The thermodynamic properties of some low molecular weight ethoxylated alkylphenol formaldehyde polymeric surfactants have been investigated. Surface tension as a function of concentration of the surfactants in aqueous solutions was measured at 28, 38, 48 and 58"C, using the spinning drop technique.From these measurements, the minimum area per molecule at the aqueous solution/air interface (Amin) was determined. The thermodynamic parameters of micellization (AG,,,, AH,,,,, ASmlc) and of adsorption (AGdd, AH,,, ASdd) for these polymeric noniontcs were calculated. Micellization IS more sensitive to ethylene oxide chain length while adsorption is more dependent on the length of the alkyl chain.
Several equation models were investigated to find the relationship between temperature (T). number of ethylene oxide (EO) units (n) or the hydrophile‐lipophile balance (HLB) and the surface and thermodynamic properties of some ethoxylated alkylphenol‐formaldehyde polymeric nonionic surfactants. These properties include critical micelle concentration (CMC), free energy of micellization (ΔGmic), surface tension at CMC (7CMC), effectiveness (γCMC) and efficiency (pC20) of surfactant to reduce the surface tension of water. The values of the ratio CMC/C2(π = 20) were also considered.
The linear multiple regression technique was employed to determine the parameters of the equations and to choose the best forms with the highest values of R2 and F‐ratio which reflect the goodness and the reliability of the fit.
Four low molecular weight nonionic polymeric surfactants were prepared by condensing octyl-, dodecyl-, tetradecyl-and hexadecylphenol with paraformaldehyde, and then reacting the resulting resins with ethylene oxide to obtain products with the desired degree of ethoxylation. The molecular weights of the prepared alkylphenol-formaldehyde resins (prior to ethoxylation) were determined by vapour pressure osmometry.The surface tensions of aqueous solutions of these nonionic polymeric surfactants were determined by using the spinning drop method. Plotting the surface tensions obtained versus the logarithm of concentrations resulted in two lines: the pre-CMC (CMC = critical micelle concentration) line (the linear portion below the CMC value) and the post-CMC line (the linear portion above the C M C value). Least squares regression analysis was performed to get the best equation for each of the two lines. Solving these two equations simultaneously resulted in the value of the CMC and the corresponding surface tension (yCMC) for each surfactant of the four polymeric nonionic groups.The CMC values obtained for these polymeric surfactants are of the same order of magnitude obtained for monomeric and other polymeric nonionic surfactants.
The natural log of critical micelle concentration (CMC) values obtained from the natural log discontinuities in surface tension‐concentration relationships, through the least‐squares regression analysis, were plotted against the respective hydrophile‐lipophile balance (HLB) values of four groups of ethoxylated octylphenol‐, dodecylphenol‐, tetradecylphenol‐ and hexadecylphenol‐formal‐dehyde polymeric surfactants.
The obtained HLB‐CMC relationship for the investigated compounds can be represented satisfactorily by the linearized equation In (CMC) = a—b (HLB). Values of the two constants a (intercept) and b (slope) for 16 of these compounds were determined at 28, 38, 48 and 58°C, using the least‐squares regression analysis of data. The study revealed that both a and b values increase with increasing number of carbon atoms in the ethoxylates of polymeric compounds having a linear alkyl chain. The influence of branching is reflected in the values of a and b of the compounds having a branched dodecyl chain.
The most striking feature of the obtained equation is that the CMC decreases with increasing HLB (negative slope). This observation is contrary to what is generally expected for both ionic and nonionic surfactants.
The interfacial tensions (IFT) of four low molecular weight groups of ethoxylated octylphenol‐, dodecylphenol‐, tetradecylphenol‐ and hexadecyl‐phenol—formaldehyde polymeric surfactants were determined using the spinning drop method. Some noteworthy features of the interfacial behaviour of dilute aqueous solutions of 16 of these compounds and homologous hydrocarbons are discussed.
An important feature is that these surfactants behave similarly to monomeric ones in their hydrocarbon scan, that is they have a minimum IFT value against a particular member of a homologous hydrocarbon series. The magnitudes of the tension at minimum (γmin) values obtained in this study are of the order of ‘ultralow’ (10−2‐10−3 mNm−1).
The nmin values of these polymeric nonionic surfactants decrease with increasing hydrophilicity, that is decrease with the increase of ethylene oxide units condensed per mole of alkylphenol unit in the polymeric surfactants studied. In this case, the downward shift in nmin is smaller and apparently not linearly related to the number of EO units.
Increasing the hydrophobicity of these polymeric nonionics, that is increasing the length of the alkyl chain from C8 to C16, resulted in an increase in the nmin values obtained.
For each of the investigated groups, the lowest γmin values are obtained with polymeric surfactants having the highest EO content. The optimum low tension performance occurs at the low end of the equivalent alkane carbon number scale (at EACNs below 6). Under the influence of added electrolytes these EACNs were shifted to higher values.
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