This paper describes the ordering degree of anionic, cationic, and zwitterionic surfactants with the increase of their packing density on Ge substrate by using Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy. This work shows new insights on the conformational order of sodium dodecyl sulfate (SDS), N-hexadecyl-N-N-dimethyl-3-ammonio-1-propane-sulfonate (HPS), hexadecyl-trimethylammonium bromide (CTAB), and dodecyl trimethylammonium bromide (DTAB). DFT and semiempirical calculations are also performed in order to evaluate the effect of headgroup hydration and counterion. The CH2 asymmetric and symmetric stretching bands for the SDS molecule show a shift of 1.7 and 0.9？cm？1 to higher frequencies as the packing density increases, while it is observed a shift of 2.6 and 2.7？cm？1 for the HPS molecule, respectively. The DTAB molecule shows a shift of 4.5？cm？1 to lower frequencies for both CH2 asymmetric and symmetric stretching bands as the packing density increases, indicating the decrease of gauche conformations and the increase of all-trans conformations over the aliphatic chain. 1. Introduction Surfactants have been widely studied due to its significance in both applied and fundamental processes: detergency, catalysis, flotation, lubrication, colloid stabilization, foaming, emulsification, protein denaturation, tension moderation in membranes, membrane permeation, and drug delivery [1]. Surfactants are also used to the syntheses of nano- and mesomaterials using their capability to form self-organized aggregate structures [2]. Therefore, the determination of the packing, ordering, and its relation to the properties of the surfactant aggregates is of fundamental importance [3, 4]. Sperline [5] and Sperline et al. [6] have studied several crystalline phases of sodium dodecyl sulfate (SDS) using transmission infrared techniques. They have pointed out some relevant aspects: (1) difficulty in assigning a degree of order to the packing of alkyl chains based on the asymmetric CH2 stretching band; (2) the molar absorptivities for the alkyl vibrational features of adsorbed structures may not be compared with those for SDS micellar solutions; (3) the relative abundances of the CH2 symmetric and asymmetric stretching bands of SDS change considerably with the crystalline phase. In other words, the nature of the packing of surfactant molecules may determine their molar absorptivities. Snyder et al. [7], Flach et al. [8], and Dicko et al. [9] show that the frequency of the CH2 asymmetric stretching feature decreases with the conformational
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