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Effect of inorganic ions on surface properties of a non-ionic surfactant at various temperatures in aqueous medium
Ram Partapa,*& O P Yadavb
aDepartment of Chemistry, GovernmentCollege,
Hisar 125 001, India
bDepartment of Chemistry,
CCS Haryana Agricultural University Hisar 125004, India
Received 14 December 2007; revised 6 June 2008
Studies on surface tension of iso-octyl phenoxy polyethoxy ethanol in water and in the presence of aqueous Na2SO4 and Na3PO4 salts having varying anion valency at 288, 293 and 298 K are reported. The values of critical micelle concentration, maximum surface excess concentration and minimum area per molecule of the surfactant at air-liquid interface have been evaluated. Thermodynamic parameters of micellization and those of adsorption have also been evaluated. The micellization process appears to be endothermic in nature while negative values of G0m and positive values of S0m support its spontaneity.
The solution characterisation and intermolecular interactions of surface active substances in aqueous medium are supposed to be landmark in the fields of interaction of medicinal solutions, agrochemicals, detergency, solubilizing power, enhanced oil recovery and in metallurgical process1-4. There have been several reports5-7 on physico-chemical properties such as surface tension, specific conductance, viscosity, fluorescence, dye solubilization, etc. on ionic as well as non-ionic surfactants in aqueous solutions. Though a number of reports have appeared on the surface and thermodynamic properties of surfactants in aqueous electrolyte solutions at temperatures 298 K or above, yet the data for such systems at low temperatures are scanty8-13. We report herein the data for critical micelle concentration (CMC), surface pressure at CMC (cmc), surface excess concentration (max), minimum area per molecule at the air-liquid interface (Amin), and thermodynamic parameters of micellezation/adsorption for iso-octyl phenoxy polyethoxy-ethanol (TX-100) in water and in aqueous sodium sulphate (Na2SO4) and sodium phosphate (Na3PO4) at 288, 293 and 298 K.
Experimental
TX-100 (SDFine, purity 98%), Na2SO4 and Na3PO4 (BDH, purity 99.9%) used were of AR grade. The solutions were prepared in doubly distilled water having a specific conductance of 2.00×10-6-1cm-1 at 298.15 K.
Surface tension of solutions was determined by dropweight method using a specially designed stalagmometer described elsewhere14. The stalagmometer was calibrated by determining the surface tension of pure liquids: benzene, carbon tetrachloride, n-hexane, acetophenone and water as standard. Reproducibility of results were within +0.2% literature values. The measurements were made in a thermostat (Tempstar, model KW201A) which provided temperature control within +0.01 K.
Results and discussion
From the plots of surface tension versus log [surfactant] for the studied systems, the CMC values have been obtained from the sharp break points. CMC values are recordrd in Table 1. The observed CMC of pure TX-100 solution at 298 K agrees well with the reported literature values15,16. It is clear from Table 1 that CMC of pure TX-100 decreases with increase in temperature. In case of non-ionic surfactants in the absence of any additive, the depression in CMC may due to the dehydration of the hydrophobic moiety of the surfactant molecules and also due to breaking of water structure17,18with increasing temperature which facilitates micellization. The process of micelle formation in non-ionic surfactants is controlled by hydrophobic interaction as well as London dispersion force.
Table 1 shows that decrease of CMC of the surfactant with the addition of Na3PO4 is more than Na2SO4. The effect of addition of Na2SO4 and Na3PO4 on the CMC value of TX-100 may be partly due to the salting out of the hydrated ethylene oxide condensate of the surfactant and partly due to ion-dipole interaction (in the Stern layer) of Na+ and negative dipole of the hydroxyl group of the surfactant19,20.
Maximum surface excess concentration (max) values at the air-liquid interface has been obtained using Gibb’s adsorption equation9:
max= – 1/2.303 nRT (d/d log C)T…(1)
Table 1 — Critical micelle concentration (CMC), surface excess concentration (max), minimum area per molecule (Amin) andsurface pressure at CMC (cmc) for Triton X-100 +electrolyte systems
[Na3PO4] / Temp. / CMC103 / max1010 / Amin102 / cmc
(mol dm-3) / (K) / (mol dm-3) / (mol cm-2) / (nm2) / (mNm-1)
Na2SO4 + H2O
0.00 / 288 / 0.49 (0.52) / 2.18 / 72.6 / 35.8
293 / 0.42 (0.44) / 2.09 / 79.4 / 36.9
298 / 0.36 (0.38) / 2.01 / 82.6 / 38.1
0.025 / 288 / 0.42 (0.42) / 1.85 / 89.8 / 41.2
293 / 0.35 (0.38) / 1.69 / 98.2 / 41.4
298 / 0.28 (0.34) / 1.58 / 105.1 / 41.8
0.050 / 288 / 0.37 (0.38) / 1.81 / 91.7 / 42.2
293 / 0.30 (0.32) / 1.60 / 103.8 / 42.5
298 / 0.22 (0.28) / 1.50 / 110.7 / 42.8
0.075 / 288 / 0.31(0.34) / 1.63 / 101.9 / 42.6
293 / 0.25 (0.28) / 1.52 / 109.2 / 43.2
298 / 0.19(0.22) / 1.40 / 118.2 / 43.2
Na3PO4 + H2O
0.025 / 288 / 0.40 (0.39) / 1.81 / 91.7 / 43.3
293 / 0.32 (0.34) / 1.66 / 100.0 / 43.7
298 / 0.25 (0.30) / 1.55 / 107.1 / 44.2
0.050 / 288 / 0.35 (0.34) / 1.65 / 100.6 / 44.4
293 / 0.27 (0.30) / 1.52 / 109.2 / 44.6
298 / 0.19 (0.28) / 1.40 / 118.6 / 44.8
0.075 / 288 / 0.28 (0.30) / 1.56 / 106.4 / 45.4
293 / 0.21 (0.26) / 1.43 / 116.1 / 45.6
298 / 0.14 (0.24) / 1.31 / 126.7 / 45.9
CMC values given in parenthesis have been obtained from the viscosity method
where ‘n’ is the number of particles released per surfactant molecule in the solution and R, the gas constant. (d/dlog C)T represents the slope of the surface tension versus log C plot below the CMC at constant temperature T. In the present investigation n=1 for non-ionic surfactant. The calculated values for maxfor the studied systems at three temperatures are also presented in Table 1. It may be seen from Table 1 that the maxvalues decrease with the increase in temperature which may be due to the enhanced molecular thermal agitation at higher temperature. These results are in conformity with results reported elsewhere21. A further decrease in maxvalues in the presence of aqueous Na2SO4 and Na3PO4 may be due to the fact that addition of these electrolytes causes a partial displacement of surfactant molecules from the air-liquid interface to the bulk phase.
The minimum area per molecule (Amin) at the liquid-air interface has been calculated using the relation:
Amin = 1014/N max…(2)
where ‘N’ is Avogadro's number. Amin values for the studied systems are given in the Table 1. An examination of these values reveals that Amin increases both with the increase in temperature as well as with the concentration of Na2SO4 and Na3PO4 in the surfactant solution. This behaviour can be explained in terms of the enhanced compatibility of surfactant with the solvent in the presence of electrolytes, thereby, causing a shift of surfactant molecules from air-liquid interface to the bulk phase.
Surface pressure at CMC (cmc), an index of surface tension reduction at CMC, has been calculated using the equation9:
cmc = 0 – cmc…(3)
where 0 = surface tension of water and cmc = surface tension of surfactant solution at CMC.cmcvalues (Table 1) show marginalincrease with increase in temperature.
The thermodynamic parameters like standard Gibb’s energy change of micellisation (G0m), standard enthalpy change (H0m) and standard entropy change (S0m)(Eqs 4-6) have been calculated using the equations9, respectively:
G0m =RT ln X…(4)
(where X is the surfactant mole fraction at CMC)
S0m = [ –d(G0m )/ dT]P…(5)
H0m =G0m + TS0 m…(6)
Table 2 — Thermodynamic parameters of the micellization/adsorption for Triton X-100 system[Na2SO4] / Temp. / –G0m/–G0ad / H0m / H0ad / S0m / S0ad
(mol dm-3) / (K) / (kJ mol-1) / (kJ mol-1) / (kJ mol-1K-1)
Na2SO4 + H2O
0.00 / 288 / 27.9 / 29.5
293 / 28.7 / 30.5 / 21.1 / 28.1 / 0.17 / 0.20
298 / 29.6 / 31.5
0.025 / 288 / 28.3 / 30.5
293 / 29.2 / 31.6 / 28.8/ 38.4 / 0.198 / 0.239
298 / 30.2 / 32.8
0.050 / 288 / 28. 6 / 30.9
293 / 29. 6 / 32.2 / 37.3/ 49.3 / 0.228 / 0.278
298 / 30.8 / 33. 7
0.075 / 288 / 28.9 / 31.5
293 / 30.0 / 32.8 / 35.1 / 50.7 / 0.222 / 0.285
298 / 31.2 / 34.4
Na3PO4 + H2O
0.025 / 288 / 28.4 / 30.8
293 / 29.4 / 32.0 / 33.6 / 44. 5 / 0.215 / 0.261
298 / 30.5 / 33.4
0.050 / 288 / 28.7 / 31.4
293 / 29.8 / 32.8 / 43.7 / 55.7 / 0.251 / 0.302
298 / 31.2 / 34.4
0.075 / 288 / 29.2 / 32.1
293 / 30.4 / 33.6 / 49.6 / 62.8 / 0.273 / 0.329
298 / 31.9 / 35.4
The various thermodynamic parameters of micellization calculated using Eqs (4)-(6) are presented in Table 2. The G0mvalues are found to be negative indicating the spontaneity of micellization process in aqueous system. The S0mvalues are positive for the studied systems. It may be due to breaking of water structure when the surfactant hydrophobic chain transfers from bulk water to the micellar core. There is a further increase in S0mon adding Na2SO4 and Na3PO4 to the surfactant solution which may be partly due to water structure breaking effect of Na2SO4 and Na3PO4 and partly due to the desolvation of prehydrated9,22 ethylene oxide chains of surfactant molecules in the presence of Na2SO4
and Na3PO4.
The standard enthalpy of micellization H0mis positive due to the hydrophobic-hydrophobic interaction of surfactant alkyl chain in the process of micellization of the non-ionic surfactant (TX-100).
Thermodynamic parameters of adsorption, viz.,G0a, H0aand S0a have been calculated using the following relations23 at constant pressure:
G0a = G0m – 6.023 10 –1 cmc .Amin …(7)
S0a = – d(G0a) / dT…(8)
H0a = G0a + T S0a…(9)
The values ofG0a, H0aand S0a are presented in Table 2. The lower G0avaluescompared to G0m(Table 2) indicate that adsorption of the surfactant molecules at the air-liquid interface is preferred over the micellization. The S0avalues are all positive and larger than S0m. This may be due to more degree of freedom of the surfactant molecules at the air-liquid interface compared to that in the cramped interior of micelle24,25.
The endothermic H0amay be due to the breaking of H-bonds between polyoxyethylene chain oxygen of surfactant and water molecules at the air-liquid interface.
Acknowledgement
One of the authors (RP) is grateful to the UGC, New Delhi, for financial assistance in the form of a minor research project, and the Principal, Government College, Hisar for providing the necessary basic research facilities.
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