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.     

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 unilamellar liposomes by betaine-type zwitterionic/anionic surfactant systems

The mechanisms governing the solubilizing interactions between zwitterionic/anionic mixed surfactant systems at different molar fractions of the zwitterionic surfactant (X_zwitter) and neutral or electrically charged unilamellar liposomes were investigated. The mixed systems were formed by N-dodecyl-N,N-dimethylbetaine and sodium dodecyl sulfate in the presence of piperazine-1,4-bis-(2-ethanesulfonic acid) buffer at pH 7.20. Unilamellar liposomes formed by egg phosphatidylcholine, in some cases together with stearylamine or phosphatidic acid, were used. Solubilization was detected as a decrease in static light-scattering of liposomes. Two parameters were regarded as corresponding to the effective surfactant/lipid molar ratios (Re) at which the surfactant system (i) saturated the liposomes, Re_sat, and (ii) led to a total solubilization of liposomes, Re_sol. From these parameters the bilayer/aqueous medium surfactant partition coefficients for the saturation (K_sat) and complete bilayer solubilization (K_sol) were determined. When X_zwitter was 0.40, The Re and K parameters showed a maximum, whereas the critical micellar concentration (CMC) of these systems exhibited a minimum, regardless of the electrical charge of bilayers. Given that the ability of the surfactant systems to saturate or solubilize liposomes is inversely related to the Re_sat and Re_sol parameters, these capacities appear to be directly correlated with the CMC of the mixed systems. The similarity of both K_sat and K_sol (particularly for X_zwitter=0.2–0.8) suggests that a similar partition equilibrium governs both the saturation and the complete solubilization of bilayers, the free surfactant concentration (S_a,S_b), remaining almost constant with similar values to the CMC for each mixed system studied.     

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.     

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.     

Structure of Mixed Reverse Microemulsions Based on Sodium Bis (2-Ethylhexyl) Sulfosuccinate and Sodium Cholate

The microenvironment of water droplets of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and sodium cholate mixed reverse microemulsions was studied. Structural changes of water pockets in mixed reverse micelles were investigated by IR spectroscopy. The O-H stretching vibrational absorption spectra in the region of 3000–3800 cm⁻¹ were fit to three subpeaks with the Monte Carlo method. It was revealed that additives of sodium cholate suppress free water fraction in the water droplets of reverse micelles from 31% to 20% and support rising of bound fraction from 53% to 65%. The binding of optical probe ortho-nitroaniline to the mixed reverse micelles was determined by UV–visible spectroscopy. It was found that introducing of additives of sodium cholate below its critical micelle concentration (CMC) causes increasing of values of binding constant K_b twice compared with reverse micelles modified with pure water. However, values of the binding constant were reduced 4-fold at concentrations of sodium cholate higher than its CMC. Electrical conductivity of the reverse mixed micellar solutions (AOT + sodium cholate) was measured. Water-induced percolation in conductance of mixed reverse microemulsions occurs at a lower value of water/surfactant molar ratio (W) under the influence of sodium cholate, viz. electrical percolation threshold decreases from W = 32 to W = 15. The size of water droplets was estimated with the dynamic light scattering method. It was found that additives of sodium cholate below and higher than the CMC results in increasing and decreasing of hydrodynamic diameters of the water droplets, respectively, but sizes of water droplets decrease at concentrations of sodium cholate higher than its CMC.     

Solubilization of cholesterol in two binary mixed micelles of bile salt and nonionic surfactant

The saturated amounts of solubilized cholesterol (C_ch) in mixed micelles of sodium cholate (NaC) and octaoxy-ethylene glycol monon-decyl ether (C₁₀E₈) and of sodium zyme assay at 25, 29, 33 and 37°C. The C_ch values in both systems increase with the total surfactant concentration. Because the mixed micelles for both systems tend to form C₁₀E₈-rich micelles near the critical micellar concentration (CMC) of the mixed system, the curves of cholesterol solubility approached the C_ch curve for C₁₀E₈ alone near the CMC. The tendency of C_ch to decrease in both systems with increasing mole fractions of bile salts resembled that of the mean aggregation number of micelles. Thermodynamic analyses of cholesterol solubilization showed that the free energy of solubilization, if considered as the transfer of cholesterol from solid state to micellar environment, increased with increasing mole fraction of bile salt. The enthalpy of cholesterol solubilization (ΔH_S→M) decreased with the mole fraction of bile salts and showed break points around the mole fraction of 0.75 for the NaC−C₁₀E₈ system and at 0.60 for the NaDC−C₁₀E₈ system, respectively. These phenomena resemble earlier hydrophobicity data for mixed micelles by fluorescence measurements. Furthermore, C_ch values for the NaDC−C₁₀E₈ system were larger than those for the NaC−C₁₀E₈ system because of the structural differences at the 7α hydroxyl group between NaC and NaDC. This fact was confirmed by thermodynamic calculations.     

Properties of aqueous solutions for two binary mixed systems between two kinds of bile salts and nonionic surfactants

The micellar properties of aqueous binary mixed solutions for two systems consisting of sodium cholate (NaC)-octaoxyethylene glycol mono n-decyl ether (C₁₀E₈) and sodium glycocholate (NaGC)-C₁₀E₈ have been studied on the basis of surface tensions, polarity of the micelle interior and the mean aggregation number. Application of two theoretical treatments, based on regular solution and excess thermodynamic quantities for critical micellar concentration (CMC) data from surface tension curves of two mixed systems showed that the mole fraction of each bile salt in the mixed micelles near the CMC is lower than that of the corresponding prepared mole fraction in the mixed solution. The polarity of the interior suggested that the hydrophobicity of intramicelles increased with the increase of the mole fraction of bile salt in the mixed solution and that the mixed micelles become dramatically more hydrophobic at a mole fraction of 0.68 for NaGC−C₁₀E₈ system and 0.75 for NaC−C₁₀E₈ system, respectively. This implies that the micelles become richer in the bile salt molecules and the tendency appears strongly for NaGC−C₁₀E₈ system due to the strong cohesion between the conjugated glycines in the NaGC molecules. The decrease of aggregation number with the increase of the mole fraction of bile salts shows that the micelles approach those of the single system of each bile salt. This supports the previously mentioned facts.     

Mixtures of anionic and cationic surfactants with single and twin head groups: Solubilization and adsolubilization of styrene and ethylcyclohexane

Mixtures of anionic and cationic surfactants with single and twin head groups were used to solubilized styrene and ethylcyclohexane into mixed micelles and adsolubilize them into mixed admicelles on silica and alumina surfaces. Two combinations of anionic and cationic surfactants were studied: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), with a twin-head cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), with a single-head cationic surfactant, dodecylpyridinium chloride (DPCl). Mixtures of SDS/PODD showed solubilization synergism (increased oil solubilization capacity) when mixed at a molar ratio of 1∶3; however, the SHD-PDS/DPCl mixture at a ratio of 3∶1 did not show solubilization enhancement over SHDPDS alone. Adsolubilization studies of SDS/PODD (enriched in PODD) adsorbed on negatively charged silica and SHDPDS/DPCl adsorbed on positively charged alumina showed that while mixtures of anionic and cationic surfactants had little effect on the adsolubilization of styrene, the adsolubilization of ethylcyclohexane was greater in mixed SHPDS/DPCl systems than for SHDPDS alone. Finally, it was concluded that whereas mixing anionic and cationic surfactants with single and double head groups can improve the solubilization capacity of micelles or admicelles, the magnitude of the solubilization enhancement depends on the molecular structure of the surfactant and the ratio of anionic surfactant to cationic surfactant in the micelle or admicelle.     

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.     

Phosphatidylcholine as substrate for human pancreatic phospholipase A2. Importance of the physical state of the substrate

The long-chain phosphatidylcholine/sodium cholate aqueous system as substrate for human pancreatic phospholipase A₂ (PLA₂) was investigated. At a constant phosphatidylcholine (PC) concentration of 8 mM, the enzyme activity increased with a decrease in cholate (C) concentration up to a PC/C ratio of approximately 0.8 and then rather abruptly decreased to lower values at a ratio above 1.5. At ratios between 0.8 and 1.5, an increasing lag phase in the PLA₂ activity was seen, indicating a progressive decrease in substrate availability to the enzyme. Reaction mixtures with a PC/C ratio of up to 0.67 were optically clear solutions composed of mixed bile salt/PC micelles of increasing mixed micellar aggregate size. Ratios between 0.67 and 1.5 were characterized by an increase in turbidity (at 330 and 450 nm) due to increasing formation of vesicles or liposomes. Above a PC/C ratio of 1.5, a sharp increase in turbidity was seen due to increasing formation of bilayer structures other than vesicles. Pure vesicles obtained by dialysis of mixed micellar solutions were not hydrolyzed by the enzyme. Addition of bile salts reversed the inhibition which was accompanied by a decrease in turbidity. Phosphatidylcholine was preferred as substrate for human PLA₂ when present in large mixed disc-like bile salt micelles. Vesicular or other types of lamellar liquid-crystalline phases of long-chain phosphatidylcholine did not serve as substrate for PLA₂.     

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.     

Solubilization as a Separation Process

Abstract

In this paper a separation process for hydrocarbon molecules is suggested, based on solubilization in aqueous solutions by surfactant micelles. A molecular thermodynamic approach to solubilization is formulated which relates the extent of solubilization and the selectivity to the structure and properties of the surfactant and of the solubilizate molecules. An evaluation of the solubilization characteristics of benzene and hexane in aqueous solutions of non-ionic octyl glucoside, anionic sodium dodecyl sulfate and cationic cetyl pyridinium chloride is made and solubilization phase diagrams for the above systems are constructed. These diagrams indicate the formation of micelles at concentrations which are lower than the critical micelle concentration of the surfactant alone. The calculations predict, for all three surfactants, preferential solubilization of (aromatic) benzene compared to (aliphatic) n-alkanes. The preferential solubilization of benzene is caused by its smaller molecular volume and lower interfacial tension against water. Preliminary experimental results using cetyl pyridinium chloride as surfactant and an equimolar binary mixture of hexane and benzene as solubilizates indicate a selectivity of over 7 for benzene compared to hexane, and a ratio of about one molecule of benzene solubilized for every surfactant molecule in the micelle.     

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.     

Mixed Micellization and Mixed Monolayer Formation of Sodium Cholate and Sodium Deoxycholate in Presence of Hydrophobic Salts Under Physiological Conditions

Mixed micellization and mixed monolayer formation of two bile salts namely sodium cholate (NaC) and sodium deoxycholate (NaDC), in the presence of sodium chloride (NaCl) and three hydrophobic salts including sodium acetate (NaAc), sodium butanoate (NaBu) and sodium hexanoate (NaHx) in 10 mM phosphate buffer (pH 6.5) at 37 °C were investigated by means of surface tension measurements. The experimental results were utilized to evaluate various parameters like critical micellar concentration (CMC), micellar and monolayer interaction parameter (β and β ^σ), micellar and monolayer mole fractions (X and Z), activity coefficients of two bile salts in mixed micelles and monolayer (f and f _(σ)), surface excess (Γ_max) and minimum surface area per molecule of bile salt (A _min). Mixed micelles and mixed monolayer were found to show slight non-ideality and both these phenomena have been found to be affected differently in the presence of various additive salts with NaHx showing larger effects. Higher efficiency of NaHx in affecting both phenomena has been attributed to its appreciable hydrophobicity and surface activity, thus showing stronger interactions with bile salt molecules.     

新型混合反胶团萃取溶菌酶的平衡行为

研究了由阴离子表面活性剂二-(2-乙基己基)琥珀酸酯磺酸钠(AOT)和非离子表面活性剂聚氧乙烯基(聚合度为4)壬基酚醚(OPE4)组成的一种新型混合反胶团体系及其萃取溶菌酶的平衡行为. 结果表明,该反胶团体系具有较大的含水量,且其含水量在较大的表面活性剂配比(0~0.8)范围内维持恒定,是由静电作用和胶束形态改变共同作用的结果. 无机盐种类和离子强度对上述混合反胶团的含水量有着显著的影响,继而影响到溶菌酶的萃取率. 它可归结为盐离子对扩散双电层和水化膜斥力的因素所致. 降低pH和提高总表面活性剂浓度均有利于溶菌酶的萃取.     

Aqueous solution properties of a silicone surfactant and its mixed surfactant systems

The aqueous solution properties of a nonionic silicone surfactant of dimethylpolysiloxane and its mixed surfactant systems were studied. It was found that the silicone surfactant has a high surface activity and forms micelles in two steps: premicelles in dilute concentrations and polymolecular micelles above 3.7 × 10⁻⁷ mol dm⁻³. In mixed systems of the silicone surfactant with anionic hydrocarbon or fluorocarbon surfactant, weak intermicellar interactions were found. They are due to electrostatic interaction between hydrophilic groups of the respective micelles. Dye solubilization measurements showed that the solubilized amount of Yellow-OB is greater than predicted by ideal systems. Hydrazo-azo tautomerism is observed in fluorocarbon-silicone surfactant systems, while Yellow-OB is solubilized only in the azo-form in the hydrocarbon-silicone surfactant system.     

The effect of micellar lifetime on the rate of solubilization and detergency in sodium dodecyl sulfate solutions

The slow relaxation time (τ₂) of sodium dodecyl sulfate (SDS) micelles, measured by the pressure-jump technique, was maximum at 200 mM concentration at 25°C, indicating that the most stable micelles are formed at this concentration. This is presumably related to the optimum molecular packing in the micelle. The rate of solubilization of benzene and Orange OT dye into SDS solutions was also maximum at 200 mM concentration. The results are explained as follows: The distance between micelles (i.e., intermicellar distance) decreases as the surfactant concentration (or the number of micelles) increases, resulting in a stronger electric repulsion between micelles. Therefore, the micelles become more rigid, due to the compressive force of intermicellar repulsion, as the concentration increases up to 200 mM SDS. With further increase in the SDS concentration, the micellar shape changes from spherical to cylindrical to accommodate more surfactant molecules in the solution and to minimize the free energy of the system. The interior of the tightly packed micelles is more hydrophobic than that of loosely packed micelles and, therefore, the tightly packed micelles induce rapid solubilization of nonpolar molecules (e.g., benzene, Orange OT) into these micelles.     

The effects of anionic and cationic surfactants on the hydrolysis of sodium barbital

Alkaline hydrolysis reactions of sodium barbital in micelles of sodium dodecyl sulfate (the anionic surfactant SDS), micelles of cetyl trimethylammonium bromide (the cationic surfactant CTAB), and mixed micelles of surfactant/n-C₅H₁₁OH/H₂O were studied by ultraviolet-visible spectrometry. The reaction rate and the activation energy of the hydrolysis of sodium barbital were calculated. The results showed that the rate of sodium barbital hydrolysis decreased with an increase in CTAB content, whereas it increased in the presence of SDS and n-C₅H₁₁OH. The different effects of CTAB and SDS on the hydrolysis of sodium barbital may be related to their interaction with sodium barbital.     

Effects of sodium glycocholate and sodium taurocholate on the mixed micelles of bile salts and nonionic surfactant

Effects of sodium glycocholate (NaGC) and sodium taurocholate (NaTC) on the mixed micelles for two systems consisting of NaGC-octaoxyethylene glycol monon-decyl ether (C₁₀E₈) and NaTC-C₁₀E₈ have been studied as a function of the mixed micelles’ compositions, polarities of the micelles’ interior and mean aggregation numbers. The compositions of the mixed micelles are calculated from critical micelle concentration (CMC) data by using excess thermodynamic quantities. The polarities and mean aggregation numbers are determined from pyrene fluorescence in the mixed micelles. Both mixed systems were nonideal, and the mole fraction of NaGC or NaTC in a mixed micelle near the CMC was less than that in the aqueous mixed solution. However, the mixed micelle of the NaTC-C₁₀E₈ system contained more bile salt molecules than that of the NaGC-C₁₀E₈ system because of a good miscibility of NaTC and C₁₀E₈ molecules. The pyrene fluorescence results suggested that the mixed micelles changed from C₁₀E₈-rich micelles to NaGC- or NaTC-rich micelles, and mean aggregation numbers of the mixed micelles decreased abruptly with increasing mole fraction of bile salts. In the low mole fraction range of bile salts, however, both the polarities and the mean aggregation numbers for the NaTC-C₁₀E₈ system are lower than those for the NaGC-C₁₀E₈ system because of the high mole fraction of NaTC in a mixed micelle, and also because of the different effect of the conjugated group between NaTC and NaGC molecules in the mixed micelles.