Vlasta Tomašić

Effect of the spacer length on the solid phase transitions of dissymmetric gemini surfactants

Three dissymmetric gemini surfactants (abbreviated as 12−s−14) in which n-dodecyldimethylammonium bromide and n-tetradecyldimethylammonium bromide are connected at the polar headgroups by a flexible −(CH2)s− spacer (s = 2, 6, or 10) have been synthesized. The influence of the spacer length on the structural and thermal properties of 12−s−14 surfactants was investigated by means of IR and NMR spectral analysis, X-ray diffraction, thermogravimetry, differential scanning calorimetry, and polarizing optical microscopy. Geminis with s = 2 or 10 form monolayers in which two alkyl chains are in the trans configuration, while the gemini with s = 6 forms interdigitated bilayers with two alkyl chains in the cis configuration with respect to the spacer. All compounds exhibited a complex polymorphism and thermotropic mesomorphism from the stable crystalline form to the liquid crystalline phases of smectic type. The number of thermal phase transitions and the sequence of phases are markedly affected by the spacer leng...

Structure of liquid crystalline aerosol-OT and its alkylammonium salts.

Aerosol-OT is a widely used anionic surfactant, and its lyotropic properties have been studied extensively. However, neat AOT is itself liquid crystalline. We carried out an X-ray study of neat AOT sodium salt, as well as of AOT-n-decyl-, n-dodecyl-, n-tetradecyl-, and n-hexadecylammonium salts. We confirm an earlier report that pure AOT forms a hexagonal columnar phase but propose a different packing model. This involves a relatively highly ordered structure with each column cross-section containing three tessellated molecules in the plane normal to the column axis. The structure is trigonal locally but hexagonal over the long-range. This mode of assembly is supported by electron density reconstruction and molecular modeling. At subambient temperatures, the AOT-alkylammonium complexes Cn-AOT, with n=10-16, also display a hexagonal columnar phase, but this is more disordered, and each column cross-section contains only two ion pairs. Unusually, molar enthalpy and entropy of the columnar-isotropic transition in Cn-AOT salts decrease with increasing n. This is attributed to a disproportionally high conformational disorder of the radial chains in the columnar phase, which is required for efficient space filling.

Interactions in aqueous mixtures of alkylammonium chlorides and sodium cholate

The interactions of alkylammonium chlorides (the number of carbon atom per chain was either 12, 14, or 16) with sodium cholate have been investigated by a combination of techniques including light and electron microscopy, surface tension, conductivity, light scattering, and microelectrophoretic measurements. The phase behavior has strongly depended on the molar ratio and actual concentration of oppositely charged surfactants. The change in the composition of the aggregates leads to a shape transformation from globular to elongated micelles to open and/or closed bilayers (vesicles) and precipitation. The length of micelles has been found to decrease dramatically with the concentration shift to the micellar regions of either surfactant. Upon a moderate excess of one surfactant, the mean hydrodynamic diameter of aggregates increases and wormlike micelles and/or open and closed bilayers are formed. Microscopic observations of alkylammonium cholates (novel catanionic surfactants precipitated in and/or close to equimolar region) have shown the presence of a variety of morphologies including twisted ribbons, tubules and bundles of tubules.

Characterization of micellar systems by the conductivity method; sodium salt of perfluoropolyether carboxylic acid

A new method for the interpretation of the conductivity of ionic surfactants is proposed. The model considers the contribution of ions and of charged micelles, and is based on the approximation of constant concentration of surfactant chains above the critical micellization concentration (CMC), and on the equilibrium constant of counterion bindings to the micelles (K). These two parameters evaluated for aqueous solutions of sodium salt of perfluoropolyether carboxylic acid are as follows:K=35800 dm3 mol−1; CMC=1.19×10−3 moldm−3. The third quantity obtained by this procedure is the product of the aggregation number (N) and the size parameter (f=e2L/6πηr):Nf=76.5 S cm2 mol−1.

Thermal behavior of dodecylpyridinium based surfactant salts with varied anionic constituent

Three novel dodecylpyridinium-based catanionic compounds were synthesized. A combination of techniques was used to examine their thermal properties: differential scanning calorimetry, powder x-ray diffraction, and polarizing microscopy. Briefly, the absence of mesomorphic properties and the formation of zig-zag blade textures were obtained in dodecylpyridinium picrate, thermotropic mesomorphism, and polymorphism in dodecylpyridinium dodecylbenzenesulfonate, and melting accompanied with degradation in combination with cholate. The anionic part of the molecule promotes behavior of novel compounds, resulting in different packing and thermal properties of catanionics in their solid state.

Precipitation and association in a mixture of dodecylammonium chloride and sodium dodecyl sulfate in aqueous medium

The precipitation boundary in aqueous mixtures of dodecylammonium chloride (DDACl) and sodium dodecyl sulfate (NaDS) was determined in the absence and in the presence of 1 mol dm−3 NaCl. The structure and the composition of the solid phase was characterized by x-ray and chemical analysis, respectively. Activities of Cl− and Na+ ions in the aqueous phase were measured by ion-selective electrodes. As determined by calorimetry, precipitation is an exothermic process.The DDA·DS precipitate was formed in the equimolar region of the precipitation components. Its crystallographic structure is described. In an excess of sodium dodecyl sulfate, the precipitate incorporated substantial amounts of NaDS, as detected by analyses of both solid and liquid phases. X-ray analysis of the dry sample showed peaks of crystalline NaDS. According to the polarization microscopy of wet samples, one may conclude that liquid crystals, containing an excess of NaDS, are incorporated in the solid DDA·DS phase. The same was found in the case of an excess of DDACl; mixed liquid crystals with an excess of DDACl were incorporated in the solid DDA·DS.Interpretation of the solubility boundary points to the presence of DDA+·DS− ion pairs. Formation of these species at low ionic strengths is characterized by the equilibrium constantKa0≈106. However, in the 1 mol dm−3 NaCl, the association of DDA+ and DS− ions into pairs is inhibited (Ka0≈0). This finding can be explained in terms of ionic clouds around the charged surfactant heads: if these heads are not in close contact, but separated due to structural effects of the chains, the dense distribution of counterions around them at high ionic strengths may compensate for electrostatic attraction and, thus, inhibit ion-pairing.

Growth of dodecylammonium dodecyl sulfate crystals

Abstract Spontaneous formation of dodecylammonium dodecyl sulfate (DDADS) has been studied in batch precipitation experiments from solutions of high ionic strength and low surfactant concentrations. Formation of crystalline DDADS is found to be an instant process. Growth of DDADS crystals from equimolar reactant solutions is controlled by recrystallization and can be described by the following relation for the volume size median ( d v 50 with time ( t ): d v 50 = 4.8 × 10 −7 × t 0.223 . In nonequimolar systems other processes which occur parallel to the DDADS growth control overall precipitation kinetics and promote secondary phase formation.

18-crown-6-sodium cholate complex: thermochemistry, structure, and stability.

18-Crown-6, one of the most relevant crown ethers, and sodium cholate, a steroidal surfactant classified as a natural bile salt, are components of a novel, synthesized coordination complex: 18-crown-6-sodium cholate (18C6·NaCh). Like crown ethers, bile salts act as building blocks in supramolecular chemistry to design new functionalized materials with a desired structure and properties. In order to obtain thermal behavior of this 1:1 coordination complex, thermogravimetry and differential thermal analysis were used, as well as microscopic observations and differential scanning calorimetry. Temperature dependent infrared (IR) spectroscopy gave a detailed view into phase transitions. The structures during thermal treatment were observed with powder X-ray diffraction, and molecular models of the phases were made. Hard, glassy, colorless compound 18C6·NaCh goes through crystalline-crystalline polymorphic phase transitions at higher temperatures. The room temperature phase is indexed to a triclinic lattice, while in the high temperature phases molecules take randomly one of the two different configurations in the unit cell, resulting in the 2-fold symmetry. The formation of cholesteric liquid crystalline phase occurs simultaneously with partial decomposition, followed by the isotropization with simultaneous and complete decomposition at much higher temperature, as obtained by IR. The results provide valuable information about the relationship between molecular structure, thermal properties, and stability of the complex, indicating the importance of an appropriate choice of cation, amphiphilic, and crown ether unit in order to synthesize compounds with desired behavior.

Influence of the pH on the Formation of the Catanionic n-Tetradecylammonium Cholate Surfactant Salt

The catanionic n-tetradecylammonium cholates, with the 1:1 and 1:2 cationic:anionic surfactant molar ratio, have been synthesized by reaction of tetradecylammonium chloride and sodium cholate. The stoichiometry of the compounds formed is strongly dependent on pH of the aqueous reaction solution. In the neutral and slightly acidic medium the isolated surfactant was detected as 1:1, at acidic medium (pH ≈ 3) as 1:2, while the cholic acid alone precipitated from the strong acidic solution (pH ≈ 2). The catanionic surfactants have been characterized by elemental and spectroscopic (IR, 1H, and 13C NMR) analyses, and microscopic observations. The mole fractions of individual components in precipitate present in the heterogeneous aqueous system have been determined as a function of pH. The data interpretation in the pH regions where two different kinds of precipitates coexist pointed to the complex equilibrium in the examined systems. The evaluation of the relevant equilibrium constants was solved by the nonlinear regression analysis. In the acidic region where cholic acid and 1:2 precipitate coexisted the equilibrium constants of dissolution were K 1 = 8.4 × 10−10 and K 4 = 3.7 × 10−27, respectively. In the region where 1:2 and 1:1 precipitate coexisted the equilibrium constant of dissolution of 1:1 tetradecylammonium cholate was K 2 = 4.2 × 10−20.

Thermal behavior of novel catanionic cholates XRPD technique in solving problems

In this article, we bring new insight into room temperature structure of catanionic cholates and complement their thermal behavior given by the conventional thermal techniques with the XRPD technique. The comparative study of the addition of each dodecyl chain and ammonium group is made bearing in mind the complete architecture of synthesized cholates. The examined samples are crystal smectic phases at room temperature, with proposed sandwich-type structure, promoted by cholates architecture. For most of the studied compounds, thermal behavior is characterized as formation of structural varieties and/or polymorphs as low-temperature phases and formation of high-temperature mesomorphic, lamellae-like phases. The exception is dimeric dicholate, which only forms SmA phase before its decomposition. The dependence of the isotropization temperatures, enthalpy and entropy changes, on the increasing ammonium headgroup number, points to the fact that thermal stability of these catanionics depends on the structure of cationic component that is its constituent, where cholate anion shows minor effect. The chemistry of amphiphiles, their supramolecular behavior and thermotropic affinity is at the frontier of the contemporary research and design of the new functional materials, because this is simple but effective way to control the nature and location of reactions. From that point of view, the systematic analysis of physico-chemical properties of various catanionic amphiphiles brings new findings of their chemical structure–properties relationship, therefore enabling simpler and reliable way of new materials synthesis with desired properties.