W/C emulsion

Figure 8.6. Emulsion stability contours (log of emulsion stability in seconds) for 50/50 (mass) C02-water (0.01 M NaCl) systems with 10.7 mM PFPE COO NH4+ (g/mol = 2500) surfactant. w/c emulsions O (c/w) emulsions. Dotted line indicates the phase boundary of the surfactant in C02 cross-hatched region indicates highly flocculated emulsions (Lee et al., 1999b).

Griffin, W.C. Emulsions in Kirk-Othmer Encyclopedia of Chemical Technology, 2nd ed., Vol. 

Figure 4 Stability contours for W/C emulsions stabilized by PDMS-6-PAA (20-0.7 K) in terms of the time for 20% settling. At low pH the surfactant prefers CO2 and limited adsorption at the interface leads to unstable emulsions. As the pH Increases, the PAA groups begin to ionize and adsorption increases, leading to more stable emulsions until the balanced state Is reached where rapid coalescence leads to unstable emulsions. (From Ref. 93.)...

Emulsions of water and CO2 have been used to form liposomes in one step without any organic solvent (57). The liposomes were made by forming a W/C emulsion stabilized with/L-i -dipalmitoylphosphatidylcholine. The pressure was reduced to form the liposomes by a reversed phase evaporation method. Large unilamellar liposomes with diameters of 0.1 to 1.2 pm were formed and used to trap D-( + )-glucose. 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

The stability of w/c emulsions, defined as the time required for the volume of the emulsion to settle from 100% to 90% based upon visual observation, has been measured for PFPE-COO"NH4" surfactants with molecular weights ranging from 667 to 7500 [17]. Figure 2.4-10 shows the stability of emulsions formed by the above microfluidizer for equal weights of water and CO2 and 1.3 wt% of 2500g/mol PFPE-COO NH4. For each experiment where non-flocculated emulsions were present during shear, the specific conductivity was less than O.lpS/cm, indicating water droplets in a CO2 continuous... 

Figure 2.4-12 Hydrolysis of benzoyl chloride to benzoic acid in a w/c emulsion formed with 7 mM Mn(PFPE)2 surfactant and equal amounts of water and CO2 at 25 °C and 270 bar. The line indicates the conversion without surfactant.

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

We chose to use a relatively low molecular wei PFPE ammonium carboxylate surfoctant (M - 567 g mol. Figure 6), since Johnston has demonstrated foat surfoctants of fois type exhibit significant solubility in water and have a propensity to form C/W rather than W/C emulsions (34). 

Following the same strategy, the formation and stability of W/C emulsions was assessed in the presence of a PEO-fc-PFOA copolymer (M p o = 2000 g/mol and M pfoa 31,000 g/mol) [40], Note that the hydrophilic/C02-philic balance (HCB) was lower, and the Af of the copolymer was higher compared to the previous study. The observation of a W/C emulsion was reported from 25°C and 143 bar while flocculation or coalescence of dispersed water phase slowly occurred upon stopping of the stirrer. Notably, the formation of a W/C emulsion was reported at the cloud point pressure of the copolymer in a binary polymer/C02 mixture. 

---