Surfactants Concentrated suspensions Dilute

The experiments in the first series were conducted with methylated surfaces immersed in aqueous solutions of alkylbenzenesulfonates. The free energy of interaction between methylated glass and quartz spherical surfaces having radii of about 1 mm was studied as described in Section 1.2. Parallel to these studies, the colloid stability of 10 mn particles of methylated Aerosil (i.e., hydrophobized nanoparticles of quartz) suspensions was monitored via turbidity measurements. A characteristic sharp increase in turbidity was observed at the coagulation threshold. This behavior was reversible an increase in the surfactant concentration resulted in a decrease in turbidity, while dilution of the solutions caused the turbidity to increase. 

When the Structured Surfactant Formulation is diluted in the quay tank, a stable, low viscosity suq>ension is formed. The herulites dissolve at the reduced spray tank sur ctant concentration. The dilute suspension can then be applied as any stable SC product... 

In the present study, hydrophobic interaction between hydroxypropylcellulose (HPC) and an ionic surfactant in an aqueous phase was discussed. HPC, as well as EHEC, is a nonionic cellulose ether which contains hydrophobic groups in its molecular structure. Therefore, it might be interesting to compare the complex-formation properties of HPC with that of EHEC. The surfactants used here were an anionic surfactant SDS and a cationic one cetylpyridinium chloride (CPC). HPC formed a complex with these surfactants, of which cloud point changed with the surfactant concentration in the same manner as that observed in the EHEC-surfactant systems [4]. Effects of the complex on stability of dilute and concentrated kaoiinite suspensions were also studied, taking physicochemical properties of the complex into account. 

Tubular Precipitator. This type of continuous operation may be employed to reduce polydispersity of precipitates (Raphael et al. 1997 Raphael and Rohani 1999). The tubular precipitator may operate either under the turbulent flow or laminar flow regime. The reactants are added into the inlet section equipped with static mixers and may also enter as a multi-port feed along the length of the tubular precipitator. If the reactant feeding streams are too concentrated or if too excessive formation of precipitate occurs in the inlet section of the precipitator, a third stream of solvent is also fed to dilute the flowing suspension. The latter may contain a protective colloid or surfactant that prevent agglomeration of precipitate. 

To fully assess the properties of suspension concentrates, three main types of measurements are required. Firstly some information is needed on the structure of the solid/solution interface at a molecular level. This requires investigation of the double layer properties (for systems stabilised by ionic surfactants and polyelectrolytes), adsorption of the surfactant or polymer as well as the extension of the layer from the interface (adsorbed layer thickness). Secondly, one needs to obtain information on the state of dispersion on standing, such as its flocculation and crystal growth. This requires measurement of the particle size distribution as a function of time and microscopic investigation of flocculation. The spontaneity of dispersion on dilution, i.e. reversibility of flocculation needs also to be assessed. Finally, information on the bulk properties of the suspension on standing is required, which can be obtained using rheological measurements. The methods that may be applied for suspension concentrates are described briefly below. 

Seven surfactants were selected with differing HLB (Hydrophile Lipophile Balance) numbers and used at concentrations of 0.5 - 2.5% w/v in aqueous solution. An intermediate concentrate was prepared by high shear mixing and subsequently diluted with 1% methylcellulose solution to give a final drug concentration of 10% w/v. Methods of analysis included laser diffraction particle size determination, optical microscopy and dissolution into an aqueous medium. The suspensions were analysed immediately after preparation and after 4 and 96 hours. 

The Carboxyfluorescein concentration of the vesicle suspension Cv and after destroying the vesicles with the surfactant Triton X 100 C/ were calculated from the fluorescence intensity of the diluted aqueous solutions. The turbidity of the vesicle solution declined within seconds after detergent addition (vesicle busting), and we obtained then a clear, aqueous solution. Typical results of these measurements are summarized in Fig. 12. Due to Ihe self-quenching properties the destruction of the vesicles with a high inner Carboxyfluorescein concentration (0.05-0.2 mol/1) led to an increase of the Fluorophore concentration in the outer phase. This occurred if a large amount of emulsion droplets coalesced with the lower water phase, thus releasing their Carboxyfluorescein content. 

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