Solubilization rate measurement

Flocculation values achieved from turbidity measurements using the light scattering technique showed improvement with nonylphenol ether carboxylic acid (4 mol EO) in particular. The oil solubilization rate has been found to be proportional to the surfactant micellar size [190]. 

Although the proposed theory has been used effectively in several practical applications, no experimental proof has been given that the oil solubilization rate is a function of surfactant aggregate size. In view of the importance of solubilization and the existence of practical methods of measuring and controlling surfactant aggregate size, we decided to correlate the solubilization rate with micellar properties for some anionic and nonionic surfactants. 

Light scattering technique was used in determining the oil solubilization rate. Debye s equation ( ) was used in the interpretation. The basic principle involves the measurement of the surfactant aggregate size during the solubilization. As the oil goes into the surfactant micelle, the increased size will be reflected by the turbidity of the solution. 

This paper presents a very basic principle in surfactant solubilization. More quantitative measurement in correlating solubilization rate with micellar properties and more applications of this principle to improve performance of various solubilization processes remain the subject of our investigation. 

In the next section a simple model is described that provides some understanding of the essentials of solubilization. Then some measurement techniques, experimental results, and theories of equilibrium solubilization are presented for micelles and microemulsions. Finally, dynamic phenomena such as solubilization rates and solubilization by intermediate phases formed after the solute contacts a surfactant solution are discussed. 

FIGURE 9.5 Schematic diagram of drop-on-fiber technique for measuring solubilization rates. Fiber radius and drop radius Z2, which varies with time, are needed to calculate the rate of solubilization. 

FIGURE 9.6 Schematic illustration of contacting experiment in which a small oil drop is injected into an aqueous surfactant solution to measure solubilization rate. 

For solutions of typical ionic surfactants with no added salts the studies of Carroll and Ward showed that solubihzation rates were much smaller than those for nonionic surfactants, presumably because the surfactant ions adsorbed at the oil-water interface repelled the micelles of like charge in the solution. Indeed, Bolsman et al. found no measurable solubilization of n-hexadecane into solutions of a pure benzene sulfonate and a commercial xylene sulfonate. They injected small oil drops into the surfactant solutions and observed whether the resulting turbidity disappeared over time due to solubilization. Similarly, Kabalnov found from Ostwald ripening experiments that the rate of solubilization of undecane into solutions of pure SDS was independent of surfactant concentration and about the same as the rate in the absence of surfactant. That is, the hydrocarbon presumably left the bulk oil phase in this system by dissolving in virtually miceUe-free water near the interface. In similar experiments TayloC and Soma and Papadopoulos observed a small increase in the solubilization rate of decane with increasing SDS concentration. De Smet et al., who used sodium dodecyl benzene sulfonate, which does not hydrolyze, found, like Kabalnov, a minimal effect of surfactant concentration. 

More recently, Pena and Miller investigated solubilzation rates of mixtures of n-decane and squalane into 2.5 wt% solutions of pure C,2Eg at 23°C using the oil drop method described above. They first measured the rate of solubilization of pure decane, confirming that the rate was controlled by interfacial phenomena as in Carroll s work, and demonstrated that pure squalane was not solubilized to any significant extent under these conditions. Next they measured solubilization rates of decane from various mixtures of the two hydrocarbons. Figure 9.7 shows results from one of these experiments together with predictions of a model based on assuming that the rate of decane... 

Selective solubilization can also occur in mixtures of polar and nonpolar oils. Using their oil drop method described previously, Chen et al. measured solubilization rates of mixtures of triolein and oleic acid in solutions of pure nonionic surfactants. As Figure 9.9 shows for a drop initially containing 85/15 triolein/oleic acid by weight injected into 2 wt% Tergitol 15-S-7 at 35°C, they observed that the solubilization process consisted of two stages. In the first stage, the drop radius... 

Wen and Papadopoulos used videomicroscopy to measure the rate of shrinkage of a single drop of pure water immersed in a surfactant-containing oil phase, which itself was in contact with an aqueous solution of NaCl. They found that the solubilization rate of water was controlled by phenomena at the drop interface, suggesting that transport in multiple emulsions is limited by interfacial effects, not diffusion. 

Hence, the objectives of this paper are to develop and explore the use of QCM technique to measure the solubilization rates in L/SC CO2, and to measure the effects of ultrasonic impulses on reactions in L/SC CO2 using the (JCM technique. 

The recommendations to measure the inter micellar dynamics are (1) to use a fast reaction with reactant concentration such that only single-ion occupation appears and (2) to choose the droplet concentration of the microemulsion low enough to be in a diffusion-controlled state. Therefore, experiments were carried out in low surfactant concentration samples at low R values with single-ion occupation for the droplets. The reactants were solubilized in freshly prepared microemulsions of the same composition. For all the rate measurements the pseudo-first-order conditions were applied so that the metal ion concentration exceeded the murexide concentration by a factor of 10. 

The solubilization rates of C-tagged linoleic acid can be measured in 1 % sodium taurodeoxycholate using a spinning liquid disc, for which... 

ATPase also catalyzed a passive Rb -Rb exchange, the rate of which was comparable to the rate of active Rb efflux. This suggested that the K-transporting step of H,K-ATPase is not severely limited by a K -occluded enzyme form, as was observed for Na,K-ATPase. Skrabanja et al. [164] also described the reconstitution of choleate solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. With the use of a pH electrode to measure the rate of H transport they observed not only an active transport, which is dependent on intravesicular K, but also a passive H exchange. This passive transport process, which exhibited a maximal rate of 5% of the active transport process, could be inhibited by vanadate and the specific inhibitor omeprazole, giving evidence that it is a function of gastric H,K-ATPase. The same authors demonstrated, by separation of non-incorporated H,K-ATPase from reconstituted H,K-ATPase on a sucrose gradient, that H,K-ATPase transports two protons and two ions per hydrolyzed ATP [112]. 

One approach to the study of solubility is to evaluate the time dependence of the solubilization process, such as is conducted in the dissolution testing of dosage forms [70], In this work, the amount of drug substance that becomes dissolved per unit time under standard conditions is followed. Within the accepted model for pharmaceutical dissolution, the rate-limiting step is the transport of solute away from the interfacial layer at the dissolving solid into the bulk solution. To measure the intrinsic dissolution rate of a drug, the compound is normally compressed into a special die to a condition of zero porosity. The system is immersed into the solvent reservoir, and the concentration monitored as a function of time. Use of this procedure yields a dissolution rate parameter that is intrinsic to the compound under study and that is considered an important parameter in the preformulation process. A critical evaluation of the intrinsic dissolution methodology and interpretation is available [71]. 

Assay of Enzymatic Hydrolysis of Synthetic Solid Polymers. Hydrolysis of solid polymers was measured by the rate of their solubilization, and the measurement process does not necessarily involve complete hydrolysis into the constituent parts. The rate was determined by measuring the water-soluble total organic carbon (TOC) concentration at 30 °C in the reaction mixture using a Beckman TOC analyzer (Model 915-B). In the substrate and enzyme controls, enzyme or substrate was omitted from the reaction mixture. 

How can chemists measure the importance of solvents Solvents solubilize solutes. The general rule for organic or nonelectrolyte inorganic solutes, whether gaseous, liquid, or solid, is that they would show adequate solubilities. Expressions for the estimation of solubilities are presented in chapter 3. The effects on the rate of reactions will be discussed there, and comparisons made among classes of solvents. 

---