Cloud point isotropic phases

Soil solubilization rates are often enhanced when surfactant-rich phases, either isotropic or liquid crystalline in nature, are present in the washing solution. Such phases exist, for instance, when nonionic surfactants are above their cloud points. These phases can either solubilize oily soils directly or interact with soil to form intermediate surfactant-rich phases such as microemulsions containing large amounts of oil. Under favorable conditions, the intermediate phases can be emulsified into the washing bath. 

For fluorescence PAH determination in tap water acid-induced cloud point extraction was used. This kind of extraction based on the phase separation into two isotropic liquid phases a concentrated phase containing most of the surfactant (surfactant-rich phase), where the solubilised solutes are exttacted, and an aqueous phase containing a surfactant concenttation closes to the critical micellar concentration. 

The aqueous micellai solutions of some surfactants exhibit the cloud point, or turbidity, phenomenon when the solution is heated or cooled above or below a certain temperature. Then the phase sepai ation into two isotropic liquid phases occurs a concentrated phase containing most of the surfactant and an aqueous phase containing a surfactant concentration close to the critical micellar concentration. The anionic surfactant solutions show this phenomenon in acid media without any temperature modifications. The aim of the present work is to explore the analytical possibilities of acid-induced cloud point extraction in the extraction and preconcentration of polycyclic ai omatic hydrocai bons (PAHs) from water solutions. The combination of extraction, preconcentration and luminescence detection of PAHs in one step under their trace determination in objects mentioned allows to exclude the use of lai ge volumes of expensive, high-purity and toxic organic solvents and replace the known time and solvent consuming procedures by more simple and convenient methods. 

Aqueous micellar solutions of many nonionic surfactants, with an increase in temperature, become turbid over a narrow temperature range, which is referred to as their cloud-point [17,277]. Above the cloud-point temperature, such solutions separate into two isotropic phases. Cloud-point extraction (CPE) is also referred to as a particular case of ATPE [278,279] and more specifically as aqueous micellar two-phase systems [10,277]. Very recently, in an extensive review, Quina and Hinze [280] have discussed in detail the emergence of CPE as an environmentally benign separation process, highlighting the basic features, experimental protocols, recent applications, and future trends in this area. 

As the temperature of dilute aqueous solutions containing ethoxylated nonionic surfactants is increased, the solutions may turn cloudy at a certain temperature, called the cloud point. At or above the cloud point, the cloudy solution may separate into two isotropic phases, one concentrated in surfactant (coacervate phase) and the other containing a low concentration of surfactant (dilute phase). As an example of the importance of this phenomena, detergency is sometimes optimum just below the cloud point, but a reduction in the washing effect can occur above the cloud point (95). However, the phase separation can improve acidizing operations in oil reservoirs (96) For surfactant mixtures, of particular interest is the effect of mixture composition on the cloud point and the distribution of components between the two phases above the cloud point. 

Figure 6.31 Phase diagram of the CH3(CH2)n(OCH2 CH2)a0H(C,2E )/H20 system A, two isotropic liquid phases B, micellar solution C, middle or hexagonal phase D, cubic phase E, neat or lamellar phase F, solid phase. The boundary between phases A and B is the cloud point.

A combination of SLS and DLS methods was used to investigate the behavior of nonionic micellar solutions in the vicinity of their cloud point. It had been known for many years that at a high temperature the micellar solutions of polyoxyethylene-alkyl ether surfactants (QEOm) separate into two isotropic phases. The solutions become opalescent with the approach of the cloud point, and several different explanations of this phenomenon were proposed. Corti and Degiorgio measured the temperature dependence of D pp and (Ig), and found that they can be described as a result of critical phase separation, connected with intermicellar attraction and long-range fluctuations in the local micellar concentration. Far from the cloud point, the micelles of nonionic surfactants with a large number of ethoxy-groups (m 30) may behave as hard spheres. ... 

In some cases, such as for C12E5, the lamellar phase La (or the HI phase) interferes with the loop (with the cloud point curve) and induces the so-called critical phase L3. L3 is an isotropic, often bluish phase, exhibiting a zero-variant three-phase critical point at its lowest temperature of existence. The three phases present at the critical conditions are W (water with a minute amount of amphiphile), L3, and La. The L3 phase seems to have a beneficial action on cleaning performance, maybe because of the presence of the critical point. 

Below temperature TB (cloud point for low polymer concentrations) and at a polymer weight fraction lower than wB, isotropic solutions are stable. Above the weight fraction wA and at a temperature below the line AD (clouds points for concentrated polymer concentrations) pure cholesteric phase separates. All other compositions are biphasic, the liquid crystals being in equilibrium with a dilute isotropic solution. At low temperature this domain exists in the small concentration range delimited by wA and wB. Note that TA may be below body or even room temperature. 

Aqueous solutions of many nonionic amphiphiles at low concentration become cloudy (phase separation) upon heating at a well-defined temperature that depends on the surfactant concentration. In the temperature-concentration plane, the cloud point curve is a lower consolution curve above which the solution separates into two isotropic micellar solutions of different concentrations. The coexistence curve exhibits a minimum at a critical temperature T and a critical concentration C,. The value of Tc depends on the hydrophilic-lypophilic balance of the surfactant. A crucial point, however, is that near a cloud point transition, the properties of micellar solutions are similar to those of binary liquid mixtures in the vicinity of a critical consolution point, which are mainly governed by long-range concentration fluctuations [61]. 

Figure 1 depicts the experimental cloud points and the nematic-isotropic transition temperatures in comparison with the theoretical phase diagram of the PMMA-OH/E7 PDLC system. The phase diagram calculation was carried out based on the combined Flory-Huggins (FH) and Maier-Saupe (MS) free energies using r = 2.25 and a = -4.0. The b value was estimated from the critic temperature using x = a + (Xc-a)Tc /T. 

Figure 1. The predicted "tea-pot" phase diagram in comparison with the experimental cloud points ( ) by LS and nematic-isotropic transition temperatures ( ) by DSC. Line ABC represents the peritectic line. The solid and dotted lines represent the binodal and spinodal, respectively.

In a study by Kahn et al. [9] on a polydisperse octadecyl amide with nine oxyethylene units, the pure surfactant is found to be a viscous liquid at room temperature. Upon solubilization in water, a clear isotropic solution phase is observed for all concentrations between the cloud point and the solidification temperature. No liquid crystal formation is observed. 

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