to appear above = 30. 

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