Dynamic interfacial tension of some sulphonate systems

Nine pure sodium phenylalkane sulphonates, namely, 3-phenyltetradecane, 5-pheyltetradecane, 7-phenyltetrdecane, 3-phenylhexadecane, 5-phenylhexadecane, 7-phenylhexadecane, 1-phenyldode-cane, 1-phenyltetrdecane and 1-phenylhexadecane sodium sulphonates, were prepared and their capabilities for lowering dynamic interfacial tension (D-IFT), were evaluated in n-octane/NaOH aqueous systems. The effects of surfactant structure and the concentration of added NaOH on the tension behaviour of these anionic surfactant systems, were studied. Tension behaviour of surfactant are considered the backbone for evaluating and designing practical formulations used for surfactant flooding systems.

Keywords: Sulphonate-alkane-NaOH system. Sulphonate structure and tension behaviour Dynamic interfacial tension and Alkalinity. prepared and the interfacial tension behaviour of these sulfonates against oil phase alkane carbon number (ACN). Also, the effect of added n-pentanol and n-octanol on the IFTs of these phenylalkane sulphonates, were studied.

2.
Preparation of 1-phenyldodecane, 1phenyltetradecane and 1-phenylhexadecane, was conducted in the steps shown in figure [1 cmc and  cmc values are given in Table (1).

Dynamic Interfacial Tension (D-IFT)
D-IFT between sulphanate aqueous systems and n-Octane, was measured at 28 o C using the spinning interfacial tensiometer developed by Cayias, Schechter and wade [Cayias, J.L. et al., (1975)] the spinning tube was filled first with the aqueous surfactant solution then a droplet of an oil phase (n-octane), was introduced through a special syring . Temperature control was achieved using the built -in cell heater and insulating muff provided on Model 300 Spining Drop tensiometer, manufactured by the University of Texas at Austin , USA. For the work reported herein, the D-IFT Values of 1-phenylalkane sulphantes , were measured at 60 o C to avoid precipitation of surfactants.

The Prepared Sulphonates
Two sets of phenyltetradecane and phenylhexadecanes sodium sulphonate isomers, were prepared following the synthesis route shown in Figure (1(a)). Each set consists of three isomers in which the benzene ring is attached to carbon atoms number 3,5 and 7. Thus , the prepared sulphantes are designated 3C 14 S, 5 C 14 S, 7C 14 S in the first set and 3C 16 S, 5 C 16 S , 7C 16 S in the second set. In Figure 1(b), 1-phenyldodecane, 1-phenyltetradecane and 1-phenylhexadecane were also prepared and designated 1C 12 S, 1C 14 S and 1C 16 S, respectively.

Surface Tension of Some Phenylalkane Sulphonates
g cmc values of 3C 14 S, 5C 14 S and 7C 14 S in pure water at 28 o C were measured from the surface tension-concentration isotherm as illustrated in Figure (2). Similarly, g cmc values of the three other isomers: 3C 16 14 S, and that of hexadecylbenzene sulphonate isomers are in the same order, i.e., 7C 16 S > 5C 16 S > 3C 16 S. It is known also that the surface energy of methyl group (CH 3 ) is smaller than that of methylene group (CH 2 ). The alkyl chain of each of these isomer has two CH 3 groups. The surfactant provides more capability for lowering surface tension if the two CH 3 groups are present on the uppermost layer. Different It is well known that molecules with the same electric charge repel each other. Having the same hydrophilic group (SO 3 -), a certain distance is assumed between the surfactant molecules. This distance is equal for the three tetradecylbenzene sulphonate isomers. 3C 14 S isomer has a long linear lipophilic group (R 2 = C 11 H 23 , Table 1) and is more spatial around molecules that would allow the single bond of carbon chain turn around freely, so that this longer chain winds itself exposing CH 2 group on the outermost layer and to have the shorter R 1 chain covered (Figure 4a), so the surface energy of 3C 14 S is the highest. The difference in length between R 1 and R 2 of 5C 14 S is smaller.
The longer R 2 chain is not easy to wind itself and the shorter R 1 chain remained uncovered by R 2 , so comparatively more CH 3 groups are on the outermost layer ( Figure 4b) and the surface energy of 5  C 14 S, is lower than 3  C 14 S. The two chains, R 1 and R 2 of 7  C 14 S are both short and similar in length. The posiibility of winding themselves or covering each other is rare. The two terminal CH 3 groups of each molecule are exposed on the outermost layer ( Figure 4C), So the surface energy of 7 C 14 S is the lowest. It can be inferred that the surface activity of 3  C 16 S > 5  C 16 S > 7  C 16 S. Based on these result, one can reach a conclusion that molecular structure of surfactant would dictate the molecular arrangement state and the fraction of coverage of the CH 3 group decide the capability of lowering the surface tension.
Terminal-position phenylalkane sodium sulphonates, 1  C 12 S, 1  C 14 S and 1  C 16 Fig. 4: An Imaginary molecular arrangement illustrating that: (a) No CH 3 groups are exposed (b) CH 3 groups of the longer chains are exposed, and (c) CH 3 groups of two chains are exposed on the outermost layer.
linear alkyl group from C 12 to C 16 resulted in a considerable decrease in cmc values, whereas an insignificant change in  cmc values, was detected.

Dynamic Interfacial Tension in Alkane/Aqueous System
In figure 3, similar tendency is shown by three curves. At the begining, the interfacial tension decreases as time goes on, then reaches the minimum; that is, the equilibrium is reached. At the beginning, the adsorption rate, at n-octane/ water interface, is faster than the desorption rate, resulting in the decrease of interfacial tension [Rosen, M.J. Chapter 2. (1989)]. As both rates become equal, the interfacial tension would not vary any more and equilibrium is established.
The minimum interfacial tension of 3  C 14 S, 5  C 14 S and 7  C 14 S isomers differ in magnitute.The interfacial tension can be lowered to 1.5x10 -1 mN/m by 3 F C 14 S, to 8.5x10 -3 mN/m by 5 C 14 S and to 2.8x10 -3 mN/m by 7C 14 S, respectively ( Figure 3). The minimum interfacial tension of 3  C 16 S, 5  C 16 S and 7  C 16 S isomers is reduced to 9.5x10 -2 mN/m by 3 F C 16 S, to 4.5x10 -3 by 5  C 16 S, and to 1.5x10 -3 by 7 F C 16 S, respectively ( Figure 5). This is in accordance with g cmc values given in Table (1). This reduction in interfacial tension values of three isomers of tetradecyl-and heaxdecylbenzene sodium sulphonates, can also be explained on molecular structure. As the benzene ring shifts toward the middle of the alkyl chain, the fraction of coverage of CH 3 groups of the surfactant molecules would increase, resulting in the increase of the capability for lowering interfacial tension. It is obvious from figure (6) and (7) that in the investigated sets of tetradecyl-and hexadecyl-benzene sodium sulphonates, the position of phenyl group along the linear alkyl chain is very important.
Dynamic interfacial tension, D-IFT, values decrease when phenyl group approaches the terminal carbon atoms of alkyl chain. D-IFT values of 3  C 14 S, 5  C 14 S and 7  C 14 S can be lowered to 15x10 -2 , 85x10 -3 and 28x10 -4 mN/m, respectively. Similarly. D-IFT values of 3 C 14 S, 5 C 14 S and 7  C 14 S can be lowered to 95x10 -3 , 45x10 -4 and 15x10 -4 mN/m, respectively. From this sequence, one can deduce that 1-phenyltetradecane and 1-phenylhexadecane sodium sulphonate isomers will have higher D-IFT values than that obtained by 3  C 14 S or 3  C 16 S, respectively.  Figures (6) and (7) show that only the dynamic interfacial tension of 3 F C 14 S and 3 F C 16 S decrease as alkalinity increases without the appearance of v-shaped curve. However, 3  C 14 S and 3 F C 16 S precipitate when alkalinity is greater than 24 and 22g/L, respectively. But each of 5C 14 S and 7FC 14 S gave (D-IFT) min values at their respective optimum alkalinities of 24 g/l and 12g/L NaOH, respectively. Similarly, each of 5C 16  As NaOH concentration increases, the molecular distance is shortened readily and the number of molecules on the unit area would also increase gradually, then the winding of long chain would tend to be "stretched" gradually, leading to the decrease of interfacial tension.

Effect of Alkalinity on D-IFT
In figure (8), increasing NaOH concentration casues a steady decrease in D-IFT of 1-phenylalkane sodium sulphonates. However, precipitation of these surfactants was observed at 24, 22, and 16g/l NaOH for 1C 12

1.
In an alkane/ aqueous system of mid-position phenyl-tetradecane and phenylhexadecane sodium sulphonate isomers, IFT decreases as the phenyl group shifts toward the middle of the alkyl chain.

2.
The minimum IFT of the investigated isomers differs in magnitude and their capabilities for lowering IFT follow the sequence: 7-phenylisomers > 5-phenyl-isomers > 3-phenylisomers. This sequence is in accordance with g cmc values of these isomers.

3.
Dynamic interfacial tension minimum (D-IFT) min values of 5-phenyl-and 7phenylalkane isomers, are achieved through v-shaped curves at their respective optimum alkalinity. D-IFT of 3-phenylalkane isomers decreases as alkalinity increases without the appearance of v-shaped curve.

ACKNOWLEDGMENT
The outher wishes to offer special word of appreciation to Prof. Dr Youssef Barakat, Emiretus professor, Egyptian petroleum Research institute, for supervising this work without his valuable teachings this study would not be possible.