Addition Of Hydrotropes Research Articles

Effect of Hydrotropic Compounds on the Self-Organization and Solubilization Properties of Cationic Surfactants

The effect hydrotropic additives (salts of aromatic acids and choline chloride) have on the micelle-forming properties (the critical concentrations of micelle formation and the Krafft temperature) of cationic surfactants, and on the solubilization capability of mono- and dicationic surfactants toward such hydrophobic compounds as a Sudan I spectral probe and curcumin natural dye, is considered. The factors that govern solubilization capacity, e.g., the structure of the head group of surfactants, the nature of the solubilizate and hydrotropic additives, and the pH of the medium are determined.

Action of hydrotropes in water-in-CO2 microemulsions

The effects of a homologous series of sodium p-n-alkylbenzoate hydrotropes in water-in-supercritical CO2 (w/c) microemulsions have been investigated, by comparing the phase behaviour and droplet structures obtained using small-angle neutron scattering (SANS). The w/c microemulsions appeared to be generally stable upon addition of hydrotropes, however, on increasing the alkyl chain length of the hydrocarbon and fluorocarbon moieties of the surfactants, different effects on stability were observed. Using high-pressure SANS (HP-SANS), the effects of hydrotrope type on the structures of microemulsion droplets were studied. Interestingly, evidence was found for multiple shell structures with a hydrotrope rich layer between the water cores and the surfactant films. These findings are relevant to the understanding of self-assembly of co-adsorbed species in supercritical CO2 (scCO2), as the hydrotrope layers potentially have significant effects on surfactant packing, and can modify the physico-chemical properties of scCO2 through formation of worm-like micellar assemblies.

Unusual pH-responsive fluid based on a simple tertiary amine surfactant: the formation of vesicles and wormlike micelles

A novel pH-responsive viscoelastic micellar system was prepared by N,N-dimethyl oleoaminde-propylamine without the addition of hydrotropes. The micellar system undergoes a gradual transition from vesicles to spherical micelles to wormlike micelles by adding an HCl solution. Rheology, Cryo-TEM and dynamic light scattering (DLS) results revealed that the pH-responsive flow behavior is attributed to the microstructural transition among the spherical micelles, vesicles and worm-like micelles. In this paper, we found out that a vesicle is extremely pH sensitive because the degree of protonation and the concentration of Cl− strongly affect the pH areas which could form worm-like micelles. The notable advantages of vesicles and worm-like micelles can be utilized to prepare pH-responsive viscoelastic fluids in the desired pH areas.

CO2-responsive anionic wormlike micelles based on natural erucic acid

Research interests highlighted in recent years a remarkable growing focus on stimuli-responsive surfactant wormlike micelles, particularly those using carbon dioxide as the trigger. In this article, we report a novel carbon dioxide–responsive wormlike micellar system based on natural erucic fatty acid and examined its solution properties through rheology, Fourier transform infrared spectroscopy and cryo–tranmission electron microscopy. When carbon dioxide is introduced into the erucic acid solution, high-viscelastic wormlike micelles shift to low-viscosity solutions in short-time scale, accompanied by decrease in pH from 11·34 to 8·90, which is accounted for the effect of pH on the existence form of erucic acid in aqueous solutions. Bubbling nitrogen under heating cannot make a converse process since the viscoelastic worms can only be obtained at a high pH. Compared with other carbon dioxide–responsive wormlike micelles, the current one is more readily obtained and more environment-friendly because it can be formed by simply changing pH value of the natural erucic acid solutions without needing complex organic synthesis or addition of hydrotropes.

Effects of small ionic amphiphilic additives on reverse microemulsion morphology

HypothesisInitial studies (Hopkins Hatzopoulos et al. (2013)) have shown that ionic hydrotropic additives can drive a sphere-to-cylinder (ellipsoid) transition in water-in-oil (w/o) microemulsions stabilized by the anionic surfactant Aerosol-OT; however the origins of this behaviour remained unclear. Here systematic effects of chemical structure are explored with a new set of hydrotropes, in terms of an aromatic versus a saturated cyclic hydrophobic group, and linear chain length of alkyl carboxylates. It is proposed that hydrotrope-induced microemulsion sphere-to-cylinder (ellipsoid) transitions are linked to additive hydrophobicity, and so a correlation between the bulk aqueous phase critical aggregation concentration (cac) and perturbation of microemulsion structure is expected. ExperimentsWater-in-oil microemulsions were formulated as a function of water content w (= [water]/[AOT]) and concentration of different hydrotropes, being either cyclic (sodium benzoate or sodium cyclohexanoate), or linear chain systems (sodium hexanoate, sodium heptanoate and sodium octanoate). Phase behaviour studies were performed as a function of w, additive type and temperature at total surfactant concentration [ST]=0.10M and constant mole fraction x=0.10 (x=[hydrotrope]/[ST]). Microemulsion domain structures were investigated by small-angle neutron scattering (SANS), and these data were fitted by structural models to yield information on the shapes (spheres, ellipsoids or cylinders) and sizes of the nanodroplets. FindingsUnder the conditions of study hydrotrope chemical structure has a significant effect on microemulsion structure: sodium cyclohexanoate does not induce the formation of cylindrical/ellipsoidal nanodroplets, whereas the aromatic analogue sodium benzoate does. Furthermore, the short chain sodium hexanoate does not cause anisotropic microemulsions, but the more hydrophobic longer chain heptanoate and octanoate analogues do induce sphere-to-ellipsoid transitions. This study shows that underlying microemulsion structures can be tuned by hydrotropes, and that the strength of the effect can be identified with hydrotrope hydrophobicity in terms of the bulk aqueous phase cac.

Hydrotropy: binding models vs. statistical thermodynamics

Hydrophobic drugs can often be solubilized by the addition of hydrotropes. We have previously shown that preferential drug-hydrotrope association is one of the major factors of increased solubility (but not "hydrotrope clustering" or changes in "water structure"). How, then, can we understand this drug-hydrotrope interaction at a molecular level? Thermodynamic models based upon stoichiometric solute-water and solute-hydrotrope binding have long been used to understand solubilization microscopically. Such binding models have shown that the solvation numbers or coordination numbers of the water and hydrotrope molecules around the drug solute is the key quantity for solute-water and solute-hydrotrope interaction. However, we show that a rigorous statistical thermodynamic theory (the fluctuation solution theory originated by Kirkwood and Buff) requires the total reconsideration of such a paradigm. Here we show that (i) the excess solvation number (the net increase or decrease, relative to the bulk, of the solvent molecules around the solute), not the coordination number, is the key quantity for describing the solute-hydrotrope interaction; (ii) solute-hydrotrope binding is beyond the reach of the stoichiometric models because long-range solvation structure plays an important role.

Mechanism of Hydrophobic Drug Solubilization by Small Molecule Hydrotropes

Drugs that are poorly soluble in water can be solubilized by the addition of hydrotropes. Albeit known for almost a century, how they work at a molecular basis is still controversial due to the lack of a rigorous theoretical basis. To clear up this situation, a combination of experimental data and Fluctuation Theory of Solutions (FTS) has been employed; information on the interactions between all the molecular species present in the solution has been evaluated directly. FTS has identified two major factors of hydrotrope-induced solubilization: preferential hydrotrope-solute interaction and water activity depression. The former is dominated by hydrotrope-solute association, and the latter is enhanced by ionic dissociation and hindered by the self-aggregation of the hydrotropes. Moreover, in stark contrast to previous hypotheses, neither the change of solute hydration nor the water structure accounts for hydrotropy. Indeed, the rigorous FTS poses serious doubts over the other common hypothesis: self-aggregation of the hydrotrope hinders, rather than promotes, solubilization.

Thermally induced structural transitions from fluids to hydrogels with pH-switchable anionic wormlike micelles

A novel pH-switchable anionic wormlike micellar system has been developed by simply changing pH value of natural erucic acid solutions without needing specialized organic synthesis or addition of hydrotropes. Through rheology, optical transmittance, and cryo-TEM observation, it was found that at 60°C, the 100mM erucic acid solutions transform from low viscous emulsion-like fluids to viscoelastic hydrogels when the pH is increased from 8.03 to 12.35, and when the pH is cycled between 9.02 and 12.35, the viscosity of the solutions varies correspondingly between 2 and 200,000mPas, five orders of magnitude in viscosity range. Such a reversible switch can be easily cycled more than four times without any deterioration. In addition, when the pH is fixed between 9.0 and 12.35, the solutions shift to highly elastic solid-like gels with decreasing temperature. These pH-switchable and temperature-sensitive rheological properties are attributed to the presence of carboxylic groups in the erucic acid molecules. With increasing pH, erucic acid is converted into an anionic surfactant, sodium erucate, which self-assembles into aggregates evolving from spherical micelles to wormlike micelles; when temperature is decreased, these disordered micellar structures become highly ordered under the effect of crystallization of hydrophobic erucate chains, forming opaque solid-like hydrogels.

Rheological characterization of mixtures of cetyl trimethylammonium bromide and sodium butyl benzene sulfonate in aqueous solutions

The rheological behavior of mixed aqueous solutions of cetyl trimethylammonium bromide (CTAB) and anionic hydrotropes, such as sodium salts of n-butyl benzene sulfonate (Na-NBBS), iso-butyl benzene sulfonate (Na-IBBS) and tert-butyl benzene sulfonate (Na-TBBS), has been studied over a range of the hydrotrope concentration at different temperatures. The zero shear viscosity measurements for the mixed systems show a bimodal curve with a first viscosity maximum at a Na-BBS/CTAB ratio of approximately 0.3 and other ones at about 2.5. The three isomers of butyl benzene sulfonate (BBS) differ in their effects on the viscosity of the solutions. The difference in the viscosities of the mixed CTAB–hydrotrope solutions is more prominent at higher values of Na-BBS/CTAB ratio. Since the viscosity depends on the microstructure generated by the CTAB-BBS mixtures in solutions, the size and shape of the mixed micelles varies with the structure of the butyl group. The linear NBBS induces higher changes even at lower concentrations. The CTAB-hydrotrope solutions in the region of the first viscosity maxima exhibit Maxwellian fluid behavior where the mixed system is highly viscoelastic and the micelles are entangled. In the region of second viscosity maxima, however, the mixed solutions are non-viscoelastic with much lower values of storage and loss modulii. The living polymer formation by linear growth of CTAB micelles, on addition of hydrotropes, to very large aggregation number is evidenced from intrinsic viscosity and rheological properties of solutions.

Effect of Organic Additives on the Phase Separation Phenomenon of Amphiphilic Drug Solutions

AbstractThe effect of various organic additives, viz. sugars, ureas, alcohols, hydrotropes and bile salts on the clouding (phase separation) phenomenon of the amphiphilic antidepressant drug amitriptyline hydrochloride was investigated in the present study. All sugars lowered the cloud point (CP) due to their water structure‐making property. Urea and alkylureas were found to lower the CP. In contrast, thioureas increased the CP slightly, but the presence of methyl group(s) had a similar effect in alkylureas. Short chain alcohols affected the CP insignificantly while higher ones decreased it, and medium chain alcohols showed peak behavior. Addition of hydrotropes and bile salts increased the CP at lower concentrations, while a decrease was observed at higher concentrations (like the medium chain alcohols). In addition, thermodynamic parameters were also evaluated but only for those additives which formed mixed micelles with the drug.

Effect of hydrotropes on the solution behavior of PEO/PPO/PEO block copolymer L62 in aqueous solutions

Effect of hydrotropes viz. sodium benzene sulfonate (NaBS), sodium toluene sulfonate (NaTS) and sodium xylene sulfonate (NaXS) on the micellization, phase behavior and structure of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer L62 in aqueous solution was studied by surface tension, dye spectral, cloud point and small angle neutron scattering measurements. The addition of hydrotropes increased the critical micelle concentration (CMC) of L62 which appears to be logistic as the added hydrotrope enhances the solubility of PPO moiety (and PEO) making it behave like a more hydrophilic block copolymer that would micellize at high copolymer concentration. Partial phase diagram of L62 in water shows two cloud point (CP) in the concentration range (0–10 wt.%). Addition of hydrotropes shifts the L62 concentration range showing double cloud points at lower side of concentration; sodium xylene sulfonate (NaXS) being more effective. SANS data for L62 in the presence of 0.4 and 0.8 M NaXS at temperatures <30 °C showed unimers which are fully dissolved Gaussian chains. The unimer-to-micelle transition takes place when temperature is increased. It is found that SANS data for L62 in the presence of 0.4 M NaXS (40 and 50 °C) and 0.8 M NaXS (45 and 50 °C) correspond to ellipsoidal structure of micelles.

Lipase purification by various techniques and its thermostabilization in presence of surface active additives

Crude porcine pancreatic lipase was purified by removing water-insoluble impurities (residual lipid material) associated with it by a conventional method, using hydrotropic additives and by liquid coacervate extraction. Maximum yield with good recovery of activity was obtained when hydrotropes were used to separate the associated lipids from lipase. The thermostability of the enzyme was also checked in the solutions of additives such as sodium butyl monoglycol sulfate (Na-BMGS), proline and Triton X-114. In Na-BMGS solutions above a concentration of 0·2 mol dm−3 the lipase activity decreased beyond 50°C whereas in 1 mol dm−3 proline solution it was retained even at 80°C, showing a good thermostabilizing effect. However, in the presence of Triton X-114 the enzyme was completely inactivated with increase in temperature. © 1998 SCI.