Solubility determinations surfactants

The high water-solubility of surfactants and their, often more polar, metabolites prevents direct application of gas chromatographic separation (GC) with appropriate detection. The necessary volatilisation without thermal decomposition can be achieved by derivatisation of the analytes, but these manipulations are time- and manpower-consuming and can be susceptible to discrimination. Additionally, each derivatisation step in environmental analysis is normally target-directed to produce volatile derivatives of the compounds to be determined. Unknown surfactants that are simultaneously present, but differ in structure and therefore cannot react with the derivatisation reagent, are discriminated under these conditions. 

Knowledge of drug properties, especially solubility in surfactants or as a function of pH, is essential. One could anticipate precipitation of the drug as the pH changes in solution, or if release from the dosage form leads to supersaturation of the test media. Be aware that preparation of a standard solution may be more difficult than expected. It is customary to use a small amount of alcohol to dissolve the standard completely. A history of the typical absorptivity range of the standard can be very useful to determine if the standard has been prepared properly. 

Due to micelle formation the total surfactant concentration undergoes an abrupt increase. Since true (molecular) solubility of surfactants, determined by the CMC, remains essentially constant, an increased surfactant concentration in solution is caused by an increase in a number of formed micelles. Micellar solubility increases with increase in temperature, and thus a continuous transition from pure solvent and true solution to micellar solution, and further to different liquid crystalline systems and swollen surfactant crystals (see below), may take place in the vicinity of the Krafft point. 

Since the cloud point of a surfactant is a structure related phenomenon, it should also be related to HLB, solubility parameter, cmc, and other parameters, as is found to be the case. Clearly, temperature can play an important role in determining surfactant effectiveness where hydration (or hydrogen bonding) is the principal mechanism of solubilization. Because of the temperature sensitivity of such materials, their activity as emulsifiers and stabilizers also becomes temperature sensitive. In particular, their ability to form and stabilize o/w and w/o emulsions may change dramatically over a very narrow temperature range. In fact, an emulsion may invert to produce the opposite emulsion type as a result of temperature changes. Such a process is termed phase inversion, and the temperature at which it occurs for a given system is its phase inversion temperature (PIT). 

Solubility of surfactants in different solvents and in water, and effect of temperature and electrolytes on the solubility of the surfactants in different solvents, determine the usability of the former for a given formulation. For example, a surfactant with great affinity for water would mean that the surfactant would form oil-in-water emulsion. 

Surfactant solubility determination Solubility limits of surfactants in brine solutions in the presence of alcohol cosurfactants were determined from visual observations of the surfactant solutions and by a spectroturbidimetric method similar to the one described by Frances et al. (2). The visual observation consisted of centrifuging surfactant solutions in a table-top centrifuge at room temperature and measuring the amount of the surfactant sediment at the bottom of the tube. Surfactant was not considered soluble in a given brine if any surfactant sediment was observed. 

No critical review of solubilities of water-soluble phosphines is available in the literature. Synthesis and purification of these compounds are rather tedious so in lack of an expressed practical need no laboratory took the burden of thorough solubility determinations. Another difficulty of the measurements is that tertiary phosphines carrying ionic substituentCs) often behave as surfactants that lead to extensive frothing and solubilization phenomena. As an example, TPPMS is able to solubilize large amounts (5-10% w/w) of unsulfonated PCCeHsla. As a result, solubility data found in the literature are sometimes erroneous (published values for TPPTS range from 200 to 1400 g/L) or represent only a lower limit (Table 1). 

The surface activity of the soluble cationic surfactants at the water-air and water-nonpolar oil interfaces is mainly determined by the structure of the hydrocarbon chains. The work of adsorption in the homologous series of alkylammonium salts obeys the Traube rule [24,46,63,76]. Shipunov studied adsorption in the homologous series of tetra-alkylammonium chlorides at the water-heptane interface. He established that the work of adsorption is proportional not to the number of methyl groups in the... 

Prajapati, H.N., D.M. Dahymple, A.T. Serajuddin. 2012. A comparative evaluation of mono-, di-and triglyceride of medium ehain fatty acids by lipid/surfactant/water phase diagram, solubility determination and dispersion testing for application in pharmaceutical dosage form development. Pharm Res 29(1) 285-305. 

Fig. 9 shows the ellipsometric isotherm A — Ao(triangles) of the cationic surfactant C12-DMP bromide. A pronounced non-monotonic behaviour is shown with an extremum at an intermidiate concentration far below the cmc. Also shown is the number density of amphiphiles adsorbed to the interface (circles) as determined by Surface second harmonic generation (SHG). At these bulk concentrations the measured number density equals the surface excess F. SHG reveals a monotoneous increase in the surface excess in qualitative agreement to a thermodynamic analysis within the Gibb s framework. The data also clearly prove that the ellipsometric quantity need not be proportional to the adsorbed amount for a soluble ionic surfactant. What causes the nonmonotonous behaviour and how can it be understood ... 

Figure 12 The interphase of a soluble cationic surfactant at the air-water interface at low (a) and high (b) bulk concentration. It consists of a charged topmost cationic monolayer, a diffuse layer of counterions and at higher concentrations a compact layer of directly adsorbed counterions. The charge density of the topmost monolayer reduced by the charge of the inner Stern layer determines the ion distribution within the diffuse layer. The prevailing ion distribution is given by solution of the nonlinear Poisson-Boltzmann equation. The excess of ions can be readily translated in a corresponding refractive index profile. The profile determines the reflectivity properties. Ellipsometric data modeled within this framework allow an estimation of the extent to which ions enter the compact layer.

By using a similar approach. Von Corswant and Thoren (23) have shown the influence of the solubilization of an active drug in lecithin-based microemulsions. By combining phase diagram determinations, NMR spectroscopic measurements and solubility determinations of the solute in aqueous and oil phases, they have determined the effect of the solute on the phase behaviour and microstructure of the microemulsion. Depending on the nature of the solute, the influence on the curvature varies. The first solute studied (felodip-ine) is water-insoluble and slightly soluble in the oil. The presence of this solute increases the polarity of the oil phase and turns the film towards the water, even if this solute has no affinity for the surfactant film. 

Arenas et al. [54] report the solubilization of polychlorocarbon solvents by alkanoylglycamines, a phenomenon that is to be expected of highly water-soluble nonionic surfactants. Miyagishi et al. [55] report determination of the CMC s of these surfactants using measurements of the fluorescence of benzophenone imine. Such determinations using, for example, pyrene have long been known the use of this particular fluorescent dye is unusual. 

Addition of cosolute can either increase or decrease the CMC, depending on the polarity of the molecules. Highly water soluble cosolutes tend to increase the CMC, since the solubility of surfactant molecules is enhanced. On the other hand, alcohols are less polar than water and are distributed between the aqueous phase and micelles. The water solubility of alcohols determines whether they are predominantly solubilized in micelles or in the aqueous phase. Medium chain length alcohols tend to be solubilized within micelles and thus increase p and lower the CMC for both ionic and nonionic surfactants. In contrast, short chain alcohols are water soluble and can either increase or decrease the CMC. 

It can be concluded that the Krafft point is the temperature at which the solubility of surfactants as monomers becomes high enough for the monomers to commence aggregation or micellization. Recall from Chapter 4 that the CMC depends on the method used for its determination, and that the CMC value should therefore be defined as a narrow temperature range even though the solubility is definitely determined by temperature... 

The chemical properties of the alkylarylsulfonates are used in its analytical determination. As anions, LAS and other anionic surfactants react with large cations to salts, which are soluble in organic solvents (e.g., CHC13). By analysis it can be seen that cations such as Hyamine 1622 (25) and methylene blue, which rearrange with LAS to complex (26), are widely spread. These reactions are the basis for the so-called two-phase titration, an extensively used method... 

Certain surface-active compounds [499], when dissolved in water under conditions of saturation, form self-associated aggregates [39,486-488] or micelles [39,485], which can interfere with the determination of the true aqueous solubility and the pKa of the compound. When the compounds are very sparingly soluble in water, additives can be used to enhance the rate of dissolution [494,495], One can consider DMSO used in this sense. However, the presence of these solvents can in some cases interfere with the determination of the true aqueous solubility. If measurements are done in the presence of simple surfactants [500], bile salts [501], complexing agents such as cyclodextrins [489 191,493], or ion-pair-forming counterions [492], extensive considerations need to be applied in attempting to extract the true aqueous solubility from the data. Such corrective measures are described below. 

BASIS OF MANUAL PHOTOMETRIC TITRATION. The determination of anionic surfactants by a photometric titration employs a cationic indicator to form a coloured complex with the surfactant which is insoluble in water but readily soluble in chlorinated solvents (1 ). The end point of the titration occurs when there is a loss of colour from the organic phase. A considerable improvement in this technique is achieved by the use of a mixture of anionic and cationic dyes (4 ), for example disulphine blue and dimidium bromide (Herring s indicator (3)). The sequence of colour changes which occurs during the two phase titration of an anionic surfactant (AS) with a cationic titrant (CT) using a mixed indicator consisting of an anionic indicator (AD) and cationic indicator (CD) is summarised in Scheme 1 ... 

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