Degree Of Counterion Binding Research Articles

ITC for Characterization of Self-Assembly Process of Cationic Dendrons for siRNA Delivery.

siRNAs are emerging as promising therapeutic agents due to their ability to inhibit specific genes in many diseases. However, these tools require specific vehicles in order to be safely delivered to the targeted site. Among different siRNA delivery systems, self-assembled nanomicelles based on amphiphilic cationic dendrons (ACDs) have recently outperformed nanovectors based on covalent carriers. This chapter describes how isothermal titration calorimetry (ITC) can be exploited as one of the best techniques to investigate the self-assembly process of ACDs. Specifically, ITC can provide, as such or via specific analysis methods, a full thermodynamic characterization of these nanomicelles, including their critical micellar concentration, micelle aggregation number, degree of counterion binding, Gibbs free energy of micellization, and its enthalpic and entropic components.

Drug-surfactant comicellization: Propranolol hydrochloride-surface active ionic liquid systems in aqueous medium

Mixed micellization behavior of an antidepressant amphiphilic drug propranolol hydrochloride (PPL) with each of two surface-active ionic liquids (SAILs), 1-decyl-3-methylimidazolium chloride (DMImCl) and 1-hexadecyl-3-methylimidazolium chloride (HDMImCl) in aqueous solutions has been studied using conductometric, and tensiometric techniques. The structures of PPL and SAILs have been optimized using Density Functional Theory (DFT) calculations and the possible coordination of PPL with the head group of the SAILs has also been studied. The critical micellar concentration (cmc) (for pure as well as mixed surfactants), micellar composition of the surfactants, interaction parameter (βR) along with the degree of counterion binding (α) have been ascertained in this study. In particular, the regular solution theory, put forward by Rubingh, was employed to obtain the SAIL-PPL interaction parameters (βR) in the micellar state from the cmc values of the corresponding SAILs and drug in their mixtures. Analyses of the results have also been made on the basis of the theories of Clint and Motomura to gain important insight into the nature of interactions prevailing among the components in the mixed micelle. Different interfacial and thermodynamic parameters for comicellization have also been derived and information as to the spontaneity of the comicellization process, thermal stability of the micellar aggregates, alteration of the surface activity of PPL in presence of the SAILs, geometrical shapes of the micellar aggregates etc. have been obtained.

Aggregation and antimicrobial properties of gemini surfactants with mono- and di-(2-hydroxypropyl)ammonium head-groups: Effect of the spacer length and computational studies

Cationic gemini surfactants, alkanediyl-α,ω-bis[(2-hydroxypropyl)dodecylammonium] dibromide (abbreviated as C12-s-C12[iso-Pr(OH)] and C12-s-C12[iso-Pr(OH)]2, with s = 2, 3, 4 and 5) have been synthesized, and their aggregation properties in aqueous solution have been studied by surface tension, electrical conductivity and dynamic light scattering methods. On the basis of the results of studying aqueous solutions of the synthesized gemini surfactants by tensiometric and conductometric methods, their principal surfactivity parameters, such as the degree of counterion binding (β), critical micelle concentration (CMC), effectiveness of surface tension reduction (πCMC), surface excess concentration (Γmax), area per molecule at the interface (Amin), changes of Gibbs free energies of adsorption (ΔGad) and micellization (ΔGmic) have been calculated. For both classes of these gemini surfactants, a character of the surfactivity parameters change with an elongation of the spacer chain was defined and comparative analysis with other ammonium-type gemini surfactants having different head-groups was performed. Using dynamic light scattering method, regularities of variation of the aggregates sizes were studied depending on the number of 2-hydroxypropyl groups and the spacer chain length was investigated. Meanwhile, it was revealed that the mentioned gemini surfactants possess antimicrobial properties. The mechanism of the epoxide ring opening and salt formation reactions were calculated by using Kohn–Sham DFT with the B3LYP functional. The epoxide ring opening via primary (dodecylamine) and secondary (INT2) amines was determined as exergonic (13.4 and 4.4 kcal/mol) reactions as observed experimentally.

Calorimetric determination of the thermodynamics of alcohol-surfactant mixed micelle formation: Temperature and concentration effects

The thermodynamic parameters of surfactant/alcohol mixed micelles were determined in mixtures of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in a series of solvents composed of varying concentrations of n-propanol (C3OH), n-butanol (C4OH), and n-pentanol (C5OH). Conductivity studies were used to determine CMC values and the degrees of counterion binding. Isoperibol solution calorimetry was used to directly determine CMC values and enthalpy of micellization (ΔmicH). From these experimentally determined values, the Gibbs energy and entropy of micellization were calculated as a function of temperature and alcohol concentration. In general, CMC values decreased with increasing alcohol carbon chain length and concentration but increased with temperature. The effects of temperature, alcohol carbon chain length, and alcohol concentration on the energetics of these mixed micellar systems are explained in terms of the relative contributions to ΔmicH by alkyl chain interactions, hydrophobic effects, and electrostatic contributions.

Thermodynamic and computational study of isomerism effect at micellization of imidazolium based surface-active ionic liquids: Counterion structure

In this work the influence of benzoate, ortho-, meta- and para-hydroxybenzoate anions, as counterions, on micellization process of 1-dodecyl-3-methyl-imidazolium cation in water is investigated by isothermal titration calorimetry (ITC) in the temeprature range between 278.15 and 318.15 K. ITC experimental data were analyzed by the help of an improved mass-action model, yielding the values of critical micelle concentration, cmc, the degree of counterion binding, β, aggregation number, n, standard heat capacity, ΔMcpθ, enthalpy, ∆MHθ, entropy, ∆MSθ, and Gibbs free energy, ∆MGθ of micellization. It was found that the investigated systems behave mainly like common ionic surfactants and already investigated SAILs: the micellization process of investigated systems is entropically driven at low temperatures, whereas at high temperatures the enthalpic contribution becomes equally important. The last is especially important in the case of ortho-hydroxybenzoate, which incorporates into micellar structure affecting also the entropy-enthalpy compensation. But evidently, the presence and position of –OH group in the counterion influence considerably the micellization process.ΔMcpθ values were further discussed in the light of the removal of water molecules from contact with nonpolar surface area upon micelle formation. All the values are negative, the most in the case of benzoate anion, which could be ascribed to the absence of –OH group.To refine the thermodynamic parameters obtained from ITC, the molecular simulations were performed. First, it is shown that the binding energies between anion and cations increase in the order from benzoate to para-, meta- and ortho-hydroxybenzoate, which coincide with ∆MHθ values. Second, it is demonstrated that water around counterions is strongly perturbed leading to differences in ΔMcpθ.

Effects of alkyl chain length on synergetic interaction and micelle formation between a homologous series of n-alkyltrimethylammonium bromides and amphiphilic drug propranolol hydrochloride

The aggregation behavior of a homologous series of n-alkyltrimethylammonium bromides (CnH2n+1TAB; n = 12, 14, and 16) in the presence of amphiphilic drug propranolol hydrochloride (PPL) in aqueous solutions were studied using conductometric technique. The critical micelle concentrations (CMCs) were determined for pure PPL and surfactant as well as mixed micelles [surfactant(1)/PPL(2)] using the Carpena method. Various physicochemical parameters such as ideal CMC (CMCid), degree of counter-ion binding (α), micellar mole fractions of surfactant (X1m and X1id), interaction parameter (βm), the activity coefficients of components (f1m and f2m), standard Gibbs free energy of micellization (ΔGmic0) and the excess free energy of micellization (GmicE) values have been calculated and compared in detail using Clint's and Rubingh's theories. The activity coefficients of surfactant and PPL are less than 1 (f1m < 1 and f2m < 1) and the values of βm are negative in all mixtures, which indicates the synergistic interaction between surfactant and PPL. Also, the results show that the synergistic effects of mixed micelles strongly depend on the chain length of cationic surfactant, and the values of interaction parameter increase with increasing the alkyl chain length of surfactant. Of the several interactive interactions, it seems hydrophobic interaction is the most dominant interaction between the molecules of the mixed systems among all observed interactive interactions in this study.

Impact of Aromatic Counter-Ions Charge Delocalization on the Micellization Behavior of Surface-Active Ionic Liquids.

The nature of counter-ions governs the micellar and structural characteristics of surface-active ionic liquids (SAILs). Especially, the introduction of aromatic counter-ions significantly increases their surface adsorption and induces the formation of various types of aggregates like prolate ellipsoidal micelles, rodlike micelles, vesicles, lamellars, etc. The present study reports the role of charge delocalization of two different aromatic counter-ions in the micellization behavior of their respective SAILs in aqueous medium. For this purpose, we have synthesized two SAILs, namely, 1-tetradecyl-3-methylimidzolium phenolate [C14mim][PO] and 1-tetradecyl-3-methylimidzolium benzoate [C14mim][BZ]. The O-atom of phenolate (PO-) possesses negative charge, which is delocalized on its phenyl ring. Conversely, the negative charge of benzoate (BZ-) is not delocalized on its phenyl ring. The more hydrophobic BZ- counter-ion increases the hydrophobic interactions and reduces the electrostatic repulsions more efficiently as compared to PO-, which results in a lower critical micelle concentration (cmc) of [C14mim][BZ] than that of [C14mim][PO]. Interfacial properties obtained by tensiometry reveal better surface activity and absorption efficiency of [C14mim][BZ] as compared to [C14mim][PO]. The increase of cmc and degree of counter-ion binding (β) with the rise of temperature for both SAILs has been observed by conductometry. The decrease in the polarity of pyrene microenvironment explains the higher compactness of [C14mim][BZ] aggregates than that of [C14mim][PO], observed by fluorimetry. The position of PO- and BZ- is in the stern and palisade layers of C14mim+ aggregates, respectively, located by 1H NMR. The existence of prolate ellipsoidal micelles for both SAILs has been established by small-angle neutron scattering measurements. Thus, the interfacial and bulk properties of [C14mim][PO] lie somewhere in between those of the SAILs having perfect aromatic counter-ions, [C14mim][BZ], and the SAILs having regular inorganic counter-ions like Cl-, Br-, etc.

Effect of head-group size of some tetradecylalkylammonium bromide surfactants on obtaining the lyotropic biaxial nematic phase.

Lyotropic quaternary mixtures of some tetradecylalkylammonium bromide surfactants were prepared to examine the effect of the size of the surfactant head group on the stabilization of different lyotropic nematic phases. The lyotropic mixtures were prepared by the addition of the tetradecylalkylammonium bromides (TTAABr) in the mixture of NaBr/decanol (DeOH)/water. The uniaxial to biaxial nematic phase transitions were determined via laser conoscopy. Some micellization parameters such as critical micelle concentration, degree of counterion binding and micellization Gibbs energy were evaluated from the electrical conductivity measurements of diluted binary surfactants/water solutions. The results indicate that the head-group size of the surfactant molecules influences the amphiphilic molecular aggregate topology. Moreover, the effective area per surfactant head group is a key parameter on stabilizing the lyotropic biaxial nematic phase.

Effect of Molecular Composition of Head Group and Temperature on Micellar Properties of Ionic Surfactants with C12 Alkyl Chain

The paper analyses influences of the temperature and hydrophilic groups on micellar properties of ionic surfactants with 12-carbonic hydrophobic chains. The aim is to assess the impact of hydrophilic groups and temperature on thermodynamic parameters and micellization. This knowledge is indispensable for the formulation of new dosage forms. The method uses conductometric measurements. The following hydrophilic groups are analyzed: trimethylammonium bromide, trimethylammonium chloride, ethyldimethylammonium bromide, didodecyldimethylammonium bromide, pyridinium chloride, benzyldimethyl-ammonium chloride, methylephedrinium bromide, cis and trans-[(2-benzyloxy)-cyclohexyl-methyl]-N, N-dimethylammonium bromide, sodium sulphate and lithium sulphate. Except for a few cases, there is a good agreement between values of critical micellar concentrations (CMC) and critical vesicle concentration (CVC) obtained here and those which were obtained by other authors and/or by other physicochemical methods. Values of the CMC are compared with respect to the molar masses of hydrophilic groups. It was found that CMC values increased non-linearly with increasing system temperature. The degrees of counterion binding and thermodynamic parameters, like the standard molar Gibbs energy, enthalpy and entropy of micellization are determined and discussed in detail. The results obtained will be incorporated into in silico processes of modeling and design of optimal dosage forms, a current interdisciplinary research focus of the team.

Aqueous solution of poly (hexamethylene guanidine hydrochloride) and poly (diethylenamine guanidine hydrochloride) as studied with acid-base indicators

In this paper, the properties of cationic polyelectrolytes as tools for governing the protolytic equilibrium of acid-base indicators in water were examined. For this purpose, water-soluble and pH-dependent poly (hexamethylene guanidine hydrochloride), PHMG, and poly (diethylenamine guanidine hydrochloride), PDEG, were studied. As molecular probes, a set of anionic indicator dyes were used; the key parameter is the so-called apparent ionization constant, Kaapp. The electrokinetic potential of the above polycationic species in the acidic pH region is substantially positive. As a rule, the polyelectrolytes display marked influence on the absorption spectra and state of the acid-base equilibrium of the anionic dyes at pH < 7, especially in the case of PHMG. Both effects resemble those known for the same dyes in aqueous solutions of cationic surfactants but are less expressed. Normally, the acid-base equilibria were studied at polyelectrolyte : dye ratio of 150 : 1, at ionic strength 0.05 M, and 25 oC. The decrease in the pKaapp (≡ –logKaapp) value on going from water to the PHMG solution is most expressed for bromocresol green (HB– ↔ B2– + H+): pKa,2app – pKa,2w = –1.93. For bromophenol blue, bromocresol purple, and sulfonefluorescein, the shift of the equilibrium is less expressed. Some kinds of specific interactions with the polyelectrolytes were revealed for methyl orange and bromophenol blue. Also, the dependence of pKaapp on logarithm of ionic strength allows estimating the degree of counterion binding by the polycation: β = 0.40.1.

Counterion-coupled gemini (Cocogem) surfactants based on dodecylisopropylol amine and dicarboxylic acids: synthesis, characterization and evaluation as biocide against SRB

New cocogem surfactants were synthesized by interaction of dodecylisopropylol amine with dicarboxylic (oxalic, succinic, adipic, sebacic, tartaric, maleic, fumaric, isophthalic) acids. By tensiometric method, the surface activity of aqueous solutions of the synthesized cocogem surfactants at the border with air was studied and, by conductometric method, the specific electrical conductivity of these solutions was determined. The degree of counterion binding (β), critical micelle concentration (CMC), effectiveness of surface tension reduction (πCMC), surface excess concentration (Γmax), area per molecule at the interface (Amin), changes of Gibbs free energies of adsorption (ΔGad) and micellization (ΔGmic) have been calculated. The character of change of the colloidal-chemical indices depending on spacer-group nature and length has been clarified. So, with an elongation of the spacer group and when passing from cis-form to trans-form, the value of CMC decreases. The obtained cocogem surfactants exhibit a considerable bactericidal effectiveness against sulfate-reducing bacteria. The bactericide properties of the cocogem surfactants containing in the spacer chain a saturated hydrocarbon fragment and benzene ring are stronger than for the others.

Effect of cationic structure of surface active ionic liquids on their micellization: A thermodynamic study

In this work the influence of cationic structure of surface active ionic liquids (SAILs) on their micellization process in water is investigated by isothermal titration calorimetry (ITC) at temperatures between 278.15 and 318.15 K. First, 1-dodecylimidazolium bromide ([C12im]Br) was studied, and then the length of alkyl chain on the N3 position is varied from methyl- (1-dodecyl-3-methylimidazolium bromide ([C12mim]Br)), ethyl- (1-dodecyl-3-ethylimidazolium bromide, [C12eim]Br), to butyl- (1-dodecyl-3-butylimidazolium bromide, [C12bim]Br). In next step, the N3 was replaced by S atom in 3-dodecylthiazolium bromide ([C12thiaz]Br) as an analogue of [C12im]Br. Due to problems at synthesis and low solubility of other thiazolium analogues no further experiments on these systems were possible.ITC experimental data were analyzed by the help of an improved model, yielding simultaneously the values of critical micelle concentration, cmc and enthalpy, ∆MHθ by the fitting procedure together with the degree of counter ion binding, β, aggregation number, n, standard heat capacity, ∆Mcpθ and Gibbs free energy, ∆MGθ of micellization. It was found that the investigated systems behave mainly like common ionic surfactants and already investigated SAILs: the micellization process of investigated systems was detected as endothermic at low temperatures and exothermic at high temperatures; the values of cmc are decreasing with the increasing length of alkyl chain on N3 position. Between [C12thiaz]+ and Br− weaker interactions were found in comparison to those between [C12im]+ and Br− what can be ascribed to more expressed aromaticity of thiazolium ring.∆Mcpθ values were further discussed, regarding the removal of water molecules from contact with nonpolar surface area upon micelle formation. It can be concluded, that the ethyl- and butyl- chain on N3 position are folded during the micellization process and thus they are (at least) partially removed from contact with water.

Densities, refractive indices and conductivities of aqueous [Cnmim][Pro] solutions (n = 4, 8, 12); micellization and the capillary electrophoresis data at 298.15 K

The densities of aqueous solutions of amino acid ionic liquids (AAILs) based on 1-alkyl-3-methylimidazolium cations and L-prolinate anions [Cnmim][Pro] (n = 4, 8, 12) in the concentration range 0-97 wt % were measured at several constant temperatures in the range 293.15-318.15 ± 0.05 K. It has been found that in the range of high AAIL concentrations the dependence of the density values on the AAIL mole fraction for binary water + [Cnmim][Pro] solutions is nonlinear. The refractive indices of aqueous solutions of these AAILs in the concentration range 0-97 wt % and the electrical conductivities of aqueous [Cnmim][Pro] (n = 8, 12) solutions were measured at temperature 298.15K. Critical micelle concentrations (cmc) of AAILs, degree of counterion binding (β), standard Gibbs energy of micellization (ΔG°m) for them were estimated from the experimental data. The dependences of the cmc, β and ΔG°m, on the length of the alkyl chain of the cations were discussed. The synthesized AAILs [Cnmim][Pro] were examined as chiral selectors for the separation of D,L-tryptophan and D,L-tyrosine enantiomers using the ligand exchange capillary electrophoresis. The highest resolution factors Rs are observed in the case of [C4mim][Pro] used as a chiral ligand: Rs = 5.3 for D,L-tryptophan and Rs = 2.2 for D,L-tyrosine.

Surfactant Interactions and Organization at the Gas-Water Interface (CTAB with Added Salt).

We have studied adsorbed layers of cetyltrimethylammonium bromide (CTAB) at air-water interfaces in the presence of added electrolyte. Fast bubble compression/expansion measurements were used to obtain the surface equation of state, i.e., the surface tension vs CTAB surface concentration dependence. We show that while a simple model where the surfactant molecules are assumed to be noninteracting is insufficient to describe the measured response of the surfactant layer, a modified Frumkin equation where the local interactions between the molecular components depend on their surface concentration captures the response. The variation of the effective interactions in the surfactant layer in the model shows that the interactions in the surfactant layer change from effectively repulsive to attractive with increasing surface concentration. Molecular dynamics simulations are performed to probe the origins of the change in the interactions. The simulations indicate that already at low surface concentrations the surfactants aggregate as highly dynamic rafts with surfactant orientation parallel to the interface. Increasing the concentration leads to a change in the assembly morphology at the interface: the surfactant layer thickens and the surfactants sample a range of tilted orientations with respect to the interfacial plane. The change from transient raftlike assemblies to dynamical aggregates at the interface involves a clear increase in the degree of counterion binding: we speculate that the flip of the effective interaction parameter in the model used to interpret the experimental results could result from this. The work here presents basic steps toward a proper understanding of the molecular organization and interactions of surfactants at an air-water interface. This is crucially important in understanding macroscopic properties of surfactant-stabilized systems such as foams, emulsions, and colloidal dispersions.

A new surfactant-copper(ii) complex based on 1,4-diazabicyclo[2.2.2]octane amphiphile. Crystal structure determination, self-assembly and functional activity.

A new surfactant-copper(ii) complex [Cu(L)Br3] (where LBr is 1-cetyl-4-aza-1-azoniabicyclo[2.2.2]octane bromide) containing a transition metal in the head group has been synthesized and characterized. Physicochemical properties, thermal stability and 3D structure were determined using X-ray diffractometry, UV-vis spectroscopy, simultaneous thermogravimetry and differential scanning calorimetry combined with mass-spectrometry of evolved vapors. The study of the self-assembly and morphological features of associated structures was performed by potentiometry using a bromide ion selective electrode and fluorescence of pyrene and 1,6-diphenyl-1,3,5-hexatriene. The influence of the metal ion embedded into the surfactant structure on critical micelle concentration, degree of counterion binding, aggregation numbers and morphology of the associates was elucidated. High solubilizing capacity and complexation ability of the metal containing micelles with respect to Orange-OT hydrophobic dye and oligonucleotide were determined. Importantly, the functional properties of this metallosurfactant complex are much better compared to those of classical cationic surfactants bearing cyclic and acyclic head groups, LBr and the LBr-CuBr2 mixture. The new cationic metallosurfactant could be recommended for investigation in gene therapy.

Distribution of zwitter-ionic tryptophan between the micelles of 1-dodecyl-3-methyl imidazolium and aqueous medium from molecular dynamic simulation.

Ionic liquids that form micelles have great potential as drug carriers and separating agents for bioactive substances. For such applications, a key issue is the distribution of the target substance between the micelle and its environment. We perform MD simulations to study solubilization of zwitter-ionic tryptophan in micelles of 1-dodecyl-3-methylimidazolium bromide. We found that the distribution of tryptophan depends strongly on the degree of counterion binding. A decrease in binding of bromide counterions leads to a substantial increase of the distribution coefficient. A dense layer of counterions at the micellar surface impedes the solubilization of the zwitter-ionic tryptophan but at the same time the presence of such a dense layer obstructs the washout of the solubilized tryptophan molecules from the micelle. Based on our simulation data, we conclude that an increase of the distribution coefficient of tryptophan between the micelle and water may be achieved by several means: by introducing counterions that bind weakly to the micelle (bulky ions whose charge is not strongly localized) and/or by employing micelle-forming ionic liquids with shorter alkyl chains to diminish the degree of counterion binding.

Association, Distribution, Liberation, and Rheological Balances of Alkyldimethylbenzylammonium Chlorides (C12-C16).

It is known that cationic surfactants have an antimicrobial effect and act as enhancers. This paper studies three cationic surfactants from the group of alkyldimethylbenzylammonium chlorides (dodecyl-, tetradecyl-, and hexadecyl). Interest is focused on the association of the surfactants with respect to temperature, partition balances and their influence on drug release, rheological properties, and the pH of hydrogels. The critical micelle concentrations (CMC) of the surfactants were estimated from dependencies of conductivity, density, spectrofluorimetry, and UV–VIS spectrophotometry on molarity in the temperature range of 25–50 °C. It was found that the temperature dependence of a CMC is U-shaped, with its minimum at 30 °C, and the CMC value decreases as the length of the chain increases. The pseudo-phase separation model was used for the calculation of various thermodynamic parameters, such as the Gibbs free energies (spontaneous process), enthalpies (exothermic process), and entropies of the micelles’ formation, CMCs, and the degree of counterion binding. All thermodynamic parameters, as functions of the temperature, were estimated. It was found that partition coefficients increase as the length of the alkyl chain and the pH = (5.0–7.0) increase. The influences of surfactants, below and above the CMC, on drug (chlorhexidine dihydrochloride) release from hydrogels, rheological properties, and pH at 30 °C were studied. Also, the amounts of the released drug increase as the alkyl chains of the surfactants prolongate. The amounts of the released drug with the surfactant below the CMC are greater than that above the CMC. All hydrogels (regardless of the length of the alkyl chain) exhibit a non-Newtonian pseudo-plastic flow. The results obtained will be used in the formulation of the drug and surfactants into dosage forms.

The Davies equation of state of ionic surfactant adsorbed monolayer and related problems

The Davies equation and relative expressions belong to the most popular tools for describing the equilibrium state of ionic surfactants on the water–air or water–hydrocarbon interface. During over half a century, this equation was repeatedly discussed and modified and various attempts of its derivation/reconsideration were proposed for both insoluble and soluble monolayers. In this paper, the corresponding publications are regarded. Basing on the previously derived modification of the fundamental Gibbs equation [Surface Eng. 50 (2014) 173–182; https://doi.org/10.3103/S1068375514020100.], another expression for the monolayer of the adsorbed ionic surfactant (2D layer) is proposed, which implies dividing the total effect into the electrostatic and non-electrostatic contributions. The dependence of the surface pressure, П, on the area per adsorbed amphiphilic ion in the interface, A, excluded area, A0, and the electrical potential Ψ of the compact monolayer against the bulk aqueous phase is obtained taking into account the degree of counter ion binding, β, and cohesion. In the following equation, the second term of the right-hand side is caused by the electrostatic energy of transfer of the charged amphiphile from the bulk to the monolayer, whereas the third one represents the electrostatic contribution to the surface pressure:Π + Πc= − kT∫∞A(1 + β)(A − A0)−2dA + ∫0Ψ(1 + β) z2eA−1dΨ + ∫0Ψ(1 − β)z2eA−1dΨ, where k, T, and e have their usual meaning. In particular, in its limited form, for β=0 and neglecting the cohesion term, Πc, the equation of state is as follows:Π=kT(A−A0)−1+2∫0ΨeA−1dΨ.This relation differs from the common Davis equation by the coefficient "2" in the last rhs term. Some limiting cases are considered, and an attempt to adapt the equation to experimental data is made.

Effect of biological buffers on the colloidal behavior of sodium dodecyl sulfate (SDS)

This study investigates the influence of three commonly used buffers (TRIS, TES and TAPS) on the conductivities and the colloidal behavior of the aqueous solutions containing a surfactant, sodium dodecyl sulfate (SDS). To find the effect of these buffers on the aggregation behavior of the aqueous SDS solutions, we experimentally determined the conductivities of the solutions containing different concentrations of SDS (0.002M–0.025M) in water and in 0.2M, 0.5M, and 1.0M TRIS, TES, and TAPS buffers, respectively, at 298.2K under atmospheric pressure. To get insight into the effect of the buffers on the aggregation behavior of the aqueous SDS solutions, the critical micelle concentration (CMC) values of the aqueous solutions are estimated from the experimental results of conductivity measurements. Interestingly, in comparison with the CMC of SDS in water, the presence of TRIS is found to increase the CMC, whereas the addition of TES or TAPS is found to decrease the CMC, especially at higher concentration (1.0M). The influence of the buffers on the CMC or aggregation of the SDS is further characterized by calculating various aggregation parameters such as the degree of ionization of the micelle (αm), the degree of counter ion binding in micelle (fm), and the standard Gibbs free energy of micellization (ΔG°m) for all the studied systems. Moreover, the outcomes from the conductivities measurement are reconfirmed by using the fluorescence probe analysis.

Developing and Validating a Set of All-Atom Potential Models for Sodium Dodecyl Sulfate.

We present a set of novel all-atom potential models for sodium dodecyl sulfate (SDS), developed within the framework of the widely used OPLS-AA and General AMBER force fields. The choice of the parameters for the models is made by rigorously following the methodology of the used force fields to ensure full compatibility with the models for other compounds. For the GAFF model, extensive quantum-chemical computations are performed to obtain reliable Boltzmann-averaged atomic point charges, and the latter are compared with the single-conformation charges. For representation of the hydrocarbon tail, we use recently published improved parameters that correctly reproduce the properties of lipids and long alkanes. The models are validated on the basis of correct reproduction of the main properties of micelles (size, degree of counterion binding) as well as diffusion coefficient of the SDS monomer. As an extended test, a simulation of a micelle with a high aggregation number (382) and unnatural initial shape is performed, and a restructuring to the correct shape is observed. This proves the suitability of the developed models for simulations of concentrated SDS solutions containing large micelles and also emphasizes importance of hydrocarbon tail parameters for the micelle properties. Finally, the developed DS- models are tested in combination with several common Na+ and water models. Their effect on the properties of SDS micelles is discussed, and suitable combinations are determined.