A microcalorimetric method of determination of critical micellar concentration and enthalpy of micellization

Summary A method is proposed for the analysis of dilution heats of micellar systems. Simultaneous determination of critical micellar concentration and of micellization enthalpy can be derived from measurements performed at a single temperature.This method, when applied to ionic and non-ionic detergents, supplies results in good agreement with those published for detergents in aqueous solutions at 25 °C.Determination of these two parameters should thus be possible for any type of solvent; further measurements were extended to detergents in solutions 0.001, 0.01 and 0.1 M in NaCl.

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Zusammenfassung Eine Methode zur Analyse der Verdünnungs-enthalpien von mizellaren Systemen wird vorgeschlagen. Messungen bei einer einzigen Temperatur ermöglichen gleichzeitig die Bestimmung der kritischen Mizellkonzentration und der Enthalpie der Mizellenbildung.Angewandt auf ionische und nichtionische oberflächenaktive Substanzen, ergibt diese Methode Resultate, die in guter Übereinstimmung sind mit den durch andere Autoren angegebenen Werten für wässerige Lösungen dieser Substanzen bei 25 °C.Die Bestimmung dieser beiden Parameter wird also in jedem beliebigen als Lösungsmittel benutzten Medium ermöglicht; wir haben unsere Messungen auch auf den Fall von oberflächenaktiven Substanzen in NaCl-Lösungen ausgedehnt.

     

On critical concentration determination

The rules formulated by Koningsveld for the determination of the critical concentration are shown to be in accord with the Flory-Huggins theory. Sometimes, however, they may not refer to stable conditions, and their application in such cases will result in misleading conclusions.     

Predicting human intestinal absorption in the presence of bile salt with micellar liquid chromatography

Understanding intestinal absorption for pharmaceutical compounds is vital to estimate the bioavailability and therefore the in vivo potential of a drug. This study considers the application of micellar liquid chromatography (MLC) to predict passive intestinal absorption with a selection of model compounds. MLC is already known to aid prediction of absorption using simple surfactant systems; however, with this study the focus was on the presence of a more complex, bile salt surfactant, as would be encountered in the in vivo environment. As a result, MLC using a specific bile salt has been confirmed as an ideal in vitro system to predict the intestinal permeability for a wide range of drugs, through the development of a quantitative partition–absorption relationship. MLC offers many benefits including environmental, economic, time‐saving and ethical advantages compared with the traditional techniques employed to obtain passive intestinal absorption values. Copyright © 2016 John Wiley & Sons, Ltd.     

System peaks in micellar electrophoresis: I. Utilization of system peaks for determination of critical micelle concentration

A new way to determine the critical micelle concentration (CMC) based on the mobilities of system peaks is presented. A general approach for the CMC determination is based on the change of the slope or on finding the inflection point in the plot of a physical property of solution as a function of surfactant concentration. The determination of CMC by system peaks in CE utilizes a "jump" instead of a continuous change in the measured quantity. This phenomenon was predicted by the program PeakMaster, which was modified for simulation of micellar systems. The simulation of the steep change in mobilities of the anionic system peaks showing the CMC value was verified experimentally in a set of measurements, where the concentration of the surfactant was varied while the ionic strength was kept constant. The experimental work fully proved our model. A comparative electric current measurement was carried out. The proposed method seems to offer easier CMC determination as compared to the standard methods.     

Highly-sensitive micellar electrokinetic chromatographic analysis of dioxin-related compounds using on-line concentration.

An application study of an on-line concentration technique of neutral analytes for micellar electrokinetic chromatography (MEKC) was carried out in environmental analysis to enhance the UV detection sensitivity. Several dioxins and related compounds, such as dibenzofuran, dibenzo-p-dioxin, 2,3- and 2,7-dichlorodibenzo-p-dioxins, and 2,3,7-trichlorodibenzo-p-dioxin, were used as test solutes. For a highly sensitive separation and detection, cyclodextrin-modified MEKC (CD-MEKC) under acidic conditions was employed as a separation mode and stacking using reverse migrating micelles and a water plug (SRW) as an on-line concentration technique. Almost a 200-fold gain in detection sensitivity was obtained for the model compounds in SRW-CD-MEKC compared to that in normal CD-MEKC without on-line concentration and the limit of detection was found to be around 0.1 ppm for each solute.     

A theory of the salt effects on the critical micelle concentration

Summary The previously proposed theory on the critical micelle concentration is extended and revised, considering the effects of coexisting small ions. Using the theory of the adsorption equilibrium instead of that of the flat electrical double layer, we obtain an equation for the salt effect on the c. m. c., which is of somewhat different form from the usual experimental formula. It is pointed out that the activities of ions should be used instead of the stoichiometric concentrations. Comparisons are made with available experimental works and the agreement is shown to be good.

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Zusammenfassung Die früher ver?ffentlichte Theorie (1) über die kritische Mizellkonzentration wurde erweitert und verbessert, indem wir die Wirkung der gleichzeitig anwesenden Ionen in Betracht zogen. Um den Effekt der Salze auf die kritische Mizellkonzentration zu untersuchen, benutzen wir die Theorie des Adsorptionsgleichgewichtes anstatt der Theorie der ebenen elektrischen Doppelschicht, und unsere Theorie führte zu einer von den üblichen experimentellen Formeln etwas verschiedenen Gleichung. Es wurde gezeigt, da? die Aktivit?t der Ionen anstatt der st?chiometrischen Konzentration gebraucht werden mu?. Unsere Theorie steht in guter übereinstimmung mit existierenden experimentellen Resultaten.

     

Use of micellar mobile phases for the chromatographic determination of melamine in dietetic supplements

Melamine is a nitrogen-rich industrial chemical which is occasionally used to increase the apparent protein content of different products destined for human and animal consumption. In this work, a liquid chromatographic procedure that uses micellar mobile phases of sodium dodecyl sulfate (SDS) buffered at pH 3, a C18 column and UV detection is reported for the determination of melamine in dietetic supplements. Samples were reconstituted with a SDS solution and were directly injected, thus avoiding long extraction and experimental procedures. Melamine was eluted in less than 10 min with no interference by other compounds of the matrices. The optimum mobile phase composition was taken by a chemometrical approach that considers the retention factor, efficiency and peak shape. Validation was performed following the indications of the European Commission (Decision 2002/657/EC). The following parameters were considered: linearity (0.02-100 μg mL(-1); R(2) = 0.9996), intra- and inter-day precisions (<12.4%), accuracy (90.0-101.3%), and robustness (less than 9.8% and 5.1%, for retention time and peak area, respectively). The limits of detection and quantification were 9 and 20 ng mL(-1), respectively. Recoveries for several spiked samples were in the 85.8-114.3% range. These results indicate that the proposed methodology is useful for routine analysis of control quality of infant formula and adult dietetic supplements.     

A theoretical model for the critical micelle concentration of bile salts

Critical micelle concentration (CMC) is a fundamental parameter in the evaluation of the biological activity of natural and synthetic bile salts. The CMC is logarithmically related to the free energy of solute micellization in aqueous solution. Hydrophobic and hydrogen bonding interaction energies were identified as the primary contributors to this free energy and the logarithm of the CMC was modeled as a linear function of relevant chemical group contributions to the solvent accessible molecular surface area of the solute. The structures (three-dimensional atomic coordinates) of 23 mono-, di-, and tri-hydroxyl bile acids were generated and optimized by energy minimization. The accessible surface area for each structure was computed and partitioned according to calculated charge distribution and polar group orientation. Experimental CMC values were fitted to these computed quantities by least squares multiple linear regression. Two regression equations, based on slightly different surface area partition schemes, were derived and compared. Their significance in explaining the aggregation process and in predicting the CMC of new bile salts is discussed.     

High-Q ultrasonic determination of the critical nanoaggregate concentration of asphaltenes and the critical micelle concentration of standard surfactants

Asphaltenes are known to be interfacially active in many circumstances such as at toluene-water interfaces. Furthermore, the term micelle has been used to describe the primary aggregation of asphaltenes in good solvents such as toluene. Nevertheless, there has been significant uncertainty regarding the critical micelle concentration (CMC) of asphaltenes and even whether the micelle concept is appropriate for asphaltenes. To avoid semantic debates we introduce the terminology critical nanoaggregate concentration (CNAC) for asphaltenes. In this report, we investigate asphaltenes and standard surfactants using high-Q, ultrasonic spectroscopy in both aqueous and organic solvents. As expected, standard surfactants are shown to exhibit a sharp break in sonic velocity versus concentration at known CMCs. To prove our methods, we measured known surfactants with CMCs in the range from 0.010 g/L to 2.3 g/L in agreement with the literature. Using density determinations, we obtain micelle compressibilities consistent with previous literature reports. Asphaltenes are also shown to exhibit behavior similar to that of ultrasonic velocity versus concentration as standard surfactants; asphaltene CNACs in toluene occur at roughly 0.1 g/L, although the exact concentration depends on the specific (crude oil) asphaltene. Furthermore, using asphaltene solution densities, we show that asphaltene nanoaggregate compressibilities are similar to micellar compressibilities obtained with standard nonionic surfactants in toluene. These results strongly support the contention that asphaltenes in toluene can be treated roughly within the micelle framework, although asphaltenes may exhibit small levels of aggregation (dimers, etc.) below their CNAC. Furthermore, our extensive results on known surfactants agree with the literature while the asphaltene CNACs reported here are one to two orders of magnitude lower than most previously published results. (Previous work utilized the terminology "micelle" and "CMC" for asphaltenes.) We believe that the previously reported high concentrations for asphaltene CMCs do not correspond to primary aggregation; perhaps they refer to higher levels of aggregation or perhaps to a particular surface structure.     

A facile surfactant critical micelle concentration determination

Liquid surface curvature variations in microplate wells due to different liquid surface tension cause significant signal change in spectroscopic measurement using a plate reader with a vertical detecting light beam. The signals have been quantitated and used to develop a method for facile surfactant critical micelle concentration determination.     

High-performance liquid chromatographic determination of diuretics in urine by micellar liquid chromatography.

The use of micellar liquid chromatography for the determination of diuretics in urine by direct injection of the sample into the chromatographic system is discussed. The retention of the urine matrix at the beginning of the chromatograms was observed for different sodium dodecyl sulphate (SDS) mobile phases. The eluent strengths of a hybrid SDS-methanol micellar mobile phase for several diuretics were compared and related to the stationary phase/water partition coefficient with a purely micellar mobile phase. The urine band was appreciably narrower with a mobile phase of 0.05 M SDS-5% methanol (v/v) at 50 degrees C (pH 6.9). With this mobile phase the determination of bendroflumethiazide and chlorthalidone was adequate. Acetazolamide, ethacrynic acid, furosemide, hydrochlorothiazide and probenecid were overlapped by the urine matrix, and the retention of amiloride and triamterene was too long.     

Studies of surfactant/water systems near the critical micellar concentration using thermal lens spectroscopy

A novel application of photothermal spectroscopy to the study of surfactant-water systems near the critical micellar concentration is reported. The thermal lens signal was induced by a slightly soluble dye and was measured with a dual-beam thermal lens spectrometer.For the two surfactants considered: nonyl phenol and Triton X-100, sharp variations of the thermal lens signal were observed at the critical micellar concentration (CMC), namely an increase for nonyl phenol and a decrease for triton X-100. These effects are arguably related to micelle formation.Our work serves as an initial assessment of the potential of the technique for the study of disperse systems of a higher complexity or dark systems where conventional techniques are impossible to apply.     

Development and validation of micellar liquid chromatographic methods for the determination of antibiotics in different matrixes

Antibiotics are the most important bioactive and chemotherapeutic compounds to be produced by microbiological synthesis, and they have proved their worth in a variety of fields, such as medicinal chemistry, agriculture, and the food industry. Interest in antibiotics has grown in parallel with an increasingly high degree of productivity in the field of analytical applications. Therefore, it is necessary to develop chromatographic procedures capable of determining various drugs simultaneously in the shortest possible time. Micellar liquid chromatography (MLC) is an RP-HPLC technique that offers advantages over conventional HPLC as far as sample preparation, selectivity, and versatility are concerned. Its main advantage is that samples can be injected directly into the chromatographic system with no previous preparation step. This paper mainly focuses on the results of the authors' own recent research and reports the chromatographic conditions for determination of various antibiotics (penicillins, quinolones, and sulfonamides) in different matrixes (pharmaceuticals, biological fluids, and food). The work of other authors on MLC-based antibiotic determination has been included.     

Photostability studies for micellar liquid chromatographic determination of nifedipine in serum and urine samples

Nifedipine is a photosensitive compound that is converted into its 4-(2-nitrophenyl) pyridine and 4-(2-nitrosophenyl) pyridine homologue. In order to obtain the most adequate conditions for handling nifedipine solutions in the analytical laboratory, a number of studies on the decomposition of this compound were performed. A simple micellar liquid chromatographic procedure was described to determine nifedipine in different biological matrices such as serum and urine, and to control its decomposition. To perform the analysis, nifedipine was dissolved in 0.1 m SDS at pH 3 and chromatographed using a mobile phase containing 0.125 m SDS-3% pentanol, pH 3 on a C18 column and UV detection at 235 nm. The chromatographic analysis time was 8 min. The response of the drug for both biological matrices was linear in the 1-100 microg/mL range, with r2>0.997 at all times. Repeatability, intermediate precision (CV, %) and limits of quantification and detection (ng/mL) were 0.19, 4.3, 104 and 31 in serum and 0.81, 2.1, 136 and 41 in urine. The method developed here does not show interferences or matrix effects produced by endogenous compounds. Micellar media and mobile phases have the advantage of stabilising the compounds, thus preventing photodegradation and allowing the direct injection of biological samples.     

A new reverse wormlike micellar system: mixtures of bile salt and lecithin in organic liquids

We report a new route for forming reverse wormlike micelles (i.e., long, flexible micellar chains) in nonpolar organic liquids such as cyclohexane and n-decane. This route involves the addition of a bile salt (e.g., sodium deoxycholate) in trace amounts to solutions of the phospholipid lecithin. Previous recipes for reverse wormlike micelles have usually required the addition of water to induce reverse micellar growth; here, we show that bile salts, due to their unique "facially amphiphilic" structure, can play a role analogous to that of water and promote the longitudinal aggregation of lecithin molecules into reverse micellar chains. The formation of transient entangled networks of these reverse micelles transforms low-viscosity lecithin organosols into strongly viscoelastic fluids. The zero-shear viscosity increases by more than 5 orders of magnitude, and it is the molar ratio of bile salt to lecithin that controls the viscosity enhancement. The growth of reverse wormlike micelles is also confirmed by small-angle neutron scattering (SANS) experiments on these fluids.     

Polarographic determination of critical micelle concentration of anionic surfactants

Summary Critical micelle concentrations of four anionic surfactants were determined by DC polarographic desorption waves of the surfactants without supporting electrolyte. The linear plot obtained between the potentials of the desorption waves and the logarithmic concentrations of the surfactant had an inflection point which corresponded to the CMC. The CMC values obtained are in good agreement with those in the literature.

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Zusammenfassung Es wurden die kritischen Mizellbildungskonzentrationen (KMK) von anionischen Tensiden durch Gleichstrompolarographie in Abwesenheit von Leitelektrolyten bestimmt.

     

The methods of determination of critical micellar concentrations of the amphiphilic systems in aqueous medium

Science is the attempt to make the chaotic diversity of our sense-experience to a logically unified system of thought. A sound theoretical basis and a rational physicochemical understanding based on experimental findings lead to a proper understanding of a topic. Colloid chemistry strengthened its footing by the extensive studies on gold sols by Michael Faraday in 1850s, however, the term “colloid” was coined by Thomas Graham in 1861. Historically, the Stone Age paintings in the Lascaux cave in France and the written records of Egyptian pharaohs were produced with stabilized colloid pigments. The part of colloid science dealing with amphiphilic molecules is so diverse on its own that it is dealt in a separate branch called the “association colloid”. A key development in the study of association colloids was the observation by James McBain that the osmotic pressure of salts of alkali metal fatty acid displayed a pronounced break in the concentration beyond a specific characteristic concentration, after which the osmotic coefficient remained almost constant (Evans and Wennerstrom, 1994). McBain attributed this fact to the self-association of these molecules among themselves (McBain, 1913, McBain, 1944) in solution. Here, we are providing a molecular as well as thermodynamic approach toward the micellization process.     

Pyrene absorption can be a convenient method for probing critical micellar concentration (cmc) and indexing micellar polarity

The critical micellar concentration (cmc) of both ionic and non-ionic surfactants can be conveniently determined from the measurements of UV absorption of pyrene in surfactant solution. The results on a number of surfactants have agreed with that realized from pyrene fluorescence measurements as well as that obtained following conductometric, tensiometric and calorimetric methods. The absorbance vs [surfactant] profiles for all the major UV spectral peaks of pyrene have been found to be sigmoidal in nature which were analyzed according to Sigmoidal-Boltzmann equation (SBE) to evaluate the cmcs of the studied surfactants. The difference between the initial and the final asymptotes (a(i) and a(f), respectively) of the sigmoidal profile, Delta a = (a(f)-a(i)) and the slope of the sigmoid, S(sig) have been observed to depend on the type of the surfactant. The Delta a has shown a linear correlation with the ratio of the fluorescence intensities of the first and the third vibronic peaks, I1/I3 of pyrene which is considered as a measure of the environmental polarity (herein micellar interior) of the probe (pyrene). Thus, Delta a values have the prospect for use as another index for the estimation of polarity of micellar interior.