Aqueous Sodium Dodecyl Sulfate Research Articles

Experimental and molecular dynamics investigation into the amphiphilic nature of sulforhodamine B.

Sulforhodamine B (SRB), a common fluorescent dye, is often considered to be a purely hydrophilic molecule, having no impact on bulk or interfacial properties of aqueous solutions. This assumption is due to the high water solubility of SRB relative to most fluorescent probes. However, in the present study, we demonstrate that SRB is in fact an amphiphile, with the ability to adsorb at an air/water interface and to incorporate into sodium dodecyl sulfate (SDS) micelles. In fact, SRB reduces the surface tension of water by up to 23 mN/m, and the addition of SRB to an aqueous SDS solution induces a significant decrease in the cmc of SDS. Molecular dynamics simulations were conducted to gain a deeper understanding of these findings. The simulations revealed that SRB has defined polar "head" and nonpolar "tail" regions when adsorbed at the air/water interface as a monomer. In contrast, when incorporated into SDS micelles, only the sulfonate groups were found to be highly hydrated, suggesting that the majority of the SRB molecule penetrates into the micelle. To illustrate the implications of the amphiphilic nature of SRB, an interesting case study involving the effect of SRB on ultrasound-mediated transdermal drug delivery is presented.

Dumbbell shaped nickel nanocrystals synthesized by a laser induced fragmentation method

Dumbbell shaped nickel nanoparticles have been synthesized for the first time by laser induced fragmentation of bulk nickel powder in aqueous sodium dodecyl sulfate. The focused output of the third harmonic from a nanosecond pulsed Nd:YAG laser operating at 35 mJ/pulse energy was used for the photo-fragmentation of micron size particles suspended in 50 mM aqueous medium. Nickel anisotropic nanocrystals have been grown by recrystallisation of nickel particles due to laser plasma generated in sodium dodecyl sulfate solution. The Vibrating Sample Magnetometer measurements of dumbbell shaped nickel nanostructure show their good ferromagnetic nature. The mechanism of the synthesis of nanostructure through melting and fragmentation has been also discussed.

Conjugated polyelectrolyte (CPE) poly[3-[6-(N-methylimidazolium)hexyl]-2,5-thiophene] complexed with aqueous sodium dodecylsulfate amphiphile: synthesis, solution structure and “surfactochromic” properties

We report on the synthesis, solution structure and photophysical properties of poly[3-[6-(N-methylimidazolium)hexyl]-2,5-thiophene] bromide (P3ImiHT) when complexed with sodium dodecylsulfate (SDS). Synthesis of P3ImiHT used a Grignard metathesis (GRIM)-type route developed by McCullough, followed by quaternisation of the bromohexyl side groups of poly[3-(6-bromohexyl)thiophene] with N-methylimidazole. P3ImiHT was mixed with either SDS or deuterated SDS to form the P3ImiHT(SDS)x complex, where x is the molar ratio of surfactant to polyelectrolyte repeat unit, in D2O and studied using small-angle neutron scattering (SANS) and optical spectroscopy. Marked differences in behaviour are observed upon interaction of P3ImiHT with SDS compared with the related poly[3-(6-trimethylammoniumhexyl)thiophene] bromide (P3TMAHT). At room temperature, P3ImiHT forms charged aggregates with electrostatic repulsion which are eliminated by the SDS addition. For x ≤ 1 P3ImiHT and SDS are molecularly mixed and form ellipsoidal (x = 1/5) or sheet-like (x = 1/2–1) P3ImiHT(SDS)x aggregates. No visible precipitation is observed around the nominal charge compensation point (x = 1). For x > 1, P3ImiHT(SDS)x aggregates coexist with SDS rich micelles which turn from thick rod-like (x = 3/2) to non-charged (x = 2) and charged ellipsoidal micelles (x = 5). This transition is driven by decreasing free ion fraction. For x = 5, P3ImiHT(SDS)x forms a lamellar phase with a periodicity of ∼270 A. The structural transitions are accompanied by an initial red-shift from 422 nm (x = 0) to 459 nm (x = 1), followed by a reverse blue-shift to 400 nm (x = 5) of the UV/vis absorption maxima. The photoexcitation spectra follow this trend but are ∼50 nm red-shifted, thus indicating energy transfer within the density of states after photoexcitation. The photoluminescence maximum is gradually blue-shifted from 643 nm to 597 nm on increasing x from 0 to 5, indicating a decrease in polymer–polymer interactions.

Stabilization of Aqueous Carbon Nanotube Dispersions Using Surfactants: Insights from Molecular Dynamics Simulations

Techniques for separating bundles of carbon nanotubes into homogeneous dispersion are still under development, although a few methods have been successful at the laboratory scale. Understanding the effective interactions between carbon nanotubes in the presence of dispersing agents will provide the necessary information to develop better methods and also to refine the existing ones. We present here results from all-atom molecular dynamics simulations for aqueous flavin mononucleotide (FMN), which has been found experimentally to efficiently separate single-walled carbon nanotubes (SWNTs) based on diameter and chirality. We report results for the aggregate morphology of FMN on SWNTs of different diameters, as well as the potential of mean force between (6,6) SWNTs in the presence of aqueous FMN. The results are compared to the potential of mean force between SWNTs in aqueous sodium dodecyl sulfate (SDS). Our detailed analysis is used to explain the role of FMN, water, and sodium ions in providing a strong repulsive barrier between approaching SWNTs.

Ionic Liquid-Induced Unprecedented Size Enhancement of Aggregates within Aqueous Sodium Dodecylbenzene Sulfonate

Physicochemical properties of aqueous micellar solutions may change in the presence of ionic liquids (ILs). Micelles help to increase the aqueous solubility of ILs. The average size of the micellar aggregates within aqueous sodium dodecylbenzene sulfonate (SDBS) is observed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) to increase in a sudden and drastic fashion as the IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) is added. Similar addition of [bmim][PF(6)] to aqueous sodium dodecyl sulfate (SDS) results in only a slow gradual increase in average aggregate size. While addition of the IL [bmim][BF(4)] also gives rise to sudden aggregate size enhancement within aqueous SDBS, the IL 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF(4)]), and inorganic salts NaPF(6) and NaBF(4), only gradually increase the assembly size upon their addition. Bulk dynamic viscosity, microviscosity, dipolarity (indicated by the fluorescent reporter pyrene), zeta potential, and electrical conductance measurements were taken to gain insight into this unusual size enhancement. It is proposed that bmim(+) cations of the IL undergo Coulombic attractive interactions with anionic headgroups at the micellar surface at all [bmim][PF(6)] concentrations in aqueous SDS; in aqueous SDBS, beyond a critical IL concentration, bmim(+) becomes involved in cation-π interaction with the phenyl moiety of SDBS within micellar aggregates with the butyl group aligned along the alkyl chain of the surfactant. This relocation of bmim(+) results in an unprecedented size increase in micellar aggregates. Aromaticity of the IL cation alongside the presence of sufficiently aliphatic (butyl or longer) alkyl chains on the IL appear to be essential for this dramatic critical expansion in self-assembly dimensions within aqueous SDBS.

Conductivity of micellar solutions of ionic surfactants and surface conductivity of micelles

Within the effective medium model, a method is proposed for determining the surface conductivity of micelles. The known experimental data on the conductivity of aqueous sodium dodecyl sulfate micellar solutions are analyzed employing the developed approach. Specific surface conductivity λs of micelles is shown to be 10−8Ω−1. The high values of λs are indicative of a noticeable contribution of the dense part of the electrical double layer, which is comparable with the contribution of its diffuse part, to the micellar solution conductivity. This estimate is strongly dependent on the initial information obtained in different studies. The marked influence of the micelle surface conductivity on the effective conductivity of micellar solutions allows one to put the question of the correctness of the method commonly used to determine the degree of counterion binding from the slopes of the dependences of the solution conductivity on the overall surfactant concentrations in the premicellar and micellar regions.

Structure and “Surfactochromic” Properties of Conjugated Polyelectrolyte (CPE): Surfactant Complexes between a Cationic Polythiophene and SDS in Water

We report on the phase transitions, solution structure, and consequent effect on the photophysical properties of poly[3-(6-trimethylammoniumhexyl)thiophene] bromide (P3TMAHT) in aqueous sodium dodecylsulfate (SDS). Polythiophene was mixed with SDS or deuterated SDS to form P3TMAHT(SDS)(x) complex (x = the molar ratio of surfactant over monomer units) in D(2)O and studied by small-angle neutron and X-ray scattering (SANS/SAXS) and optical spectroscopy. At room temperature, P3TMAHT forms charged aggregates with interparticle order. The addition of SDS eliminates the interparticle order and leads to rod-like (x = 1/5) or sheet-like polymer-SDS aggregates (x = 1/2 to 1) containing rod-like (x = 1/5 to 1/2) or sheet-like (x = 1/2 to 1) polymer associations. Partial precipitation occurs at the charge compensation point (x = 1). Ellipsoidal particles without interparticle order, reminiscent of SDS micelles modified by separated polymer chains, occur for x = 2 to 5. Free SDS micelles dominate for x = 20. Structural transitions lead to a concomitant variation in the solution color from red (P3TMAHT) to violet (x = 1/5 to 1) to yellow (x > 2). The photoluminescence fingerprint changes progressively from a broad featureless band (x = 0) through the band narrowing and appearance of vibronic structure (x = 1/5 to 1) to the return to a blue-shifted broad emission band (x = 5). The polymer stiffness reaches a maximum for x = 1, which leads to minimization of the Stokes shift (0.08 eV). This work gives fundamental information upon how surfactant complexation can influence both the solution structure and photophysical properties of a water-soluble polythiophene.

Volumetric studies of some electrolytes in water and in aqueous sodium dodecylsulfate solutions at different temperatures

The apparent molar volumes (φ v) of KCl, KNO3, MgCl2, and Mg(NO3)2 have been determined in water and in aqueous sodium dodecylsulfate solutions from density measurements at 303.15, 308.15, 313.15, 318.15, and 323.15 K. The limiting apparent molar volumes ( $$ \varphi_{v}^{0} $$ ) and experimental slopes (S v) were derived from the Masson equation. The partial molar volume transfer $$ (\Updelta {\bar{V}}_{\text{tr}} ) $$ of the electrolytes were obtained from limiting apparent molar volume data from water to aqueous sodium dodecylsulfate solutions and have been interpreted in terms of ion–ion, hydrophilic–hydrophilic, and hydrophobic–hydrophobic interactions on the basis of a co-sphere overlap model. It is shown that the transfer volumes $$ (\Updelta {\bar{V}}_{\text{tr}} ) $$ are positive and increase with increasing sodium dodecylsulfate concentration for all electrolytes. The structure making or breaking capacities of the electrolytes have been inferred from the sign of [∂2 φ v 0 /∂T 2]p, i.e., the second derivative of the limiting apparent molar volume with respect to temperature at constant pressure. In water, KCl and KNO3 exhibit structure breaking and MgCl2 and Mg(NO3)2 exhibit structure making behavior. All the studied electrolytes were found to act as structure makers in aqueous sodium dodecylsulfate solutions.

Laser photolysis study of the transient products of 4-carboxybenzophenone-sensitized photolysis of chlorophenoxyacetic acid-based herbicides in aqueous micellar solutions

The kinetics of the reaction of the triplet states and radicals of 4-carboxybenzophenone (CB) in aqueous sodium dodecyl sulfate micellar solutions in the absence of additives and in the presence of 2,4-dichlorophenoxyacetic acid (DCPA) or 2-methyl-4-chlorophenoxyacetic acid (MCPA) were studied. The rate constants of 3CB quenching by DCPA and MCPA were found to be 8.5 × 105 and 1.4 × 106 s−1, respectively (as calculated for a reactant pair in the micelle). As a result of quenching, radical pairs are produced in the micelle, as well as the ketyl radicals CBH· that lost their partners because of the fast disappearance of some of the DCPA and MCPA radicals. An applied external magnetic field substantially retards the recombination of radical pairs in the micelle.

Effects of dodecanol on the adsorption kinetics of SDS at the water–hexane interface

Even though sodium dodecyl sulphate (SDS) is the most frequently studied surfactant, its properties at liquid interfaces are not easily accessible. This is mainly caused by the fact that in aqueous solution SDS is subject to hydrolysis, by which the homologous dodecanol (C12OH) is formed. Due to its enormously high surface activity it competes with SDS at the interface. We demonstrate here that this "natural" impurity C12OH does not remarkably affect the adsorption dynamics of SDS at the water/hexane interface, due to its high solubility in hexane. Therefore, the dynamic adsorption properties can be determined independent of disturbing dodecanol effects. The surfactant adsorbs diffusion controlled and the interfacial tension isotherm at the water/hexane interface is well described by a Frumkin model. However complementary experiments via direct admixture of dodecanol in hexane indicate a significant decrease in interfacial tension of the water-hexane interface at concentrations higher than 10(-3) mol/l in hexane. This condition may happen when the oil phase is distributed as small droplets in a high concentrated solution of SDS. The distribution coefficient of C12OH between water and hexane is estimated from adsorption experiments to be K(p)=c(o)/c(w)=6.7×10(3).

Effect of Temperature on the Interactions of Glycyl Dipeptides with Sodium Dodecyl Sulfate in Aqueous Solution: A Volumetric, Conductometric, and Fluorescence Probe Study

The interactions of glycyl dipeptides (2-[(2-aminoacetyl)amino]acetic acid (commonly known as glycylglycine), 2-[(2-aminoacetyl)amino]-3-methylbutanoic acid (commonly known as glycyl-l-valine), and (2S)-2-[(2-aminoacetyl)amino]-4-methylpentanoic acid (commonly known as glycyl-l-leucine)) with sodium dodecyl sulfate (SDS) as a function of temperature in aqueous solution have been investigated by a combination of density, conductivity, and fluorescence methods. The standard partial molar volume (V2,ϕo), standard partial molar volumes of transfer for dipeptide from water to aqueous SDS solutions (ΔtVo), partial molar expansibility (Eϕo), and Hepler’s constant have been calculated from density data. Electrical conductivity was used to estimate the critical micellar concentration (cmc) and the thermodynamic parameters of micellization of SDS in aqueous peptide solutions. The change of micropolarity produced by the interaction was monitored by the measurement of emission intensity ratio between the first and th...

Part II: Prediction of the dialysis process performance using Artificial Neural Network (ANN)

In the present research, an experimental based Artificial Neural Network (ANN) model is developed to describe the dialysis process performance under different operating conditions in the charged and neutral states of dialysis. The transfer behavior of charged micelles through the charged and neutral conductive membranes was investigated using this model. The parameters are highly interconnected in this system. Moreover, using the available deterministic models for tracking the process performance and the switch of the diffusion mechanisms is not completely realizable. Therefore, using neural networks is highlighted as a recommended model for this type of situations. The concentration gradient, absolute feed concentration and membrane electrical charge are the main parameters which affect the process performance. The experimental system consists of aqueous sodium dodecyl sulphate (SDS) solution above critical micelle concentration (CMC) and commercial micro-filtration membrane (GVHP) coated by conductive poly-pyrrol (PP) or silver. The amounts and the mechanisms of mass transfer were analyzed for the two types of membranes in a variety of operating conditions. Moreover, the study over the competitive state of the process was performed where the parameters are manipulated simultaneously. The developed ANN is able to predict the process performance under individual manipulation and simultaneous manipulation of the parameters. Therefore it is able to track the governing mechanisms any time. The experimental data and the developed model show that in low concentration and concentration gradients, the diffusion mechanism and value, are different in comparison to the state with high values of concentration and concentration gradient. The neural network approach was found to be capable of modeling this complex process accurately.

Photocatalytic Hydrogen Evolution from Rhenium(I) Complexes to [FeFe] Hydrogenase Mimics in Aqueous SDS Micellar Systems: A Biomimetic Pathway

To offer an intriguing access to photocatalytic H(2) generation in an aqueous solution, the hydrophobic photosensitizer, Re(I)(4,4'-dimethylbpy)(CO)(3)Br (1) or Re(I)(1,10-phenanthroline)(CO)(3)Br (2), and [FeFe] H(2)ases mimics, [Fe(2)(CO)(6)(mu-adt)CH(2)C(6)H(5)] (3) or [Fe(2)(CO)(6)(mu-adt)C(6)H(5)] (4) [mu-adt = N(CH(2)S)(2)], have been successfully incorporated into an aqueous sodium dodecyl sulfate (SDS) micelle solution, in which ascorbic acid (H(2)A) was used as a sacrificial electron donor and proton source. Studies on the reaction efficiency for H(2) generation reveal that both the close contact and the driving force for electron transfer from the excited Re(I) complexes and [FeFe] H(2)ases mimics are crucial for efficient H(2) generation with visible light irradiation. Steady-state and time-resolved investigations demonstrate that the electron transfer takes place from the excited Re(I) complex 1 or 2 to [FeFe] H(2)ases mimic catalyst 3, leading to the formation of the long-lived Fe(I)Fe(0) charge-separated state that can react with a proton to generate Fe(I)Fe(II).H, an intermediate for H(2) production. As a result, a reaction vessel for the photocatalytic H(2) production in an aqueous solution is established.

Micellar TLC of inorganic ions: Simultaneous separation of lead(II), zinc(II), and cobalt(II) and spectrophotometric estimation of zinc(II)

Micellar thin-layer chromatography has been used to investigate the separation and migration behavior of metal ions. Thin layers (0.25 mm) of silica gel G on glass plates (5 cm × 20 cm) were used as adsorbent. Ascending development of the plates was performed with aqueous sodium dodecyl sulfate (SDS) and two-component mixtures of aqueous SDS (0.2 m ) and 0.04 m , 0.08 m , or 0.1 m carboxylic acids in different ratios (1:9, 1:1, and 9:1) as mobile phases. The R F values of metal ions were measured. The mobile phase 0.2 m SDS–0.08 m tartaric acid 1:1 was best for separation and identification of Pb 2+ , Zn 2+ , and Co 2+ . The order of mobility ( R F ) was Pb 2+ < Zn 2+ < Co 2+ . On-plate identification of Pb 2+ , Zn 2+ , and Co 2+ in the presence of impurities was achieved. The method was successfully applied to identification and separation of Pb 2+ , Zn 2+ , and Co 2+ in drugs, river water, sea water, and tap water. TLC-spectrophotometry was used for quantitative analysis of Zn 2+ in spiked drug samples ...

Lateral confinement effects on the structural properties of surfactant aggregates: SDS on graphene

The structure of aqueous sodium dodecyl sulfate (SDS) surfactant aggregates formed on small graphene sheets and graphene nanoribbons has been studied using all-atom molecular dynamics simulations. Because the edges of the carbonaceous supports confine laterally the surfactant aggregates, by changing the size of the support (diameter of graphene sheets and width of graphene nanoribbons) it is possible to investigate lateral confinement effects on the aggregate morphology. The results are compared to those available on graphite, with no lateral confinement. Aqueous SDS aggregates were studied on 2.0 nm, 5.0 nm, and 10.0 nm circular graphene sheets and on 2.0 and 5.0 nm wide graphene nanoribbons. For the first time our results show that, because of lateral confinement provided by the graphene edges, SDS yields multiple layers, hemispheres, hemicylinders or multiple hemispheres depending on the graphene size and shape. Results are quantified in terms of morphology of the surfactant aggregates, order parameter of the adsorbed surfactant aggregates, and number of water molecules at contact with the carbonaceous support.

Development of a green microwave assisted extraction method for triazine herbicides determination in soil samples

It was developed a green microwave-assisted extraction method for determination of triazine herbicides in soil samples. The extraction strategy was based on micellar medium associated to microwaves followed by the herbicides determination by HPLC in such way that the procedure could be considered as green. Two portions (10 and 8 mL) of aqueous SDS (sodium dodecyl sulphate) 0.06% (m/v) were used as extracting solutions associated to 0.500 g of sample and the following microwave oven program: 540 W (6 min), 0 W (1 min), 630 W (2 min). The analytical curves were linear in the analytical range of 0.05 up to 10.0 mg L-1 for atrazine, ametryne and simazine. The limits of detection and quantification were estimated as 0.90, 0.57 and 0.235 µg g-1 and 3.1, 1.9 and 0.8 µg g-1 for atrazine, ametryne and simazine, respectively. The proposed method is precise (RSD < 8%, n = 3) and the obtained results are in agreement with the classical one. Recovery tests for soil samples indicated that the percentage of recuperation were between 82.0 e 94.0%.

Partition equilibrium of phenol red in aqueous polymer–surfactant system: Determination of critical aggregation concentration

Acid–base equilibrium of phenol red (PR), a sulphonephthalein dye, was studied spectrophotometrically in aqueous media containing water soluble nonionic polymers viz., polyvinyl alcohol (PVA) and polyethylene glycol (PEG) in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A partition equilibrium method was utilized to determine the equilibrium constant of partition of the PR between micellar pseudo phase and aqueous phase and the critical aggregation concentration (CAC) of SDS in buffered aqueous systems containing the neutral polymers. The pH dependent association constants, K ass of PR with SDS–PVA, and SDS–PEG system increase in the order PEG 200 < PEG 400 < PEG 600 < PVA. The value of K ass was found to be greater in Tris buffer system than in phosphate buffer system of same pH. Interaction of the dye with the aqueous SDS–PVA and SDS–PEG systems is found to be endothermic and entropy oriented.

Phase formation in sodium dodecylsulfate solutions in the presence of salicylic acid for preconcentration purposes

AbstractThe occurrence of surfactant-rich phases in aqueous sodium dodecylsulfate solutions in the presence of salicylic acid was investigated. The effect of acidity, electrolyte and salicylic acid concentrations on the parameters of phase formation were studied. Optimal conditions for microcomponent preconcentration were found. The features of solubilization efficiency changes of organic substances depending on their charge and hydrophobicity were established. High efficacy of preconcentration of strongly hydrophobic, positively charged substrates by low-temperature surfactant-rich phases obtained in the sodium dodecylsulfate-salicylic acid-sodium chloride ternary system is shown.

Conformational heterogeneity of a leucine enkephalin analogue in aqueous solution and sodium dodecyl sulfate micelles: comparison of time-resolved FRET and molecular dynamics simulations.

We have undertaken time-resolved Förster resonance energy transfer (FRET) and molecular dynamics simulations to analyze conformations and conformational heterogeneity of an analogue of leucine enkephalin in solution and in the presence of sodium dodecyl sulfate (SDS) micelles. Enkephalins are opioid pentapeptides that interact with opioid receptors in the central nervous system. We used time-correlated single-photon counting to detect energy transfer between the N-terminal tyrosine and a tryptophan residue substituted for phenylalanine at the 4 position. FRET from Tyr to Trp was measured over a temperature range from 5 to 55 degrees C in aqueous solution. By taking into account Tyr rotamer interconversion rates measured previously, we determined average distances between Tyr and Trp for the two populated rotameric conformations of Tyr. Molecular dynamics simulations (100 ns) support this analysis and indicate extensive conformational heterogeneity. The simulations also predict that the FRET orientational factor is correlated with the Tyr-Trp separation. Failure to account for the correlation between orientation and distance results in errors that appear to be largely offset in the leucine enkephalin analogue (YGGWL) by a weighting bias inherent in the R(-6) dependence of the energy-transfer rate. The Tyr lifetimes decrease upon titration of the peptides with SDS, indicating formation of compact conformations of the peptide in the micelle environment. This result is consistent with the conjecture that the lipid environment may induce formation of bioactive conformations of the peptide.

Photo-oxidations initiated by UV radiation of urocanic acid and its methyl ester in solution, micelles, and lipid bilayers: TYPE I (free radical) or TYPE II (singlet oxygen) mechanisms depend on the medium

Photo-oxidative reactions on methyl linoleate (ML) initiated by UV radiation of urocanic acid (UCA) or methyl urocanate (UCAME) in a non-polar solvent, aqueous sodium dodecyl sulfate (SDS) micelles or ML in lipid bilayers of dimyristoylphosphatidylcholine (DMPC) or of dilinoleoylphosphatidylcholine (DLPC) are investigated to determine medium effects on reaction mechanisms. Experiments include: kinetics of oxygen uptake, effects of radical trapping antioxidants or singlet oxygen quenchers and product analysis of cis,trans to trans,trans ( c,t// t,t) isomer ratios of oxidized linoleate to distinguish free radical, TYPE I and singlet oxygen TYPE II reactions. Irradiation of the system toluene/ML/UCAME reaction occurred by TYPE I according to kinetics studies and inhibiting effects of 2,6-di- t-butyl-4-methoxyphenol (DBHA) and the four typical c,t// t,t isomer ratios formed. Similarly irradiation of the system SDS/ML/UCAME (or UCA) resulted in the TYPE I reaction. The latter system was used to evaluate the antioxidant activities, k inh , showing the relative activities as: Trolox ≥ 2,2,5,7,8-pentamethylhydroxychroman (PMHC) > DBHA. The low k inh values of these antioxidants compared to those in a non-polar solvent are interpreted by quantitative methods of “solvent effects”. Photo-oxidation of the systems DMPC/ML/UCA or DLPC/UCA occurred by singlet oxygen (TYPE II) in contrast to those in solution or micelles according to effects of a singlet oxygen quencher, sodium azide, and lack of inhibition by antioxidants. This contrast is explained by the structure of lipid bilayers where the excited 3n,π* of UCA produces singlet oxygen in the aqueous phase which diffuses into the bilayer phase to cause TYPE II reactions.