A new type of renewable surfactants for enhanced oil recovery: Dialkylpolyoxyethylene ether methyl carboxyl betaines

Accepted Manuscript Title: A new type of renewable surfactants for enhanced oil recovery: dialkylpolyoxyethylene ether methyl carboxyl betaines Author: Binglei Song Xin Hu Xiangqiang Shui Zhenggang Cui Zhijun Wang PII: DOI: Reference:

S0927-7757(15)30341-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.11.018 COLSUA 20289

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

20-8-2015 5-11-2015 9-11-2015

Please cite this article as: Binglei Song, Xin Hu, Xiangqiang Shui, Zhenggang Cui, Zhijun Wang, A new type of renewable surfactants for enhanced oil recovery: dialkylpolyoxyethylene ether methyl carboxyl betaines, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.11.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A new type of renewable surfactants for enhanced oil recovery: dialkylpolyoxyethylene ether methyl carboxyl betaines Binglei Songa, Xin Hua, Xiangqiang Shuia, Zhenggang Cuia* [email protected], Zhijun Wangb a

The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical

and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu 214122, P.R. China. b

Solvay (China) Co., Ltd., 3968 Jindu Road, Xinzhuang Industrial Zone, Shanghai 201108, P.R. China.

*

Corresponding author.

1

Graphical Abstract

10

IFT/mN m-1

1

0.1

0.01 EO groups

0.001 6

8

10 ACN

2

12

14

Highlights 1. Carboxyl betaines with double alkylpolyoxyetheylene ether chains were synthesized 2. Effects of EO number on solubility and surface activity were evaluated 3. IFT behavior of these products for surfactant-polymer flooding was evaluated 4. Inserting EO groups improves solubility and reduces adsorption loss on sandstone 5. The new surfactants with EO groups keep small cross section area at interfaces

3

Abstract To avoid side effects in Alkali-Surfactant-Polymer (ASP) flooding due to use of caustic alkalis, Surfactant-Polymer (SP) flooding free of alkali has been paid more attention in recently years. This calls surfactants more hydrophobic than those used in ASP flooding. A carboxyl betaine with double long alkyls, didodecylmethylcarboxyl betaine (diC12B), is such a candidate which behaves very well in reducing crude oil/water interfacial tension (IFT) but suffers from poor aqueous solubility and large adsorption loss by negatively charged sandstone. In this paper a new type of carboxyl betaines derived from renewable materials were synthesized by inserting EO groups between the long alkyl chains and head groups, and both homogenous compounds (diC12EnB, n=2, 3, 4) and a mixture of homologues diC12-14EnB (n=2.2) derived from commercially available coconut alcohol polyoxyethylene ether were obtained and their properties as surfactants for SP flooding free of alkali were evaluated. The results show that with increasing EO number the solubility of the surfactants increases whereas the adsorption loss decreases, and accordingly the hydrophilicity of the surfactants increases, leading to a decrease of their preferred alkane carbon number (Nmin). Using these surfactants solely ultra low IFT (<0.01mN/m) can be achieved by diC12E2B, diC12E4B, and diC12-14E2.2B against C13~15, C7~9, and C8~9 n-alkanes, respectively, and an order of 10-2 mN/m IFT between Daqing crude oil and connate water can be achieved. However, ultra low IFT between Daqing crude oil and connate water can be achieved by using these surfactants via formulations. In comparison with diC12B the new surfactants with EO groups have both improved aqueous solubility and lower adsorption loss when employed in SP flooding free of alkali.

Keywords: Surfactants for enhanced oil recovery; Double chain carboxyl betaines; EO groups; SP flooding; Ultra low interfacial tension; Adsorption

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1. Introduction Enhanced oil recovery by surfactant flooding has been a sustained subject in the past decades [1-3], especially in China [3-4], where the output of several giant oil fields has come down naturally [5] and the self-sufficient in crude oil supply has successively dropped. On the other hand, about 60% of the original oil in place (OOIP) remains underground after water flooding as separated oil droplets trapped in porous rocks by capillary force[3, 6-7]. Researches have indicated that among the residual oil ca. 20% OOIP can be recovered by surfactant flooding [3]. The mechanism involves mainly adsorption of surfactants at the crude oil/connate water interface and achievement of ultra low (<0.01mN/m) interfacial tension (IFT), which enable the trapped oil droplets overcome the capillary force and mobilize at water flooding pressure[1, 3, 7]. During the past half century various surfactant flooding techniques based on the ultralow IFT mechanism have been developed [3, 8-11], such as alkali flooding [10], microemulsion flooding [12-14], and alkali-surfactant-polymer (ASP) flooding [3, 8]. Among these techniques the ASP flooding has been paid more attention [3-4, 8-9] due to both high effectiveness and low cost, where the polymer is used to increase the viscosity of the flooding water, and the alkali is expected to react with saponifiable components in crude oil to produce surfactants in situ, which together with the surfactants added reduce the IFT to ultra low. Experiments have shown that in the presence of alkali, ultra low IFT is readily achieved by using some low cost surfactants [3, 9], such as petroleum sulfonates [9], alkylbenzene sulfonates [3], thanks to synergistic effects between the surfactants produced in situ and that added [3, 15-16]. However, side effects of using alkali especially the caustic alkali (NaOH) have been noticed in recent pilot and field tests [3, 17], such as scaling and corrosion in equipment and pipelines, and reduction of core permeability, mainly due to the formation of insoluble substances by the reaction of alkali with divalent cations from rock and connate water. To avoid the side effects, using mild or organic alkalis to replace the caustic alkalis [17-19] and using SP flooding free of alkali to replace ASP flooding [17, 20-28] have been paid more attention recently. 5

It has been recognized that in the absence of alkali the surfactants used in ASP flooding are mostly not effective or less effective [17, 28-29] due to lack of synergisms. The achievement of ultra low IFT thus relies on solely the performances of the surfactants added. According to the Winsor’s R ratio theory [30] such a surfactant should have both a big hydrophobe and a strongly hydrophilic head group, which are well balanced each other, and a large adsorption at the oil/water interface. For anionic surfactants, however, in the absence of alkali both the lipophilicity and the adsorption at oil/water interface decrease due to reduction of ionic strength, which seems to be not compensable by simply adding neutral electrolytes, whereas for nonionic surfactants with polyoxyethylene ether or EO chains as hydrophilic groups, the large cross section area of the EO chains at the interface limited their adsorption. Thus many efforts have be made to design and synthesize suitable surfactants for SP flooding, and zwitterionic surfactants [29, 31-37], both carboxyl betaines and sulfobetaines and some nonionic surfactants without long EO chain such as alkyl alcohol amide and alkylpolyglycoside [26-28] have been found to be effective. In general zwitterionic surfactants have an adsorption much larger than anionic surfactants at oil/water interface due to lack of electrostatic repulsion between the head groups [29]. But for the crude oils in Daqing, China, the conventional zwitterionic surfactants with single long alkyl chain (C12 ~ C18) are less hydrophobic for SP flooding. Increasing the hydrophobicity of the surfactants by lengthening the alkyl chain is not only limited by the supply of the raw materials (the major components in renewable oils and fats have hydrocarbon chains C18) but also easily leads to precipitation or crystallization of the molecules from aqueous phase [30]. Many efforts have then been made to increase the hydrophobicity of the surfactants. One route is to use unsaturated hydrocarbon chains such as olefin and that derived from oleic acid in synthesizing surfactants, where the double-bonds can be opened to connect with an aromatic hydrocarbon such as benzene or alkyl benzene by Friedel-Crafts alkylation [36-38], and surfactants with branches are found to perform even better than the lineal ones [39-40]. Another route is to introduce a large-hydrophobe into surfactant molecules, such as in Cuebert alkoxy sulfate [41-42], 6

where the starting material Cuebert alcohols have a larger branch and the total carbon number can be far greater than C18. Different from these routes, we have designed and synthesized hydrophobic surfactants with double long alkyl chains in a molecule [29, 43], which made it possible to make use of commercially available raw materials to constitute large hydrophobes in surfactant molecules and is relatively easy to realize for zwitterionic surfactants where the N atom is connected to three alkyls. As an example, didodecylmethyl carboxyl betaine (diC12B) has been synthesized and proved to be good at reducing crude oil/connate water IFT by mixing with suitable hydrophilic surfactants in the absence of alkali [29]. It is a pity that the double long-alkyl carboxyl betaine surfactants have a poor aqueous solubility and high adsorption on negatively charged sandstones [44], which limited their use in SP flooding. It has been well known that the solubility and interfacial properties of a surfactant can be significantly modified by embedding a number of ethylene oxide (EO) groups into the alkyl chains, as seen for anionic surfactants such as alkylpolyoxyethylene ether sulfate (AES) and alkylpolyoxyethylene ether carboxylates (AEC) [40], and for zwitterionic surfactants like alkylpolyoxyethylene ether dimethyl betaines and alkylpolyoxyethylene ether dimethyl oxide [45]. In this paper we report a new type of carboxyl betaine surfactants with double alkylpolyoxyethylene ether chains: didodecylpolyoxyethylene (n) ether methyl carboxyl betaines (diC12EnB) as homogeneous compounds (n=2, 3, 4) derived from lauryl alcohol, and dicoconut alcohol polyoxyethylene ether methyl carboxyl betaine (diC12-14E2.2B) as a mixture of homologues derived from commercially available coconut alcohol polyoxyethylene ether. The effect of EO number on the aqueous solubility and their interfacial properties are examined. The results show that these surfactants keep generally high surface activity of diC12B but possess improved aqueous solubility and lower adsorption on negatively charged sandstones. These surfactants which can be derived from renewable raw materials (fatty alcohols) come to be an excellent type of surfactants for SP flooding.

7

2. Experimental 2.1 Materials Bromododecane (AR, 98%) was supplied by J&K Scientific Ltd., China; diethylene glycol (AR), triethylene glycol (AR), tetraethylene glycol (CP) and sodium metal (CP) were purchased from Sinopharm Chemical Reagent Co., China; methylamine in ethanol (30~33 wt.%) was from Aladdin, Shanghai, China; commercial coconut alcohol polyoxyethylene ether with an average EO number of 2.2 (C12-14E2.2) was supplied by Sinolight Chemical Co. Ltd.; thionyl chloride, pyridine, sodium hydroxide, sodium iodide monohydrate, anhydrous sodium carbonate, hydrochloric acid (36%), all AR grade, and thin layer silica plate (GF254) were purchased from Sinopharm Chemical Reagent Co., China; chloroacetic acid of 98% purity was supplied by Solvay Feixiang Chemicals Co. Ltd., China; Sodium dodecyl sulphate (SDS, 99%), Hyamine 1622 (97%), dimidium bromide (95%), acid blue-1 (50% dye content), and quartz (50-70 mesh) particles were purchased from Sigma; all solvents and other reagents used in synthesis and purification and a series of n-alkanes (n=7~16) were AR grade from Sinopharm Chemical Reagent Co., China. Dodecyldimethyl carboxyl betaine (C12B), hexadecyldimethyl carboxyl betaine (C16B), and palmityldimethyl carboxyl betaine (C16-18B) were supplied by Solvay (Zhangjiagang) Specialty Chemicals Co. Ltd. Palmitoyl diglycol amide (PDGA) was synthesized in our lab [46]. Crude oil, connate water, polyacrylamide (PAM) with an average molar mass of 25 millions, as well as sandstones particles were supplied by Daqing Oilfield, China. All chemicals were used as received except specified. 2.2 Synthesis of intermediates and target products The synthesis of the new carboxyl betaines with double alkylpolyoxyethylene ether chains and involved intermediates are described in detail in supporting information. However a brief introduction is given below with the principle and routes shown in Scheme 1. Initially the intermediate, dodecylpolyoxyethylene (n) ether (C12En) with n=2, 3 and 4 respectively, were synthesized by Williamson reaction [47] at a yield of ca. 70%. 8

Then the second intermediates, dodecylpolyoxyethylene (n) ether chloride (C12EnCl) and C12-14E2.2Cl were synthesized by reaction of the C12En/C12-14E2.2 with thionyl chloride (SOCl2) in the presence of pyridine at a yield of 88%. After that the third intermediates, didodecylpolyoxyethylene (n) ether methyl amine (diC12EnA) and diC12-14E2.2A were synthesized by reaction of C12EnCl/C12-14E2.2Cl with methylamine (dissolved in ethanol) in a closed hydrothermal metal reactor with anhydrous ethanol as solvent to form C12EnNHCH3/C12-14E2.2NHCH3 (with a yield of 50%), followed by reaction with C12EnI/C12-14E2.2I (obtained by reaction of C12EnCl/C12-14E2.2Cl with KI) with a yield of 50~60%. The crude tertiary amines C12EnA/ C12-14E2.2A were further purified by passing through a column (45cm (L) 7.5 cm (d)) filled with chromatograph silica (FCP 300-400 mesh) with petroleum ether (60-90C)/ethyl acetate(v/v=3/2) (for washing off C12EnCl) and petroleum ether (60-90C)/ethyl acetate(v/v=1/4) (for washing off C12EnA) as eluting solvents with a yield of 50~60%. The target products, dialkylpolyoxyethylene ether methyl carboxyl betaines were then synthesized by reaction of the diC12EnA/diC12-14E2.2A with ClCH2COOLi (obtained by neutralizing ClCH2COOH with LiOH in ethanol) in ethanol with KI as catalyst at a yield of 90%. The crude betaines were further purified by either recrystallization from ethyl acetate (diC12E2B) or passing through a column filled with chromatograph silica (FCP300-400 mesh) (all other target products) with ethyl acetate/methanol (v/v=10/1) (for washing off C12EnA) and ethyl acetate/methanol (v/v=1/1) (for washing off C12EnB) as eluting solvents with a yield of 60%. 2.3 Characterization of the target products and related intermediates The structures of the pure intermediates, dodecylpolyoxyethylene (n) ether (C12En) with n=2, 3, 4, respectively, and purified carboxyl betaines with double alkylpolyoxyethylene ether chains were characterized by ESI-MS (ZMD-4000 LC/MS, Waters, USA) and 1HNMR (AVANCE III, Bruker, Switzerland) respectively. The trace amount of tertiary amines in the carboxyl betaines were determined by titration with 0.1M standard HCl-in-isopropanol solution with bromophenol blue as indicator [29], and the contents of the carboxyl betaines were measured by two-phase titration method using dimidium bromide/acid blue-1 mixed indicator in strongly acidic 9

aqueous media, where the zwitterionic surfactants were transformed to cationic surfactants [29]. 2.4 Evaluation of properties 2.4.1 Solubility measurement A series of aqueous solutions of a surfactant were prepared by dissolving the surfactant in pure water at heating, followed by settling at 250.2C for more than 4 hr. The transparence (T) at 600 nm of the solutions was then measured using a spectrophotometer. When the concentration is lower than the solubility the T is nearly 100%, whereas when the concentration is beyond the solubility the solution becomes turbid and the T decreases significantly. The T was then plotted against logarithm of concentration (log C), and the concentration corresponding to the break point of the T-logC curve was regarded as aqueous solubility of the surfactant at 25C. 2.4.2 Surface and interfacial tension measurements The surface tension of the aqueous solutions of surfactants was measured using du Noüy ring method at 250.1C. The interfacial tension (IFT) between surfactant-polymer solutions in connate water and n-alkanes or Daqing crude oil was measured using a Texas 500 model tensiometer at 450.1C. 2.4.3 Adsorption of the surfactants at sandstone/water interface Approximately 1.0 g sandstones were weighed into a series of glass bottles (7.5 cm (h) by 2.5 cm (d)), followed by adding 9 cm3 aqueous solutions of a surfactant at different concentrations. The particles were dispersed using an ultrasound probe (JYD-650, Shanghai) for 1 min. at 50 W output and the bottles were then agitated by rotating on a tube rotator at 60 rpm for 24 hr. at 450.5C and then allowed to settle for 2 hr. at the same temperature. The supernatant was then centrifuged at 5,000 rpm for 20 min. and the middle layer of solution was forced to pass through a micro-sized film of 0.45m followed by measuring the concentration by two-phase titration [29, 44] or HPLC [44] or spectrophotometric methods (C  1mM) [44]. The adsorbed amount  of a surfactant at sandstone/water interface was calculated by: 

V

C  C   mmol g  w o

eq

-1

(1) 10

where V (mL) is the volume of the solution, Co and Ceq (M) are initial and equilibrium concentrations of the surfactant, respectively, and w is the weight (g) of the sandstone.

3. Results and discussion 3.1 Characterization of the surfactants The chemical structure and abbreviated name of the carboxyl betaine surfactants with double alkylpolyoxyethylene ether chains are listed in Table 1 together with that of other surfactants involved. Their structures were characterized by ESI-MS and 1

HNMR (shown in Figures S1 to S7) and the spectra shows that the synthesized

products have structures in good agreement with that designed. For diC12EnB (n=2, 3, 4), the ESI-MS spectra indicate that they are all homogeneous compounds, with m/z = [M+Na]+ or [M+H]+, and the total number of hydrogen in a molecule is in good agreement with the formula based on the 1HNMR. For diC12-14E2.2B derived from a commercial raw material, coconut alcohol polyoxyethylene (2) ether, the ESI-MS spectrum indicates that it is a mixture of three series of homologues, C12/C12, C12/C14 and C14/C14, each with an EO number distribution. The average molar mass of the purified product is estimated to be 642.43 g/mol by two-phase titration, which gives an average EO number of 2.3 in each alkylpolyoxyethylene chain, a little higher than that (2.1) estimated by measuring the hydroxyl value (197.8) of the coconut alcohol polyoxyethylene (2) ether on the basis of an average molar mass 192 g/mol of the mixed fatty alcohol. This slightly increased EO number suggests that a little part of carboxyl betaines with zero or lower EO number may have been washed off in column chromatography together with tertiary amine due to their high hydrophobicity. Taking an average this product is abbreviated as diC12-14E2.2B. The compositions of the purified products together with that of diC12B are listed in Table 2 based on chemical analysis, where diC12EnB (n=2, 3, 4) and diC12B are white powders and diC12-14E2.2B is a yellow paste, each with small amount of solvent and moisture. These products were used in the following property measurements, which all have a purity of more than 98% after further drying.

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3.2 Aqueous solubility The aqueous solubility at 25C of the carboxyl betaines synthesized together with that of diC12B was estimated by measuring transparence of the aqueous solutions at different concentration. Solutions with a concentration lower than the solubility are transparent, with a transparency near to 100%, but that with a concentration beyond the solubility are turbid and the transparency decreases dramatically with increasing concentration. The concentration corresponding to the break point in a transparencyconcentration curve (shown in Figure S8) is taken as the solubility, as listed in Table 3 together with the surface activity parameters. It is seen that diC12B without EO group gives a small solubility, 0.048 mmol/L, in pure water at 25C, and with the increase of EO number, the solubility increases and the diC12-14E2.2B gives the highest solubility, 0.570 mmol/L, nearly 12 times that of diC12B, thanks to the presence of the members with larger EO numbers according to ESI-MS spectra shown in Figure S7. 3.3 Surface activity parameters The surface tensions of the aqueous solutions of the carboxyl betaines as a function of concentration measured by Du Noüy ring method at 25C are shown in Figure 1. The surface activity parameters including critical micelle concentration (cmc), effectiveness in reducing surface tension (cmc), saturated adsorption at air/water interface (), as well as the cross section area (a) of a molecule at air/water interface at saturated adsorption are listed in Table 3 together with that of diC12B for comparison. It is seen that by inserting 2 or 3 EO groups into each long alkyl chain in diC12B, the cmc of the surfactant increases slightly due to an increase of hydrophilicity of the surfactant, but the cmc keeps unchanged. With the EO number coming up to 4, the cmc of the surfactant increases further and the cmc increases too. As a mixture of homologues diC12-14E2.2B gives a similar cmc but larger cmc in comparison with diC12B, probably due to the presence of homologues with larger EO number which posses larger cross section area at air/water interface. Nevertheless, the saturated adsorption  of the carboxyl betaines with EO groups is still much higher than that of typical anionic or nonionic surfactants, giving a cross section area smaller than 0.3 nm2/molec. In considering of two alkyl chains present in a molecule, the 12

cross section area of each alkyl chain is less than 0.15 nm2/molec., about 1/3 that of typical surfactants such as SDS (0.53 nm2/molec.)[48]. A dense monolayer at oil/water interface with both high density of hydrophobic chains towards oil phase and large hydrophilic head groups towards water phase may then be expected, which will be beneficial for reducing crude oil/water IFT. 3.4 Reducing water/n-alkane IFT The equilibrium IFT at 45C (reservoir temperature in Daqing oil field) between aqueous solution of each surfactant dissolved in Daqing connate water (5 mM plus 1000 mg/L PAM) and a series of n-alkanes is illustrated in Figure 2. The connate water has a total salinity of 5334 mg/L and contains 34.1 mg/L Ca2+ and 6.4 mg/L Mg2+ and, respectively [29]. Normally the IFT between a surfactant solution and n-alkanes is a function of the alkane carbon number (ACN) and a minimum IFT can be achieved to a preferred n-alkane at specified conditions (the ACN of the alkane is referred to Nmin)[30]. This is true for the carboxyl betaines with double alkyl polyoxyethylene ether chains. For the homogeneous carboxyl betaines diC12EnB, the Nmin decreases with increasing EO number (n), and ultra low IFT (<0.01mN/m) can be achieved by diC12E2B against C13~15 n-alkanes and by diC12E4B against C7~9 n-alkanes, respectively. Although no ultra low IFT is achieved by diC12E3B against n-alkanes, a minimum close to ultra low (0.013 mN/m) is achieved, and the preferred n-alkane directs to n-decane (Nmin=10), which is between that of diC12E2B and diC12E4B and is quite reasonable. For diC12-14E2.2B, ultra low IFT is achieved to C8~9 n-alkanes. As a comparison the IFT between diC12B aqueous solution and n-alkanes between C9 and C16 is relatively high (>0.1 mN/m). It is seen that diC12B is not well matched with the n-alkanes in reducing IFT due to its high lipophilicity. On the other hand, the insertion of EO groups into the alkyls of diC12B enhances its hydrophilicity, and thus makes diC12EnB as well as diC12-14E2.2B well matched with the n-alkanes. 3.5 Reducing water/crude oil IFT It is well known that the IFT behavior of a crude oil is equivalent to a n-alkane, and a crude oil has usually an equivalent alkane carbon number (EACN) [30]. With n-alkanes replaced by Daqing crude oil, equilibrium IFT (the dynamic IFT measured 13

after spinning 120 min) in an order of 10-2 mN/m can be achieved by the individual carboxyl betaines with double alkylpolyoxyethylene ether chains (5mM) at 45C in the absence of alkali, as listed in Table 3. The data show that for the homogeneous products, diC12EnB, the IFT decreases with increasing EO number, and for the mixture of homologues, diC12-14E2.2B, the lowest IFT close to ultra low (1.410-2 mN/m) is obtained, probably due to the presence of homologues with larger EO number. It has been reported that the Daqing crude oil has an EACN between C9 and C10 [29], which is in general matched with the IFT data shown in Figure 2. In considering the composition complexity of the crude oil, it is usually difficult to achieve an ultra low IFT by using a single or individual surfactant, and formulations by mixing with other surfactants to adjust the hydrophile-lipophile balance for matching well with the crude oil are necessary [22, 25]. Figures 3 to 6 illustrate both dynamic and equilibrium IFT behavior of some formulations containing the carboxyl betaines with double alkylpolyoxyethylene ether chains against Daqing crude oil at 45C in absence of alkali, from which the effects of EO number can be distinctly evaluated. It has been previously reported [29] that for the Daqing crude oil and connate water ultra low IFT can be achieved by using diC12B mixed with hexadecyl dimethyl carboxyl betaine (C16B) and dodecylpolyoxyethylene (3) ether sulfate (AES) at a total concentration range of 0.025~0.3 wt% (0.625~7.5 mM), where the diC12B has a molar fraction of 0.45, and both the C16B and AES are strongly hydrophilic surfactants. Figure 3 shows that ultra low IFT can be achieved by using diC12E2B mixed with C16B at a total concentration range of 1.25~7.5 mM, and the molar fraction of diC12E2B in total surfactants is increased to 0.7, indicating an increase of the hydrophilicity of the diC12E2B compared with diC12B. With diC12E2B replaced by diC12E3B, the molar fraction of diC12E3B should be increased to 0.8 in total surfactants for achieving ultra low IFT (Figure 4), and the IFT at low concentration (<2.5 mM) is beyond ultra low (>0.01mN/m). Further increasing the EO number leads to a surfactant (diC12E4B) too hydrophilic to achieve ultra low IFT against Daqing crude oil, as indicated by the fact that it should be mixed with hydrophobic surfactant 14

(diC12B) at a molar fraction of 0.2 for achieving ultra low IFT (Figure 4). It

was

observed

that

using

the

carboxyl

betaines

with

double

alkylpolyoxyethylene ether chains as main component of a formulation the ultra low IFT behavior at low concentrations is in general not good (Figure 4), either not achievable or taking relatively long time to achieve (Figure 3), probably duo to their large molecular mass which reduces the moving speed of the surfactants towards interface. This disadvantage can usually be avoided by adding surfactants with low molecular mass into the formulation. Thus the behavior at low concentration of the formulation with diC12E4B is significantly improved by adding C16B, as shown in Figure 4. The diC12-14E2.2B is more hydrophilic than diC12E4B as reflected by the composition of the formulation for achieving ultra low IFT, where it needs to be mixed with diC12B at a lower molar fraction of 0.15 and C16B at a similar molar fraction (0.25) as shown in Figures 5. With this formulation ultra low IFT can be obtained in a wide total surfactant range, from 1.25 mM to 12.5 mM. It is interesting to notice that diC12B can be removed from this formulation and replaced by a nonionic surfactant, palmitoyl diglycol amide (PDGA)[46], which is easy to prepare and cheap, and the C16B can be replaced by a commercial product C16-18B (C16/C18=0.7/0.3). The molar fraction of diC12-14E2.2B in total surfactants is reduced to 0.375 and the new formulation also gives ultra low IFT at a wide total concentration range, from 1.25 mM to 12.5 mM, as shown in Figure 5. However, at low total concentration it usually takes a long time for the IFT being reduced to ultra low, as shown by Figures 3 and 6. 3.6 Adsorption loss by sandstones It is well known that the adsorption of surfactants by sandstones not only reduces effective concentration of surfactants, but also leads to chromatographic separation of mixed surfactants, making a formulation deviating from its optimized composition. The Daqing sandstones have a complicated composition, including various rocks (90%) and clays (10%) [44], and are strongly negatively charged in pure water (= -35.3mV) and therefore have a large adsorption to cationic surfactants and carboxyl 15

betaines like C16B and diC12B but low adsorption to anionic and nonionic surfactants [44]. To evaluate the effect of insertion of EO group into diC12B on the adsorption loss by the sandstone, we have used pure quartz particles (50-70 mesh) for diC12EnB for avoiding complexity and Daqing sandstones for diC12-14E2.2B for applicability. To compare the difference of the two types of particles the adsorption isotherms of an aqueous soluble carboxyl betaine, dodecyldimethylcarboxyl betaine(C12B) on two particles were measured and shown in Figure S9. It is seen that the adsorption follows Langmuir type and the saturated adsorption of C12B on sandstone (0.019 mmol/g) is approximately 5 times that on quartz particles (0.0036 mmol/g). Since the determination of the low concentration (<1mM) of the carboxyl betaines is relatively difficult, we focused on the saturated adsorption at relatively high concentration range which can be measured using two-phase titration or HPLC methods [44]. To ensure the dissolving of the surfactants in water at relatively high concentration, the carboxyl betaines diC12EnB (n=0, 2, 3, 4) were mixed with C12B at a molar fraction ratio of 0.5/0.5. The adsorption of each component on quartz/water interface can be determined simultaneously by measuring the equilibrium concentration using HPLC method as described in Supporting Information. The experimental results are shown in Figures S10 to S13 from which we can see that in designed concentration ranges both components do reach saturated adsorption. The saturated adsorption of each component and the total saturated adsorption are listed in Table 4. The data show that the saturated adsorption of C12B is almost a constant (0.00310.00018 mmol/g), which means that the adsorption of C12B depends only on its equilibrium concentration in the systems, or the systems behave as ideal mixing which is true for mixture of homologues. On the other hand the saturated adsorption of diC12EnB decreases with increasing EO number (n), from 0.00416 mmol/g (diC12B, n=0) to 0.00200 mmol/g (diC12E4B, n=4), achieving a reduction of 52%, and the total saturated adsorption decreases accordingly. In measuring saturated adsorption of diC12B on Daqing sandstones by mixing with C12B we have found that with the molar fraction of diC12B in total surfactant increasing from 0.5 to 0.8, the total saturated adsorption is almost a constant. The total saturated adsorption can then be regarded as 16

the saturated adsorption of diC12B solely. Supposing this inference is also applicable to quartz particles, we then obtain a reduction of 35% of the adsorption of diC12E4B compared with diC12B. We therefore are able to conclude that the insertion of EO groups into the alkyl chains of diC12B will reduce the adsorption of the surfactant, and in the case of n=4, the saturated adsorption can be estimated to reduce by 35% to 52%. The diC12-14E2.2B has a better solubility in water and its adsorption loss due to adsorption by sandstone can be directly measured. The results are shown in Figure 7, where the equilibrium concentration of diC12-14E2.2B was measured using two-phase titration (1mM) [29, 44] and spectrophotometric method (<1mM) [44] respectively, and the data for diC12B is from our previous measurements [44]. The results show that the saturated adsorption of diC12-14E2.2B at Daqing sandstone/water interface is 0.00775 mmol/g, a reduction of 75% in comparison with the predicted saturated adsorption of diC12B (0.03 mmol/g, dashed line). The predicted saturated adsorption of diC12B on sandstone (0.03mml/g) is approximately 4 times that on quartz, well equivalent with that measured for C12B (5 times), indicating that the predicted saturated adsorption of diC12B are reasonable.

4. Conclusions A new type of carboxyl betaine surfactants, didodecylpolyoxyethylene (n) ether methyl carboxyl betaines (diC12EnB, n=2, 3, 4) as homogenous compounds, and dicoconut alcohol polyoxyethylene ether methyl carboxyl betaine (diC12-14E2.2B) as a mixture of homologues were synthesized and their properties as surfactants for surfactant-polymer flooding free of alkali were examined. The experimental results indicate that the solubility of the surfactants increases whereas their adsorption loss by the negatively charged sandstones decreases respectively with increasing the EO number. On the other hand these surfactants possess high surface activity such as low cmc and high adsorption at air/water interface. With increasing EO number the hydrophilicity of the surfactants increases and accordingly their preferred alkane carbon number (Nmin), decreases. Ultra low IFT (<0.01mN/m) can be achieved solely 17

by diC12E2B, diC12E4B, and diC12-14E2.2B against C13~15, C7~9, and C8~9 n-alkanes, respectively. Although only an order of 10-2 mN/m IFT between Daqing crude oil and connate water can be achieved by using these surfactants solely, ultra low IFT can be obtained using these surfactants via formulations. This type of surfactants derivable from renewable materials (fatty alcohols) have both improved aqueous solubility and lower adsorption loss by the sandstones in comparison with diC12B when used in SP flooding free of alkali.

Acknowledgement Fundamental Research Funds for key laboratories of Ministry of Education (No. JUSRP51507) and for Central Universities (No. JUSRP51405A) are gratefully acknowledged. Thanks also to Laboratory of Oil Recovery, Institute of Petroleum Exploring and Development of Daqing, China, for proving crude oil, connate water, sandstones and PAM for experiments.

18

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22

Figure Captions Figure 1. Surface tension of aqueous solutions of carboxyl betaines as function of concentration at 25C

80 diC12B diC12EO2B diC12EO3B diC12EO4B diC12-14EO2.3B

γ/mNm

-1

70 60 50 40 30 20 1.E-07

1.E-06

1.E-05

1.E-04 -1

C/molL

23

1.E-03

Figure 2. Interfacial tension at 45C between n-alkanes and the aqueous solutions of carboxyl betaines at 5 mM in Daqing connate water containing 1000 mg/L PAM

10 diC12B diC12EO3B diC12-14EO2B

diC12EO2B diC12EO4B

IFT/mNm

-1

1

0.1

0.01

0.001 6

8

10

24

12 ACN

14

16

Figure 3 Dynamic interfacial tension at 45 C between Daqing crude oil and the aqueous solutions of diC12E2B mixed with C16B a molar fraction ratio of 0.70/0.30 in Daqing connate water containing 1000 mg/L PAM at different total concentration as shown in the legend

1 1.25 mM 2.5 mM 5 mM 7.5 mM

IFT/mNm-1

0.1 0.01 0.001 0.0001 0

20

40

60 80 t/min

25

100

120

140

Figure 4 Equilibrium interfacial tension at 45 C as function of total surfactants between Daqing crude oil and the connate water solutions of diC12EnB (n=2,3,4) mixed with C16B, diC12B or both at various molar fraction ratios as shown in the legend. The aqueous phase contains 1000 mg/L PAM.

equilibrium IFT/mNm-1

1 diC12E2B/C16B=0.7/0.3 diC12E3B/C16B=0.8/0.2 diC12E4B/diC12B=0.8/0.2 diC12E4B/diC12B/C16B=0.37/0.37/0.26

0.1

0.01

0.001

0.0001 0

2

4

6

8

total surfactant concentration/mmol L-1

26

10

Figure 5 Equilibrium interfacial tension at 45 C as function of total surfactants between Daqing crude oil and the connate water solutions of diC12-14E2.2B mixed with C16B and diC12B or with C16-18B and PDGA at specified molar fraction ratios as shown in the legend. The aqueous phase contains 1000 mg/L PAM.

0.1 diC12-14E2.2B/diC12B/C16B=0.6/0.15/0.25

equilibrium IFT/mNm

-1

diC12-14E2.2B/C16-18B/PDGA=0.375/0.25/0.375

0.01

0.001

0.0001 0

2

4

6

8

10

12

total surfactant concentration/mmol L-1

27

14

Figure 6 Dynamic interfacial tension at 45C between Daqing crude oil and the aqueous solutions of diC12-14E2.2B/C16-18B/PDGA ternary mixture at a molar fraction ratio of 0.375/0.25/0.375 in Daqing connate water containing 1000 mg/L PAM at different total concentration as shown in the legend

1 1.25 mM 2.5 mM 5 mM 7.5 mM 10 mM 12.5 mM

IFT/mNm

-1

0.1 0.01 0.001 0.0001 0

20

40

60

80 t/min

28

100

120

140

Figure 7 Adsorption isotherms of diC12B and diC12-14E2.2B at Daqing sandstone/water interface at 45C

adsorbed amount/mmol g-1

1.0E-01

1.0E-02

diC12-14E2B

1.0E-03

diC12B diC12B predicted

1.0E-04 1.E-06

1.E-05

1.E-04

1.E-03 -1

Ce/molL

29

1.E-02

1.E-01

Scheme 1. Route of synthesizing carboxyl betaines with double alkylpolyoxyethylene (n) ether chains H(OCH2CH2)n OH

R(OCH2CH2)n R(OCH2CH2)n

Na Na(OCH2CH2)nOH

N

CH3 CH2COO

ClCH2COOLi

C12H25 Br R(OCH2CH2)n R(OCH2CH2)n

R(OCH2CH2)nOH SOCl2 R(OCH2CH2)nCl

KI

R(OCH2 CH2)n I

Acetone

NH2CH3 R(OCH2CH2)nNHCH3

30

N CH3

Tables

Table 1 Chemical structure and abbreviated name of the surfactants involved Abbreviation

Chemical Structure

Characteristics

diC12B

(C12H25)2N+(CH3)CH2COO

diC12E2B

[C12H25(OCH2CH2)2]2N+(CH3)CH2COO

homogeneous

diC12E3B

[C12H25(OCH2CH2)3]2N+(CH3)CH2COO

homogeneous

diC12E4B

[C12H25(OCH2CH2)4]2N+(CH3)CH2COO

homogeneous

diC12-14E2.2B

[CmH2m+1(OCH2CH2)n]2N+(CH3)CH2COO

C12B

C12H25N+(CH3)2CH2COO

mixture of homologues with m = 12~14, n = 2.2 homogeneous

C16B

C16H33N+(CH3)2CH2COO

homogeneous

C16-18B

CmH2m+1N+(CH3)2CH2COO

mixture of homologues with m = 16~18

PDGA

C15H31CONH(CH2CH2O)2H

homogeneous

-

homogeneous -

-

-

-

-

-

-

31

Table 2 Compositions of carboxyl betaines synthesized based on chemical analysis Carboxyl betaine /wt%

Tertiary amine /wt%

Moisture and volatiles /wt%

Total /wt%

Purity after drying wt%

diC12B

92.7

1.60

5.93

100.3

98.3

diC12E2B

97.5

<0.1

2.58

100.2

99.9

diC12E3B

94.1

<0.1

6.10

100.3

99.9

diC12E4B

91.9

<0.1

8.14

100.1

99.9

diC12-14E2.2B

85.2

<0.1

14.7

100.1

99.9

32

Table 3 Solubility and surface activity parameters of carboxyl betaines at 25C (mol cm )

a (nm2 molec.-1)

IFT* (mN m-1)

27.0

8.010-10

0.21

-

1.310-5

27.0

6.210-10

0.27

0.094

1.410-4

1.210-5

27.3

5.810-10

0.29

0.080

diC12E4B

1.510-4

2.010-5

29.4

5.710-10

0.29

0.021

diC12-14E2.2B

5.710-4

2.510-6

35.2

6.410-10

0.26

0.014



cmc

Solubility (mol L-1)

cmc (mol L-1)

(mN m )

diC12B

4.810-5

3.710-6

diC12E2B

8.910-5

diC12E3B

Surfactants

-1

-2

*Equilibrium IFT of aqueous solution of carboxyl betaines in Daqing connate water (5 mM) with 1000 mg/L PAM against Daqing crude oil at 45C

33

Table 4 Saturated adsorption of diC12EnB/C12B (dodecyl dimethyl carboxyl betaine) at a molar fraction ratio of 0.5/0.5 at quartz/water interface at 45 C System

by HPLC diC12EnB

C12B

C12B

by titration Total

Total

0.00365

0.0038

diC12B/C12B

0.00416

0.00315

0.00731

0.0075

diC12E2B/C12B

0.00382

0.00323

0.00705

0.0077

diC12E3B/C12B

0.00219

0.00323

0.00542

0.0062

diC12E4B/C12B

0.00200

0.00279

0.00479

0.0050

34