Hydration repulsion

The stability of dispersions in aqueous media can often be described by the DLVO theory, which contains the double-layer repulsion and the van der Waals attraction. In some applications other effects are important, which are not considered in DLVO theory. At short range and for hydrophilic particles the hydration repulsion prevents aggregation. Hydrophobic particles, in contrast, tend to aggregate due to the hydrophobic force. 

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. 

Reduced Hydration-Repulsion Causes Enhanced Transfection... 

The results described above show that adding cholesterol and certain analogs increases TE more than the resulting increase in membrane charge density would predict. Previous work has demonstrated that CL-DNA complexes at low aM transfect poorly due to inefficient endosomal escape (which involves fusion) [21, 25]. Thus, our findings suggest that cholesterol and certain analogs facilitate fusion of the membranes of the complex and the endosome, independent of their effect on aM. A possible explanation for this is the overall reduction of the hydration repulsion layer of the membrane. 

It is known that the hydration repulsion layer of cholesterol is much smaller than that of DOPC [42, 44], Therefore, exchanging DOPC for cholesterol enhances fusion [42]. For CL-DNA complexes, this enhanced fusion of the membranes of... 

Hydration stabilisation. Due to the polarisability of the water molecules, even in a deionised aqueous solution, stabilisation may occur. Positively charged alumina particles, for example, bind preferentially to the negative oxygen of the water molecule. As a result, a double layer is formed, similar to ionic electrostatic repulsion. In ionic solutions, the hydration repulsive force occurs simultaneously with the electrostatic force, while the proportion of the two forces depends on the ionic concentration [22],... 

The second contribution to the energy comes from the hydration repulsion between the interfaces of the water layers, which is given by... 

For neutral bilayers, there are no long-range doublelayer forces which, coupled with the van der Waals attraction, could explain the stability of the lamellar structure. At small separations, the required repulsion is provided by the hydration force, which was investigated both experimentally6-8 and theoretically.9,10 However, it was experimentally observed that the lipid bilayers could be swollen in water up to very large interlayer distances,11 where the short-range exponential hydration repulsion becomes negligible. 

The dependence of the interaction force between two undulating phospholipid bilayers and of the root-mean-square fluctuation of their separation distances on the average separation can be determined once the distribution of the intermembrane separation is known as a function of the applied pressure. However, most of the present theories for interacting membranes start by assuming that the distance distribution is symmetric, a hypothesis invalidated by Monte Carlo simulations. Here we present an approach to calculate the distribution of the intermembrane separation for any arbitrary interaction potential and applied pressure. The procedure is applied to a realistic interaction potential between neutral lipid bilayers in water, involving the hydration repulsion and van der Waals attraction. A comparison with existing experiments is provided. 

Another limitation of the Poisson-Boltzmann approach is that the interaction between two surfaces immersed in water might not be exclusively due to the electrolyte ions. For instance, water has a different structure in the vicinity of the surface than in the bulk and the overlapping of such structures generates a repulsion even in the absence of electrolyte [20]. In this traditional picture, the hydration repulsion is not related to ion hydration actually it is not related at all to electrolyte ions. However, as recently suggested [21], this hydration interaction can still be accounted for within the Poisson-Boltzmann framework, assuming that the polarization is not proportional to the macroscopic electric field, but depends also on the field generated by the neighboring water dipoles and by the surface dipoles. 

A phenomenological treatment of the hydration repulsion, based on a Landau expansion of the free energy density, was proposed by Marcelya and Radic.9 They showed that, if the free energy density is a function of an order parameter that varies continuously from the surface, and if only the quadratic terms in this parameter and its derivative are nonnegligible, an exponential decay... 

Another mechanism for the hydration repulsion between lipid bilayers was more recently proposed by Marcelja.22 It is based on the fact that in water the ions are hydrated and hence occupy a larger volume. The volume exclusion effects ofthe ions are important corrections to the Poisson— Boltzmann equation and modify substantially the doublelayer interaction at low separation distances. The same conclusion was reached earlier by Ruckenstein and Schiby,28 and there is little doubt that the hydration of individual ions modifies the double-layer interaction, providing an excess repulsion force.28 However, while the hydration of ions affects the double-layer interactions, the hydration repulsion is strong even in the absence of an electrolyte, or double-layer repulsion. 

One can therefore conclude that there is no commonly accepted explanation for the microscopic origin of the hydration repulsion. The main purpose of this paper is to show that a suitable model for the polarization of water layers, based on the earlier work of Schiby and Ruckenstein,10 is compatible with both simulations and experiments on hydration repulsion. 

While one cannot rule out that there are contributions of different origins to the hydration repulsion, the polarization contribution might be the dominant one, at least for not too small separations, and this can explain, as shown later in the paper, the quadratic dependence, determined experimentally by Simon and McIntosh,24 of the hydration repulsion on the surface dipolar potential. 

One cannot yet rule out that other interactions contribute to the hydration, such as the disruption of the hydrogen bond networks when two surfaces approach each other. However, at least a part of this disruption is already contained in the dipole—dipole interactions included in the polarization model. In addition, the polarization model of hydration can relate the magnitude of the hydration force to the density of dipoles on the surface. This can explain the dependence of the hydration repulsion on the surface dipolar potential18 or the restabilization of some colloids at high ionic strength16 observed experimentally.10... 

It is usually assumed that the total repulsion is the sum between a double layer repulsion, due to the charges on the interface, and a hydration repulsion, due to the structuring of water in the vicinity of the interface, and that the two effects are independent of each other. This is, however, not accurate when the hydration is induced by the orientational correlation of neighboring dipoles, because both forces depend on polarization. 

It was shown that the interaction between dipoles increases markedly the repulsion at high ionic strength and large separation distances, when compared to the DLVO theory. When both charges and dipoles are present on the surface, the repulsion is not provided by the sum of two independent repulsions, a double layer and a hydration repulsion. The presence of dipoles on the surface can even decrease the repulsion. 

The values of the interaction energy at the maximum and at the secondary minimum are proportional with the radius a ofthe particle or droplet and depend strongly on the Hamaker constant AH and on the hydration repulsion. As shown in the previous section, small modifications in the ratio (pje ) (and hence in the hydration repulsion) can lead to a large increase in the potential barrier between the primary and secondary minima, thus affecting the stability of the system. 

The hydration repulsion is expected to be affected by the hydrogen bonding. When two surfaces approach each other, the increase ofthe free energy (due to the disruption ofthe hydrogen bonds) generates repulsion. Indeed, Monte... 

As already noted, the restabilization of some colloids at high ionic strength found in previous experiments20 can be explained in the traditional framework of the additivity between double layer and hydration forces, by a slight increase of the hydration repulsion caused by the increase in surface ion pair (dipole) density with electrolyte concentration. However, the increase in repulsion due to this mechanism is much too low to explain the strong increase of the second virial coefficient. 

The effect of electrolyte concentration on the transition from common to Newton black films and the stability of both types of films are explained using a model in which the interaction energy for films with planar interfaces is obtained by adding to the classical DLVO forces the hydration force. The theory takes into account the reassociation of the charges of the interface with the counterions as the electrolyte concentration increases and their replacements by ion pairs. This affects both the double layer repulsion, because the charge on the interface is decreased, and the hydration repulsion, because the ion pair density is increased by increasing the ionic strength. The theory also accounts for the thermal fluctuations of the two interfaces. Each of the two interfaces is considered as formed of small planar surfaces with a Boltzmannian distribution of the interdistances across the liquid film. The area of the small planar surfaces is calculated on the basis of a harmonic approximation of the interaction potential. It is shown that the fluctuations decrease the stability of both kinds of black films. 

A stable film can be obtained only if the hydration repulsion is stronger than the above critical value, hence if Aj > A, and the maximum disjoining pressure occurs at a distance d < d = 3A,. 

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