Dissolution of surfactants

Aqueous micelles 3D 20-40 A diameter Dissolution of surfactant in water at concentrations above the critical micelle concentration (CMC) Dynamic equilibrium between monomers and micelles Particle formation was only possible at the micellar surface or close to it 55, 101... 

A class of solutions where the combined properties of water make it a rather unique solvent is micellar solutions. These solutions arise from the dissolution of surfactants in water at above a certain concentration (the critical micelle concentration, CMC). The micelles are structures that contain a few tens to a few hundreds of surfactant molecules, arranged so as to have the hydrophobic long... 

Until recently, only three chlorofluorocarbon (CFC) propellants, namely CFCs 11, 12 and 114 (Table 1), had been approved worldwide for use in medical MDIs. Their widespread acceptance was due to their ability to substantially meet the ideal propellant properties. All the CFC MDIs that are currently marketed employ CFC 12 as the major constituent mixed with either CFC 11 or with a mixture of CFC 11 and CFC 114. These mixtures of propellants closely obey Raoult s law and therefore the blend selected can be used to give a defined vapor pressure (Table 1). The inclusion of CFC 11 in the formulation also offered advantages in that it increased the solvency of most propellant systems, thereby facilitating the dissolution of surfactants in suspension formulations. By virtue of it being a liquid below 24° C, it was used as the primary dispersion medium for either suspending or dissolving the drug. 

Neither the dissolution of surfactants into CO2 nor the addition of pentane into CO2 in order to increase tri-n-butyltin fluoride (TBTF) concentration resulted in any sigfnificant increase in CO2 viscosity. 

This method (Figure 34.7) involves the dissolution of surfactant and cosurfactant mixtures in apolar solvent, resulting in the formation of reverse micelles. Thereafter, addition of water forms tubular... 

The simple dissolution of surfactant molecules in water makes the micellar media. These media are easy to prepare, but their physico-chemical properties should be known in order to be able to use them correctly in MLC. The two important points to remember are (i) surfactants adsorb at any interface, and (ii) surfactant associations are dynamic they form and break more or less rapidly. The first point partly explains the uncommon solute retention and selectivity obtained in MLC due to modifications of the stationary phase by surfactant adsorption. The second point may explain the slow mass transfer often observed in MLC and may be responsible for the reduced chromatographic efficiency. 

It should be noted that the adsorption experiment described above involves systems that only contain surfactant, cosurfactant, brine and reservoir rock. There is no oil present in the system, and this is the reason why the ratio of cosurfactant to surfactant is higher than usually reported by other researchers. It was found that the higher ratio was necessary for complete dissolution of surfactants in the brine since a lower alcohol/surfactant ratio caused the solutions to become cloudy. This latter condition resulted in substantial increases in surfactant retention in flow experiments. The observation of the relationship between the solution condition and retention lead to experiments which could better define the phenomenon. 

In the previous chapters, the dissolution and micellization of surfactants in aqueous solutions were discussed from the standpoint of the degrees of freedom as given by the phase rule. The mass-action model for micelle formation was found to be better for explaining the phenomena of surfactant solutions than the phase-separation model. Two models have similarly been used to explain the Krafft point, one postulating a phase transition at the Krafft point and the other a solubility increase up to the CMC at the Krafft point. The most recent version of the first approach is a melting-point model for a hydrated surfactant solid. The most direct approach to the second model of the Krafft point rests entirely on measurements of the solubility and CMC of surfactants with temperature. From these mesurements the concept of the Krafft point can be made clear. This chapter first reviews the concepts used to relate the dissolution of surfactants to their micellization, and then shows that the concept of a micelle temperature range (MTR) can be used to elucidate various phenomena concerning dissolution... 

Our assumption that the surfactant used is insoluble during the whole duration of the experiment may not be fuUy correct. Unfortunately, we have been unable to estimate the desorption time, T. If that time is reached during our experiments, dissolution of surfactant in the bulk may be significant at the higher front edge. 

Nanoparticles of the semicondnctor titanium dioxide have also been spread as mono-layers [164]. Nanoparticles of TiOi were formed by the arrested hydrolysis of titanium iso-propoxide. A very small amount of water was mixed with a chloroform/isopropanol solution of titanium isopropoxide with the surfactant hexadecyltrimethylammonium bromide (CTAB) and a catalyst. The particles produced were 1.8-2.2 nm in diameter. The stabilized particles were spread as monolayers. Successive cycles of II-A isotherms exhibited smaller areas for the initial pressnre rise, attributed to dissolution of excess surfactant into the subphase. And BAM observation showed the solid state of the films at 50 mN m was featureless and bright collapse then appeared as a series of stripes across the image. The area per particle determined from the isotherms decreased when sols were subjected to a heat treatment prior to spreading. This effect was believed to arise from a modification to the particle surface that made surfactant adsorption less favorable. 

The structure of these globular aggregates is characterized by a micellar core formed by the hydrophilic heads of the surfactant molecules and a surrounding hydrophobic layer constituted by their opportunely arranged alkyl chains whereas their dynamics are characterized by conformational motions of heads and alkyl chains, frequent exchange of surfactant monomers between bulk solvent and micelle, and structural collapse of the aggregate leading to its dissolution, and vice versa [2-7]. 

Ong et al. [134] found that several hydrophilic anionic, non ionic, or cationic surfactants can alleviate the deleterious effect of magnesium stearate over-mixing on dissolution from capsules when added with the lubricant in a ratio as low as 1 5 (w/w). These successful surfactants were sodium A-lauroyl sarcosinate, sodium stearoyl-2-lactylate, sodium stearate, polox-amer 188, cetylpyridinium chloride, and sodium lauryl sulfate. The lipophilic surfactant glyceryl monostearate did not alleviate the magnesium stearate mixing effect. A reduction in thier particle size was shown to enhance effectiveness, particularly in the case of surfactants with low solubility and slow dissolution rate. 

The effectiveness of surfactants in overcoming the hydrophobic effect of magnesium stearate may not be a result solely of an increase in the wetting properties of the bulk phase. Compared to putting the surfactant in the dosage form, Botzolakis [71] and Wang and Chowhan [135] found that adding an equivalent amount of surfactant to the dissolution medium was not effective. The possible impact of the surfactant at... 

Dissolution test data will be required in all cases (and for all strengths of product) for development and routine control and should be based on the most suitable discriminatory conditions. The method should discriminate between acceptable and unacceptable batches based on in vivo performance. Wherever possible Ph Eur test methods should be used (or alternatives justified). Test media and other conditions (e.g., flow through rate or rate of rotation) should be stated and justified. Aqueous media should be used where possible and sink conditions should be maintained. A small amount of surfactant may be added where necessary to control surface tension or for active ingredients of very low solubility. Buffer solutions should be used to span the physiologically relevant range—the current advice is over pH 1 6.8 or perhaps up to pH 8 if necessary. Ionic strength of media should be reported. The test procedure should employ six dosage forms (individually) with the mean data and a measure of variability reported. 

V. DISSOLUTION OF POORLY SOLUBLE COMPOUNDS IN SURFACTANT SOLUTIONS... 

A method used to describe the enhanced dissolution rate following micelle-facilitated dissolution is to compare the dissolution of the drug in the surfactant solution to that of the dissolution rate in water this is often termed the reaction factor method. The reaction factor, ( vM, which is the total flux of the micelle-solubilized solute plus the free solute divided by the flux of the free solute, is given by... 

VP Shah, JJ Konecny, RL Everett, B McCullough, AC Noorizadeh, JP Skelly. In vitro dissolution profile of water-insoluble drug dosage forms in the presence of surfactants. Pharm Res 6 612-618, 1989. 

JR Crison, VP Shah, JP Skelly, GL Amidon. Drug dissolution into micellar solutions Development of a convective diffusion model and comparison to the film equilibrium model with application to surfactant-facilitated dissolution of carbama-zepine. J Pharm Sci 85 1005-1011, 1996. 

JR Crison, ND Weiner, GL Amidon. Dissolution media for in vitro testing of water-insoluble drugs Effect of surfactant purity on in vitro dissolution of carbama-zepine in aqueous solutions of sodium lauryl sulfate. J Pharm Sci 86 384-388, 1997. 

It has been reported by Celik and Somasundaran T381 that the interaction of divalent (and trivalent) cations with sulfonate surfactants causes surfactant precipitation followed by dissolution of the precipitate at higher concentrations. The precipitate redissolution phenomenon is not observed with monovalent ions. Indeed, some surfactant precipitation in the spinning drop tube was observed above concentrations corresponding to the first minimum of Figure 8 it is not known whether redissolution took place at higher concentrations resulting in the second tension minimum. 

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