Critical Micelle Concentration Of Surfactant Research Articles

Advanced synthesis for monodisperse polymer nanoparticles in aqueous media with sub-millimolar surfactants

Highly monodisperse polystyrene nanoparticles with mean diameters of less than 100 nm are synthesized via aqueous emulsion polymerization using an amphoteric initiator (VA-057) in the presence of sub-millimolar concentrations of anionic surfactant. Since the net charge on the initiator is almost zero at neutral pH, the resultant latex particle size is mainly determined by surfactant adsorption. Polymerizations were performed in the presence of a range of anionic surfactants with differing critical micelle concentrations (CMC) by varying the concentrations of surfactant, initiator and monomer, and also the ionic strength. Sodium dodecyl benzene sulfonate (SDBS), sodium hexadecyl sulfate (SHS), and sodium octadecyl sulfate (SOS) have relatively low CMCs and so enable formation of highly monodisperse nanoparticles at relatively low (sub-millimolar) surfactant concentrations, CS (i.e. below the CMC in each case). Empirically, it was found that the particle number, Np, and coefficient of variation of the particle size, CV, were strongly dependent on the CS/CMC ratio: Np increased almost in proportion with the square of this ratio, while the CV exhibited a minimum at approximately CS/CMC = 0.20. Higher ionic strength reduced the particle size, which is consistent with the above relationship because the addition of salt lowers the CMCs of ionic surfactants. Polymer latex particles produced using such formulations form highly regular, close-packed colloidal arrays.

Surfactants solubility, concentration and the other formulations effects on the drug release rate from a controlled-release matrix

Surfactant effects on the drug release from controlled release systems have been widely studied. These effects are dependent on the surfactant physical properties such as structure, charge, solubility and concentration. In addition, presence of excipients in the matrices can modify the surfactants effects. Here we investigated the effects of surfactant solubility and concentration of excipients on the drug release. Two cationic surfactants (cetrimide and cetylpyridinium chloride), two anionic surfactants (sodium lauryl sulfate and sodium taurcholate) and the amphoteric surfactant betaine were used. The used dissolution medium was simulated gastric fluid pH 1.2. The results revealed that surfactants of the same charge with the drug showed increase of drug release rate in concentration below the surfactant critical micelle concentration (CMC), while the increase in the drug release was to a less extent in surfactant concentration above its CMC. On the other hand, drug release rate was increased with surfactant solubility and vice versa. Surfactants of different charges with that of the drug resulted in a decrease in the drug release rate, depending on surfactant solubility and the excipients. The amphoteric surfactant increased the drug release rate depending on surfactant solubility and concentrations. Key words: Propranolol hydrochloride, surfactant solubility, controlled-release, surfactant concentration, drug release rate.

콘택트렌즈 관리용액의 미셀임계농도 및 표면장력과 세척력 간의 상관관계

목적: 본 연구에서는 콘택트렌즈 관리용품의 세척력 평가를 위해 미셀임계농도 평가법, 표면장력 평가법 및 단백질세척효율평가법의 상관관계에 대해 알아보고자 하였다. 방법: 계면활성제의 미셀임계농도와 실제 콘택트렌즈 관리용액의 계면활성제 농도를 문헌과 관련 자료들을 이용하여 조사하였다. 표면장력기기로 <TEX>$25{\pm}1^{\circ}C$</TEX>에서 콘택트렌즈 관리용액의 표면장력을 측정하였으며, lotrafilcon A, comfilcon A, balafilcon A재질의 렌즈를 인공누액에 14일 동안 침착시킨 후 콘택트렌즈 관리용액으로 세척하여 세척효율을 비교하였다. 결과: 콘택트렌즈 관리용품 22제품 중 계면활성제 농도가 표시된 제품은 9제품이었으며 문헌상의 미셀임계농도보다 계면활성제 농도가 더 높은 제품은 7제품이었다. 표면장력을 측정한 결과 대체적으로 소프트렌즈용 관리용품의 표면장력이 다른 관리용품보다 낮음을 알 수 있었으며, 렌즈에 침착된 단백질 제거효과 분석 결과에서는 표면장력이 더 낮은 제품의 단백질 제거효율이 더 높은 것을 알 수 있었다. 결론: 콘택트렌즈 관리용액의 계면활성제 농도가 높은 경우 표면장력이 낮으며, 표면장력이 낮을수록 침착된 단백질 세척 효율이 높아져 세척력 평가법 간에 서로 상관관계가 있음을 밝혔다. Purpose: The present study was aimed to figure out the correlation amongst the evaluation methods for critical micelle concentration, surface tension and protein cleaning efficacy to evaluate cleaning efficacy of contact lens care products. Methods: The critical micelle concentration of surfactants and the actual concentration of surfactants in contact lens care solutions were investigated by employing references published and related information. Surface tension of contact lens care solutions was measured by surface tension device at <TEX>$25{\pm}1^{\circ}C$</TEX>, and contact lenses made of lotrafilcon A, comfilcon A and balafilcon A were washed with contact lens care solutions after the incubation in artificial tears for 14 days and their cleaning efficacy was compared. Results: Among the 22 contact lens care products, 9 products provided the label of the concentration of surfactant, and 7 products showed higher concentration of surfactant than the critical micelle concentration reported in references. As a result of measuring surface tension, the surface tension of lens care products for soft contact lens was generally lower than other care products. When examined the removal effect of protein deposited on lens surface, it was known that the care products having lower surface tension showed higher protein removal efficiency. Conclusions: The surface tension is low when surfactant concentration in contact lens care solutions is high, and the removal effect of protein deposited is accordingly increased with the decrease of surface tension. Thus, these indicate the correlation amongst the evaluation methods for cleaning efficacy.

Insights into the Interactions among Surfactin, Betaines, and PAM: Surface Tension, Small-Angle Neutron Scattering, and Small-Angle X-ray Scattering Study

The interactions among neutral polymer polyacrylamide (PAM) and the biosurfactant Surfactin and four betaines, N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SDDAB), N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (STDAB), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SHDAB), and N-dodecyl-N,N-dimethyl-2-ammonio-acetate (C12BE), in phosphate buffer solution (PBS) have been studied by surface tension measurements, small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and rheological experiments. It has been confirmed that the length of alkyl chain is a key parameter of interaction between betaines and PAM. Differences in scattering contrast between X-ray and neutrons for surfactants and PAM molecules provide the opportunity to separately follow the changes of structure of PAM and surfactant aggregates. At concentrations of betaines higher than CMC (critical micelle concentration) and C2 (CMC of surfactant with the presence of polymer), spherical micelles are formed in betaines and betaines/PAM solutions. Transition from spherical to rod-like aggregates (micelles) has been observed in solutions of Surfactin and Surfactin/SDDAB (αSurfactin = 0.67 (molar fraction)) with addition of 0.8 wt % of PAM. The conformation change of PAM molecules only can be observed for Surfactin/SDDAB/PAM system. Viscosity values follow the structural changes suggested from scattering measurements i.e., gradually increases for mixtures PAM → Surfactin/PAM → Surfactin/SDDAB/PAM in PBS.

Determination of Critical Micel Concentration of PEG-10 Tallow Propane Amine: Effects of Salt and pH

Most applications of surfactants depend on formation and characteristics of micelle. In this study we measured critical micelle concentration (CMC) of cationic surfactant N, N’, N’-polyethylene (10)-N tallow-1, 3-di amine propane (PEG-10 tallow) in aqua solution by using electrical conductivity and surface tension methods. The CMC value of PEG-10 tallow with two different methods was found very close to 1.40 × 10?3 M with electrical conductivity, and 1.41 × 10?3 M with surface tension. It also investigated the effect of inorganic salt (NaCl) and pH on micellization of cationic surfactant PEG-10 tallow. With increasing addition of NaCl, formation of micelle structure resulted in a higher CMC. However, at low salt concentrations smaller CMC value was observed comparing the critical micelle concentration of individual PEG-10 tallow. When it comes to the effect of changing pH in CMC of the PEG-10 tallow , it was clearly found that micelle formation or CMC of surfactant was independent on alkaline pH (between 6 and 10), but it was dependent on acidic pH (below 5).

Effects of Branched-Chain Alcohols on Surface Activity and Micellization of Gemini Surfactants

Gemini surfactants are a kind of novel and efficient surfactants and alcohols are one of the most widely used additives to surfactant products in industries and daily life. Understanding the effects of alcohols on surface activities and micellization of gemini surfactants will promote the applications of gemini surfactants. Effects of linear-chain alcohol (1-pentanol) and branched-chain alcohols (2-hexanol and 3-heptanol) with the same main chain as 1-pentanol on the surface activity and micellization of cationic ammonium gemini surfactants C12CSC12Br2 (S=2, 4, 6, 8, 10, 12) in aqueous solution have been investigated by surface tension, electrical conductivity, isothermal titration microcalorimetry (ITC), cryogenic transmission electron microscopy (Cryo-TEM), and NMR techniques. The branched-chain alcohols have not shown obvious effects on the critical micelle concentration of surfactants (CMC), the size and the morphology of micelles due to loose microstructure and weak interactions between the alcohols and the gemini surfactant molecules. However, the addition of branched-chain alcohols greatly decreases the surface tension of surfactants and the surface tension decreases with the increase of the branching factor of the alcohols. In addition, the surfactant concentration required to reduce the surface tension of the solvent by 20 mN/m (C20) and the surface tension values of the surfactants at CMC (γCMC) decrease obviously with the increment of the branching factor of the alcohols at a fixed alcohol concentration due to the increase of the hydrophobic chain density at the air/solution interface. The results suggest that the branched-chain alcohols influence the self-assembly of surfactants more obviously at the air/solution interface than in micelles. Moreover, there is a second critical concentration in the surface tension curves of C12C10C12Br2 and C12C12C12Br2 with a longer spacer, indicating that the hydrophobic chains of these gemini surfactants are not packed tightly even above CMC due to the flexibility of their spacers and low CMC, and are packed more tightly with the increase of the surfactant concentration. The addition of alcohols decreases the second critical concentration remarkably with the increase of their branching factor, which means that the branched alcohols have a more obvious effect on the surface activity of gemini surfactants with a longer spacer. This work helps us to understand the effects of branched-chain alcohols on the self-assembly of gemini surfactants at air/solution interface and in bulk solution, and may provide some guidance on how to choose alcohols to adjust surface activities and micellization of gemini surfactants.

Enhanced Surfactant Supramicellar Assembly by Hydrophobic Dopants

The light scattering peak (LS peak) near the surfactant critical micelle concentration (cmc) demonstrates that low hydrophobic dopant levels can cause supramolecular assembly of surfactants into metastable structures much more massive than dopant-free micelles. These supramicellar assemblies (SA) exist over the entire LS peak region, which extends from above the cmc down to well below the cmc. Dodecanol (D) was the dopant, and sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTABr) were the surfactants in this work. Well above the cmc dopant is solubilized by micelles. The SA appear above the cmc, dependent on dopant level and solution ionic strength, as seen by an abrupt increase in LS. Hence, the micelles do not simply release their hydrophobic payload at the cmc; rather, the dopant causes a morphological change from normal micelles above the LS peak concentration regime into SA as the LS regime is entered from above the cmc. Below the cmc the LS peak has a cutoff concentration corresponding to the solubility limit of dodecanol in water, which can be termed a "critical supramicellar assembly concentration" (csac). A three-component model is proposed that self-consistently yields maximum micellar dopant loading, SA mass, and dopant solubility in solution. The constancy of SA molar mass under widely different ionic strength and dopant levels conditions is surprising and not currently understood.

Micellar-enhanced ultrafiltration of copper ions using sodium dodecyl sulfate and its mixture with Brij 35, Tween 80 and Triton X-100

The performance of copper ion removal using sodium dodecyl sulfate (SDS) and its mixtures with Brij 35, Tween 80 (TW80) and Triton X-100 (TX100) by micellar-enhanced ultrafiltration (MEUF) was investigated. The effects of the molar ratio of nonionic surfactant to SDS on the critical micelle concentration (CMC) of SDS/Brij 35, SDS/TW80 and SDS/TX100, the removal efficiency of Cu(2+), the residual concentration of SDS in the permeate solution and the permeate flux were tested. The results showed that the CMCs of the mixed surfactants were sharply less than that of pure SDS. The removal efficiencies of Cu(2+) were up to the maximum values 98.3 and 95.8% when the molar ratios of Brij 35 and TW80 to SDS were 0.3, and it was 93.5% given 0.7 molar ratio of TX100 to SDS. The concentration of SDS in the permeate decreased dramatically with the addition of nonionic surfactant, and the permeate flux decreased slightly as the molar ratio increased. Compared with the performance by single SDS, the mixed SDS/Brij 35, SDS/TW80 and SDS/TX100 at an optimum composition could result in not only higher rejection of Cu(2+) but also much less dosage of surfactant and concentration of SDS in the permeate.

Surface Activity Research of Cationic Gemini Surfactants

Cationic Gemini surfactant concentration, the inorganic salts added and the pH value of surface tension obtained cationic gemini surfactant critical micelle concentration is 0.4mmol / L;by adding three kinds of inorganic salts NaCl, MgCl2, and Na2SO4 ,which Na2SO4 has the greatest impact on surface tension, followed by MgCl2.The surface minimum tension of the pH ranged from 9 to 11 , indicating that the surface activity of cationic gemini surfactants achieved the highest.

Micellar properties of β-casein–cationic surfactant solutions

The micellar properties of b-casein in solution with a homologous series of cationic surfactants have been investigated using turbidity, fluorescence spectroscopy, and quenching fluorescence measurements. Turbidity mea- surements indicated that monomeric cationic surfactants initially bind cooperatively with a site on the b-casein micelle in negatively charged amino acids. This leads to the formation of low-soluble b-casein micelle-monomeric surfactants complexes. Further addition of surfactant leads to the formation of more highly soluble complexes of b- casein micelle-cationic surfactant. The fluorescence experiments were also used to study the formation of b- casein-cationic surfactant complexes and to determine critical micelle concentration of surfactants and b-casein. The mean aggregation number of b-casein and cationic surfactant micelles at various experimental conditions and the dependence of aggregation phenomenon on the chain length of cationic surfactants were investigated. The mean aggregation number for b-casein micelle has been deter- mined to be 45.70 using quenching fluorescence.

Surface Activity Research of Cationic Gemini Surfactants

Cationic Gemini surfactant concentration, the inorganic salts added and the pH value of surface tension obtained cationic gemini surfactant critical micelle concentration is 0.4mmol / L;by adding three kinds of inorganic salts NaCl, MgCl2, and Na2SO4 ,which Na2SO4 has the greatest impact on surface tension, followed by MgCl2.The surface minimum tension of the pH ranged from 9 to 11 , indicating that the surface activity of cationic gemini surfactants achieved the highest.

Experimental Study of Surface and Solution Properties of Gemini -conventional Surfactant Mixtures on Solubilization of Polycyclic Aromatic Hydrocarbon

Experimental data are presented on the enhanced solubilities of fluorene (FLR) resulting from solubilization in aqueous solutions of two conventional surfactants: cationic cetyltrimethylammonium bromide (CTAB) , anionic sodium dodecyl sulfate (SDS), nonioinic polyethylene glycol dodecyl ether (Brij35) and a cationic gemini bis (hexadecyldimethylammonium) pentane dibromide (G5). The critical micellar concentration of surfactants was determined by surface tension measurements and aqueous solubilities of fluorene compound in surfactant solutions were measured spectrophotometrically. Solubilization of PAH compound commenced at the surfactant critical micelle concentration and was proportional to the concentration of surfactant in micelle. The results of the mixed systems were analyzed with the help of regular solution theory, in which the deviation of CMCexp values for mixed surfactant systems from CMCideal was measured by evaluating the interaction parameter, βm. Negative values of βm were observed in all equimolar binary systems which show synergism in the mixed micelle. Attraction force between two oppositely charged head groups lead the strongest synergism effect between cationic gemini and anionic conventional surfactant. In addition to molar solubilization ratio (MSR) solubilization efficiency is also quantified in terms of micelle-water partition coefficient (Km).

Dynamics of poly-nipam chains in competition with surfactants at liquid interfaces: from thermoresponsive interfacial rheology to foams

We report results on the interfacial viscoelasticity and foaming of solutions of a thermoresponsive polymer (poly-n-isopropylacrylamide) with and without added surfactants, and as a function of temperature. With pure polymer solution, a clear transition is evidenced: both interfacial shear and dilational rheology shift from a fluid-like to a solid-like behavior at a well-defined temperature. The high temperature regime shows that the layer shares many features with soft glassy systems. At all temperatures, the foaming is low and the foam produced is unstable. Adding a surfactant not only helps to foam and to stabilize the foam, but also removes the thermal responsivity of the interfacial viscoelasticity. Under the conditions used here, we observe that the surfactant concentration threshold for altering the high temperature interfacial viscoelasticity is low, and is of the order of 1% of the surfactant critical micelle concentration. It turns out to be very different from critical values for the polymer–surfactant association found previously by structural studies (in bulk and at interface), and also below the threshold value required to stabilize the foam.

QSPR with extended topochemical atom (ETA) indices: Exploring effects of hydrophobicity, branching and electronic parameters on logCMC values of anionic surfactants

In this study, quantitative structure-property relationship (QSPR) models have been developed to establish the relationship between the molecular structures and the critical micelle concentration (CMC) of 37 anionic surfactants using extended topochemical atom (ETA) indices along with computed hydrophobicity descriptors. The ETA models have also been compared to those developed with non-ETA topological descriptors along with hydrophobicity terms. The selection of the training and test compounds (n=28 and 9, respectively) was done by using a principle component analysis (PCA) score plot. The best ETA model (Q2=0.938; Rpred2=0.923) could outperform in statistical quality and predictive ability the models developed with non-ETA (Q2=0.910; Rpred2=0.885) and combined set (ETA and non-ETA) of descriptors (Q2=0.926; Rpred2=0.915). In this study, it is observed that hydrophobicity plays a major role in the model development for CMC of anionic surfactants while the branching character and electronic nature of the surfactants are also important as evidenced from different ETA parameters.

QSPR with extended topochemical atom (ETA) indices, 3: Modeling of critical micelle concentration of cationic surfactants

The ability of surfactant molecules to facilitate solubilization of essential chemical entities accounts for their wide application in the pharmaceutical industry. The solubilization ability begins at concentrations exceeding the critical micelle concentration (CMC) which in turn is influenced by the molecular structure of the surfactants. Here, attempts have been made to explore the essential molecular fragments determining CMC of cationic surfactants using in silico technique. Different classes of descriptors (ETA and non-ETA) were correlated with the CMC data of a series of cationic surfactants to develop predictive models. Models were developed using genetic function approximation technique and were extensively validated employing different validation statistics. Comparative analysis was performed between the best model developed using the ETA descriptors and that obtained with the non-ETA variables. The results revealed that the external predictive potential of the ETA model was superior to the non-ETA model. The results also indicated that the ETA model could efficiently determine the CMC profile of the untested molecules. Finally, the two classes of descriptors were combined and the QSPR model thus obtained exhibited improved external predictive potential. The ETA descriptors implicated the importance of molecular size and their degree of unsaturation while the non-ETA descriptors signified impact of the bond order and the number of fragments bearing hydrogen atoms attached to carbon atoms with variable hybridization. Thus, addition of ETA descriptors improved the model quality extensively identifying essential molecular fragments contributing to their CMC profile and can be comprehensively utilized for property prediction of new cationic surfactants.

Vapour–liquid equilibrium relationship between toluene and mixed surfactants

Micellar partitioning of volatile organic compounds (VOCs) in surfactant solutions and its effects on vapour–liquid equilibrium is fundamental to the overall design and implementation of surfactant-enhanced remediation. Knowledge of the vapour–liquid equilibrium partitioning coefficients for VOCs, especially in contaminated soils and groundwater in which they exist, is required. Headspace experiments were performed to quantify the effect of three mixed surfactants, cetyltrimethyl ammonium bromide (CTMAB) with tetrabutylammonium bromide (TBAB), sodium dodecyl sulph ate (SDS) with Triton X-405 (TX405), and CTMAB with Triton X-100 (TX100), on the apparent Henry's constants (H c ) of toluene at temperatures ranging from 25 °C to 40 °C. The H c values were significantly reduced in the presence of all three mixed surfactants at concentrations above their critical micelle concentrations (CMC). Mixed micellar partitioning, showing effects on the vapour–liquid equilibrium of toluene, was primarily responsible for the significant reduction of H c in their mixed systems. The mixed surfactants CTMAB–TX100 had the greatest effect on H c above the CMC, followed by SDS–TX405, then CTMAB–TBAB. Mixed systems of CTMAB–TX100 decreased H c at concentrations significantly lower than the SDS–TX405 and CTMAB–TBAB concentrations, because of to the lower CMC of CTMAB–TX100. Vapour–liquid equilibrium data were also tested against the model (H c =H/(1+K(X−CMC)) that described the partitioning of VOCs in vapour–water–micelle phases. The correlation of H c with mixed surfactant concentrations (X) and CMC can be utilized as an effective tool to predict the H c by mixed surfactants.

Surfactant solubility and micellization in ternary eutectic melt (acetamide + urea + ammonium nitrate)

Cationic surfactants such as dodecyltrimethylammonium bromide (DTAB), tetradecyltrimehtylammonium bromide (TTAB) and hexadecyltrimethylammonium bromide (HTAB); and anionic surfactants such as sodium decyl sulphate (SDeS), sodium dodecyl sulphate (SDS) and sodium tetradecyl sulphate (STDS) have been used to determine their solubility and micellization in ternary eutectic melt (acetamide+urea+ammonium nitrate) at 50°C. We employed the electrical conductivity and the surface tension measurement techniques to determine the critical micelle concentration (CMC). The deviation in the slope of the specific conductance/surface tension against surfactant concentration plots indicated the aggregations of surfactants and hence, their CMC. CMC decreases with increase of alkyl chain length due to the increased van der Waals forces. The calculated increment in Gibb's energy per methylene group for cationic and anionic surfactants is about −6kJmol−1 and −4kJmol−1 respectively. It is found that, the CMCs of the surfactants in the ternary melt are higher than the CMCs of same surfactants in water (∼25°C).

Interactions of Ionic Surfactants With PEO-PBO-PEO Triblock Copolymers in Aqueous Solutions

The interactions of triblock copolymers (TBP) with ionic surfactants were studied employing surface tensiometry, electrical conductivity, steady-state fluorescence (SSF), and dynamic light scattering (DLS) techniques. An increasing trend in the critical micelle concentration (CMC) of SDS/CTAB in the presence of triblock copolymers was observed especially at higher polymer to surfactant ratio. The delay in the CMC of surfactants was more pronounced in the presence of E48B10E48 possibly due to its less hydrophobic nature. The negative values of free energy of micellization (ΔGm) both in case of SDS and CTAB confirmed the spontaneity of the processes. The aggregation number (Nagg) and hydrodynamic radius (Rh) of polymer/surfactant mixed systems were determined by SSF and DLS. The suppression of the surfactant micelle size in the presence of TBP was confirmed by SSF and DLS studies.

Influence of Surfactant Concentration on Laser-Based Gold Nanoparticle Formation and Stability

The time evolution of gold nanoparticle (AuNP) yields by in-situ laser irradiation from bulk gold in aqueous solutions containing the surfactant sodium dodecylsulfate (SDS) at concentrations above and below the critical micelle concentration in water is reported. These studies are augmented by transmission electron microscopy images of AuNP samples at each SDS concentration recorded after 90 min of laser irradiation. The results show that while a low concentration of SDS plays a role in the formation kinetics, there is no apparent influence of the SDS concentration around the surfactant critical micelle concentration on particle size during AuNP production.

Effect of Metal Ion Hydration on the Interaction between Sodium Carboxylates and Aluminum(III) or Chromium(III) Ions in Aqueous Solution

The interaction between sodium octanoate, decanoate, and dodecanoate and aluminum(III) and chromium(III) has been studied in water at natural pH values, starting well below the surfactant critical micelle concentration, using electrical conductivity, turbidity, and potentiometric measurements. With decanoate or dodecanoate, maximum interaction occurs at 3:1 stoichiometry, corresponding to charge neutralization. Although the solutions become turbid with both metal ions, indicating phase separation, differences are observed and attributed to the fact that aluminum(III) is relatively labile to substitution and rapidly replaces its water ligands, whereas chromium(III) is substitution inert. This shows up in well-defined floc formation with Al(3+), whereas Cr(3+) suspensions do not precipitate, probably because that replacement of coordinated water by carboxylate ligands is impeded. This can be overcome by increasing temperature, and differences in the thermal behavior with Al(3+) and Cr(3+) are suggested to be due to increased involvement of substitution reactions in the latter case. The effect of octanoate on the trivalent metal ions is less clear, and with Cr(3+) interaction only occurs when the carboxylate is in excess. Hydrophobic interactions between alkyl chains play a major role in driving phase separation. At high surfactant concentrations, the solid phases do not dissolve, in contrast to what is observed with the corresponding alkylsulfates. This has implications for use of these systems in metal separation through froth flotation. The concentration of metal ions in supernatant solution has been determined for sodium dodecanoate and sodium dodecylsulfate with Al(3+) and Cr(3+) over the whole surfactant concentration range by inductively coupled plasma-mass spectrometry (ICP-MS). From this, association constants have been determined and are found to be larger for the carboxylate than the alkylsulfate, in agreement with the greater Lewis basicity of the -CO(2)(-) group.