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.     

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.     

Effect of a Hydrophilic PEO–PPO–PEO Copolymer on Cetyltrimethyl Ammonium Tosylate Solutions in Water

Cetyltrimethyl ammonium tosylate (CTAT) in water forms long flexible wormlike micelles at concentrations above 10 mM, leading to highly viscous solutions and viscoelastic stiff gels above 100 mM. In the presence of a nonmicellar hydrophilic PEO–PPO–PEO triblock copolymer F87 (TBC-F87, Total mol.wt. = 7,700, EO = 70%) these wormlike micelles RE transformed into smaller structures, as evident from a sharp decrease in viscosity and increase in specific conductance. These results are quantified by small angle neutron scattering (SANS) measurements. The PPO middle block of TBC-F87 gets inserted in the CTAT micelle, the size and total aggregation number of CTAT/TBC-F87 mixed micelles decreased but the number of TBC-F87 molecules in the mixed micelles increased with an increase in [TBC-F87]. Two break points in the typical specific conductance versus CTAT concentration plot at various [TBC-F87] amounts represent interactions between CTAT and TBC-F87. The penetration of PPO of TBC-F87 inside CTAT micelles decreases hydrophobicity of the core while the presence of PEO end blocks enhances hydrophilicity each favoring smaller micelles.     

Temperature-dependent aggregation of bio-surfactants in aqueous solutions of galactose and lactose: Volumetric and viscometric approach

Modulation in the aggregation behavior of bio-surfactants(bile salts), sodium cholate(NaC) and sodium deoxycholate(NaDC) in aqueous solutions of carbohydrates(galactose and lactose) have been investigated by measuring the density(ρ), speed of sound(u) and viscosity(η) of the mixtures at different temperatures293.15, 298.15, 303.15, 308.15 and 313.15 K. The density and speed of sound data have been used to calculate various volumetric and compressibility parameters such as apparent molar volume(V_φ), isentropic compressibility(κ_s), apparent molar adiabatic compression(κ_(s, φ)) to get a better insight into the micellization mechanism of bile salts. Further, the viscosity data have been studied in the light of relative viscosity(η_r) and viscous relaxation time(τ). Some derived parameters such as free volume(V_f), internal pressure(πi) and molar cohesive energy(MCE) of Na C and Na DC in aqueous solution of saccharides have also been calculated from viscosity data in conjunction with density and speed of sound values. All the calculated and derived parameters provide qualitative information regarding the nature of interactions i.e. solute–solute, solute–solvent and solvent–solvent in the solution.     

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.     

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.     

Aggregation and Adsorption at the Air–Solution Interface of the Cetyltrimethyl Ammonium Tosylate With Two Poly(oxyethylene)–Poly(oxypropylene)–Poly(oxyethylene) Block Copolymers Aqueous Mixtures

The aqueous mixed systems (EO₇₆PO₃₀EO₇₆) (TBCP8400)—cetyltrimethyl ammonium tosylate (CTAT), and (EO₉₇PO₆₉EO₉₇) (TBCP12600)—CTAT were studied to determine both the bulk aggregation and the adsorbed monolayer at the air/water interface. Results were interpreted with the pseudophase separation model plus the regular solution theory for aggregates and monolayers. The behavior is different for TBCP8400–CTAT and TBCP12600–CTAT mixtures, but it is strongly non-ideal in both cases. In bulk, TBCP8400–CTAT mixtures produce aggregates more close to CTAT micelles having TBCP8400 as a solubilizate than the inverse. At low CTAT content, the interaction is repulsive becoming attractive at high TBCP8400 content. The TBCP12600–CTAT aggregates strongly differ from the structure of both pure component micelles, and the interaction is always repulsive. In both cases, the interaction seems not to be cooperative but gradual. CTAT effect on copolymers aggregates seems to be more similar to that of a zwitterionic surfactant than to that of an ionic one. However, CTAT is not included in the aggregates as an ion pair, as revealed by the ionization degree results. It seems that cetyltrimethyl ammonium and tosylate ions have different effects on aggregates which in part are opposite. The adsorbed monolayers also show different behavior. In TBCP8400–CTAT system, the monolayer is mainly a CTAT one with inclusion of TBCP8400 as a monolayer-soluble impurity. However, the inclusion of the non-ionic surfactant alters the structure of the monolayer, which differs from that of the pure CTAT one. The area per adsorbed molecule (A₀) is systematically higher than the ideal and computed ones. The system TBCP12600–CTAT shows a monolayer composition which is almost the same that the overall surfactant mixture composition, and the monolayer structure differs from both the pure-TBCP12600 and the pure-CTAT monolayers ones. The experimental A₀ values are systematically lower than the ideal and the computed ones. Then, in both cases the A₀ values for the pure components do not remain invariable in the mixed monolayer. The phenomenon is interpreted on the basis of the conformation of the copolymers adsorbed at the air/solution interface.     

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.     

Adsorption of Anionic–Cationic Surfactant Mixtures on Metal oxide Surfaces

This 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.

---

Spectroscopic and conductometric studies on the interactions of thionine with anionic and nonionic surfactants

In this study, the interaction of thionine, a cationic dye, with anionic [sodium dodecyl sulphate (SDS), lithium dodecyl sulphate (LiDS), and sodium dodecylbenzene sulphonate (SDBS)], nonionic (Tween 20 and Triton X‐100), and binary mixtures of anionic and nonionic surfactants was studied by conductometric and spectrophotometric measurements. The degree of ionisation, the counterion binding parameters, and the equilibrium constants in the premicellar region were obtained from conductivity data. Binding constants of thionine to anionic, nonionic, and mixtures of anionic and nonionic micelles were determined by spectrophotometric measurements. The binding tendency of thionine to anionic micelles followed the order SDBS > SDS > LiDS. The presence of nonionic surfactants increased significantly the binding affinity of thionine to anionic micelles, and the highest binding constant was calculated in the presence of Tween 20. The results obtained from conductometric studies correlated with those obtained from spectroscopic studies. Data concerning dye–surfactant interaction are important for a fundamental understanding of the performance of single and mixed surfactants and for their industrial application.     

Change in the aggregational character of the cationic surfactant cetyl trimethyl ammonium bromide in the presence of <Emphasis Type="Italic">o</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-itrophenolates

The effect of the addition of sodium salts of p-nitrophenol and o-nitrophenol on the structural transition of aqueous micellar solutions of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) was studied by viscosity, ¹H nuclear magnetic resonance, and dynamic light scattering measurements. The rapid increase in the relative viscosity of CTAB solutions on the addition of sodium salts of o-nitrophenol and p-nitrophenol was due to the transition in micellar shape from spheres to rods. Dynamic light scattering studies indicated a decrease in the diffusion coefficient of the micelles with the addition of p-nitrophenol. The adsorption and orientation of the salts on the micellar surface and subsequent growth of the micelles was studied by nuclear magnetic resonance.     

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.     

Lecithin inhibits fatty acid and bile salt absorption from rat small intestine in vivo

During digestion of a fatty meal, long chain free fatty acids (FFA) and lecithin are among the lipids solubilized in intestinal contents as mixed micelles with bile salts. We hypothesized that if lecithin were not hydrolyzed, the mixed micelles would be abnormal, and absorption of FFA and bile salts would be depressed. To test this hypothesis, isolated segments of rat small intestine were infused in vivo with micellar solutions of 2 mMolar linoleic acid and 10 mMolar taurocholate to which was added 3 mMolar 1-palmitoyl, 2-oleoyl lecithin (a common lecithin in bile and food), or 1-palmitoyl lysolecithin (the hydrolytic product of lecithin). Absorption of FFA and bile salt was measured under steady state conditions using a single-pass technique. Lecithin depressed the rate of FFA absorption by 40% (p<0.025) in jejunal and ileal segments whereas lysolecithin was associated with normal rates of FFA absorption. Lecithin also reduced taurocholate absorption from the ileum by 30% (p<0.05). These data support the idea that lecithin may depress FFA and bile salt absorption from the small intestine in pancreatic insufficiency. The following trivial names are used: lecithin (1,2-diacyl-sn-glycero-3-phosphorylcholine); lysolecithin (1-acyl-sn-glycero-3-phosphorylcholine).     

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.     

Substrate specificity of lysosomal cholesteryl ester hydrolase isolated from rat liver

The effect of various physicochemical forms of substrate on the activity of acid cholesteryl ester hydrolase isolated from rat liver lysosomes was studied. The amount of sodium taurocholate was varied in the substrate mixture which contained constant amounts of egg phosphatidylcholine (PC) and cholesteryl oleate. The resulting substrate forms produced were PC vesicles, PC vesicles with incorporated sodium taurocholate, mixed micelles, and mixed micelles together with free bile salt micelles. Gradually increasing amounts of sodium taurocholate activated cholesteryl oleate hydrolysis until the molar sodium taurocholate/PC ratio of ca. 0.6; thereafter hydrolytic activity decreased rapidly. The presence of sodium taurocholate micelles clearly inhibits cholesteryl oleate hydrolysis. We therefore propose that the activation observed at low bile salt concentrations depends on bile salt interaction with the substrate vehicle, whereas the inhibition observed at high bile salt concentrations depends on sodium taurocholate interacting with the enzyme. When comparing different phospholipid components in the supersubstrate, the enzyme activity was highest in the presence of dioleyl PC and decreased when present with dipalmitoyl PC and egg PC. Egg lysoPC completely inhibited the enzyme activity. A net negative charge on the surface of the vesicle substrate increased cholesteryl ester hydrolase activity while a net positive charge on the surface inhibited the enzyme activity. Only part of the product inhibition of cholesteryl oleate hydrolase caused by Na-oleate was reversible when tested with bovine serum albumin present in the incubation mixture.     

Interaction Between Nonionic and Gemini (Cationic) Surfactants: Effect of Spacer Chain Length

The interaction between mixtures of nonionic surfactant polyethylene glycol p-(1,1,3,3-tetramethyl butyl)-phenyl ether and cationic gemini surfactants alkanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-s-12, where s = 2, 4 and 6) was studied using surface tension and small-angle neutron scattering measurements. Marked interaction was observed for the investigated surfactants mixtures which depend upon the hydrophobic spacer length of the gemini surfactant and also on the fraction of nonionic surfactant in the mixed systems. The results are discussed in terms of interaction parameters calculated according to the theory of regular solutions which uses the critical micelle concentration determined tensiometrically to calculate the molecular interaction parameter and the mole fractions of the two components in the mixed micelles. A relatively high negative molecular interaction parameter value (up to −3.40) obtained for mixtures of nonionic and cationic gemini surfactant indicates a presence of strong attractive interaction in the mixed system that increases with the spacer length of the gemini surfactant. Micellar parameters deduced from small-angle neutron scattering measurements also compliment the surface tension results.     

Micelle formation in mixtures of nonionic and cationic detergents

The effect of a homologous series of polyoxyethylene n-dodecanols on the critical micelle concentration (cmc) of mixtures with n-dodecyl trimethyl ammonium bromide (DTAB) and of polyoxyethylene n-hexadecanols on the cmc of mixtures with n-hexadecyl trimethyl ammonium bromide (CTAB) has been studied in terms of the composition of the mixtures. The cmc of the nonionic component of the mixed micelles is approximately one hundredth of that of the cationic component. Only a gradual increase in the cmc values of the mixed micelles above the values of the nonionic components was observed in the composition range of 0–75 mole % or 0–90 mole %, respectively, of cationic detergent. This is followed by an abrupt transition to the high cmc values of the cationic component. This abrupt transition is considerably reduced by addition of NaBr. It is postulated that in the absence of added electrolyte the degree of ionic repulsion of the cationic component in mixed micelles is markedly decreased as the proportion of nonionic component reaches a threshold range of 25 or 10 mole %, respectively. Swamping of charges reduces ionic repulsion in NaBr solutions and, consequently, the abrupt transition in the cmc vs. composition curves is reduced.     

Incorporation ofall-trans-or 9-cis-β-carotene into mixed micellesin vitro

Various studies suggest thatall-trans- and 9-cis-β-carotene are absorbed in the intestine to different extents. The present study was undertaken to evaluate the degree ofin vitro incorporation of the two isomers into intestinal mixed micelles, which is an essential early step in the absorption process. The micelles were designed to simulate those present during fat digestion in the lumen of the human small intestine with respect to bile salts, lipids, pH and temperature. Solutions ofall-trans-and 9-cis-β-carotene at various ratios were added to the lipid mixture. A direct correlation was seen between the 9-cis-β-carotene level in the mixture and the degree of total β-carotene incorporation into micelles. An increased level ofall-trans-β-carotene incorporated into the micelles. In contrast, when carotene mixtures enriched with the 9-cis isomer were used, an increase in the level of total carotene in the solution was accompanied by a constant or even enhanced carotene incorporation. The results indicate that the differences in the absorption of β-carotene isomers might be the result of their different ability to be incorporated into the lipid micelles of the intestinal lumen. In addition, the results point toward the possibility that ingestion of 9-cis-β-carotene by humans may increase carotene availability.     

Adsorption, Desorption and Adsolubilization Properties of Mixed Anionic Extended Surfactants and a Cationic Surfactant

Anionic and cationic surfactant mixtures exhibit desirable synergism, but are limited by their tendency to form precipitates. This research evaluates the adsorption, adsolubilization and desorption of mixtures of carboxylate-based anionic extended surfactants and a pyridinium-based cationic surfactant. The mixture of cetylpyridinium chloride (CPC), selected as the cationic surfactant, with four anionic extended surfactants were studied. The anionic surfactants studied were alkyl propoxylated ethoxylated carboxylate with average number of carbon chain length of 16 and 17 or 16 and 18 with 4?mol of propylene oxide groups and either 2 or 5?mol of ethylene oxide groups. The adsorption of anionic extended and cationic surfactant mixtures onto a negatively charged metal oxide surface (silica dioxide) was evaluated. The adsolubilization of phenylethanol, styrene and ethylcyclohexane were evaluated for these mixed surfactant systems. The desorption potential of individual and mixed surfactant systems was also evaluated by varying the number of washing (desorption) steps. It was found that the plateau adsorption of mixed anionic extended surfactant and cationic surfactant occurred at lower surfactant concentration than that of the CPC alone, although the maximum adsorption capacity of CPC was not enhanced in our mixed surfactant systems. Adsolubilization capacities of these mixed surfactant systems are higher than that of the individual surfactant system. For desorption studies, these mixed surfactant systems showed lower stability than the individual surfactant system.