Mixed Micelle Systems Research Articles

Simulating micelle self-assembly to assess potential for viscosity build in surfactant formulations

Self-assembly of surfactants into complex structures is key to the performance of many formulated products, which form a significant fraction of the world’s manufactured goods. Here we adopt the dissipative particle dynamics simulation approach to explore the self-assembly process of surfactants, with the aim of understanding what information can be obtained that may correlate with an increased zero-shear viscosity of surfactant based products. To this end we experimentally measured the zero-shear viscosity of mixed micelle systems comprised of cocoamidopropyl betaine (CAPB) and sodium lauryl sarcosinate (SLSar), as a function of the CAPB/SLSar mass ratio and pH, and characterised the early stages of self-assembly of the same systems computationally. From simulation we identify three distinct behaviors in the micellar self-assembly process (logarithmic, linear and cubic growth) which we find show some degree of correlation with the experimental zero-shear viscosity. Owing to the relatively short simulation times required, this may provide formulation scientists with a practical route to identify regions of interest (i. e. those with a desired zero-shear viscosity) prior to synthesising de novo (potentially natural) surfactants.

Tween 80-Based Self-Assembled Mixed Micelles Boost Valsartan Transdermal Delivery.

Valsartan (Val) is an important antihypertensive medication with poor absorption and low oral bioavailability. These constraints are due to its poor solubility and dissolution rate. The purpose of this study was to optimize a mixed micelle system for the transdermal delivery of Val in order to improve its therapeutic performance by providing prolonged uniform drug levels while minimizing drug side effects. Thin-film hydration and micro-phase separation were used to produce Val-loaded mixed micelle systems. A variety of factors, including the surfactant type and drug-to-surfactant ratio, were optimized to produce micelles with a low size and high Val entrapment efficiency (EE). The size, polydispersity index (PDI), zeta potential, and drug EE of the prepared micelles were all measured. The in vitro drug release profiles were assessed using dialysis bags, and the permeation through abdominal rat skin was assessed using a Franz diffusion cell. All formulations had high EE levels exceeding 90% and low particle charges. The micellar sizes ranged from 107.6 to 191.7 nm, with average PDI values of 0.3. The in vitro release demonstrated a uniform slow rate that lasted one week with varying extents. F7 demonstrated a significant (p < 0.01) transdermal efflux of 68.84 ± 3.96 µg/cm2/h through rat skin when compared to the control. As a result, the enhancement factor was 16.57. In summary, Val-loaded mixed micelles were successfully prepared using two simple methods with high reproducibility, and extensive transdermal delivery was demonstrated in the absence of any aggressive skin-modifying enhancers.

Study of the properties of binary systems: Selected derivatives of B-vitamins-cationic surfactant

Aqueous binary solutions consisting of CTAB and niacinamide or d-panthenol were tested using conductometric, spectroscopic and tensiometric methods in two concentration variants. Based on the conductometric tests, the CMC values were determined and the interaction parameter (β) was estimated. Slight antagonistic interactions were found in mixed micelle systems. Based on tensiometric measurements, the parameters of the adsorption at the interface were determined, the adsorption process was thermodynamically more preferred for the CTAB-niacinamide system. The Wilhelmy method was used to determine the effect of niacinamide and d-panthenol addition on the value of the contact angle and thus on the wetting properties of the binary systems tested. UV–vis analysis enabled the identification and understanding of structural changes and interactions occurring in the studied binary systems. Moreover free Radical Scavenging Capacity of investigated systems was evaluated using DPPH• assay. IC50 value indicated weak antioxidant activity of niacinamide. An increase in Free Radical Scavenging Capacity of CTAB-N system was observed compared to pure niacinamide.

Comparative studies on the aggregate formation of synthesized zwitterionic gemini and monomeric surfactants in the presence of the amphiphilic antipsychotic drug chlorpromazine hydrochloride in aqueous solution: an experimental and theoretical approach.

The formation of aggregates, which are widely used in the field of biochemistry and the medical industry, was studied with different compositions of alkyl betaine gemini surfactant (C14Ab) in conjugation with chlorpromazine hydrochloride (CPZ). The results were compared with those of a single-chain zwitterionic surfactant (C12DmCB) of the same type with CPZ. Dynamic light scattering (DLS), confocal laser scanning microscopy (CLSM), and transmission electron microscopy (TEM) methods were used to distinguish the aggregates for the CPZ/C14Ab system in aqueous solutions above a certain mole fraction of the drug CPZ (αCPZ = 0.2). Time-resolved fluorescence decay measurements of acridine orange revealed relative polarity near the head group regions of mixed micelle (CPZ/C14Ab and CPZ/C12DmCB) systems. The hydrophilic environment around the head group regions of the CPZ/C14Ab system was different from that in the case of the CPZ/C12DmCB system. On the other hand, several theoretical models were employed (Clint, Rubingh, Motomura, and SPB) for mixed micellar systems to elucidate the different interaction parameters. Such a systematic study of a zwitterionic gemini amphiphile and its interaction with other amphiphiles and an amphiphilic drug molecule is rare in the literature.

Investigating the effect of dipropylene glycol and mixed-surfactant concentrations on perfume release

HypothesisThe solubilization and release of perfume from aqueous micellar solutions are to a large extent dependent on the geometric, surface and electrostatic properties of the micelle. These properties are influenced by the molecular structure of the surfactant, the solvent properties, and additives (hydrotropes). Both surfactant and co-solvent are expected to affect solvent polarity, hence addition of a co-solvent of lower polarity than water should reduce the size of micelles, whereas addition of surfactant should increase the size of micelles. ExperimentThe effect of dipropylene glycol and surfactant concentration on micelle size and perfume release were systematically studied on (a) branched-chain sodium trideceth-2 sulfate (ST2S)/cocamidopropyl betaine (CAPB), and (b) linear-chain sodium laureth-1 sulfate (SLE1S)/CAPB mixed-surfactant systems. Combined small-angle neutron scattering (SANS) and GC-MS studies were conducted to elucidate the impact of co-solvent and surfactant on fractional charge/size of micelle and perfume release from formulations. Principal component analysis (PCA) was done to determine if the 12-PRM perfume accord could be simplified into smaller groups of PRMs. Statistical analysis was performed to determine if perfume release was significantly correlated to any aspects of the colloidal domain as a function of DPG or surfactant concentration. FindingsThe micelles shrank, aggregation numbers decreased, and fractional charges increased with increasing DPG concentration, whereas the opposite occurred with increasing surfactant concentrations. These trends were largely consistent for both mixed-micelle systems. GC-MS headspace concentration results further demonstrated that although the perfume headspace concentrations typically follow certain trends, performance is largely dependent on surfactant and DPG concentration. PCA showed that the 12-PRM accord could be treated as two components of perfume molecules, with florol strongly contributing to the second component. Statistical analysis showed that the first component significantly correlated to the micelle size and volume fraction as a function of DPG and surfactant concentration in both mixed-surfactant systems. This is the first time the release of a multi-component perfume accord was systematically studied as a function of DPG and surfactant concentration, and how DPG and surfactant variation affect the colloidal domain in the presence of a multi-component perfume accord.

Solubilization of direct black 2 in mixed micellar media: insights from spectroscopic and conductometric measurements

The core-shell architecture of surfactant micelles serves as a super solvent to remove inevitable organic pollutants from industrial wastewater. In the present study, the synergistic mechanism improving the aqueous solubility of Direct Black 2 dye by a single cationic surfactant, cetyltrimethylammonium bromide (CTAB), and its composite solutions with nonionic surfactants, Triton X-100 (TX-100) and Tween 20 (Tw-20), has been studied in detail. The spectral behavior of dye was monitored in the presence of a surfactant from its pre-to-post micellar concentration. It was found that solubilization was greatly enhanced in the cationic-nonionic mixed micelle systems. The value of the binding constant (Kb ) was 9.10 × 104 dm3/mol for CTAB, which was increased significantly increased to 58.45 × 104 dm3/mol for CTAB + Tw-20 and 86.82 × 104 dm3/mol for CTAB + TX-100. The value of the partition coefficient (Kx ) also increased in a similar manner. The observed order of the extent of dye solubilization in micellar system was: CTAB < CTAB + Tw-20 < CTAB + TX-100. The enhanced solubility of dye in mixed surfactant solutions was attributed to higher aggregation number, lower critical micelle concentration (CMC) and more hydrophobic interior of micelle. Moreover, the degree of ionization degree (β), the CMC and related thermodynamic parameters were calculated from conductometric measurements, the results of which showed that the solubilization process in all types of surfactant systems is spontaneous and thermodynamically feasible, which may contribute to the sustained release of active-ingredients through self-assemblies.

Why is glycocholic acid sodium salt better than deoxycholic acid sodium salt for the preparation of mixed micelle injections?

Classical mixed micelle systems make excellent parenteral drug carriers for lipophilic or poorly soluble drugs, but many formulations details are not fully understood and need further study. Thus, we constructed mixed micelle systems with lecithin and either glycocholic acid sodium salt or deoxycholic acid sodium salt in order to investigate the differences between the bile salts. Vitamin K1, a lipid‐soluble drug, was encapsulated in the mixed micelles, and the influence of bile salts on the quality and stability of the mixed micelle systems was analyzed. Both bile salts displayed similar profiles, and the amounts of bile salts used in formulating clear solutions did not differ. Mixed micelle systems formed from glycocholic acid sodium were physically stable at low pH levels (5.5), whereas those formed from deoxycholic acid required higher pH (>8.5). High pH levels hurt active pharmaceutical ingredients that are prone to hydrolytic and oxidative degradation. Hence, when mixed micelle systems formed from deoxycholic acid sodium were sterilized, unexpected chemical unstability occurred. Therefore, we conclude that glycocholic acid sodium salt is more suitable than deoxycholic acid sodium salt for the preparation of mixed micelle injections.

&lt;p&gt;Solutol&lt;sup&gt;®&lt;/sup&gt;HS15+pluronicF127 and Solutol&lt;sup&gt;®&lt;/sup&gt;HS15+pluronicL61 mixed micelle systems for oral delivery of genistein&lt;/p&gt;

Purpose: We aimed to prepare two oral drug delivery systems consisting of polyoxyl 15 hydroxystearate (HS15) with pluronicF127 (F127) and HS15 with pluronicL61 (L61) to overcome the challenges of genistein’s poor oral bioavailability. This provides a good strategy for enhancing the potential value of genistein.Methods: We designed two binary mixed micelle systems employing the organic solvent evaporation method using surfactants (HS15, L61, and F127). Formulations (GEN-F and GEN-L) were characterized by transmission electron microscopy. Drug content analysis, including entrapment efficiency (EE%), drug loading (DL%), and the cumulative amount of genistein released from the micelles, was performed using HPLC. The permeability of optimum formulation was measured in Caco-2 cell monolayers, and the oral bioavailability was evaluated in SD rats.Results: The solutions of GEN-F and GEN-L were observed to be transparent and colorless. GEN-F had a lower EE% of 80.79±0.55% and a DL% of 1.69±0.24% compared to GEN-L, which had an EE% 83.40±1.36% and a DL% 2.26±0.18%. TEM results showed that the morphology of GEN-F and GEN-L was homogeneous and resembled a spherical shape. The dilution and storage conditions had no significant effect on particle size and EE%. Genistein demonstrated a sustained release behavior when encapsulated in micelles. Pharmacokinetics study showed that the relative oral bioavailability of GEN-F and GEN-L increased by 2.23 and 3.46 fold while also enhancing the permeability of genistein across a Caco-2 cell monolayer compared to that of raw genistein.Conclusion: GEN-F and GEN-L as a drug delivery system provide an effective strategy for enhancing and further realizing the potential value of GEN.

Synergism and Physicochemical Properties of Anionic/Amphoteric Surfactant Mixtures with Nonionic Surfactant of Amine Oxide Type

The physicochemical properties of initial formulation, that is anionic/amphoteric surfactants mixture SLES/AOS/CAB (sodium lauryl ether sulfate (SLES), α-olefin sulfonates (AOS) and cocamidopropyl betaine (CAB) at ratio 80 : 15 : 5) with nonionic surfactant of amine oxide type (lauramine oxide (AO)) in various concentration (1–5%) were studied. To characterize the surfactants mixture, the critical micelle concentration (CMC), surface tension (γ), foam volume, biodegradability and irritability were determined. This study showed that adding of AO in those mixtures lowered both γ and CMC as well as enhanced SLES/AOS/CAB foaming properties, but did not significantly affect biodegradability and irritability of initial formulation. Moreover, an increase in AO concentration has a meaningful synergistic effect on the initial formulation properties. All those results indicates that a nonionic surfactant of amine oxide type significantly improves the performance of anionic/amphoteric mixed micelle systems, and because of that anionic/amphoteric/nonionic mixture can be used in considerably lower concentrations as a cleaning formulation.

Precipitate-Coacervate Transformation in Polyelectrolyte-Mixed Micelle Systems.

The polycation/anionic-nonionic mixed micelle, poly(diallyldimethylammonium chloride)-sodium dodecyl sulfate/Triton X-100 (PDADMAC-SDS/TX100), is a model polyelectrolyte-colloid system in that the micellar mole fraction of SDS (Y) controls the micelle surface charge density, thus modulating the polyelectrolyte-colloid interaction. The exquisite temperature dependence of this system provides an important additional variable, controlling both liquid-liquid (L-L) and liquid-solid (L-S) phase separation, both of which are driven by the entropy of small ion release. In order to elucidate these transitions, we applied high-precision turbidimetry (±0.1 %), isothermal titration calorimetry, and epifluorescence microscopy which demonstrates preservation of micelle structure under all conditions. The L-S region at large Y including precipitation displays a remarkable linear, inverse Y-dependence of the L-S transition temperature Ts. In sharp contrast, the critical temperature for L-L coacervation Tφ, shows nearly symmetrical effects of positive and negative deviations in Y from the point of soluble complex neutrality, which is controlled in solution by the micelle charge and the number of micelles bound per polymer chain n (Zcomplex = Zpolymer + nZmicelle). In solid-like states, n no longer signifies the number of micelles bound per polymer chain, since the proximity of micelles inverts the host-guest relationship with each micelle binding multiple PE chains. This intimate binding goes hand-in-hand with the entropy of release of micelle-localized charge-compensating ions whose concentration depends on Y. These ions need not be released in L-L coacervation, but during L-S transition their displacement by PE accounts for the inverse dependence of Ts on micelle charge, Y.

The formation of mixed micelles of sugar surfactants and phospholipids and their interactions with hyaluronan

The aggregation of sugar surfactants and the incorporation of phospholipids into sugar surfactant micelles were investigated by means of fluorescence spectroscopy. Two representatives from the family of alkyl glucosides were studied: dodecyl-β-D-maltoside and octyl-β-D-glucopyranoside. The presence of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) in the sugar surfactant system promoted the formation of new structures at sugar surfactant concentration below its critical micelle concentrations. According to the aggregation number measurements studied by a fluorescence quenching of hexadecylpyridinium chloride measurements, the premicellar aggregates formed from octyl-β-D-glucopyranoside and DPPC were composed of the third of molecules compared to the octyl-β-D-glucopyranoside micelles. Interactions of the formed mixed micelle systems composed of sugar surfactant and DPPC with hyaluronan were explored. The addition of hyaluronan had different effects on the dodecyl-β-D-maltoside/DPPC and octyl-β-D-glucopyranoside/DPPC mixed systems. In addition, the mixed system of dodecyl-β-D-maltoside and lecithin was studied, but possibly, only a coexistence of lipid aggregates and sugar surfactant micelles was observed.

Mixed reverse micellar extraction and effect of surfactant chain length on extraction efficiency

Abstract This study investigated the effect of surfactant chain length on mixed reverse micellar extraction of cellulase. The mixed reverse micelle systems were formed by biosurfactant rhamnolipid and a nonionic surfactant tween (tween-20, tween-40, tween-60, or tween-80). Electron spin resonance (ESR) method was used in order to find the suitable chain length of tween and the best ratio of rhamnolipid/tween mixture. The key parameters such as pH and temperature in forward and backward extraction were studied, and the results showed that mixed reverse micelle with different surfactant chain length had different pH sensitivities. Rhamnolipid/tween-80 system was more sensitive to pH than other systems, and rhamnolipid/tween-20 system was the least sensitive to pH among four systems. The experimental results also showed that chain length had limited sensitivity to temperature. Rotation correlation time of cellulase solutions (original and extraction) were investigated, which proved the superiority of longer chain length system (rhamnolipid/tween-80 system).

Performance and Efficiency of Anionic Dishwashing Liquids with Amphoteric and Nonionic Surfactants

AbstractPerformance and efficiency of anionic [sodium lauryl ether sulfate (SLES) and sodium α‐olefin sulfonate (AOS)] and amphoteric [cocamidopropyl betaine (CAB)] as well as nonionic [cocodiethanol amide (DEA), various ethoxylated alcohols (C12–C15–7EO, C10–7EO and C9–C11–7EO) and lauramine oxide (AO)] surfactants in various dishwashing liquid mixed micelle systems have been studied at different temperatures (17.0, 23.0 and 42.0 °C). The investigated parameters were critical micelle concentration (CMC), surface tension (γ), cleaning performance and, foaming, biodegradability and irritability of anionic (SLES/AOS) and anionic/amphoteric/nonionic (SLES/AOS/CAB/AO) as well as anionic/nonionic (SLES/AOS/DEA/AO, SLES/AOS/C12‐C15‐7EO/AO, SLES/AOS/C10–7EO/AO and SLES/AOS/C9–C11–7EO/AO) dishwashing surfactant mixtures. In comparison to the starting binary SLES/AOS surfactant mixture, addition of various nonionic surfactants promoted CMC and γ lowering, enhanced cleaning performance and foaming, but did not significantly affect biodegradability and irritability of dishwashing formulations. The anionic/nonionic formulation SLES/AOS/C9–C11–7EO/AO shows both the lowest CMC and γ as well as the best cleaning performance, compared to the other examined dishwashing formulations. However, the results in this study reveal that synergistic behavior of anionic/nonionic SLES/AOS/ethoxylated alcohols/AO formulations significantly improves dishwashing performance and efficiency at both low and regular dishwashing temperatures (17.0 and 42.0 °C) and lead to better application properties.

A proof-of-concept for folate-conjugated and quercetin-anchored pluronic mixed micelles as molecularly modulated polymeric carriers for doxorubicin

Pluronic, F127 (PEG–PPO–PEG, Mn = 12,500 g/mol) and reverse pluronic, 10R5 (PPO–PEG–PPO, Mn = 2000 g/mol) were molecularly modulated to reach multifunctional mixed micelle systems aiming to overcome some of the inherent weaknesses of pluronic based drug delivery systems. Targeting function was introduced by covalent attachment of folic acid to F127 (F127-FA), while quercetin was anchored to 10R5 (P-Q). The successful syntheses were evidenced by 1H NMR, FTIR, DSC and UV–Vis. The proof-of-concept for the mixed micelles prepared from the drug anchored pluronics was demonstrated through reduced CMCs, slower release rates and increased Doxorubicin (DOX) encapsulation capacity from ∼19% to ∼43%. Quercetin therefore boosted the interactions of DOX with the hydrophobic core of the micelles. This was further evidenced by colloidal probe AFM which demonstrated almost doubled adhesion forces between the DOX coated probe and the quercetin modified pluronic as compared to the plain pluronic. The pre-biological essay of the DOX-modulated mixed micelle demonstrates promising properties. In addition quercetin has previously been proposed as combinatory drug to DOX enhancing its therapeutic function and reducing the side effects to normal cells.

Affinity purification of recombinant human cytochrome P450s 3A4 and 1A2 using mixed micelle systems

Recombinant cytochrome P450 (CYP or P450) enzymes are useful for drug metabolism research and thereby many expression and purification systems have been developed. Here, we provide a method for the purification of human P450s 3A4 and 1A2 expressed in Escherichia coli using mixed micelles containing anionic phospholipids. This method does not require any protein-tagging system for protein isolation and has a further advantage that the purification is concomitantly conducted with reconstitution of the enzymes into a phospholipid environment, which is crucial for the catalytic activity assay of P450 enzyme. This method may also be applied to high-throughput catalytic assays of the enzymes because the purification procedures can be undertaken in a 96-well plate.

Partition Coefficients of Ionizable Solutes in Mixed Nonionic/Ionic Micellar Systems

Surfactant solutions in practical applications usually are mixtures of ionic and nonionic surfactants. Because of synergistic effects, the solubilization of hydrophobic compounds can be enhanced while decreasing the needed amount of surfactant at the same time. In this work, the influence of the composition of Brij 35/CTAB and Brij 35/SDS mixed micelles on the partition coefficient log D(MW) of various acids and bases over the entire pH range was investigated. Two experimental methods (MLC, micellar liquid chromatography; MEUF, micellar enhanced ultrafiltration) are evaluated for the determination of partition coefficients in mixed-micelle systems. Although MLC stands out because of its automation and easy handling, MEUF is applicable to a broader log D(MW) range. It is shown that the partitioning can be influenced dramatically by the two investigated parameters. By adjusting the pH value and the composition of the micelles, we can tailor the partition behavior of solutes for virtually any application. The thermodynamic model COSMO-RS gives valuable predictions of the partition coefficients if the composition of the micelle is available. Different approaches for the description of the micellar composition are evaluated in this work. On the basis of the cmc value of the single surfactants and the mixture only, it is shown that the regular solution approximation gives reasonable micellar compositions. The partition coefficients between water and the mixed micelles are predicted with the COSMO-RS model, in good agreement with the experimental data. Moreover, the micellar composition can be evaluated by fitting the prediction to the experimentally determined partition coefficients.

Nonideal Mixed Micelles of Fluorinated and Hydrogenous Surfactants in Aqueous Solution. NMR and SANS Studies of Anionic and Nonionic Systems

Contrast variation SANS and (19)F chemical shifts were measured for three mixed equimolar micelle systems: sodium perfluorooctanoate (SPFO) and sodiumdecylsulfate (SDeS) in 200 mM NaCl, lithium perfluorononanate (LiPFN) and lithium dodecylsulfate (LiDS) in 200 mM LiCl, and a nonionic system C(8)F(17)C(2)H(4)(OC(2)H(4))(9) and C(12)H(25)(OC(2)H(4))(8) in water, all at 25 degrees C. The chemical shift measurements allow the calculation of the average fraction of nearest neighbors of each kind around the reporter group (the trifluoromethyl group). A preference for like neighbors were found in all systems, smallest in the SDeS/SPFO system and largest in the nonionic system, but in all cases substantially smaller than expected at critical conditions. From the SANS measurements the width of the micelle composition distribution was obtained. For the ionic systems similar values were obtained, showing a broadening compared to ideal mixtures, but not broad enough for demixing or clearly bimodal distributions. In the nonionic system the width was estimated as sigma = 0.18 and 0.22 using two different evaluation methods. These values suggest that the system is close to critical conditions. The lower value refers to a direct modeling of the system, assuming an ellipsoidal shape and a Gaussian composition distribution. The modeling showed the nonionic mixed micelles to be prolate ellipsoids with axial ratio 2.2 and an aggregation number larger than 100, whereas the two ionic systems fitted best to oblate shapes (axial ratios 0.8 and 0.65 for SDeS/SPFO and LiDS/LiPFN, respectively) and aggregation numbers of 60 for both.

Mixed micelle systems formed from critical micelle concentration and temperature-sensitive diblock copolymers for doxorubicin delivery

Mixed micelles formed by critical micelle concentration (Cmc) character's diblock copolymer, and temperature-sensitive character's diblock copolymer were studied for their capabilities in improving stability with and without drug. Experimental results showed that this type of mixed micellar systems possessed complementary effects in adjusting external temperature shift (storage vs. body temperature) and concentration change (dilution after intravenous injection). Therefore, they provided excellent stability as drug carriers. Further results obtained from physicochemical property and drug release kinetics studies demonstrated such systems could also be applied to physiological conditions, in compliance with varied pH environments. We concluded that the newly developed mixed micelles can serve as a potential injectable drug delivery system for anticancer drugs, such as doxorubicin and many others.

Using micellar mole fractions to assess membrane protein stability in mixed micelles

The increased focus on the structural and physical properties of membrane proteins has made it critical to develop methods that provide a reliable estimate of membrane protein stability. A simple approach is to monitor the protein's conformational changes in mixed detergent systems, typically consisting of an anionic (denaturing) and non-ionic (non-denaturing) component. Linear correlations between, e.g., the melting temperature and the bulk mole fraction of the anionic component have been observed. However, a potential complication is that the bulk mole fraction is not identical to the mole fraction in the mixed micelle, which is the local environment experienced by the membrane protein. Here, we present an extensive analysis of the thermal stability of the membrane-integrated domain of the outer membrane protein AIDA in the presence of different mixed micelles. In the micelle system SDS-octyl-polyoxyethylene, the melting temperature in the absence of SDS extrapolates to 113 °C using bulk mole fractions. However, for mixed micelles involving short-chain detergents or phospholipids, the melting temperature calculated using bulk mole fractions reaches values up to several hundred degrees higher than 113 °C and can only be obtained by extrapolation over a narrow mole fraction interval. Furthermore, there is a non-linear relationship between the melting temperature and bulk mole fractions for mixed micelle systems involving cationic detergents (also denaturing). We show that if we instead use the micellar mole fraction as a parameter for denaturing detergent strength, we obtain linear correlations which extrapolate to more or less the same value of the melting temperature. There remains some scatter in the extrapolated values of the melting temperature in different binary systems, which suggest that additional micellar interactions may play a role. Nevertheless, in general terms, the mixed micellar composition is a good parameter to describe the membrane protein's microenvironment. Note, however, that for the mixed micelle system involving SDS and dodecyl maltoside, which has been used by several research groups to determine membrane protein stability, the estimate provided by bulk mole fraction leads to similar values as that of micellar mole fractions.

Modified micellar electrokinetic chromatography in the analysis of catechins and xanthines in chocolate.

Modified micellar electrokinetic chromatography (MEKC) analysis of monomeric flavanols (catechin and epicatechin) and methylxanthines (caffeine and theobromine) in chocolate and cocoa was performed by using sodium dodecyl sulfate (SDS) as a principal component of the running buffer. Because of the reported poor stability of catechins in alkaline solutions, acidic conditions (pH 2.5) were chosen and consequently the electroosmotic flow (EOF) was significantly suppressed; this resulted in a fast anodic migration of the analytes partitioned into the SDS micelles. Under these conditions, variations of either pH value in acidic range or SDS concentration, showed to be not suitable to modulate the selectivity. To overcome this limit, use of additives to the SDS-based running buffer was successfully applied and three different systems were optimized for the separation of (+)-catechin, (-)-epicatechin, caffeine, and theobromine in chocolate and cocoa powder samples. In particular, two mixed micelle systems were applied; the first consisted of a mixture of SDS and 3-[(3-cholamidopropyl)dimethylammonio]-1-propansulfonate (CHAPS) with a composition of 90 mM and 10 mM, respectively; the second was SDS and taurodeoxycholic acid sodium salt (TDC) with a composition of 70 mM and 30 mM, respectively. A further MEKC approach was developed by addition of 10 mM hydroxypropyl-beta-cyclodextrin (HP-beta-CD) to the SDS solution (90 mM); it provided a useful cyclodextrin(CD)-modified MEKC. By applying the optimized conditions, different separation profiles of the flavanols and methylxanthines were obtained showing interesting potential of these combined systems; their integrated application showed to be useful for the identification of the low level of (+)-catechin in certain real samples. The CD-MEKC approach was validated and applied to the determination of catechins and methylxanthines in aqueous extracts from four different commercial chocolate types (black and milk) and two cocoa powders.