Polymorphism and mesomorphism of oligomeric surfactants: effect of the degree of oligomerization.

A series of cationic oligomeric surfactants (quaternary dodecyldimethylammonium ions with two, three, or four chains connected by an ethylene spacer at the headgroup level, abbreviated as dimer, trimer, and tetramer) were synthesized and characterized. The influence of the degree of oligomerization on their polymorphic and mesomorphic properties was investigated by means of X-ray diffraction, polarizing optical microscopy, thermogravimetry, and differential scanning calorimetry. All compounds display layered arrangements with interdigitated dodecyl chains. The increase in the degree of oligomerization increases the interlayer distance and decreases the ordering in the solid phase; whereas the dimer sample is fully crystalline with well-developed 3D ordering and the trimer and tetramer crystallize as highly ordered crystal smectic phases. The number of thermal phase transitions and sequence of phases are markedly affected by the number of dodecyl chains. Anhydrous samples exhibit polymorphism and thermotropic mesomorphism of the smectic type, with the exception of the tetramer that displays only transitions at higher temperature associated with decomposition and melting. All hydrated compounds form lyotropic mesophases showing reversible phase transitions upon heating and cooling. The sequence of liquid-crystalline phases for the dimer, typical of concentrated ionic surfactant systems, comprises a hexagonal phase at lower temperatures and a smectic phase at higher temperatures. In contrast, the trimer and tetramer reveal textures of the hexagonal phase.

Structural and thermal study of mesomorphic dodecylammonium carrageenates.

Structural characteristics and thermal stability of a series of dodecylammonium carrageenates formed by stoichiometric complexation of dodecylammonium chloride and differently charged carrageenans (kappa-, iota- and lambda-carrageenan, respectively) were investigated. IR spectral analysis confirmed the electrostatic and hydrogen bond interactions between the dodecylammonium and carrageenan species. X-ray diffraction experiments show increased ordering in the complexes compared to that in the parent carrageenans. Dodecylammonium carrageenates have a layer structure, in which a polar sublayer contains layers of carrageenan chains and a nonpolar sublayer consists of conformationally disordered dodecylammonium chains electrostatically attached to the carrageenan backbone. The major factor that determines the dodecylammonium carrageenate structure is cationic surfactant, while the carrageenans moiety plays a major role in determining thermal properties.

Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions.

The interactions between a hydrophilic anionic polysaccharide, dextran sulfate, and oppositely charged surfactants, n-alkylammonium chlorides (the number of carbon atoms per chain being 10, 12, and 14), were investigated by optical microscopy, X-ray diffraction, microelectrophoretic mobility, conductivity, surface tension, and light-scattering measurements at 303 K. The increase of surfactant alkyl chain length shifts both the critical aggregation (cac) and the critical micelle concentrations (cmc) toward lower surfactant concentration. Light-scattering and microelectrophoretic data revealed the coexistence of differently structured complexes beyond the cac. The presence of giant vesicles indicates that at least one type of species is ordered in bilayers. X-ray analysis of dry n-alkylammonium dextran sulfates exhibited mesomorphous ordering and interplanar spacings typical for lamellar structures; i.e., n-alkylammonium molecules form more or less disordered bilayers interconnected with dextran sulfate chains, thus forming multilamellar stacks. The average basic lamellar thickness increased linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayers in the stack of lamellae decreases.

Effect of the spacer length on the association and adsorption behavior of dissymmetric gemini surfactants.

A series of dissymmetric gemini surfactants with the general formula [C12H25(CH3)2N(CH2)sN(CH3)2C14H29]Br2 designed as 12-s-14, where s=2, 6, and 10, were synthesized and their physicochemical properties investigated. The effect of spacer length on Krafft temperature, adsorption at the air/solution interface, and association in aqueous solution was studied by tensiometry, conductometry, and cryo-transmission electron microscopy. The Krafft temperature was found to increase linearly with spacer length. In the submicellar concentration range the dissymmetric 12-s-14 surfactants display ion pairing and premicellar association. Adsorption at air/solution interfaces and micellization in aqueous solution are similar to the behavior of their symmetric counterparts and depend strongly on spacer length.

Physicochemical Properties of Dodecylammonium Picrate.

A novel surfactant, dodecylammonium picrate (DDAP), was synthesized and its crystal structure was determined by X-ray diffraction analysis. DDAP's physicochemical properties were examined by spectral (infrared and nuclear magnetic resonance), thermal, microscopic, and conductometric studies. The results revealed the influence of counterion specificity on thermal solid-state transitions and solution properties: the Krafft point, the aqueous solubility, the critical micelle concentration, the degree of counterion binding, and thermodynamic parameters of micellization. Copyright 2000 Academic Press.

Solid State Transitions of Asymmetric Catanionic Surfactants.

A series of asymmetric surfactants were prepared from cationic surfactant, hexadecyltrimethylammonium bromide, and anionic surfactant, sodium alkyl sulfate (the number of carbon atoms per chain being 10, 12, or 14). The influence of the alkyl chain asymmetry on the thermal properties of formed hexadecyltrimethylammonium alkyl sulfates was investigated by means of polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. Asymmetric catanionic surfactants exhibited a complex polymorphism and thermotropic mesomorphism from the stable crystalline form to the isotropic phase. On heating, successive phase transitions (several solid crystalline-solid crystalline, solid crystalline-liquid crystalline, and liquid crystalline-isotropic liquid) were observed. The number of polymorphs, in all of the bilayered structure, depended on the asymmetry between the lengths of surfactant tails. The basic lamellar thickness varied linearly with the increase of alkyl sulfate chain length. An increase of the basic lamellar thickness with temperature was determined. Polarizing microscopy revealed a characteristic texture of the smectic phase. On cooling, all compounds underwent reversibly the isotropic liquid-liquid crystalline transitions, while crystallization from melted samples was kinetically controlled. Copyright 1999 Academic Press.

Interactions in Calcium Oxalate Hydrate/Surfactant Systems.

Phase transformation of calcium oxalate dihydrate (COD) into the thermodynamically stable monohydrate (COM) in anionic (sodium dodecyl sulfate (SDS)) and cationic (dodecylammonium chloride) surfactant solutions has been studied. Both surfactants inhibit, but do not stop transformation from COD to COM due to their preferential adsorption at different crystal faces. SDS acts as a stronger transformation inhibitor. The general shape of adsorption isotherms of both surfactants at the solid/liquid interface is of two-plateau-type, but differences in the adsorption behavior exist. They originate from different ionic and molecular structures of crystal surfaces and interactions between surfactant headgroups and solid surface. Copyright 1999 Academic Press.

Induction of crystallization of calcium oxalate dihydrate in micellar solutions of anionic surfactants.

Calcium oxalate dihydrate (CaC2O4 center dot (2+x)H2O; COD; x < or = 0.5) does not readily crystallize from electrolytic solutions but appears as a component in crystalluria. In this paper, we review in vitro studies on the factors responsible for its nucleation and growth with special attention given to the role of surfactants. The following surfactants were tested: dodecyl ammonium chloride (cationic), octaethylene mono-hexadecylether (non-ionic), sodium dodecyl sulfate (SDS, anionic), dioctyl sulphosuccinate (AOT, anionic), and sodium cholate (NaC, anionic). The cationic and some of the anionic surfactants (SDS, AOT) induced different habit modifications of growing calcium oxalate crystals by preferential adsorption at different crystal faces. In addition, the anionic surfactants effectively induced crystallization of COD at the expense of COM, the proportion of COD in the precipitates abruptly increasing above a critical surfactant concentration, close to, but not necessarily identical with the respective CMC. A mechanism is proposed, whereby crystallization of COD in the presence of surfactants is a consequence of the inhibition of COM by preferential adsorption of surfactant hemimicelles (two-dimensional surface aggregates) at the surfaces of growing crystals.

Precipitation of calcium phosphates from electrolyte solutions. III. Radiometric studies of the kinetics of precipitation and aging of calcium phosphates.

Precipitation and precipitate transformation in the system sodium phosphate (pre-adjusted to pH 7.4)--calcium chloride (25 degrees) was studied by means of radiometric analysis using 45Ca and 32P as tracers. Changes in the pH and the total concentrations of calcium and phosphate were followed during solid phase formation and the data were used to calculate composition changes of the precipitates and their supernatants. In all investigated systems two-step precipitation was observed, the precursor being more basic than the secondary precipitate. The composition of the latter was mostly within the range of the composition of octacalcium phosphate. The course of further chemical changes was dependent on the pH established during secondary precipitation. The heterogeneous exchange of the radionuclides between the solid phase and their supernatant solutions was also followed as a function of time. The results indicate that recrystallization through the mother liquid accompanied by composition changes is the dominant mechanism of equilibrium of the solid phases.