Hydrotropic Enhancement of Rate of Ninhydrin‐α‐Amino Acid Reaction: A Kinetic Study

The kinetics of ninhydrin‐α‐amino acid (alanine, phenylalanine, and valine) reaction have been studied in the absence and in the presence of a common hydrotrope, sodiumbenzoate. A strong enhancement in the rate of reaction has been observed in the presence of the hydrotrope above its minimal hydrotrope concentration (MHC). The order of reaction in ninhydrin was always found to be unity while that in amino acid decreases from unity to zero at higher [Aminoacid]. A mechanism consistent with kinetic data and involving the formation of a mixed aggregation assemblies between amino acid and hydrotrope has been proposed.     

The Coupling Action of a Hydrotrope and Structure Transition from Lamellar Liquid Crystal

The influence of SDS (a hydrotrope) on the region of stability of a microemulsion of water, sodium dodecyl sulfate (SDS), pentanol (C3OH) and p-xylene was determined. The addition of the hydrotrope united the O/W and W/O regions of the phase diagram enlarging the bicontinuous region by reducing the lamellar liquid crystal one. Hence, the phase behavior and the low angle x-ray measurements show that the stratum corneum coupling action of a hydrotrope is in     

Electroosmotic Flow of a General Electrolyte Solution through a Fibrous Medium

Although hydrotropy is extensively used in industry, the molecular mechanism of hydrotropic solubilization has not been completely elucidated yet. In this paper the interaction between a nonionic surfactant (ethoxylated fatty alcohol containing between five and six oxyethylenic units) and sodium p-toluene sulfonate is examined. Surface tension measurements confirm that the hydrotropic effect occurs at a concentration in which the hydrotropes self-associate. Photon correlation spectroscopy studies show that for this concentration of hydrotropes a drastic reduction in the surfactant micellar radius occurs. Furthermore the luminescence of the hydrotrope used as a fluorescence probe indicates that at low concentrations p-toluene sulfonate dissolves in the surfactant micelles but beyond the minimum concentration for hydrotropic solubilization the hydrotrope is present in the aqueous phase. These results suggest that the hydrotropic effect is related to alterations in the water structure induced by the hydrotrope molecules and to the presence of hydrotrope aggregates that furnish an appropriate niche for the surfactant amphiphile.     

PLURONIC x TETRONIC polyols: study of their properties and performance in the destabilization of emulsions formed in the petroleum industry

In this work, a new family of branched poly(ethylene oxide-propylene oxide) (PEO-PPO) block copolymers designed as TETRONIC polyols is evaluated and compared to linear PEO-PPO block copolymers designed as PLURONIC polyols. Additives have been employed as well in order to improve solubility of these materials in aqueous solution. Such additives include the sodium p-toluene sulfonate (NaPTS) hydrotrope and concentrated hydrochloric acid. Solubility tests and aqueous solution surface tension data showed consistent results: the structure of the block PEO-PPO copolymers exerts a huge influence on their solubility in water. The solubility of such copolymers is increased by the presence of the sodium toluene sulfonate (NaPTS) hydrotrope. The presence of HCl caused increased solubility for the copolymer TETRONIC polyol only, the effect being less than that observed for the hydrotrope. It is concluded that as regards emulsion stabilization, TETRONIC copolymer polyols perform better. Correlation between structure and properties leads to the optimization of block PEO-PPO copolymer selection aiming at using these materials for the separation of petroleum industry emulsions.     

The physicochemical and thermophysical properties of sodium dodecyl sulfate: The influence of p-toluene sulfonyl chloride and temperature

In the present study, we report on the interaction between a hydrotrope, p-toluene sulfonyl chloride (p-TSC), and an anionic surfactant, sodium dodecyl sulfate (SDS) which has been performed using electrical conductivity, Fourier transform infrared (FTIR), ¹H NMR, density, dynamic viscosity, and kinematic viscosity measurements. The effect of p-TSC on the micellization of SDS in non-aqueous (ethanol) medium at various temperatures (viz., 298.15, 303.15, 308.15, 313.15, 318.15, and 323.15 K) was investigated using the electrical conductivity method. The results show that the CMC value increases as the concentration of the hydrotrope is increased. It is noteworthy that at higher concentrations of hydrotrope, the trend of micelle formation is reversed (i.e., reverse micelles are formed). The thermodynamic parameters in micellization have also been evaluated. The FTIR and ¹H NMR data reveal the physicochemical properties of the pure and mixed systems and confirm no covalent bond formation takes place. Density, dynamic viscosity, and kinematic viscosity of the pure as well as mixed systems at various temperatures were also reported.     

The pH and temperature dual-responsive micelle transition in the mixture of hydrotrope potassium phthalic acid and quaternary ammonium surfactants cetyltrimethylammonium bromide

A facile and effective route to prepare a pH and temperature dual-responsive surfactant micelle has been introduced based on hydrotrope potassium phthalic acid (PPA) and quaternary ammonium surfactants cetyltrimethylammonium bromide (CTAB). The system can be switched between viscoelastic solution and water-like fluid by adjusting pH values. In this paper, different binding abilities between the hydrotrope and surfactants have been demonstrated during the process of adjusting pH because the hydrotrope has different ionization degrees with the pH variation. And with the aid of rheological measurements and cryo-TEM observation, we discussed the different effects of the hydrotrope on surfactants at various pH values in detail and analyzed the possible aggregate transition mechanism of CTAB–PPA system induced by pH. Besides, this CTAB–PPA system has temperature-responsive property when pH?<?2.50. The temperature sensitivity and reversible control of rheological properties may greatly facilitate practical applications of such responsive viscoelastic solution.     

Hydrotropic Behavior of Sodium Salicylate in Presence of Additives

It is well known that, like surfactants aggregating at a certain concentration (called critical micelle concentration, CMC), hydrotropes also have minimum hydrotrope concentration (MHC). However, unlike surfactant CMC, this MHC value is usually very high, thereby reducing their application. In this paper we report the results of conductivity and viscosity measurements with a well known hydrotrope sodium salicylate (NaSal) solutions in presence and absence of additives (propanol, PrOH; butanol, BuOH; pentanol, PeOH and tetrabutylammonium bromide, Bu₄NBr). We have found that MHC value of NaSal decreases in presence of additives. Alcohols increase the hydrophobic interactions and decrease the MHC while Bu₄NBr, in addition to ameliorating the hydrophobic interactions, reduces the charge on head groups and MHC decreases more steeply in its presence. Increased solubility of riboflavin in NaSal containing the above additives corroborates the results.     

Minimum Hydrotrope Concentration Behavior of Aqueous Solution of Sodium Salicylate in Presence of Additives

Hydrotropes are regaining importance in the currently blossoming field of self-organizing molecular systems. Their hydrophile-lipophile balance being on the higher side, their water solubility is significantly more than that of traditional surfactants, and, yet, they provide a sequestered niche or microenvironment for the solubilizate. Hydrotropes start aggregating at a certain concentration known as minimum hydrotrope concentration (MHC). The MHC values fall in the molar range because of which hydrotropes applications are reducing. In this article, the effect of various additives such as alcohols, electrolytes, surfactants, amines, and organic compounds on the behavior of sodium salicylate (NaSal) was investigated using viscosity measurement.     

Kinetics of vesicle formation

 The kinetics of vesicle formation from a hydrotrope (sodium xylenesulfonate) solution of a surfactant (Laureth 4) is studied by the use of a stopped-flow apparatus combined with a dynamic light scattering device to determine vesicle size in the system. The hydrotrope system studied presents a system with a high surfactant solubilization combined with vesicle formation simply by dilution with water. The kinetic results show a single exponential decay time. The kinetic analysis indicates that the vesicles are formed from a molecular solution which resulted from the shear in the stopped-flow device and grow by monomeric association. Received: 1 October 1996 Accepted: 22 November 1996     

Influence of the hydrotrope structure on the physical chemical properties of polyoxide aqueous solutions

The physical chemical properties of block substituted poly(ethylene oxide-propylene oxide) (PEO-PPO) block copolymer aqueous solutions were evaluated in the presence of two hydrotropes of different structures: sodium p-toluene sulfonate (NaPTS) and butyl monoglycol sodium sulfonate (NaBMGS). The critical micelle concentration and the cloud point of the copolymer solutions were displaced to higher concentration values, indicating that the solubility of the copolymer was increased in the presence of the hydrotropes. Temperature increased the micelle hydrodynamic radius, but concentration had a limited effect. Carbon-13 nuclear magnetic resonance (13C NMR) permitted the interaction between the surface-active agent and the hydrotrope to be evaluated: NaBMGS, which presented a more pronounced hydrotropic effect, interacts more effectively with the hydrophobic moiety of the surfactant, while NaPTS interacts rather mainly with the hydrophilic oxyethylenic groups. The results furnish experimental evidence to conclude that the hydrotropic phenomenon is specific in relation to both the hydrotrope and the solubilizate.     

Synthesis of sodium tungstate from the system Na<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>-NaNO<Subscript>3</Subscript>-WO<Subscript>3</Subscript>

The physicochemical basis of the heterogeneous reaction between the components of the system (sodium oxalate)-(sodium nitrate)-(tungsten(VI) oxide) has been studied. The reaction of WO₃ with sodium oxalate-nitrate mixtures occurs at a lower temperature and at a higher rate than the reaction with pure sodium carbonate. A high-yield process for the synthesis of high-purity sodium tungstate has been developed on the basis of this study.     

Toward the Rationalization of Facilitated Hydrotropy: Investigation with the Ternary Dimethyl Isosorbide / Benzyl Alcohol / Water System

Solubility enhancement has been achieved by facilitated hydrotropy for the dimethyl isosorbide (DMI) / benzyl alcohol / water system. Facilitated hydrotropy has been studied via three different approaches: the solubilization in water of a hydrophobic dye, the evolution of the surface tension and dynamic light scattering, all as a function of the benzyl alcohol concentration. The facilitated hydrotropy has been rationalized from the solubilization properties of the system according to the ratio between the insoluble hydrotrope (here benzyl alcohol, a preservative used in parenteral injections) and the bio-sourced co-solvant (here the dimethyl isosorbide ether, DMI, a solvent used in pharmaceutical formulation). The presence of self-associated nanostructures has been detected by dynamic light scattering (DLS). It appears that the cosolvent, DMI, has an antagonistic action: DMI increases the facilitated hydrotrope (benzyl alcohol) solubility in the aqueous solution (favoring solute solubilization) but simultaneously decreases the hydrotropic efficiency of benzyl alcohol.     

Unexpected effect of alkoxides on the reactivity of sodium borohydrid in the reduction of perfluoropolyether carboxylic esters

The reduction of perfluoropolyether carboxylic esters to the corresponding alcohols with sodium borohydride in ethanol has been investigated. This reaction does not follow the expected stoichiometry but a large excess (>60% molar) of reducing agent is required for a complete conversion of the ester; this excess parallels the massive built up of hydrogen observed during the reaction.Interestingly, the addition of catalytic amount of sodium ethoxide strongly influences the course of the reaction, so that both the excess of reducing agent and the evolution of hydrogen during the reaction are strongly reduced. These experimental evidences suggest that different mechanisms are active depending on the selected experimental conditions, so indicating that the considered reduction can follow different pathways.     

Dilution induced thickening in hydrotrope-rich rod-like micelles

Dilution induced changes in the microstructure and rheological behavior of micelles formed by a cationic surfactant-anionic hydrotrope mixture has been investigated in the hydrotrope-rich region. The surfactant used is cetyltrimethylammonium bromide (CTAB) and the hydrotropic salt is sodium 3-hydroxy naphthalene 2-carboxylate (SHNC). The concentration of the mixture is varied from 0.5% to 10.0% w/w (φ=0.005-0.100) at a fixed weight ratio of hydrotrope to surfactant (85:15). Rheological studies indicate Newtonian flow behavior at low and high volume fractions (0.005 and 0.100) while a shear thinning behavior is observed at intermediate volume fractions. The zero-shear viscosity η(0) also passes through a maximum upon changes in the concentration. The most striking feature in our study is that a low viscosity Newtonian fluid transforms to a viscoelastic fluid, upon dilution, and then again to a Newtonain fluid. Small angle neutron scattering studies of 10.0% micellar solution show the presence of rod-like aggregates. Upon dilution, the scattering intensity per unit concentration shows an increase in the low q-region. The nature of pair distance distribution function and subsequent model fitting indicates a transition from rod-like micelles to unilamellar vesicles upon dilution. This behavior is explained in terms of the volume fraction dependant solubilization of hydrotropes in the rod-like micelles and consequent changes in the composition of the mixed micelles.     

Kinetics of the acid hydrolysis of isoproturon in the absence and presence of sodium lauryl sulfate micelles

Kinetics of the hydrolysis of isoproturon by hydrochloric acid has been studied spectrophotometrically in the absence and the presence of anionic sodium lauryl sulfate (NaLS) micelles. The anionic micelle was found to increase the rate of reaction. The reaction followed first-order kinetics with respect to isoproturon and was linearly dependent upon [HCl]. In both aqueous and micellar pseudophases, the reaction was started with the protonation of the amino group of isoproturon followed by attack of water to yield phenylcarbamic acid and the corresponding amine, thus obeying the addition–elimination mechanism. The surfactant decreased the activation entropy. The binding constant in consistence with the rate constants was evaluated on the basis of pseudophase ion-exchange model. The added salts (NaCl and KCl) decreased the rate of reaction due to the exclusion of H⁺ from micellar surfaces.     

Kinetics of oxidation of acidic amino acids by sodium N-bromobenzenesulphonamide in acid medium: A mechanistic approach

Kinetics of oxidation of acidic amino acids (glutamic acid (Glu) and aspartic acid (Asp)) by sodium N-bromobenzenesulphonamide (bromamine-B or BAB) has been carried out in aqueous HClO₄ medium at 30°C. The rate shows first-order dependence each on [BAB]_o and [amino acid]_o and inverse first-order on [H⁺]. At [H⁺] > 0·60 mol dm⁻³, the rate levelled off indicating zero-order dependence on [H⁺] and, under these conditions, the rate has fractional order dependence on [amino acid]. Succinic and malonic acids have been identified as the products. Variation of ionic strength and addition of the reaction product benzenesulphonamide or halide ions had no significant effect on the reaction rate. There is positive effect of dielectric constant of the solvent. Proton inventory studies in H₂O-D₂O mixtures showed the involvement of a single exchangeable proton of the OH⁻ ion in the transition state. Kinetic investigations have revealed that the order of reactivity is Asp > Glu. The rate laws proposed and derived in agreement with experimental results are discussed.     

Kinetic and mechanistic studies of oxidation of alanine and phenylalanine by sodium N-chlorobenzene sulfonamide (chloramine-B) in hydrochloric acid medium

The kinetics of oxidation of alanine and phenylalanine by sodium N-chlorobenzene sulfonamide (CAB) has been investigated at 30°C in two ranges of acid concentrations. The reactions follow identical kinetics for both amino acids. At low acid concentration (0.03–0.10M), simultaneous catalysis by H⁺ and Cl^? ions is noted. The rate shows a first-order dependence on [CAB], but is independent of [substrate]. A variation of the ionic strength or the dielectric constant of the medium or the presence of the added reaction product benzene sulfonamide (BSA) has no pronounced effect on the rate. At [HCl] > 0.2M, the rate is independent of [H⁺], but shows a first-order dependence on [CAB] and a fractional-order dependence on [amino acid]. The addition of BSA or Cl^? ions, or a change in the ionic strength of the medium has no influence on the rate. Upon decreasing the dielectric constant of the medium, the rate increased, indicating positive ion–dipole interaction in the rate-determining step. The reaction was studied at different temperatures, and activation parameters have been computed. Rate laws in agreement with experimental results have been derived. Suitable mechanisms to account for the observed kinetics are proposed. The rate constants obtained from the derived rate laws as [H⁺], [Cl^?], and [substrate] vary are in excellent agreement with the observed rate constants, thus justifying the proposed rate laws and hence the suggested mechanistic schemes.     

The photochemistry properties on interaction silver with tryptophan

UV-vis and fluorescence spectra for interaction silver(I) ion with tryptophan (Trp) have been studied. The influence of pH of media, multicomponent concentration, including amino acids, silver(I) ion, formaldehyde, sodium dodecyl sulfate (SDS), and trihydroxymethylaminomethane (Tris) as well as temperature, illumination, time, etc. on reaction were investigated, and the mechanism of reaction has been explored. The results shown that the optimum condition for the interaction between tryptophan and silver(I) ion were found.     

Effect of salts on the phase behavior and the stability of nanoemulsions with rapeseed oil and an extended surfactant

For many decades, the solubilization of long-chain triglycerides in water has been a challenge. A new class of amphiphiles has been created to overcome this solubilization problem. The so-called "extended" surfactants contain a hydrophilic-lipophilic linker to reduce the contrast between the surfactant-water and surfactant-oil interfaces. In the present contribution, the effects of different anions and cations on the phase behavior of a mixture containing an extended surfactant (X-AES), a hydrotrope (sodium xylene sulfonate, SXS), water, and rapeseed oil were determined as a function of temperature. Nanoemulsions were obtained and characterized by conductivity measurements, light scattering, and optical microscopy. All salting-out salts show a transition from a clear region (O/W nanoemulsion), to a lamellar liquid crystalline phase region, a clear phase (bicontinuous L(3)), and again to a lamellar liquid crystalline phase region with increasing temperature. For the phase diagrams with NaSCN and Na(2)SO(4), only one clear region (O/W nanoemulsion) was observed, which turns into a lamellar phase region at elevated temperatures. Furthermore, the stability of the nanoemulsions was investigated by time-dependent measurements: the visual observation of phase separation, droplet size by dynamic light scattering (DLS), and optical microscopy. The mechanism of the different phase transitions is also discussed.