Abstract: The thermodynamics of the solution of binary mixture of surfactants has been studied.Considering the effect of counterion on the interactions between the surf- ace active ions in the micelle and at the interface,the general equations for calcu- lating the molecular interaction parameters in the micelle (β_m) and at the interface (β_σ) have been obtained:
β_m=ln[(cmc_1)/(cmc_1~0x_(1m))(c″_1/c′_1)~(K_1)]/x_(2m)~2=ln[(cmc_2)/(cmc_2~0x_(2m)(c″_2)/(c′_2)~(K_2)]/x_(1m)~2 β_σ=ln[((c_1(π))/(c_1~0(π)x_(1σ)))(c″_1/c′_1)~(K_1)]/x_2σ~2=ln[((c_2(π))/(c_2~0(π)x_(2σ)))(c″_2/c′_1)~(K_2)]/x_(1σ)~2
For the nonionic surfactants mixtures or the ionic ones in the presence of excess of inorganic electrolytes, as a limited case, the general equations reduce to
β_m=ln[(cmc_1)/(cmc_1~0x_(1m)]/x_2m~2=ln[(cmc_2)/(cmc_2~0x_(2m)]/x_(1m)~2
β_σ=ln[(c_1(π))/(c_1~0(π)x_(1σ)]/x_2σ~2=ln[(c_2(π))/(c_2~0(π)x_(2σ)]/x_(1σ)~2
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