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Effect of Added Tetraalkylammonium Counterions on the Dilational Rheological Behaviors of N-Cocoyl Glycinate

: Ion specific effect, which is also known as Hofmeister effect, has been reported in numerous systems including ionic surfactant aggregates. Acyl amino acid surfactants have attracted growing attentions in the field of novel surfactants research due to their environmentally benign characteristics. The objective of this study was to investigate the effect of different salts containing NH 4+ and tetraalkylammonium (TAA + ), where alkyl = methyl (TMA + ), ethyl (TEA + ), and propyl (TPA + ), cations on the dilational rheological properties of interfacial film are stabilized by potassium N-cocoyl glycinate (KCGl). The interfacial behaviors were studied using oscillating drop shape analysis method. The interfacial tensions (IFTs) and dilational rheological parameters results illustrate that KCGl in the presence of salts has better interfacial activity and stronger intermolecular interaction, indicating that added cations contribute to denser molecular packing at oil-water interface. Ion specific effects were observed in the system. Among the cations, KCGl shows highest dilational modulus in the presence of NH 4+ . The overall interaction between cations and headgroups of KCGl decreases in the sequence NH 4+ >TMA + >TEA + ≈TPA + , which follows Hofmeister series. The increasing hydrophobicity of TAA + prevents the interaction between cations and KCGl’s headgroup, and therefore prevent amphiphiles from packing closely at interface. The results present a theoretical origin for useful application of KCGl in cosmetics, petroleum and daily chemical industries.


Introduction
Ion specific effect, which was first reported by Hofmeister in 1888 1 , has been reported in tons of systems with charged interfaces or surfaces, including surfactant aggregates, supramolecular structures, enzymes, and polymers over the past several decades 2 5 . Counterions in added salts are an important factor affecting the intermolecular interaction, surface activity, micellization, and aggregate morphological transition of ionic surfactants. Various organic and inorganic counterions are often used because they not only affect the interfacial behaviors of the surfactants, but also affect their self-aggregation structures 6 11 .
Added salts affect the interfacial behaviors of ionic surfactants in different ways such as, electrolytes increase their oil-soluble nature and enhance their diffusion to oilwater interface from bulk or stabilize oil-water interface by Abstract: Ion specific effect, which is also known as Hofmeister effect, has been reported in numerous systems including ionic surfactant aggregates. Acyl amino acid surfactants have attracted growing attentions in the field of novel surfactants research due to their environmentally benign characteristics. The objective of this study was to investigate the effect of different salts containing NH 4 + and tetraalkylammonium (TAA + ), where alkyl = methyl (TMA + ), ethyl (TEA + ), and propyl (TPA + ), cations on the dilational rheological properties of interfacial film are stabilized by potassium N-cocoyl glycinate (KCGl). The interfacial behaviors were studied using oscillating drop shape analysis method. The interfacial tensions (IFTs) and dilational rheological parameters results illustrate that KCGl in the presence of salts has better interfacial activity and stronger intermolecular interaction, indicating that added cations contribute to denser molecular packing at oil-water interface. Ion specific effects were observed in the system. Among the cations, KCGl shows highest dilational modulus in the presence of NH 4 + . The overall interaction between cations and headgroups of KCGl decreases in the sequence NH 4 + >TMA + >TEA + ≈TPA + , which follows Hofmeister series. The increasing hydrophobicity of TAA + prevents the interaction between cations and KCGl's headgroup, and therefore prevent amphiphiles from packing closely at interface. The results present a theoretical origin for useful application of KCGl in cosmetics, petroleum and daily chemical industries. increasing their adsorption through electrostatic interaction with counterions in added electrolytes 12 . Tetraalkylammonium ions are a class of kosmotropic cations, and their effects on surfactants are even more pronounced 12 17 . Interfacial tensions IFT can be reduced to an ultralow value with added tetrabutylammonium bromide 13 . Yuan S. et al. have studied interfacial aggregations of different TAA counterions with sodium dodecyl sulfate SDS and found that mixed interfacial layers are formed with methyl groups of TAA penetrate into the monolayer 17 . Therefore, it is of great importance to study the specific effect of different TAA on the interfacial behaviors of ionic surfactants.
To study the adsorption progresses of surfactants, dynamic interfacial tensions are often used. Only the adsorption amounts cannot give a detail description about the interaction between the molecules at interface. Cur-2 rently, the adsorption of surfactants at interface is illustrated by their interfacial dilational rheology properties 18 23 . Information of the molecular packing at interfacial absorption film is hidden in the interfacial properties, and can be decoded by studying the influence of many factors, i.e., disturbance frequency, time, concentration, interfacial pressure, on the interfacial rheology properties 24 26 . Therefore, for the assessment of interfacial behaviors of amphiphile systems, like surfactants and proteins, interfacial dilational rheological experiments are used and have been widely studied in foam and emulsion stability 27 30 .
Amino acids bearing both carboxyl and amine groups are excellent natural building blocks used for the preparation of surfactants 31 . Acyl amino acid surfactants are a class of environmentally friendly surfactants 32 34 . They have been widely applied to various industry, including personal care, cosmetics, and pharmaceutics 32, 35 41 .
Here, we study the interfacial dilational rheological properties of acyl amino acid surfactant in the presence of various ammonium and tetraalkylammonium salts. The interfacial dilational rheological behaviors of a representative acyl amino acid surfactant potassium N-cocoyl glycinate KCGl in the presence of different salts, namely ammonium chloride NH 4 Cl , tetramethylammonium bromide, where alkyl methyl TMAB , ethyl TEAB , and propyl TPABr , are investigated respectively by the oscillating drop method. By analyzing the results of the above salts, the influences of ammonium and tetraalkylammonium ion on the interfacial behaviors of KCGl are revealed. Table 1 lists the chemicals used in the experiments. The chemicals are used as received without further purification. The potassium N-cocoyl glycinate average molecular weight 522.82 g/mol is bought as a 30 wt surfactant solution. Water used in these experiments was ultrapure water resistivity 18.2MΩ.cm . n-Decane with purity of 99.0 was used as oil phase.

Experimental
A response in interfacial tension IFT is observed when the interfacial area is disturbed by a periodic compression and expansion. Gibbs has defined the interfacial dilational modulus, ε, to depict the IFT response with the changes of area, which can be expressed by Equation 1: Where γ represents IFT, and A denotes the interfacial area. Dilational modulus, ε, reveals the capability to oppose the deformation of the interface. The IFT and interfacial dilational rheology data are all measured by the oscillating drop shape analysis method, with ODG-20 from Data Physics Instruments GmbH, as we described elsewhere 42 . An oil drop injected from a syringe into an optical glass cuvette containing surfactant aqueous solution was captured by a CCD camera and digitized by Fourier transform analysis. The glass cuvette was always thermo stated at 25 0.1 . The profile of the oil droplet was fitted to the Laplace equation to determine the IFT. The accessible frequencies between 0.005 and 0.1 Hz for the drop oscillations were carried out in sinusoidal mode and the oscillations amplitude was 10 ΔA/A .

Results and Discussion
3.1 Dynamic IFT and dilational moduli of KCGl in the presence of different salts containing tetraalkylammonium cations MR 4 , R 1-3 and NH 4 Figure 1 shows the results of dynamic IFTs and dilational moduli of KCGl with added 5 10 2 mol/L NH 4 Cl, or tetraalkylammonium bromide TAABr , where alkyl denotes methyl TMABr , ethyl TEABr , and propyl TPABr . The concentration of KCGl varies from 10 7 -10 3 mol/L. The interfacial adsorption of KCGl molecules increases over time as soon as an oil-water interface was created, resulting in the gradual decrease in the values of IFT. The intermolecular interactions become stronger with the enhanced interfacial coverage and cause higher IFT gradient when the interface deforms. Therefore, the interfacial dilational Table 1 Purities and suppliers of chemicals.   Fig. 2. As expected, IFTs decrease gradually with the increase of surfactant concentration. The oil-water IFT versus KCGl concentration curve in pure water is presented as a dotted line. Interestingly, the IFTs for KCGl in electrolyte solutions are clearly lower than that in pure water, demonstrating that salts have improved the interfacial activity of KCGl and contribute to denser molecular packing at oil-water interface. As reported in previous researches, this may be attributed to the screening electrostatic repulsion between anionic headgroups of KCGl molecules by the counterions, which increases the interfacial concentration of the surfactants 13, 44 46 .
However, as shown in Fig. 2, the ability of KCGl to reduce IFT varies significantly in the presence of different salts, suggesting the hydrophobicity and size of the cations may have significant influence on the interfacial behaviors of KCGl. Ammonium ion, NH 4 , as the most hydrophilic cation, performs differently at low and high KCGl concentration. At low KCGl concentrations, the effect of NH 4 on the IFT is less obvious than other cations because it has less tendency to adsorb to the interface compared to other hydrophobic cations bearing methyl and methylene groups. Above 5 10 6 mol/L KCGl concentrations, where the interfacial KCGl concentrations are also enhanced, NH 4 is mixed adsorbed with anionic headgroups of KCGl at interface. The electrostatic repulsion between headgroups of surfactant is screened by NH 4 , which contributes to a denser adsorption of the KCGl molecules at interface. The IFT values for KCGl in the presence of TMA and TEA are higher than these in the presence of NH 4 . Although this can be explained by the increasing of cation size, to some extent, prevent surfactant molecules from packing closely, there are other possibilities, Scheme 1. First, based on Collins concept 47, 48 , the strongly hydrated carboxylate headgroup tends to interact strongly, and form possibly ion-pairs, with strongly hydrated cations, than weakly hydrated cations. Therefore, the interaction between KCGl headgroup and NH 4 is stronger than other hydrophobic cations. Second, possible hydrogen bonding 49 between NH 4 and amino acid headgroups may also contribute to the packing between amphiphiles with added NH 4 . Thus, the IFT at high surfactant concentration is lower for ammonium ions compared to TMA and TEA . However, TPA , with longer alkyls, itself has certain interfacial activity. To some extent, TPA behaves as co-surfactant and mixed adsorb together with KCGl. Therefore, TPA also decreases the IFT to a very low value.
Even though very low IFT was observed for KCGl in the presence of either NH 4 or TPA , the interfacial arrangement of KCGl is essentially different, which can be further demonstrated by the interfacial dilational moduli. Figure 3 shows the changes of the equilibrium dilational moduli with increasing KCGl concentration. Usually, with increasing surfactant concentration, the interfacial dilational modulus increases to a maximum followed by decreases. The initial increase of interfacial surfactant concentration results in the deformation of interface and stronger interactions between molecules at interfaces, thus the IFT values increase at first. However, the diffusion exchange of surfactant molecules between interfacial region and bulk region also increases with increasing bulk surfactant molarities. As a result, the rapid diffusion of surfactant molecules to the interface can reduce the IFT gradient and leads to a decrease of the dilational modulus at high surfactant concentrations. Thus, as surfactant concentration increases, the modulus runs through a maximum 42,50 . Hydration shells are formed around the headgroups of KCGl that adsorbs at interface. Consequently, the molecules organize with long distance and the intermolecular interaction of the surfactant molecules is not significant. In the absence of salts, the maximum dilational modulus of pure KCGl determined by us is only 18.5 mN/m. Prominent maximum increase in dilational modulus is observed in the presence of NH 4 and TMA with KCGl. As discuss above, the cations that have smaller size and less hydrophobicity have stronger interaction with the KCGl and cause surfactant molecules to pack tightly at interface by screening the electrostatic repulsion between the headgroups. As a result, the interactions of the molecules at interface, namely the Van der Waals interaction between hydrophobic tails and hydrogen bonds between amidos, are enhanced, which results in higher dilational modulus. However, as for TEA and TPA , the interactions between added cations and headgroups are weak. Hence, the relaxation processes, such as diffusion exchange, play more significant roles in determining the interfacial properties and dilational modulus begins to decrease. Therefore, KCGl with added TEA and TPA shows lower dilational modulus than other counterions which can be further proved from the viscoelasticity of the adsorption films.

Frequency dependence of the interfacial dilational
properties for KCGl in the presence of added salts The interfacial dilational moduli versus oscillation frequency for KCGl with NH 4 , TMA , TEA and TPA are characterized in Fig. 4, because the changes of dilational moduli with oscillating frequency further characterize the viscoelasticity of interfacial film 20 . The experiments were carried out by varying the frequency in the range of 0.005 -0.1 Hz. The salt concentration was fixed at 5 10 2 mol/L, and the KCGl concentration varies from 10 6 to 10 3 mol/L.
As increasing frequency, the dilational moduli of all systems increase as shown in Fig. 4. At low frequencies, the dilational modulus is small, indicating the time for surfactants to eliminate the IFT gradient caused by the deformation in area is sufficient. But at higher frequencies, the dilational modulus is also higher. The interfacial deformation is much faster and there is no enough time for the recovery of interfacial film. As oscillating frequency increases at moderate rate, change in interfacial area becomes faster than the relaxation progresses, making the IFT gradient become increasingly difficult to eliminate.

Interfacial dilational modulus as a function of interfa-
cial pressure for KCGl in the presence of added salts To determine the interfacial behaviors of amphiphiles, the changes of interfacial dilational modulus with interfacial pressure are usually evaluated 28,42,51 . Therefore, the interfacial dilational moduli versus interfacial pressure for the KCGl/salts systems have been illustrated in Fig. 5.
Usually, under fixed interfacial pressure, the interfacial dilational moduli of amphiphiles decrease as their concentrations increase. At high surfactant concentration, the strong diffusion exchange between interfacial region and bulk region decreases the dilational modulus value. In the presence of salts, the characteristics of KCGl s dilational modulus versus interfacial pressure profiles are divided into two notable regions. At low KCGl concentrations, dilational modulus vs. interfacial pressure coincide on a single curve, representing the interfacial properties are primarily decided by the interactions between molecules, but not the diffusion-exchange process. As increasing surfactant concentration, the diffusion exchanges of surfactant molecules between interfacial region and bulk region becomes more and more significant. Thus, the dilational modulus versus  interfacial pressure curves starts to disperse from the main branch at higher concentrations. For KCGl in the presence of different counterions, the dispersion degree of the curves increases with the length of the alkyl/hydrogen arms on the couterions, suggesting the diffusion of surfactant monomers between interfacial region and bulk region becomes more significant, and the interfacial film becomes more viscoelastic.

Conclusions
In the current work, effects of different salts NH 4 Cl, TMABr, TEABr and TPABr on the interfacial dilational rheological properties of KCGl have been investigated. The results demonstrate that the participations of salts in KCGl solutions have strongly decreased the IFTs and increased the dilational moduli of the system, due to the interaction between the added cations and surfactant headgroups. Interestingly, various cations bearing various sizes and hydrophilicities behave differently. In case of relatively small and hydrophilic cations, e.g., NH 4 , KCGl packs closer at interface, and decreases the diffusion between interfacial region and bulk region. However, those relatively large and hydrophobic cations, such as, TEA and TPA , interact weakly with surfactant headgroups and fail to induce a closer arrangement of molecules at interface. Relaxation processes are easy to take places and thus play an important role in determine interfacial properties. Therefore, viscoelastic films are formed in the presence of TEABr and TPABr. The experimental data of KCGl with salts may provide a hypothetical basis for its useful application in detergents, pharmaceutical, cosmetic, petroleum and daily chemical industries.