Partial Molecular Area Research Articles

Mixing Properties of Sphingomyelin Ceramide Bilayers: A Simulation Study

Ceramide is the simplest molecule in the class of glycosphingolipids composed of a sphingosine backbone and acyl moiety. It plays significant roles in cell signaling; apoptosis; binding of hormones, toxins, and viruses; and many other biologically important functions. Sphingomyelin, ceramide with a phosphotidylcholine headgroup, is another biologically vital lipid present in the myelin sheath of nerve cell axons. Regions with high concentrations of ceramide can be formed in biological membranes composed of sphingomyelin by enzymatic catalysis with sphingomyelinase. To better understand the biophysical and thermodynamic properties of these molecules and their mixtures, we have preformed NPT molecular dynamics simulations of hydrated 16:0 sphingomyelin bilayers with increasing concentrations of 16:0 ceramide at 323, 332, 340, and 358 K. From analyses of electron densities, hydrogen bonding, NMR order parameters, partial molecular volume, and partial molecular area, we have identified possible structural changes corresponding to liquid ordered and liquid disordered phases. These structural changes are the results of changes in intra- and intermolecular hydrogen bonds between SM and Cer molecules. Our results correspond to DSC experiments for sphingomyelin bilayer concentrations up to 50% Cer. Above 50% concentration, we observe conformational changes in the SM headgroup similar to that of the umbrella model for lipid cholesterol mixtures.

Head group effects on molecular packing in lamellar liquid crystals

In this study, molecular packing in lamellar liquid crystals in poly(oxyethylene) dodecyl ether(C12EOn) pure systems and the two surfactant mixtures of C12EO8/1-dodecanol(C12EO0), C12EO8/lipophilic sucrose laurate (L-595), hydrophilic sucrose laurate (L-1695)/C12EO2 is investigated in terms of mean molecular area and partial molecular area (PMA). Lamellar liquid crystals formed in the C12EO8/C12EO0 mixed system show higher melting temperatures than those in the C12EOn pure systems, even though the average number of EO units in the mixed surfactant system is the same as in the pure system. We compared the mean molecular area at the interface between hydrophilic and lipophilic moieties in the lamellar liquid crystals in each system. In the mixed system, the molecules are packed more tightly than in the pure system. Among the C12EOn and sucrose laurate mixtures, the L-1695/C12EO2 mixed system showed a smaller mean molecular area per lipophilic chain than the C12EO8/L-595 mixed system. We investigated the effect of mixing two surfactants with different head group geometry on molecular packing by comparing the PMA of each surfactant.

Mixed monolayer of dipalmitoylphosphatidylcholine and stage-specific embryonic antigen-1 (SSEA-1)

The mixing behavior of dipalmitoylphosphatidylcholine (DPPC) and stage-specific embryonic antigen-1 (SSEA-1) in air/water interface monolayer was estimated by measuring the surface pressure. For comparison, a DPPC and synthetic maltohexaosyl lipid (Mal 6(C 12) 2) mixed monolayer was also measured. The DPPC–SSEA-1 system exhibits expanded–condensed phase transition. A thermodynamic analysis revealed that the DPPC–SSEA-1 system exhibits a negative azeotropic phase transition. This result indicates an attractive interaction between DPPC and SSEA-1, whereas SSEA-1 molecules show little specific interaction between themselves. Evaluation of the partial molecular area suggests the attractive interaction due to the steric effect of packing. Even on enough divalent salt solution, there is little change in the π– A curve. The attractive interaction between the SSEA-1 molecules requires a suitable arrangement of Le X determinant (Gal1 → 4[Fucα1 → 3]GlcNAcβ1 → R).

Cholesterol packing around lipids with saturated and unsaturated chains: a simulation study.

The fundamental role of cholesterol in the regulation of eukaryotic membrane structure is well-established. However the manner in which atomic level interactions between cholesterol and lipids, with varying degrees of chain unsaturation and polar groups, affect the overall structure and organization of the bilayer is only beginning to be understood. In this paper we describe a series of Molecular Dynamics simulations designed to provide new insights into lipid-cholesterol interactions as a function of chain unsaturation. We have run simulations of varying concentrations of cholesterol in dipalmitoyl phosphatidylcholine (DPPC), palmitoyl-oleyol phosphatidylcholine (POPC), and dioleyol phosphatidylcholine (DOPC) bilayers. Structural analysis of the simulations reveals both atomistic and systemic details of the interactions and are presented here. In particular, we find that the minimum partial molecular area of cholesterol occurs in POPC-Chol mixtures implying the most favorable packing. Physically, this appears to be related to the fact that the two faces of the cholesterol molecule are different from each other and that the steric cross section of cholesterol molecules drops sharply near the small chain tails.

水性２成分混合の界面活性剤系に関する最近の研究の総覧

Recent studies on mixed surfactant systems were systematically overviewed, paying special attention to synergism observed in micellization as well as adsorbed film formation upon mixing of a few nonionic surfactants with a variety of surfactants (such as anionics including bile salts and a hybrid type surfactant, cationics including a Gemini type surfactant, different types of nonionics and a zwitterionic surfactant used as a membrane solubilizer) in addition to various combinations of anionics. Through the text, it was shown for each given binary mixed system composed of surfactants 1 and 2 how to estimate not only the composition of mixed micelles (Y(2)) equilibrated with singly dispersed surfactant species in bulk solution phase, where the mole fraction of 2 in the surfactant mixture is denoted as X(2), but also the composition of adsorbed film phase (Z(2)). Almost all combinations were discussed in terms of the respective interaction parameters, omega(R) and omega(A), in mixed micelles (3-D phase) and in mixed adsorbed film (2-D phase), respectively, surface excess concentration (Gamma), partial molecular area (PMA), minimum surface Gibbs energy (G(s)min), and such defined measures as pC(20), CMC/C(20) etc. for evaluation of synergism.

Intermolecular interaction of phosphatidylinositol with the lipid raft molecules sphingomyelin and cholesterol.

Diacylphosphatidylinositol (PI) is the starting reactant in the process of phosphatidylinositide-related signal transduction mediated through the lipid raft domain. We investigated intermolecular interactions of PI with major raft components, sphingomyelin (SM) and cholesterol (Chol), using surface pressure–molecular area (π–A) isotherm measurements. The classical mean molecular area versus composition plot showed that the measured mean molecular areas are smaller in PI/Chol mixed monolayers and larger in PI/SM mixed monolayers than those calculated on the basis of the ideal additivity. These results indicate that PI interacts attractively with Chol and repulsively with SM. In addition, we energetically evaluated the interaction of PI with SM/Chol mixtures and found that the mixing energy of PI/SM/Chol ternary monolayers decreased as the molar ratio of Chol to SM increased. In order to quantitatively analyze the distribution of PI we calculated the chemical potentials of mixing of PI into the SM/Chol mixed monolayer and into the dioleoylphosphatidylcholine (DOPC) monolayer, which was used as a model for the fluid matrix, on the basis of partial molecular area analysis. Analysis using the chemical potential of mixing of PI suggested that partition of PI molecules between these two monolayers can be changed by a factor of about 1.7 in response to change in Chol molar fraction in the SM/Chol mixed monolayer from 0.3 to 0.6 when the concentration of PI in the DOPC monolayer is kept constant at 7 mol%.

Blending effects on adsorption and micellization of different membrane protein solubilizers: II. A thermodynamic study on a mixed system of CHAPS with a bile salt in pH 7.4 phosphate buffer solution

For a mixed system of a typical membrane protein solubilizer CHAPS (a derivative of a bile acid cholic acid) combined with a bile salt (sodium salt of glycocholic acid, NaGC), which is also a candidate as a membrane protein solubilizer, micellization and adsorbed film formation in a phosphate buffer solution of pH 7.4 at 303 K were studied paying special attention to the synergistic effect upon mixing. The collection of sufficient data based on plots of surface tension ( γ) versus logarithmic concentration ( C t or m t) in total molality at discrete mole fractions ( X 2) in the mixture of surfactants 1 and 2 (where 1 and 2 correspond to CHAPS and NaGC, respectively) allowed us to accurately determine critical micelle concentration (CMC), minimum surface tension at CMC ( γ CMC), and the slope (d γ/d ln C t) from the γ–ln C t curves in the concentration range just below CMC. These data enabled us to estimate surface excess ( Γ t), and mean molecular area ( A m) in addition to such parameters as the minimum surface Gibbs energy ( G min ( S ) ), p C 20 and CMC/ C 20 related to synergism accompanied by blending. Applying the regular solution theory (RST), the relation of compositions of the singly dispersed phase ( X 2) and the micellar phase ( Y 2) as well as the interaction parameter ( ω R) (by using the Rubingh's equations) were estimated. The relation between the composition in the adsorbed film ( Z 2) and X 2 together with the interaction parameter ( ω A) in the adsorbed film was also estimated. The partial molecular area (PMA), γ CMC, and G min ( S ) were examined as functions of X 2 and/or Z 2. The resultant CMC– X 2 and CMC– Y 2 curves and ω R and ω A values have demonstrated that mixed micelles and adsorbed film formation are attained accompanying to some extent enhanced intermolecular interaction (with negative ω R and ω A values). Comparing with previous results for mixed systems of CHAPS with n-acyl (octanoly, nonanoyl, and decanoyl)- N-methylglucamides [MEGA- n's ( n = 8, 9, and 10)] and of sodium chenodeoxycholate (NaCDC) with sodium ursodeoxycholate (NaUDC), the synergism observed for the mixed system of CHAPS with NaGC lies between both combinations. However the expected properties as a membrane protein solubilizer are judged to be sufficient.

Monolayers (Langmuir films) behavior of multi-component systems composed of a bile acid with different sterols and with their 1:1 mixtures

Different physicochemical properties of Langmuir films (monolayers) composed of 10 mixed systems of a bile acid, deoxycholic acid (DC) with various plant sterols, such as stigmasterol (Stig), β-sitosterol (Sito) and campesterol (Camp) and a stanol, cholestanol (Chsta) in addition to an animal sterol, cholesterol (Ch) [these sterols and Chsta are abbreviated as St] and DC with 1:1 St mixtures; (Ch + Chsta), (Ch + Stig), (Stig + Chsta), (Ch + Sito) and (Ch + Camp) on the substrate of 5 M aqueous NaCl solution (pH 1.2) at 25 °C, were investigated in terms of mean surface area per molecule ( A m), the partial molecular area (PMA), surface excess Gibbs energy (Δ G (ex)), interaction parameter ( I p) as well as activity coefficients ( f 1 and f 2) in 2-D phase of each binary (or ternary) component system and elasticity (Cs −1) of formed films; these were analyzed on the basis of the respective surface pressure ( π) versus A m isotherms as a function of mole fraction of Sts ( X st) in the DC/St(s) mixtures at discrete surface pressures. Notable findings are: (i) all the binary component systems did form patched film type monolayers consisting of (a) DC-dominant film solubilizing a trace amount of St molecules and (b) St dominant film dissolving a small amount of DC molecules, (ii) DC in 2-D phase exhibited a transition from LE film to LC film at a constant pressure ( π C 1 ) accompanied by compression and (iii) Δ G (ex) as well as I p was found to be greatly dependent on (a) the combinations of DC with different St species and (b) to be markedly varied by a difference in mixing ratio of DC to Sts. Compressibility (or elasticity) analyses and fluorescence microscopy images could support the above findings as well as interpretation.

Mode of interaction of two fluorinated-hydrogenated hybrid amphiphiles with dipalmitoylphosphatidylcholine (DPPC) at the air–water interface

Two-component Langmuir monolayers formed on 0.02 M Tris buffer solution (pH 7.4) with 0.13 M NaCl at 298.2 K were investigated for two different fluorinated-hydrogenated hybrid amphiphiles (F6PH5PPhNa and F8PH5PPhNa or F6 and F8, respectively) with DPPC. Surface pressure ( π), surface potential (Δ V) and dipole moment ( μ ⊥) as a function of molecular surface area ( A) were measured by employing the Whilhelmy method and an ionizing electrode method. From the A– and Δ V– X F6 (or X F8) curves, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined as a function of surface mole fraction ( X F n ) at discrete surface pressures. Then, the behavior of occupied surface areas and surface potentials of the respective components could be made clearer. Compressibility ( C s), elasticity ( C s − 1 ) , and excess Gibbs energy (Δ G (ex)) as a function of X F6 (or X F8) were estimated at definite pressures. These physico-chemical parameters were found to reflect the mechanical strength of monolayer films formed. The regular solution theory being applied to Δ G (ex), the activity coefficients ( f) as well as the interaction parameter ( I p) between DPPC and two hybrid amphiphiles in the binary monolayers were evaluated. I p values thus obtained indicated that F8 molecules interact more strongly with DPPC molecules than F6. Moreover, in order to better understand the morphological monolayer state, Langmuir–Blodgett (LB) films made from DPPC and fluorinated-hydrogenated hybrid amphiphiles were examined by atomic force microscopy (AFM). The miscibility of the two components in the monolayer state is evidenced by these thermodynamic quantities and AFM observations. Furthermore, AFM images demonstrated that F8 could more effectively disperse the ordered domains of DPPC than F6.

Mode of interaction of ganglioside Langmuir monolayer originated from echinoderms: Three binary systems of ganglioside/DPPC, ganglioside/DMPE, and ganglioside/cholesterol

The surface pressure ( π)–area ( A), the surface potential (Δ V)– A, and the dipole moment ( μ ⊥)– A isotherms were obtained for monolayers made from a ganglioside originated from echinoderms [ Diadema setosum ganglioside (DSG-1)], dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylethanolamine (DMPE), cholesterol (Ch), and their combinations. Monolayers spread on several different substrates were investigated at the air/water interface by the Wilhelmy method, ionizing electrode method, fluorescence microscopy (FM) and atomic force microscopy (AFM). Surface potentials (Δ V) of pure components were analyzed using the three-layer model proposed by Demchak and Fort [R.J. Demchak, T. Fort, J. Colloid Interface Sci. 46 (1974) 191–202]. The new finding was that DSG-1 was stable and showed a liquid-expanded film and that its monolayer behavior of Δ V was sensitive for the change of the NaCl concentration in the subphase. Moreover, the miscibility of DSG-1 and three major lipids in the two-component monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the DSG-1 molar fraction ( X DSG-1), using the additivity rule. From the A– X DSG-1 and Δ V m– X DSG-1 plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the discrete surface pressure. The PMA and APSP with the mole fraction were extensively discussed for the miscible system. The miscibility was also investigated from the two-dimensional phase diagrams. Furthermore, a regular surface mixture, for which the Joos equation was used for the analysis of the collapse pressure of two-component monolayers, allowed calculation of the interaction parameter ( ξ) and the interaction energy (−Δ ɛ) between them. The observations using fluorescence microscopy and AFM image also provide us the miscibility in the monolayer state.

Condensation effect of cholesterol, stigmasterol, and sitosterol on dipalmitoylphosphatidylcholine in molecular monolayers

The effects of the incorporation of cholesterol, sitosterol, and stigmasterol into dipalmitoylphosphatidylcholine (DPPC) monolayers were investigated by Langmuir monolayer film technique in this study. The compressibility coefficient was assessed at various surface pressures. The observations indicated that sitosterol and stigmasterol interacted less effectively than cholesterol with the phospholipid. Nevertheless, these sterol molecules could all cause condensation effect on DPPC monolayers. Attractive interactions between DPPC and sterol molecules, or hydrophobic effect, was found to play an important role, testified by negative excess molecular areas at particularly low surface pressures and negative partial molecular area of three sterols at low surface pressures. The minimum extreme points for the excess area were all located at around 0.3 mol fractions for the three sterols at 30 mN/m, which suggest a 2:1 ratio of DPPC/sterol in the ordered structures.

Blending effects on adsorption and micellization of different membrane protein solubilizers: A thermodynamic study on three mixed systems of CHAPS with MEGA-8, -9 and -10 in pH 7.2 phosphate buffer solution

By means of surface tension measurement (Wilhelmy method), micellization and adsorbed film formation were investigated for three combinations of mixed surfactant systems, all of which are used for solubilizing membrane proteins: a typical zwitterionic surfactant, CHAPS (a derivative of cholic acid) with n-alkyl (octyl, nonyl and decyl)- N-glucamides, MEGA- n ( n = 8, 9, 10). The data based on plotting of surface tension ( γ) versus logarithmic total molarity (or molality) ( C t or M t) as a function of mole fraction of surfactant 2 (2 corresponds to MEGA- n's) enabled us to determine critical micellization concentration (CMC), minimum surface tension at CMC ( γ CMC), surface excess ( Γ t), mean molecular surface area ( A m), the minimum Gibbs energy ( G min ( S ) ) of adsorbed film of both single and mixed surfactant systems and partial molecular area (PMA) in addition to parameters such as p C 20 and CMC/ C 20 being related to synergism accompanied by blending (mixing) in regard to surface activity as well as micelle forming ability. On the basis of the regular solution theory, the relations of compositions of singly dispersed phase ( X 2), micellar phase ( Y 2) and adsorbed film ( Z 2) were estimated, and then the interaction parameters in micelles ( ω R) and in the adsorbed film phase ( ω A) were also calculated. From both the CMC– X 2 and CMC– Y 2 curves, it was found for all the combinations to show synergistically enhanced ability of mixed micelle formation as well as surface tension reduction. The resultant synergism coming from blending CHAPS with MEGA- n's was discussed in comparison with different combinations of various types of surfactants including membrane proteins solubilizers.

Langmuir monolayers of cerebroside originated from Linckia laevigata: Binary systems of cerebrosides and phospholipid

The surface pressure ( π)–area ( A), the surface potential (Δ V)– A and the dipole moment ( μ ⊥)– A isotherms were obtained for six cerebrosides of LLC-2, LLC-2-1, LLC-2-8, LLC-2-10, LLC-2-12, and LLC-2-15, which were isolated from Linckia laevigata, and two-component monolayers of two different cerebrosides (LLC-2 and LLC-2-8) with phospholipid of dipalmitoylphosphatidylcholine (DPPC) on a subphase of 0.15 M sodium chloride solution as a function of cerebroside compositions in the two-component systems by employing the Wilhelmy method, the ionizing electrode method, and the fluorescence microscopy. The new finding was that LLC-2 showed a stable and liquid expanded type film. Four of them (LLC-2-8, -10, -12, and -15) had the phase transition from the liquid-expanded (LE) to the liquid-condensed (LC) states at 298.2 K. The apparent molar quantity changes (Δ s γ , Δ h γ , and Δ u γ ) on their phase transition on 0.15 M at 298.2 K were calculated. The miscibility of cerebroside and phospholipid in the two-component monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the cerebroside molar fraction ( X cerebroside), using the additivity rule. From the A– X cerebroside and Δ V m– X phospholipid plots, a partial molecular surface area (PMA) and an apparent partial molecular surface potential (APSP) were determined at the discrete surface pressure. The PMA and APSP with the mole fraction were extensively discussed for the miscible systems. Judging from the two-dimensional phase diagrams, these were found to be one type, a positive azeotropic type; all the cerebrosides were miscible with DPPC. Furthermore, assuming a regular surface mixture, the Joos equation for the analysis of the collapse pressure of two-component monolayers allowed calculation of the interaction parameter ( ξ) and the interaction energy (−Δ ɛ) between the cerebrosides and DPPC. The miscibility of cerebroside and phospholipid components in the monolayer state was also supported by fluorescence microscopy.

Cerebroside Langmuir monolayers originated from the echinoderms: II. Binary systems of cerebrosides and steroids

Two-component Langmuir monolayers formed on a subphase of 0.5 M sodium chloride solution were investigated for two different cerebrosides (LMC-1 and LMC-2) with steroids of cholesterol (Ch) and cholesteryl sodium sulfate (Ch-S); i.e. LMC-1/Ch, LMC-1/Ch-S, LMC-2/Ch, and LMC-2/Ch-S were examined in terms of surface pressure ( π), the surface potential (Δ V) and the dipole moment ( μ ⊥) as a function of surface area ( A) by employing the Langmuir method, the ionizing electrode method, and the fluorescence microscopy. Surface potentials (Δ V) of steroids were analyzed using the three-layer model proposed by Demchak and Fort [R.J. Demchak, T. Fort Jr., J. Colloid Interface Sci. 46 (1974) 191–202]. The miscibility of cerebrosides and steroids in the insoluble monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the steroid molar fraction ( X steroid) based upon the additivity rule. From the A– X steroid and Δ V m– X steroid plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the different surface pressures. The PMA and APSP with the mole fraction were discussed for the miscible system. Judging from the two-dimensional phase diagrams, they can be classified into two types. The first is a completely immiscible type; the combination of cerebrosides with cholesterol. The second is a negative azeotropic type, where cerebrosides and cholesteryl sodium sulfate are completely miscible both in the expanded state and in the condensed state. In addition, a regular surface mixture (the Joos equation for the analysis of the collapse pressure of two-component monolayers) allowed calculation of the interaction parameter ( ξ) and the interaction energy (−Δ ɛ) between the cerebrosides and Ch-S. The miscibility of cerebroside and steroid components in the monolayer state was also supported by fluorescence microscopy.

Cerebroside Langmuir monolayers originated from the echinoderms: I. Binary systems of cerebrosides and phospholipids

The surface pressure ( π)–area ( A), the surface potential (Δ V)– A and the dipole moment ( μ ⊥)– A isotherms were obtained for two-component monolayers of two different cerebrosides (LMC-1 and LMC-2) with phospholipids of dipalmitoylphosphatidylcholine (DPPC) and with dipalmitoylphosphatidylethanolamine (DPPE) on a subphase of 0.5 M sodium chloride solution as a function of phospholipid compositions by employing the Langmuir method, the ionizing electrode method, and the fluorescence microscopy. Surface potentials (Δ V) of pure components were analyzed using the three-layer model proposed by Demchak and Fort [J. Colloid Interf. Sci. 46 (1974) 191–202]. The contributions of the hydrophilic saccharide group and the head group to the vertical component of the dipole moment ( μ ⊥) were estimated. The miscibility of cerebroside and phospholipid in the two-component monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the phospholipid molar fraction ( X phospholipid), using the additivity rule. From the A– X phospholipid and Δ V m– X phospholipid plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the discrete surface pressure. The PMA and APSP with the mole fraction were extensively discussed for the miscible system. Judging from the two-dimensional phase diagrams, these can be classified into two types. The first is a positive azeotropic type; the combinations of cerebrosides with DPPC are miscible with each other. The second is a completely immiscible type: the combination of cerebrosides with DPPE. Furthermore, a regular surface mixture, for which the Joos equation was used for the analysis of the collapse pressure of two-component monolayers, allowed calculation of the interaction parameter ( ξ) and the interaction energy (−Δ ɛ) between the cerebrosides and DPPC component. The miscibility of cerebroside and phospholipid components in the monolayer state was also supported by fluorescence microscopy.

Adsorption and Micelle Formation of Mixed Surfactant Systems in Water. IV. Three Combinations of SDS with MEGA-8, -9 and -10

By means of surface tension measurement (Wilhelmy method), micellization and adsorbed film formation were investigated for three combinations of mixed surfactant systems: a typical anionic surfactant, sodium dodecylsulfate (SDS) was combined with three nonionic surfactants used as membrane-protein solubilizers, n-alkyl (octyl, nonyl and decyl)-N-methylglucamides (MEGA-8, MEGA-9 and MEGA-10, respectively) in water at 30°C. The data of surface tension (γ) versus logarithmic total molality of both single and mixed surfactant systems (mt) plots as a function of mole fraction of surfactant 2 (2 corresponds to MEGA-n’s), X2, enabled us to determine critical micellization concentration (CMC), minimum surface tension at CMC (γCMC), surface excess (Γt), mean surface area occupied by a molecule (Am) and parameters related to synergism in surface activity such as pC20 and CMC/C20. Based on the regular solution theory, the relation of compositions of the singly dispersed phase (X2) and the composition of the adsorbed film phase (Z2) were estimated, and then the interaction parameters in micelles (ωR) and in adsorbed film (ωA) were also calculated. Both the CMC-X2 and CMC-Y2 curves were found for all combinations to show a negative deviation from ideal mixing, and even the curve of mt (bulk phase concentration) versus Z2 produced a prominent negative ωA. A marked synergism in surface tension reduction was observed for these MEGA-n’s mixtures with SDS; the extent of the synergism increased in the order of MEGA-8, -9 and -10. A positive synergism was observed, even from the partial molecular area (PMA) and the minimum free energy at surface G(S)min= (γCMC·Am·L) where, L is the Avogdro’s number.

Adsorption and Micelle Formation of Mixed Surfactant Systems in Water. III. A Comparison between Cationic Gemini / Cationic and Cationic Gemini / Nonionic Combinations

A surface tension study (drop volume method) was performed at 30°C on micellization and adsorbed film formation of a mixed system of a cationic Gemini-type surfactant with a cationic surfactant in comparison with another mixed system of the same Gemini-type surfactant with a nonionic surfactant. The systems studied were Bis-ammonium Gemini derived from tartaric acid dibromide salt [BAGTB, 1,4-Bis(trimethylammonio)-2,3-dodecyloxy butane dibromide] with hexadecyltrimethylammonium bromide (HTAB): BAGTB / HTAB mixed system and with n-decanoyl-N-methylglucamide (MEGA-10): BAGTB / MEGA-10 mixed system. The data of surface tension (γ ) vs logarithmic molality plots as a function of mole fraction of surfactant 2 (2 corresponds to HTAB or MEGA-10), X 2, enabled us to determine the critical micellization concentration (CMC), the surface tension at CMC (γCMC), surface excess concentration (Γt ), the mean molecular surface area (A m), the partial molecular area (PMA), and the measures of efficiency of adsorption (pC20 = -log C20) and CMC / C20 which is available for evaluating the facilitating balance between adsorption and micellization. In addition, a newly defined measure of synergism in surface activity, i.e., the minimum surface Gibbs energy (G (S)min) was employed. Based on these data, the examination of synergism in micelle formation and in surface tension reduction elucidated that the Gemini-type surfactant does not exhibit any positive synergism for either BAGTB / HTAB or BAGTB / MEGA-10 mixed systems. Most of these parameters are found to depend conspicuously on the mixing ratio for both mixtures; indicating that the state of adsorbed film is divided into three ranges of X 2; the lower, the middle and the higher. The compositions of micelles formed at CMC (Y 2) and of adsorbed film (Z 2) equilibinated with bulk solution at a fixed surface tension were estimated.

Adsorption and Micelle Formation of Mixed Surfactant Systems in Water II: A Combination of Cationic Gemini-type Surfactant with MEGA-10

Mixed micellization and mixed adsorbed film formation were investigated for the combination of a Gemini type cationic and a nonionic surfactants mixture: Bis-trimethyl ammonium Gemini derived from tartaric acid bromide (BAGTB) and n-Decanoyl-N-methylglucamide (MEGA-10). The surface tension of the aqueous mixed surfactant solution was measured at every 0.1 mole fraction of MEGA10 in the surfactant mixture applying a drop volume method at 30°C. From the curves of surface tension (γ) vs logarithmic concentration in molality (ln m ), critical micellization concentration (CMC), minimum surface tension at CMC (γCMC), surface excess (Γ), mean surface area occupied by a molecule (A m) and parameters related to synergism in surface activity such as pC20 and CMC / C20 were determined. Based on the regular solution theory, the relation of compositions of the singly dispersed phase (X MEGA10) and the micellar phase (Y MEGA10), and the relation of X MEGA10 with the composition in adsorbed film phase (Z 2) were estimated, and along with these, the interaction parameters in micelles (ωR) and in adsorbed film (ωA ) were calculated. Both the CMC-X MEGA-10 and CMC-Y MEGA-10 curves showed a negative deviation from ideal mixing and even the curve of Z MEGA10-m t (bulk phase concentration) produced a slightly negative ωA . However, the synergism in surface tension reduction was found to be rather weak from examination of partial molecular area (PMA) and the minimum free energy at surface G min(S) = (γCMC·A m·L ). As for the adsorbed film, the interaction mode between molecules, as well as two dimensional molecular packing, was observed to be separated into three regions; i. e. at X STDS = 0.45 and at X STDS = 0.75 different properties changed discontinuously.

Adsorption and Micelle Formation of Mixed Surfactant Systems in Water. I. A Combination of Top-heavy Type Anionic Surfactants.

The micellization and adsorbed film formation for an aqueous binary mixed system of anionic top-heavy type surfactants: sodium salt of α-sulfonatomyristic acid isopropyl ester (α-SMy·iPr) and sodium 1-oxo-1 [4-(tridecafluorohexyl) phenyl]-2-hexanesulfonate (FC6-HC4), were studied by means of surface tension measurement (drop volume method). The surface tension (γ) was precisely measured for the solutions of the respective single and eleven mixed systems at 30.00 ± 0.03°C in order to determine critical micellization concentrations (CMC) and surface excesses (Γ) as a function of net mole fraction of FC6-HC4 (XFC6-HC4). The effect of added NaCl on CMC was examined for each single system to determine the degree of counterion binding (βi) from the Corrin-Harkins plot. Using the data of CMC and βi, we estimated the micellar composition (YFC6-HC4), activity coecients (fi) of the two surfactants in mixed micelles, and the interaction parameter (ω0), the calculation of which was made on the basis of our own theoretical equations taking into account of mutual counterion effect in binary ionic surfactant mixtures. From the CMC-YFC6-HC4 curve, markedly positive and slightly negative deviations from ideal mixing micelle formation were found, i.e. the positive deviation at the range of 0 < XFC6-HC4 < ca. 0.3 and the slightly negative deviation at the range of ca. 0.3 < XFC6-HC4 < 1. On the other hand, the composition of adsorbed film (ZFC6-HC4) was also estimated as a function of XFC6-HC4 using the surface tension data from below CMCs. In addition, Γ was determined as functions of XFC6-HC4 and ZFC6-HC4 and the partial molecular area (PMA) was evaluated from the plot of mean molecular surface area (Am) vs. ZFC6-HC4. All the data obtained showed without exception that an addition of 10 to 20% α-SMy·iPr to FC6-HC4 results in a marked enhancement of surface activity and micelle formation ability, accompanying a favorable packing in two-dimensional or three dimensional structuring by the two top-heavy type surfactants, and that the two or three dimensional structures are thermodynamically more stable due to higher FC6-HC4 content.

Equations of state for POPX lipids at the air/water interface. A comprehensive study

We have investigated monomolecular fluid-like films of palmitoyl oleoylphosphatidyl lipids with choline, glycerol and serine head groups, respectively. Conventional Langmuir trough experiments have been evaluated towards a thermodynamic analysis applying a novel approach that was recently developed in this laboratory. Our work involves elaborate efforts to exclude possible error sources of the basic measuring parameters. By means of pertinent mass conservation plots it could then be shown that the present lipids form a practically insoluble monolayer. Relative deviations of the lateral pressure from its ideal (gaseous) value are seen to be a very pronounced linear function of the surface concentration (between 1 and about 35 mN/m). They reveal a clearly manifested Boyle point around 4 mN/m, indicating formation of aggregates in the very low pressure range. The results are discussed in terms of a rather simple quantitative formulation of the underlying equation of state including fit curves of the related partial molecular area and Gibbs free energy.