Surfactant-selective Electrode Research Articles

Influence of Dodecylsulfate Adsorption on the Stability of Cerium Oxide Nanoparticle-Based Colloidal Aqueous Dispersions

In this contribution, the influence of the adsorption of dodecylsulfate, an anionic surfactant, on the stability of colloidal aqueous dispersions containing ceria (CeO2) nanoparticles (NPs) was investigated using zetametry, UV-visible spectrophotometry, and potentiometry involving an ionic surfactant-selective electrode (ISSE). In particular, thanks to absorbance follow-ups carried out as a function of time, aqueous dispersions containing a given loading of CeO2 NPs were found simultaneously to stabilize more quickly with time and to adopt a higher opacity and a more pronounced light-yellow color as the sodium dodecylsulfate (SDS) concentration increased. Knowing that this absorbance was attributed undoubtedly to CeO2 NPs, the fact that the measured absorbance is lower for a higher amount of CeO2 NPs in suspension, as revealed by a higher opacity of the studied dispersions, is somewhat counterintuitive. Besides the higher opacity of the dispersions, a shield effect of the adsorbed SDS layer toward UV-visible light may also explain this observation. The adsorption of dodecylsulfate on CeO2 NPs was indeed demonstrated using zetametry measurements in the presence of SDS and the potentiometric method combined with an ISSE. This latter method did not only allow the accurate determination of impoverishment in freely diffusing dodecylsulfate (DS) anions resulting from DS adsorption on CeO2 NPs but it also showed that this latter obeys a Freundlich isotherm.

The aggregation and micellization of ionic surfactants in aqueous solution detected using surface-confined redox and ion-pairing reactions

This article describes the use of a redox-induced ion-pairing reaction confined to an electrode surface to detect the micellar aggregation of ionic surfactants in water. Cyclic voltammetry characterizes the Faradaic response of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of the sodium salts of the anionic surfactants n-decyl sulfate, n-decyl sulfonate, and taurodeoxycholate, the bromide salt of the cationic surfactant decyltrimethylammonium, and the sodium salt of the azo dye Orange II. These electrolytes present different aggregation behaviors in water. The cyclic voltammograms indicate that the surfactant and dye anions and the bromide counteranion of the cationic decyltrimethylammonium pair with the electrogenerated SAM-bound ferrocenium. The voltammetric signatures are sensitive to the concentration and aggregation state of the ions in solution. The concentration dependence of the apparent redox potential (E°’SAM) presents the same features observed by potentiometry using surfactant ion-selective or bromide electrodes, indicating that E°’SAM tracks the activity of the unaggregated or free surfactant, dye or bromide anions in solution. The formation of micellar aggregates above the critical micelle concentration (cmc) causes the free anion activity to deviate from the molar concentration of added surfactant, resulting in a break in the plot of E°’SAM versus the logarithm of the surfactant concentration. The quantity of surfactant anions associated to the oxidized SAM increases with the solute concentration and plateaus out at concentrations > cmc. These findings point to the potential application of ferrocene-terminated SAMs to the investigation of ionic micelle-forming systems and the determination of their cmc’s.

Electroactive Self-Assembled Monolayers Detect Micelle Formation

The interfacial electrochemistry of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold (FcC12SAu) electrodes is applied to detect the micellization of some common anionic surfactants, sodium n-alkyl sulfates, sodium n-alkyl sulfonates, sodium diamyl sulfosuccinate, and sodium dodecanoate, in aqueous solution by cyclic voltammetry. The apparent formal redox potential (E°'SAM) of the FcC12SAu SAM is used to track changes in the concentration of the unaggregated surfactant anions and determine the critical micelle concentration (cmc). The effect of added salt (NaF) on the sodium alkyl sulfate concentration dependence of E°'SAM is also investigated. Weakly hydrated anions, such as ClO4-, pair with the electrogenerated SAM-bound ferroceniums to neutralize the excess positive charge created at the SAM/electrolyte solution interface and stabilize the oxidized cations. E°'SAM exhibits a Nernstian-type dependence on the anion activity in solution. Aggregation of the surfactant anions into micelles above the cmc causes the free surfactant anion activity to deviate from the molar concentration of added surfactant, resulting in a break in the plot of E°'SAM versus the logarithm of the concentration of anionic surfactant. The concentration at which this deviation occurs is in good agreement with literature or experimentally determined values of the cmc. The effects of Ohmic potential drop, liquid junction potential, and surfactant adsorption behavior on E°'SAM are addressed. Ultimately, the E°'SAM response as a function of the anionic surfactant concentration exhibits the same features reported using potentiometry and surfactant ion-selective electrodes, which provide a direct measure of the free surfactant anion activity, thus making FcC12SAu SAM electrodes useful for the detection of surfactant aggregation and micelle formation.

A novel approach to the measurement of surfactant parameters in arthropod digestive juices

In arthropods, the determination of two important parameters of digestive juices, i.e. the total surfactant concentration and the critical micelle concentration (CMC), is challenging due to small sample volumes and low surfactant concentrations. In this work, we report a successful implementation of potentiometric titrations using the surfactant ion-selective electrode (SISE) and the pyrene fluorescence method (PFM) for the determination of the total surfactant concentration and CMC in the digestive juice of terrestrial isopod crustaceans Porcellio scaber. Pooled digestive juice extracts of four (SISE) or two (PFM) animals were used per measurement run. In both cases, digestive juice extracts in 100μL of deionized water were sufficient for one measurement run. The total surfactant concentration of P. scaber digestive juice was determined to be 9.2±3.5mM and the CMC was approximately 90μM. Our work presents an important improvement towards easy CMC determination in small volume samples in comparison with the commonly used stalagmometric technique, where much larger sample volumes are usually needed. To date, the total surfactant concentration was not measured in the digestive juices of arthropods other than Homarus vulgaris, Astacus leptodactylus and Cancer pagurus, for which complex separation and analytical techniques were required. Our results obtained by SISE and PFM therefore present the first successful quantification of surfactants and their CMC in small volumes of arthropod digestive juice without prior separation or purification techniques.

Interactions between a series of pyrene end-labeled poly(ethylene oxide)s and sodium dodecyl sulfate in aqueous solution probed by fluorescence.

The interactions between a series of poly(ethylene oxide)s covalently labeled at both ends with pyrene pendants (PEO(X)-Py2, where X represents the number-average molecular weight of the PEO chains and equals 2K, 5K, 10K, and 16.5K) and an ionic surfactant, namely, sodium dodecyl sulfate (SDS), in water were investigated at a fixed pyrene concentration of 2.5 μM corresponding to polymer concentrations smaller than 21 mg/L and with an SDS concentration range between 5 × 10(-6) and 0.02 M, thus encompassing the 8 mM critical micelle concentration (CMC) of SDS in water. The steady-state fluorescence spectra showed that the I1/I3 ratio decreased from 1.73 ± 0.06 for SDS concentration smaller than 2 mM where pyrene was exposed to water to 1.43 ± 0.03 for SDS concentration greater than 6 mM where pyrene was incorporated inside SDS micelles. The ratio of excimer-to-monomer emission intensities (the IE/IM ratio) of all PEO(X)-Py2 samples remained constant at low SDS concentrations, then increased, passed through a maximum at the same SDS concentration of 4 mM before decreasing to a plateau value that is close to zero for PEO(10K)-Py2 and PEO(16.5K)-Py2 but nonzero for PEO(2K)-Py2 and PEO(5K)-Py2. The pyrene end groups of these two latter samples could not bridge two different micelles due to the short PEO chain, and excimer was formed by intramolecular diffusion inside the same SDS micelle. Time-resolved fluorescence decays of the pyrene monomer and excimer of the PEO(X)-Py2 samples were acquired at various SDS concentrations and globally fitted according to the "Model Free" analysis over the entire range of SDS concentration. The molar fractions of various excited pyrene species and the rate constant of pyrene excimer formation retrieved from the analysis of fluorescence decays were obtained as a function of SDS concentration. Interactions between SDS and PEO could not be detected by isothermal titration calorimetry, potentiometry with a surfactant selective electrode, and conductance measurements.

Novel surfactant-selective membrane electrode based on polyelectrolyte–surfactant complex

Novel class of active ionophores for surfactant selective electrodes is proposed. PVC membrane doped with polyelectrolyte–surfactant stoichiometric complex is used for ion-selective electrode construction responsive to cetyltrimethyl ammonium bromide and related surfactants. New ionophore is quite stable and completely insoluble in aqueous media in wide range of pH. The electrode displays nearly Nernstian slope in CTAB concentration range 10−6–10−3M. Polyelectrolyte platform allows to design wide range of different ionophores responsive to cationic organic substances.

Binding of cationic surfactants to a thermo-sensitive copolymer below and above its cloud point

The binding interactions between cationic surfactants and a statistical thermo-sensitive copolymer based on oligo(ethylene glycol) methacrylates were studied by isothermal titration calorimetry (ITC), surfactant selective electrode (SSE) and dynamic light scattering (DLS). Strong binding interactions were observed between this copolymer and cationic surfactants. The binding is driven by the hydrophobic effect, occurring on the hydrophobic backbone instead of the ethylene–glycol side chains. The surfactant saturation concentration C2 depended on the polymer concentration, but was not affected by temperature. Three types of cationic alkyltrimethylammonium bromide surfactants (RTAB with R equal to C12, C14, and C16), namely, dodecyltrimethylammonium bromide (DoTAB), tetradecyltrimethylammonium bromide (TTAB), and cetyltrimethylammonium bromide (CTAB), were examined, and the results confirmed that the binding affinity followed the sequence: CTAB>TTAB>DoTAB.

Preparation and Application of Cationic Surfactant Ion-selective Electrode

Abstract In this study, a cationic surfactant ion-selective electrode was prepared; its performance was determined. The electrode showed good Nernstian response to some kinds of cationic surfactants and had lower detection limits (10− 6 mol/L order of magnitude). Potentiometric titration was employed to determine the concentrations of three cationic surfactants (DDMBAC, CTAB, OTMAC) using the electrode as indicator electrode and sodium tetraphenylborate solution as titrant. The titration results were satisfactory compared with the two-phase titration method. The relative errors between the electrode and two-phase titration were 2.12 %, 3.45 %, and 4.21 %, respectively. In addition, the electrode was used to determine the critical micelle concentration of cetyl trimethyl ammonium bromide (CTAB) in organic solvent/water, and the CMC measured using the electrode method was nearly identical with that measured using the conductivity method.

Potential changes during in situ formation of carriers for cationic surfactant ion-selective electrodes by conditioning

The potential changes were monitored during symmetric conditioning and asymmetric conditioning of the polyvinyl chloride membrane containing sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB) or sodium tetraphenylborate (NaTPB) in dodecyltrimethylammonium chloride (DTMACl) solutions at 10−5.0–10−3.5moldm−3. A slow potential increase, which reflected a change in local concentration of DTMA+ in the conditioning solution proximate to the membrane, was observed for NaTFPB at a DTMACl concentration of 10−5.0–10−4.5moldm−3. For NaTFPB at a DTMACl concentration of 10−4.0–10−3.5moldm−3 or for NaTPB, the potential decreased after a rapid increase reflecting the change in DTMA+:Na+ ratio as a counter ion in the membrane by the ion-exchange. In asymmetric conditioning, the leakage of DTMA+ into the inner solution overlapped with the change in DTMA+:Na+ ratio for NaTPB but was delayed from the change for NaTFPB. Some criteria for completion of the conditioning are described.

Surfactantion-selective electrodes: A promising approach to the study of the aggregation of ionic surfactants in solution

The potentiometric method based on surfactant ion-selective electrodes (SISEs) was broadly applied to ionic surfactants in solution due to the change in their activities, i.e., concentration in dilute solution, of both the counterions and ionic surfactants at the point of aggregation. The monomeric surfactant concentration can be determined by SISEs with almost no pretreatment, even if the sample solutions were colored or viscous. Ion-sensitive electrodes are usually used to investigate the dimerization, aggregation, and micelle formation of ionic surfactants in aqueous solution. By using SISEs, the binding isotherm of ionic surfactants bound to other materials such as polymers and biomolecules (DNA and proteins) can be calculated easily. According to the binding isotherms, the different binding and aggregation processes between ionic surfactants and other materials can be identified and characterized, which is important for understanding the interaction between ionic surfactants and polymers and/or biomolecules. Herein we present a comprehensive review of the literature devoted to SISEs used in the aggregation of surfactants in solution over the last two decades, looking back the developmental history of SISEs, reviewing the applications of SISEs in the studies of surfactant/surfactant mixtures, the adsorption of surfactants on polymers, including neutral polymers, polyelectrolytes, and biopolymers and the interactions between surfactants and lipid vesicles, etc., prospecting the new wave of SISEs.

Surfactant Self-Assembly Nanostructures in Protic Ionic Liquids

The existence and properties of mesoscopic self-assembly structures formed by surfactants in protic ionic liquid solutions are reported. Micellar aggregates of n-alkyltrimethylammonium (n = 10, 12, 14, 16) chlorides and bromides and of n-alkylpyridinium (n = 12, 16) chlorides in ethylammonium nitrate and propylammonium nitrate were observed by means of several experimental techniques, including surface tension, transmission electron micrography, dynamic light scattering, and potentiometry using surfactant-selective electrodes. The effect of the alkyl chain length of both solute and solvent molecules on the critical micelle concentration is discussed, and a Stauff-Klevens law is seen to apply to surfactant solutions in both protic ionic liquids. The counterion role is also a matter of study in the case of alkyltrimethylammonium-based surfactants, and the presently reported evidence suggests that the place of the surfactant counterion in the Hoffmeister's series could determine its effect on micellization in IL solution. The size distribution of the aggregates is also analyzed together with the Gibbs free energies of micellization and the minimum surface area per monomer in all of the studied cases. All of the hereby reported evidence suggests that the negative entropic contribution arising from the release of the solvent layer upon micellization is also the driving force of conventional surfactant self-association in protic ionic liquids.

PH-dependent polyampholyte SDS interactions

Aqueous solutions of sodium dodecylsulfate (SDS) and poly(N,N′-diallyl-N,N′-dimethyl-alt-maleamic carboxylate) (PalH), a synthetic pH-tuneable polyelectrolyte (PEL), have been investigated by various techniques at different pH-values in absence and presence of NaCl. Potentiometric measurements using a surfactant-selective electrode indicate a quite complex interaction mechanism, which can be subdivided into different regions, where non-cooperative, electrostatic and cooperative hydrophobic interactions are of relevance. It was concluded, that in dependence on pH, conformational changes are responsible for the different interaction behavior in the NaCl-free system. Isothermal titration calorimetry (ITC) suggests that early stage hydrophobic binding is an exothermic process followed by electrostatic interactions, which are endothermic in nature and entropy driven. After NaCl addition the interaction mechanism becomes independent of pH due to a screening of (i) attractive interactions between the surfactant head groups and oppositely charged binding sites and (ii) repulsive forces between the surfactant head groups. Furthermore, the ITC investigations have revealed that after salt-addition surfactant micelles interact with the polymer instead of separated SDS molecules due to a depression of the CMC.

Adsorption of CTAB on Zeolite A Detected by Surfactant Ion-selective Electrode

Abstract Adsorption of cationic surfactant, cetyltrimethylammonium bromide (CTAB) on zeolite A has been investigated using surfactant-selective electrode technique. The electrode consisted of a polyvinyl chloride (PVC) membrane based on neutral ion-pair carrier complex of cetyltrimethylammonium tetraphenylborate (CTA+TPB−). Adsorbance of CTAB on zeolite A was calculated from the potential and concentration response characteristics of the surfactant-selective electrode to get the surfactant adsorption isotherms. Adsorption isotherm of CTAB was S-shaped. The adsorbance was related to pH and electrolyte in the surfactant solutions.

Interaction of anionic azo dye and TTAB: cationic surfactant

Interaction between three anionic azo dyes, C.I. Acid Red 14 (AR14), C.I. Acid Red 1 (AR1), C.I. Acid Orange 7 (AO7) and a cationic surfactant tetradecyltrimethylammonium bromide (TTAB) has been investigated using surfactant selective electrode technique. The TTAB selective electrode was constructed and used to determine the concentration of TTAB monomers and also surfactant ions bounded to dyes by electromotive force data. The dye-surfactant complex formation constant, K1, and the standard free energy change, ΔG1o, could be obtained from the experimental data. The results indicate that in the TTAB/AO7 system, where the more hydrophobic dye AO7 is used, the dye-surfactant interactions are stronger than in other systems. The results also indicate that both the electrostatic attractive and the hydrophobic interactions are very important for the formation of the complex between the oppositely-charged dyes and the surfactant. Maximum binding capacities of the dyes and also the degree of binding of the surfactant to the dyes were calculated from the experimental data.

Surfactant binding by humic acids in the presence of divalent metal salts

The binding of metal ions and surfactant to humic substances plays an important role in the distribution of toxic metal ions and toxic organic contaminants in the environment. We measured the binding of dodecyl- (C12TA +) and decyltrimethylammonium (C10TA +) surfactants to humic acid in the presence of heavy metal salts (Cu(II), Cd(II), Zn(II), Pb(II), and Ca(II)) at pH 8 using a surfactant ion-selective electrode. The binding curves of the surfactant cation were analyzed using the Langmuir equation to determine the binding constant K and capacity m. From the dependence of K on ionic strength, the hydrophobic interaction term is inferred to have Gibbs energy of −28 kJ/mol for the binding of C12TA + ion to HA, and the dependence of K on the surfactant chain length revealed a 4 kJ/mol difference in Gibbs energy between C12TA + and C10TA + ion binding. In the presence of 0.5 mM divalent metal salts, K decreased significantly, partly because of territorial binding and partly because of the irreversible binding of metal ions. Aldrich humic acid had a smaller binding constant and capacity.

Thermodynamic and binding study of hemoglobin, oxy-hemoglobin and carbamino-hemoglobin upon interaction with cationic surfactants, using surfactant membrane selective electrodes

The interaction of tetradecyltrimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB), two cationic surfactants, with hemoglobin (Hb) has been studied using potentiometric technique based on a surfactant ion selective membrane electrode in the presence of 2.7 mM buffer solution, pH 6.4 phosphate and pH 8.3 glycine. Binding isotherms were obtained from the potentiometric technique and using the Wyman binding potential model, apparent binding constants were calculated. Thermodynamic parameters were obtained for binding process from the Wyman binding potential analysis. Binding of surfactant molecules induced the unfolding of hemoglobin accompanied with exposure of the heme pocket and facilitating the oxidation of heme. The Gibbs free energy change calculated on the basis of the Wyman binding potential concept decreases in the initial stages of binding and passes through a minimum, followed by occupation of the different binding set. The affinity of binding of surfactant to Hb increased with increasing temperature, indicating an endothermic and essentially entropy driven process. On the other hand, a comparison between hemoglobin (Hb) oxy-hemoglobin (O 2-Hb) and carbamino-hemoglobin (CO 2-Hb) upon the interaction with CTAB was carried out.

Effect of various additives on the performance of a newly developed PVC based potentiometric sensor for anionic surfactants

A new poly(vinyl chloride) PVC membrane electrode to determine monomer concentrations of dodecylbenzenesulphonate ions (DBS −) based on a neutral ion-pair carrier complex of dodecyltrimethylammonium–dodecylbenzenesulphonate (DTA +–DBS −), is reported here. The electrode exhibits a slope of 51.25 mV per decade for DBS − ions. The DBS − ion selective electrode (ISE) can determine monomer units down to concentrations as low as 3.32 × 10 −4 M. The effect of three kinds of additives, i.e. alcohols, glycols and triblock polymers on the performance of the surfactant selective electrode is studied systematically. The effect of foreign anions along with primary ions on the performance of ion-selective electrode is investigated in terms of potentiometric selectivity coefficients, which were determined using the fixed interference method (FIM) at 1.0 × 10 −2 M concentration of foreign anions. The sensor responds well to the surfactant ions in the presence of additives at lower concentration. The Gibbs free energy of micelle formation (Δ G m) of sodium dodecylbenzenesulphonate (SDBS) in the presence of various additives is calculated and found to vary differently with respect to the increase in the amount of additives. The sensor worked in the acidic pH range with a short response time of 30 s. The lifetime of the sensor is more than three months. The sensor was further used to determine the amount of DBS − in local detergents. This method of determining anionic surfactants was found to be quite accurate when compared with classical methods.

Study of Adsorption of Sodium Dodecyl Sulfate on Alumina Using Surfactant Ion-selective Electrode

Abstract The adsorption of sodium dodecyl sulfate (SDS) on alumina was studied by using the surfactant ion-selective electrode. The adsorbance could be calculated from the potential and concentration response characteristics of surfactant ion-selective electrode to get surfactant adsorption isotherm. Further experiment results show that the adsorbance was greatly influenced by pH of the solution. It is demonstrated that surfactant ion-selective electrode could be utilized to measure surfactant concentrations in solution after adsorption equilibrium without solid-liquid separation.

Study of β-cyclodextrin/fluorinated trimethyl ammonium bromide surfactant inclusion complex by fluorinated surfactant ion selective electrode

The construction and performance of a liquid membrane electrode responsive to N-(1,1,2,2-tetrahydroperfluorooctyl)- N, N, N-trimethylammonium bromide (FTABr) and its use for the study of β-cyclodextrin/fluorinated surfactant inclusion complex is described. The electrode is based on the use of tetrahydroperfluorooctyltrimethylammonium-tetraphenylborate ion pair as electro active material in polyvinyl chloride (PVC) matrix plasticized using 2-Nitrophenyl octyl ether (NPOE). The electrode exhibits a fast, stable, reproducible and “Nernstian” response (59 ± 2 mV) for FTABr over the concentration range of 10 −5 to 2 × 10 −3 mol L −1 at 298 K. The lowest detection limit is 2 × 10 −6 mol L −1 and the response time is around 20–30 s. The validity of the electrode, for detection of fluorinated surfactant ions and hence to carry out electrochemical measurements to study micellization of fluorinated surfactant, is verified by comparing the critical micelle concentration (cmc) value of FTABr obtained by using the electrode, with that obtained by surface tension measurements. Association constant K for β-cyclodextrin/FTABr complex is evaluated from the potentiometric measurements carried out using this electrode and is observed to be ∼1.26 × 10 5. The results suggest that β-cyclodextrin forms an equimolar association complex with the FTA + surfactant ion.

Potentiometric Studies of Micellization Behavior of Cationic Surfactants in the Presence of Glycol Additives and Triblock Polymer (Pluronic F68), Using Surfactant-Selective Sensors Based on Neutral Ion-Pair Complexes

Poly(vinyl chloride) (PVC) membrane electrodes that are used to determine monomer concentrations of hexadecyltrimethylammonium ion (HTA+) and dodecyltrimethylammonium ion (DTA+), based on neutral ion-pair carrier complexes of hexadecyltrimethylammonium dodecylsulfate (HTA+−DS-) and dodecyltri-methylammonium dodecyl sulfate (DTA+−DS-), respectively, are reported here. The electrodes exhibit a sub-Nernstian slope of 34.59 mV per decade for HTA+ and 35.10 mV per decade for DTA+ ions. The HTA+-ISE and DTA+-ISE can determine the monomer units down to concentrations as low as 3.32 × 10-4 and 4.52 × 10-4 M, respectively. The effect of three glycol additives (i.e., ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG)) and a triblock polymer (Pluronic F68) on the performance of two surfactant selective electrodes is studied systematically. The effect of the presence of foreign cations, along with primary ions, on the performance of both ion-selective electrodes is investigated in terms of potentiometric selectivity coefficients, which were determined using a fixed interference method (FIM) at a foreign cation concentration of 1.0 × 10-2 M. The sensors respond well to the surfactant ions in the presence of a lower concentration of additives. The Gibbs free energy of micelle formation (ΔGm) of both surfactants, in the presence of various additives, is calculated and determined to be less favorable, with respect to the increase in the amount of additives. The sensors worked in the pH range of 8−13 with a short response time of 30 s. The lifetime of the sensors is more than three months.