Regular Solution Approximation Research Articles

Preparation of some Ethoxylated Alkyl Sulfosuccinate Surfactants and Investigate their Mixed Solutions

Abstract A series of ethoxylated sodium monoalkyl sulfosuccinate (ESMASS) ester surfactants were prepared by reaction of polyethylene glycol (M. wt. 600) with sodium mono alkyl sulfosuccinate (SMASS). The esters were prepared by reaction of octyl, lauryl or cetyl alcohol with sodium sulfosuccinate (SSS) to prepare E(14)SMOSS, E(14)SMLSS and E(14)SMCSS. The chemical structures of the prepared surfactants were confirmed by FTIR and 1HNMR spectroscopy. The surface tension of the synthesized surfactants was measured at 25°C individually or mixing at different molar fraction with the sodium dioctyl sulfosuccinate. The surface active properties were calculated and micellization process of the mixture was investigated. The molar ratio of anionic (SDOSS) and anionic-nonionic surfactants moiety [E(14)SMOSS, E(14)SMLSS and E(14)SMCSS] in the mixed aggregates were deduced using the regular solution approximation equations. Depending on the critical micelle concentration values for each surfactant individually and mixed systems, the minimum surface tension was exhibited at mixture of [0.6 SDOSS and 0.4 E(14)SMCSS]. The micellar composition of the mixed aggregates were explained and discussed based on the effect of their chemical structures. Activity coefficient (f1, f2), interaction parameter (b) and ideality of anionic-nonionic mixed aggregates were evaluated.

Properties of Surface and Micelle in a pH‐Mediated Ternary Surfactant Mixture in Salt Solution

We have investigated the mixing behavoir of a pH‐mediated ternary surfactant mixture at constant ratio of dodecyldimethylamine oxide (DDAO) and Triton X‐100 (9:1). From the equilibrium surface tension measurements at different pHs, the critical micelle concentration (cmc) data were obtained as functions of the pH. Values of the cmc and composition of the micelles were predicted using the regular solution approximation. To some extent, the experimental cmc values agree with the predicted cmc. The average degree of ionization of dodecyldimethylamine oxide in the mixed surfactant systems was estimated using potentiometric titrations. The surface electric potential of the micelles (Ψo) was determined using two methods, one by hydrogen ion titration and the other by the dissociation constants of an acid‐base indicator. In a high degree of ionization of DDAO in the micelles phase (am), Ψo estimated from acid‐base indicator is much higher than that from hydrogen ion titration. In the protonated dodecyldimethylamine oxide/TX‐100 binary surfactant system, Ψo estimated from hydrogen ion titration was as high as 89 mV. The micellar aggregation numbers evaluated by the steady‐state fluorescence probe method increase with pH except at pH=5.03. At pH=5.03, the maximum micelle aggregation number was observed.

On the finite-volume approximation of regular solutions of the p-Laplacian

We consider finite-volume schemes on rectangular meshes for the p-Laplacian with Dirichlet boundary conditions. In Andreianov et al. (2004a, Math. Model. Numer. Anal., 38, 931-959), we constructed a family of schemes and proved discrete W 1,p error estimates in the case of W 2,p solutions of the homogeneous problem. Here we improve these estimates in the case of W 4,1 solutions on uniform meshes for p > 3, using symmetry properties of the schemes. The proof also works for the Laplace equation, giving O(R 2 ) convergence for a family of nine-point finite-volume schemes. With the same ideas, using the improved coercivity inequalities of Barrett and Liu, we obtain even better W 1,p , W 1,1 and L°° convergence rates for special classes of regular solutions to the inhomogeneous problem-in particular, for solutions without critical points in Q, for all p ∈ (1, ∞). Numerical examples are given. They suggest the optimality of the L°° estimates, of order h 2 , obtained for solutions without critical points.

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.

Effect of hydrophobicity of zwitterionic surfactants and triblock polymers on their mixed micelles: A fluorescence study

The pyrene fluorescence measurements of various zwitterionic + triblock polymer surfactant mixtures have been carried out at 25 °C. From the variation of I 1/ I 3 intensity ratio of pyrene fluorescence, the critical micelle concentration (cmc) and other related micellar parameters of single and mixed surfactants have been determined. The nature of the mixed micelles has been evaluated with the help of regular solution and Motomura's approximations. It has been observed that the magnitude of synergistic mixing of zwitterionic and triblock polymer components in the mixed state is mainly governed by both the hydrophobicity of zwitterionic and triblock polymer components. The synergism reduces with an increase in the hydrophobicity of either of the two or both the components. The origin of synergistic interactions has been attributed to the cationic part of the polar head group repulsions by the intercalation of PEO units of triblock polymers.

The Hg—Ra (Mercury—Radium) System

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synergistic behavior of sodiumdodecylsulfate and 1,2-diheptanoyl-sn- glycero-3-phosphocholine in an aqueous medium: interfacial and bulk behavior

The synergistic behavior of sodiumdodecylsulfate (SDS) and 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC) binary mixtures has been studied with interfacial and pyrene fluorescence(I1/ I3) intensity measurements. From the interfacial data, the interfacial parameters; the maximum surface excess (Γmax), or the minimum area per molecule (Amin), and the surface pressure at the critical micellar concentration (πcmc) have been evaluated. The cmc value has been used for evaluating the free energies of micellization (ΔGom). The mixed micelle formation was evaluated with the help of the Clint equation. The SDS plus DHPC mixed micelles showed negative departure from ideality indicating synergistic interactions between the unlike components. The quantitative analysis of mixed micelle, mixed monolayer and the composition of the mixed micelle was carried out with the help of regular solution approximation. The interaction parameters, β and βσ, in the mixed micelle as well as in the mixed monolayer, respectively showed negative values indicating synergistic behavior of SDS and DHPC molecules.

Composition modulation in GaInNAs quantum wells: Comparison of experiment and theory

Composition modulation observed in GaInNAs quantum wells imposes an important handicap to their potential application within optical components, particularly as the indium and nitrogen contents are increased to reach longer wavelengths. In this paper, we compare our experimental results of phase separation in GaInNAs quantum wells grown at different temperatures with recent theoretical models of spinodal decomposition from the literature. This comparison has shown that the regular solution approximation, which explains the higher composition modulation compared to GaInAs samples, provides a more appropriate explanation of GaInNAs decomposition than the usual delta lattice-parameter approximation. Transmission electron microscopy shows no composition modulation contrasts with the chemical sensitive 002 dark field reflection and a strong increase in the intensity of the strain contrasts observed with 220 bright field reflection as the growth temperature increases from 360to460°C. These observations can be explained by an uncoupling between N and In composition profiles forming separate In-rich and N-rich regions according to the regular solution approximation model. We therefore believe that the compositional fluctuations in GaInNAs are not only due to GaInAs decomposition, but that an uncoupled modulation of the III and V elements is also present.

The Hg-Ra (Mercury-Radium) System

The Hg-Ra phase diagram is not known. The melting points of Hg and Ra are −38.8290 and 700 °C, respectively. Information about the crystallographic features of both metals is given in Table 1. [1910Cur] prepared ∼0.8 at.% Ra amalgam by electrolysis. It looked completely fluid at room temperature. When heated, it became solid at 400 °C and emanated the rest of the Hg at 700 °C. [1962Koz] calculated the solubility of Ra in liquid Hg of 1.1 at.% Ra at 25 °C using a semiempirical equation relating metal solubility to its entropy of melting. Calculations by the same author [1992Koz], using a regular solution approximation and the cellular model of Miedema et al. [1983Nie], resulted in solubility values both equal to 0.5 at.% Ra. In the opinion of this evaluator, these estimations give the proper order of magnitude for Ra solubility. One may predict a probable Hg-Ra phase relationships by an extrapolation of the systematic changes in the behavior of Mg, Ca, Sr, and Ba in their binary alloys with Hg [2004Gum]. A distinct tendency to formation of the Hgricher compounds is observed with an increase in the atomic number of the alkaline earth metal series. Therefore, it is not clear whether HgRa2 exists, but most probably the congruently melting HgRa and Hg2Ra compounds should be the most stable phases of the Hg-Ra system. Moreover, a formation of Hg31Ra7, Hg11Ra, and Hg∼6Ra, by analogy to the Ba-Hg compounds, is expected. [1992Koz] reported the formation of HgRa, Hg2Ra, and Hg4Ra, but the experimental evidence underlying this report is unknown. Strong affinity between Hg and Ra is manifested in the partial excess Gibbs energy of Ra in its very diluted amalgam. This energy was found to be −255 ± 10 kJ/mol Ra near room temperature, and −243.8 kJ/mol Ra at 25 °C, respectively, according to [1975Nug] and [1990Kor]. These values point to stronger Hg-Ra interactions in the Hg-rich RaHg alloys than was observed in the Ba-Hg system [2000Gum]. The partial excess Gibbs energy was calculated from the difference between the standard potential of Ra/ Ra(II) and the polarographic half-wave potential of Ra, which was experimentally determined by a radiopolarographic method [1975Nug]. [1977Koz] reported the partial excess Gibbs energy (−250 kJ/mol) and enthalpy (−313 kJ/ mol) of Ra in its diluted amalgam by an extrapolation of the corresponding values for Ca, Sr, and Ba compared with the electronegativity or atom radii of these elements to the position of Ra. Because neither the electronegativity nor the atomic radius of Ra is precisely known, the strongly negative partial enthalpy value should be treated very tentatively. The activity coefficient of Ra in a dilute amalgam was estimated by [1989Gum] to be on the order of 10. This also points to a very strong interaction between Hg and Ra.

Thermodynamic Study of Phase Equilibria in the Ni–Fe–B System

The phase equilibria in the Ni-Fe-B ternary system have been studied experimentally and using thermodynamic calculations. The Gibbs energy of the individual phases was described by the regular solution approximation and the two-sublattice model. The thermodynamic parameters for each phase were evaluated using the experimental data on phase boundaries obtained from differential scanning calorimetry (DSC) and other available literature data. The evaluated parameters enabled us to obtain reproducible calculations of the isothermal section diagrams. The calculated isopleth at the 26 mol% B section agreed with the DSC data obtained in this study, whereas the calculated liquidus and sulidus temperatures were higher than the reported values at the Ni 2 B-Fe 2 B pseudo-binary section.

Synergistic interactions in mixed micelles of alkyltriphenylphosphonium bromides and triblock polymers

The mixed micelle formation of hexadecyltriphenylphosphonium (HTPB) and tetradecyltriphenylphosphonium (TTPB) bromides with triblock polymers such as (PEO) 18(PPO) 31(PEO) 18 (18–31–18), (PEO) 2(PPO) 15(PEO) 2 (2–15–2), (PEO) 2(PPO) 31 (PEO) 2 (2–31–2), and (PEO) 77(PPO) 29(PEO) 77 (77–29–77) have been carried out with the help of fluorescence, relative viscosity, and cloud point measurements. The binary mixtures have been analyzed on the basis of regular solution approximation and by determining the aggregation numbers. It has been observed that the mixtures of HTPB + 18–31–18/2–15–2 and TTPB + 18–31–18/2–15–2 undergo mixed micellization due to synergistic interactions, whereas the mixtures of HTPB/TTPB + 2–31–2/77–29–77 demonstrate antagonistic or ideal mixing. The relative viscosity and cloud point measurements fully support these results. The results also suggest that the favorable mixed micellization with triblock polymers can be achieved if the number of PPO groups are not very large from that of the PEO groups and vice versa as in the case of mixtures of 18–31–18 and 2–15–2 in comparison to that of 2–31–2 and 77–29–77.

Thermodynamic evaluation of the phase equilibria and glass-forming ability of the Fe–Si–B system

A thermodynamic study has been carried out on the Fe–Si–B ternary system, which is important in the development of transformer core materials and Ni-based filler metals. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. Thermodynamic parameters for each phase were evaluated by combining the experimental results from differential scanning calorimetry with literature data. The evaluated parameters enabled us to obtain reproducible calculations of the isothermal and vertical section diagrams. Furthermore, the glass-forming ability of this ternary alloy was evaluated by introducing thermodynamic quantities obtained from the phase diagram calculations into Davies–Uhlmann kinetic formulations. In this evaluation, the time–temperature-transformation (TTT) curves were obtained, which are a measure of the time required to transform to the minimum detectable mass of crystal as a function of temperature. The critical cooling rates calculated on the basis of the TTT curves enabled us to evaluate the glass-forming ability of this ternary alloy. The results show good agreement with the experimental data in the compositional amorphization range.

Mixed adsorption and surface tension prediction of nonideal ternary surfactant systems

To deal with the mixed adsorption of nonideal ternary surfactant systems, the regular solution approximation for nonideal binary surfactant systems is extended and a pseudo-binary system treatment is also proposed. With both treatments, the compositions of the mixed monolayer and the solution concentrations required to produce given surface tensions can be predicted based only on the γ-LogC curves of individual surfactants and the pair interaction parameters. Conversely, the surface tensions of solutions with different bulk compositions can be predicted by the surface tension equations for mixed surfactant systems. Two ternary systems: SDS/Hyamine 1622/AEO7, composed of homogeneous surfactants, and AES/DPCl/AEO9, composed of commercial surfactants, in the presence of excess NaCl, are examined for the applicability of the two treatments. The results show that, in general, the pseudo-binary system treatment gives better prediction than the extended regular solution approximation, and the applicability of the latter to typical anionic/cationic/nonionic nonideal ternary surfactant systems seems to depend on the combined interaction parameter, \( {\mathop {(\beta }\nolimits_{an}^s } + {\mathop \beta \nolimits_{cn}^s })/2 - {\mathop \beta \nolimits_{ac}^s }/4 \): the more it deviates from zero, the larger the prediction difference. If \( {\mathop {(\beta }\nolimits_{an}^s } + {\mathop \beta \nolimits_{cn}^s })/2 - {\mathop \beta \nolimits_{ac}^s }/4 \)→0, good agreements between predicted and experimental results can be obtained and both treatments, though differently derived, are interrelated and tend to be equivalent.

Thermodynamic Analysis of the Ni-Si-Ti System Using Thermochemical Properties Determined from Ab Initio Calculations

A thermodynamic analysis has been carried out on the Ni-Si-Ti ternary system using the CALPHAD method. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. The thermodynamic parameters for each phase were evaluated using available experimental data on the phase boundaries and other related thermochemical properties. In addition to the experimental data, the enthalpy of formation for some binary and ternary compound phases as determined by ab initio calculations was incorporated in the present analysis. There was good agreement between the calculated and the experimental phase equilibria in the binary and ternary systems.

A thermodynamic analysis of the hydrogen ion titration of micelles

A thermodynamic analysis of hydrogen ion titration is presented for association colloids with particular emphasis on surfactant micelles. When a particular type of the micellar Gibbs–Duhem relation (MGD), α M dμ I +(1−α M ) dμ N =0 [ α M: the degree of ionization of micelles; μ I, μ N: chemical potentials of ionized and nonionized species], holds, the free energy change accompanying the ionization of the micelle G ex can be evaluated from the titration data in the same manner as for covalently bonded colloids such as linear polyions. In the case where the regular solution approximation is valid for mixed micelles, the titration curve should be a straight line with a slope yielding the interaction parameter, and G ex is given as a function of α M 2. For dodecyldimethylamine oxide micelles for which the MGD relation has been shown to hold, values of the calculated electrostatic free energy G el were close to but significantly greater than experimental G ex values when the former were calculated on the basis of the Poisson–Boltzmann equation for either a sphere or a plate with smeared charges in a salt solution of infinite volume. When the critical micelle concentration (cmc) data are combined with the hydrogen ion titration data, we obtain a criterion to judge whether the above MGD relation holds or not. When the MGD relation holds, the monomer concentration C 1 can be evaluated from the hydrogen ion titration. For most cases examined, the C 1/ C 1( α M=0) from the titration agrees well with cmc/cmc( α M=0), suggesting cmc= C 1 above the cmc. For tetradecyldimethylamine oxide, the MGD relation does not hold in the range of low ionic strength and even at 0.1 M NaCl it has been found that C 1/ C 1( α M=0)<cmc/cmc( α M=0), due probably to enormous micelle growth.

Unlike surfactant?polymer interactions of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate with water-soluble polymers

Single and mixed micelle formation by sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) and their mixtures in pure water and in the presence of water-soluble polymers such as Synperonic 85 (triblock polymer, TBP), hydroxypropylcellulose (HPC), and carboxymethylcellulose sodium salt (CMC) were studied with the help of conductivity, pyrene fluorescence, cyclic voltammetry, and viscosity measurements. Conductivity measurements showed a single aggregation process for pure surfactants and their mixtures both in pure water as well as in the presence of water-soluble polymers. Triple breaks corresponding to two aggregation processes for SDS, SDBS, and their mixture in the presence of TBP were observed from fluorescence measurements. The first one demonstrated the critical aggregation process due to the adsorption of surfactant monomers on TBP macromolecule. The second one was attributed to the participation of surfactant–polymer aggregates formed at the first one, in the micelle formation process. The aggregation number (N agg ) of single and mixed micelles and diffusion coefficient (D) of electroactive probe were computed from the fluorescence and cyclic voltammetry measurements, respectively. Both parameters, along with the viscosity results, indicated stronger SDS–polymer interactions in comparison to SDBS–polymer interactions. Mixed surfactant–polymer interactions showed compensating effects of both pure surfactants. The nature of mixed micelles was found to be ideal in all cases, as evaluated by applying the regular solution and Motomura's approximations.

Synergism in mixtures of cationic surfactant and anionic copolymers

Surface tension measurements have been made in aqueous solutions of anionic hemiesters of an alternating copolymer of maleic acid and styrene, MAS- n with n=0–12, in the presence of dodecyltrimethylammonium bromide, DTAB. A synergistic aspect of surface tension reduction efficiency was observed for all systems studied. The pseudo-phase separation approach and regular solution approximation have been applied, and the interaction parameter, β, and the mole fraction of DTAB in the adsorbed layer (on a surfactant/repetitive unit basis), X, were obtained. Negative values of β, ranging from −3 to −11, were calculated. On the other hand, the molar fraction of DTAB varies from 0.52 to 0.26. These results are discussed in terms of hydrophobic effects on the distribution of the aggregates between the interface and the bulk of the solution. The conditions predicted by the model to obtain synergism in the tension reduction efficiency are completely satisfied in all cases.

Head-group-induced structural micellar transitions in mixed cationic surfactants with identical hydrophobic tails

The mixed micelle formation by hexadecyltrimethylammonium bromide (HTAB), hexadecylpyridinium bromide (HPyBr), and hexadecylpyridinium chloride (HPyCl) with benzylhexadecyldimethylammonium chloride (BHDACl) was studied with the help of conductivity, κ, and viscometric measurements. Each κ curve for HTAB, HPyBr, and HPyCl showed a single break, whereas double breaks were observed in the case of BHDACl and its binary mixtures with HTAB, HPyBr, and HPyCl. The first break, c1,M, was due to mixed-micelle formation, whereas the second one, c2,M, was due to the structural micellar transitions in the mixed micelles formed during the first break. The quantitative analysis of the mixed-micelle formation corresponding to the first break was carried out with the help of regular solution and Motomura's approximations. The BHDACl plus HTAB and BHDACl plus HPyBr mixtures showed weak synergistic interactions, whereas the BHDACl plus HPyCl mixture was close to ideal. A variation in the degree of dissociation corresponding to the first and second breaks demonstrated a similar variation over the whole mixing range and this was attributed to the identical nature of synergism in both kinds of micellar aggregates available at c1,M, and c2,M. They were rich in BHDACl over most of the mole fraction range. Unlike conductivity measurements, the relative viscosity demonstrated that the micellization of HTAB, HPyBr, and HPyCl proceeded through a minimum, whereas that for BHDACl and its mixtures showed a strong maximum in each case, being stronger in the case of the BHDACl plus HTAB and BHDACl plus HPyCl mixtures.

Phase Equilibria in the Ni-Si-B System

A thermodynamic study has been carried out on the Ni-Si-B ternary system, which is an important system in view of the development of Ni-base filler metals. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. Thermodynamic parameters for each phase have been evaluated using the available experimental information on phase boundaries and other related thermodynamic properties. Thermal analysis experiments have also been conducted on several ternary alloys to re-examine the available ternary experimental data on phase boundaries. The set of evaluated parameters in this study enables reproducible calculations of the liquidus and solidus temperatures and vertical section diagrams satisfactorily. Ni-base brazing filler metals are widely used in the fields of aircraft, various engines, and nuclear engineering due to their high strength at high temperatures, corrosion resistance and oxidation resistance. At present, the study on the development of Ni-base filler metals is done by trial and error. The melting point has to be measured for many alloy specimens in spite of only a few candidate alloys out of them are used for brazing processes. Therefore, the prediction of liquidus and solidus temperatures and phase equilibria is significantly useful in the design of candidate alloy compo- sition. The CALPHAD (CALculation of PHAse Diagrams) approach 1) of phase equilibria calculation using thermody- namic descriptions from databases provides a powerful tool for obtaining such information. In the present study, a thermodynamic analysis of the phase equilibria in the Ni-Si-B system, which is an important system of Ni-base brazing filler metals, has been carried out. And also thermal analysis experiments have been conducted on several ternary alloys in order to re-examine the available experimental data on phase boundaries. 2. Experimental Procedures The phase boundaries of the ternary system were deter- mined by differential scanning calorimetry (DSC). The starting materials are powders of Ni(99.9%), Si(99.9%), and B(99%). The alloys were prepared by arc melting of cold-pressed pellets in an atmosphere of argon. A titanium button was melted to getter the residual oxygen in the chamber, prior to melting the actual charges. The arc-melted alloys were re-melted in vacuum by induction heating in order to ensure homogeneity. The as-cast alloys were encapsulated in quartz tubes under vacuum and then annealed at 850 � C for 18 days before water quenching. No chemical analysis for the alloys was conducted, since the weight losses in preparing alloys were generally less than 1%. The prepared alloy compositions are shown in Table 1. Thermal analysis (DSC) was carried out using a Rigaku ThermoPlus DSC8270 (Rigaku Corp., Tokyo, Japan). A cylindrical specimen was heated and cooled in Al2O3 crucible at a rate of 5 � C/min under a purified argon-flow atmosphere with � -Al2O3 as the standard materials. However, in order to avoid experimental error caused by supercooling, the peak temperature values during heating were adopted in a thermodynamic analysis. The peak temperatures in heating processes are shown in Table 1. In addition, in order to confirm the eutectic temperature of the Si-B system, two alloys of the Si-B binary system were Table 1 Experimental results for phase boundaries of the Si-B binary and the Ni-Si-B ternary alloys determined by DSC. Alloy composition (mol%) Peak temperature ( � C)

Mixed-micelle formation by strongly interacting surfactant binary mixtures: effect of head-group modification

The mixing behavior of several cationic plus anionic surfactant binary mixtures, viz., sodium dodecyl benzene sulfonate (SDBS) plus hexadecyltrimethylammonium bromide (HTAB), SDBS plus tetradecyltrimethylammonium bromide (TTAB), SDBS plus tetradecyltriphenylphosphonium bromide (TTPB), sodium dodecyl sulfate (SDS) plus TTPB, SDBS plus hexadecylpyridinium chloride (HPyCl), and SDS plus HPyCl was studied in pure water with the help of conductivity, interfacial tension, turbidity, and viscosity measurements. The mixed-micelle formation was evaluated with the help of the Clint equation and the regular solution approximation. It was observed that head-group modification had a significant influence on the nature of the mixed-micelle formation. The SDBS plus TTAB mixed micelles showed positive departure from ideality and had positive values for the regular solution interaction parameter, β. The SDS plus HPyCl mixture showed both negative and positive departures in the SDS- and HPyCl-rich regions of the mixtures, respectively, from ideality. The SDBS+TTPB mixture remained close to ideal, and the SDBS plus HTAB, SDBS plus HPyCl, and SDS plus TTPB mixtures showed synergistic behavior. The positive β value in the case of the SDBS plus TTAB mixture was not the result of antagonistic behavior, but was due to the higher value of the mixed critical micelle concentration compared to the ideal mixed cmc values in the ideal state (obtained from the Clint equation) and which was the consequence of unavailability of unlike monomers in the formation of insoluble salt owing to the neutralization of the unlike monomers with opposite polarities. The SDBS plus HTAB mixture did not show similar behavior, most probably owing to the larger size and lower charge density at HTA+ in comparison to at TTA+, which could have resulted in the former having weaker electrostatic interactions with the DBS– anion in comparison to the latter. The weaker electrostatic interactions in conjunction with the packing geometrical constraints are responsible for the close-to-ideal behavior of the SDBS plus TTPB mixture.