Hydration pressure of a homologous series of nonionic alkyl hydroxyoligo(ethylene oxide) surfactants Helge Pfeiffer,* a Hans Binder, b Gotthard Klose c and Karel Heremans a a Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200 D, B-3001, Leuven, Belgium. E-mail: Helge.Pfeiffer@fys.kuleuven.ac.be; Fax: (32) 16 32 79 82; Tel: (32) 16 32 75 26 b Universita ¨ t Leipzig, Interdisciplinary Centre for Bioinformatics, Kreuzstr. 7b, D-04103, Leipzig, Germany c Universita ¨ t Leipzig, Institute of Experimental Physics I, Linne ´str. 5, D-04103 Leipzig, Germany Received 23rd October 2003, Accepted 1st December 2003 F|rst published as an Advance Article on the web 19th December 2003 The hydration pressure of a homologous series of nonionic surfactants of the type CH 3 (CH 2 ) n1 (OCH 2 CH 2 ) m OH (C n E m ) was determined by using sorption gravimetry. The hydration pressure shows a non-exponential decay on hydration and the curves are best fitted with a rational function. The non-exponential character is explained by contributions arising from the bending energy of non-lamellar liquid crystalline phases. The parameters obtained show that hydration of pure nonionic surfactants is correlated with the number of the oxyethylene groups in the headgroup. However, they are almost independent of the length of the alkyl chain. Exceptions are surfactants in the solid crystalline state that almost prevent hydration. Furthermore, the respective mesophase structures do not dominate the hydration behaviour. The Gibbs free energy (free enthalpy) of hydration is 1.1 kJ mol 1 per oxyethylene group. 1 Introduction Hydration pressure is correlated with important interactions in colloid systems and it was originally introduced by Lang- muir. Later, it was extensively discussed for the disjoining pressure of phospholipid membranes because it is important for the approach of biological surfaces (e.g. cell fusion, stress on cartilage, osmotic dehydration, freezing induced dehydra- tion 1 ). Hydration pressure (also expressed by the hydration force) is a general phenomenon, i.e. it also plays a role for sur- factants, DNA, proteins, polyelectrolytes and polysacchar- ides. 2,3 It represents a measure of the resistance to remove hydration water from a hydrophilic surface. A number of pos- sible origins of this phenomenon have been discussed in var- ious theoretical and experimental descriptions. 4–8 We use the empirical definition: hydration pressure is the hydrostatic pres- sure which maintains the chemical equilibrium of hydration water that is attached to a hydrophilic surface with a water phase existing under reference conditions (see also refs. 9 and 10). Biomembranes are complex systems and therefore, model systems are required to study the basic properties of lipid mem- branes in detail. One of these model systems are phospholipid membranes modified by nonionic surfactants such as alkyl hydroxyoligo(ethylene oxide) surfactants, CH 3 (CH 2 ) n1 - (OCH 2 CH 2 ) m OH (C n E m ). Many studies were performed to investigate the structural and thermodynamical consequences of the incorporation of surfactants of this oxyethylene-type in model membranes. 11–16 Moreover, C n E m are also useful tools for applications in biochemistry 17,18 such as for the solubilisation of biomembrane components. 19,20 This paper will present a systematic study on the hydration of pure C n E m surfactants as a function of the hydrophobic alkyl chain length, n, and of the number of the hydrophilic oxyethylene groups, m. The paper continues previous investi- gations from Klose et al. 21 In this study, the hydration behaviour of the homologous series C 12 E m was examined for different headgroup sizes (m ¼ 2–8) and two selected rela- tive humidities (84.2 and 97.0%). Klose et al. found that the number of adsorbed water molecules grows linearly with the number of the oxyethylene groups and that the effective water binding energy increases by 7 kJ mol 1 by adding one oxyethylene group. The present study reports sorption isotherms performed over the whole humidity range and it includes the investigation of surfactants with varied hydrophobic chain length (C n E m ; C n E 4 with n ¼ 8, 10, 12, 14, 16 and C 12 E m with m ¼ 2, 4, 6, 8). It should be expected that the variation of the hydrophobic chain length has little influence on the hydration parameters in contrast to a variation of the size of the hydrophilic head- group. This study reports these differences for the given homologous series. The hydration pressure was derived from the sorption iso- therms according to the ‘‘ osmotic stress method ’’. 22 Hydration pressure shows for phospholipids an exponential decay on hydration in the case of lamellar phases, 9 and an exponential decay 6 or a rational decay in the case of hexagonal phases. 23,24 Another study performed on C n E m surfactants fitted the hydra- tion pressure curves (hydration force curves) of lamellar phases and hexagonal phases by an exponential function. 25 The C n E m surfactants show in fact a variety of liquid crystalline phases 17,18 and we found an empirical description to fit all of these curves by a simple rational function. We try to give an explanation for the rational decay behaviour and we present the corresponding parameters. Furthermore, the sorption iso- therms gave the Gibbs free energy (free enthalpy) of hydration. 2 Materials and methods The nonionic surfactants of the type CH 3 (CH 2 ) n1 - (OCH 2 CH 2 ) m OH C n E m were obtained from Fluka Chemie PCCP www.rsc.org/pccp RESEARCH PAPER 614 Phys. Chem. Chem. Phys. , 2004, 6, 614–618 This journal is Q The Owner Societies 2004 DOI: 10.1039/b313445h