Solubilization of phospholipid bilayer caused by surfactants

The interaction of surfactants with liposomes eventually leads to the rupture of such structures and the solubilization of the phospholipid components. In this paper, solubilization is regarded as a decrease in light scattering of liposome suspensions. To this end, in accordance with the nomenclature, adopted by Lichtenberg, three parameters were considered as corresponding to the effective surfactant/lipid molar ratios (Re) at which light scattering starts to decrease, Re_sat; reaches 50% of the original value, Re₅₀; and shows no further decrease, Re_sol. These parameters corresponded to the Re at which the surfactant (i) saturated the liposomes, (ii) resulted in a 50% solubilization of vesicles and (iii) led to a total solubilization of liposomes. The surfactants tested were the nonionic surfactant octylphenol ethoxylated with 10 units of ethylene oxide or Triton X-100 (OP-10EO), two anionic surfactants, sodium dodecyl sulfate and sodium dodecyl ether sulfate, and an amphoteric surfactant dodecyl betaine (D-Bet). Unilamellar liposomes formed by egg phosphatidylcholine containing increasing amounts of phosphatidic acid were used. The Re parameters were the lowest for D-Bet, followed by OP-10EO, whereas the anionic surfactants always showed the highest values regardless of the electrical charge of the lipid bilayers. These parameters seem also to be inversely related to the critical micelle concentration (CMC) of the surfactant, except for OP-10EO. Moreover, the CMC values of the surfactant/lipid systems at 0.5 mM lipid concentration corresponded in all cases to the surfactant concentration at which liposomes were saturated by surfactants. As a consequence, this ratio can be regarded as an interesting parameter associated with the mixed micelle formation in liposome solubilization.     

Solubilization of model stratum corneum liposomes by quaternary ammonium surfactants

The solubilizing interactions of a series of quaternary ammonium surfactants [alkyl chain lengths C-12 (DoTAB), C-14 (TeTAB), and C-16 (HeTAB)] with liposomes formed by a mixture of lipids modeling the stratum corneum (SC) lipid composition (40% ceramides, 25% cholesterol, 25% palmitic acid, and 10% of cholesteryl sulfate) were investigated. Surfactant/lipid molar ratios (Re) and bilayer/aqueous phase partition coefficients (K) were determined by monitoring changes in static light scattering of the system during solubilization. Free surfactant concentration was always similar to the critical micelle concentration (CMC). A general assumption for phosphatidylcholine (PC) liposomes suggests that the free surfactant concentration must reach CMC for solubilization to occur. This assumption can be applied to SC liposomes in this study, and indicates that liposome solubilization was mainly driven by mixed micelle formation. The Re and K parameters fell as the surfactant alkyl chain length decreased or CMC increased. Thus, a higher CMC corrsponds to an increased ability of these surfactants to saturate or solubilize SC liposomes and to a lower degree of partitioning into liposomes or affinity with these bilayer structures. The overall balance of these opposing tendencies shows that TeTAB had the highest effectiveness with respect to the saturation and solubilization of SC structures in terms of total surfactant needed to produce these effects. Different trends in surfactant interaction with SC liposomes were observed when comparing Re and K parameters with those for PC liposomes. Because SC liposomes were more resistant to the surfactant action, the affinity of surfactants with these bilayer structures was higher in all cases.     

Changes in phospholipid bilayers caused by sodium dodecyl sulfate/nonionic surfactant mixtures

The interaction of mixtures of sodium dodecyl sulfate (SDS) and oxyethylenated nonylphenol (30 mol of ethylene oxide) [NP(EO)₃₀] with phosphatidylcholine liposomes was investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles, and bilayer solubilization was measured as a decrease in the static light scattered by liposome suspensions. Three parameters were described as the effective surfactant/lipid molar ratios (Re) at which the surfactant system: (i) resulted in 50% CF release (Re _50%CF); (ii) saturated the liposomes (Re _SAT); (iii) led to complete solubilization of these structures (Re _SOL). The corresponding surfactant partition coefficients (K _50%CF, K _SAT, and K _SOL) were determined from these parameters. The free surfactant concentrations S _W were lower than the mixed surfactant critical micellar concentration at subsolubilizing levels, whereas they remained similar to these values during saturation and solubilization of bilayers. Although the Re values increased linearly as the mole fraction of the SDS rose (X _SDS), the K parameters showed maximum values at X _SDS 0.6 for K _50%CF and approximately at X _SDS 0.2 for K _SAT and K _SOL, respectively. Thus, the lower the surfactant contribution in the surfactant/lipid system, the higher the X _SDS at which the maximum bilayer/water partitioning of added mixed surfactant systems occurred. As a consequence, the influence of SDS in this partition appears to be more significant at the sublytic level (monomeric effect), whereas the influence of NP(EO)₃₀ seems to be greater during saturation and solubilization of liposomes via formation of mixed micelles.     

Solubilization of phosphatidylcholine unilamellar liposomes caused by alkyl glucosides

Solubilizing alterations caused by a series of alkyl glucosides (alkyl chain lengths ranging from C₈ to C₁₂) in phosphatidylcholine (PC) unilamellar liposomes were investigated. Surfactant-to-phospholipid molar ratios (Re) and bilayer/aqueous phase partition coefficients (K) were determined by monitoring changes in static light scattering (SLS) of the system during solubilization. At the two interaction levels investigated (surfactant concentrations producing SLS values of 100 and 0% for each surfactant/PC system studied) the free surfactant concentration for each surfactant was always comparable to its critical micelle concentration (CMC). This indicates that liposome solubilization was mainly ruled by mixed micelle formation. A rise in CMC (or decrease in the surfactant alkyl chain length) resulted in an increase in the ability of these surfactants to saturate or solubilize PC liposomes and, inversely, in an abrupt decrease in their affinity with these bilayer structures. The overall balance of these opposite tendencies shows that the octyl glucoside had the highest ability to saturate and solubilize liposomes (lowest Re values), whereas the dodecyl glucoside exhibited the highest degree of partitioning into liposomes or affinity with bilayer structures (highest K values). From a practical viewpoint, the use of nonyl glucoside reduced approximately 2.5 times the concentration needed to saturate and solubilize 1.0 mM PC liposomes with respect to that needed using the conventional octyl glucoside.     

Permeability alterations in unilamellar liposomes due to betaine-type zwitterionic and anionic surfactant mixed systems

The alterations caused by betaine-type zwitterionic and anionic surfactant mixed systems in the permeability of unilamellar liposomes have been investigated. The partition coefficient of these systems, at different molar fractions, between the aqueous phase and the lipid bilayer of liposomes has been determined. These surfactant mixed systems were formed byN-dodecyl-N,N-dimethylbetaine (C₁₂-Bet) and sodium dodecyl sulfate (SDS) in the presence of 20 mM PIPES buffer and 110 mM Na₂SO₄, at pH 7.21. Unilamellar liposomes were prepared from egg phosphatidylcholine and phosphatidic acid (9:1 molar ratio). The release of the fluorescent agent 5-(6)-carboxyfluorescein induced by the systems has been studied at sub-solubilizing concentrations. When the molar fraction of C₁₂-Bet/SDS is about 0.4, the critical micelle concentration values of these systems exhibit a minimum, whereas their partition coefficient between the aqueous phase and lipid bilayer of lipid bilayers shows a maximum. There is a consistent correlation between the partition coefficient and the ability of the different systems of surfactants to modify the permeability of liposomes.     

Solubilizing effects caused by the nonionic surfactant octyl glucoside in phosphatidylcholine liposomes

The mechanisms governing the interaction of the nonionic surfactant octyl glucoside (OG) on phosphatidylcholine (PC) liposomes were investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles, and bilayer solubilization was determined as a decrease in the static light scattered by liposome suspensions. A direct relationship was established in the initial interaction steps (10–50% CF release) between the growth of vesicles, the leakage of entrapped CF, and the effective molar ratio of surfactant to phospholipid in bilayers (Re). This dependence was also detected during the solubilization range of Re values between 1.3 and 3.0, where the decrease in the surfactant-PC aggregate size and in the light scattering of the system depended on the Re parameter and, hence on the composition of these aggregates. The free OG concentrations at subsolubilizing and solubilizing levels showed lower and similar, respectively, values than its critical micelle concentration (CMC). These findings indicated that the alterations in bilayer permeability were due to the action of surfactant monomers, whereas bilayer solubilization was determined by the formation of mixed micelles. This finding supports the generally accepted assumption that the concentration of free surfactant must reach the CMC for solubiliation to occur.     

Permeability changes caused by surfactants in liposomes that model the stratum corneum lipid composition

The alterations caused by different surfactants in the permeability of liposomes formed by a lipid mixture that models the stratum corneum (SC) composition (40% ceramides, 25% cholesterol, 25% palmitic acid, and 10% cholesteryl sulfate) were investigated. The surfactant/lipid molar ratios (Re) and the bilayer/aqueous phase surfactant partition coefficients (K) were determined at two sublytic levels. The selected surfactant were sodium dodecyl sulfate (SDS); sodium dodecyl ether sulfate (SDES) to assess the influence of the ethylene oxide groups on the anionic surfactant’s behavior; Triton X-100 (OP-10EO) and dodecyl betaine (D-Bet) as representatives of nonionic and amphoteric surfactants. Permeability alterations were determined by monitoring the increase in the fluorescence intensity of liposomes due to the 5(6) carboxyfluorescein (CF) released from the interior of vesicles. The SC liposomes/surfactant sublytic interactions were mainly ruled by the action of surfactant monomers. OP-10EO showed the highest ability to alter the permeability of bilayers and the highest affinity with these structures, whereas D-Bet showed the lowest tendencies. Although SDS and SDES exhibited similar activity at 50% CF release (similar Re values), SDES appeared to be more active at 100% CF release, its affinity with bilayers being also increased. The different ability exhibited by SDS, SDES, and D-Bet (same alkyl chainlength) to alter the permeability of SC liposomes emphasizes the role played by the polar part of these surfactants in this interaction. Different trends in the evolution of Re and K were observed when comparing the results with those reported for phosphatidylcholine (PC) liposomes. Thus, whereas SC liposomes appeared to be more resistant to the action of surfactants, the surfactant affinity with SC bilayers was always greater than that reported for PC bilayers.     

Assembly properties of the aggregates resulting in the solubilization of phosphatidylcholine bilayers by sodium cholate

The interaction of sodium cholate with phosphatidylcholine liposomes was investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light-scattered by liposomes. Free surfactant concentration at subsolubilizing and solubilizing levels showed, respectively, values that were lower than and similar to the surfactant critical micelle concentration, and indicated that permeability alterations and solubilization were determined, respectively, by the action of surfactant monomer and by the formation of mixed micelles. A direct relationship was established in the initial steps [effective surfactant/lipid molar ratio (Re) lower than 0.07] between the growth of vesicles, the leakage of entrapped CF (fluidity of vesicles), and Re. These changes could be correlated with the increasing presence of surfactant molecules in the outer monolayer of vesicles and its saturation. The subsequent increase in Re led to a lower growth of vesicles in coexistence with a similar increase in the CF release. This behavior could be related to a increased rate of flipflop of the sodium cholate molecules, making the inner monolayer also available for the interaction with added surfactant. A direct dependence was also established in the initial solubilization steps (Re values up to 0.6) between the surfactant-phosphatidylcholine aggregate size, the static light-scattering of the system, and Re. In the Re interval 0.36–0.84 mixed vesicles and mixed micelles coexisted.     

Synergisms in Binary Mixtures of Anionic and pH‐Insensitive Zwitterionic Surfactants and Their Precipitation Behavior with Calcium Ions

This work aims to investigate synergy in anionic and zwitterionic surfactant mixtures, as they result in better interfacial properties and micellization behavior. Various mixtures of the pH‐insensitive zwitterionic surfactant 3‐(decyldimethylammonio) propanesulfonate (Zwittergent 3–10) and sodium dodecylsulfate (SDS) were prepared in aqueous solution at a range of pH values between 2 and 13. The thermodynamic parameters during mixed surfactant adsorption at the air/water interface are obtained and the results show the mixed surfactant systems having superior properties to the constituent surfactants. Experimentally, the mixed surfactant solutions clearly improve the surface activities by lowering the critical micelle concentration (CMC) and lowering the surface tension at the air/water interface. The synergisms are investigated through the interaction parameters estimated from regular solution theory that is used to quantitatively describe the nonideality of surfactant mixtures. High negative interaction parameters are obtained from these surfactant mixtures. Experimental precipitation phase boundaries of SDS in the presence of CaCl₂ were also investigated in mixtures containing pH‐insensitive zwitterionic surfactant at different pH levels from 2 to 13 and SDS mole fractions of 0.25, 0.50, 0.75, and 1.00. Changes in the precipitation phase boundaries are due to the changes in the speciation or activities of the major components both below and above the CMC. As a result, the precipitation phase boundaries are pH dependent. In addition, mixed micellization and counterion binding to the micelle also change the precipitation phase boundary above the CMC. The activity‐based solubility product of calcium dodecylsulfate is also determined from the precipitation phase boundaries below the CMC. X‐ray diffraction patterns and SEM images confirm that only calcium dodecylsulfate precipitates in the soap scum for all pH and surfactant compositions studied.     

Solubilization of phospholipid vesicular structures into mixed micelles of zwitterionic surfactants

Interactions between the binary combination of dimethyltetradecylammoniopropanesulfonate (TPS) and l-α-phosphatidylcholine (PC), 1,2-didecanoyl-sn-glycero-3-phosphocholine, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine in the aqueous bulk phase were evaluated with the help of pyrene fluorescence (l ₁/l ₃) measurements by studying the aggregation processes of TPS in pure water and in the presence of 7–36 μM of fixed concentrations of each lipid. The fluorescence measurements showed that TPS monomers undergo two kinds of aggregation process, which were identified by the three breaks. The first break, C₁, and the second, C₂, indicated the onset and completion of bilayer solubilization, respectively, on the incorporation of TPS monomers into the bilayer assemblies, which led to bilayer solubilization in the form of mixed micelles. This process was not clearly visible in the presence of PC, whereas some kinds of structure transitions were observed upon the incorporation of surfactant monomers. The partition coefficient (K), which defines the degree of partitioning of surfactant monomers into the bilayers with respect to the aqueous medium, was evaluated. A high K value of TPS-lipid aggregates indicated stronger interactions between surfactant and bilayer assemblies of lipid. The K values determined for the three phospholipids are close to each other, which indicates that K values do not depend on the hydrocarbon chain length of the phospholipid but of the surfactant used.     

Synergistic Behavior and Microstructure Transition in Mixture of Zwitterionic Surfactant,Anionic Surfactant,and Salts in Sorbitol/H<Subscript>2</Subscript>O Solvent: 1. Effect of Surfactant Compositions

The phase behavior and rheological properties of a multi-component system, made of a zwitterionic surfactant cocoamidopropyl betaine (CAPB), an anionic surfactant sodium lauryl sulfate (SLSS), and mixed salts (tetrasodium pyrophosphate, sodium acid pyrophosphate, sacharrin, and sodium fluoride) in sorbitol/H₂O mixed solvent at different mass fraction of SLSS (X _SLSS) were systematically investigated by steady and dynamic rheology, dynamic light scattering, and diffusion ordered spectroscopy (DOSY). When fixing the salt concentration and the mass ratio of sorbitol in mixed solvent (R), the zero-shear viscosity increases first and then decreases showing a maximum with increasing X _SLSS, resulting from the formation and entanglement of wormlike micelles. Especially when X _SLSS is between 0.33 and 0.80, the mixture is dominated by entangled wormlike micelles coexisting with small micelles and separated wormlike micelles, and shows high viscoelasticity. The maximum of the zero-shear viscosity is ca. 5 orders of magnitude larger than that of sorbitol/H₂O mixed solvent or the CAPB/SLSS aqueous solution. The characteristic structural parameters for the micellar solutions at different X _SLSS are also estimated from further analysis of the rheological results, and indicate the stronger network structures of the wormlike micelles are formed in our systems compared with the wormlike micelles formed by a traditional zwitterionic/anionic surfactant aqueous solutions. The great improvements of rheological properties are attributed to the strong screening effects of the mixed salts and the strong solvophobic effect of sorbitol on the electrostatic and hydrophobic interaction between the CAPB and SLSS molecules. The present work has improved our understanding about the aggregation behavior of zwitterionic/anionic mixed surfactants with salts in less polar solvent/H₂O mixture, which would be of widely practical importance to optimize the formulation of products for personal care and household cleaning.     

Physicochemical Investigation of Biocompatible Mixed Surfactant Reverse Micelles: III. Aqueous NaCl Solubilization, Thermodynamic Parameters of Desolubilization Process and Conductometric Studies

Solubilization of water and aqueous NaCl in mixed reverse micelles (RMs) comprising sodium bis(2‐ethylhexyl) sulfosuccinate (AOT), and polyoxyethylene (20) sorbitan trioleate or polyoxyethylene (20) sorbitan monooleate has been studied at different compositions (X_nonionic = 0–1.0) at a total surfactant concentration, S_T = 0.10 × 10³ mol m^?3 in biocompatible oils of different chemical structures; viz., ethyl oleate (EO), isopropyl myristate (IPM) and isopropyl palmitate (IPP) at 303 K. The enhancement in water solubilization (i.e., synergism) has been evidenced by the addition of nonionic surfactant to dioctyl sulfosuccinate/oil(s)/water systems. Addition of NaCl in these systems at different X_nonionic enhances their solubilization capacities further until a maximum, ω_NaCl,max is reached. ω_NaCl,max and [NaCl]_max (concentration at which maximization of NaCl solubilization occurs) depend on type of nonionic surfactant, its content (X_nonionic) and oil. A new solubilization efficiency parameter (SP*_water or SP*_NaCl) has been proposed to compare solubilization phenomena in these oils. The energetic parameters of the desolubilization process of water or aqueous NaCl in single and mixed RMs have been estimated. Energetically, the water dissolution process in oil has been found to be more exothermic as well as more organized in IPP. Overall, the dissolution of water and aqueous NaCl in mixed RMs is entropically driven process. Conductance behavior of these systems in the presence of NaCl has been investigated under different [NaCl] at 303 K. An attempt has been made to give an insight to the mechanism of solubilization phenomena, percolation in conductance and microstructures vis‐à‐vis role of biocompatible oils in these systems.     

Rheological Properties and Microstructure Transition in Mixture of Zwitterionic Surfactant,Anionic Surfactant and Salts in Sorbitol/H2O Solvent: 2. Effect of Salts and Sorbitol

The impact of mixed salts and sorbitol on the viscoelastic properties of a multi‐component system, made of a zwitterionic surfactant cocoamidopropyl betaine (CAPB), an anionic surfactant sodium lauryl sulfate (SLSS) and mixed salts (tetrasodium pyrophosphate, sodium acid pyrophosphate, saccharin and sodium fluoride) in sorbitol/H₂O mixed solvent are systematically investigated by steady state and dynamic rheology. As reported previously, the viscosity of the mixed system passes through a maximum with increase in the SLSS mass fraction (X_SLSS) at a fixed total surfactant concentration, salt concentration (C_salt) and mass ratio of sorbitol in mixed solvent (R). The shape of the X_SLSS‐dependent viscosity curve does not change regardless of C_salt and R, but adding salts or sorbitol has different effects on the rheological properties of this system. The former due to a high screening effect plays an important role in the elongation and entanglement of the wormlike micelles, facilitating the enhancement of rheological properties and the formation of Maxwell fluids. The latter has a dual effect on the rheological properties and phase behavior of the mixtures. A certain amount of sorbitol can promote the formation entangled wormlike micelles, while the effect is reversed if the sorbitol content is too large. The electrostatic and hydrophobic interaction between CAPB and SLSS are the prerequisite for the aggregate formation and transition. Meanwhile, the aggregation behaviors are strongly influenced by the balance between low dielectric constant, strong solvophobic interaction and steric effect of sorbitol with the ability to form hydrogen bonds which favors the growth of micelles, and appearance of aqueous two‐phase systems with smaller amounts of wormlike micelles in CAPB‐rich regions which oppose enhancement of rheological properties. Our findings provide a new insight and approach to control and adjust the phase behavior of such a complicated applied multi‐component system.     

Micellar and Interfacial Behavior of Cationic Benzalkonium Chloride and Nonionic Polyoxyethylene Alkyl Ether Based Mixed Surfactant Systems

Micellar and interfacial properties of mixed surfactant systems comprising benzalkonium chloride, a cationic surfactant and nonionic polyoxyethylene alkyl ether surfactants (POE: C₁₀E₇, C₁₀E₈, C₁₀E₉, C₁₀E₁₀) have been investigated by surface tension, fluorescence and dynamic light scattering techniques. Critical micelle concentration (CMC) for different mixing mole fractions has been investigated by surface tension and fluorescence measurements. Ideal CMC, mixed micellar composition (X ₁ ^m , X ₁ ^σ ), interaction parameters for mixed micelles (β ^m) and adsorption monolayer (β ^σ ), surface excess concentration (Г_max), minimum area per molecule (A _min) and related thermodynamic properties have also been determined. Lowering of the CMC and negative interaction parameter values indicate synergism in the mixed micelle and monolayer formed, whereas, thermodynamic parameters evaluated for the proposed mixed systems indicate stability of the resulting micelles and monolayer. Micellar aggregation number (N _agg) and hydrodynamic diameter (D _h) computed from fluorescence and dynamic light scattering measurements respectively illustrate micellar growth in the mixed state. Results obtained for the proposed mixed systems can be helpful in designing smart materials for industrial surfactant based formulations.     

α-Sulfonato palmitic acid methyl ester-hexaoxyethylene monododecyl ether mixed surfactant system: Interfacial,thermodynamic, and performance property study

Interfacial, thermodynamic, and performance properties of aqueous binary mixtures of α-sulfonato palmitic acid methyl ester, C₁₄H₂₉CH(SO₃Na)COOCH₃(PES), and hexaoxyethylene monododecyl ether, CH₃(CH₂)₁₁(OCH₂CH₂)₆OH (C₁₂E₆), were investigated with tensiometric, conductometric, fluorimetric, and viscometric techniques. The critical micelle concentration (CMC), maximum surface excess, minimum area per molecule of surfactant at the air/water interface, and the thermodynamics of micellization and adsorption were determined. The CMC was very low for mixed systems, indicating probable use as a detergent with less effect on the environment because of surfactant biodegradability and less amount in the environment. The interaction parameter β^m, computed by using the theory of Rubingh and Maeda, indicated an attractive interaction (synergism) between the surfactant molecules, which was also confirmed by proton nuclear magnetic resonance studies in the mixed micelle. The micellar aggregation number (N _agg), determined by using a steady-state fluorescence quenching method at a total surfactant concentration of about ∼10 mM at 25°C, was almost independent of the surfactant mixture composition. The micropolarity and the binding constant (K _sv) for the C₁₂E₆/PES mixed system were determined by the ratio of the intensities (I ₁/I ₃) of the pyrene fluorescence emission spectrum, and the local microenvironment inside the micelle was found to be polar. The viscosity of the mixed system at all mole fractions suggested that mixed micelles are nonspherical in nature. The cloud point of oxyethylene group-containing surfactants was increased by the addition of PES. Foaming was temperature dependent, and a 1∶1 mixed system showed minimum foaming. All performance properties were composition dependent.     

Effect of Polyoxyethylene Chain Length on the Physicochemical Properties of N,N‐Dimethyl‐N‐dodecyl Polyoxyethylene Amine Oxide Hybrid Surfactants (C12EOnAO,with n = 1–4)

In order to determine the structure‐performance relationship of nonionic‐zwitterionic hybrid surfactants, N,N‐dimethyl‐N‐dodecyl polyoxyethylene (n) amine oxides (C₁₂EO_nAO) with different polyoxyethylene lengths (EO_n, n = 1–4) were synthesized. For homologous C₁₂EO_nAO, it was observed that the critical micelle concentration (CMC), the maximum surface excess (Γ_m), CMC/C₂₀, and the critical micelle aggregation number (N_m,c) decreased on going from 1 to 4 in EO_n. However, there were concomitant increases in surface tension at the CMC (γ_CMC), minimum molecular cross‐sectional area (A_min), adsorption efficiency (pC₂₀), and the polarity ([I₁/I₃]_m) based on the locus of solubilization for pyrene. The values of log CMC and N_m,c decreased linearly with EO_n lengthening from 1 to 4, although the impact of each EO unit on the CMC of C₁₂EO_nAO (n = 1–4) was much smaller than that typically seen for methylene units in the hydrophobic main chains of traditional surfactants. Compared to the structurally related conventional surfactant N,N‐dimethyl‐N‐dodecyl amine oxide (C₁₂AO), C₁₂EO_nAO (n = 1–4) have smaller CMC, A_min, and CMC/C₂₀, but larger pC₂₀, Γ_m, and N_m,c with a higher [I₁/I₃]_m. This may be attributed to the moderately amphiphilic EO_n (n = 1–4) between the hydrophobic C₁₂ tail and the hydrophilic AO head group.     

Study of the Interaction Between Methyl Orange and Mono and Bis-Quaternary Ammonium Surfactants

The solubilization and interaction of an azo-dye (methyl orange) with dodecyl trimethyl ammonium bromide and cationic gemini surfactants in the series of alkanediyl α,ω-bis[(dimethyl alkyl ammonium)bromide)] referred to as (m-s-m), m = 10, 12, 14 and s = 2, 3, 4 were investigated by means of UV–Vis spectroscopy. Aggregation with the anionic dye was reflected by a hypsochromic shift with a decrease in the intensity of the absorption band. The results also show a bathochromic shift followed by a sharp increase in the intensity of the maximum absorption band λ_max after the critical micellar concentration (CMC). This indicates that the dye solubility increased with increasing surfactant concentration. It was also observed that the aggregation of surfactant and dye takes place at a surfactant concentration far below the CMC of the individual surfactant. The effects of the chain length as well as the spacer length of gemini surfactants on the critical aggregation concentration and CMC were also examined. Moreover, the partition coefficients between the bulk water and surfactant micelles K _S and K _X as well as the Gibbs energies of distribution of dye between the bulk water and surfactant micelles were determined using the pseudo-phase model. The effect of the hydrophobic chain length and spacer of gemini surfactants on the distribution parameters is also reported.     

Solubilization of selected polycyclic aromatic compounds by nonionic surfactants

Solubilization of selected polycyclic aromatic compounds (PAC) by biodegradable nonionic surfactants, Tergitol 15-S-X (X=7 or 9) and Neodol 25–7, was investigated and correlated with micellar properties of these surfactants. These PAC include dibenzofuran, phenanthrene, acenaphthene, fluoranthene, and 9-chloroanthracene. Tergitol surfactants are mixtures of secondary ethoxylated alcohols, and Neodol 25–7 is a mixture of similar species but has the alcohol group in the primary position. These surfactants have the same chain length of hydrophobic tails and similar numbers of ethylene oxides. The results show that the Neodol surfactant yields micelles having larger hydrophobic core volume and renders a higher solubilization capacity for the PAC solubilizates in comparison with Tergitol surfactants. In general, aggregation numbers and micellar sizes both increase at elevated temperatures still below the cloud point. The micellewater partition coefficients of these PAC by the nonionic surfactants were well correlated to their octanol-water partition coefficients. Moreover, an estimated log K _ow value of 9-chloanthracene is 4.78.     

Ethoxy Carboxylate Extended Surfactant: Micellar,Adsorption and Adsolubilization Properties

The micellar, adsorption, and adsolubilization properties of a novel ethoxy carboxylate extended surfactant are measured and compared to an extended sulfate surfactant. The critical micelle concentration (CMC) of the ethoxy carboxylate extended surfactant is measured to be 0.02 mM while it is 0.07 mM for the extended sulfate surfactant. Adsorption and adsolubilization studies are carried out on alumina oxide surfaces. The extended sulfate surfactant has a higher maximum adsorption capacity onto the aluminum than the ethoxy carboxylate extended surfactant (0.47 vs. 0.14 mmol/g, respectively). For adsolubilization, the extended sulfate surfactant shows a slightly higher phenanthrene adsolubilization compared to the ethoxy carboxylate extended surfactant (log K_adm of 6.15 vs. 5.71, respectively). In contrast, for solubilization, the ethoxy carboxylate extended surfactant exhibits higher phenanthrene solubilization capacities than the extended sulfate surfactant (log K_mic of 5.61 vs. 5.42, respectively). Relative to surfactant loss from the solid surface, the ethoxy carboxylate extended surfactant shows a higher desorption capacity as compared to the extended sulfate surfactant. From these measurements, the ethoxy carboxylate extended surfactant has better properties for micellar applications (lower CMC, higher K_mic), while the extended sulfate surfactant has better properties for admicellar applications (higher q_max and K_adm values, and less desorption).