Microemulsion, hydration

Enzymes are suspended in hydrated microemulsion surrounded by a monolayer of surfactant molecules dispersed in an apolar solvent [53-60,135] [Fig. 1(b)]. Micelles ( 2 nm sphere) are formed when lyophilized or aqueous preparation of enzymes are introduced with stirring or shaking into a solution of synthetic or natural surfactant in an organic solvent. 

For Cu(ll) in water adsorbed on silica gels it was reported 5 that in gels with small pores isolated hydrated ions are detected at 77 K5 in pores larger than -4 nm a broad signal is superimposed on the spectrum of isolated ions. The appearance of the broad signal indicates aggregation of cations and the presence of bulk or freezable water. In a recent publication the Cu(ll) probe was used to test the possibility of ice formation in microemulsions, jce formation was detected in one of the microemulsions studied for very slow cooling rates from -300 K to 77 K. 

It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984 El Seoud, 1984). 

In the field of biology, the effects of hydration on equilibrium protein structure and dynamics are fundamental to the relationship between structure and biological function [21-27]. In particular, the assessment of perturbation of liquid water structure and dynamics by hydrophilic and hydrophobic molecular surfaces is fundamental to the quantitative understanding of the stability and enzymatic activity of globular proteins and functions of membranes. Examples of structures that impose spatial restriction on water molecules include polymer gels, micelles, vesicles, and microemulsions. In the last three cases since the hydrophobic effect is the primary cause for the self-organization of these structures, obviously the configuration of water molecules near the hydrophilic-hydrophobic interfaces is of considerable relevance. 

The viscosity of microemulsions has been studied several times in order to determine hydration and interactions between the dispersed droplets. It was found that an increase in hydration of the surfactant molecules resulted in rheological behavior more similar to that of suspensions containing solid particles in low concentrations. In any case, the microemulsions showed Newtonian flow characteristics. 

The low temperature properties of a dodecane-hexanol-K.oleate w/o microemulsion from 20°C to -190°C were studied vs. increasing water content (C,mass fraction) in the interval 0.021+-0.1+, by Differential Scanning Calorimetry and dielectric analysis (5 Hz-100 MHz). A differentiation between w/o dispersions is obtained depending on whether they possess a "free water" fraction. Polydispersity is evidenced by means of dielectric loss analysis. Hydration processes occurring, at constant surface tension, on the hydrophilic groups of the amphiphiles, at the expenses of the free water fraction of the droplets, are shown to develop "on ageing" of samples exhibiting a time dependent behavior. 

Recently, Steinbach and Sucker (23) reported about the formation of l+-H20-molecule structures that may develop on the hydrophilic groups of surface active compounds upon dilatation of a l-H20-molecu-le structure, by adsorbing 3-water molecules from the subphase at a water-air interface. In the case of the water-oil interphase of the microemulsion, the dispersed droplet consits of an interphasal choro-na that surrounds an inner water core the free water fraction of the latter (bulk-H20)is the subphase that, acting as a reservoir, supplies H2O molecules to the interphase region. Since the formation of hydrated structures takes place at ons ant sur ace tension (23), the above mechanism allows the water-oil interface to expand without affecting the surface pressure necessary to maintain the system s equilibrium. In this way while the area of every polar head of the amphi-phile remains constant, the interphase area stabilized by a single polar head increases up to the amount corresponding to the definite area requirement of the it-H20-molecule structure (23) (3-6). 

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.). 

The effect of hydrated radii, valency and concentration of counterions on oil-external and middle phase microemulsions was investigated by Chou and Shah (40). It was observed that 1 mole of CaCl2 was equivalent to 16-19 moles of NaCl for solubilization in middle phase microemulsion, whereas for solubilization in oil-external microemulsions, 1 mole of CaCl2 was equivalent to only 4 moles of NaCl. For monovalent electrolytes, the values for optimal salinity for solubilization in oil-external and middle phase microemulsions are in the order LiCl > NaCl > KC1 > NH Cl, which corre-... 

On the contrary, this set of experimental results would provide some ground for a theoretical and thermodynamical explanation of the evolution swollen micelle-microemulsion. Indeed each type of structure seems to reflect a domination of one or other component of the free energy of these nonionics at room temperature. Although a calculation and a discussion of these energy effects are well beyond the scope of the present paper, we can point out the importance of the forces specific to the hydrocarbon chain and to the oil beside the pure hydration forces. Van der Waals forces would favour the formation of a water layer, while entropic effects seem very important as far as the transitions hank-lamella and lamella-globule are concerned. These effects due to the solvent concentration (but also to the nature of the oil (2,5) are quite evident from the fine evolution of the phase diagrams, especially for water/surfactant ratios in the range 0.5-1.2. 

The transport properties of microemulsions are of great interest both for the information they provide about the physical properties of the systems, and in industrial applications of these materials. The transport of matter or energy through oil in water (0/W) microemulsions is determined both by the volume fraction and geometry of the oil and emulsifier microdroplets (the structure effect") and by possible modifications in the transport properties of the continuous water phase by its interaction with the hydrophilic groups in the surfactant and cosurfactant that stabilize the microemulsion (the "hydration effect"). Through the use of appropriate mixture theories, these two effects can in part be separated. 

We will also consider the apparent phase volume p which is calculated from the mixture theories as the total volume fraction of the microemulsion that is excluded from the transport. Assuming that the transport property of the hydration water is negligible compared to that of the bulk liquid, p would include the hydration water as well as the oil and emulsifier. 

Figure 1 shows the dielectric relaxation properties of the Tween microemulsions plotted on the complex permittivity plane (from Foster et al ( 1). The mean relaxation frequency (corresponding to the peak of each semicircle) decreases gradually from 20 GHz for pure water at 25°C to ca. 2 GHz for a concentrated microemulsion containing 20% water. Since the permittivity of the suspended oil/ emulsifier is 6 or less at frequencies above 1 GHz, this relaxation principally arises from the dipolar relaxation of the water in the system. Therefore, the data shown in Figure 1 clearly show that the dielectric relaxation times of the water in the microemulsions are slower on the average than those of the pure liquid. The depressed semicircles indicate a distribution of relaxation times (9), and were analyzed assuming the presence of two water components (free and hydration) in our previous studies. 

The striking observation is that the ionic conductivity and water self-diffusion coefficient, but not the thermal conductivity, deviate significantly from the predictions of the mixture theories. This could arise from structural effects, such as a gradual transition from 0/W to W/0 structure with decreasing water content. We argue instead that these deviations principally result from hydration effects, and not from structural properties of the microemulsions. This would be expected because of the similarity of the data from the microeraulsions and PEO, in which structure effects would be quite different. 

Physical Mechanisms. The simplest interpretation of these results is that the transport coefficients, other than the thermal conductivity, of the water are decreased by the hydration interaction. The changes in these transport properties are correlated the microemulsion with compositional phase volume 0.4 (i.e. 60% water) exhibits a mean dielectric relaxation frequency one-half that of the pure liquid water, and ionic conductivity and water selfdiffusion coefficient one half that of the bulk liquid. In bulk solutions, the dielectric relaxation frequency, ionic conductivity, and self-diffusion coefficient are all inversely proportional to the viscosity there is no such relation for the thermal conductivity. The transport properties of the microemulsions thus vary as expected from simple changes in "viscosity" of the aqueous phase. (This is quite different from the bulk viscosity of the microemulsion.)... 

Effect of Microemulsion Structure on the Transport Properties. It appears from the discussion above that the reduction in the ionic conductivity and water self-diffusion coefficient is primarily attributable to hydration effects, not principally to changes in the structure of the microemulsion with higher phase volume. 

Lastly, we would like to point out that the head group of the ionic surfactant have to be hydrated by a minimum amount of water in order to dissolve into a low polarity solvent (e.g. short chain alcohols). In the hydrocarbon oil rich corner of a microemulsion phase diagram, micellization occurs as long as the minimum water required to hydrate the ionic head group is added (5). Hence the minimum water to surfactant molar ratio required for such hydration can be determined by light scattering measurement. The ratio has been found to be 10 for sulfate surfactants in toluene and 8 for carboxy-... 

We took a clear microemulsion sample of 0.22 water mass fraction near the phase boundary, first titrated with IPA till the sample just became turbid, then titrated with water till the sample became clear again. Repeating these procedures many times and plotting the ratio of Nw/Ng versus Na/Ng, we then obtained a straight line as shown in Figure 9. The slope yields the constant k and the intercept on y-axis corresponds to the minimum number of water molecules per surfactant molecule required for dissolution. It was concluded that minimum 8 water molecules are needed to hydrate each sulfate group for dissolution of SDS into IPA. It should be noted that this titration method can only be used in the miscibility range of the short chain alcohol with water. 

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