Aqueous Phase Analysis

In common practice the aqueous phase, or serum, of a polymer dispersion is only investigated for its pH (Sect 3.2.1). On the other hand, the aqueous phase contains a host of substances which play an important role in many applications. These substances include (a) emulsifiers, (b) initiator residues, (c) electrolytes from the neutralization process or from initiator decomposition (for example sodium sulfate from sodium peroxodisulfate), (d) unreacted water-soluble monomers such as acryHc add or vinyl sulfonic acid, and (e) water-soluble oligomers formed from this kind of monomers. 

To analyze the aqueous phase for any of these substances, it must first be separated from the polymer particles. Both flocculation and membrane filtration techniques can be used for this purpose and they are described in more detail below. The detection of the substances listed above can then be performed with the usual array of analytical methods used for characterizing aqueous media. For the determination of emulsifiers, electrolytes and water-soluble monomers, ion chromatography (IC) and high-performance liquid chromatography (HPLC) are particularly suitable. The techniques of choice for characterizing oligomers are gel permeation chromatography (GPC) and capillary electrophoresis (CE). As these analytical techniques are not specific to colloidal chemistry, they will not be described further here and the reader should consult the literature for more information. 

The dispersion is for instance flocculated by the addition of acids or salts (typically containing polyvalent ions). Examples of salts of this type are aluminum sulfate or 

In this case, the polymer particles are separated from the aqueous phase by a membrane through which the particles cannot permeate. Suitable membranes include dialysis tubes (molecular weight cut-off 10000-15 000 g mol ) or, for example, Nucleopore membranes, which are available with pore diameters from 15 nm to several micrometers. 

In dialysis the dispersion is placed in a well-sealed tube and immersed for several days in water, which should be changed regularly. Before being analyzed, the dialysate usually has to be concentrated. Changing the water and concentrating the dialysate can both be carried out easily if the dialysis tube is placed inside a Soxhlet apparatus. 

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To achieve the latter, spectroscopists have exploited the fact that when in a coiled conformation in aqueous solution, certain polyelectrolytes can solubilize [6,12,17-21] low molar organic molecules. On changing the pH, the chain expands releasing this material into the aqueous phase. Analysis of the resultant emission from the probe can reveal information concerning its environment and thence, the conformation of the polymer [6,12,17-21],... 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

Recently, many experiments have been performed on the structure and dynamics of liquids in porous glasses [175-190]. These studies are difficult to interpret because of the inhomogeneity of the sample. Simulations of water in a cylindrical cavity inside a block of hydrophilic Vycor glass have recently been performed [24,191,192] to facilitate the analysis of experimental results. Water molecules interact with Vycor atoms, using an empirical potential model which consists of (12-6) Lennard-Jones and Coulomb interactions. All atoms in the Vycor block are immobile. For details see Ref. 191. We have simulated samples at room temperature, which are filled with water to between 19 and 96 percent of the maximum possible amount. Because of the hydrophilicity of the glass, water molecules cover the surface already in nearly empty pores no molecules are found in the pore center in this case, although the density distribution is rather wide. When the amount of water increases, the center of the pore fills. Only in the case of 96 percent filling, a continuous aqueous phase without a cavity in the center of the pore is observed. 

A solution of 1.0 mmol of 2-acetyl alkenoate in 2.5 mL of CH2C1, is added slowly to a solution of 4.0 mmol of titanium(IV) chloride in 7.5 mL of CH-CL under an atmosphere of nitrogen at — 78 °C. The mixture instantaneously turns deep red. and is stirred at — 78 °C before being quenched by the addition of 5 mL of sat. aq potassium carbonate. The mixture is then partitioned between 10 mL of bt20 and 10 mL of water. The aqueous phase is extracted with three 10-mL portions of Et2(), and the extracts are combined, washed with 10 mL of brine, and dried over anhyd potassium carbonate. Concentration under reduced pressure gives the crude product. Product analysis is by capillary GC. 

The low TTA dependence at 35.0°C probably is attributable to dissolution of TTA in the aqueous phase. Observation of fourth-power dependence on acidity argues against any change in the extraction mechanism (e.g., Pu(IV) reduction or NO3 involvement). An aqueous Pu(TTA)3+ complex has been reported (14, 15) and this possibility has been considered in the error analysis of the Pu(IV)-sulfate stability constants. 

Analysis of sulfonic acid species in sulfonated olefins. Kupfer and Kuenzler [108] reported the determination of acid species following partition between a 6.5% hydrochloric acid solution in 40% ethanol and a 1 1 (v/v) propan-2-ol-hexane mixture. The organic fraction contains alkenesulfonic and hydroxy-alkanesulfonic acids and the aqueous phase disulfonic acids and sulfato-sulfonates. The monosulfonic acids were converted to methyl esters and separated by column chromatography. To determine sulfatosulfonates the aqueous fraction was hydrolyzed and then partitioned and chromatographed. The separation is controlled using IR spectroscopy. 

After extraction, each phase may be studied independently in order to obtain a useful qualitative evaluation of the components in the original sample. The selectivity and specificity of fluorescence analysis can be especially beneficial in identification of PAHs. For example, some components could be identified by examining the fluorescence spectra of the organic and aqueous phases. Characteristic peak shapes may reveal identities of the components. For more complicated systems in which the spectra overlap, lifetime measurements may be used to identify components (27). 

Cyclodextrin-modified solvent extraction has been used to extract several PAHs from ether to an aqueous phase. Data evaluation shows that the degree of extraction is related to the size of the potential guest molecule and that the method successfully separates simple binary mixtures in which one component does not complex strongly with CDx. The most useful application of cyclodextrin-modified solvent extraction is for the simplification of complex mixtures. The combined use of CDx modifier and data-analysis techniques may simplify the qualitative analysis of PAH mixtures. 

Twelve, 36-inch soil cores of the Lakeland sand were selected for chemical analyses in December 1970. Twenty-five gram samples of 6-inch increments were acidified and extracted with 1 1 hexane acetone. Each sample was extracted with IN KOH, and the aqueous phase was saved for 2,4-D and 2,4,5-T analysis. The hexane phase was extracted repeatedly with concentrated H2SO4 until the acid was clear. The H0SO4 was removed, and the extract was drained through NaHCOs and anhydrous... 

Similarly to experiments under potentiostatic conditions, success in the analysis of ET kinetics relies on the fact that neither products nor reactants can transfer across the interface. Various redox couples in the aqueous phase have been studied, including Fe(CN) /, Ru(CN) / Mo(CN) /, FeEDTA / IrClg and Co(III)/(II)... 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

The previous analysis indicates that although the voltammetiic behavior suggests that the aqueous phase behaves as a metal electrode dipped into the organic phase, the interfacial concentration of the aqueous redox couple does exhibit a dependence on the Galvani potential difference. In this sense, it is not necessary to invoke potential perturbations due to interfacial ion pairing in order to account for deviations from the Butler Volmer behavior [63]. This phenomenon has also been discarded in recent studies of the same system based on SECM [46]. In this work, the authors observed a potential independent ket for the reaction sequence. 

Within the potential range where Ru(bpy)3 remains in the aqueous phase, photocurrent responses are clearly observed with a slow rising time of the order of 10 s as shown in Fig. 14(a). According to the convention employed by these authors, positive currents correspond to the transfer of a negative charge from water to DCE. No photoresponses were observed in the absence of either the dye in the aqueous phase or TCNQ in DCE. Further analysis of the interfacial behavior of the product TCNQ revealed that the ion transfer occurred outside of the polarizable window [cf. Fig. 14(d)], confirming that these photoresponses are not affected by coupled ion-transfer processes. An earlier report also showed photoeffects for the photoreduction of the viologen under similar conditions [131]. 

The concentration of the transferred ion in organic solution inside the pore can become much higher than its concentration in the bulk aqueous phase [15]. (This is likely to happen if r <5c d.) In this case, the transferred ion may react with an oppositely charged ion from the supporting electrolyte to form a precipitate that can plug the microhole. This may be one of the reasons why steady-state measurements at the microhole-supported ITIES are typically not very accurate and reproducible [16]. Another problem with microhole voltammetry is that the exact location of the interface within the hole is unknown. The uncertainty of and 4, values affects the reliability of the evaluation of the formal transfer potential from Eq. (5). The latter value is essential for the quantitative analysis of IT kinetics [17]. Because of the above problems no quantitative kinetic measurements employing microhole ITIES have been reported to date and the theory for kinetically controlled CT reactions has yet to be developed. 

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