Interaction Between A Series Of Alky-betaines And Sodium Dodecyl Sulphate

Dialkylmethylcarboxybetaine(diC₁₂B) and monoalkyldimethylcarboxybetaine(C_nB) have been found to be good surfactants applicable in alkali-free surfactant polymer flooding to enhance crude oil recovery. By mixing with other surfactants, they can reduce the crude oil/water interfacial tension (IFT) to ultralow. In this thesis the interactions and synergisms between a series of alkylbetaines and sodium dodecyl sulfate (SDS) are studied to reveal the dependences of the interactions and synergisms on molecular structure of the alkylbetaines.A series of alkylbetaine samples were first prepared and purified. Then the surface activity parameters of the individul surfactants were obtained by measuring surface tension of aqueous solutions of them and IFT between aqueous solutions of them and nonane as function of concentration at 25°C in the presence of 1:1(molar ratio)NaCl. After that by measuring both the surface tensions and IFTs and appliying nonideal mixed micelle theory and nonideal mixed monolayer theory to calculate the interaction parameter in mixed micelle,β^M, and that in mixed monolayer,β^S, and to analyze various synergistic effects.The results show that the presence of small amount of NaCl and trace of tertiary amine in the samples will not signicatly affect theγ-LogC curves of the alkylbetaines. At 25°C and in the presence of 1:1NaCl, the cmc of the individual surfactants measured via surface tension measurements and IFT measurements are similar. Strong attractive interaction in both mixed micelles and mixed monolayers was detected between C_nB (n=12¹8) and SDS, and relatively big negativeβM (-8.32～-10.65, air water interface) andβ^S (-7.47^-10.60, oil/water interface) values were calculated. The interactions are in general stronger than that in anionic-nonionic surfactant mixtures but weaker than that in anionic-cationic surfactant mixtures. Specifically the attractive interaction in mixed monolayer at oil/water interface(β^S = -0.27～-6.34) is less strong than that in mixed monolayer at air/water interface, probably resulted from the inserting of oil molecules in between the alkyltails to enhance the distance between surfactant molecules in the mixed monolayer. The synergistic effect in reducing surface tension effectiveness or reducing IFT effectiveness between C_nB and SDS at equal molar ratio depends on the alkyl length in the C_nB. Both C₁₂B and C₁₄B give strong synergistic effect while further increasing the alkyl length results in loss of this synergistic effect.Double long alkyl betaine, diC₁₂B, displays relatively high cmc and lowerγ_cmc, the molecular structure of which seems to be beneficial to enhancing the efficient and effectiveness in reducing surface tension and IFT. However, no synergistic effects were observed in either efficient or effectiveness in reducing surface tension and IFT when mixed with SDS. The surface tensions or IFTs measured for mixed systems were a little higher than that calculated based on ideal mixing theory, and positiveβ^S values, +0.601 for air/water interface and +1.33 for nonane/water interface, were then predicted, or apparently an antagonism was obtained. Similar results were obtained when mixing diC₁₂B with cationic and nonionic surfactants. The strong interaction between C_nB and SDS is mainly due to the electrostatic interactions between the quaternary ammonium in C_nB and negative headgroup in SDS. For diC₁₂B molecule, however, which has double long alkyls and is normally more closely packed in its individual monolayer and thus has a cross section area of alkyl much smaller than that of conventional monoalkyl surfactant, the inserting of any conventional surfactant molecule to form mixed monolayer will results in an increase of the cross section area of the alkyl area on average, and thus results in an increase of the surface tension or IFT.