Effects of surfactant micelles on solubilization and DPPH radical scavenging activity of Rutin

The interaction of the antioxidant Rutin with the radical DPPH (2,2-diphenyl-1-picrylhydrazyl) in presence of cationic (CTAB, TTAB, DTAB), non-ionic (Brij78, Brij58, Brij35), anionic (SDS) and mixed surfactant systems (CTAB-Brij58, DTAB-Brij35, SDS-Brij35) has been followed by spectrophotometric and tensiometric methods to evaluate the DPPH radical scavenging activity (RSA) of Rutin in these model self-assembled structures. The results show that the solubilization capacity of various single surfactant systems for both DPPH as well as Rutin followed the order cationics > non-ionics > anionic. The radical scavenging activity of Rutin in the solubilized form was higher within ionic micelles than in non-ionic micelles. However, the antioxidant exhibited enhanced activity for the radical in mixed cationic-non-ionic micelles compared with any of the single component micelles. In contrast, anionic-non-ionic mixed micelles modulated the activity of Rutin in-between that seen for pure anionic and non-ionic micelles only.     

Characterization of micelles of polyoxyethylene nonylphenol (Igepal) and its complexation with 3,7-diamino-2,8-dimethyl- 5-phenylphenazinium chloride

This paper explores the association of a nonionic surfactant, Igepal (polyoxyethylene nonylphenol), in aqueous media by means of absorption and fluorescence spectroscopy. The critical micellar concentration (CMC) of the aggregate formed in the aqueous medium has been determined, using three different methods: UV-visible spectroscopy, fluorescence spectroscopy, and Stokes shift. The correlation of CMC with hydrophile-lipophile balance (HLB) indicates that the CMC decreases with decreased HLB. The obtained CMC values from different methods are close to each other and have allowed the determination of DeltaG values associated with the micellization. The association constant of the dye molecule Safranine T (ST) with the nonionic micelle of Igepal, aggregation number of the surfactant monomer, and location of the fluorophore in the micellar environment have been determined. The vertical ionization potential of Igepal, electron affinity of the dye, and degree of charge transfer from the micellar aggregate to the dye molecule have been determined by AM1 calculation. The experimental charge transfer transition energies are well correlated with the determined ionization potential values (ID) of Igepal. The degree of charge transfer (ground state complexes) has been found to be low. The polarity of the micelle solubilization sites has been estimated from the solvatochromic shift, Kosowar Z value, and ET30, and ETN values.     

Cyclic voltammetry and viscosity measurements of aggregated assemblies of anionic surfactants with nonionic surfactants and triblock copolymers

Cyclic voltammetry (CV) and viscosity measurements have been employed to study the aggregation behavior of mixed micellar systems of anionic surfactant (dioctyl sulfosuccinate sodium salt, AOT) with conventional nonionic surfactants such as Brij 35/TritonX-100/Tween 20/Tween 80/Myrj 45 and two triblock copolymers (L64 and F68). Critical micelle concentration (cmc) values have been determined for various micellar systems from CV measurements using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as an electroactive probe at 25 °C. Diffusion coefficient (D) has been evaluated from Randles–Sevcik equation which showed an overall decrease for most of the binary systems. The negative values of interaction parameters (β) obtained from regular solution theory suggest the synergistic behavior in all the binary systems except AOT + Tween 80 mixtures. The mixed systems of AOT with triblock copolymers showed stronger synergistic interactions than that of mixed systems of AOT with nonionic surfactants. A comparative evaluation of mixed systems of anionic surfactants AOT and sodium dodecyl sulfate with Myrj 45 and AOT + L64 and F68 has been made on the basis of different micellar parameters and structural properties of surfactants. Viscosity measurements also show similar type of interactions in the mixed micelles.     

Activity of α‐Chymotrypsin in Cationic and Nonionic Micellar Media: Ultraviolet and Fluorescence Spectroscopic Approach

α‐Chymotrypsin (α‐CT) activity was measured in aqueous buffer with the following alkyltriphenylphosphonium bromide surfactants in the series cetyl, tetradecyl, and dodecyl as a tail length. For the sake of comparison with mixed micellar investigation on activity of α‐CT, cationic cetyltriphenylphosphonium bromide (CTPB) and nonionic surfactant Triton X‐100, Brij‐56, Brij‐35, Tween 20, and Igepal Co‐210 have been used. The p‐nitrophenyl acetate (PNPA) hydrolysis rate was determined at the surfactant concentration of both cationic and mixed micellar systems by a UV–vis spectrophotometer. The catalytic reaction follows the Michaelis–Menten mechanism, and the catalytic efficiency (k_cat/K_M) was evaluated for both homogeneous and mixed‐micellar media. The maximum catalytic efficiency was observed at 5 mM concentration of CTPB, but the highest catalytic efficiency, 572 M^?1 s^?1, was measured in the presence of mixed micellar (7.5 mM CTPB + 2.5 mM Tween‐20). The fluorescence (FL) spectra showed the differences of α‐CT conformations in the presence of cationic surfactants. The FL results suggest that the influence of cationic surfactant on proteolysis arises from the interaction with the α‐CT. The binding constant, k_sv, of α‐CT with cationic aggregates was determined in the buffer using the Stern–Volmer equation by the fluorescence spectroscopic approach.     

Novel fluorescent probe as aggregation predictor and micro-polarity reporter for micelles and mixed micelles

Aggregational behaviour of micelles sodium dodecyl sulphate (SDS and Triton X-100, TX-100 both in pure and mixed form) and micelle like aggregates such as polymer-surfactant system [polymer poly(vinyl pyrrolidone), PVP]-SDS have been studied by using fluorescence characteristics of a newly synthesized probe. The critical micelle concentration (CMC) values determined at various surfactant compositions are lower than the ideal values indicating a synergistic effect. The value of the interaction parameter for the surfactant mixture has been determined which agrees well with the value calculated according to molecular thermodynamic theory. The total aggregation number of surfactant in mixed micelle shows a drastic variation in the SDS mole fraction range 0 < or = alpha1 < or = 0.3 and beyond the range it remains practically constant. Molar-based partition coefficients for the dye between the micellar and aqueous phase have been determined and a non-linear variation is obtained for the mixed micellar system. Variations of micro-polarity in the mixed micellar region have been investigated as a function of surfactant composition and results have been explained in terms of a suitable realistic model.     

Separation and selectivity in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or Tween 20-modified mixed micelles

The separation and selectivity of eight aromatic compounds ranging from hydrophilic to hydrophobic properties in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or Tween 20-modified mixed micelles were investigated. The effect of different operation conditions such as SDS and Tween 20 modifier surfactant concentration, buffer pH, and applied voltage was studied. The resolution and selectivity of analytes could be markedly affected by changing the SDS micelle concentration or Tween 20 content in the mixed micelles. Applied voltage and pH of running buffers were used mainly to shorten the separation time. Complete separation of eight analytes could be achieved with an appropriate choice of the concentration of SDS micelles or Tween 20-modified mixed micelles. Quicker elution and better precision could be obtained with SDS-Tween 20 mixed micelles than with SDS micelles. The mechanisms that migration order of those analytes was mainly based on their structures and solute-micelle interactions, including hydrophobic, electrostatic, and hydrogen bonding interactions, were discussed.     

The interaction of isomeric hexanediols with sodium dodecyl sulfate and dodecyltrimethylammonium bromide micelles

The micelle formation process for a typical anionic surfactant, sodium dodecyl sulfate, and a typical cationic surfactant, dodecyltrimethylammonium bromide, has been investigated in a series of mixed solvents consisting of different concentrations of isomeric hexanediols (1,2-hexanediol and 1,6-hexanediol) in water. The critical micelle concentrations and the degrees of counterion dissociation of the mixed micelles were obtained from conductance experiments. Luminescence probing experiments have been used to determine the concentration of micelles in solution and, hence, the micellar aggregation numbers of the surfactants in the mixed solvent systems. The alcohol aggregation numbers were determined by combining the partition coefficients (obtained using NMR paramagnetic relaxation enhancement experiments) with the micellar concentrations from the luminescence probing experiments. All these results are interpreted in terms of the difference in the interaction of the isomeric hexanediols with the surfactant as a function of the position of the hydroxyl groups on the six-carbon chain of the alcohol. Received: 28 June 2000/Accepted: 5 July 2000     

Thermodynamics of micellization of ionic surfactant/alkoxyethanol mixed micelles as a function of temperature

Estimates of the thermodynamic parameters of micellization ((_micG°, (_micH°, and (_micS°) have been determined for a series of mixed micelles consisting of ionic surfactants (sodium dodecylsulfate and dodecyltrimethylamonium bromide) and medium chain length alkoxyethanols as the co surfactant. The enthalpies of micellization have been measured directly for the above systems using isoperibol solution calorimetry; the Gibbs energies and entropies of micellization are obtained by application of the mass-action model to the critical micelle concentration values from the calorimetric titration experiments. The thermodynamic properties of mixed micelle formation with alcohol concentration and temperature are in excellent agreement with our previous results. However, there does appear to be some dependence of the thermodynamic properties of mixed micelle formation on the charge of the ionic surfactant. These dependencies are discussed in terms of the manner in which the ethylene oxide group of the alcohol interacts with the ionic head groups and the location of the solubilizates in the micellar interior.     

Effect of Micelle Composition on Acidic Drugs Separation Behavior by Micellar Electrokinetic Capillary Chromatography

Micellar electrokinetic capillary chromatography (MECC) is a branch of capillary electrophoretic techniques, in which surfactant micelles are added to the electrolyte solution as pseudostationary phase. Separation in MECC is based on electrophoretic mobilities of the analytes when partitioned into micelles1. In this work, four acidic drugs similar in structure with aryl carboxylic acid were separated by MECC. The effects of type of surfactant, such as anionic surfactant SDS, nonionic …     

Spectroscopic investigations on the binding of ammonium salt of 8-anilino-1-naphthalene sulfonic acid with non-ionic surfactant micelles in aqueous media

The anionic dye 8-anilino-1-napthalensulfonic acid ammonium salt, or ANS, was used as a fluorescent probe to investigate the behaviour of dye-surfactant interactions in aqueous solutions of Triton X-100 and the Brij and polyoxyethylene tridecyl ether (POE TDE) series of polyoxyethylene non-ionic surfactants. The fluorescence behaviour of the dye with the non-ionic surfactants was examined in micellar media. The concentration of surfactant was kept well above the cmc to investigate the interaction of the dye with surfactant micelles. In this investigation, the relative fluorescence enhancements, binding constants of the dye to the surfactant micelles and aggregation numbers of the micelles were determined, from the analysis of spectroscopic data.     

Effects of non-ionic surfactants on isotachophoretic separations of 2-arylpropionic acids

Non-ionic surfactant (Brij 35, Tween 20, Tween 80 and Tergitol NPX) modified capillary isotachophoresis was investigated for the separation of 2-arylpropionic acids (fenoprofen, flurbiprofen, ibuprofen, ketoprofen and naproxen) and benzoic acid and its derivatives (salicylic, acetylsalicylic and gallic acids). The relative step height (RSH) values of analytes were found to be dependent on the type and concentration of the surfactant. The strength of the affinity of the 2-arylpropionic acids to the non-ionic micelles was found to be as follows: flurbiprofen > fenoprofen > ibuprofen > naproxen > ketoprofen. In general, the RSH values of 2-arylpropionic acids increase with an increase in the concentration of surfactants. However, the RSHs of benzoic, salicylic and gallic acids are not considerably affected. Separation of all acids was obtained with the Tween 20 (1.5%, w/v) in the leading electrolyte 10 mmol L(-1) hydrochloric acid/L-histidine (pH 6.0). Changes in the fluorescence intensity of fenoprofen, flurbiprofen and naproxen were also investigated in micellar media (Tween 20, Tween 80 and Brij 35). The strength of the affinity of the 2-arylpropionic acids to the Tweens micelles was found to be as follows: flurbiprofen > fenoprofen > naproxen, which is consistent with the isotachophoretic results. On the contrary, the strength of the affinity to the Brij micelles was found to be as follows: fenoprofen > naproxen > flurbiprofen.     

混合表面活性剂在非极性溶剂中的聚集行为

表面活性剂在非极性溶剂中的聚集行为比在水溶液中复杂得多. 水溶液中表面活性剂有一明确的临界胶束浓度(CMC),而在非极性溶剂中至今对CM C概念仍有怀疑^[1], 但已有多种手段如染料增溶法、水增溶法、光散射法、荧光偏振、紫外和核磁共振谱等证实并测定了非极性溶剂中 CMC 的存在^[1～5]. 表面活性剂在非极性溶剂中以非离子化状态存在, 其缔合主要靠两亲分子之间的偶极-偶极以及离子对相互作用, 那么在一种表面活性剂溶液中加入另一种表面活性剂, 即表面活性剂的复配, 必然对其聚集行为产生重大影响, 但迄今为止, 尚未见关于混合表面活性剂在非极性溶剂中聚集行为的报道. 本文采用碘光谱法和水增溶法测定了阴离子表面活性剂AOT 和非离子表面活性剂 Brij30 混合后在正庚烷中形成反胶束的 CMC, 以期考察表面活性剂的复配对其聚集行为的影响。     

Small-angle neutron scattering of mixed ionic perfluoropolyether micellar solutions

Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.     

Association (micellization) and partitioning of aglycon triterpenoids

The reaction between methyl 4-nitrobenzenesulfonate and bromide ions has been studied in mixed single-chain-gemini micellar solutions of n-dodecyltrimethylammonium bromide, DTAB, and dodecyl tricosaoxyethylene glycol ether, Brij(35), with alkanediyl-alpha-omega-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br(-) (s=3,4,5). Kinetic micellar effects show that an increase in the solution mole fraction of the single-chain surfactant, X(single-chain), results in a diminution of the mixed micelles tendency to form spherocylindrical aggregates upon increasing surfactant concentration. The dependence of the surfactant concentration at which the sphere-to-rod transition occurs, C(*), on X(single-chain) showed through kinetic data was in agreement with results obtained by means of fluorescence measurements.     

Trifluoperazine effects on anionic and zwitterionic micelles: a study by small angle X-ray scattering

In this work small angle X-ray scattering (SAXS) studies on the interaction of the phenothiazine trifluoperazine (TFP, 2-10 mM), a cationic drug, with micelles of the zwitterionic surfactant 3-(N-hexadecyl-N,N-dimethylammonium) propane sulfonate (HPS, 30 mM) and the anionic surfactant sodium dodecyl sulfate (SDS, 40 mM) at pH 4.0, 7.0, and 9.0 are reported. The data were analyzed through the modeling of the micellar form factor and interference function, as well as by means of the distance distribution function p(r). For anionic micelles (SDS), the results evidence a micellar shape transformation from prolate ellipsoid to cylinder accompanied by micellar growth and surface charge screening as the molar ratio TFP:SDS increases in the complex for all values of pH. Small ellipsoids with axial ratio nu=1.5+/-0.1 (long dimension of 60 A) grow and reassemble into cylinder-like aggregates upon 5 mM drug incorporation (1 TFP:8 SDS monomers) with a decrease of the micelle surface charge. At 10 mM TFP:40 mM SDS cylindrical micelles are totally screened with an axial ratio nu approximately 4 (long dimension approximately 140 A at pH 7.0 and 9.0). However, at pH 4.0, where the drug is partially diprotonated, 10 mM TFP incorporation gives rise to a huge increase in micellar size, resulting in micelles at least 400 A long, without altering the intramicellar core. For zwitterionic micelles (HPS), the results have shown that the aggregates also resemble small prolate ellipsoids with averaged axial ratio approximately nu=1.6+/-0.1. Under TFP addition, both the paraffinic radius and the micellar size show a slight decrease, giving evidence that the micellar hydrophobic core may be affected by phenothiazine incorporation rather than that observed for the SDS/TFP comicelle. Therefore, our results demonstrate that the axial ratio and shape evolution of the surfactant:TFP complex are both dependent on surfactant surface-charge and drug:surfactant molar ratio. The results are compared with those recently obtained for another phenothiazine drug, chlorpromazine (CPZ), in SDS and HPS micelles (Caetano, Gelamo, Tabak, and Itri, J. Colloid Interface Science 248 (2002) 149).     

Micropartioning and solubilization enhancement of 1,2-bis(bis(4-chlorophenyl) methyl)diselane in mixed micelles of binary and ternary cationic-nonionic surfactant mixtures

The aqueous solubilization of the organoselenium compound viz., 1,2-bis(bis(4-chlorophenyl)methyl)diselane [(ClC(6)H(4))(2)CHSe](2) has been investigated experimentally in micellar solutions of two cationic (hexadecyltrimethylammonium bromide, CTAB, hexadecyltrimethylammonium chloride, CTAC) and one nonionic (polyoxyethylene(20)mono-n-hexadecyl ether, Brij 58) surfactants possessing the same hydrocarbon "tail" length and in their single as well as equimolar binary and ternary mixed states. Solubilization capacity determined with spectrophotometry and tensiometry has been quantified in terms of molar solubilization ratio and micelle-water partition coefficient. FTIR, UV-vis, fluorescence and zeta potential measurements have been utilized to ascertain the interaction of organochalcogen compound with surfactants. Equimolar cationic-nonionic surfactant combinations show better solubilization capacity than pure cationics or nonionics, whereas equimolar cationic-cationic-nonionic ternary surfactant systems exhibit intermediate solubilization efficiency between their single and binary counterparts. Locus of solubilization of [(ClC(6)H(4))(2)CHSe](2) in different micellar solutions was probed by UV-visible spectroscopy. The investigation has presented precious information for the preference of mixed surfactants for solubilizing water-insoluble compounds. Indeed the solubilization aptitude of these surfactants is not merely related to molar capacity. The results furnish adequate support to justify comprehensive exploration of the surfactant properties that influence solubilization.     

A simple rule for describing the composition dependence of the aggregation number of mixed micelles

The mean aggregation numbers of mixed micelles composed of hydrocarbon surfactants (nonionic/nonionic and ionic/nonionic surfactants) have been determined by the intensity light-scattering method, in order to compare them with the values calculated by using the equations derived. The equations have been derived for representative micellar shapes (disk-like, rod-like, and spherical shapes) by making the assumptions that (i) the surface area of the hydrocarbon core of a mixed micelle is built up by independent contributions from each surfactant monomer, and (ii) the dimension of the hydrocarbon core is determined by the number of carbon atoms of a surfactant. The closest agreement of the observed aggregation numbers with the calculated ones has been obtained for the mixed micelle of an oblate ellipsoidal shape as a geometrical model for a disk-like micelle. This suggests that an oblate ellipsoidal shape may be more probable for a micelle formed at a moderate range of surfactant concentration than a prolate ellipsoidal (a rod-like) and a spherical shape if the assumptions (i) and (ii) hold. The equations presented here are useful, since they make it possible to calculate an accurate aggregation number of the mixed micelle of any composition from the aggregation numbers of the pure micelles of the components and the number of carbon atoms of component surfactants as long as there is no highly specific interaction between different surfactant components.     

SDS对SB3-12胶束表面电荷密度的调控作用及对药物增溶的影响

两性离子甜菜碱表面活性剂(SB3-12)胶束具有较好的生物相容性,由于相反电荷的极性头之间具有静电中和作用,胶束表面具有小的负电荷密度。当加入阴离子的十二烷基硫酸钠(SDS)以后,负离子SD-与SB3-12胶束极性区内层季铵正电荷的静电中和作用,能连续地调节胶束表面磺酸基的负电荷密度,这有利于对药物分子的选择性增溶和调节在生理条件下的药物的输送。等温滴定量热(ITC)研究发现SB3-12和SDS有强的协同效应,混合临界胶束浓度(CMC)和胶束化焓明显降低,并得到两者协同效应的弱静电作用机理。当模型药物分子芦丁(Rutin)与SB3-12/SDS混合胶束作用时,芦丁7位羟基的氢解离后的阴离子与SDS共同作用于SB3-12形成混合胶束。UV-Vis吸收光谱和~1H NMR谱研究发现,在SB3-12胶束中,芦丁分子的A环位于季铵阳离子附近,B环位于两个相反电荷之间的弱极性区域。在SDS胶束中,B环位于栅栏层,而A环和二糖暴露于水相侧。在混合胶束中,随着SDS摩尔分数增加,对A环的静电吸引变弱。离子表面活性剂对两性离子表面活性剂胶束表面电荷密度的调节作用,本质上是对胶束极性区域的物理及化学性质的微调,进而实现对药物的可控增溶。     

Effect of self‐assemblies of various surfactants in their single and mixed states on the BZ oscillatory reaction

Micelles of different surfactants are well known to affect chemical equilibria and reactivities by selectively sequestering the reagent substrates through electrostatic and hydrophobic interactions. In this article, the effects of micelles of various surfactants on different parameters of the Ce(IV)‐catalyzed Belousov–Zhabotinsky (BZ) oscillatory reaction at 35°C in nonstirred closed conditions are studied by employing spectrophotometry and tensiometry. Surfactants used in this study are the cationics hexadecyltrimethylammonium bromide (CTAB) and pentamethylene‐1,5‐bis(N‐hexadecyl‐N,N‐dimethylammonium)bromide gemini (Gemini), anionic sodium dodecylbenzene sulfonate (SDBS), and nonionic Brij58, whereas the binary surfactant systems used are cationic–nonionic CTAB+Brij58 and anionic–nonionic SDBS+Brij58. The results revealed that the induction period shows a definite variation with increasing concentration of different surfactants above their critical micelle concentration (cmc). The amplitudes of oscillation and absorbance maxima and minima are enhanced in the presence of micelles of CTAB and Gemini surfactants, whereas micelles of SDBS and Brij58 have almost no effect on the nature of the oscillations. However, mixed micelles of CTAB+Brij58 and SDBS+Brij58 binary mixtures show a quite different effect on the overall behavior of the oscillations. The enhanced effect of CTAB and Gemini surfactants on the overall nature of oscillations has been attributed to the positive charge on the surface of their micelles and to some extent on the presence of nitrogen in their head group. The effect of mixed binary micelles may be attributed to their synergistic nature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 659–668, 2010     

Interaction of pyrimethamine and sulfadiazine with ionic and neutral micelles: Electronic absorption and fluorescence studies

The binding of two antitoxoplasmosis drugs, pyrimethamine (PYR) and sulfadiazine (SDZ) to cationic cetyltrimethylammonium chloride (CTAC), anionic sodium dodecylsulfate (SDS), zwiterionic N-hexadecyl-N,N-dimethyl-2-ammonium-1-propanesulfonate (HPS) and neutral polyoxyethylene-dodecyl-ether (Brij-35^®) micelles was studied using absorption and fluorescence spectroscopic methods. The pK_a of PYR changed in the presence of charged anionic, cationic and zwiterionic micelles, indicating that interaction is influenced by the micellar charge. For SDZ, pK_a changes were lower than 1 for all micelles, suggesting the occurrence of low binding constants in addition to a reasonable influence of the micellar charge. The values of binding constants K_b, obtained from fluorescence measurements, for PYR to CTAC micelles were very low at pH 4.0, where the drug is in a complete protonated state, increasing at pH 9.0 to long-chained CTAC and HPS micelles since this factor also favors accomodation of the neutral drug in the hydrophobic compartments. For SDZ the binding constants were determined from optical absorption measurements. Low binding constants were observed to charged surfactant micelles, with influence of micellar charge. It must be stated however that those values can be underestimated due to the relatively low sensitivity of the method based on absorption measurements.