Adsorption behavior of branched polyoxyethylene ether carboxylate surfactants

Accepted Manuscript Title: Adsorption behavior of branched polyoxyethylene ether carboxylate surfactants Authors: Qinghong Zhang, Yunling Li, Yongbo Song, Jun Li, Zhifei Wang PII: DOI: Reference:

S0927-7757(17)30879-8 https://doi.org/10.1016/j.colsurfa.2017.09.050 COLSUA 21953

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

21-8-2017 26-9-2017 26-9-2017

Please cite this article as: Qinghong Zhang, Yunling Li, Yongbo Song, Jun Li, Zhifei Wang, Adsorption behavior of branched polyoxyethylene ether carboxylate surfactants, Colloids and Surfaces A: Physicochemical and Engineering Aspects https://doi.org/10.1016/j.colsurfa.2017.09.050 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.

Adsorption behavior of branched polyoxyethylene ether carboxylate surfactants

Qinghong Zhang, Yunling Li*, Yongbo Song, Jun Li and Zhifei Wang

China Research Institute of Daily Chemical Industry, Taiyuan 030001, P. R. China

Graphical abstract air Branched chain Hydrophilic head with -COOH

Hydrogen bond COOH

1s

HOOC

water

air

Branched chain Hydrophilic head with -COONa



Electrostatic repulsion COO-

COO-

1s

water

Author to whom correspondence should be addressed, E-mail: [email protected].

Tel: +86-351-4046827.

Fax: +86-351-4040802.

Address: 34# Wenyuan Str. Taiyuan, Shanxi Province P. R. China: 030001

Abstract: In our work, branched anionic surfactant (3, 5, 7-trimethyl decanol polyoxyethylene ether carboxylate (A13EC5-Na)) has been successfully synthesized and characterized by fourier transform infrared (FT-IR) spectra. Static equilibrium surface tension, contact angle and dynamic surface tension of alcohol ether carboxylic acid (A13EC5-H), alcohol ether carboxylate (A13EC5-Na) and linear chain carboxylate (A12EC5-Na, as comparison) were investigated to study their spreading and adsorption behaviors. Electrolyte tolerance of anionic surfactants (A13EC5-Na and A12EC5-Na) was also studied to explore the application in enhanced oil recovery. The results

indicated that critical micelle concentration (CMC) and minimum area per molecule (A0) are the lowest and the surface excess concentration (m) is the highest for

nonionic surfactants (A13EC5-H). A13EC5-Na can decrease the surface tension of water to 27.43 mN/m. Due to the introduction of branched chains, m of A13EC5-Na is smaller than that of A12EC5-Na. And the adsorption efficiency and the efficiency of forming micelles of A13EC5-Na molecules are superior to that of A12EC5-Na. By contact angle and dynamic surface tension measurements, we can know that for branched products, the wetting ability is stronger, diffusion rate to the interface is faster and the time of surface tension reaching equilibrium is shorter. The adsorption process of A13EC5-Na is diffusion controlled, while the adsorption process of A12EC5-Na is mixed diffusion-kinetic adsorption mechanism. CaCl2 tolerance of A13EC5-Na is stronger than that of A12EC5-Na but NaCl tolerance of A12EC5-Na is stronger.

Key words: branched anionic surfactants; spreading; adsorption behaviors; diffusion.

1. Introduction Surfactants are indispensable components in laundry, cosmetics and household cleaning products, accounting for 15%-40% in total detergent formulation [1]. Anionic surfactants are especially important which have the ration of 60% in the world production [2]. Exploring low surface energy surfactants has attracted much attention due to their applications in printing, painting, emulsification/suspension used in cosmetics and medicines and washing etc. [3-5]. It is reported that chain branching can lead to lower surface tensions (γcmc) in comparison with linear analogues [6-9]. Mohamed et al. [10] studied short branched hydrocarbon surfactants like the trichain (TC14) can obtain surface energy of 27 mN/m which is said to be low for hydrocarbon surfactants and comparable to a fluorocarbon surfactant DiHCF4 (γcmc = 26.8 mN/m) [11,12]. Zhang et al. [13] synthesized a series of polyoxyethylene alkyl ether carboxylates then investigated their properties, and they raised that the

introduction of branching in the hydrocarbon chain contributes to higher surface activity and higher foam volume than linear chain surfactants. Alexander et al. [14] also concluded that branched hydrophobic chain anionic surfactants can decrease the surface tension of water to 24-27 mN/m. These values can match by a certain fluorocarbon surfactant with serious environmental issues. So the study of branched chain surfactants is of great significance. Wetting ability on low energy solid surface is crucial in our daily life and some industrial applications [15-17]. As is known to us all, wetting on the solid surface of water is difficult, so surfactants are generally used to improve the wettability of water on solid surface by changing the surface tension of water. There are many literatures about the study of various wetting. Liu et al. [18] studied wetting behavior on paraffin film of anionic-nonionic surfactants with different ionic headgroups (carboxylate, sulfonate and sulfate) and they concluded the surface activity and spreading ability of the three surfactants decrease in the order AE3C > AE3S > AE3SO. Chang et al. [19] proposed the adsorption on solid surface polytetrafluoroethylene (PTFE) would rely on hydrophobic interaction and the van der Waals force. But Paraffin is a petroleum-based alkane mixture, which makes its surface hydrophobic. Nascimento [20] investigated five nonionic surfactants with different oxyethene (EO) number to evaluate their wettability and they found with the increasing of EO number, their wettability on paraffin showed a drop. It is quite interesting to study wetting behavior of surfactants. Now anionic surfactants have attracted considerable interest due to their excellent performance at water-solubility, low Krafft point, high salinity and temperature tolerance. They generally contain two hydrophilic groups: nonionic hydrophilic groups (polyethylene oxides groups) and anionic hydrophilic groups (carboxylate group, sulfonic group and sulfate group) which make them possess characteristics of both anionic and nonionic surfactants. And there are a great deal of studies on fat alcohol ether sulfates (AES) [21-23]. Alkyl polyoxyethylene ether

carboxylates are also a kind of anionic-nonionic surfactant whose terminal group is COOH. They show high surface activity, excellent biodegradability and salt resistance [24]. Many papers were mainly focused on the application performances and surface properties of alcohol ether carboxylates [25-27]. In the present work, branched alkyl polyoxyethylene ether carboxylates (3, 5, 7-trimethyl decanol polyoxyethylene ether carboxylates (A13EC5-Na)) have been synthesized by one step oxidation. The structures of raw material A13EO5 and synthesized product A13EC5-Na are displayed in Scheme 1. The static surface tension, dynamic surface tension, dynamic wetting ability on the parafilm of different concentration A13EC5-H, A13EC5-Na and A12EC5-Na solutions (A12EC5-Na as comparison) were investigated to explore the spreading and adsorption of surfactant solutions at air/water interface. What’s more, their electrolyte tolerance was also tested in the paper. OH

O O

4

ONa

O O

A13EO5

4

A13EC5-Na

O

Scheme 1. The structures of raw material (A13EO5) and synthesized product (A13EC5-Na). 2. Experimental section 2.1. Chemicals and materials Branched alcohol polyoxyethylene ether (A13EO5, hydroxyl value = 128.86, averaged molecular weight = 435.37) with purity higher than 99% was obtained from Sinolight Surfactants Technology Co., Ltd. Straight chain alcohol ether with twelve carbon (A12EO5) was natural fatty alcohol ether obtained from Sinolight Surfactants Technology Co., Ltd. This nonionic surfactant was thought as raw material of oxidation reaction. Oxygen was purchased from Taiyuan Iron & Steel Co., Ltd of China. Catalyst was multi-component palladium carbon catalyst made by ourselves.

2.2 Experimental techniques The structure of synthesized products was characterized by Fourier transform infrared (FT-IR) spectrometer (Bruker Vertex 70, Germany) by KBr plate method. Equilibrium surface tension of surfactant aqueous solutions was operated on a Krüss K12 Processor Tension meter (Wilhelmy plate method) at the temperature of 25 ± 0.2 ºC. Before measurement, surface tension of triply water was measured to reach 72.5 ± 0.5 mN/m to adjust the instrument. Every surfactant solution was stabilized for 5 min in the thermostatic bath before measurement. The dynamic surface tension of surfactant aqueous solutions was measured at 25 ± 0.1ºC using a Krüss bubble pressure tensionmeter BP100 method, which involves measuring the maximum pressure necessary to blow a bubble in a liquid from the tip of a capillary. Effective surface ages of the measurements range from 0.01 s to 250 s. The contact angle measurements of surfactant solutions were performed on a Drop Shape Analyzer DSA255 (Krüss Company, Germany) at 25ºC to measure the wetting ability on parafilm of surfactant solutions. A piece of parafilm was placed on glass pane in front of microscope connected to a horizontal CCD camera which displays dynamic spreading process of drop. The contact angle was obtained by software calculation. Each experiment was repeated at least three times to ensure the accuracy of results. The electrolyte tolerance experiment was carried out by the transmittance measurement of surfactant solutions at different concentrations using a UV-1601 spectrophotometer (Beijing Beifen-Ruili Analytical Instrument Company, China) at 300 nm. The temperature was kept at 25 ± 0.1ºC. Each measurement was repeated at least three times to reduce errors. 2.3. Preparation of sample Branched alcohol polyoxyethylene ether carboxylate (A13EC5-Na) and straight chain alcohol ether carboxylate (A12EC5-Na) were synthesized by oxidation process [26, 27]. Synthesized products (A12EC5-Na) were directly evaporated and dried to

remove water to make comparison. A certain amount of products were taken out to prepare branched alcohol polyoxyethylene ether carboxylic acids (A13EC5-H). Firstly, they were adjusted to the pH of 3 to ensure full conversion of alcohol ether carboxylic acids. Secondly, they were placed into water bath with the temperature of 90ºC to make organic phase stratified. After stratifying completely, the yellow organic phase (alcohol ether carboxylic acid) was rapidly extracted by separating funnel. Then, deionized water of 90ºC was added into organic phase and repeat above operation for two times. Our aim was to remove sodium chloride, hydrochloric acid and a part of fatty alcohol ether adequately. Finally the products was evaporated and dried to anhydrate. So A13EC5-H was obtained for a series of research and A13EC5-Na can be obtained by the reaction of A13EC5-H with 20% sodium hydroxide solution. The reaction equation was as follows:

R(OCH2CH2)nOH + O2

Pd/C NaOH

R(OCH2CH2)n-1OCH2COONa

3. Results and Discussion 3.1. Characterization of sample The significant changes in structure of synthesized products have been confirmed by FT-IR spectra. The FT-IR spectra of acid products (A13EC5-H) and raw material (A13EO5) were shown in Fig. 1. The peak of 2855 cm-1 was corresponding to stretching vibration of -C-H both in A13EC5-H and A13EO5. The absorption band of -C-O-C- appeared at 1117 cm-1 both in the spectra of A13EC5-H and A13EO5, which was due to the existence of polyoxyethylene ether groups in the structure of A13EC5-H and A13EO5. An obvious absorption peak at 1740 cm-1 was regarded as the stretching vibration of -C=O. The reason why the strong absorption band appeared at 3410 cm-1 of A13EO5 is the existence of -OH group in A13EO5. We can see the broad peak of 3410 cm-1, which is for the influence of carboxyl group in A13EC5-H. These characteristic peaks implied that A13EC5-H was obtained from the reaction of A13EO5.

Fig. 1. FTIR spectra of A13EO5 and A13EC5-H 3.2. Equilibrium surface tension Surface activity is the most basic property of surfactants. Surface tensions of A13EC5-H and A13EC5-Na aqueous solutions were measured to evaluate their surface abilities and that of linear dodecyl fatty alcohol polyoxyethylene ether carboxylates (A12EC5-Na) was also measured to make comparison. Static surface tensions of different concentration of A13EC5-H, A13EC5-Na and A12EC5-Na aqueous solutions were investigated at 25°C (seen as Fig. 2). Critical micelle concentration (CMC) and surface tension at this concentration (γcmc) can be shown in surface tension curves. From Fig. 2, we can know surface tension of water declines sharply once surfactant solutions are added and it remains relatively constant above the CMC. The curve is different from that of single-tailed anionic surfactant sodium 3, 6, 9, 12, 15-pentaoxaheptacosanoate (AEC4-Na) and monoalkyl phosphate (MAP) salts reported by Takaya Sakai [28, 29]. Their curves have two break points, that is, surface tension firstly decreases and then increases and finally keeps balance. Takaya Sakai thought vesicles formed in the first break point and transition of vesicle-to-micelle occured in the second break point. The results were listed in the table 1, γcmc of A13EC5-Na is 27.43 mN/m, which is in accord with that of A12EC5-Na (27.14 mN/m) and lower than that of A13EC5-H (28.13 mN/m). The value is lower than γcmc of AEC9-Na (29 mN/m). It can be concluded that surface activity of A13EC5-Na is

excellent, that is, the effect of decreasing surface tension of water is great. So our branched anionic surfactants show more excellent properties than their acid products. Considering CMC, A13EC5-H is lowest and the value is 0.175 mM. The CMC of A13EC5-Na is 0.201 mM, which is basically the same with that of A12EC5-Na (0.193 mM). As is known to all, CMC can also be used as an indicator of surfactant surface activity and the smaller CMC, the lower the concentration of forming micelles for surfactants and the lower the concentration of the surface saturated adsorption. So the concentration of forming micelles for non-ionic surfactants (A13EC5-H) is the lowest which is very valuable and the concentration of forming micelles of A13EC5-Na and A12EC5-Na is comparative. In a word, the surface properties of branched surfactants A13EC5-Na are excellent.

Fig. 2. Static surface tension versus concentration of A13EC5-H, A13EC5-Na and A12EC5-Na aqueous solutions. We also calculated the maximum surface excess concentration (Γm) and the surface areas (A0) at the air-water interface of surfactants to further study their properties. They were calculated according to Gibbs adsorption equation [30]:

Γm  

A0 

1 d ( )T 2.303nRT d log C

1 Γm N A

(1)

(2)

Where R is the gas constant (8.314 J/mol·K), T is the absolute temperature (298.15 K), NA is Avogadro’s constant and (dγ/dlogC)T is the slope of the surface tension γ versus logC below the CMC. The parameter n is the number of solute species. So n is taken as 1 for non-ionic surfactants and 2 for anionic surfactants by ionization of ionic surfactants without other electrolyte. All the results are listed in Table 1. From the table, we can know that the maximum surface excess concentration of non-ionic surfactant (A13EC5-H) is higher than anionic surfactants. This is because repulsive force between ion head groups with the same charge in ionic surfactants makes ionic surfactant molecules arrange loosely on the surface adsorption layer, so the value Γm is smaller than that of non-ionic surfactants. After all, the smaller the surface area, the higher the maximum surface excess concentration. We also observed

Γm of A13EC5-Na is smaller than that of A12EC5-Na. The reason is that the steric hindrance between branched chains in A13EC5-Na molecules leads to loose arrangement on the air-water interface compared with linear surfactants A12EC5-Na. Thus the surface area of A13EC5-Na is bigger than that of A12EC5-Na. What’s more, we studied the adsorption efficiency of surfactants at air/water interface pC20 (the negative logarithm of surfactant molar concentration required to decrease the surface tension of water by 20 mN/m) and the efficiency of surfactant molecules’ adsorption to the air/water interface then forming micelles CMC/C20. The larger the pC20, the greater the adsorption efficiency of the surfactant at the air/water interface. And the larger the CMC/C20, the more efficiency of forming micelles. From Table 1, we can know pC20 and CMC/C20 value of A13EC5-H are largest, which indicates that its adsorption to the air/water interface is fastest and forming micelles is fastest. For A12EC5-Na, the pC20 and CMC/C20 value is smallest, which shows the adsorption efficiency and the efficiency of forming micelles of A12EC5-Na molecules are inferior to that of our synthesized products. In short, the adsorption efficiency of branched A13EC5-H at the air/water interface is the fastest and the formation of micelles is the easiest, correspondingly, its CMC is also the smallest. The adsorption behaviors were

illustrated in scheme 2.

Table 1 Surface properties of three surfactant solutions. CMC

γ cmc

Γm

A0

(mM)

(mN/m)

(μmol/m2)

(nm2)

A13EC5-H

0.175

28.13

2.67

A13EC5-Na

0.201

27.43

A12EC5-Na

0.193

27.14

sample

pC20

CMC/C20

0.622

5.27

32.59

1.53

1.09

5.13

27.31

1.81

0.918

4.91

15.56

air Branched chain Hydrophilic head with -COOH

Hydrogen bond COOH

HOOC

water

1s

air

Branched chain Hydrophilic head with -COONa

Electrostatic repulsion COO-

COO-

1s

water

Scheme 2. Schematic representation of adsorption behavior for A13EC5-H and A13EC5-Na molecules at air/water interface. 3.3. Wetting ability Wetting is an important process in human production and life. It is of great value to study the spreading of surfactant solutions on the solid surface due to its wide application in coating, herbicide and painting [31,32]. The wetting ability of surfactants was measured by contact angle measurement at 25°C. Fig. 3 showed the dynamic contact angles of different concentrations of A13EC5-H, A13EC5-Na and A12EC5-Na aqueous solutions on parafilm. From Fig. 3a, b, c, we can know contact angle is 100ºat the lower concentration with increasing of time. With the increasing

of time, contact angle is firstly decreased and then kept changeless at higher concentrations. We also found when the concentration is 0.05 mM, contact angle of A13EC5-H aqueous solutions begins to decrease to about 80º. But contact angle of A12EC5-Na aqueous solutions can decrease to 90ºwhen the concentration is 0.5 mM. Fig. 3d showed that dynamic contact angle curves versus different concentrations for different samples at the same time of 120 s from which we intuitively see contact angles of the three samples decrease first and then keep almost constant. Contact angle of A13EC5-H aqueous solutions is smallest (30º) and that of A12EC5-Na aqueous solutions is the largest (60º), so wetting ability of A13EC5-H aqueous solutions is more excellent than that of anionic surfactants. Young's equation was usually used to explain the wetting behavior:

 sg   lg cos    sl

(3)

Where θ is the contact angle of surfactants, γsg is gas-solid interface free energy, γlg is liquid surface free energy, and γsl is solid-liquid interface free energy. γsg and γlg can be regarded as constant because in the experiment water is the liquid media and parafilm is the solid media, whose γ is their inherent quality. γsl will decrease when surfactants are added into water, that is, γsl would decrease all the way, so in the equation, cosθ must increase and this promotes the spreading of surfactant drop on solid surface. Considering the correlation between wetting ability and diffusion of surfactant molecules, we conducted the study of dynamic surface tension of different concentration surfactant solutions.

130

a

Concentration/mmol/L 0.01 0.5

120

Contact angle/degree

Contact angle/degree

110

b

0.2

0.1 5

0.05 2

0.02 1

100 90 80 70 60 50 40

130

Concentration/mmol/L 0.01 0.5

120 110 100 90 80 70 60

40 0

20

40

60

100

80

120

0

20

Time/s

80

60

40

120

100

Time/s

130

110 Concentration/mmol/L

120

0.01 0.5

0.02 1

0.2

0.1 5

0.05 2

A13EC5-H

d

110

Contact angle/degree

Contact angle/degree

0.2

0.1 5

0.05 2

50

30

c

0.02 1

100 90 80 70

100

A13EC5-Na

90

A12EC5-Na

80 70 60 50 40

60

30 0

20

40

60

80

Time/s

100

120

0

1

2

3

4

5

Concentration/mM

Fig. 3. Dynamic contact angle curves as a function of time for different concentrations of a) A13EC5-H b) A13EC5-Na c) A12EC5-Na. d) Dynamic contact angle curves versus different concentration for different samples at the same time of 120 s. 3.4. Dynamic surface tension The dynamic surface tension method by a maximum bubble pressure technique can be used to measure the change of surface tension with the increase of time and investigate the kinetics of surfactant adsorption. Fig. 4 showed dynamic surface tension curves with surface age for A13EC5-H, A13EC5-Na and A12EC5-Na solutions of various concentrations at 25ºC. From Fig. 4, we can know that dynamic surface tension curves of the three surfactant solutions all decrease rapidly first and then keep equilibrium slowly. From Fig. 4b and c, we can surmise for surfactant system with similar structure, branched surfactants (A13EC5-Na) make the time of their surface tension reaching equilibrium shorter and make the rate faster. What’s more, the higher the concentration of surfactant solutions, the faster the rate of surface tensions reaching equilibrium (Fig. 4a, b, c). We also conclude that when the concentration is above the CMC, the rate of surface tensions reaching equilibrium is affected

minimally by the concentration. This is because that monomer concentration is approximately constant after the micelle formation. We can infer that A13EC5-H molecules diffuse fastest to the interface, and A12EC5-Na is the slowest, which is consistent with the wetting process in Fig. 3. A13EC5-H 0.01mM 0.05 mM 0.1 mM 0.2mM 1mM 5mM

Surface tension/mN/m

70 60 50 40 30

b Surface tension/mN/m

80

a

A13EC5-Na

70

0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

60

50

40

30 0

50

100

150

200

250

0

50

c

150

200

250

A12EC5-Na

90

0.01mM 0.05mM 0.1 mM 0.2mM 1 mM 5 mM

80

Surface tension/mN/m

100

Surface age/s

surface age/s

70 60 50 40 30 0

50

100

150

200

250

surface age/s

Fig. 4. Dynamic surface tension versus surface age (s) for (a) A13EC5-H (b) A13EC5-Na (c) A12EC5-Na solutions of different concentrations at 25ºC. Rosen [33] divided the DST curve into four stages when he studied changes of the dynamic surface tension with time, which were respectively induction region, rapid fall region, meso-equilibrium region and equilibrium region. The dynamic surface tension data of the first three regions fit the following equation.

 0  t t  (  )n t   m t

(4)

Where γ0 is surface tension of solvent, γt is surface tension of surfactant solutions at surface age t and γm is meso-equilibrium surface tension. n and t* are constant, n is dimensionless quantity and t* is a unit of time the same as t. Taking the logarithm on both sides of equation (4):

lg( 0  t ) /(t   m )  n lg t  n lg t 

(5)

We got three figures about A13EC5-H, A13EC5-Na and A12EC5-Na solutions using the above model (seen as Fig. 5). We can calculate the n and t* value from the fitting straight line. The time parameters ti and tm also can be obtained from the equations. lg ti  lg t   1

(6)

n

lg t m  lg t   1

(7)

n

Where ti is time for induction region to end. tm is time of the meso-equilibrium region to begin. The fall rate in surface tension rapid decline region at t*, that is R1/2, is defined by the following equation:

R1  2

0  m 2t 

(8)

The dynamic surface tension characteristic parameters are all listed in Table 2. For pure surfactant, with the increase of concentration, n value increases first, then decreases, which indicates that the diffusion rate of surfactants becomes faster in the 3

lg0-t)/(t-m)]

2

1

b

A13EC5-H 0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

3.0 2.5 2.0 1.5

lg0-t)/(t-m)]

a

0

-1

1.0 0.5

A13EC5-Na 0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

0.0 -0.5 -1.0 -1.5 -2.0

-2 -2.0

-2.5 -1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

lgt 3

c

2

lg0-t)/(t-m)]

1

A12EC5-Na 0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

-1 -2

-1.5

-1.0

-0.5

0.0

lgt

-2.0

-1.5

-1.0

-0.5

lgt

0

-3 -2.0

-3.0 -2.5

0.5

1.0

1.5

2.0

2.5

0.0

0.5

1.0

1.5

2.0

2.5

Fig. 5. lg[(γ0-γt)/(γt-γm)] versus lgt for (a) A13EC5-H (b) A13EC5-Na (c) A12EC5-Na solutions of different concentrations. high concentration region. Because n is a measure of inherent tendency of surfactant molecules to absorb at the air/water interface. Thus when the concentration increases, more surfactant molecules will diffuse to the interface layer, which contributes to larger diffusion rate. t* value is smaller and smaller and R1/2 value is larger and larger with increasing concentration of surfactant. The decrease of t* is the result of increasing adsorption barrier. Because when the concentration increases, more surfactant molecules are absorbed onto the surface of the solution, but in the later stage of adsorption, a significant number of surfactant molecules occupied on the surface hinder the adsorption of new surfactant molecules due to the increasing of repulsive interaction of molecules. So adsorption barrier increases. The increase of R1/2 value shows the rate of surface tension decreasing increases, that is, dynamic surface activity enhances with the increase of concentration. For surfactants with the same concentration, n value of A12EC5-Na is smaller than that of branched products (A13EC5-H and A13EC5-Na) and n value of A13EC5-Na is smaller than A13EC5-H. It reveals the diffusion rate of A12EC5-Na is the largest, and A13EC5-H is the slowest. We can know t* of A13EC5-Na is smaller than that of A12EC5-Na, which indicates more energy is needed to absorb to surface for A13EC5-Na. tm is the time of the meso-equilibrium region beginning. The tm of A12EC5-Na is larger than that of A13EC5-Na because surface excess value of A12EC5-Na is larger and adsorption efficiency (pC20) is smaller compared with A13EC5-Na (seen as Table 1), which needs more time to reach the meso-equilibrium region. ti is the time for the induction region to end and is related to the surface coverage of the air/water interface and to the apparent diffusion coefficient of the surfactant at short times Dap. ti of A12EC5-Na is the largest. 3.5. Diffusion Coefficient In order to test the adsorption rate at air/water interface of surfactant, we usually

use the diffusion-controlled adsorption model by Ward and Tordai [34] to analyze the dynamic surface tension. Γt  2C 0

Dt D 2  t



t

C sd( t   )

0

(9)

Where Γt is the surface excess concentration at time t, t is time, D is the apparent diffusion coefficient, C0 is the bulk concentration, Cs is the concentration in the subsurface and  is a dummy time delay variable. Because experimental data can’t fit the equation, the short and long-time adsorption behaviors could be obtained by the following asymptotic equations.

Dt 

Short time: (t )t 0   0  2nRTC 0

Long time: (t )t     eq

nRTΓeq  C0

2

(10)

 4Dt

(11)

Where γ(t) is surface tension at time t, γ0 is surface tension of solvent water, γeq is the equilibrium surface tension, Γeq is the equilibrium surface excess concentration and n is 1 for nonionic surfactants and 2 for ionic surfactants. Fig. 6 shows the plots of the DST versus short-time (t1/2) and long-time (t-1/2) for A13EC5-H, A13EC5-Na and A12EC5-Na solutions at different concentrations.

Table 2 Dynamic surface tension parameters of A13EC5-H, A13EC5-Na and A12EC5-Na with different concentrations. C(mM)

A13EC5-H

Parameter

0.01

0.05

0.1

0.2

1

5

n

0.53

0.80

0.75

0.82

0.82

0.80

t*(s)

802.93

18.89

5.32

1.76

0.26

0.07

ti (s)

10.43

1.06

0.25

0.11

0.02

3.84×10-3

tm(s)

6.18×104

335.93

112.34

29.02

4.31

1.22

R1/2 (mN/m·s)

0.028

1.17

4.16

12.58

84.48

320.80

1

5

C(mM)

A13EC5-Na

Parameter

0.01

0.05

0.1

0.2

n

0.54

0.69

0.72

0.76

0.70

0.34

t*(s)

873.48

44.72

13.50

3.23

0.16

4.14×10-3

ti (s)

12.05

1.58

0.55

0.16

5.83×10-3

4.93×10-6

tm(s)

6.30×104

1264.77

329.11

66.56

4.39

3.48

R1/2 (mN/m·s)

0.03

0.50

1.67

6.96

140.53

5431.20

1

5

C(mM)

A12EC5-Na

Parameter

0.01

n

0.51

0.56

0.55

0.54

0.48

0.31

t*(s)

6148.90

329.60

110.50

30.11

2.05

0.06

ti (s)

65.62

5.20

1.64

0.42

0.02

3.38×10-5

tm(s)

5.77×105

2.09×104

7435.70

2169.80

259.78

116.24

R1/2 (mN/m·s)

3.68×10-3

0.07

0.21

0.75

11.06

359.21

0.05

0.1

0.2

a

70

b

70

60

Surface tension/mN/m

Surface tension/mN/m

60 50 40 A13EC5-H 0.01 mM 0.05 mM 0.1 mM 20 0.2 mM 1 mM 5 mM

30

10 0.0

50 40 30 20 10

1.2

2.4 1/2

0 0.0

3.6

A13EC5-Na 0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

1.2

1/2

50

30 20 0.0

70 60

Surface tension/mN/m

Surface tension/mN/m

d

60

40

A12EC5-Na 0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

50 A13EC5-H

40

0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

30 20

1.2

2.4 1/2

0.0

3.6

1.2

1/2

2.4 -1/2

t (s )

3.6

-1/2

t (s )

70

e

3.6

1/2

t (s )

70

c

2.4 1/2

t (s )

f

70

Surface tension/mN/m

Surface tension/mN/m

60 50 40 A13EC5-Na

30

0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

20 10 0 0.0

1.2

2.4 -1/2

-1/2

t (s )

3.6

60

50 A12EC5-Na 0.01 mM 0.05 mM 0.1 mM 0.2 mM 1 mM 5 mM

40

30

20 0.0

1.2

2.4 -1/2

3.6

-1/2

t (s )

Fig. 6. Dynamic surface tension as a function of a short-time (t1/2) at various concentration of surfactant solutions: (a) A13EC5-H (b) A13EC5-Na (c) A12EC5-Na and a long-time (t-1/2) at various concentrate of surfactant solutions: (d) A13EC5-H (e) A13EC5-Na (f) A12EC5-Na. The apparent diffusion coefficient can be calculated from the slope of the DST plots versus t1/2 (short time) and t-1/2 (long time) as shown in Fig. 6 based on equation (10) and (11). The calculated results are listed in Table 3, from which we can know Dshort value of A13EC5-H increases first then decreases, but Dlong value decreases all the way with the increase of concentration. Overall, both Dshort and Dlong value of A13EC5-Na and A12EC5-Na decrease when their concentrations increase. This may be

because for ionic surfactants, electrostatic repulsion between intermolecular headgroups is enhanced when their concentrations increase, which slows down the adsorption kinetics of surfactant molecules at the air/water surface. What’s more, at the same concentration, Dshort and Dlong values of A13EC5-H are the largest which is due to the lack of effect of electrostatic repulsion and Dshort value of A13EC5-Na is larger than that of A12EC5-Na. While for Dlong, when CA13EC5-Na; when C>CMC, A12EC5-Na
A13EC5-Na

A12EC5-Na

Dshort

Dlong

Dshort

Dlong

Dshort

Dlong

(m2/s)

(m2/s)

(m2/s)

(m2/s)

(m2/s)

(m2/s)

0.01

6.12×10-12

3.07×10-10

2.22×10-12

6.88×10-11

6.78×10-13

8.09×10-10

0.05

1.52×10-11

1.80×10-11

1.90×10-12

5.45×10-12

4.91×10-13

2.74×10-11

0.1

1.55×10-11

8.96×10-12

1.19×10-12

1.72×10-12

3.56×10-12

5.45×10-12

0.2

1.32×10-11

7.95×10-12

1.11×10-12

1.18×10-12

1.75×10-13

6.77×10-13

1

2.82×10-12

2.39×10-12

5.15×10-13

7.49×10-13

2.34×10-13

4.23×10-14

5

2.17×10-13

2.56×10-13

3.07×10-14

4.83×10-13

2.43×10-14

1.30×10-14

3.6. Electrolyte Tolerance The test of electrolyte tolerance is of significance for its application in industry, detergency and surfactant enhanced oil recovery [37]. Generally, Na+, Mg2+ and Ca2+ are used to study surfactant tolerance. We chose the surfactant concentration as 6 mM,

which is close to 3 g/L. The curves of salt tolerance of A13EC5-Na and A12EC5-Na solutions against transmissivity of solutions are shown in Fig. 7. As shown in Fig. 7a, the turbidity decreased critical concentration of NaCl for A13EC5-Na is 130 g/L and for MgCl2, it is around 80 g/L. NaCl and MgCl2 tolerance of A12EC5-Na are excellent. Although added NaCl reaches saturation and MgCl2 reaches 180 g/L, A12EC5-Na solution is still clarified. NaCl tolerance of A13EC5-Na is also preferable than alky carboxylates [38] although it cannot compare with that of A12EC5-Na. For A13EC5-Na, transmissivity declined but salting-out didn’t occur when larger concentration NaCl and MgCl2 were added. NaCl tolerance was stronger than MgCl2 tolerance. From Fig. 7b, we can conclude that the turbidity decreased critical concentration of CaCl2 of A13EC5-Na is 0.4 g/L, which is stronger than that of A12EC5-Na (0.2 g/L). In summary, the order of influence of inorganic counter ions on surfactants was Na+ < Mg2+ < Ca2+. It is due to the fact that the binding of surfactant ions to monovalent counterions is dominated by regional binding, while the binding of surfactant ions to divalent counterions is mainly localized binding. When the charge of inorganic counterions is the same, it may be the effect of radius of inorganic counterions. Interestingly, after 3 months, we found that there were precipitation occuring in all the A12EC5-Na solutions with CaCl2. However, A13EC5-Na solutions were still homogeneous. This is because the existence of EO chain strengthens hydrophily of alcohol ether surfactants, so their salt tolerance is great. What’s more, the existence of branched chain in branched surfactant molecules may make themselves possess dendritic three-dimensional structure, which is chelation for metal ions. Under such circumstances, branched surfactants were not easily precipitate. Considering practical application, the salt tolerance of branched A13EC5-Na is better.

110

110 A13EC5-Na

a

b

100

A13EC5-Na

100

A12EC5-Na

90 80

90

T/%

T/%

70

80

60 50 40

70

30 60 60

20 70

80

100

90

110

120

130

140

150

160

0.1

10

1

100

CaCl2/ g/L

NaCl/ g/L 100

c

A13EC5-Na

T/%

A12EC5-Na

80

0

20

40

60

80

100

120

140

160

180

MgCl2/ g/L

Fig. 7. Transmittance curves at 300 nm for 6 mM A13EC5-Na and A12EC5-Na solutions against (a) NaCl concentration (b) CaCl2 concentration (c) MgCl2 concentration. 4. Conclusions A kind of branched polyoxyethylene ether carboxylates (A13EC5-Na) have been successfully synthesized by one step oxidation. Alcohol ether carboxylic acid (A13EC5-H) shows the lowest CMC and A0 and the highest Γm. And the adsorption efficiency of branched A13EC5-H at the air/water interface is the fastest and the formation of micelles is the easiest. The surface energy of A13EC5-Na is great, of which CMC is 0.201 mM and γcmc is 27.43 mN/m. These values are excellent for anionic surfactants and can comparable with linear chain A12EC5-Na. From contact angle and dynamic surface tension measurements, we can know the wetting ability is stronger, diffusion rate to the interface is faster and the time of surface tension reaching equilibrium is shorter for branched products. For A12EC5-Na, the adsorption process is mixed diffusion-kinetic adsorption mechanism; for A13EC5-Na, the adsorption process is diffusion controlled. The electrolyte tolerance of A13EC5-Na and

A12EC5-Na is superior. The order of influence of inorganic counter ions on surfactants was Na+ < Mg2+ < Ca2+. CaCl2 tolerance of branched A13EC5-Na is better than that of A12EC5-Na. Acknowledgment The project is funded by the Science and Technology Innovation Foundation of China SinoLight Corporation (2016) and the National Key Research and Development Program of China (No. 2017YFB0308804) and JALA Research Fund (Jala 2015). We would like to express our great gratitude to these foundations.

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