Free Energy Of Micelle Formation Research Articles

Parallel (competitive) reactions of micelle formation and cyclodextrin–surfactant inclusion complex formation in aqueous solution: A consequence of the Gibbs–Duhem equation—invariance of the mass action law and the possibility of determining the equilibrium constant of the inclusion complex

The Gibbs–Duhem equation results in the independence between the standard Gibbs free energy of micelle formation and the standard Gibbs free energy of monocomponent surfactant’s and cyclodextrin’s (CD’s) inclusion complex formation in aqueous solution. The experimental data show that the surfactant’s critical micellar concentration increases in the presence of CD. At a constant temperature, the standard Gibbs free energy of micelle formation remains constant, irrespective of the presence of parallel competitive processes such as inclusion complex formation. This indicates that the critical micellar concentration remains unchanged at the corresponding concentration scale. As a result, the equilibrium constant for surfactant–CD inclusion complex formation can be determined by changing the critical micellar concentration in the presence and absence of CD. A graphical method for determining the equilibrium constant of the inclusion complex is also presented. An analytical approach for analyzing the influence of the formation of the inclusion complex with CD on the formation of the binary mixed micellar pseudophase is presented. Suppose that different values exist for the equilibrium constants of the formation of the inclusion complex for two surfactants from their binary mixture. In this case, there is a change in the initial mole fraction from the initial mixture of surfactants, which should be considered when applying the regular solution theory protocol when determining the parameters of a binary mixed micelle in the presence of CD.

Structure-activity correlation for new phosphorylated quaternary ammonium salts: To antimicrobial activity via self-organization

The impact of the hydrocarbon tail length on the physicochemical properties of N-((diisopropoxyphosphoryl)methyl)-N,N-dimethyl-N-alkylammonium bromides (CnPN+Br-, where n = 10, 12, 14, 16, 18) was studied through surface tension, electrical conductivity, and spectral characteristics of hydrophobic probes (Orange OT, pyrene) measurements. In homologous series, the values of the critical micelle concentration (cmс) decrease and reach the micromolar range for hexadecyl and octadecyl derivatives. Surface tension isotherms were used to determine the maximum surface excess (Γmax), the minimum area per surfactant molecule at the air/water interface (Amin), and the Gibbs free energy of micelle formation (∆Gmic) and adsorption (∆Gad). Thermodynamic parameters (ΔGmic, ΔHmic, ΔSmic) and counterion binding degree values (β) were calculated from the temperature dependences of specific electrical conductivity on the CnPN+Br- concentration. It was established that micellization is a spontaneous and entropy-driven process. Additionally, aggregation numbers of CnPN+Br- were obtained from steady-state fluorescence quenching and dye solubilization techniques. It was established that the investigated cationic surfactants exhibit membranotropic activity against gram-positive bacteria Staphylococcus aureus, caused by depolarization of bacterial cytoplasmic membrane.

Synthesis of tri‐cationic surfactants containing two amide functional groups and inhibition effectiveness against metal corrosion in acidic medium

AbstractIn this study, two tri‐cationic surfactants containing two amide functional groups and three positively charged quaternary aminium nitrogens were synthesized. The structures of the synthesized surfactants were proved by various spectroscopic methods (FT‐IR, 1H NMR, and 13C NMR). Inhibition activities of two synthesized products against mild steel corrosion were determined using the weight loss method in 1.0 M HCl acidic medium. As a result of corrosion tests performed by immersing metal coupons in acidic solutions with different inhibitor concentrations for 24 h at room temperature, close and effective inhibitions were obtained for both surfactants. It has been found that the number of carbons in the long carbon chain in their structures is an effective factor in corrosion protection. In the acid environment, the corrosion inhibition efficiency of the inhibitor with more carbon in the long chain was found slightly higher than the other. While the compound containing 16 carbon atoms in its long chain showed corrosion inhibition over 92%, the compound containing 12 carbons in its long chain exhibited corrosion inhibition of 90% and below. In addition, some physicochemical parameters such as critical micelle concentration, surface tensions at this concentration, and free energy of micelle formation were calculated for the tri‐cationic surfactants in order to correlate them with their anti‐corrosion activities. Surface images were also taken by scanning electron microscope, which is an important supporting evidence that surfactants adsorb on the metal surface and protect the surface from corrosion, to determine the roughness conditions on the metal surfaces immersed in the acid solution.

Solubilization of cationic dye in single and mixed micellar media

The effect of mixed-micellization of anionic/non-ionic surfactants on the solubilization of a cationic dye (Methylene blue) is presented in this manuscript. Interactions of dye with the single anionic surfactant sodium dodecyl sulfate (SDS), and its mixture with the non-ionic surfactant (Triton X-100), were investigated. The non-ionic surfactant was initially screened, and then its concentration was kept constant at its critical micellar concentration. The critical micellar concentration, entropy, enthalpy, and free energy of micelle formation (ΔGmic°) were determined by electrical conductivity measurements. The solubilization parameters in terms of the partition coefficient (Kx) and binding constant (Kbind) were assessed by UV–Visible spectroscopy. The role of non-ionic surfactants on the electrostatic interactions of cationic dye and surfactant was also elaborated. Mixed micellization significantly enhanced the solubility of dye molecules as shown by conductivity and absorbance analysis. The disruption of structured water in the vicinal environment of hydrophobic groups of surfactants was responsible for the increase in critical micelle concentration (CMC). The negative values of energies of binding, partitioning, and micellization indicated the spontaneity and feasibility of micellar systems. The experimental outcomes supported the superiority of anionic/non-ionic mixed micellar solubilization over a single anionic surfactant. Thus, the mixed micellar system (SDS + TX-100) assured a higher degree of solubilization than the corresponding single system (SDS).

Effects of selenium atom on the solution properties of N-alkyl-N-methylpyrrolidinium bromide

Selenium-containing surfactants have recently attracted considerable interest, but the influences of a selenium (Se) atom and its site on the solution properties are still a challenge. Herein, we systematically studied the micellization behavior and foam/emulsion properties of ionic liquid surfactants (CmSeCnMPB, m = 14,12,10, m + n = 16, MPB = N-methylpyrrolidinium bromide) bearing a Se atom at different sites of the hydrophobic tail, and compared with N-cetyl-N-methylpyrrolidinium bromide (C16MPB). The results showed that the micellization behavior and foam/emulsion properties of CmSeCnMPB are closely related to the site of the Se atom in the hydrophobic tail. With increasing the spacer between Se atom and headgroup, the critical micelle concentration (cmc) and minimum area per surfactant molecule at the air–water interface (Amin) all increased, whether for the reduced or oxidized form, and the pC20 values, standard Gibbs free energy of micelle formation (ΔGmo) decreased accompanied by an increasing contribution of the Se atom, reflecting a shortening in the effective length of the hydrophobic chain. Upon increasing temperature, the micellization of CmSeCnMPB was transformed from entropy-driven mode to enthalpy-driven mode, and the transformation temperature decreased with increasing the spacer length. The longer the spacer, the worse the foam/emulsion performance, and the greater the oxidation-induced changes in cmc, Amin, pC20, and foam/emulsion properties. Oxidation can completely switch off the foam/emulsion properties of C10SeC6MPB, but only impair those of C12SeC4MPB or C14SeC2MPB. By comparison with C16MPB, it is confirmed that the Se atom brings two competing effects: enhancing hydrophobicity akin to one methylene and decreasing hydrophobicity by disturbing the regularity. The former prevails over the latter for C14SeC2MPB, but the latter surpasses the former for C10SeC6MPB.

Isooctane-based anionic and zwitterionic surfactant: Synergistic interaction of mixed reverse micelle and solubilisation of erythromycin

In the present study, the reverse micelle of anionic (sodium bis(2-ethylhexyl) sulfosuccinate) (AOT) and zwitterionic (3-N, N-Dimethyllaurylammonio propanesulfonate) (SB3–12) surfactants was mixed in an isooctane organic phase, and their behaviour towards solubilisation of erythromycin was investigated. Erythromycin is used as an alternative for gram-positive infections in patients resistant to, or allergic to penicillin for skin treatment, soft tissue infection, mycoplasma pneumonia and legionella disease. The critical micelle concentration of the mixture (CMCmix) was analysed by applying phase separation and Rubingh's model of the surfactants to evaluate the synergistic effect between AOT and SB3–12 in isooctane solution. The critical micelle concentration of AOT and SB3–12 was 22.5 g/L and 0.9 g/L, respectively in isooctane phase. The critical micelle concentration of the mixed AOT/SB3–12 reverse micelle was 0.70 g/L. The mixed reverse micelle has efficiently increased the solubility of erythromycin, by which 94.2% transfer was achieved from 0.8:0.2 proportion of AOT to SB3–12. Thermodynamic parameters describing the free energy of micelle formation (ΔGm) and standard Gibbs free energy of adsorption (ΔGads) were found to be negative which suggest that micellization and adsorption were spontaneous. The excess Gibbs free energy of mixing (ΔGoex) value indicates that the formation of the AOT/SB3–12 mixed reverse micelle was thermodynamically stable. Conclusively, the mixed reverse micelle enhances erythromycin solubilisation in isooctane organic phase at low surfactant concentration, and the synergism of the combination between AOT and SB3–12 was positive.

Synergistic Properties of a Mixed Micelle System Consisting of a Nonionic Fluorinated Surfactant and a Cationic Surfactant

Abstract The physicochemical properties of aqueous surfactant mixtures containing a nonionic fluorosurfactant (undecafluoro-n-pentyldecaoxyethylene ether (C5F11EO10)) and various amounts of a cationic surfactant (decyltrimethylammonium bromide (DeTAB)) were determined by surface tension and conductivity measurements. All values of the critical micelle concentrations of the mixtures turn out to be smaller than those of both pure surfactants revealing the presence of a significant synergy for all DeTAB proportions in the mixed system. The analysis of the experimental data was performed on the basis of three different well established thermodynamic models of mixed micelle formation to determine several relevant parameters, especially the micelle composition, the interaction parameters, and the free energy of micelle formation. The results indicate that the dominant interactions between DeTAB and C5F11EO10 molecules are attractive. The main reason for this behavior could be attributed to the complexation between the polyoxyethylene chain of C5F11EO10 and the quaternary ammonium group of DeTAB giving rise to stable structures.

Imidazolium‐Based Ionic Liquid as Modulator of Physicochemical Properties of Cationic, Anionic, Nonionic, and Gemini Surfactants

AbstractThe aggregation morphology of 2 cationic surfactants (cetyldimethylethanolammonium bromide and cetyldiethylethanolammonium bromide), an anionic surfactant (sodium dodecylbenzenesulfonate), a nonionic surfactant (Triton X‐100), and 2 gemini surfactants (16‐4‐16,2Br−[butanediyi‐1,4‐bis(dimethyldohexylammonium bromide)] and 16‐6‐16,2Br−[hexanediyi‐1,6‐bis(dimethyldohexylammonium bromide)]) in the presence of the ionic liquid (IL) 1‐ethyl‐3‐methylimidazoliumbromide [Emim][Br] is studied using various techniques such as surface tension, conductivity, and UV–visible and fluorescence spectroscopy. Increasing the concentration of [Emim][Br] results in a decrease in the critical micelle concentration (CMC) value of the surfactants. Various interfacial properties, namely the surface excess concentration (Гmax), minimum area per molecule at the air–water interface (Amin), and surface pressure at the CMC (πcmc), as well as the thermodynamic parameters such as free energy of the given air/water interface (), Gibbs free energy of micelle formation (), Gibbs free energy of micellization per alkyl tail (), Gibbs energy of transfer (), and standard free energy of adsorption () were also investigated. The aggregation number (Nagg) was determined by the fluorescence method. It was observed that Nagg decreased with increasing weight‐percent of the IL.

Synthesis and Properties of a Novel Linear Alkylated Diphenylmethane Sulfonate Gemini Surfactant

AbstractThe linear alkylated diphenylmethane sulfonate (C12‐DSDM) was synthesized by a four‐step reaction with lauric acid, diphenylmethane and chlorosulfonic acid as raw materials. The structure of the final product was characterized by MS. The air–liquid surface tensions at various temperatures and salt solutions (NaCl) were measured by using the drop‐volume technique and the thermodynamic parameters of the micellization were calculated. The results show that the critical micelle concentration (CMC) and γCMC of the surfactant are 1.452 mmol L−1 and 38.49 mN m−1 at 298 K. With an increase in temperature, the CMC gradually increases, the γCMC and the maximum surface adsorption capacity Γmax decrease. The free energy of micelle formation is negative (−51.2 to −60.5 kJ mol−1).

Adsorption and micellar phase properties of anionic surfactant in the presence of electrolyte and oil at different temperatures

The aggregate structures and adsorption of anionic surfactant on interfacial region between water–hexane and –heptane as aliphatic hydrocarbons and –toluene as an aromatic hydrocarbon have been investigated by surface tension and conductometry at different electrolyte concentrations at different temperatures. Cyclic voltammetry and dynamic light scattering measurements have been used in order to explore the effect of aromatic hydrocarbons on the size of micellar aggregates. Also, the effect of hydrocarbons on particle size has been investigated.The presence of alkanes decreases the surface tension of water and the amount of adsorption of surfactant at interface of water–alkane. The values of free energy of adsorption are more negative than free energy of micelle formation, so in the presence of alkanes, the free energy of adsorption is affected more. In all systems and independent of the light phase, critical micelle concentration increases with rise of temperature due to breaking of water structure. Also, the results indicate a decreased adsorption with increase of temperature as molecular motions increase. However, free energy of micelle formation becomes more positive with the addition of salt.

Study on the Synthesis and Surface Active Properties of a Novel Surfactant with Triple Quaternary Ammonium Groups and Triple Dodecyl Chains Derived from Glycerin

AbstractThe new triple chain surfactant was firstly prepared by ring‐opening and a quaternization reaction with 2‐(chloromethyl)‐oxirane, tertiary amine and propane‐1,2,3‐triol as starting materials. The surfactant was characterized by MS and elementary analysis. Air–Liquid surface tension was measured by using a tensiometer at temperatures from 298 to 318 K respectively and in various salt solutions (NaX). The results show that with the increase of temperature, the values of critical micelle concentration (cmc) can achieve a minimum at 308 K. The maximum surface adsorption capacity Γ decreases, while the minimum molecule Amin area increases. With the increase in temperature and the decrease in counterionic diameter, the values of cmc decrease from 8.09 × 10−5 to 5.52 × 10−5 mol/L, and the adsorption capacity Γ increase from 1.80 × 10−10 to 2.74 × 10−10 mol/m2, while there are almost no influences to surface tension (γcmc). The free energy of micelle formation is negative (−85.2 to −92.0 kJ/mol).

Interactions and influence of α-cyclodextrin on the aggregation and interfacial properties of mixtures of nonionic and zwitterionic surfactants

The interactions of α-cyclodextrin (α-CD) with the nonionic surfactant decanoyl-N-methyl-glucamide (Mega-10) and the zwitterionic surfactant dimethyldodecylammoniopropanesulfonate (DPS) in their mixed system have been studied using interfacial tension, fluorescence, and nuclear magnetic resonance measurements. From the plots of interfacial tension vs. log of total surfactant concentration, we have obtained values of the surface excess of surfactant, the critical micellar concentration (cmc), the standard free energy of micelle formation, and association constant of surfactant/α-CD inclusion complexes (assuming a 1:1 stoichiometry). A comparison of the Ka values obtained for the interaction between α-CD and DPS and Mega-10, respectively, shows that DPS interacts stronger with α-CD than Mega-10. The experimental mixed cmc was analyzed by the pseudophase separation model and regular solution theory for the evaluation of ideality or nonideality of the mixed micelle formation. The interaction parameters in the mixed micelle and the micelle composition at different mole fractions of DPS were also computed. The fluorescence anisotropy (r) values of rhodamine B decreases with the increase of α-CD concentrations.

Thermodynamics of Micelle Formation of the Counterion Coupled Gemini Surfactant Bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine Salt and Its Dynamic Adsorption on Sandstone

A novel counterion-coupled gemini (cocogem) surfactant, DBSJ, was synthetized via the 2:1 coupling reaction between 4-(2-dodecyl)benzenesulfonic acid (Lutensit A-LBS) and polypropyleneglycol-bis(2-aminopropyl) ether (Jeffamine D230). The surfactant had a polydispersity index of Mw/Mn = 1.04, as determined by electrospray-ionization mass spectrometry. The micellar properties of DBSJ in water were investigated in the temperature range 283-348 K by conductometry and titration microcalorimetry. The critical micelle concentration (cmc) of the cocogem was found to be more than 1 order of magnitude less than that of monomeric sodium 4-(2-dodecyl)benzenesulfonate (SDBS). The mean degree of dissociation in the temperature range studied proved to be alpha = 0.39. The calorimetric enthalpies of micelle formation agreed well with the enthalpies calculated via the van't Hoff relation. The cmc versus T curve passes through a minimum just below room temperature, after which the micelle formation changes from endothermic to exothermic. The Gibbs free energy of micelle formation was nearly constant as the temperature was increased, due to enthalpy/entropy compensation. The isotherm for DBSJ adsorption from aqueous solution onto sandstone was determined by continuous flow frontal analysis solid/liquid chromatography at 298 K and 60 bar. The adsorption of DBSJ on sandstone followed an S-type isotherm. Surface aggregation occurred over an extended range of concentration. Surface saturation was reached at a solution concentration more than 1 order of magnitude less than for monomeric SDBS. This finding is a point of concern in the chemical flooding of oil reservoir rocks to enhance oil recovery.

Effect of various additives on the performance of a newly developed PVC based potentiometric sensor for anionic surfactants

A new poly(vinyl chloride) PVC membrane electrode to determine monomer concentrations of dodecylbenzenesulphonate ions (DBS −) based on a neutral ion-pair carrier complex of dodecyltrimethylammonium–dodecylbenzenesulphonate (DTA +–DBS −), is reported here. The electrode exhibits a slope of 51.25 mV per decade for DBS − ions. The DBS − ion selective electrode (ISE) can determine monomer units down to concentrations as low as 3.32 × 10 −4 M. The effect of three kinds of additives, i.e. alcohols, glycols and triblock polymers on the performance of the surfactant selective electrode is studied systematically. The effect of foreign anions along with primary ions on the performance of ion-selective electrode is investigated in terms of potentiometric selectivity coefficients, which were determined using the fixed interference method (FIM) at 1.0 × 10 −2 M concentration of foreign anions. The sensor responds well to the surfactant ions in the presence of additives at lower concentration. The Gibbs free energy of micelle formation (Δ G m) of sodium dodecylbenzenesulphonate (SDBS) in the presence of various additives is calculated and found to vary differently with respect to the increase in the amount of additives. The sensor worked in the acidic pH range with a short response time of 30 s. The lifetime of the sensor is more than three months. The sensor was further used to determine the amount of DBS − in local detergents. This method of determining anionic surfactants was found to be quite accurate when compared with classical methods.

Nonideal mixing of alkylammonium chloride and decyldimethylphosphine oxide surfactants in adsorbed films and micelles

Miscibility and interaction of decyldimethylphosphine oxide (DePO) with ammonium chloride (AC), hexylammonium chloride (HAC), and dodecylammonium chloride (DAC) in adsorbed films and micelles were studied by surface tension measurements. Phase diagrams were drawn for the mixed adsorption, mixed micelle formation, and equilibrium between adsorbed films and micelles. Nonideal mixing of DAC and DePO was characterized by a negative excess Gibbs free energy and positive excess area of adsorption and negative excess Gibbs free energy of micelle formation. It is concluded that the interaction between DAC and DePO in adsorbed films and micelles is larger than those between the same surfactants alone due to two factors: ion–dipole interactions between the head groups of DAC and DePO and alkyl-chain/alkyl-chain interactions.

Potentiometric Studies of Micellization Behavior of Cationic Surfactants in the Presence of Glycol Additives and Triblock Polymer (Pluronic F68), Using Surfactant-Selective Sensors Based on Neutral Ion-Pair Complexes

Poly(vinyl chloride) (PVC) membrane electrodes that are used to determine monomer concentrations of hexadecyltrimethylammonium ion (HTA+) and dodecyltrimethylammonium ion (DTA+), based on neutral ion-pair carrier complexes of hexadecyltrimethylammonium dodecylsulfate (HTA+−DS-) and dodecyltri-methylammonium dodecyl sulfate (DTA+−DS-), respectively, are reported here. The electrodes exhibit a sub-Nernstian slope of 34.59 mV per decade for HTA+ and 35.10 mV per decade for DTA+ ions. The HTA+-ISE and DTA+-ISE can determine the monomer units down to concentrations as low as 3.32 × 10-4 and 4.52 × 10-4 M, respectively. The effect of three glycol additives (i.e., ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG)) and a triblock polymer (Pluronic F68) on the performance of two surfactant selective electrodes is studied systematically. The effect of the presence of foreign cations, along with primary ions, on the performance of both ion-selective electrodes is investigated in terms of potentiometric selectivity coefficients, which were determined using a fixed interference method (FIM) at a foreign cation concentration of 1.0 × 10-2 M. The sensors respond well to the surfactant ions in the presence of a lower concentration of additives. The Gibbs free energy of micelle formation (ΔGm) of both surfactants, in the presence of various additives, is calculated and determined to be less favorable, with respect to the increase in the amount of additives. The sensors worked in the pH range of 8−13 with a short response time of 30 s. The lifetime of the sensors is more than three months.

Quantifying the Hydrophobic Effect. 2. A Computer Simulation−Molecular-Thermodynamic Model for the Micellization of Nonionic Surfactants in Aqueous Solution

In this article, the validity and accuracy of the CS-MT model is evaluated by using it to model the micellization behavior of seven nonionic surfactants in aqueous solution. Detailed information about the changes in hydration that occur upon the self-assembly of the surfactants into micelles was obtained through molecular dynamics simulation and subsequently used to compute the hydrophobic driving force for micelle formation. This information has also been used to test, for the first time, approximations made in traditional molecular-thermodynamic modeling. In the CS-MT model, two separate free-energy contributions to the hydrophobic driving force are computed. The first contribution, gdehydr, is the free-energy change associated with the dehydration of each surfactant group upon micelle formation. The second contribution, ghydr, is the change in the hydration free energy of each surfactant group upon micelle formation. To enable the straightforward estimation of gdehydr and ghydr in the case of nonionic surfactants, a number of simplifying approximations were made. Although the CS-MT model can be used to predict a variety of micellar solution properties including the micelle shape, size, and composition, the critical micelle concentration (CMC) was selected for prediction and comparison with experimental CMC data because it depends exponentially on the free energy of micelle formation, and as such, it provides a stringent quantitative test with which to evaluate the predictive accuracy of the CS-MT model. Reasonable agreement between the CMCs predicted by the CS-MT model and the experimental CMCs was obtained for octyl glucoside (OG), dodecyl maltoside (DM), octyl sulfinyl ethanol (OSE), decyl methyl sulfoxide (C10SO), decyl dimethyl phosphine oxide (C10PO), and decanoyl-n-methylglucamide (MEGA-10). For five of these surfactants, the CMCs predicted using the CS-MT model were closer to the experimental CMCs than the CMCs predicted using the traditional molecular-thermodynamic (MT) model. In addition, CMCs predicted for mixtures of C10PO and C10SO using the CS-MT model were significantly closer to the experimental CMCs than those predicted using the traditional MT model. For dodecyl octa(ethylene oxide) (C12E8), the CMC predicted by the CS-MT model was not in good agreement with the experimental CMC and with the CMC predicted by the traditional MT model, because the simplifying approximations made to estimate gdehydr and ghydr in this case were not sufficiently accurate. Consequently, we recommend that these simplifying approximations only be used for nonionic surfactants possessing relatively small, non-polymeric heads. For MEGA-10, which is the most structurally complex of the seven nonionic surfactants modeled, the CMC predicted by the CS-MT model (6.55 mM) was found to be in much closer agreement with the experimental CMC (5 mM) than the CMC predicted by the traditional MT model (43.3 mM). Our results suggest that, for complex, small-head nonionic surfactants where it is difficult to accurately quantify the hydrophobic driving force for micelle formation using the traditional MT modeling approach, the CS-MT model is capable of making reasonable predictions of aqueous micellization behavior.

Thermodynamics of aqueous solutions of dodecyldimethylethylammonium bromide

The thermodynamic properties of the aqueous solutions of dodecyldimethylethylammonium bromide (DEDAB) were determined as a function of concentration by means of direct methods. Dilution enthalpies at 298 and 313 K, densities and sound velocities at 298 K were measured, allowing the determination of apparent and partial molar enthalpies, volumes, heat capacities and compressibilities. Changes in thermodynamic quantities upon micellization were derived using a pseudo-phase transition approach. These data allow for the determination of the effect of the CH 2 group, when added to the polar head of alkyltrimethylammonium bromides. The properties mainly affected by this addition are the enthalpies and, as a consequence, the entropies. The lowering of the charge density on the quaternary nitrogen due to the inductive effect of the ethyl group, greater than that of the methyl one, raises the plateau value of apparent and molar enthalpy by a quantity similar to that due to the removing of a methylene group from the hydrophobic chain. This effect does not play a great role in the value of the cmc (i.e. on the free energy of micelle formation), since the small decrease in cmc of DEDAB compared to DTAB reflects the increase in the overall hydrophobicity of the molecule. Volumes of DEDAB are greater than those of DTAB by about 15 cm 3 mol −1, both at infinite dilution and at micellar phase, a value in agreement with that generally accepted for a methylene group. The trends of apparent molar heat capacities and compressibilities vs m are the same as for DTAB: in fact, these quantities are related to the number of water molecules involved in the hydrophobic processes in solution, not very greatly affected by the substitution of a methyl group by an ethyl one on the polar head. In summary, this substitution affects to a significant extent the first derivatives of the free energy, but does not affect the second derivatives.

A molecular dynamics study of free energy of micelle formation for sodium dodecyl sulfate in water and its size distribution

Free energy of micelle formation has been evaluated for spherical sodium dodecyl sulfate (SDS) in water by a thermodynamic integration method combined with a series of large-scale molecular dynamics calculations following the chemical species model. In particular, free energy change delta mu(n+1)0 with respect to the addition of one surfactant molecule to the spherical micelle of size n was obtained as a function of n. The free energy profile showed a minimum followed by a maximum, which corresponds to a peak in the size distribution. The calculated peak size n = 57 near its critical micelle concentration is in good agreement with the experimental averaged aggregation number n = 55-75 of the SDS micelle. The distribution showed a rather sharp peak, indicating that the size is almost a monodisperse one. The size is likely to be insensitive to the total concentration of the surfactant.

Interactions between anionic mixed micelles and α-cyclodextrin and their inclusion complexes: conductivity, NMR and fluorescence study

Mixed micelle formation of anionic surfactants sodium dodecyl sulfate (SDS) and sodium lauroyl sarcosine (SLAS) have been studied in water and in 5, 10, and 15 mM concentrations of α-cyclodextrin (α-CD) over mole fraction range of αSDS from 0 to 1. From the conductivity curves, the critical micellar concentration (CMC) for the pure and binary mixtures were evaluated. The degree of counterion association (χ) or counterion dissociation (δ), the equivalent ionic conductivities of the monomeric species (Λm), the associated species (Λassc), and the micelle (Λmic) were evaluated from the slope of the conductivity vs concentration plots. The CMC values have been used to calculate the thermodynamic parameters such as the standard free energy of micelle formation \(\Delta G^{{\rm{0}}}_{{{\rm{mic}}}}\) and a transfer of standard free energy of micelle \({\left( {\Delta G^{{\text{0}}}_{{{\rm{M,tr}}}} } \right)}\) from the aqueous medium to additive medium computed. The apparent CMC of the surfactants varies linearly with α-CD concentrations. From the dependence of CMC of the surfactants on α-CD concentration, we are able to determine the association constant (K) of surfactant-α-CD inclusion complexes assuming 1:1 stoichiometry. Mixed micelle behaves ideally in the pure water as well as at the different concentrations of α-CD, which was evaluated by using the Clint equation, the regular solution approximation, and Motomura’s formulation. Self-diffusion coefficients of the micelle increased upon the induction of SDS into the micelle. 2D-rotating frame Overhauser effect spectroscopy spectra of SDS and SLAS were recorded in the presence of α-CD to investigate the interaction between H-atoms of the alkyl chain of the surfactants and H-atoms of the hydrophobic cavity of α-CD indicating multiple complexation. The fluorescence anisotropy of rhodamine B has been measured to observe the structural behavior of mixed micelle.