J. Dey

Aggregation and adsorption behavior of cetylpyridinium chloride in aqueous sodium salicylate and sodium benzoate solutions

The critical micelle concentration (cmc) values of cetylpyridinium chloride (CPC) were determined in the presence of salicylate and benzoate ions in the less explored concentration region where viscosity is Newtonian. The cmc of CPC decreased from 9 x 10(-4) to 7 x 10(-7) and 3 x 10(-6) mol kg(-1) by adding about 0.3 mol kg(-1) of salicylate and benzoate, respectively. The ortho hydroxyl group in salicylate thus has a remarkable influence on the micellization of CPC and the extent of this favorable effect is found to be about 3.5 kJ mol(-1). The Corrin-Harkins equation was modified to explain the variation of cmc with electrolyte concentration in the presence of mixed counterions. The slope of the equation does not provide the value of the total counterion binding constant (beta), but gives us information about the lower limit to the value of beta, which is found to be 0.66. Addition of salicylate and benzoate increases the counterion binding to CPC micelles compared to that in the presence of chloride alone. An adsorption isotherm was derived to estimate the surface excess of CPC in the presence of mixed counterions.

Additive induced core and corona specific dehydration and ensuing growth and interaction of Pluronic F127 micelles.

Pluronic F127 is considered to be the most prominent member amongst Pluronics owing to its uses in the fields of drug delivery and fabrication of mesoporous materials. Though extensive studies were carried out on the phase behavior of this copolymer, the effect of additives on restructuring and growth processes of its micelles is not understood yet. In this manuscript we report DLS, SANS, fluorescence and rheological studies on the effects of NaCl and butan-1-ol on the properties of Pluronic F127 micelles in the aqueous medium. The studies show that corona specific micellar dehydration by NaCl induces inter micellar attraction and consequent formation of micellar clusters. Core specific micellar dehydration by butan-1-ol on the other hand, brings about sphere-to-rod micellar shape transition on approaching the cloud point of copolymer solutions. A room temperature sphere-to-rod shape transition of Pluronic F127 micelles can also be induced in the combined presence of butan-1-ol and NaCl. Observation of such micellar shape transition in aqueous Pluronic F127 system is first of its kind, which can have important bearing with their application in mesoporous structure formation and drug delivery.

Solubilization of hydrophobic alcohols in aqueous Pluronic solutions: investigating the role of dehydration of the micellar core in tuning the restructuring and growth of Pluronic micelles

Pluronics® are considered to be good carriers for poorly water soluble substances on account of their superior solubilization capacity over other ionic and nonionic surfactants. Understanding the influence of these substances on the aggregation characteristics of Pluronics® is therefore of overriding importance. In this manuscript we report dynamic light scattering, small angle neutron scattering, fluorescence and viscometry studies on the effect of adding hydrophobic alcohols viz. hexan-1-ol, octan-1-ol and decan-1-ol to the aqueous solutions of two Pluronics® (P85 and P123) with different hydrophilic–lipophilic balances. Both these Pluronics® exhibit a sphere to rod micellar shape transition with increasing temperature, the dynamics of the transition being significantly slower in the case of P123 because of its higher hydrophobicity and molecular weight. The aim of our studies was to investigate the restructuring and growth of Pluronic® micelles upon addition of hydrophobic alcohols that have applications in fields ranging from personal care/food products to pharmaceutical formulations. The studies show that alcohol induced micellar restructuring and growth for the two Pluronics® slow down as the concentration of alcohol increases and in addition their aqueous solubility decreases progressively from hexan-1-ol to decan-1-ol. These observations, which were manifested by a decreasing rate of sphere to rod micellar growth, have been attributed to more effective dehydration of the micellar core by alcohols with higher levels of solubilization and hydrophobicity. The results thus shed light on the specific role of additive induced dehydration of micellar cores in the restructuring and growth characteristics of micelles, for both hydrophilic (P85) and hydrophobic (P123) Pluronics®.

Tetraalkylammonium ion induced micelle-to-vesicle transition in aqueous sodium dioctylsulfosuccinate solutions

The aggregation behavior of sodium dioctylsulfosuccinate (AOT) in aqueous media containing tetraalkylammonium bromide (TAAB, where alkyl = ethyl (TEAB), propyl (TPAB) and butyl (TBAB)) was studied by surface tension, fluorescence (with pyrene as the probe), small-angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements. A comparison of the critical micelle concentration (cmc) values of AOT in the presence of the salts showed the order TBAB < TPAB < TEAB < NH4Cl < NaCl. Synergism in the cmc occurs when the solution contains a mixture of sodium and tetraalkylammonium counterions. The counterion binding behavior was examined by applying the modified Corrin–Harkins (CH) equation which revealed that a special counterion binding behavior of AOT exists in aqueous solutions with tetraalkylammonium salts. The modified CH equation and DLS data indicate a change in the shape of the surfactant aggregates, which was confirmed by the SANS data. Dehydration of the head group and the counterion during their interaction appears to induce a micelle-to-vesicle transition in the aggregates.

Effect of sodium salicylate, sodium oxalate, and sodium chloride on the micellization and adsorption of sodium deoxycholate in aqueous solutions

The salicylate ion increases the rate of bile flow (choleretic effect) and bile salts are known to affect the colonic absorption of oxalate. Owing to this physiological relevance of salicylate and oxalate ions, critical micelle concentration (cmc) values of sodium deoxycholate (NaDC) were determined in aqueous sodium oxalate, sodium salicylate, and sodium chloride solutions by using surface tension, fluorescence, and EMF methods. The results indicate, besides a counterion effect, the influence of coanions on the cmc. In the range from 25 to 40 °C, cmc increases almost linearly with temperature. In the temperature range from 30 to 40 °C, the counterion binding constant β of NaDC micelles has the same value (0.17±0.01) in the presence of sodium chloride and sodium salicylate. On the other hand, in sodium oxalate solution β=0.05±0.02 when oxalate concentration is less than or equal to c* and β=0.48±0.04 above c*, where c*≈0.038 mol kg(-1). EMF measurements also supported this type of counterion binding to NaDC micelles in sodium oxalate solutions. In sodium oxalate solution, at c* a change in the shape of deoxycholate micelles is expected to take place. Salicylate, oxalate, and chloride coanions have a similar effect on the adsorption of NaDC. This study reveals that the choleretic effect of salicylate is not due to the influence of salicylate ions on the micellization of NaDC.

Aggregation and adsorption of sodium dioctylsulfosuccinate in aqueous ammonium chloride solution: Role of mixed counterions

The critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (AOT) was determined at 25 °C from surface tension and fluorescence methods in aqueous NH(4)Cl solution for assessing the influence of mixed counterions on the special counterion binding behavior (SCB) of AOT. The SCB of AOT refers to a sudden twofold increase in the value of the counterion binding constant (β) in aqueous medium when the concentration (c(*)) of the added 1:1 sodium salt is about 0.015 mol kg(-1), and it has been tested so far for sodium ion only. In the presence of sodium and ammonium mixed counterions also the SCB of AOT exist, but with lower c(*) (0.009 mol kg(-1) NH(4)Cl). Synergism in the cmc occurs due to mixed counterions. In the case of inorganic counterions, unlike the case with organic counterions, the cmc is dependent on the total counterion concentration in solution and negligibly on the specific type of counterion. Na(+) and NH(4)(+) bind almost equally to the micelle in the region of low β (below c(*)), but in the region of high β (above c(*)) NH(4)(+) binds predominantly. It has been shown that the theoretical expression for the surface excess of ionic surfactant+electrolyte system containing a single counterion can also be used to evaluate the surface excess in the presence of mixed counterions if the two counterions are considered to undergo Henry-type adsorption at the air-solution interface.

Effect of ethylene glycol on the special counterion binding and microstructures of sodium dioctylsulfosuccinate micelles

Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior (SCB), which refers to the abrupt twofold increase in the counterion binding constant (β) at a critical concentration (c*) of added NaCl (in water c* ≈ 0.015 mol kg(-1)). In this paper, the SCB of AOT has been studied in a mixture of water and ethylene glycol (EG) by applying surface tension, fluorescence, and small angle neutron scattering (SANS) methods. The SCB exists in water + 10% (w/w) EG as well, but disappears when the EG% is ≥20. It has been found out that the SCB of AOT occurs in media having cohesive energy density values in the range of 2.3-2.75 J m(-3). SANS data indicate co-existence of vesicles and cylindrical micelles of AOT in water + 10% EG when the added NaCl concentration is greater than c* thereby revealing that change in the morphology of aggregated species is the probable cause for the SCB of AOT. From this study it has become clear that the Corrin-Harkins (CH) equation, commonly used for determining β, can be applied only above a limiting concentration (ce(#)) of added electrolyte. In aqueous organic or pure organic polar solvents below ce(#) sharp deviation from the CH equation occurs with reversal of slope rendering this equation inapplicable for the determination of β.

Aggregation of sodium dodecylsulfate in aqueous nitric acid medium

Nitric acid medium is invariably used for nitration of organic molecules. Although surfactants are known to influence reaction rates, little is known about the aggregation behavior of surfactants in nitric acid medium. Micellization characteristics of sodium dodecylsulfate (SDS) in aqueous nitric acid are investigated in this work by using the conductance method. The critical micelle concentration (cmc) and the aggregation number were also determined by the surface tension and the steady-state fluorescence methods, respectively. This study reveals that in acidic medium SDS exhibits both normal and unusual conductivity behaviors. Equations developed on the basis of the mixed electrolyte model, Debye-Hückel-Onsager approach, and the pseudophase ion-exchange model successfully simulate the conductivity data. The exchange of sodium and hydrogen counterions at the micellar surface has no significant effect on the cmc of SDS. Acid concentration, surfactant concentration, and cmc control the competitive binding of sodium and hydrogen counterions. Analysis of conductivity data revealed hydrolysis of about 12% SDS when [HNO(3)]≥0.02 mol dm(-3). Hydrolysis of SDS has been confirmed by nitrating some of the substituted phenols. It has been predicted that SDS+aqueous HNO(3) medium with [HNO(3)]≥0.02 mol dm(-3) may be used as a green medium for nitration.

Controlling the aggregation of sodium dodecylsulphate in aqueous poly(ethylene glycol) solutions

The interaction between polymers and surfactants has been a topic of study due to their various applications, though for this purpose, a better understanding of these interactions and the ability to control them is also important. In the present work, we explore the interaction of a well known polymer, poly(ethylene glycol) (PEG) of different molecular weights (400, 6000, 10 000 and 20 000 g mol−1) and an anionic surfactant sodium dodecylsulphate (SDS), by using the techniques of surface tension, conductance and small angle neutron measurements. The surface tension isotherms indicate the presence of two breaks in the case of high molecular weight PEGs. At the first break point a surface tension minimum occurs which has been attributed to the onset of PEG–SDS interaction leading to the formation of SDS aggregates on the PEG chains. The second break point is due to the formation of free SDS micelles. These observations have been well supported by conductance and small angle neutron scattering (SANS) measurements. SANS indicates the formation of a bead-necklace structure of micelle-like clusters of the surfactant formed along the unfolded polymer chain in the case of PEG-20 000. The role of added NaCl in these interactions and its ability to control the aggregation of SDS has also been addressed in the present work, thus throwing light on the importance of electrostatic interactions in polymer–surfactant interactions.

Aggregation and surface behavior of aqueous solutions of cis-bis(1,3-diaminopropane)bis(dodecylamine)cobalt(III) nitrate. A double-chained metallosurfactant

Metallosurfactants or amphiphilic metal complexes are emerging as a new class of material with a range of properties inherent to both metal complexes and surfactants. Looking at the potential applications of these materials in diverse fields, studying the fundamental aspects of their adsorption and aggregation is necessary. cis-Bis(1,3-diaminopropane)bis(dodecylamine)cobalt(III) nitrate (DDCN), a double-chained cationic metallosurfactant, was synthesized and its critical micelle concentration values were determined in aqueous medium as a function of sodium nitrate concentration by using surface tension, conductivity and spectrophotometric methods. Thermal gravimetric analysis showed stability of DDCN up to about 183 °C. DDCN has a salt dependent counterion binding constant, a low value equal to 0.16 becomes more than double (0.43) above 0.025 mol kg−1 NaNO3. The counterion binding constant value of DDCN is however surprisingly low compared to other ionic surfactants. Dynamic light scattering measurements revealed large size aggregates (hydrodynamic diameter = 116 nm with polydispersity index = 0.23) of DDCN which grow even larger on adding NaNO3. Small angle neutron scattering measurements also showed the presence of large size DDCN aggregates existing probably as micellar clusters. Adsorption behavior of DDCN was assessed by calculating surface excess and area per molecule at the air/water interface.