Yinglu Gu,Shuang Chen*,Hui'e Liu,Yubin Wang,Pingping Zhou

State Key Laboratory of Heavy Oil Processing,China University of Petroleum(East China),Qingdao 266580,China

Keywords:Gemini surfactant Monovalent anions Micro-emulsion Phase behavior Solubilization

A B S T R A C T Micro-emulsion usually consists of water,oil,surfactants and co-surfactants,and each component has an effect on the phase behavior and solubilization of the micro-emulsion.When the surfactant in the micro-emulsion system is quaternary ammonium cationic Gemini surfactant,the surfactant mainly combines with the anions in the salt.With the increase of salt concentration,the phase transformation of Winsor I→ Winsor III→ Winsor II occurred,but the optimum salinity and salt width are different because of the type of salt.The effects of 5 different kinds of monovalent anions,including C6H5SO3-,I-,Br-,NO3-and Cl-,on the phase behavior and solubilization of quaternary ammonium cationic Gemini micro-emulsion are researched by Winsor phase diagram.It is found that the effects of organic anions C6H5SO3-and I-on the phase behavior and solubilization of quaternary ammonium cationic Gemini micro-emulsion are most significant,and the effects of Br-,NO3-and Cl-are less significant.Meanwhile,when the optimum solubilization is achieved,the amount of sodium benzoate is the least,indicating that the organic anion has stronger self-organization behavior with quaternary ammonium cationic Gemini surfactants.

1.Introduction

Micro-emulsion is composed of water,oil,surfactants and cosurfactants.Its appearance is transparent or translucent.It's a thermodynamically stable dispersion system,which has the characteristics of ultrahigh capacity of solubilization and ultra-low interfacial tension[1,2].According to the characteristics of different phases,the micro-emulsion can be divided into Winsor I,Winsor II,Winsor III and Winsor IV.The upper phase of Winsor I is oil phase,and the lower phase is O/W microemulsion.The upper phase of Winsor II is W/O micro-emulsion,and the lower phase is water phase.The upper phase of Winsor III is the oil phase,the medium phase is micro-emulsion phase and the lower phase is water phase.Winsor IV is a uniform single micro-emulsion system[3].The phase of the micro-emulsion system can be converted by changing some factors,such as temperature,the concentration and type of salts in the water phase and the concentration of the surfactants.The addition of inorganic salts in aqueous solution can shield the charge of the ionic head groups of the surfactants,resulting in its adsorption and accumulation enhanced in the gas/liquid interface[4].The commonly used monovalent salts include sodium chloride(NaCl)and sodium bromide(NaBr).Yuan Ying et al.[5]investigated the effects of different kinds of inorganic salts on sodium dodecyl benzene sulfonate(SDBS)micro-emulsion phase behavior by using Winsor phase diagram and ichthyomorphic phase diagram.The results showed that the phase transition of Winsor I→ Winsor III→ Winsor II occurred with the increase of the concentration of inorganic salts or alcohol in the micro-emulsion system.But the phase transition can occur when the amount of the organic counterions added in the aqueous solution is far less than that of the inorganic counter-ions.It is because that the strong combination between organic counter-ions and cationic surfactants makes the micelles grow faster and produce wormlike micelles.As a result,its aqueous solution has significant viscoelastic properties[6,7].The common organic salts are sodium benzo(or naphthalene)sulfonic acid sodium salt[7]and its derivatives[8],sodiumbenzoate and its derivatives[9],sodiumsalicylate and its derivatives[10],etc.Jiang Rong[11]found that benzene sulfonic acid sodium salt(SNzS)and naphthalene sulfonic acid(SNphS)were strongly combined with C12-s-C12·2Br.Thereby,the hydrophobicity of Gemini surfactant molecules is increased,and its adsorption in the gas/liquid interface and aggregation in solution are enhanced.

Compared with the traditional single chain surfactants,Gemini surfactant has many excellent properties due to its special double continuous structure,such as ultra-high capacity of solubilization and ultra-low interfacial tension[12].Gemini surfactant is a new surfactant that connects 2 or more than 2 traditional surfactants in the hydrophilic group or near the hydrophilic group.Its molecular structure is shown in Fig.1.Among Gemini surfactants,Cm-s-Cm·2Br with polymethylene as the attachment chain of quaternary ammonium cationic Gemini surfactant has been studied both here and abroad,but until now,the research on the effects of the type and concentration of monovalent counter-ions on the phase behavior and solubilization of Gemini micro-emulsion has seldom been reported.In this paper,the effects of NaBr,NaCl,NaI,NaNO3and sodium benzo sulfonate on the solubilization and phase behavior of quaternary ammonium cationic Gemini micro-emulsion under different oil phase systems were investigated.That is to say,the effect of different kinds of univalent ion on the system under the oil phase of same alkyl carbon number and the effect of the same ion on the system under the oil phase of different alkyl carbon number were investigated.

Fig.1.The molecular structure of Gemini surfactant.

2.Experimental

2.1.Experimental reagents and instruments

The instruments and reagents used are as follows:A 50 ml plug test tube,electronic balance,METTLER TOLEDO AL204/00(Calibrating staff division value:0.0001 g),constant temperature water-bathing,10 ml buret,a new type of viscoelastic Gemini surfactant,purity 98%,n-butanol(CH3(CH2)3OH),AR,cyclohexane(C6H12),AR,n-hexane(CH3(CH2)4CH3),AR,n-heptane(CH3(CH2)5CH3),AR,n-octane(CH3(CH2)6CH3),AR,sodium chloride(NaCl),AR,sodium bromide(NaBr),AR,sodium iodide(NaI),AR,benzene sulfonic acid sodium salt(C6H5SO3Na),purity 99%,Deionized water and so on.The EACN values of the hydrocarbons used in the experiment are listed in Table 1.EACN is the equivalent number of alkanes in the oil phase.When measuring the interfacial tension between oil and surfactant system,if the interfacial tension between oil and surfactant system is equal to the interfacial tension between n-alkanes and surfactant system,the carbon number of n-alkane is called equivalent alkane carbon number[13,14].

Table 1The EACN of hydrocarbon used in the experiment

2.2.Experimental methods

Water and oil with a volume ratio of 1:1 were added to the 50 ml plug test tube,then NTX-05 and n-butanol were added to the plug test tube too,making the concentration(based on the total volume of deionized water and oil phase)of NTX-05 0.05 g·ml-1and the mass fraction(based on the total mass of deionized water and oil phase,the same below)of n-butanol 11.97%.The salinity was scanned by NaBr,NaCl,NaI,NaNO3and sodium benzo sulfonate,and the volume of each phase was recorded after the system was stable.Then,the amount of inorganic salt was gradually increased and the above operation was repeated.The volume of solubilized oil or water of per gram of surfactant in micro-emulsion is defined as the solubilization parameter SP[15,16],which is shown in the formula(Eqs.(1)and(2)).

Vorefers to the volume of solubilized oil in the medium microemulsion,ml.msrefers to the mass of surfactants,g.

Vwrefers to the volume of solubilized water in the medium microemulsion,ml.msrefers to the mass of surfactants,g.

As shown in Fig.2,with the salinity(the concentration of monovalent anions,the same below)as the transverse coordinates and the SP as the longitudinal coordinates,the curves of the solubilization parameters were plotted.When Winsor I micro-emulsion is formed,the phase of micro-emulsion will change from Winsor I to Winsor III to Winsor II with the increase of salinity(the same as anionic charge concentration).In the process of phase conversion,the solubilization parameter SPoincreases gradually,and the solubilization parameter SPwdecreases gradually.When the volume of the solubilized oil in the medium microemulsion is equal to the volume of the solubilized water,that is,SPo=SPw,the micro-emulsion reaches the maximum solubilization capacity and the lowest interfacial tension.At this time,the solubilization parameter is called the optimum solubilization parameter which is recorded as SP*,and the unit is ml·g-1.And the salinity is called the optimum salinity which is recorded as S*,and the unit is mol·L-1[17].In addition,the optimum solubilization parameters are different when the cations of the inorganic salts are the same and the anions are different.

Fig.2.The relation between solubilization parameters and charge concentration of monovalent anions.

The effects of different kinds of monovalent anions on the phase behavior and solubilization of quaternary ammonium cationic Gemini micro-emulsion system were investigated by the Winsorphase diagram[18].With the concentration of monovalent anions as the transverse coordinates and the volume fractions of each phase when the equilibrium is balanced as the longitudinal coordinates,the Winsor phase diagram is plotted,which is shown in Fig.3.

It can be seen from Fig.3 that when the concentration of monovalent anionic charge increases continuously,the phase of the micro-emulsion system changes from Winsor I to Winsor III to Winsor II.In Fig.3,c1is the point that the Winsor III micro-emulsion has just formed,and c2is the point that the Winsor III micro-emulsion has just disappeared,and the D-value between the two points of c1and c2is called the salt width,which is represented by Δc,and the value of the abscissa of c1is called the minimum salinity.From the Winsor phase diagram which is scanned by salt,the minimum salinity c1,the salt width Δc and the changing of system phase and phase volume with the increase of salinity can be seen clearly.

3.Results and Discussion

Through the analysis of the Winsor phase diagram,the optimum solubilization parameter SP*,salt width Δc and initial salinity c1,the effects of NaBr,NaCl,NaI,NaNO3and benzene sulfonic acid sodium salt on the phase behavior of cationic quaternary ammonium salt Gemini micro-emulsion were researched.

Fig.3.Winsor phase diagram.

3.1.Winsor phase diagram

The Winsor phase diagrams of the Gemini micro-emulsion systems with cyclohexane,n-hexane,n-heptane and n-octane as oil phase are shown in Fig.4.

Fig.4.Winsor phase diagram of the Gemini–oil–water–n-butanol micro-emulsion system.■ — C6H5SO3Na;—NaBr;—NaNO3;—NaCl;—NaI.

From Fig.4 we can see that in the micro-emulsion systems of cyclohexane,n-hexane,n-heptane or n-octane as oil phase,the phase transition of Winsor I to Winsor III to Winsor II will take place with the increase of the concentration of different kinds of anionic charge.It is because the charges of the micro-emulsion droplets formed by the quaternary ammonium cationic surfactants are positive.The higher the anion concentration,the greater the pressure effect on the electric double layer of the micro-emulsion droplets formed by the cationic surfactants.So the electrostatic repulsion between droplets of microemulsion is reduced,which makes it easier to split the phase.As a result,the performance of single-phase micro-emulsion is stronger and the solubilization capacity of the system is greatly improved.In addition,the effects of different kinds of salts on the phase behavior of the micro-emulsion system are different due to the fact that different kinds of anions have different effects on quaternary ammoniumcationic surfactants.In addition to the phase of cyclohexane as oil phase,the influences of different kinds of monovalent salts on the micro-emulsion phase behavior are in the order of C6H5SO3Na＞NaBr＞NaNO3＞NaI＞NaCl,which is due to the EACN value of cyclohexane being 1.When the value of EACN is in the range of 1–3,the content of alcohol needed to reach the best middle phase decreases with the increase of the EACN value.And the volume of solubilized oil phase/water phase in micro-emulsion phase increases gradually.When EACN＞6,the larger the value of EACN,the less alcohol it needs to reach the best middle phase,but the volume of water and oil solubilized by the medium micro-emulsion will be smaller[19].The effects of different kinds of salts with different EACN on the phase behavior of micro-emulsion were in the order of C6H5SO3Na＞NaI＞NaBr＞NaNO3＞NaCl.When the kinds of monovalent salts are different,the minimum salinity for the formation of Winsor III micro-emulsion and salt width are different,which can be seen in Table 2.

As can be seen from Table 2,in the micro-emulsion of different types of oil,when monovalent salts have the same cations and different anions(NaCl,NaBr,C6H5SO3Na,NaNO3,NaI),the initial salinity c1andthe salt widthΔc are different.At the same time,initial concentration c1and salt width Δc under the condition of different kinds of monovalent salts are in the orderofcyclohexane＜n-hexane＜n-heptane＜n-octane,which shows that the monovalentanion has a stronger influence on the alkanes with smaller EACN values than the alkanes with larger equivalent alkyl carbon number(EACN value).It is because with the increase of the number of carbon atoms in alkanes,the cohesive energy between the oil molecules and the cohesive energy between the surfactant molecules and the oil molecules both increase,but the cohesive energy between the oil molecules increases more rapidly[20].Therefore,when the number of alkyl carbon EACN increases,the cohesive energy between the oil molecules is larger than that between the surfactant molecules and the oil molecules,resulting in the decrease of the ability to form a micelle.At the same time,the increase of the length of carbon chain and the number of alkyl carbon EACN decreases the ability of the oil molecules to penetrate the interfacial film,thus reducing the lipophilicity of the interfacial film[21,22].As a result,more surfactants and inorganic salts are needed in equilibrium phase interfacial film to compress the electric double layer,resulting in the decrease of the solubilization ability of the micro-emulsion systems,that is,causing the increase of initial concentration c1and salt width Δc.

Table 2Salinity range and salt width of Gemini–cyclohexane(n-hexane/n-heptane/n-octane)–water–n-butanol system for the formation of Winsor III micro-emulsion

It also can be seen from Table 2,in the micro-emulsion of the same oil phase,when monovalent salts have the same cations and different anions(NaCl,NaBr,C6H5SO3Na,NaNO3,NaI),the salt widths corresponding to the formation of Winsor III micro-emulsion are in the order of C6H5SO3Na＜NaI＜NaBr＜NaNO3＜NaCl.It shows that the order of the influence of monovalent anions on the phase behavior of micro-emulsion is C6H5SO3-＞I-＞Br-＞NO3-＞Cl-.It can be seen that the effect of organic salt counter-ions is more obvious than that of inorganic salt counter-ions on the solubilization of alkanes.On the one hand,compared with inorganic counter-ions,organic counter-ions can greatly reduce CMC,because they can exist in the barriers of micelles and reduce the electrostatic repulsion between the ionic headers of the surfactants,so that micelles are easy to generate.On the other hand,the order is due to the salt effect.The salts dissolved in the water core of the micro-emulsion droplets compress the ionic head group of the Gemini surfactant molecules,resulting in the decrease of the hydrophilicity of the Gemini surfactants[23].The type of salts also has a significant influence on the intensity of the compression degree of the ionic head group in the Gemini surfactant molecules of the micro-emulsion droplets.The benzene ring in organic counter-ion C6H5SO3-has hydrophobicity and its charge is opposite to the surfactant molecules,resulting in the partial aromatic counter-ions entering into the micelle cores.At the same time,the attraction of cation-π generated between the aromatic rings and the cations of the Geminisurfactant micelles[9].Therefore,the degree of compression of the organic counterions to the ionic head groups of the surfactants is greater than that of the inorganic counter-ions,indicating that the organic salts and the Gemini surfactants have stronger self-organization behavior[11].That is,the transformation of phase requires little salt.And different kinds of inorganic salt anions also have differences in the degree of compression of the ionic head groups in the Gemini surfactant molecules.

3.2.Optimum solubilization parameter SP*

Under the same oil phase,the optimum solubilization parameters of Gemini micro-emulsion are shown in Table 3.

Table 3The optimum solubilization parameter(SP*)for the Gemini–cyclohexane(n-hexane/n-heptane/n-octane)–water–n-butanol micro-emulsion system

It can be seen from Table 3,under the action of C6H5SO3Na,NaI,NaBr and NaNO3,the Gemini micro-emulsion has better effect on the solubilization of cyclohexane,which can achieve 100%solubilization of oil phase in a certain salinity range,that is,the Winsor IV micro-emulsion is formed.In addition,when n-hexane,n-heptane and n-octane are used as oilphase,the order of the optimum solubilizing parameter SP*of Gemini micro-emulsion under different salts is NaI＞C6H5SO3Na＞NaBr＞NaNO3＞NaCl,indicating that inorganic salt I-has great influence on the solubilization ability of the micro-emulsion system.When adding inorganic electrolytes into the micro-emulsion system,the increase of counter-ion concentration leads to the double layer compressed,the electrostatic repulsion between the polar head groups decreased and the cohesive energy increased.The degree of compression of I-is stronger than that of other inorganic salts.After adding a large number of counter-ions,the ionization equilibrium of the surfactants is affected,which promotes the formation of neutral molecules,thereby,the number of the charges on the unit area is decreased.Compared with the inorganic ion,the organic counter-ion has a higher binding coefficient and increases with the increase of relative molecular mass.When the relative molecular mass increases to a certain extent,it also has a parent property,so the effect on the system is not only the effect of the counter-ions.Because of the strong combination between positive and negative charges,the system is actually an anion/cation mixed surfactant system,so the solubilization ability is stronger than before.

3.3.Optimum salinity S*

Under different oil phases,the optimum salinities of the Gemini micro-emulsion are shown in Table 4.

Table 4The optimum salinity(S*)of the Gemini–cyclohexane(n-hexane/n-heptane/n-octane)–water–n-butanol micro-emulsion system

As can be seen from Table 4,under different oil phases,the salinity for the optimum solubilization is in the order of C6H5SO3Na＜NaI＜NaBr＜NaNO3＜NaCl,which corresponds to the order of the salt width.It shows that the influences of anionic species on the quaternary ammonium cation Gemini micro-emulsion system are different.That is to say,the self-organization behavior of anionic and cationic surfactants is affected by the type of anions.Compared with inorganic anions,organic anions have stronger compression on the ionic head groups of Gemini surfactants and can reduce the electrostatic repulsion between the ionic head groups of surfactants,which make micelles easy to generate.Therefore,C6H5SO3Na is more sensitive to the phase behavior of micro-emulsion.Different kinds of inorganic salts have different effects on the phase behavior of the micro-emulsion system,which due to their degrees of compression on the ionic head groups of the Gemini surfactant being different[5].

4.Conclusions

1)With the increase of salinity,the phase of Gemini–cyclohexane(n-hexane/heptane/n-octane)–water –n-butanol system changed from Winsor I to Winsor III to Winsor II.

2)In NaCl,NaBr,C6H5SO3Na,NaNO3and NaI,the effects of monovalent anions on the phase behavior of cationic Gemini micro-emulsion were in the order of C6H5SO3-＞I-＞Br-＞NO3-＞Cl-,and the Gemini surfactants had strong self-organization behaviors with the organic anion C6H5SO3-.

3)Under the action of different kinds of monovalent anions,the optimum solubilization amount of cationic Gemini micro-emulsion was different.For different kinds of anions,the order of the optimum solubilization parameters was I-＞C6H5SO-3＞Br-＞NO-3＞Cl-.

4)Under different oil phases,the salinities for the optimum solubilization were in the order of C6H5SO3Na＜NaI＜NaBr＜NaNO3＜NaCl,which correspond to the order of the salt widths of Winsor III micro-emulsion.