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.
The adsorption behaviour of some ethoxylated alkylphenolformaldehyde polymeric non-ionic surfactants, at the aqueous solution-air interface, was investigated by measuring the surface tension (y) as a function of concentration (C) at four temperatures. By applying the Gibbs adsorption equation to the y versus C data, the adsorption isotherms of these polymeric surfactants were obtained.Surface concentration (r) of the investigated polymeric non-ionics was found to increase with decreasing temperature and decreasing hydrophobic group (R) chain length, while varying the length of polyoxyethylene chain (% EO) appeared to have an insigificant effect. Effects of temperature, % EO and chain length R on the surface pressure (w) of the surfactant solutions were also investigated. K e y words: polymeric nonionic surfactants, ethoxylated alkylphenolformaldehyde, adsorption of polymeric nonionics
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.
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