Mole Fraction Of Surfactant Research Articles

Upper critical solution temperatures of immiscible phase with dodecyl trimethylammonium bromide, methyltrioctylammoniumchloride, trimethylsulphoxoniumiodide cationic surfactants

Upper critical solution temperatures (UCST) and mutual solubilities of phenol and water were studied with 0.45 mmol kg−1 (mm kg−1) dodecyl trimethylammonium bromide, trimethylsulphoxonium iodide and methyltrioctylammonium chloride. The surfactants decrease the UCSTs by about 0.51–1.98°C with slight enhancement in mutual solubilities. Mole fractions of surfactants were restricted to 0.002 to 0.005 range, their hydrophilic and hydrophobic interactions enhance solubility of phenol in water by about 0.0456% as compared to water-phenol systems.

Adsorption of Anionic–Cationic Surfactant Mixtures on Metal oxide Surfaces

AbstractThis research evaluates the adsorption of anionic and cationic surfactant mixtures on charged metal oxide surfaces (i.e., alumina and silica). For an anionic‐rich surfactant mixture below the CMC, the adsorption of anionic surfactant was found to substantially increase with the addition of low mole fractions of cationic surfactant. Two anionic surfactants (sodium dodecyl sulfate and sodium dihexyl sulfosuccinate) and two cationic surfactants (dodecyl pyridinium chloride and benzethonium chloride) were studied to evaluate the effect of surfactant tail branching. While cationic surfactants were observed to co‐adsorb with anionic surfactants onto positively charged surfaces, the plateau level of anionic surfactant adsorption (i.e., at or above the CMC) did not change significantly for anionic–cationic surfactant mixtures. At the same time, the adsorption of anionic surfactants onto alumina was dramatically reduced when present in cationic‐rich micelles and the adsorption of cationic surfactants on silica was substantially reduced in the presence of anionic‐rich micelles. This demonstrates that mixed micelle formation can effectively reduce the activity of the highly adsorbing surfactant and thus inhibit the adsorption of the surfactant, especially when the highly adsorbing surfactant is present at a low mole fraction in the mixed surfactant system. Thus surfactant adsorption can be either enhanced or inhibited using mixed anionic–cationic surfactant systems by varying the concentration and composition.

Unusual behavior of CMC for binary mixtures of alkyltrimethylammonium bromides: Dependence on chain length difference

The critical micelle concentrations (CMC) of binary mixtures of alkyltrimethylammonium bromides (CnTAB) were measured by a conductivity method. The CMCs of C12TAB-C14TAB and C14TAB-C16TAB systems exhibit the usual behavior, namely a monotonic decrease of the CMC with the mole fraction of the longer chain surfactant. However, the CMC behaviors of C10TAB-C16TAB, C11TAB-C16TAB, C12TAB-C16TAB, and C11TAB-C14TAB are unusual. The behaviors of the CMCs with mole fraction for these systems consist of three regions, of which the first is characterized by a very small decrease of the CMC in the range of low mole fraction, followed by a second where there is an abrupt decrease of the CMC, and a third where the CMCs exhibit their usual behavior. The molecular interaction parameter omega is almost equal to zero for mixtures that have the usual CMC behavior, but is small and positive for those systems with unusual CMCs. We infer that for very low mole fractions of C16TAB, the C16TA ion in the C12TAB-C16TAB system penetrates imperfectly into the micelle and its two methylene groups exist outside the micelle.

The Properties of a Binary Mixture of Nonionic Surfactants in Water at the Water/Air Interface

The behavior of mixed nonionic/nonionic surfactant solutions, that is, p-(1,1,3,3-tetramethylbutyl)phenoxy poly(ethylene glycol)s Triton X-100 (TX100) and Triton X-165 (TX165) have been studied by surface tension and density measurements. The obtained results of the surface tension measurements were compared with those calculated from the relations derived by Joos, Miller, and co-workers. From the comparison, it appeared that by using these two approaches the adsorption behavior of TX100 and TX165 mixtures at different mole fractions can be predicted. The negative deviation from the linear relationship between the surface tension and composition of TX100 and TX165 mixtures in the concentration range corresponding to that of the saturated monolayer at the interface, the values of the parameters of molecular interaction, the activity coefficients, as well as the excess Gibbs energy of mixed monolayer formation calculated on the basis of Rosen and Motomura approaches proved that there is synergism in the reduction of the surface tension of aqueous solutions of TX100 and TX165 mixture when saturation of the monolayer is achieved. The negative parameters of intermolecular interaction in the mixed micelle and calculations based on MT theory of Blankschtein indicate that there is also synergism in the micelle formation for TX100 and TX165 mixture. It was also found that the values of the standard Gibbs energy of adsorption and micellization for the mixture of these two surfactants, which confirm the synergetic effect, can be predicted on the basis of the proposed equations, which include the values of the mole fraction of surfactant and excess Gibbs energy TX100 and TX165 in the monolayer and micelle.

The adsorption at solution–air interface and volumetric properties of mixtures of cationic and nonionic surfactants

Surface tension, density and conductivity measurements were carried out for systems containing mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol), Triton X-100 (TX100). The obtained results of the surface tension measurements were compared with those calculated from the relations derived by Joos, Miller and co-workers. From the comparison it appeared that using the modified adsorption isotherm derived by Joos the adsorption behaviour of CTAB and TX100 mixture can be predicted satisfactorily; however, the values of the surface tension of aqueous solutions of these mixtures calculated on the basis of the simple relationships of Miller et al. are a little higher than those measured in contrast to many two component systems of surfactants studied earlier. On the basis of the results obtained from the measurements and calculations it was found that there is a linear relationship between the surface tension and composition of CTAB and TX100 mixtures at their low concentration; however, in the concentration range corresponding to that of the saturated monolayer at the interface a negative deviation from the linear relationship is observed. This fact and the values of the parameters of molecular interactions in the mixed monolayer suggest that there is synergism in the reduction of the surface tension of aqueous solutions of CTAB and TX100 mixture when saturation of the monolayer is achieved. The negative parameters of intermolecular interaction in the mixed micelle and calculations based on MT theory of Blankschtein indicate that there is also synergism in the micelle formation for CTAB and TX100 mixture. It was also found that the values of the standard free energy of micellization for the mixture of CTAB and TX100 are somewhat lower than for individual components and they can be predicted on the basis of Maeda equation and the mole fraction of surfactant and free enthalpy of mixing CTAB and TX100 in the micelle. Our measurements and calculations indicate that the volume both of the individual surfactants and mixture studied decreases during the micellization process; however, at the concentration of the surfactants higher than CMC an increase of this volume is observed.

Investigation of Mixed Micellar Systems of Ionic+C12En Surfactants

The micellization process of binary mixtures formed by polyoxyethylene monododecyl ether (C12En; n=12 and 15) and different ionic surfactants, anionic sodium dodecyl sulfate (SDS) and cationic hexadecylpyridinium bromide (CPB), was examined by using surface tension and viscosity measurements. Rubingh's nonideal solution theory predicted nonideal mixing and attractive interaction between the constituent surfactants in the mixed micelle. In the single systems, the relative viscosity of nonionic surfactant is greater than that of the ionic surfactant and increases with the increasing level of ethylene oxide. In the mixed surfactant systems, the relative viscosities vary at a mixed molar fraction between 0.2 and 0.3, and then the relative viscosities of mixed systems decrease with the increasing mole fraction of ionic surfactants. The experimental viscosity values show a positive deviation from ideal behavior because of mixed micelle formation and the electroviscous effect. This effect could be suppressed by the addition of NaCl.

Effects of Polyelectrolyte Chain Stiffness, Charge Mobility, and Charge Sequences on Binding to Proteins and Micelles

The binding affinities of polyanions for bovine serum albumin in NaCl solutions from I = 0.01-0.6 M, were evaluated on the basis of the pH at the point of incipient binding, converting each such pH(c) value into a critical protein charge Zc. Analogous values of critical charge for mixed micelles were obtained as the cationic surfactant mole fraction Yc. The data were well fitted as Yc or Zc = KI a, and values of K and a were considered as a function of normalized polymer charge densities (tau), charge mobility, and chain stiffness. Binding increased with chain flexibility and charge mobility, as expected from simulations and theory. Complex effects of tau were related to intrapolyanion repulsions within micelle-bound loops (seen in the simulations) or negative protein domain-polyanion repulsions. The linearity of Zc with radicalI at I < 0.3 M was explained by using protein electrostatic images, showing that Zc at I < 0.3 M depends on a single positive "patch"; the appearance of multiple positive domains I > 0.3 M (lower pH(c)) disrupts this simple behavior.

Mixed micellization and interfacial properties of dodecyltrimethylammonium bromide and tetraethyleneglycol mono-n-dodecyl ether in absence and presence of sodium propionate

Mixed micelle formation and interfacial properties of aqueous binary surfactant combinations of dodecyltrimethylammonium bromide (C12TAB) and tetraethyleneglycol mono-n-dodecyl ether (C12E4) at 30 °C in absence and presence of sodium propionate (NaPr) have been investigated. The critical micelle concentration, aggregation number, micropolarity and interfacial adsorption have been quantitatively estimated by surface tension and steady-state fluorescence measurements. The micellar and adsorption characteristics like composition, activity coefficients and mutual interaction parameters have been estimated following different theoretical treatments like that of Clint, Rubingh, Rodenas, Maeda, Blankschtein and Rosen. The analysis reveals very small mole fraction of cationic surfactant in both the mixed micelles and mixed monolayer inspite of synergism. Blankschtein's model predicts a continuous decrease in synergism due to the salt effect of NaPr; Rubingh's approach, on the contrary, indicates an increase in it above 30 mM of NaPr concentration. Aggregation number variation with NaPr indicates the same. Mixed monolayer shows better synergism compared to that in mixed micelles which increases with the addition of sodium propionate above 30 mM concentration.

The role of the surfactant head group in the emulsification process: Binary (nonionic–ionic) surfactant mixtures

Dilute emulsions of dodecane in water were prepared under constant flow rate conditions with binary surfactant systems. The droplet size distribution was measured as a function of the mixed surfactant composition in solution. The systems studied were (a) the mixture of anionic sodium dodecyl sulfate (SDS) with nonionic hexa(ethyleneglycol) mono n-dodecylether (C 12E 6) and (b) the mixture of cationic dodecyl pyridinium chloride (DPC) with C 12E 6. At a constant concentration of SDS or DPC surfactant in solution (below the CMC) the mean emulsion droplet size decreases with the increase in the amount of C 12E 6 added to the solution. However, a sharp break of this droplet size occurs at a critical concentration and beyond this point the mean droplet size did not significantly change upon further increase of the C 12E 6. This point was found to corresponded to the CMC of the mixed surfactant systems (as previously determined from microcalorimetry measurements) and this result suggested the mixed adsorption layer on the emulsion droplet was similar to the surfactant composition on the mixed micelles. The emulsion droplet size as a function of composition at the interface was also studied. The mean emulsion droplet size in SDS–C 12E 6 solution was found to be lower than that in DPC–C 12E 6 system at the equivalent mole fraction of ionic surfactant at interface. This was explained by the stronger interactions between sulphate and polyoxyethylene head groups at the interface, which facilitate the droplet break-up. Counterion binding parameter ( β) was also determined from zeta-potential of dodecane droplets under the same conditions and it was found that ( β) was independent of the type of the head group and the mole fraction of ionic surfactant at interface.

Surface Properties of Gleditsia Saponin and Synergisms of Its Binary System

A gleditsia saponin (GS) is isolated and purified from the fruits of Gleditsia sinensis Lam. Its surface and thermodynamic properties in aqueous solution at 0, 15, 25, 35, 45, and 60°C have been investigated by surface tension measurement. The major data at 25°C are the critical micelle concentration (cmc), 1.10E‐3 mol · L−1; the surface tension at cmc (γ cmc ), 34.4 mN · m−1; the maximum surface excess concentration (Γmax), 2.33E‐6 mol · m2; the minimum area per molecule (A cmc ), 0.712 nm2; the standard adsorption energy (ΔG ad 0), −32.9 kJ · mol−1; and the standard micellization energy (ΔG m 0), −16.9 kJ · mol−1. The molecular interaction parameters, β s and β m , for the binary systems of GS with a cationic surfactant (C16H33NMe3Br), an anionic surfactant (C12H25SO3Na), and a nonionic surfactant (C8H17Ph(EO)10OH) have been determined. Based on the regular solution treatments of Rubingh, Rosen, and their coworkers, the conditions of synergism in surface tension reduction effectiveness have been extended. The condition to produce this synergism and the corresponding parameters at the maximum synergism point, i.e., the mole fractions of surfactant 1 in the solution phase (α1*), the critical micelle concentration of mixtures (cmc 12*), and the surface tension at this concentration (γ cmc12*), are given. The results show that of the three binary systems, synergism in micelle formation can occur only in the mixture of GS with cationic surfactants; synergism in surface tension reduction efficiency can occur in the mixture of GS with both cationic surfactant and nonionic surfactants, but, all the binary systems of GS with ionic and nonionic surfactants can show remarkable synergism in surface tension reduction effectiveness.

Solubilization by different-sized surfactant mixtures

The solubilization phenomenon was investigated in mixed surfactant systems. The solubilization power of a mixed surfactant reaches its maximum at a particular temperature at each mixing ratio of surfactants. When the mole fraction of C 4E 1 in the total surfactant ( w 1 value) was varied in a water/C 12E 5/C 4E 1/decane system, the minimum mole fraction of total surfactant in the system necessary to obtain a single microemulsion phase ( ξ value) was almost unchanged for w 1 < 0.3 , whereas it increased remarkably for w 1 > 0.8 . The molar solubilization capacity ( C s = ( 1 − ξ ) / ξ ) of the mixed surfactant decreased remarkably for w 1 < 0.3 , whereas it decreased gradually for w 1 > 0.8 . The result | d ξ / d w 1 | w 1 < 0.3 < | d ξ / d w 1 | w 1 > 0.8 is due largely to the characteristic of the function ξ ( C s ) = 1 / ( 1 + C s ) , specifically, | d ξ / d C s | w 1 < 0.3 < | d ξ / d C s | w 1 > 0.8 , where d ξ / d w 1 = ( d ξ / d C s ) ( d C s / d w 1 ) . The partial molar solubilization capacity ( C ¯ s ) of C 4E 1 was negative at almost all w 1 , but the C ¯ s value of C 12E 5 went through a maximum on the addition of C 4E 1. Propanol (a cosurfactant) has the same effect on the solubilization phenomenon in the water/C 12E 6/propanol/heptane system. In the water/C 12E 5/C 12E 7/decane system, the C ¯ s value of each surfactant did not vary greatly as the mixing ratio of surfactants was varied. The C s and ξ values were close to molar additivity for each mixing ratio.

Effect of NaCl and temperature on the water solubilization behavior of AOT/nonionics mixed reverse micellar systems stabilized in IPM oil

Solubilization of water in mixed reverse micellar systems formed with anionic surfactant (AOT) and nonionic surfactants (Brij-30, Brij-35, Brij-52, Brij-56, Brij-58, Brij-76, Tween-20, Tween-40, Span-20, Span-40, Span-60, Span-80) in isopropyl myristate (IPM) oils has been studied. The enhancement in water solubilization (i.e. synergism) has been evidenced by the addition of nonionic surfactants to AOT/IPM system. The maximum water solubilization capacity ( ω 0,max) and the mole fraction of nonionic surfactant at which maximization occurs ( X nonionic,max) in these mixed reverse micellar systems has been found to depend on surfactant component (size and nature of polar group and hydrocarbon moiety of the surfactant). The addition of electrolyte (NaCl) in these systems has been found to enhance the solubilization capacity ( ω max) depending upon the content and their EO chains and type of polar head group of nonionic surfactant used. The maximum solubilization of electrolyte, ω max was obtained at an optimal concentration of it ([NaCl] max), which depends on the content and their EO chains and type of polar heads of the nonionic surfactants. The temperature induced solubilization of water (in absence and presence of additives) for AOT blended with nonionic surfactants (Brijs, Tweens, Spans) at different compositions has been investigated. The stability of these systems with respect to temperature has been found to be dependent on the content and nature of the polar head group as well as the hydrophobic moiety of the nonionics. The energetic parameters ( Δ G ¯ ° , Δ H ¯ ° and Δ S ¯ ° ) of the desolubilization process of water in the mixed systems have been estimated from the solubilization capacity–temperature profile. An attempt has been made to shed more insight into the process of solubilization–desolubilization of water (or aqueous NaCl) in mixed reverse micelles stabilized in IPM oil on the basis of the model proposed by Shah et al. [M.J. Hou, D.O. Shah, Langmuir 3 (1987) 1086; R. Leung, D.O. Shah, J. Colloid Interf. Sci. 120 (1987) 320, 330] and thermodynamic approach.

Study on the interaction of anionic dye–nonionic surfactants in a mixture of anionic and nonionic surfactants by absorption spectroscopy

The interaction between an anionic dye C.I. Reactive Orange 16 (RO16) and different surfactants, i.e., the anionic surfactant sodium dodecylsulfate (SDS) and nonionic surfactants polyoxyethylene ethers (C m POE n ; m = 12, 16 and 18, n = 4, 10 and 23) has been researched spectrophotometrically in a certain micellar concentration range. Light absorbances in the visible spectral range are measured as a function of mole fraction of surfactant at four different temperatures and at 24 h over the periodic time intervals. For this reason, a typical system occurred at 1.0 × 10 −2 mol/l for surfactants and at 1.0 × 10 −4 mol/l for dye concentrations. It is evident from the experimental measurements that while nonionic surfactants form a complex with the anionic dye RO16 in aqueous solution in studied concentration range, anionic surfactant does not. The formation of C m POE n –RO16 complex in the SDS solutions of different mole fractions in its micellar concentration range has been determined and compared to those obtained in the dye–surfactant binary mixtures. Spectrophotometric measurements indicated that the value of absorbance of SDS–RO16 is greater than that of nonionic surfactant–RO16 in the binary systems. This state explains that the ionic and hydrophobic interactions between SDS–RO16 are less than that of nonionic surfactant–RO16 interactions. The addition of SDS to the mixture of C m POE n –RO16 causes an increase in the value of absorbance. This increase is due to changed hydrophobic–hydrophilic balance of the studied mixtures. Furthermore, we observed that the alkyl chain length and the number of polyoxyethylene in nonionic surfactants do not have an enormous effect on the values of absorbance.

Salt effect on the complex formation between cationic gemini surfactant and anionic polyelectrolyte in aqueous solution.

Salt effect on the interaction of anionic polyelectrolyte sodium carboxymethylcellulose (NaCMC) with cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (C12C6C12Br2) has been investigated using turbidimetric titration, steady-state fluorescence, and mobility measurement. It is found that the critical aggregation concentration(cac) for C12C6C12Br2/NaCMC complexes depends little on addition of sodium bromide (NaBr). However, in the presence of nonionic surfactant Triton X-100 (TX100), the critical ionic surfactant mole fraction for the onset of complex formation (Yc) increases markedly with increasing NaBr concentration. These salt effects are supposed as the overall result from competition between the increase of interaction and the screening of interaction. The increase of interaction is referred to as the effect that the larger micelle with higher surface charge density induced by salt has a stronger interaction with oppositely charged polyelectrolyte. The screening of interaction is referred to as the salt screening of electrostatic attraction between the polymer chain and the surfactant. For complex formation between C12C6C12Br2 and NaCMC, the increase of interaction probably compensates the screening of interaction, leading to constant cac values at different salt concentrations. For complex formation between the C12C6C12Br2/TX100 mixed micelle and NaCMC, the screening of interaction probably plays a dominant role, leading to higher suppression of electrostatic binding of micelles to polyelectrolyte.

Study of the electrostatic and steric contributions to the free energy of ionic/nonionic mixed micellization

We have investigated mixed micelle formation in mixtures of ionic and nonionic surfactants. Three types of ionic surfactant were used: CTAB (hexadecyltrimethylammonium bromide), CPC (hexadecylpyridinium chloride) and CPB (hexadecylpyridinium bromide). These surfactants all have a hexadecyl chain but contain different head groups. The use of CPC and CPB, which differ only in counter ion, allowed investigation of counter ion effects. TritonX-100 ( p-(1,1,3,3-tetramethylbutyl) polyoxyethylene) was used as the nonionic surfactant in all experiments. PFG-NMR was used to measure the self-diffusion coefficients of the mixed micelles as a function of solution composition and total surfactant concentration. The aggregation number and hydrodynamic radius of each type of mixed micelle were determined by combining viscosity and self-diffusion coefficient measurements. The electrostatic and steric contributions to the free energy of mixed micellization, which are considered to be the most important contributions for mixtures of ionic and nonionic surfactants with different head groups, were determined. The electrostatic free energy was determined by solving an analytical approximation to the Poisson–Boltzmann equation. The electrostatic free energy varied dramatically with increasing mole fraction of ionic surfactant in the CTAB/TritonX-100 system but increased more gradually in the CPC/TritonX-100 and CPB/TritonX-100 systems. The more pronounced change for CTAB/TritonX-100 system can be attributed to the trimethylammonium headgroup of CTAB, which confers a high surface charge density and thus a high electrostatic free energy.

Spectrophotometric studies of anionic dye–cationic surfactant interactions in mixture of cationic and nonionic surfactants

The interaction between an anionic dye C.I. Reactive Orange 16 (RO16) and a cationic surfactant dodecylpyridinium chloride (DPC) in mixtures of DPC and nonionic surfactants poly(oxyethylene)ethers (C m POE n ; m = 12, 16 and 18, n = 4, 10 and 23) are investigated spectrophotometrically in a certain micellar concentration range. The spectrophotometric measurements of dye–surfactant systems are carried out as function of mole fraction of surfactant at four different temperatures. For this reason, a typical system was occurred at 1.0 × 10 −2 mol l −1 for surfactants and at 1.0 × 10 −4 mol l −1 for dye concentrations. The formation of DPC–RO16 complex in the C m POE n solutions of different mole fractions in its micellar concentration range have been determined and compared to those obtained in the binary mixtures. From the spectrophotometric measurements has been observed that the addition of nonionic surfactant in to the mixture of DPC–RO16, causes a significant increase of the value of absorbance. This increase explains that the stability of DPC–RO16 complex is reduced in the presence of nonionic surfactant micelles. It can be seen from results; in mixed surfactant solutions, there are DPC–C m POE n and RO16–C m POE n interactions in addition to DPC–RO16 interaction. Since the solubilizaton of the DPC–RO16 complex has been appeared in the C m POE n solution, our results support the conclusion that adding C m POE n influences the hydrophobic–hydrophylic balance of the studied complex. Furthermore effect of the alkyl chain length and the number of poly(oxyethylene) in nonionic surfactant on values of absorbance have been investigated.

Variegated micelle surfaces: correlating the microstructure of mixed surfactant micelles with bulk solution properties.

Electron paramagnetic resonance, viscosity, and small-angle neutron scattering (SANS) measurements have been used to study the interaction of mixed anionic/nonionic surfactant micelles with the polyampholytic protein gelatin. Sodium dodecyl sulfate (SDS) and the nonionic surfactant dodecylmalono-bis-N-methylglucamide (C12BNMG) were chosen as "interacting" and "noninteracting" surfactants, respectively; SDS micelles bind strongly to gelatin but C12BNMG micelles do not. Further, the two surfactants interact synergistically in the absence of the gelatin. The effects of total surfactant concentration and surfactant mole fraction have been investigated. Previous work (Griffiths et al. Langmuir 2000, 16 (26), 9983-9990) has shown that above a critical solution mole fraction, mixed micelles bind to gelatin. This critical mole fraction corresponds to a micelle surface that has no displaceable water (Griffiths et al. J. Phys. Chem. B 2001, 105 (31), 7465). On binding of the mixed micelle, the bulk solution viscosity increases, with the viscosity-surfactant concentration behavior being strongly dependent on the solution surfactant mole fraction. The viscosity at a stoichiometry of approximately one micelle per gelatin molecule observed in SDS-rich mixtures scales with the surface area of the micelle occupied by the interacting surfactant, SDS. Below the critical solution mole fraction, there is no significant increase in viscosity with increasing surfactant concentration. Further, the SANS behavior of the gelatin/mixed surfactant systems below the critical micelle mole fraction can be described as a simple summation of those arising from the separate gelatin and binary mixed surfactant micelles. By contrast, for systems above the critical micelle mole fraction, the SANS data cannot be described by such a simple approach. No signature from any unperturbed gelatin could be detected in the gelatin/mixed surfactant system. The gelatin scattering is very similar in form to the surfactant scattering, confirming the widely accepted picture that the polymer "wraps" around the micelle surface. The gelatin scattering in the presence of deuterated surfactants is insensitive to the micelle composition provided the composition is above the critical value, suggesting that the viscosity enhancement observed arises from the number and strength of the micelle-polymer contact points rather than the gelatin conformation per se.

Thermodynamic studies of mixed ionic/nonionic surfactant systems

Mixtures of alkyltrimethylammonium bromide (C n TAB, n=12,14,16,18) and Triton X-100 were studied at a range of mole fractions of ionic surfactant per nonionic surfactant. For each mixture, the cmc obtained from surface tension measurements differed from that obtained using potentiometry. The behavior of these mixed-surfactant systems showed three different regions with increasing total surfactant concentration. From the surface tension and potentiometry data, we obtained the free monomer concentration of ionic surfactant ( m 1), the micellar mole fraction of surfactant ( x i ), and the degree of dissociation ( α) of ionic surfactant. We also obtained the free monomer concentration of Triton X-100 ( m 2) using PFG-NMR technique. A new equation was introduced to evaluate the activity coefficient in the micellar phase. The excess free energy ( G E) and the synergetic parameters of mixtures were determined at various mole fractions of C n TAB/Triton X-100. Finally, the complexity of the synergism parameters was investigated.

Balance of pH and Ionic Strength Influences on Chain Melting Transition in Catanionic Vesicles

We study the thermotropic phase transition of “salt-free” mixtures of single-tailed cationic and anionic surfactants, called “catanionic systems” in this paper, for concentrated (10 wt %) and diluted (0.5 wt %) samples. Small-angle neutron scattering (SANS) and differential scanning calorimetry (DSC) demonstrate that the chain melting phase transition is identical for diluted and concentrated samples at the same mole fraction (r) of anionic surfactants. The addition of anionic surfactants (for r = 0.5−0.7) induces an increase of the global surface charge, followed by an increase of the main transition temperature. This evolution may be interpreted as the hydrogen bonding formation between carboxylic acid headgroups. At r ∼ 0.6, corresponding to the region where facetted vesicles are formed, the “melting” of the alkyl chains is a progressive process extending over 15 °C. Here, molten and crystallized bilayers coexist in swollen lamellar phases of catanionic bilayers. This coexistence explains the increase ...

Determination of Isotherms for Binding of Surfactants to Vesicles Using a Surfactant-Selective Electrode

Mixtures of the nonionic lipid glycerol monooleate with four cationic surfactants have been investigated. The surfactants were cetyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, and dodecyltrimethylammonium chloride, and one with a partially fluorinated chain, N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride. With a low addition of the cationic surfactant, GMO easily forms vesicles. Using a surfactant selective electrode, binding isotherms of the surfactants to a suspension of preformed vesicles were determined. The mole fractions of surfactant at which the micelle−vesicle coexistence region and the micelle region were reached, as obtained from the binding isotherms, were compared to the location of the corresponding phase borders in the phase diagrams of the respective mixtures. The different stages of the solubilization process were directly followed using cryo-TEM. Instead of a micelle−vesicle coexistence region, perforated vesicles were observed during the solubilization w...