Model calculations of aggregation numbers and radii of w/o microemulsions

Summary A method of calculating aggregation numbers and radii of reversed micellar aggregates is presented and applicated on the alcohol-rich solution phaseL ₂ in the model system water/decanol/sodium octanoate at 293 K. The result showing good agreement with experimental data illustrate the extension of the micellar core at different decanol contents and molar ratios water to sodium octanoate. The influence of a distribution of water between the interior of the micelle and the decanolic medium is demonstrated.

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Zusammenfassung Es wird eine Methode zur Berechnung von Aggregations zahlen und Radien invertierter mizellarer Aggregate angegeben und auf die Alkohol-reichenL ₂-Lösungen im Modellsystem Wasser/Dekanol/Natrium-Oktanoat bei 293 K angewendet. Die Resultate sind in guter Übereinstimmung mit experimentellen Messungen; sie zeigen die Ausdehnung der Mizellkerne bei verschiedenen Dekanol-Konzentrationen und bei verschiedenen molaren Verhältnissen Wasser/Oktanoat. Der Einfluß der Verteilung von Wasser zwischen dem dekanolischen Medium und den Mizellen wird diskutiert.

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With 4 figures and 3 tables     

Some dielectric properties of reversed micelles in the system sodium octanoate/water/n-decanol

We have found it possible to estimate the dielectric constant of the aqueous micellar core by means of quite simple calculations. The presented method requires assumptions about the particle shape and about the distribution of the different species in order to obtain the right order of magnitude. The presented calculations deal only with limited conditions in the model system sodium octanoate/water/decanol. Here we find that a spherical micelle model can be applied to surprisingly broad concentration intervals. At the alcohol-poor phase boundary other kinds of aggregates might occur. It is our purpose to develop the presented model to involve other particle shapes and also other model systems.     

Partial Phase Diagrams in the System Water/Formamide/Sodium Dodecyl Sulfate/Decanol

The phase regions were determined in the systems of sodium dodecyl sulfate, decanol and a polar solvent. The latter was a) water, b) water and formamide (1/1 weight ratio) and c) formamide

The water system showed an aqueous micellar solution, a decanol solution with inverse micelles as well as a lamellar and a hexagonal liquid crystal. The formamide system gave a formamide solution, a lamellar liquid crystal and a decanol solution while the system with water plus formamide in a 1/1 ratio still retained the hexagonal liquid crystal.     

Linear solvation energy relationships of mixed micelles of sodium dodecyl sulfate and decanol: towards a better model of octanol/water partitioning.

We show that we can alter the mechanism of micelle/water partitioning by the addition of decanol as a co-surfactant to an SDS micellar solution. Linear solvation energy relationship (LSER) studies indicate that as we increase the amount of decanol added to sodium dodecyl sulfate solution, the hydrogen bond donating ability of the aqueous phase increases and the cavity term of the micellar phase increases. We obtain a better correlation with octanol/water partitioning using the mixed micelle system compared to normal micelle solution. Choosing the appropriate micelle marker is very important. Significant changes in the LSER equations can occur if a different compound is used as the micelle marker.     

Enthalpies of solution in sodium octanoate + water + alcohol mixtures

Enthalpies of solution of sodium octanoate in water, 1-propanol and aqueous mixtures of 1-propanol, 1-butanol, 1-pentanol and 1-hexanol, and of the alcohols in aqueous solutions of sodium octanoate at various concentrations were determined calorimetrically at 35 °C. MostH(soln) values are exothermic and strongly dependent on the solute concentration. The main energetic factor governing the process of dissolution of the surfactant is associated with changes in the water structure caused by the presence of alcohol. That governing the process of the alcohol dissolution in surfactant solutions is due to the effect alcohols have on the CMC of the octanoate. There is no indication of the alcohol being either solubilized in the interior of the aqueous micelle, or becoming part of the micellar film.The solubility at 35 °C of sodium octanoate in water, 1-propanol and their mixtures has also been determined.     

First-order transition between nematic phases in lyotropic liquid crystals

Experimental evidence for the existence of a first order transition between N_c-N_d uniaxial lyonematic phases as a function of the variable M_d (number of decanol molecules per amphiphilic molecule) is reported. The relevance of this evidence to molecular models for micellar aggregates is discussed. The evidence is for a change in micellar symmetry at this transition.     

有机盐对水/AOT/醇反相微乳体系电导行为的影响

用二(2-乙基己基)琥珀酸酯磺酸钠(AOT)为表面活性剂研究了以正构醇(己醇、庚醇、辛醇、癸醇)为连续相的微乳体系的电导行为, 结果表明只有水/AOT/癸醇体系有水诱导的电导渗滤现象. 研究了有机盐(胆酸钠、水杨酸钠)及温度对电导行为的影响, 发现庚醇、辛醇体系电导率随胆酸钠浓度的增加而减小, 而癸醇体系电导率不受影响; 庚醇、癸醇体系的电导率随水杨酸钠浓度的增加而增大; 在5～40 ℃范围内lnσ(电导率的自然对数)与温度成很好的线性关系, 无论有机盐存在与否都没有温度诱导的渗滤现象. 根据Arrhenius-type公式估算了体系的电导活化能.     

Micellar behavior in solutions of bile-acid salts

Summary The density, electrical conductivity, viscosity, lightscattering and the low-angle X-ray diffraction have been examined in the solution region of the ternary systems of sodium cholate or desoxycholate and n-decanol and water at 20°. The phase diagrams show a continuous transition from homogeneous solutions of bile-acid salt in water to homogeneous solutions of bile-acid salt and water in decanol. Along the bile-acid salt-water axis there are micellar solutions containing micelles of the normal type where the polar groups are directed outwards, and these micelles can solubilize decanol without initially undergoing any change in their basic structure. Above a critical value, however, the micellar structure is determined by the decanol molecules, and at large decanol contents there is an inversion that results in the formation of reversed micelles, with the polar groups and water now located within the core of the micelles.

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Zusammenfassung Dichte, elektrische Leitfähigkeit, Viskosität, Lichtstreuung und Kleinwinkelstreuung von Röntgenstrahlen wurden im Lösungsgebiet der ternären Systeme Natriumcholat bzw. Natriumdesoxycholat, n-Dekanol und Wasser studiert. Das Phasendiagramm zeigt einen kontinuierlichen Übergang von der homogenen Gallensalzlösung in Wasser bis zur homogenen Lösung von Gallensalz und Wasser in Dekanol. Entlang der Gallensalz-Wasserachse kommen mizellare Lösungen vor, die Mizellen vom normalen Typ enthalten, bei denen die polaren Gruppen nach außen gerichtet sind. Diese Mizellen können recht große Mengen Dekanol solubilisieren, ohne daß ihre Grundstruktur verändert wird. Oberhalb einer kritischen Menge bestimmen jedoch die Dekanolmoleküle die Mizellstruktur. Bei großen Mengen Dekanol erfolgt eine Umstrukturierung, die zur Bildung von Mizellen mit den polaren Gruppen im Mizellkern führt.

     

Small angle neutron scattering studies on micellar systems part 1. Ammonium octanoate,ammonium decanoate and ammonium perfluoroocatanoate

Small angle neutron scattering has been used to examine the size and shape of micelles of ammonium octanoate, ammonium decanoate and ammonium perfluoro-octanoate. Ammonium octanoate was found to form micelles with a micellar weight of 1640 and ammonium decanoate with a micellar weight of 12,576; both materials appeared to form spherical micelles. Ammonium perfluoro-octanoate formed micelles with a micellar weight of 17,610. Evidence from the scattering experiments suggested that the micelles were cylindrical and a model for the micelle is proposed.     

Hydrogen-bond-assisted excited-state deactivation at liquid/water interfaces

The excited-state dynamics of eosin B (EB) at dodecane/water and decanol/water interfaces has been investigated with polarization-dependent and time-resolved surface second harmonic generation. The results of the polarization-dependent measurements vary substantially with (1) the EB concentration, (2) the age of the sample, and (3) the nature of the organic phase. All of these effects are ascribed to the formation of EB aggregates at the interface. Aggregation also manifests itself in the time-resolved measurements as a substantial shortening of the excited-state lifetime of EB. However, independently of the dye concentration used, the excited-state lifetime of EB at both dodecane/water and decanol/water interfaces is much longer than in bulk water, where the excited-state population undergoes hydrogen-bond-assisted non-radiative deactivation in a few picoseconds. These results indicate that hydrogen bonding between EB and water molecules at liquid/water interfaces is either much less efficient than in bulk water or does not enhance non-radiative deactivation. This strong increase of the excited-state lifetime of EB at liquid/water interfaces opens promising avenues of applying this molecule as a fluorescent interfacial probe.     

Non-cylindrical symmetry of the ordering matrix of some spectroscopic probes in a lyotropic mesophase studied by 2H N.M.R. spectroscopy

Deuterium N.M.R. spectroscopy has been used to investigate the orientational ordering of perdeuteriated naphthalene and two perdeuteriated fluorescent probes (pyrene and perylene) dissolved in a discotic nematic micellar phase of potassium laurate/potassium chloride/decanol/water. The results show that the ordering tensors of these probes are not cylindrically symmetric.     

溶剂性质对DBS/C~1~0H~2~1OH/溶剂体系的相行为 与结构的影响

在DBS/C~1~0H~2~1OH/溶剂体系中,以甲酰胺、水或甘油为溶剂时,在一定浓度范围内能生成O/W,W/O微乳液和层状液晶。C~1~0H~2~1OH在各种溶剂的DBS胶束中的分配系数K均很高。以甲酰胺、水为溶剂时,DBS能在一定浓度范围内生成层状液晶,但以甘油为溶剂时,则不能生成层状液晶。C~1~0H~2~1OH的加入有助于层状液晶的生成,但过量的C~1~0H~2~1OH却使层状液晶失稳。在相同质量比C~1~0H~2~1OH/DBS下,按顺序H~2O,FA,Gly,层状液晶中溶剂的渗透率α降低,d~0值亦降低;以FA为溶剂时,表示其表观有序度的Δν值却较高;以水为溶剂时,Δν值却较低;以Gly为溶剂时,Δν具有中等值。不同溶剂层状液晶两亲双层内的分子取向与溶剂性质对层状液晶稳定性的影响相一致。     

The structure and properties of medium-chain surfactant solutions: a case study of sodium octanoate

In the paper the most characteristic properties of binary sodium octanoate solutions of variable ion strengths are discussed. Both pre- and postmicellar association is investigated with reference to available scattering, spectroscopic and thermodynamic data. The focus is then placed on references dealing with, or referring to, results including sodium octanoate. A picture is offered illustrating the sodium octanoate micelle as a rather unstable entity including micelle sizes from oligomeric pre-associates up to some 20 monomers. Due to this circumstance the molecular features obtained with different scattering and spectroscopic techniques are rather divergent. However, the structure emerging describes the micelle as consisting of a small “dry” hydro-carbon core with a radius shorter than the fully extended heptyle chain of the octanoate. Instead a “rough shell” encompassing the hydrated polar head groups, some “wet” methylene/methyle groups and the hydrated counterions separates the micelle from the surrounding solvent. On purely geometric grounds this hydrated shell represent, however, more than 50% of the micellar volume. The numerous distinct properties of medium-chain surfactants should thence not, as is frequently done, be related to those of long-chain surfactants! At high concentrations of octanoate or salt, or upon solubilization, the average degree of water contact seem to diminish due to an increased aggregation number or a lack of available “free” hydration water. The latter effect is suggested to produce observable effects at the second critical concentration as well as close to the phase boundary. The influence of solubilizates are dealth with only when the results provide significant information on the properties of the binary system.     

Study of the reciprocal space image of the biaxial nematic phase in the plane of the amphiphilic bilayer

High resolution synchrotron X-ray measurements have been performed on the uniaxial and biaxial phases of the lyotropic mixture potassuim laurate, decanol and water. An elegant magnetic orientational procedure allows us to obtain the cut of the reciprocal space image of the biaxial phase at the plane perpendicular to the amphiphilic bilayer. The analysis of densitometric profiles of the diffracted bands indicate an anisotropic micellar correlation in the plane of the amphiphilic bilayer. It is suggested that an anisotropic distribution of decanol and potassium laurate in this plane could be responsible for this effect.     

Neutron scattering study of pentanol solubilization in sodium octanoate micelles

We extend a previous small-angle neutron scattering study of sodium octanoate (NaC₈) micelles to the ternary system sodium octanoate/pentanol/water. The use of contrast variation through selective deuteration of individual components together with explicit computation of interference effects, permits us to deduce the location of pentanol (C₅OH) in the micelles. Our main conclusion is that, although the micelles grow as (C₅OH) is solubilised, there is no concomitant variation in the NaC₈ aggregation number. At low alcohol concentrations, the C₅OH is located near the NaC₈ polar heads, while at higher concentration the -OH groups are distributed throughout the micellar core.     

Characterization of structural transitions in the SLS/decanol/water system

The ternary system sodium-dodecylsulphate (SLS)/decanol/water has been investigated at three different water contents and varying ratios of cosurfactant to surfactant by means of polarized optical microscopy,²H-NMR quadrupole splittings and small angle x-ray scattering. Upon addition of decanol a hexagonal phase transforms into a lamellar phase. For the highest water content of 0.65 no intermediate two-phase regions are detected but nematic phases are formed between. The lamellar phase at low cosurfactant content is very sensitive to changes of temperature and seems to be a so-called defective one with curved interfaces. From the scaling behavior it is concluded that the building units seem to be ribbons of increasing width on addition of cosurfactant or amphiphilic substance. By reaching a decanol mole fraction of 0.4 a classical lamellar phase with well-known behavior is formed. During these transformations the position of the first diffraction maximum changes gradually irrespective of phase transitions. The maximum mole fraction of cosurfactant the lamellar phase of our system can incorporate is 0.77.     

Deformation of cylinders in the hexagonal phase of sodium decyl sulphate/decanol/water on the introduction of decanol

X-ray scattering studies show that aggregates of amphiphilic molecules in the hexagonal phase of the binary system sodium decyl sulphate/water are infinite cylinders whose radius is very close to that of the extended molecule. When sodium decyl sulphate molecules are progressively substituted by decanol molecules the phase remains hexagonal, up to a decanol/soap molar ratio of 0.15, but the radius of its cylinders becomes larger than the molecular length. We have developed specific experiments to investigate this growth and see if it is isotropic or anisotropic. We have focused our attention on the evolution of the configuration of soap and decanol molecules within the aggregates. Neutron scattering experiments show that the two molecules do not distribute themselves uniformly and D.M.R. measurements show that they stay anchored at the amphiphile/water interface by their polar heads. The distribution of the two molecules along the interface is not therefore uniform. This induces an inhomogeneity of the interfacial curvature and the growth of the section of the cylinders cannot be isotropic. This anisotropy increases when the decanol/soap molar ratio increases. It provides a clue to understand the transformation of the hexagonal phase into a recangular one whose aggregates are infinite ribbons with non-circular section, above the decanol/soap molar ratio of 0.15.     

甲苯对水/AOT/醇反相微乳液体系电导行为的影响

采用电导法研究了以二(2-乙基己基)琥珀酸酯磺酸钠(AOT)为表面活性剂、庚醇和癸醇分别与甲苯的混合溶剂为油相的反相微乳液体系在有无添加剂氯化钠时的电导行为。 结果表明,甲苯对水/AOT/癸醇体系电导渗滤有明显的抑制作用,而对水/AOT/庚醇体系没有影响;添加氯化钠对水/AOT/庚醇（癸醇）/甲苯体系的电导率基本无影响。     

SDS/正癸醇/甘油与SDS/正癸醇/甲酰胺液晶结构

以极性有机物为溶剂制备展状液晶已经引起重视,但关于极性有机溶剂对层状液晶结构和稳定性影响的研究还很不够.我们曾以~2H NMR测量研究了以甲酰胺代替甘油为溶剂,对非水层状液晶SDS/C_(10)H_(21)OH/溶剂体系稳定性的影响.本文则以小角X射线衍射测量,研究以甲酰胺代替甘油后,对层状液晶SDS/C_(21)H_(21)OH/溶剂体系结构的影响。     

Partial molar enthalpies in “inverted micellar” solutions of water and sodium octanoate in decanol

Summary The relative molar enthalpy of solutions with three constant molar ratios of water to sodium octanoate in the isotropic decanol-rich solution phase, L₂, of the system water — sodiumn-octanoate -n-decanol have been determined by means of calorimetry at 2° C. A method of calculating the relative partial enthalpies of the components from these data is described. It is the thermic interaction of the sodium octanoate that dominates the properties throughout the extent of the phase studied. Where the decanol content is highest, the partial enthalpies of the sodium octanoate and the water increase sharply while the partial enthalpy of the decanol remains relatively constant throughout the extent of the phase. This agrees well with the general view that aggregation into inverted micelles takes place as a continuous process and that the properties of the aggregates remain constant.

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Zusammenfassung Die relative molare Enthalpie von Lösungen mit drei konstanten Molverhältnissen Wasser/Natriumoktanoat in der isotropen dekanolreichen Lösungsphase, L₂, in dem System von Wasser-NatriumN-Oktanoat —N-Dekanol wurde kalorimetrisch bei 25°C bestimmt. Außerdem wurde ein Verfahren angegeben, um die relativen partiellen Enthalpien der Komponenten aus diesen Daten zu berechnen. Die thermische Wechselwirkung des Natriumoktanoates bestimmt die Eigenschaften der untersuchten Phase. Wo die Dekanolgehalte am größten sind, nehmen die partiellen Enthalpien von Natriumoktanoat und Wasser noch zu, während die partielle Enthalpie von Dekanol relativ konstant bleibt, Das stimmt gut überein mit der allgemeinen Auffassung, daß die Aggregation zu inverted micelles durch einen kontinuierlichen Prozeß geschieht und daß die Eigenschaften der Aggregate konstant bleiben.

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With 5 figures and 3 tables