Effect of Triton X-100 surfactant on the interfacial activity of ionic surfactants SDS, CTAB and SDBS at the air/water interface: A study using molecular dynamic simulations

Highlights

• MD simulations demonstrated that TX-100 surfactant in mixed monolayer at the air/water produce a contraction of the water layer.

• The presence of TX-100 surfactant at the air/water interface produce a reduction of the interfacial thickness of water layer.

• TX-100 surfactant at the air/water interface increment the hydrophobic film thickness in mixed monolayers with ionic surfactants.

• TX-100 surfactant reduce the interaction between hydrophilic headgroup of SDS and SDBS surfactants with the water.

Abstract

Molecular dynamics simulations were carried out to investigate the effect of TX-100 surfactant on the interfacial activity of ionic surfactants SDS, CTAB and SDBS at the air/water interface. Here, interfacial properties, molecular orientation of hydrocarbon chains of ionic surfactant and radial distribution functions between hydrophilic headgroups and water molecules at the air/water interface was evaluated by means of MD simulations. Particularly, TX-100 surfactant produces a reduction of the interfacial film thickness obtained using the water layer and increases the hydrophobic film thickness of the systems. In this investigation, we found that TX-100 surfactant present in mixed monolayers and distributed randomly on the water surface can modify the hydrophilic-lipophilic balance of the monolayer increasing the hydrophobic film thickness and reducing the adsorption of ionic surfactants at the air/water interface. Furthermore, the molecular array of these mixed monolayers obtained by the TX-100 surfactant improve the elasticity of the liquid film, which avoid the rupture of foams reducing the liquid drainage velocity of these systems. Finally, the results suggest that hydrophobic film thickness of the mixed monolayers produce by the TX-100 surfactant can reduce the diffusion of gas molecules from air phase toward the water phase, which increase the stability of foams.

Introduction

Surfactants are molecules that contain a hydrophilic group linked to a hydrophobic part [1]. These species are adsorbed at the air/water and oil/water interfaces producing the reduction of surface and interfacial tensions [2]. These species are widely used in different industrial applications such as enhanced oil recovery [[3], [4], [5]], food processing [6,7], preparation of pharmaceutics compounds [8,9] and personal care products [10,11].

These species are used in preparation of foams, which are colloidal systems that contain gas bubbles dispersed in a continuous phase [12]. In those systems, gas bubbles are separated by liquid films stabilized by surfactant molecules [13]. In these systems, surfactant molecules are adsorbed at the air/water interface with hydrophilic groups located into the water phase and hydrophobic tails oriented to the air phase [14]. Different investigations have demonstrated that ionic surfactants such as sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) produce the highest height of foams compared with nonionic surfactants [15]. At the same time, in foams prepared with mixed nonionic and ionic surfactants, the presence of nonionic surfactants in mixed monolayers enhance or reduce the stability of the foam films with a certain synergism or antagonism [[16], [17], [18], [19]]. Specifically, synergism is present in a system when mixed surfactants produce a considerable reduction of surface tension at lower concentration than those observed by a simple surfactant [1]. Also, synergism is achieved when mixed surfactants form a compact monolayer with high surface elasticity and high surface viscosity, preventing the rupture of thin liquid films that separate the bubbles [1,2].

The evaluation of synergism or antagonism requires of the determination of molecular interactions between mixed surfactants located at the air/water interface, which is difficult to explore from experiments. In this sense, different techniques such as small-angle neutron scattering (SANS), Vibrational sum-frequency generation (SFG) spectroscopy and neutron reflectivity have been used to attempt characterize the molecular interactions, adsorption and conformations of mixed surfactants at the air/water interface [[20], [21], [22], [23], [24]]. Nevertheless, these experimental techniques give incomplete information about molecular arrangement and molecular interactions between mixed surfactants when those are located at the air/water interface, whose behavior of surfactants are important for understanding of the interfacial phenomena.

Triton X-100 (Polyethylene glycol tert-octylphenyl ether, TX-100) is a nonionic surfactant with a great industrial importance used in the preparation of foams [18,19]. This nonionic surfactant contains a hydrophilic chain with 9–10 ethylene oxide units linked to an aromatic ring with a branched hydrocarbon chain. The properties of TX-100 surfactant such as surface activity, micellar morphology, drugs solubilization capacity and phase behavior in presence of ionic surfactants has been studied using experimental techniques [[25], [26], [27]]. Instead, TX-100 surfactant has been used with ionic surfactant SDS and CTAB for the preparation of aqueous solutions, where the synergism between TX-100 surfactant and ionic surfactants has been determined using surface tension measurements [28,29]. Here, SDS/TX-100 and CTAB/TX-100 mixtures have synergism in the efficiency for the reduction of surface tension. Also, these mixtures show a packing compact in the air/water interfacial region with the highest interfacial activity. For instance, interaction parameter between TX-100 surfactant and ionic surfactant sodium dodecyl benzene sulfonate (SDBS) was determined using conductance measurements [30,31]. Here, SDBS surfactant has an attractive interaction with the TX-100 surfactant in the systems studied. In addition, the effect of anionic surfactants on the adsorption of TX-100 surfactant at the air/water interface has been evaluated [32,33]. In this case, an increase of the concentration of ionic surfactants in aqueous solutions cause an increment of the adsorption of TX-100 surfactant at the air/water interface. In addition, TX-100 surfactant has the capacity of producing a high height of foams in aqueous solution and, furthermore, certain studies have been performed to explore the stabilization of foams prepared with TX-100 surfactant in presence of SDS and CTAB surfactants [18,19]. For this study, the foamability was enhanced by the ionic surfactant and the stability by the concentration of TX-100 surfactant. Here, the capability in the process of adsorption, surface concentration and interfacial activity at the air/water interface of mixed surfactants to affect the stability of foams was studied. In other cases, it was observed that the increment of the CTAB concentration in aqueous solutions of mixed CTAB/TX-100 surfactants reduce the height of foams prepared with TX-100 surfactant [30] and the presence of TX-100 surfactant has no influence on the height of foams prepared with SDBS surfactant [30,31].

Additionally, Molecular Dynamics (MD) simulations have been used to study molecular array of single surfactants at the air/water and oil/water interfaces allowing to establish a relationship between molecular structure and interfacial properties [[34], [35], [36], [37], [38], [39], [40], [41]]. For instance, Xu et al. [42] and Klein et al. [43] studied the molecular interaction between alkyl benzene sulfonate (SDBS) and water at the air/water interface. They found that counterions penetrate into the solvation shell of hydrophilic headgroup and reduce the mobility of water molecules in the interfacial region. Also, the degree of branching of alkyl tails affect the average tilt angles of surfactant chains at the air/water interface. Striolo et al. [44] studied the morphology and structural behavior of monolayers of hexaethylene glycol monododecyl ether (C₁₂E₆) and SDS surfactants at the vapor/water interface for different surface coverages. They found that hydrocarbon chains of C₁₂E₆ surfactant are oriented less perpendicular to the interfacial region compared with hydrocarbon chains of SDS surfactant. At the same time, MD simulations have been used to study the molecular interactions between mixed surfactants at the air/water. In this case, Wang et al. [45,46] studied mixed dodecylamine/dodecanol molecules by means of MD simulations with different mixture ratios to evaluate their molecular arrangement and explained the synergistic effect at the air/water interface. They determined the molecular aggregation, orientation, angle distributions, and hydrogen bond distributions of those mixed surfactants at the air/water interface. Also, Wang et al. [47] studied the behavior of mixed cationic dodecylamine/anionic sodium oleate surfactants at different molar ratios at the air/water interface. They found that this monolayer formed by those mixed surfactants have the highest interfacial activity. Furthermore, Du et al. [48] evaluated the stability of multilayer films prepared with 1-dodecanol, sodium dodecyl sulfonate and polyvinyl alcohol with MD simulations. They determined that this system decrease the gas diffusion rate. In other work, Xue et al. [49] studied the synergism of mixed surfactants in monolayers prepared with sodium lauroyl glutamate, dodecyl trimethyl ammonium bromide, nonionic surfactant laurel alkanolamide and sodium dodecyl sulfonate. They evaluated the molecular array and interfacial behavior of those mixed surfactants at the air/water interface. Moreover, Dominguez [50] studied the molecular arrangement of SDS/dodecanol and SDS/hexadecanol mixtures located at the air/water interface. He demonstrated that hydrocarbon chains of alcohols are more ordered than hydrocarbon chains of SDS surfactant, while alcohol molecules are closer to each other at the interface. Particularly, in these works were studied the molecular arrangement and interactions between mixed surfactants with hydrophilic groups of the same molecular size at the air/water interface. In these cases, the hydrophilic group of nonionic surfactants was located between hydrophilic group of ionic surfactants forming hydrogen bonds with the water molecules linking to the hydrophilic group of ionic surfactants. However, in systems with mixed surfactants at the air/water interface, the exploration of molecular interactions between ionic surfactants with nonionic surfactants that have hydrophilic groups with polymeric characteristics of great size such as TX-100 surfactant have not been widely evaluated by means of MD simulations. In fact, we have observed that the molecular aggregation in water of this representative nonionic surfactant has been studied using MD simulations [[51], [52], [53]].

Experimentally, the ionic surfactants such as SDS, CTAB and SDBS have a high foamability that depend of their adsorption rate, interfacial activity and the total amount of surfactant molecules adsorbed at the air/water interface [[15], [16], [17], [18], [19]]. These studies have been carried out with high concentrations of ionic surfactants at the interface above of the CMC (air/water interface saturated with surfactants molecules). However, foams produced with these ionic surfactants have poor stability and very short lifetime. Like this, foams stability prepared with these ionic surfactants can be enhanced by the addition of polymeric nonionic surfactants at the air/water interface. At the same time, the distribution of these polymeric nonionic surfactants in the interfacial region is important to increment the foams stability prepared with these ionic surfactants. Therefore, the evaluation of the effect of TX-100 surfactant on the interfacial activity of ionic surfactants at the air/water interface and the distribution of this representative polymeric nonionic surfactant at the air/water interface is important for the understanding of the mechanism involved in the stability of these foams. For this reason, we explore the structural properties and adsorption behavior of SDS, CTAB and SDBS surfactants at the air/water interface as function of the number of TX-100 surfactant (representative nonionic surfactant) present in mixed monolayers by means of MD simulations to assess the influence of this nonionic surfactant on the foams stability prepared with these ionic surfactants. The evaluation of this effect was carried out for the ionic surfactant/TX-100 surfactant mixture ratios equal to 3:1, 2:1 and 1:1 that correspond to surface coverage of 78 Ų/molecule, 69 Ų/molecule and 50 Ų/molecule, respectively. In this study, solely, structural properties and molecular interactions of mixed surfactants at air/water interface were explored using mass density profiles, interfacial film thickness, molecular orientation of carbon chains and radial distribution functions between water molecules and the hydrophilic group of ionic surfactants. The determination of the interfacial tension was not considered in this study.