Mixed Surfactant Microemulsions Research Articles

Effects of a Multicomponent Perfume Accord and Dilution on the Formation of ST2S/CAPB Mixed-Surfactant Microemulsions.

Perfume mixtures contain perfume raw materials (PRMs) with varying structures and hydrophobicities, which influence PRM localization within a surfactant-based formulation and thereby affect the phase behavior. In rinse-off products, the addition of water can further affect the phase behavior. In this study, a mixture of 12 PRMs was used as the oil phase in an aqueous system consisting of sodium trideceth-2 sulfate as a primary surfactant, cocamidopropyl betaine as a cosurfactant, and dipropylene glycol as a cosolvent. A series of phase diagrams were constructed with increasing water content, simulating the use conditions for rinse-off products, to determine how the phase boundaries shift with dilution. Using these phase diagrams, the compositions of interest in the micelle without perfume, micelle with perfume, microemulsion, and micelle-microemulsion transition regions were identified at each dilution level. The structural changes were probed through combined small-angle neutron scattering (SANS) and cryo-transmission electron microscopy analyses. The SANS results showed that ellipsoidal micelles were maintained as the perfume content and the dilution level increased. With ≥50 wt % water, increasing the perfume content increased the micelle volume. Interestingly, a higher rate of volume increase was observed at ≥70 wt % water. Notably, the volumes of the micelles with and without perfume increased steadily with dilution, whereas the volumes of the assemblies in the transition region and the microemulsion region increased more rapidly once diluted to 70 and 80 wt % water, respectively.

Formation, thermodynamic properties, microstructures and antimicrobial activity of mixed cationic/non-ionic surfactant microemulsions with isopropyl myristate as oil

HypothesisModification of the interface by blending of surfactants produces considerable changes in the elastic rigidity of the interface, which in turn affects the physicochemical properties of w/o microemulsions. Hence, it could be possible to tune the thermodynamic properties, microstructures and antimicrobial activity of microemulsions by using ionic/non-ionic mixed surfactants and polar lipophilic oil, which are widely used in biologically relevant systems. ExperimentsThe present report was aimed at precise characterization of mixed cetyltrimethylammonium bromide and polyoxyethylene (23) lauryl ether microemulsions stabilized in 1-pentanol (Pn) and isopropyl myristate at different physicochemical conditions by employing phase studies, the dilution method, conductivity, DLS, FTIR (with HOD probing) and 1H NMR measurements. Further, microbiological activities at different compositions were examined against two bacterial strains Bacillus subtilis and Escherichia coli at 303K. FindingsThe formation of mixed surfactant microemulsions was found to be spontaneous at all compositions, whereas it was endothermic at equimolar composition. FTIR and 1H NMR measurements showed the existence of bulk-like, bound and trapped water molecules in confined environments. Interestingly, composition dependence of both highest and lowest inhibitory effects was observed against the bacterial strains, whereas similar features in spontaneity of microemulsion formation were also evidenced. These results suggested a close relationship between thermodynamic stability and antimicrobial activities. Such studies on polar lipophilic oil derived mixed surfactant microemulsions have not been reported earlier.

Effects of water content and oil on physicochemical and microenvironmental properties of mixed surfactant microemulsions

In this report, phase behavior, conductivity, viscosity, dynamic light scattering, fluorescence lifetime, steady state fluorescence anisotropy and Fourier transform infrared spectroscopy (FTIR) techniques were employed for understanding of the physicochemical properties and microenvironment of water-in-oil microemulsion comprising equimolar (1:1) cetyltrimethylammonium bromide (C16TAB) and polyoxyethylene (20) cetyl ether (C16E20)/1-butanol/heptane or decane, with varying water content (ω) at 303K. Both conductivity and viscosity of these systems were increased with increase in ω in both oils. Droplet size was also increased with increase in ω and corroborated well with the conductance and viscosity measurements, and depends on oil chain length. The physicochemical changes in the microenvironment with increase in ω were presented by measuring the changes in decay time using a fluoroprobe (7-hydroxycoumarin). The effect of hydration on the microstructure of these systems was studied by polarized fluorescence measurements. FTIR measurements reveal three states of water molecules, viz. trapped, bound and bulk water, in water pool of these systems. Further, the interfacial composition and free energy of transfer of 1-butanol from oil to the interface were evaluated by the dilution method. Changes in interfacial composition as a function of ω corroborate well with FTIR results indicating bound and bulk water.

Preparation of Silica-Encapsulated ZnSe Nanocrystals by Mixed Surfactant Microemulsions

Silica-encapsulated ZnSe nanocrystals (NCs) have been prepared by employing sodium (2-ethylhexyl)sulfonate (AOT)/water/cyclohexane microemulsions containing ZnSe quantum dots with polyoxyethylenenonylphenylether (NP5)/water/cyclohexane microemulsions containing tetraethylorthosilicate (TEOS). Size tunable silica nanoparticles were achieved by using various water-to-surfactant ratios, W ([H2O]/[surfactant]). In order to characterize as-synthesized nanocrystals, photoluminescence (PL) spectroscopy, UV-visible spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were employed. Cubic zinc blende quantum dots (QDs) (2.5 nm in diameter) were successfully encapsulated by silica nanoparticles (14.3-28.9 nm in diameter). The enhanced photoluminescence efficiency about 10% has been observed when compared with the results from the similar work using AOT microemulsions. The entire size of the silica-encapsulated nanocrystals increased with increasing W, then decreased slightly when free water exists in the core of the NP5/AOT microemulsion. On the other hand, the ZnSe NCs in the silica did not change their size during the synthesis. In addition, the possible mechanisms for growth of silica particles and the kinetics of silica particles formation were discussed.

Physicochemical investigation of mixed surfactant microemulsions: Water solubilization, thermodynamic properties, microstructure, and dynamics

In this contribution, we report on a systematic investigation of phase behavior and solubilization of water in water-in-heptane or decane aggregates stabilized by mixtures of polyoxyethylene (20) cetyl ether (Brij-58) and cetyltrimethylammonium bromide (CTAB) surfactants with varying compositions in conjugation with 1-pentanol (Pn) at fixed surfactant(s)/Pn ratio and temperature. Synergism in water solubilization was evidenced by the addition of CTAB to Brij-58 stabilized system in close proximity of equimolar composition in both oils. An attempt has been made to correlate composition dependent water solubilization and volume induced conductivity studies to provide insight into the solubilization mechanism of these mixed systems. Conductivity studies reveal the ascending curve in water solubilization capacity-(Brij-58:CTAB, w/w) profile as the interdroplet interaction branch indicating percolation of conductance and the descending curve is a curvature branch due to the rigidity of the interface in these systems. The microstructure of these systems as a function of surfactant composition has been determined by dynamic light scattering (DLS) and Fourier transform infrared spectroscopy (FTIR) measurements. FTIR study reveals increase and decrease in relative population of bound and bulk-like water, respectively, with increase in Brij-58:CTAB (w/w). DLS measurements showed that the droplet hydrodynamic diameter (Dh) decreases significantly with the increase in Brij-58:CTAB (w/w). Further, the interfacial composition and energetic parameters for the transfer of Pn from bulk oil to the interface were evaluated by the dilution method. Formation of temperature-insensitive microemulsions and temperature invariant droplet sizes are evidenced in the vicinity of the equimolar composition. The results are interpreted in terms of a proposed mechanism.

Interfacial composition and characterization of a quaternary water-in-oil mixed surfactant (cationic of different alkyl chain lengths + polyoxyethylene type nonionic) microemulsions in absence and presence of inorganic salts

In this contribution, phase behavior and the Schulman's cosurfactant titration of a quaternary water-in-oil microemulsion, formed from equimolar cationic [alkyltrimethylammonium bromide (CnTAB)] and nonionic [polyoxyethylene (20) cetyl ether (C16E20)] surfactants, n-pentanol (Pn) and n-heptane or isopropyl myristate, have been studied along with the variation in alkyl chain length of the cationic surfactant, Cn (n=12, 14, 16 and 18) at a fixed water content (ω=25) and temperature (303K). The synergies in single-phase microemulsion zone, interfacial composition and the spontaneity of formation of mixed surfactant microemulsions have been observed with increase in Cn (n=12→18). Further, the effect of the variation of Cn (n=12→18) on the interface vis-à-vis interdroplet interaction, morphology and the dynamics of confined water of these systems have been examined by means of viscosity, dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR) measurements. The increase in the Cn (n=12→18) leads to shrink the droplet size as well as increase in the droplet numbers. Subsequently, the change in droplet size affects the states of water organization (bulk and bound) inside the pool. Additionally, the influence of different inorganic salts [normal salt (NaCl), precursor salt for nanoparticle synthesis (NiCl2, FeCl3), water structure affective salt (KF, KI)] on the transfer process of Pn (oil→ interface) at equimolar composition (of both surfactants) has been investigated. FTIR measurement (via D2O probing) indicates that the presence of salts significantly modifies the water structure in confined environment.

Physicochemical studies of mixed surfactant microemulsions with isopropyl myristate as oil

The present study is focused on evaluation of interfacial compositions and thermodynamic properties of w/o mixed surfactant [(sodium dodecylsulfate, SDS/polyoxyethylene (23) lauryl ether, Brij-35)/1-pentanol (Pn)/isopropyl myristate (IPM)] microemulsions under various physicochemical conditions by the dilution method. The number of moles of Pn at the interface (nai) and bulk oil (nao), and various thermodynamic parameters [viz. standard Gibbs free energy (ΔGo→i0), standard enthalpy (ΔHo→i0), and standard entropy (ΔSo→i0) of the transfer of Pn from bulk oil to the interface] have been found to be dependent on the molar ratio of water to surfactant (ω), concentration of Brij-35 (XBrij-35), and temperature. Temperature-insensitive microemulsions with zero specific heat capacity (ΔCp0)o→i have been formed at specific compositions. The intrinsic enthalpy change of the transfer process (ΔH0)o→i∗ has been evaluated from linear correlation between ΔHo→i0 and ΔSo→i0 at different experimental temperatures. The present report also aims at a precise characterization on the basis of molecular interactions between the constituents and provides insight into the nature of the oil/water interfaces of these systems by conductivity and dynamic light scattering studies as a function of ω and XBrij-35. Conductivity studies reveal that incorporation of Brij-35 in non-percolating water/SDS/Pn/IPM systems makes them favorable for ω-induced percolation behavior up to XBrij-35⩽0.5. But further addition of Brij-35 causes a decrease in conductivity with increasing ω. Furthermore, the hydrodynamic diameters of the microemulsion droplets increase with increase in both XBrij-35 and ω. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.

Interfacial composition, thermodynamic properties, and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed surfactants

The present study is focused on the evaluation of the interfacial composition, thermodynamic properties, and structural parameters of water-in-oil mixed surfactant microemulsions [(cetylpyridinium chloride, CPC+polyoxyethylene (20) cetyl ether, Brij-58 or polyoxyethylene (20) stearyl ether, Brij-78)/1-pentanol/n-heptane, or n-decane] under various physicochemical environments by the Schulman method of cosurfactant titration of the oil/water interface. The estimation of the number of moles of 1-pentanol at the interface (nai) and bulk oil (nao) and its distribution between these two domains at the threshold level of stability have been emphasized. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. nai,nai, standard Gibbs free energy (ΔGt0), standard enthalpy (ΔHt0), and standard entropy (ΔGt0) of transfer process have been found to be dependent on the molar ratio of water to surfactant (ω), type of nonionic surfactant and its content (XBrij-58 or Brij-78), oil and temperature. A correlation between (ΔHt0) and (ΔSt0) is examined at different experimental temperatures. Bulk surfactant composition dependent temperature insensitive microemulsions have been reported. Associated structural parameters, such as droplet dimensions and aggregation number of surfactant and cosurfactant at the droplet interface have been evaluated using a mathematical model after suitable modifications for mixed surfactant systems. In light of these parameters, the prospect of using these microemulsion systems for the synthesis of nanoparticles and the modulation of enzyme activity has been discussed. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.

Physicochemical investigations of anionic–nonionic mixed surfactant microemulsions in nonaqueous polar solvents: I. Phase behavior

Phase behaviors of AOT/heptane (Hp)/formamide (FA), ethylene glycol (EG), propylene glycol (PG), triethylene glycol (TEG) and glycerol (GLY) have been investigated in the absence and presence of a nonionic surfactant, polyoxyethylene(2) cetyl ether (Brij-52) at 303 K. The phase characteristics of (AOT+Brij-52)/Hp/(EG or PG or TEG) have been found to be different from that of AOT/Hp/FA systems in respect of both the area of monophasic domain and the appearance of other mesophases. The area of monophasic domain of (AOT+Brij-52)/Hp/EG depends on the content of Brij-52 (X Brij-52) and shows a maximum at X Brij-52=0.4. A negligible effect on the area of the monophasic domain has been shown by more hydrophobic surfactants, polyoxyethylene(2) stearyl ether (Brij-72) and polyoxyethylene(2) oleyl ether (Brij-92). The effect of oils (dodecane and hexadecane) on the mixed systems stabilized by (AOT+Brij-52) in EG has been investigated. The area of monophasic domain has been found to be dependent on the type of nonaqueous solvents and follows the order GLY>EG>PG>TG. A systematic investigation on the measurement of phase volumes of mixed surfactant systems [AOT+nonionic surfactant(s)] stabilized in oils of different chain lengths (heptane, dodecane and hexadecane) and polar solvent (EG) has been carried out at different compositions of the ingredients to identify the phase transitions of these systems as a function of X Brij-52. The threshold point of phase transition (both W I→W IV and W IV→W II transitions) has been found to be a function of the configuration of added nonionic surfactant, nature of the polar solvent and oil. The conversion of the initial oil/EG droplets into EG/oil droplets with increasing X nonionic has been facilitated for hydrophobic surfactants polyoxyethylene(4) lauryl ether (Brij-30), Brij-52, and Brij-72 in comparison to the hydrophilic surfactants polyoxyethylene(10) cetyl ether (Brij-56) and polyoxyethylene(20) cetyl ether (Brij-58).

Interfacial Compositions and Phase Structures in Mixed Surfactant Microemulsions

Well-defined mixtures of didodecyldimethylammonium bromide (DDAB), with either n-dodecyltrimethylammonium bromide (DTAB), sodium n-dodecyl sulfate (SDS), or pentaethylene glycol monododecyl ether (C12E5), have been used to stabilize water-in-heptane microemulsions. Using DTAB or C12E5 with DDAB causes a synergistic effect, and the water solubilization capacity of the microemulsion is increased. However, with SDS there is a destabilization, indicating surfactant antagonism. Structure and dynamics in these systems have been studied as a function of surfactant type and composition using small-angle neutron scattering (SANS) and pulsed-field gradient NMR (PFG-NMR). Compositions of mixed films at these oil−water interfaces have also been measured, by blending deuterated DDAB with proteated DTAB or C12E5, and carrying out contrast variation SANS experiments. For both the DDAB/DTAB and DDAB/C12E5 mixtures, all the added surfactant was strongly adsorbed and there was no evidence for any significant partitioning i...