Comparison of the performance of non-ionic and anionic surfactants as mobile phase additives in the RPLC analysis of basic drugs

Surfactants added to the mobile phases in reversed-phase liquid chromatography (RPLC) give rise to a modified stationary phase, due to the adsorption of surfactant monomers. Depending on the surfactant nature (ionic or non-ionic), the coated stationary phase can exhibit a positive net charge, or just change its polarity remaining neutral. Also, micelles in the mobile phase introduce new sites for solute interaction. This affects the chromatographic behavior, especially in the case of basic compounds. Two surfactants of different nature, the non-ionic Brij-35 and the anionic sodium dodecyl sulfate (SDS) added to water or aqueous-organic mixtures, are here compared in the separation of basic compounds (β-blockers and tricyclic antidepressants). The reversible/irreversible adsorption of the monomers of both surfactants on the stationary phase was examined. The changes in the nature of the chromatographic system using different columns and chromatographic conditions were followed based on the changes in retention and peak shape. The study revealed that Brij-35 is suitable for analyzing basic compounds of intermediate polarity, using "green chemistry", since the addition of an organic solvent is not needed and Brij-35 is a biodegradable surfactant. In contrast, RPLC with hydro-organic mixtures or mobile phases containing SDS required high concentrations of organic solvents.     

Mixed micelles of polyethylene glycol (23) lauryl ether with ionic surfactants studied by proton 1D and 2D NMR

(1)H NMR chemical shift, spin-lattice relaxation time, spin-spin relaxation time, self-diffusion coefficient, and two-dimensional nuclear Overhauser enhancement (2D NOESY) measurements have been used to study the nonionic-ionic surfactant mixed micelles. Cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used as the ionic surfactants and polyethylene glycol (23) lauryl ether (Brij-35) as the nonionic surfactant. The two systems are both with varying molar ratios of CTAB/Brij-35 (C/B) and SDS/Brij-35 (S/B) ranging from 0.5 to 2, respectively, at a constant concentration of 6 mM for Brij-35 in aqueous solutions. Results give information about the relative arrangement of the surfactant molecules in the mixed micelles. In the former system, the trimethyl groups attached to the polar heads of the CTAB molecules are located between the first oxy-ethylene groups next to the hydrophobic chains of Brij-35 molecules. These oxy-ethylene groups gradually move outward from the hydrophobic core of the mixed micelle with an increase in C/B in the mixed solution. In contrast to the case of the CTAB/Triton X-100 system, the long flexible hydrophilic poly oxy-ethylene chains, which are in the exterior part of the mixed micelles, remain coiled, but looser, surrounding the hydrophobic core. There is almost no variation in conformation of the hydrophilic chains of Brij-35 molecules in the mixed micelles of the SDS/Brij-35 system as the S/B increases. The hydrophobic chains of both CTAB and SDS are co-aggregated with Brij-35, respectively, in their mixed micellar cores.     

Studies on the adsorption of Brij-35 and CTAB at the coal-water interface

The adsorption behavior of polyoxyethylene (23) lauryl ether (Brij-35) and cetyl trimethyl ammonium bromide (CTAB) on coal sample has been studied. The adsorption process is found to be sensitive to pH, temperature, electrolyte concentration, and the amount of surface active agent. An attempt has been made to explain the adsorption behavior of the surfactants using the Langmuir equation. The extent of adsorption of Brij-35 on coal is found to be the highest at pH 2, which decreases with increase in pH and remains constant in the neutral and alkaline pH regions. But, the adsorption of CTAB exhibits the opposite behavior of that of Brij-35. Adsorption of any of the surfactant at the coal/water interface sharply decreases the apparent viscosity of 55 wt% coal-water slurry (CWS) at a shear rate of 100 s(-1). Electrostatic adsorption of the surfactants on the coal surface decreases the surface charge and renders the coal surface hydrophobic which is manifested in the form of high apparent viscosity of the coal-water slurry under the test conditions.     

Micellar versus reversed phase liquid chromatography for the determination of desferrioxamine and its chelates with aluminium and iron in uremic serum

Micellar liquid chromatography with sodium dodecyl sulphate or Brij-35 as surfactants in the mobile phase was evaluated and compared with reversed phase liquid chromatography using conventional acetonitrile-water eluents for the separation and determination of desferrioxamine (DFO) and its complexes with aluminium (AIDFO) and iron (FeDFO) in uremic serum. Reversed phase liquid chromatography proved to be superior in terms of sensitivity and selectivity. The three solutes investigated were separated with a mobile phase of 13% (v/v) acetonitrile/phosphate buffer (5 mM, pH = 3.5) on a C(18) column and detected by ultraviolet absorption at 210 nm (DFO) and 220 nm (AlDFO and FeDFO). Limits of detection of 0.1 mug ml(-1) and relative standard deviation of 3-4% were obtained. The recovery from serum samples after ultramicrofiltration was around 90%. The method was applied to the determination of DFO, AlDFO and FeDFO in uremic serum.     

Surfactant aggregation within room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

On the basis of the response of solvatochromic probes [Reichardt's betaine dye, pyrene, and 1,3-bis(1-pyrenyl)propane], we have investigated the aggregation behavior of common anionic, cationic, and nonionic surfactants when solubilized within a low-viscosity room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N). We observed possible aggregate formation by all nonionic surfactants included in the study (Brij-35, Brij-700, Tween-20, and Triton X-100), while no aggregation was observed for the cationic surfactant cetyltrimethylammonium bromide. The anionic surfactant sodium dodecyl sulfate does not appear to solubilize within emimTf2N at ambient conditions.     

Effect of micelles on the acid hydrolysis of N-phenylbenzohydroxamic acid

The effect of cationic (cetylpyridinium bromide and cetylpyridinium chloride) anionic (sodium dodecyl sulfate and lithium dodecyl sulfate) and non-ionic (Brij-35 and Triton-X-100) micelles on the acid-catalyzed hydrolysis of N-phenylbenzohydroxamic acid in 20 vol.% dioxane medium has been investigated. The kinetic results are explained by both pseudo-phase and Piszkiewicz cooperativity models.     

胶束电动毛细管色谱法测定硫酸多粘菌素E药物中的多粘菌素E1和E2

采用胶束电动毛细管色谱法(MECC)对硫酸多粘菌素E药物中的主要成分多粘菌素E1和E2进行了分离,并测定了多粘菌素E1、E2的含量。分别考察了电泳电压、表面活性剂种类、Brij-35(月桂醇聚氧乙烯醚)浓度、乙腈含量、磷酸盐缓冲液的pH值、氯化钠浓度等实验参数对实验结果的影响,从而确定了最佳的分离条件: 电泳电压为10 kV,运行缓冲液为含有30 mmol/L Brij-35、5%(体积分数)乙腈、0.167 mol/L氯化钠的磷酸二氢钠缓冲液(0.01 mol/L,pH 4.1)。在优化的实验条件下,E1和E2得到了较好的分离,分离度达到1.94。以多粘菌素E1为例,柱效和峰面积的日间及日内测定的相对标准偏差(RSD)均小于5%。E1和E2在硫酸多粘菌素E药物中的含量分别为67%和32%。该方法简便、快速、准确、重现性好。     

Interface of AOT/Brij mixed reverse micellar systems: conductometric and spectrophotometric investigations

Solubilization and conductivity studies are carried out with AOT/Brijs (Brij-30, Brij-35, Brij-52, Brij-56, Brij-58, Brij-72, Brij-76, Brij-78)/isooctane/water mixed reverse micellar systems. Replacement of AOT molecules with large head group Brij molecules (Brij-30, Brij-35, Brij-56, Brij-58, Brij-76, Brij-78) decreases the solubilization capacity, whereas those with smaller polar head groups (Brij-52 and Brij-72) increases it. The former blends assist the conductance percolation whereas the latter retard it. An attempt has been taken to obtain more insight on the interfacial composition of the mixed interface with the help of spectrophotometric studies using 7-hydroxycoumarin as the fluorophore. The results obtained from the solubilization and conductometric studies have been correlated with those obtained from the spectroscopic studies.     

Measurement of the elution strength and peak shape enhancement at increasing modifier concentration and temperature in RPLC

Two approaches are proposed to measure the effect of different experimental factors (such as the modifier concentration and temperature) on the elution strength and peak shape in reversed-phase liquid chromatography, which quantify the percentage change in the retention factor and peak width (referred to the weakest conditions) per unit change in the experimental factor. The approaches were applied to the separation of a set of flavonoids with aqueous micellar mobile phases of the surfactant Brij-35 (polyoxyethylene(23)dodecanol), in comparison with acetonitrile–water mixtures, using an Eclipse XDB-C18 column. The particular interaction of each flavonoid with the oxyethylene chains of Brij-35 molecules (adsorbed on the stationary phase or forming micelles) changed the elution window, distribution of chromatographic peaks and partitioning kinetics, depending on the hydroxyl substitution in the aromatic rings of flavonoids. At 25?°C, peak shape with Brij-35 mobile phases was significantly poorer with regard to acetonitrile–water mixtures. At increasing temperature, the efficiency of Brij-35 increased, approaching at 80?°C the values obtained at equilibrium conditions, already reached with acetonitrile at 25?°C.     

Flow-injection system for determination of critical micelle concentrations of ionic and nonionic surfactants

A practical technique is presented for the rapid, accurate determination of the critical micelle concentrations (CMCs) of ionic and nonionic surfactants. The precision, speed and instrumental simplicity of a flow-injection system are combined with a gradient chamber and flow-through conductance and absorbance detection to produce a system capable of determining the CMC of surfactant solutions in less than 30 min. The exponential response gradients from the resulting system are monitored by a chart recorder and simple manual calculations yield the CMC. The validity of the technique is verified by determination of the CMC values for both ionic (cetyltrimethylammonium bromide and chloride and sodium dodecyl sulfate) and nonionic (Brij-35, Brij-56, Brij-99, Triton X-100) surfactants. The proposed technique does not require the extensive solution preparation, repetitive measurements, complex instrumentation and data manipulation typical of other methods for determining CMCs.     

Properties of polyethylene glycol (23) lauryl ether with cetyltrimethylammonium bromide in mixed aqueous solutions studied by self-diffusion coefficient NMR

NMR self-diffusion coefficient measurements have been used to study the properties of polyethylene glycol (23) lauryl ether (Brij-35) with cetyltrimethylammonium bromide (CTAB) in the mixed aqueous solutions with different mole fractions of CTAB. By fitting the self-diffusion coefficients to the two-state exchange model, the critical micelle concentrations of the two solutes in the mixed solutions (cmc*1 and cmc*2) were obtained. The critical mixed micelle concentrations (cmc*) were then evaluated by the sum of cmc*1 and cmc*2, which are in good agreement with the results measured by the surface tension method. The cmc* values are lower than those of the ideal case of mixing, which indicates that the behavior of the CTAB/Brij-35 system is nonideal. Moderate interactions between CTAB and Brij-35 in their mixtures can be deduced from the interaction parameters (betaM) based on the cmc* obtained by the NMR self-diffusion method. The compositions (x1) of the mixed micelles at different total surfactant concentrations were also evaluated. By using these results, a possible mechanism of mixed micellar formation and a picture of the formation of nonsimultaneous CTAB/Brij-35 binary mixed micelle were proposed. In contrast to the case of CTAB/TX-100 system, Brij-35 molecules have a tendency to form micelles first at any mole fraction of CTAB. The mixed micellar self-diffusion coefficients (Dm) increase slightly at lower CTAB molar ratios, and then speed up with increasing CTAB mole fraction.     

Reversed-phase liquid chromatography without organic solvent for determination of tricyclic antidepressants

The chromatographic behavior of seven tricyclic antidepressants (amitryptiline, clomipramine, doxepin, imipramine, maprotiline, nortryptiline, and trimipramine) was examined with micellar mobile phases containing the nonionic surfactant Brij-35. Acetonitrile-water mixtures were also used for comparison purposes. Tricyclic antidepressants are moderately polar basic drugs, which are positively charged in the usual working pH. This gives rise to a strong association with the alkyl chains and residual ionized silanols in silica-based stationary phases, which is translated in a high consumption of organic solvent to get appropriate retention times. Brij-35 modifies the surface of the stationary phases creating a neutral bilayer that masks silanols and reduces the polarity. Consequently, the retention times are decreased. A simple chromatographic procedure for the control of tricyclic antidepressants in pharmaceutical formulations was developed, using 0.02 M Brij-35 at pH 3 and UV detection. Satisfactory recoveries were achieved, with intra- and inter-day relative standard deviations usually below 1 and 2%, respectively. The preparation of the samples was simple and only required solubilization and filtration steps previous to injection. The proposed procedure has the advantage of not using an organic solvent in the mobile phase, and the biodegradable character of Brij-35. This makes an example of "green" liquid chromatographic analysis.     

Measurement of the peroxidation of Brji-35 in aqueous solution by hemin and horseradish peroxidase catalyzed fluorogenic reaction

The non-ionic surfactant Brij-35 can easily be oxidized in solution by dissolved oxygen upon light radiation during its storage. Various factors, such as light radiation, temperature, pH, ionic strength as well as dissolved oxygen influencing the stability of Brij-35 were studied. The results showed that light radiation is the most important factor inducing the oxidation by the dissolved oxygen. The oxidized product was confirmed, indirectly, to be the peroxide of Brij-35 based on a number of evidences. The extent of the oxidation can be traced by a peroxidase or hemin catalyzed fluorogenic reaction with p-hydroxyphenylacetic acid as a substrate. The concentration of the peroxide in a Brij-35 solution (0.8%, w/v) stored at room temperature for a month is in the range of 9 × 10^–5 mol/L.     

Measurement of the peroxidation of Brji-35 in aqueous solution by hemin and horseradish peroxidase catalyzed fluorogenic reaction

The non-ionic surfactant Brij-35 can easily be oxidized in solution by dissolved oxygen upon light radiation during its storage. Various factors, such as light radiation, temperature, pH, ionic strength as well as dissolved oxygen influencing the stability of Brij-35 were studied. The results showed that light radiation is the most important factor inducing the oxidation by the dissolved oxygen. The oxidized product was confirmed, indirectly, to be the peroxide of Brij-35 based on a number of evidences. The extent of the oxidation can be traced by a peroxidase or hemin catalyzed fluorogenic reaction with p-hydroxyphenylacetic acid as a substrate. The concentration of the peroxide in a Brij-35 solution (0.8%, w/v) stored at room temperature for a month is in the range of 9 × 10^–5 mol/L. Received: 8 February 1999 / Revised: 13 April 1999 / Accepted: 17 April 1999     

Adsorption at the air/water interface in dodecylammonium chloride/sodium dodecyl sulfate mixtures

The composition and properties of the adsorption films of dodecylammonium chloride/sodium dodecyl sulfate at the air/water interface depend on interactions between the film molecules and equilibria in the bulk phase (monomer-micelle and/or monomerprecipitate equilibria).The negative value of surface molecular interaction parameter ^mon calculated using the regular solution theory indicates strong attractive interactions between adsorbed molecules. Electrostatic interactions between oppositely charged ionic head groups enhance the adsorption of surfactants and decrease the minimum molar area of surfactant molecules at the air/water interface. The addition of an oppositely charged surfactant enhances packing at the air/water interface and transition from a liquid expanded to a liquid condensed state. Surface potential measurements reveal positive values for the mixtures investigated, implying the cationic surfactant ions are closer to the surface than the anionic ones.     

Adsorption of sodium dodecyl benzenesulfonate onto coal

The adsorption behavior of sodium dodecyl benzenesulfonate (NaDDBS) on a raw (as received) coal sample and its demineralized variety with 11.3% and 1.2% ash contents respectively has been studied. The samples have been characterized by their proximate analysis, particle size distribution, surface area, porosity, density, points of zero charge, etc. Adsorption of NaDDBS on these two samples has been studied as a function of concentration of NaDDBS, temperature, pH, and presence of indifferent electrolyte in the medium. It has been observed that the isotherm exhibits two adsorption plateaus below and above the critical micelle concentration (CMC) of NaDDBS. Low heats of adsorption suggest weak hydrophobic bonding between adsorbent and adsorbate. The present work aims to correlate the adsorption of surfactant onto coal particles with the rheological behavior of coal-water slurry (CWS). The results reveal that addition of a very small amount of NaDDBS (0.3 wt% of coal) to 60% (w/w) CWS results in a marked reduction of the apparent viscosity of the CWS at a shear rate of 100 s(-1). The effect of pH on the apparent viscosity of CWS with and without the presence of the surfactant is also investigated.     

Miscibility of calcium chloride and sodium dodecyl sulfate in the adsorbed film and aggregates

The adsorption, micelle formation, and coagel-particle formation of sodium dodecyl sulfate in the presence of calcium chloride were studied from the viewpoint of mixed adsorption and aggregate formation of inorganic salt and surfactant. Judging from the phase diagrams of adsorption and aggregate formation, negative azeotropy takes place in the mixed adsorption and aggregate formation of calcium chloride and sodium dodecyl sulfate due to electrostatic attraction between calcium and dodecyl sulfate ions. The miscibility of calcium chloride and sodium dodecyl sulfate in the oriented states increases in the order, particle > adsorbed film > micelle. The difference in the miscibility was ascribed to the difference in geometry between the adsorbed film and micelle and to the interaction between bilayer surfaces in the particle. The particle-micelle equilibrium was thermodynamically considered by using the equilibrium composition of aggregates.     

Adsorption of sodium dodecyl sulfate and sodium dodecyl phosphate on aluminum, studied by QCM-D, XPS, and AAS

The adsorption of two anionic surfactants, sodium dodecyl sulfate (SDS) and sodium dodecyl phosphate (SDP), at surfaces of aluminum and aluminum oxide has been studied by means of atomic absorption spectrometry (AAS), X-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance with dissipation monitoring (QCM-D). It was shown that more SDP than SDS binds to the surface and that SDP prevents dissolution of aluminum in water whereas SDS does not. This was not obvious, since the adsorption isotherms of the two surfactants to aluminum pigment powder are quite similar, as shown in an earlier work. The decreased aluminum dissolution with SDP compared to SDS was explained by the formation of a more compact protective layer with less permeability on the aluminum surface with SDP than with SDS. This is explained by differences in complexing ability between the surfactants and the aluminum pigment surface. While SDP is expected to form an inner-sphere complex with aluminum, leading to a lower accessibility of aluminum sites to water, SDS is likely to form a weaker outer-sphere complex.     

Dynamic surface tension and its diffusional decay of dodecyl ethoxylates with different homologue distribution

Dynamic surface tension and its diffusional decay have been studied with four different polydisperse C12E7 at different temperatures and different concentrations. The CMC and the headgroup area from equilibrium surface tension were shown with polydispersity and temperature. The chain length of oxyethylene on the surface was derived from comparison between the headgroup area of monodisperse dodecyl ethoxylates and that of polydisperse C12E7. The values for (Deff/D) were deduced with a diffusion-controlled adsorption model using parameters obtained from equilibrium surface tension. It was shown at short adsorption time that molecules were really adsorbed onto the surface in a diffusion-controlled manner. At a comparably long adsorption time, the ratios (Deff/D) were calculated by assuming the selective adsorption onto the surface. The modified Arrhenius-type equation was proposed by putting a concentration term in front of the exponential terms. The modified Arrhenius-type equation gave Ea=30 kJ/mole for this system. Ea directly derived without an Arrhenius plot was between 9 to 11 kJ/mole. It was an indication that the activation energy alone was not enough to explain the decay of dynamic surface tensions.     

Aminoacetal derivative of calix[4]resorcinarene. Acid-base properties and reactions with copper(II) and lanthanum(III) in various media

Studies were carried out by means of pH-metry, spectrophotometry, and mathematic simulation of equilibria (program CPESSP) on the state, acid-base properties and complexing ability [with respect to copper(II) and lanthanum(III)] of calix[4]resorcinol with acetal groups in the aminomethyl substituent [H₈L: R¹ = C₇H₁₅, R² = CH₂N(CH₃)CH₂CH₂CH(OCH₃)₂] in water-alcohol solution [80 vol % of i-PrOH] and in solutions of nonionogenic and ionic surfactants [Triton X-100, Brij-35, sodium dodecyl sulfate]. In all environments protonated and deprotonated tetrameric, dimeric, and monomeric species were found (pH range 3–10.6). In the presence of sodium dodecyl sulfate dimer (H₈L)₂ did not form that was the main speciation form in the other solvents. The sodium dodecyl sulfate like also Brij-35 favors formation of a hexadecamer (H₈L)₁₆ (pH about 6.3–6.5) in relatively small maximum accumulation fractions, 20 and 16% respectively. The solubility of H₈L in acid medium in the presence of cetyltrimethylammonium bromide, and in the presence of sodium dodecyl sulfate in alkaline medium, originated from the formation of mixed cationic and mixed anionic aggregates respectively. In a water-alcohol solution six complexes of copper(II) were detected: four mononuclear, one binuclear, and one tetranuclear complex with neutral and deprotonated forms of the ligand. Lanthanum(III) formed nine mononuclear complexes and in general coordinated more ligands than copper(II) favoring association (aggregation) of the compound under study.