Molecular Organization of an Adsorbed Layer: A Zwitterionic, pH

Oct 2, 2016 - ... and Sr by Ion Flotation, Using a Surface-Active Derivative of DTPA (C12-DTPA). Alireza Eivazihollagh , Javier Tejera , Ida Svanedal ...
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Molecular organization of an adsorbed layer: a zwitterionic, pH-sensitive surfactant at the air/water interface Ida Svanedal, Fredrik Andersson, Erik Hedenström, Magnus Norgren, Håkan Edlund, Sushil K. Satija, Bjorn Lindman, and Adrian R. Rennie Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02598 • Publication Date (Web): 02 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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Molecular organization of an adsorbed layer: a zwitterionic, pH-sensitive surfactant at the air/water interface Ida Svanedal,*,† Fredrik Andersson,† Erik Hedenström,† Magnus Norgren,† Håkan Edlund,*,† Sushil K. Satija,‡ Björn Lindman,† Adrian R. Rennie§ Fibre Science and Communication Network (FSCN), Mid Sweden University, SE-851 70 Sundsvall, Sweden. †

NIST Center for Neutron Research, 100 Bureau Drive, MS 6100, Gaithersburg, MD 208996100, USA. ‡

§

Materials Physics and Centre for Neutron Scattering, Uppsala University, Ångström Laboratory, Box 516, SE-75120 Uppsala, Sweden. ABSTRACT Neutron and X-ray reflection measurements have been used to study the structure of the adsorbed layer of a chelating surfactant at the air/liquid interface. The chelating surfactant 2dodecyldiethylenetriaminepentaacetic acid (C12-DTPA) has a large head group containing eight donor atoms that can participate in the coordination of metal ions. The donor atoms are also titrating, resulting in an amphoteric surfactant that can adopt a number of differently charged species depending on the pH. Very strong coordination complexes are formed with metal ions, where the metal ion can be considered as part of the surfactant structure, in contrast to monovalent cations that act as regular counter-ions to the negative net charge. Adsorption was investigated over a large concentration interval, from well below the critical micelle concentration (cmc) to five times the cmc. The most striking result is the maximum in the surface excess found around the cmc, which is consistent with previous indications from surface tension measurements. Adding divalent metal ions has a limited effect on the adsorption at the air/liquid interface. The reason is the coordination of the metal ion, resulting in compensating deprotonation of the complex. Small variations in the head group area of different metal complexes are found, correlating to the conditional stability constants. Adding sodium chloride has a significant effect on the adsorption behavior and the results indicate that the protonation equilibrium is more important than the ionic strength effects. From combined fits of the neutron and X-ray data, a model that consists of a thick head group region and a relatively thin dehydrated tail region is found, and it indicates that the tails are not fully extended and that the limiting area per molecule is determined by the bulky head group.

1. INTRODUCTION Chelating surfactants are functional molecules that can form coordination complexes with a variety of metal ions, usually with very high stability constants. They may also show 1 ACS Paragon Plus Environment

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selectivity towards specific metal ions. In contrast to monovalent cations that act as regular counter-ions to the negatively charged groups, multivalent cation can be incorporated in the surfactant structure with strong coordination.1-4 For an equimolar concentration of metal ions, 1:1 complexes are formed exclusively. However in the case of an excess of metal ions, we have found that a single surfactant molecule can coordinate up to two metal ions.5 When not otherwise stated, the 1:1 complexes are discussed in the present work. The second functionality of chelating surfactants is the surface activity. This class of surfactants is very interesting from a technology point of view since they are efficient for metal ion recovery even from very dilute aqueous solutions.6 In technical applications, the target metal ions in the solution are captured by the strongly chelating head group. This stage is followed by separation of the complex from the aqueous phase by e.g. foam flotation, where the metal complexes are brought to the surface of the solution in the form of foam, due to the surface active properties. The collected metal complexes are further treated by electrophoresis to recover the metal ions and the cheating surfactant separately. The common feature of chelating surfactants is the ability to coordinate metal ions. The classification, including anionic, cationic, zwitterionic, pH-responsive and amphoteric, depends on the types of donor atoms present in the head group. Chelating surfactants are a rather unusual and complex group of surfactants that are not described extensively in the literature. C12-DTPA has a large head group, containing eight donor atoms that can become protonated or deprotonated depending on the pH, see Figure 1. There are two types of donor atoms present; five from the carboxylate groups and three from the amine groups. This results in an amphoteric surfactant with a charge that changes from +3 at very low pH, over a number of zwitterionic species, to -5 at very high pH. Except at extremely high or low pH, there will actually be a distribution of differently charged surfactant species due to overlapping pKavalues of the functional groups. This leads to different mixed surfactant systems at as the pH varies. The dominating species depends on the pH, but the degree of protonation will also be different in the three different states; monomers, micelles and surfactant molecules adsorbed at planar surfaces where monomers will be most and surface molecules least ionized. At intermediate pH, the dominating species is zwitterionic with increasingly negative net charge as the pH increases. The head group area and ionization can be tuned by varying the pH, adding a simple salt such as NaCl or by chelating multivalent metal ions, all of which will affect the adsorption behavior. When metal ions are added these will be coordinated by the head group and this means that in equimolar concentration to the surfactant, the metal ions will be present at the surface in the same concentration as the surfactant. This is a very interesting system for studies regarding structural properties of adsorbed surfactant layers at the air/liquid interface using neutron and X-ray reflection.

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COO

-

N

COO

-

COO

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N COO

-

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Figure 1. Structure of the chelating surfactant. The adsorption of a wide range of anionic, cationic and non-ionic surfactants at the surface of aqueous solutions has been studied by a range of techniques. The comparison between surface tension data and calculations using the Gibbs’ equation has been examined in a number of recent papers that have used neutron reflectivity to determine the surface excess.7-8 There are particular challenges as regards the unknown degree of ionization for ionic surfactants. Complementary data from different techniques are needed to provide a complete picture of the adsorption. Zwitterionic surfactants have been investigated less but present some particularly interesting behavior. Packing of zwitterionic surfactants, which generally have bulky head groups, leads to compact micelles and different interfacial layers.9-11 A consequence of the recent studies has been the understanding that the surface concentration of a planar interface with adsorbate that is limited by packing of molecules does not necessarily remain constant beyond the critical micelle concentration. The critical micelle concentration rather reflects the assembly of surfactants in solution. The specular reflection of neutrons and X-rays from liquid surfaces depends on the structure and composition of the interface.12 The observed intensity changes with incident angle, θ, and wavelength, λ, and this can be calculated using conventional optical methods for multilayer structures if the appropriate refractive index and thicknesses are known. Reflectivity is usually analyzed as a function of the momentum transfer, Q (= (4π/λ) sin θ). For neutrons the refractive index, nn, is given by: nn = 1 - 2 /2 where  = i bi / V,  is the wavelength and bi are the coherent scattering lengths for the nuclei that form a molecule with volume V. The values of b are well known and are tabulated.13 Of particular note is that different isotopes can have very different values of b. Some values of scattering length and scattering length densities are shown in Table 1 and 2. Table 1. Neutron and X-ray scattering lengths for different elements Element H D O Na

neutron b (fm) -3.74 6.67 5.80 3.63

X-ray b (fm) 2.81 2.81 22.68 31.37

X-ray bimag (fm) 0.0 0.0 0.095 0.35

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Ni Co Zn Mg

10.3 2.49 5.68 5.38

70.5 69.2 80.0 34.4

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1.43 10.17 1.98 0.51

Table 2. Neutron and X-ray scattering length densities for different components Component C12-DTPA C12D25-DTPA H-tail C12H25 D-tail C12D25 Head Water H2O Heavy water D2O Null reflecting water

ρ × 10-6 (Å-2) neutron 1.54 4.96 -0.37 6.58 3.68 -0.56 6.35 0.00

ρ × 10-6 (Å-2) X-ray 10.90 10.90 7.31 7.31 14.30 9.44 9.44 9.44

V (Å3) 760.50 760.50 374.91 374.91 385.60 30.00 30.00 30.00

Normal hydrogen, 1H, and deuterium, 2H or D, have b with opposite sign. This allows a mixture of normal water, H2O, and heavy water, D2O, to be prepared with a value of ρ equal to zero. This water has a refractive index equal to 1 and is known as null reflecting water (NRW) as there is no specular reflection at the interface of air and water with the same refractive index. Under these conditions the reflection arises only from an adsorbed surface layer. The refractive index for X-rays, nX, is calculated in an analogous way nX = 1 –  - i where = reNAZm /2A, and m is the mass density, Z is the number of electrons per atom, A is the atomic mass, re is the Thomson electron radius. Alternatively this can be expressed simply as an X-ray scattering length density as for the neutrons. The real and imaginary parts of the scattering length are listed in Table 1. The term  is more significant for X-rays and depends on the absorption. Fitting of models to reflectivity data is performed by calculations using standard optical methods.14 The interface is divided into layers with a defined refractive index to describe the structure as regards composition and density normal to the surface. For neutrons and x- ray the refractive index is simply related to composition by the values of the scattering length as given above. The model profile is then fitted to the observed data by least squares. In practice constraining fits to multiple data sets with measurements with both X-rays and neutrons and using a model that represents features of the surfactant such as hydrophobic tails and hydrophilic head regions allows the structure of the interface as well as the amount of adsorbate to be determined. While the deuterium labelling of the surfactant provides unambiguous information about the surface excess when neutron measurements are made 4 ACS Paragon Plus Environment

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with null reflecting water, the situation is slightly more complicated for X-rays. The electron density determines the scattering for each atom. The surfactant alkyl tails with an assumed density of 0.75 g cm-3 would have a scattering length density for X-rays that is lower than that of water. When both contrasts are available the combined fit agrees well with a model of the stated surface excess with the surfactant largely above the water. It would be convenient if the signal from the counter-ions gave clear information about the metal binding but unfortunately the scattering from the transition metals ions that have been studied is roughly equivalent to that of two to three water molecules. As the density of the surfactant at the interface is not known independently, the present experimental data are insufficient to determine unambiguously the location of the metal ions. Thus the substitution of hydrating water molecules around the head of the surfactant with a counter-ion could not be distinguished in models. In principle measurements with D2O as well as H2O might provide this information but the characteristic signal is expected to be at larger momentum transfer, Q, than can be measured with a significant signal above background. Future work may need to use spectroscopic methods such as X-ray photoelectron spectroscopy (XPS) or the fluorescence method that could be developed for particular ions as described by Witala15 to locate and quantify the metal ions. In previous work we investigated the solution behavior of C12-DTPA and we showed that this surfactant behaves unconventionally as regards the surface tension of its aqueous solutions.16 The present study uses both neutron and X-ray reflection to examine the correlation between surface tension and surface excess for the chelating surfactant as well as to determine the structure of the adsorbed layer of surfactant molecules at the air/liquid interface. In particular the effects of adding monovalent or divalent cations or changing the pH are examined. The effect of varying the molar ratio between the chelating surfactant and the metal ion is also examined.

2. MATERIALS AND METHODS 2.1 Materials and Sample preparation The chelating surfactant 2-dodecyldiethylenetriaminepentaacetic acid (C12-DTPA) was delivered by Syntagon AB. The synthesis and analyses have been reported earlier.16 The deuterated analogue to 2-dodecyldiethylenetriaminepentaacetic acid (C12D25-DTPA) was prepared at Mid Sweden University by the route shown in Scheme 1. The 2-dodecyldiethylenetriamine-d25 pentaacetic acid (C12D25-DTPA) was obtained in 44% total yield over seven steps using 1-bromododecane-d25 (98 atom% D) for introduction of the deuterium labelled part of C12D25-DTPA. The method used was described by us previously for the undeuterated analog.17 The synthesis commenced with a malonic ester synthesis of diethyl acetamidoacetate and 1-bromododecane-d25 (Scheme 1), followed by an acidic hydrolysis/decarboxylation step resulting in an alkyl-d25 amino acid hydrochloride. This alkyl-d25 amino acid hydrochloride was esterified to methyl 2-aminotetradecanoate-d25 hydrochloride (1), using MeOH and SOCl2, in 81% total yield over three steps, using the 5 ACS Paragon Plus Environment

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method of Culbertson et al.,18 for a similar compound. 2-aminotetradecanoic acid-d25 ethylenediamine amide (2) was prepared from deprotonated 1 and ethylendiamine, in 94% yield, using the method of McMurry et al.19 for a similar compound. By reduction of 2 with BH3·THF, 2-dodecyldiethylenetriamine-d25 trihydrochloride (3) could be obtained in 79% yield, after a work-up procedure with EtOH and HCl (g), using the method of McMurry et al.,19 for a similar compound. 3 was acylated with tert-butyl bromoacetate using diisopropylethylamine as base, using the method of McMurry et al.,19 for a similar compound. The 2-dodecyldiethylenetriamine-d25 penta-tert-butyl acetate (4), was purified by repeated flash chromatography to remove traces of starting material and incompletely reacted compound 3, and was obtained in 86% yield. Finally, 2-dodecyldiethylenetriamine-d25 pentaacetic acid (C12D25-DTPA) was obtained by acidic hydrolysis, using a modified procedure of Grote et al.20 All intermediate products and the end product C12D25-DTPA could be identified and compared spectroscopically with the undeuterated analogues presented earlier.17 In 1H and 13C NMR spectroscopy the signals of the C12D25-alkyl moiety are absent and only some trace signals due to incomplete deuteration are present. In IR spectroscopy signals from C-D stretching appears in the region around 2100 cm-1. ESI-HRMS (Electrospray Ionization-High Resolution Mass Spectrometry) analyses on C12D25-DTPA and all intermediate products were performed and m/z-values correlated well to the calculated ones. A full description of the synthesis of C12D25-DTPA and analytical details are given in the Supporting information.

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Scheme 1. Synthesis of 2-dodecyldiethylenetriamine-d25 pentaacetic acid (C12D25-DTPA). Nickel chloride, cobalt chloride, zinc chloride, magnesium chloride, and sodium chloride were of analytical grade and used without further purification. Water was of Milli-Q grade. Deuterium oxide (D2O), containing 99.9% D, was used as supplied. Null reflecting water with 0.088 mole fraction of deuterium oxide in normal water (H2O) was used for the neutron reflection measurements. Samples were prepared in water or null reflecting water (for neutron reflection) and the pH was measured and adjusted to the appropriate pH by addition of sodium hydroxide. Samples of metal complexes were prepared by adding metal chloride in the appropriate molar ratio to C12D25-DTPA (0.5:1, 1:1, and 2:1 for the zinc complex and 1:1 for the others). Samples of C12D25-DTPA in NaCl were prepared in 1.2 M concentration background electrolyte. 2.2 Surface tension The surface tension was measured with a Krüss K6 tensiometer and a platinum/iridium du Noüy ring at a temperature of approximately 22 °C. Measurements were made on the same samples as used for the neutron reflection, i.e., on C12D25-DTPA in NRW unless otherwise stated. Each sample was measured five times consecutively, the mean values are reported. The chelating surfactant with deuterated tail showed the same surface tension behavior as that 7 ACS Paragon Plus Environment

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previously described for the hydrogenous analog,16 as shown in Figure 2, confirming that this is the true surface tension behavior of the chelating surfactant, it is not an effect of an impurity and that there is no significant effect caused by the change in isotopic composition. The uncertainty is shown by bars representing one standard deviation. This convention is used throughout the paper. 80 70

(mN/m)

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Figure 2. Surface tension of C12-DTPA (●) and C12D25-DTPA (Δ) in H2O at pH 5.

2.3 Neutron and X-ray reflection Neutron reflection experiments were performed using the NG7 horizontal surface reflectometer at the NIST Center for Neutron Research, Gaithersburg, Md., USA.21 A description of the instrument can be found in the paper by Ankner et al.22 The technique is described in the paper by Penfold and Thomas.23 Measurements are made by scanning the incident angle on a sample with a monochromatic beam (wavelength 4.77 Å with a wavelength resolution of 2.5 % Δλ/λ) and measuring the reflected intensity on a linear detector. The background was subtracted by evaluating observed intensity on the detector on either side of the specular peak. In the scans, slits were opened with increasing angle so as to maintain constant illumination of the sample and constant resolution. Data were corrected and normalized using the ‘Reflpak’ software.24 Samples were placed in PTFE troughs and the height was adjusted to allow for different levels of the meniscus. To avoid contamination and reduce evaporation, the troughs had an aluminum cover and the beam could enter and exit through thin aluminum foil windows. An active anti-vibration table was used to avoid disturbance of the interface. A Bruker D8 diffractometer with an active anti-vibration table was used for the complementary X-ray reflection measurements. Copper K radiation (1.54 Å) was used with -2 scans of the source and the detector. Calibrated attenuators were used for the low angle measurements that were then scaled to the other data measured at high angles. The background was measured with the detector offset and was subtracted using the ‘Reflpak’ software.24 Reflection measurements were made on equilibrated samples and care was taken to ensure the aging time for each sample was sufficient. 8 ACS Paragon Plus Environment

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3. RESULTS AND DISCUSSION 3.1 Correlation between surface tension and surface excess 3.1.1

Neutron reflection data described by a single layer model

In a previous study on the behavior of C12-DTPA, we showed that this surfactant has an unusual surface tension versus concentration behavior in aqueous solutions.16 Instead of the normal sharp break followed by a constant level in the plot of surface tension against concentration, we found an increase in surface tension from around the critical micelle concentration, cmc, which is 20 ± 3 mmol L-1 at pH 5,16 see Figure 2. This was interpreted as indicating a decreasing amount of surfactant at the surface once micelles had formed. NMR diffusometry was used to determine cmc for the chelating surfactant.5-6, 16, 25 In the present paper we use neutron reflection to examine directly the surface excess concentration and the thickness of the adsorbed layer for a series of varying concentrations. We investigate three different series of samples over a large concentration interval: the pure chelating surfactant (C12D25-DTPA), the 1:1 complex with nickel (Ni2+-C12D25-DTPA), and the chelating surfactant in 1.2 -mol L-1 sodium chloride background electrolyte (C12D25-DTPA in 1.2 mol L1 NaCl). At fixed concentrations, we also examine the effects of varying the pH as well as adding different metal ions to form different metal complexes. The neutron reflection data for surfactant at the surface of solutions of NRW are well described by a model that consists of a single layer and the amount of material at the interface dominates the observed signal: the surface excess is thus usually determined to an accuracy of a few percent.26 Some information about the average thickness is determined to lower precision but the single contrast does not provide any distinction as to water or air in the layer. The neutron reflection data for C12D25DTPA at three different concentrations with model fits for a single layer are shown in Figure 3a. Models were always constrained so that the product of the overall thickness and the molecular area were equal to or greater than the estimated volume of a surfactant molecule. The reflection increases with concentration from 0.05 to 10 mmol L-1 and then decreases in the range from 10 to 100 mmol L-1. These results are in agreement with our previous assumptions based on surface tension measurements. Similar trends were found for the other two series of samples with coordinated nickel ions and with NaCl as a background electrolyte. In Figure 3b the reflection data with fits for the 10 mmol L-1 sample from each series are showing nearly overlapping plots for the two series with and without coordinated nickel ions, and a significantly lower reflection for the sample with NaCl.

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Figure 3. Neutron reflection data with fits (solid lines). (a) C12 D25-DTPA at varying concentrations: 0.05 mmol L-1 (○), 10 mmol L-1 (●), and 100 mmol L-1 (●). (b) C12-DTPA (●), Ni2+-C12-DTPA (●), and C12D25-DTPA in 1.2 mol L-1 NaCl (○) at 10 mmol L-1. Note that C12D25-DTPA and Ni2+-C12-DTPA are nearly overlapping. 3.1.2 Surface excess concentration and thickness of the adsorbed layer The changes in surface excess and in thickness are correlated to the previous results from surface tension measurements as shown in Figure 4. A maximum around the cmc (20 ± 3 mmol L-1 for the pure chelating surfactant) is found for both the surface excess and the thickness of the adsorbed layer, corresponding to the minimum in surface tension. The studies with neutrons support the idea that the amount of material at the surface decreases as the surface tension increases, i.e. above the cmc. This implies that the activity in the solution changes above the cmc. This is especially pronounced for the pure chelating surfactant and the nickel-complex. With NaCl added, the decrease in surface excess above the cmc is less pronounced, and this correlates with the smaller increase in surface tension. In Gibbs adsorption equation, a region of positive slope in the surface tension plot can only occur if there is a reduction in the bulk solution activity of one of the species adsorbed at the surface. The neutron data show that this species has to contain a deuterated chain. The chelating surfactant can form several differently charged ionic species, and the likely explanation is an increase in the fraction of one or more ionic species bound in the micelles. Most studies on adsorption of surfactant at the surface of solutions are focused on concentrations up to the cmc and there has been less discussion about what happens above the cmc, although there are some studies, for example those of Lu et al.8, 27-28 There has been recent recognition that the adsorption to interfaces while necessarily obeying the thermodynamics described by Gibbs can be complicated when several components are present and because the activity in solution changes, for example because of pre-aggregation below the critical micelle concentration as 10 ACS Paragon Plus Environment

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well as variation of the ionic dissociation.8 There is also no a-priori reason that the maximum packing density at a planar surface is reached exactly at the critical micelle concentration. For example, at an alumina/solution interface there has been clear evidence for a maximum in the adsorption of various anionic surfactants above the critical micelle concentration.29 This implies that a variety of patterns of change in the surface excess could reasonably be expected for zwitterionic surfactants with polyvalent ions.

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Figure 4. Correlation between (a) Surface excess, (b) thickness, and (c) surface tension for the pure C12D25-DTPA (●), Ni2+-C12D25-DTPA (Δ), and C12D25-DTPA in 1.2 mol L-1 NaCl (■). Typical uncertainties reported from the fitting program are 1% for surface excess and 3% for thickness, except for the 100 mmol L-1 samples where the errors in thickness amount to 8%. For the lowest thickness, where the thickness is fixed at 7 Å, the uncertainties are a few Å. Typically random errors from the surface tension measurements are 0.2% except around 50 mN m-1 where they amount to 2%. 11 ACS Paragon Plus Environment

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Calculations of surface excess from Gibbs adsorption equation would not be very accurate in these systems and has therefore not been done. The calculations require determination of the prefactor for the Gibbs equation, which is the number of species at the surface whose concentration changes with the surfactant concentration. Determining the prefactor for the surfactant studied in this work is not straightforward due to overlapping pKa-values and complex dissociation behavior. Further, very accurate surface tension measurements are necessary.28 This has shown to be difficult in the studied systems, especially at concentrations below the cmc. The reason seems to be an unusually slow adsorption process, which requires hours to reach equilibrium. We have discussed this elsewhere.5, 16 Even dipping the ring in the solution prior to surface tension measurements disturbs the equilibrium, which is why each sample is measured five times and mean values are reported. However, the surface tension plots only shifts vertically but not laterally during equilibration and can therefore be compared with surface excess and thickness from reflection measurements performed at equilibrium. We have not been able to catch this kinetics in the studies with neutrons, except for very weak indications on the surface excess at low concentrations. The X-ray measurements on the other hand show some changes at the surface during aging. The evolution with time of the surface tension and that this corresponds with changes at high momentum transfer in the X-ray reflections suggests that internal structural changes could be responsible for the changes in surface energy. We will explore the kinetics of the surface adsorption further in future studies. 3.1.3 Adding metal ions results in complex formation From Figure 4 it is clear that the nickel-complex behaves similar to the pure chelating surfactant. The cmc is also slightly reduced, from 20 ± 3 mmol L-1 to 14 ± 3 mmol L-1. Making a comparison with conventional surfactants, one might have expected a larger effect on the adsorption when adding a divalent counter-ion as nickel. The reason for the small effect on the adsorption behavior is the strong coordination of the metal ion by the head group, resulting in 1:1 complexes since nickel was added in equimolar concentration to the surfactant. The coordination of nickel results in a release of protons from the head group, which is detectable by pH measurements,5 thus the formed complex does not differ much in net charge from the chelating surfactant molecule. The metal ions do not act as normal counter-ions, but instead they are incorporated in the surfactant structure due to the high stability constant. Almost every surfactant will hold exactly one metal ion. The metal ions are therefore not free to move around in the solution like normal counter-ions, but always present at the surface in the exact same concentration as the chelating surfactant. 3.1.4 Significant effects on the adsorption when adding NaCl Adding the simple salt, NaCl, changes the adsorption behavior significantly, shown in Figure 4. While nickel was present in equimolar concentration to the chelating surfactant, NaCl was added in a constant and relatively high concentration, 1.2 mol L-1, in order to screen the electrostatic interactions between the numerous functional groups. The monovalent sodium ions are not coordinated but act as regular counter-ions to the negatively charged groups. With NaCl added, the cmc is reduced from 20 ± 3 to 1.7 ± 1 mmol L-1 due to screening of the 12 ACS Paragon Plus Environment

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electrostatic repulsions. For the same reason, salt addition to an ionic surfactant solution would normally increase the adsorption at the surface.30 Here the salt addition leaves the surface excess virtually unaffected at low concentrations and at the cmc the adsorption levels off, resulting in a lower surface excess at concentrations above cmc. Adding NaCl diminishes the concentration dependence on the surface excess above the cmc. With NaCl added the maximum in the surface excess is less pronounced. Whether or not this is solely an effect of the reduced cmc limiting the adsorption of molecules at the surface, is hard to say. Salt addition also facilitates the ionization, which increases the electrostatic repulsions and it seems like this effect is dominating for the adsorption. We found that the surface excess plots mirror the surface tension plots quite well, which is seen when comparing the data shown in Figure 4b and 4c. 3.1.5. A limited effect of varying the pH The dependence of the head group area and the surface tension on the pH was examined at a surfactant concentration of 0.5 mmol L-1, see Table 3. Increasing the pH from 5 to 6.5 reduces the surface excess, and consequently increases the head group area and it seems that the results are very sensitive to this first change in pH. The reduced surface excess is a natural consequence of the stronger electrostatic repulsion between head groups at the higher pH, where the negative net charge is higher. Apparently there is a limit to this effect on the surface excess as increasing the pH further to 8 and to 10, does not cause any further significant change. This is consistent with the results from surface tension measurements and previous measurements of cmc showed a similar trend. Table 3. Effect of varying the pH pH

0.5 mmol L-1 Amolecule (Å2) γ (mN/m) 59 ± 2 50 88 ± 2 63 91 ± 2 60 87 ± 2 62

5 6.5 8 10 a from ref 16

cmc (mmol L-1)a 20 ± 3 35 ± 10 38 ± 12 37 ± 12

3.1.6 The correlation between head group areas and stability constants Also for the examined metal complexes of the chelating surfactant there is a weak correlation between the area per molecule at the surface, Amolecule, as shown in Table 4. The role of the metal ion is however not a strong effect. When looking at different metal complexes there is a difference in how the metal ions are coordinated by the chelating head group of C12D25DTPA, which is clearly seen from the variation in the strength of the complexes expressed as the conditional stability constant, log K.5 Generally, the relative strength between the coordination of different metal ions with a specific chelating agent correlates to the classification of the metal ions on the scale from hard to soft acids according to the theory of hard and soft acids and bases (HSAB).31 A higher log K value means that the metal ion is bound stronger to the head group and this could affect the molecular area. Although not a 13 ACS Paragon Plus Environment

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strong effect, this can be seen in the studied complexes, compared at 35 mmol L-1 in Table 4. Firstly, coordination of metal ions increases the head group area above its original 61 Å2 without metal ions. Furthermore, there is a week correlation between the log K and the molecular area for the metal complexes; the area is smaller the stronger the complexes are. This means that for the stronger complexes, the metal ions are coordinated more tightly making the head group slightly smaller. This has not been shown previously in a direct way, even for conventional chelating agents. The effect of adding NaCl, which is found to increase the head group area, is also shown for comparison. With NaCl added the cmc is however much lower, leading to lower surface excess as already discussed. The uncertainties quoted in the table and subsequently elsewhere in the paper represent two standard deviations. Table 4. Ionic radius, conditional stability constant, molecular area and surface tension for C12D25-DTPA, its 1:1 metal complexes and in 1.2 mol L-1 NaCl at 35 mmol L-1, pH 5. Metal Ions Ionic radiusa (Å) log Kb Amolecule (Å2) No salt 61 ± 2 2+ Ni 0.83 11.4 62 ± 2 2+ Co 0.88 10.0 63 ± 2 2+ Zn 0.88 9.9 65 ± 2 Mg2+ 0.86 1.9c 66 ± 2 +d Na 1.16 70 ± 2 a 32 b 5 c From ref . For C12-DTPA from ref . For DTPA from ref 33. dNot coordinated.

3.2 The structure of the adsorbed layer as described by a two-layer model 3.2.1

Comparison of thickness against surface excess concentration

Compared at the same bulk concentrations, chelation of Ni2+ affects neither the surface excess nor the thickness of the adsorbed layer significantly, as shown above in Figure 4. Adding NaCl results in higher thickness at low concentrations, at about the same surface excess. This indicates that the molecules at the surface become elongated, in the z-direction, at the addition of salt. Most likely the change happens in the head group region since that part is most affected by the electrostatics. When analyzing the data at higher concentrations, one must keep in mind the reduction of the cmc from 20 to 1.7 mmol L-1 in the presence of salt. When micelles start to form, this plot deviates from the other two. As seen in Figure 4, the addition of sodium chloride results in lower thickness and lower surface excess when compared at higher concentrations. This is most likely a consequence of the formation of micelles in the bulk, which levels off the adsorption at the surface. If a comparison is made at the same surface excess rather than for the same bulk concentration, the situation looks slightly different. This is illustrated in the plot of thickness against surface excess shown in Figure 5. The two data sets, with and without Ni2+, look quite similar. Adding NaCl clearly results in a higher thickness over the whole measured interval when compared in this way. This supports the idea that the molecules become elongated in the presence of NaCl.

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Even though the thickness from neutron reflection is an average for the entire surfactant layer, it seems most likely that it is the head group region that expands when salt is added. 20

thickness (Å)

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15

10

5 1.5

2.0

2.5 -6

max

3.0

3.5

2

(10 mol/m )

Figure 5. Surface excess vs thickness at pH 5 for C12D25-DTPA (●), Ni2+-C12D25-DTPA (Δ) and C12D25-DTPA in 1.2 mol L-1 NaCl (■).Typical uncertainties reported from the fitting program are 1% for surface excess and 3% for thickness, except for the 100 mmol L-1 samples where the errors in thickness amount to 8%. For the lowest thickness, where the thickness is fixed at 7 Å, the uncertainties are a few Å. The results in Figure 5 can be interpreted as showing that there is a reduced surface excess when salt is added, if the data are compared at a given thickness instead. Normally, reduced head group area (increased surface excess) and/or thickness would be expected from adding salt to an ionic surfactant solution, due to less loss of counter-ion entropy. Here the situation is reversed, which implies that the repulsive interactions within or between adjacent head groups at the surface increases with salt. The unconventional reduction in surface excess at concentrations above the cmc, discussed in section 3.1.2, could also be related to this mechanism. 3.2.2 Combined fit from neutron and X-ray reflection measurements Better refinement of structural models for the interfacial surfactant layers is achieved by including further data from X-ray reflection in a combined fit with the neutron data,34 see Figure 6 for an example. The neutron and X-ray reflectivity profiles were modeled together using an optical matrix calculation in which two surface layers with variable thickness that are parameterized as containing (i) a certain number of surfactant tails and (ii) the stoichiometric number of surfactant head groups with counter-ions and a fittable number of water molecules. As previously constraints are applied in the software that physically reasonable and adequate volumes for the surfactant are used, in this case the estimated tail and head volumes, taking in to account water molecules, are considered separately. The models correspond to layers containing about 10 water molecules within the hydrophilic layer of head groups. While the overall surface excess is still constrained largely by the neutron data measured with NRW, the further parameters are determined as a consequence of the different contrast for X-rays and the extended range of momentum transfer Q. The combined fit in the models shown 15 ACS Paragon Plus Environment

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constrains the amount of material in the layers measured with neutrons and with X-rays to be the same. It separates the structure in to regions of hydrophobic tails and hydrated heads. The tails have a lower X-ray scattering length density than water and the heads have a higher scattering length density. This contrast and the larger range of momentum transfer that is accessible allows details of the structure to be distinguished in model that are fitted to the data. The neutron reflection data are dominated by the amount of surfactant at the interface.

log10 (reflectivity)

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1 0 -1 -2 -3 -4 -5 -6 -7 -8 0.0

0.1

0.2

0.3

0.4

0.5

0.6

-1

Q (Å ) Figure 6. Neutron (●) and X-ray (○) reflection with fits for Zn2+:C12D25-DTPA 2:1 at 0.1 mmol L-1 concentration of C12D25-DTPA. Two concentrations were chosen for the combined measurements and fits; one below and one above the cmc, see Table 5. The combined fit suggests a two-layer model, with part of the tails in the head group layer. The average thickness at both 0.5 mmol L-1 and 35 mmol L-1 is 20 ± 2 Å, of which the dehydrated tail region is 8 ± 1 Å, see Table 5. One must however not forget that the thickness vs concentration plot goes through a maximum, as seen in Figure 4b and discussed above. The two chosen samples lie on either side of this maximum and can therefore show the same thickness despite the concentration dependence. It is interesting to note that the thicknesses of the two layers are the same for both the high and the low concentration, indicating that the structure of the layer is not dependent on the bulk concentration. There is a thick head group region, even thicker than the tail region, as illustrated in Figure 7. The thickness of the tail region, on the other hand, is rather low. According to Tanford’s expression for saturated hydrocarbon chains of n carbon atoms, the maximum chain length of the tail, lc,t, is the effective length in the liquid state and is given in Ångström by:35 lc,t ≤ lmax = 1.54 + 1.265 n The length of the fully extended C12 hydrocarbon chain, lmax, is 16.7 Å. Even though a part of the chain is included in the head group region, a tail region of only 8 Å means that the hydrocarbon chains are not fully extended, as indicated in Figure 7. For the nickel complex at 35 mmol L-1 the thickness is 21 ± 2 Å, of which the dehydrated tail region is 8 ± 1 Å. Reducing the concentration to 0.1 mmol L-1 decreases the average thickness slightly, to 20 ± 2. However the extent of the tail region remains the same. This, in 16 ACS Paragon Plus Environment

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combination with the tails not being fully extended, indicates that the packing is limited by the bulky hydrophilic moiety. The head group determines the area and the hydrocarbon tail will pack closely on that area to minimize contact with air. Table 5. Thickness of the two layers and hydration of head groups from combined fits Species c (mmol L-1) τtotal (Å) τtail (Å)a τhead group (Å) nwater C12D25-DTPA 35 20 ± 2 8±1 12 ± 2 10 ± 2 C12D25-DTPA 0.5 20 ± 2 8±1 12 ± 2 10 ± 2 2+ Ni -C12D25-DTPA 35 21 ± 2 8±1 13 ± 2 12 ± 2 2+ Ni -C12D25-DTPA 0.1 20 ± 2 8±1 12 ± 2 15 ± 2 a An interlayer roughness of 4.5 Å was included in the model to account for intermixing of tails with the head groups. air

tail region interlayer roughness head group region

bulk solution z-direction

Figure 7. Illustration of the structure of the adsorbed surfactant layer showing the upper tail region, the interlayer roughness, and the thicker, lower head group region. 3.2.3 The effect of varying the molar ratio between metal ion and chelating surfactant So far only 1:1 coordination complexes have been discussed. One can assume that the molar ratio between metal ion and chelating surfactant will affect the adsorption behavior. The effect of varying the ratio between the metal ion and the chelating surfactant was investigated for Zn2+-C12D25-DTPA at a fixed surfactant concentration of 0.1 mmol L-1, as shown in Table 6. The three molar ratios were 0.5:1, 1:1, and 2:1. Adding less than the equimolar ratio of the metal ion will result in a mix of free chelating surfactant molecules and 1:1 coordination complexes. When the metal ion is added in more than the equimolar ratio, the chelating surfactant can coordinate up to two metal ions per molecule. When increasing the amount of zinc compared to chelating surfactant in the mentioned sequence the surface excess increases, which means that the head group area decreases. This is most likely due to reduced electrostatic repulsion and increased hydrophobicity. The negative net charge of the chelating surfactant is reduced by the coordination of positively charged metal ions. At 35 mmol L-1 the effect was the reversed; adding metal ions increased the head group area, as shown above in Table 4. Apparently, coordination of metal ions has different effects on the head group area depending on the concentration. The main difference between 0.1 and 35 mmol L-1 is that there are micelles present at the higher concentration and clearly the effect is related to that. 17 ACS Paragon Plus Environment

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The thickness of the adsorbed layer, on the other hand, is not significantly affected by the variation in zinc content and the variation in the thickness of both layers is the same within uncertainty. This means that increasing the zinc addition implies a closer packing at the surface while leaving the thickness of the adsorbed layer virtually unaffected. Table 6. Parameters for the structure of the adsorbed layer from combined model fits to X-ray and neutron reflectivity showing the effect of varying the molar ratio between zinc (Zn2+) and C12D25-DTPA (at 0.1 mmol L-1). ratio Amolecule (Å2) τtotal (Å) τtail (Å)b τhead group (Å) nwater 0:1 82 ± 2a 0.5:1 71 ± 2 17 ± 2 7±1 11 ± 2 11.9 ± 1.5 1:1 69 ± 2 17 ± 2 8±1 9±2 8.0 ± 1.5 2:1 66 ± 2 19 ± 2 8±1 11 ± 2 10.5 ± 1.5 a b From neutron reflection data only without zinc addition. An interlayer roughness of 4.5 Å was included in the model to account for some intermixing of tails with the head groups.

4

CONCLUSIONS

Both the surface excess and the thickness obtained from the reflection measurements show a maximum at concentrations around the cmc for the chelating surfactant, which correlates to the previously observed minimum in surface tension. The reflection experiments confirm that the amount of material at the surface decreases with increasing concentration once micelles are formed in the bulk solution. Adding divalent metal ions in equimolar concentration to the surfactant had a limited effect on the adsorption behavior. This is a consequence of the complexes binding of the metal ions, since the ions are not free to move around in the solution but are bound to the chelating surfactant in 1:1 complexes. The formation of the complex leads to compensating deprotonation of the complex, resulting in a net charge of the complex similar to that of the pure chelating surfactant molecule. A small variation was seen between the molecular areas for different metal complexes and it was found that the smaller head group areas correlate to larger conditional stability constants for those complexes, i.e., stronger binding of the metal ions. The adsorption behavior was altered significantly by addition of NaCl. The salt increased the thickness of the adsorbed layer, meaning that the molecules become more elongated in the zdirection perpendicular to the surface, see figure 7, in the presence of simple salt. Apparently the NaCl increases the repulsions in the head group region, which is the opposite of what is expected for an ionic surfactant. Increasing the pH reduces the surface excess due to increased electrostatic repulsions, the maximum effect is however reached already at pH 6.5. Combined fits from neutron and X-ray reflection measurements suggest a structure that can be described by a two layer model for the adsorption at the air/liquid interface. A thin upper layer consists of the dehydrated tails and a thicker lower layer is formed of the hydrated head 18 ACS Paragon Plus Environment

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groups. There is some intermixing between the layers represented by 4.5 Å roughness in the model fits. The thicknesses of the two layers indicate that the tails are not fully extended and that for most conditions, the packing of the surfactant is limited by the bulky, hydrated head groups. Increasing the molar ratio between zinc and the chelating surfactant in the sequence 0.5:1, 1:1, and 2:1 increases the surface excess due to reduced electrostatic repulsions. The thickness of the adsorbed layer was however essentially unaffected. ASSOCIATED CONTENT Supporting Information. Full description of the synthesis of C12D25-DTPA and analytical details. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

*

E-mail: [email protected]

ACKNOWLEDGEMENTS We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. REFERENCES 1. Scrimin, P.; Tecilla, P.; Tonellato, U.; Vendrame, T., Aggregate Structure and Ligand Location Strongly Influence Cu2+ Binding Ability of Cationic Metallosurfactants. J. Org. Chem. 1989, 54 (25), 5988-5991. 2. Griffiths, P. C.; Fallis, I. A.; Willock, D. J.; Paul, A.; Barrie, C. L.; Griffiths, P. M.; Williams, G. M.; King, S. M.; Heenan, R. K.; Gorgl, R., The structure of metallomicelles. Chem.--Eur. J. 2004, 10 (8), 2022-2028. 3. Bordes, R.; Holmberg, K., Physical chemical characteristics of dicarboxylic amino acidbased surfactants. Colloids Surf., A 2011, 391 (1-3), 32-41. 4. El-Sukkary, M. M. A.; Soliman, E. A.; Ismail, D. A.; El Rayes, S. M.; Saad, M. A., Synthesis and Properties of Some N-Acylethylenediamine Triacetic Acid Chelating Surfactants. Tenside, Surfactants, Deterg. 2011, 48 (1), 82-86. 5. Svanedal, I.; Boija, S.; Almesaker, A.; Persson, G.; Andersson, F.; Hedenstrom, E.; Bylund, D.; Norgren, M.; Edlund, H., Metal Ion Coordination, Conditional Stability Constants, and Solution Behavior of Chelating Surfactant Metal Complexes. Langmuir 2014, 30 (16), 4605-4612. 6. Svanedal, I.; Boija, S.; Norgren, M.; Edlund, H., Headgroup Interactions and Ion Flotation Efficiency in Mixtures of a Chelating Surfactant, Different Foaming Agents, and Divalent Metal Ions. Langmuir 2014, 30 (22), 6331-6338. 7. Li, P. X.; Li, Z. X.; Shen, H. H.; Thomas, R. K.; Penfold, J.; Lu, J. R., Application of the Gibbs Equation to the Adsorption of Nonionic Surfactants and Polymers at the Air-Water Interface: Comparison with Surface Excesses Determined Directly using Neutron Reflectivity. Langmuir 2013, 29 (30), 9324-9334. 19 ACS Paragon Plus Environment

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8. Li, P. X.; Thomas, R. K.; Penfold, J., Limitations in the Use of Surface Tension and the Gibbs Equation to Determine Surface Excesses of Cationic Surfactants. Langmuir 2014, 30 (23), 67396747. 9. Chevalier, Y.; Melis, F.; Dalbiez, J. P., Structure of zwitterionic surfactant micelles: micellar size and intermicellar interactions J. Phys. Chem. 1992, 96 (21), 8614-8619. 10. Zajac, J.; Chorro, C.; Lindheimer, M.; Partyka, S., Thermodynamics of micellization and adsorption of zwitterionic surfactants in aqueous media. Langmuir 1997, 13 (6), 1486-1495. 11. Ma, K.; Li, P. X.; Dong, C. C.; Thomas, R. K.; Penfold, J., Unusual Adsorption at the AirWater Interface of a Zwitterionic Carboxybetaine with a Large Charge Separation. Langmuir 2016, 32 (14), 3340-3347. 12. Thomas, R. K., Neutron reflection from liquid interfaces. Annu. Rev. Phys. Chem. 2004, 55, 391-426. 13. Rauch, H.; Waschkowski, W., Neutron scattering lengths. In Low Energy Neutrons and their Interaction with Nuclei and Matter. Part 1, Schopper, H., Ed. Springer Berlin Heidelberg: Berlin, 2000. 14. Lu, J. R.; Lee, E. M.; Thomas, R. K.; Penfold, J.; Flitsch, S. L., Direct determination by neutron reflection of the structure of triethylene glycol monododecyl ether layers at the air-waterinterface. Langmuir 1993, 9 (5), 1352-1360. 15. Witala, M.; Konovalov, O.; Nygård, K., Relative adsorption excess of ions in binary solvents determined by grazing-incidence x-ray fluorescence. J. Colloid Interface Sci. 2016, 484, 249– 253. 16. Svanedal, I.; Persson, G.; Norgren, M.; Edlund, H., Anomalies in Solution Behavior of an Alkyl Aminopolycarboxylic Chelating Surfactant. Langmuir 2013, 29 (45), 13708-13716. 17. Högberg, I.; Andersson, F.; Hedenström, E.; Norgren, M.; Edlund, H., The Interaction Parameter in Binary Surfactant Mixtures of a Chelating Surfactant and a Foaming Agent. Prog. Colloid Polym. Sci. 2011, 138, 17-20. 18. Culbertson, S. M.; Porter, N. A., Unsymmetrical azo initiators increase efficiency of radical generation in aqueous dispersions, liposomal membranes, and lipoproteins. J. Am. Chem. Soc. 2000, 122 (17), 4032-4038. 19. McMurry, T. J.; Parmelee, D. J.; Sajiki, H.; Scott, D. M.; Ouellet, H. S.; Walovitch, R. C.; Tyeklar, Z.; Dumas, S.; Bernard, P.; Nadler, S.; Midelfort, K.; Greenfield, M.; Troughton, J.; Lauffer, R. B., The effect of a phosphodiester linking group on albumin binding, blood half-life, and relaxivity of intravascular diethylenetriaminepentaacetato aquo gadolinium(III) MRI contrast agents. J. Med. Chem. 2002, 45 (16), 3465-3474. 20. Grote, C. W.; Kun, D. J.; Rapoport, H., Stereocontrolled Synthesis of DTPA Analogues Branched in the Ethylene Unit. J. Org. Chem. 1995, 60 (21), 6987-6997. 21. https://www.ncnr.nist.gov/instruments/ng7refl/ (accessed 12/15/2015). 22. Ankner, J. F.; Majkrzak, C. F.; Satija, S. K., Neutron reflectivity and grazing angle diffraction Journal of Research of the National Institute of Standards and Technology 1993, 98 (1), 4758. 23. Penfold, J.; Thomas, R. K., The application of the specular reflection of neutrons to the study of surfaces and interfaces Journal of Physics-Condensed Matter 1990, 2 (6), 1369-1412. 24. Kienzle, P. A.; O'Donovan, K. V.; Ankner, J. F.; Berk, N. F.; Majkrzak, C. F. http://www.ncnr.nist.gov/reflpak/ (accessed 06/10/2015). 25. Svanedal, I.; Persson, G.; Norgren, M.; Edlund, H., Interactions in Mixed Micellar Systems of an Amphoteric Chelating Surfactant and Ionic Surfactants. Langmuir 2014, 30, 1250-1256. 26. http://www.reflectometry.net/fitprogs/mono.htm (accessed 07/09/2015). 27. Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J., Structure of the Surface of a Surfactant Solution above the Critical Micelle Concentration. J. Phys. Chem. 1993, 97 (51), 1390713913. 28. Xu, H.; Li, P. X.; Ma, K.; Thomas, R. K.; Penfold, J.; Lu, J. R., Limitations in the Application of the Gibbs Equation to Anionic Surfactants at the Air/Water Surface: Sodium 20 ACS Paragon Plus Environment

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Dodecylsulfate and Sodium Dodecylmonooxyethylenesulfate Above and Below the CMC. Langmuir 2013, 29 (30), 9335-9351. 29. Li, N. N.; Thomas, R. K.; Rennie, A. R., Neutron reflectometry of anionic surfactants on sapphire: A strong maximum in the adsorption near the critical micelle concentration. J. Colloid Interface Sci. 2016, 471, 81-88. 30. Prosser, A. J.; Franses, E. I., Adsorption and surface tension of ionic surfactants at the air-water interface: review and evaluation of equilibrium models. Colloids Surf., A 2001, 178 (1-3), 140. 31. Pearson, R. G., Hard and Soft Acids and Bases. J. Am. Chem. Soc. 1963, 85 (22), 35333539. 32. Wulfsberg, G., Principles of Descriptive Inorganic Chemistry. University Science Books: Sausalito, 1991. 33. Keys to Chelation; Form No. 298-717-80; The Dow Chemical Company: Midland, MI, 1974. 34. http://www.reflectometry.net/fitprogs/jakt.htm (accessed 07/10/2015). 35. Tanford, C., Micelle Shape and Size. J. Phys. Chem. 1972, 76 (21), 3020-&.

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Q incident

θi

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reflected

tail region Ni

head group region

chelating surfactant

TOC graphic

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