Interfacial interactions between oleic acid and betaine molecules at decane-water interface: A study of dilational rheology

https://doi.org/10.1016/j.molliq.2020.113784Get rights and content

Highlights

  • The betaine adsorption film is determined by both the fast diffusion-exchange and slow re-arrangement process.

  • 18PC film is tighter than that of 18PS because the size of carboxyl is smaller than sulfonyl.

  • Oleic acid and betaine surfactants can form mixed adsorption films with less strength but more elastic.

  • Oleic acid/18PS film is tighter than oleic acid/18PC film.

Abstract

The interfacial interactions between organic acid and betaine molecules play an important role in reducing interfacial tension (IFT). To deeply detect the arrangements of organic acid and betaine molecules at the interface, the dilational rheological properties of mixed adsorption films formed by oleic acid and octadecyl carboxyl betaine (18PC) or octadecyl sulfobetaine (18PS) were investigated using drop shape analysis method at the decane-water interface. The influences of aging time, oscillation frequency and surfactants concentration were detected. The experimental results show that the properties of 18PC and 18PS adsorption film are controlled by both the fast diffusion-exchange process between the bulk and the interface and the slow re-arrangement process in the interface. 18PC film is tighter than that of 18PS because the size of carboxyl is small than sulfonyl. Oleic acid and betaine surfactants can form mixed adsorption films with less strength caused by the rapid transfer of oleic acid between the bulk and the interface, but more elastic due to the enhancement of the intermolecular interaction. In this case, the oleic acid molecules are easier to insert into the spaces between 18PS molecules on the interface, which results in a more tight mixed film than oleic acid/18PC film.

Introduction

Crude oil is not only one of the most important energy supplies of the world, but also an indispensable raw material in people's daily life and production. At present, many studies have revealed that the interfacial tension (IFT) between oil and water [1,2] and the formation of crude oil emulsions [3] are two significant important factors in the investigation of enhanced oil recovery (EOR). As known, the ultralow IFT (<10−2 mN/m) between the displacement fluid and crude oil is beneficial to the increase of capillary number (Nc), which can greatly enhance the efficiency of oil recovery [2,4,5]. The stable O/W emulsion can improve the mobility ratio between oil and water and improve the sweep coefficient, thereby increasing the efficiency of oil recovery [[6], [7], [8]]. Ultralow IFT and stable emulsions can be obtained with the employment of appropriate surfactants in the EOR process [[9], [10], [11], [12], [13]].

Zwitterionic surfactants, containing both positive and negative charges in one hydrophilic part, have attracted many scholars' attentions for their special structures [14]. Compared with conventional surfactants, zwitterionic surfactants present high foam stability, good resistance to salt and temperature, excellent compatibility with other surfactants, good biodegradation and low toxicity [[15], [16], [17], [18], [19], [20]]. Betaine is an important zwitterionic surfactant applied in the personal care product industry for the traits of less irritation to skin and eyes and low toxicity [[21], [22], [23]]. Besides, researchers have focused on the application of the zwitterionic surfactants in enhanced oil recovery (EOR), especially in high-temperature and high-salinity reservoirs, due to its high interfacial activity [24,25].

In addition, studies established that the active fractions present in crude oil, such as asphaltenes, resin and acidic fraction, are key factors in the reduction of IFTs and the formation of emulsion as natural surfactants [16,[26], [27], [28], [29]]. Among the active components, the acid fraction is considered to be the most interfacial activity agent and contributes the most to the reduction of interfacial tension [30,31].

The reports about the interaction between betaines and acid fraction are lacking till now. Zhou [32] studied the effect of fatty acids on interfacial tensions of novel sulfobetaine solutions and found that the fatty acid and sulfobetaines can form a tight film at the interface, which results in a decrease in the IFT. They also pointed out that the whole hydrophilic part of the betaine molecule is almost flat at the interface and the betaine, BSB, with larger sized hydrophobic part can form a more compact adsorption film than linear betaine, ASB. Cao [10] and Zhou [26] studied the effects of crude oil fraction on the IFTs of betaine solutions and found that crude oil fractions, especially acidic fractions and resins, have a synergistic effect with liner betaine and form a mixed adsorption film at the interface, which can produce ultralow IFTs. However, an obvious antagonistic effect exists between the active fractions and branched betaines, increasing the IFT. It should be pointed out that interfacial tension can't comprehensively reflect the properties of the adsorption layer, especially the structural changes.

Interfacial dilational rheology is a credible method to investigate the nature of adsorption layer, by which one can obtain the information about the microcosmic structure of interfacial film [33,34]. The interfacial dilational rheological properties of betaine solutions have been studied by our group and the experimental results ensure the parallel orientation of hydrophilic part at the interface [35,36]. In this paper, the behavior of betaine and oleic acid molecules at the decane-water interface have been studied by the dilational rheological method, aiming to understand the mechanism responsible for their interfacial interaction.

Section snippets

Theoretical background

The interfacial dilational modulus ε is defined as a change in the interfacial tension γ for a small relative change of interfacial area A at constant temperature. It is a method to measure the interfacial tension response to changes in area and is given by:ε=dlnAwhere ε is the dilational modulus, γ is the interfacial tension and A is the interfacial area. The dilational modulus can also be presented as a complex function, in which real part εd represents the storage modulus, called the

Materials

The betaine surfactants, with different polar groups, employed in this paper were synthesized in our laboratory, which purity checked by 1H NMR was above 90%. The structures and abbreviations are listed in Scheme 1. Compare with commercial betaines, 18PC and 18PS have hydrophilic parts with larger sizes, which may leads to the synergism with different organic surface-active components in the oil phase. All the solutions in our experiments were prepared with ultrapure water (resistivity

Equilibrium value of interfacial tension

For the two systems of decane/aqueous and decane/oleic acid/aqueous, the influence of surfactant concentration on the interfacial tension (IFT) was firstly investigated and the results present in Fig. 1.

As shown in Fig. 1, the effects of two betaine surfactants, 18PC and 18PS, on the IFT of decane/water interface are quite similar, which may come from their similar molecular structures. As the increase of bulk concentration, the number of surfactant molecules adsorbed onto the interface

Conclusion

In the present work, we investigated the dilational behaviors of mixed films formed by betaine surfactants and oleic acid at the water-decane interface. Based on the experimental results, the following conclusions can be drawn:

The two betaine surfactants of 18PC and 18PS have a remarkable interfacial activity and can reduce the interfacial tension to about 10 mN/m at the concentration of 5 × 10−6 mol/L and 3 × 10−6 mol/L for 18PC and 18PS, respectively. The nature of betaine adsorption film is

CRediT authorship contribution statement

Huan-Quan Sun: Project administration, Conceptualization, Writing - review & editing, Funding acquisition. Zhao-Yang Guo: Writing - original draft. Xu-Long Cao: Conceptualization, Funding acquisition. Yang-Wen Zhu: Conceptualization, Funding acquisition. Bin-Lin Pan: Conceptualization. Miao Liu: Investigation. Lei Zhang: Visualization. Lu Zhang: Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank financial support from the National Science & Technology Major Project of China (2016ZX05011-003) and National Natural Science Foundation of China (21703269).

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