Isotropic liquid hydrocarbons

This means that the phase changes observed have comparatively less Importance for the thermodynamics of the system. On the other hand, the changes and modifications of the association structures within the isotropic liquid hydrocarbon or alcohol phase pose a series of interesting problems. Some of these have recently been treated in review articles by Fendler — who focussed on surfactant inter-association emphasizing consecutive equilibria and their thermodynamics. The following description will focus on the Intermolecular interaction between different kinds of molecules and the Importance of these interactions for the "inverse" association structures. 

Fig. 11. In a system of water and hydrocarbon a nonionic emulsifier with a polyethylene glycol) chain as the polar part dissolves in the aqueous phase at low temperatures (a) and in the oil phase at high temperatures (c). At an intermediate temperature (b) three isotropic liquid phases maybe found.

Phase diagrams of water, hydrocarbon, and nonionic surfactants (polyoxyethylene alkyl ethers) are presented, and their general features are related to the PIT value or HLB temperature. The pronounced solubilization changes in the isotropic liquid phases which have been observed in the HLB temperature range were limited to the association of the surfactant into micelles. The solubility of water in a liquid surfactant and the regions of liquid crystals obtained from water-surfactant interaction varied only slightly in the HLB temperature range. 

In this context it is instructive to ruminate on the structure of the surfactant phase. A representative composition of the phase would be 10% emulsifier and equal amounts of water and hydrocarbon. The conclusions giving a layer structure (31, 32, 33) appear to be a reasonable basis for discussing the energy conditions implied in the structure. If an area per molecule of 10-18 m2 is considered reasonable (39), the water and oil layers are approximately 1.2 X 10"8 m thick. Low angle x-ray determinations have shown that the structure does not consist of regular layers with constant spacings a structure which would accommodate the factors which determine stability would be difficult to envision. Further, since the phase is an isotropic liquid, a regularly layered structure is excluded. 

According to the present results the marked changes of isotropic liquid regions at the PIT described by Shinoda and co-workers (29, 30, 31, 32, 33) appear limited to micellar associations and solubilization. The cosolvency of water and hydrocarbon in the emulsifier and the liquid crystalline phase region seem to undergo only small changes. 

The behavior of a series of polyoxyethylene alkyl ether nonionic surfactants is also illustrative. According to Figure 11 the dioxyethylene (A) compound does not form liquid crystals when combined with water. Its solutions with decane dissolve water only in proportion to the amount of emulsifier. The tetraoxyethylene dodecyl ether (B) forms a lamellar liquid crystalline phase and is not soluble in water but is completely miscible with the hydrocarbon. The octaoxyethylene compound (C) is soluble in both water and in hydrocarbon and gives rise to three different liquid crystals a middle phase, an isotropic liquid crystal, and a lamellar phase containing less water. If the hydrocarbon p-xylene is replaced by hexadecane (D), a surfactant phase (L) and a lamellar phase containing higher amounts of hydrocarbon are formed in combination with the tetraoxyethylene compound (B-D). 

Microemulsions are clear (transparent and translucent are also used in the literature), thermodynamically stable, isotropic liquid mixtures of oil, water, and surfactant, frequently in combination with a cosurfactant. The aqueous phase may contain salt(s) and/or other ingredients, and the oil may actually be a complex mixture of different hydrocarbons and olehns. In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require high shear conditions generally used in the formation of ordinary emulsions. Microemulsions tend to appear clear due to the small size of the disperse phase. However, clear appearance (transparency) may not be a fundamental property. Sometimes microemulsion may not look clear to the naked eye in the case where dark viscous oil exists. The solution may not be purely transparent because it contains aggregates of micelles. Quite often, we still use these terms, even in this book. Probably we should simply use the term homogeneous solution. 

A different thermal behavior is observed for those examples in which the hydrocarbon chain length exceeds the length of the fluorocarbon part. Here, no phase transitions other than that to the isotropic liquid occur at elevated temperatures. Indeed, in case of 6, F(CF2)i2(CH2)ir)H, the thermally induced molecular motion of the long hydrocarbon chain is comparable to that of short chained examples at room temperature, shown by Raman spectra, for... 

Triblock partially fluorinated hydrocarbons F/nHiiFin(m = 12, n = 8 m = 10, n = 10 m = 12, n = 12) have also been examined and all are found to exhibit very narrow (< ) range of thermotropic smectic B phases between their crystalline and isotropic liquid phases [88]. When the polymethylene portions of such molecules are replaced by a much stiffer pam-substituted aromatic ring, the mesogenic character is lost [89]. 

Here the R 1 values provided in the Supplementary Content (Leftin and Brown 2011) for the (CH2) segments of DLPC and the liquid hydrocarbon -dodecane are compared. Whereas scaling of the relaxation rates for the lipid depends on segmental motion, anisotropic molecular motion, and collective membrane motion, the relaxation rate of the alkane depends on isotropic, fast segmental, and molecular motions only. The frequency dispersion for the liquid is linear with a slope nearly equal to zero at all frequencies. However, for DLPC the slopes of the dispersion depend significantly on temperature. This shows that the phase behavior of the membrane contributes to the structural dynamics observed, and that the rate of the acyl chain motion becomes more like the isotropic alkane with increasing temperature, thereby highlighting the contribution of order fluctuations to... 

The hydrocarbon chain melting transition is facilitated by factors that reduce the polar headgroup network cohesion. The addition of water to cetyltrimethylam-monium tosylate produces a peak at 23°C, which is related to the melting of CTAT crystals (embedded in saturated aqueous solution below 23 C) to produce a liquid crystalline phase (in highly concentrated CTAT systems) or micellar solutions (in dilute systems). The peak is broad, probably due to the existence of a biphase transition zone. No melting peak related to the polar network was detected, probably because of the relatively weak cohesive forces in this particular polar network. The second peak detected in concentrated water-surfactant samples was due to the hexagonal mesophase-isotropic liquid transition [53]. 

The nature of the headgroup plays a crucial role in determining the phase transition, for two main reasons. One is the electrostatic interaction among head-groups at the bilayer surface, which may differently affect the stability of the bilayer structure. The other is the influence of the headgroup on the alignment of the hydrocarbon chains. For example, protonated pyridinium salts show a simple single phase transition from the solid crystalline state to an isotropic liquid, while methylated pyridinium salts exhibit solid crystalline-solid crystalline, solid crystalline-liquid crystalline, and liquid crystalline-isotropic liquid transitions [77],... 

This large continuous isotropic liquid region at first appears highly appealing, but effective utilization of shorter chain length alcohols as cosurfactants is countered by another factor. Butanol certainly destabilizes the lamellar liquid crystal efficiently (Figure 1.6), but when the hydrocarbon is added to form the microemulsion, the butanol is too water soluble and does not reach and reside at the oil/ water interface sufficiently. As a result, the system forms two separate phases a traditional macroemulsion of oil and water. 

Addition of the hydrocarbon oils to the various surfactant pairs resulted in the formation of different phases. Representative diagrams are illustrated in figures 5 and 6. There was generally a large area of isotropic liquid phase whose viscosity decreased as the amount of hydrocarbon was increased. Some of the three-component systems exhibited a gel region while many also exhibited regions containing emulsions (water + oil) of low viscosity (fig. 5). 

It is well known that many compounds containing hydrocarbon chains such as liquid crystals or lipid membranes undergo solid state phase transitions prior to melting. In these intermediate phases, between the completely ordered crystal and the isotropic liquid, large amplitude motions of the hydrocarbon chains are effective and give rise to a strong dynamical disorder. 

---